Long-range atmospheric transport of mercury to ecosystems,and the importance of
anthropogenic emissions—a critical review and evaluation of the published evidence
T.A.Jackson
Abstract :Literature on the long-range atmospheric transport of both anthropogenic and naturally occurring mercury (Hg)to terrestrial and aquatic ecosystems was reviewed for the purpose of assessing the quantitative importance and environmental significance of the anthropogenic emissions.The weight of evidence,comprising many different kinds of data that
corroborate each other independently,supports the following conclusions.(i )Approximately 5000t of anthropogenic Hg are introduced into the atmosphere every year,both by direct emission from sources of pollution and by reemission of
previously deposited Hg from diffuse secondary sources in the environment.The primary emissions (~4000t)result from various human activities,especially the combustion of fossil fuels (notably coal)and solid wastes.Natural emissions amount to about 2000t/year.(ii )Although some of the annual anthropogenic Hg output (about half the quantity emitted by primary sources)is deposited near its points of origin,the rest (a total of ~3000t)is subject to transport over great distances by atmospheric circulation,resulting in measurable contamination of terrestrial and aquatic environments and organisms up to several thousand kilometers from the sources of pollution.Indeed,a number of remote ecosystems receive most of their Hg input from the atmosphere.(iii )The available evidence supports the generally accepted conclusion that the Hg enrichment commonly seen in the uppermost horizons of sediment cores from remote lakes is due primarily to contamination by airborne anthropogenic Hg in the recent past rather than postdepositional redistribution of Hg.Although postdepositional alteration may result in detectable remobilization of sedimentary Hg,its effects on total Hg profiles in lake sediment cores have been found,thus far,to be negligible.(iv )Atmospheric transport of anthropogenic Hg to aquatic and terrestrial
ecosystems is a cause for concern,as the Hg is accumulating,to a greater or lesser extent,in organisms (e.g.,fish in remote lakes).Moreover,the Hg is associated with,and interacts with,other by-products of fossil fuel combustion,including the strong acids responsible for acid precipitation.The acids aggravate the effects of Hg pollution by furthering the accumulation of methyl Hg in fish inhabiting ill-buffered lakes.Contamination of the atmosphere with Hg and associated pollutants is a serious international problem that calls for reduction or elimination of emissions.Key words :mercury,atmospheric transport,pollution.
Résumé:Afin de déterminer l’importance quantitative et la signification environnementale des émissions anthropogènes,l’auteur revoie la littérature sur le transport atmosphérique àlongue distance du mercure (Hg),anthropogène aussi bien que naturel,vers les écosystèmes terrestres et aquatiques.Une lourde preuve,comprenant différents types de données qui se
corroborent mutuellement de fa?on indépendante,supporte les conclusions suivantes.(i )Environ 5000t de Hg anthropogène sont introduites annuellement dans l’atmosphère,àla fois par des émissions directes provenant de sources de pollution et par des réémissions dans l’environnement de Hg préalablement déposé,en provenance de sources secondaires diffuses.Les émissions primaires (~4000t)proviennent de diverses activités humaines,surtout de la combustion d’énergie fossile
(principalement du charbon)et de déchets solides.Les émissions naturelles sont d’environ 2000t/an.(ii )Bien qu’une partie des émissions anthropogènes annuelles de Hg (environ la moitiédes quantités émises par les sources primaires)soit déposée près de son point d’émission,le reste (un total de ~3000t)fait l’objet d’un transport sur de grandes distances par la
circulation atmosphérique,ce qui se traduit par une contamination mesurable des environnments terrestres et aquatiques,ainsi que des organismes,jusqu’àplusieurs milliers de kilomètres àpartir des sources de pollution.En effet,un nombre d’écosystèmes éloignés re?oivent la majeure partie des arrivées de Hg,de l’atmosphère.(iii )Les preuves accumulées supportent la conclusion généralement acceptée que l’enrichissement en Hg,communément observée dans les horizons supérieurs des carottes de sédiments provenant de lacs éloignés,est principalement d?àla contamination par du Hg anthropogène venu des airs au cours des récentes années,plut?t que d’une redistribution par post-déposition.Bien que
l’altération venant d’une post-déposition puisse conduire àune remobilisation décelable du Hg sedimentaire,ses effets sur le Hg total du profil dans les carottes de sédiments lacustres se sont avérés jusqu’ici négligeables.(iv )Le transport
atmosphérique de Hg anthropogène vers les écosystèmes terrestres et aquatiques est préoccupant,puisque le Hg s’accumule,
Received September 27,1996.Accepted April 17,1997.
T.A.Jackson.Aquatic Ecosystem Restoration Branch,National Water Research Institute,P.O.Box 5050,Burlington,ON L7R 4A6,Canada.
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?1997NRC Canada
àdes degrés plus ou moins importants,dans les organismes(p.ex.,poissons de lacéloignés).De plus,le Hg est associéavec,et interagit avec,d’autres sous-produits originant de la combustion de sources d’énergie fossile,incluant les acides forts responsables des précipitations acides.Les acides aggravent les effets de la pollution par le Hg en encourageant
l’accumulation du Hg méthylédans les poissons des lacs mal tamponnéhttps://www.doczj.com/doc/c5547496.html, contamination de l’atmosphère par le Hg et les polluants associés est un problème international sérieux qui commande la réduction ou l’élimination desémissions.
Mots clés:mercure,transport atmosphérique,pollution.
1.Introduction
It appears to be the consensus among scientists(e.g.,Expert Panel on Mercury Atmospheric Processes1994)who have investigated the origin and regional or global distribution of airborne mercury(Hg)that the following conclusions are supported by the available evidence:(i)winds transport Hg (mostly in the form of Hg(0)vapour)up to thousands of kilo-meters from its points of origin,depositing appreciable quan-tities of it in remote terrestrial and aquatic ecosystems;(ii)the anthropogenic component of this Hg flux is comparable in magnitude to the natural component;and(iii)the rates at which Hg has been emitted into the atmosphere and deposited in natural environments have increased progressively over time as a result of human activities(especially the burning of fossil fuels)since the onset of the Industrial Revolution,accelerating in the mid-20th century(although there are indications that in some regions,at least,the emission or deposition of anthropo-genic Hg peaked in the latter part of the20th century and is now declining).These generalizations are founded on a wide range of complementary and mutually corroborating measure-ments and calculations.Much of the evidence for historical changes in rates of atmospheric Hg pollution comes from profiles of Hg and associated contaminants in dated cores of undisturbed sediment sequences in remote lakes where no contamination from local sources is known to have occurred. The Hg content of such a core is commonly highest at the top (i.e.,in the youngest layer of sediment)and decreases down-ward(i.e.,with increasing age),leading to the widely held opinion that core data of this kind comprise records of rela-tively recent temporal increases in Hg loading due to atmos-pheric pollution(Johnson et al.1986).In some cases there is a buried Hg maximum,implying a temporal increase in the rate of Hg deposition followed by a decrease(Munthe et al.1995; Engstrom and Swain1997).Other contaminants show com-parable trends(Lockhart et al.1993).
Recently,however,Rasmussen(1994)has questioned the quantitative significance of emissions due to human activities in the long-range atmospheric distribution of Hg,claiming that insufficient attention has been paid to the assessment of natural sources of Hg in the Earth’s crust.She has also challenged the proposition that Hg enrichment in the uppermost horizons of sediment cores from remote lakes is a record of recent in-creases in Hg loading due to air pollution,advancing the alter-native hypothesis that postdepositional redistribution of Hg can account for the observed Hg profiles.But,as discussed below,an impressive amount of evidence comprising many different kinds of information supports the conclusion that the anthropogenic contribution to the long-range atmospheric flux of Hg rivals or exceeds the natural contribution in quantitative importance.There are also grounds for concluding that Hg profiles in cores representing undisturbed sequences of fine-grained lake sediments are reliable records of temporal vari-ations in Hg loading and that postdepositional alteration of sediments has,at most,only negligible effects on the vertical distribution of Hg.Furthermore,the literature provides ample proof that environmental scientists who investigate anthropo-genic Hg emissions are well aware of the existence of natural Hg emissions and generally take them into account as a matter of course(Heit et al.1981;Johnson et al.1986;Nriagu1989; Lindqvist et al.1991;Nater and Grigal1992;Nater et al.1992; Rognerud and Fjeld1993;Mason et al.1994;Hudson et al. 1995;Lockhart et al.1995;Semkin et al.1996).
The purpose of this review is to resolve the controversy by systematically examining the available evidence and,insofar as that evidence permits,determining(i)to what extent long-range atmospheric transport is responsible for the Hg found in remote ecosystems,(ii)to what extent human activities are responsible for the atmospheric transport of Hg to these eco-systems,(iii)how Hg profiles in lake sediment cores should be interpreted,and(iv)what effect atmospheric Hg contami-nation is having on organisms.A wide range of relevant pub-lished information has been studied,and an effort has been made to develop a coherent interdisciplinary synthesis.This paper is a revised version of a Canadian government report (Jackson1995).
2.Global Hg emissions to the atmosphere 2.1.The atmospheric Hg cycle
An instructive,though drastically simplified,diagram of the biogeochemical cycle of Hg is shown in Fig.1.Detailed treat-ment of the subject is beyond the scope of this review,but a brief overview of the major pathways of atmospheric Hg will serve as a useful preliminary to our discussion of the issues of primary concern to us,i.e.,the importance of anthropogenic Hg emissions.
2.1.1.Sources of atmospheric Hg
The primary natural sources of Hg are the Earth’s crust and mantle.Volatile Hg is vented into the atmosphere from volca-noes,faults,and other openings in the crust,which serve as conduits for the degassing of the Earth’s interior,while Hg enclosed in rocks exposed at the surface is released by weath-ering(Jonasson and Boyle1972;Nriagu1989,1992;D’Itri 1991;Painter et al.1994;Rasmussen1994;Friske and Coker 1995).Hg is widely dispersed as a trace element in various rocks,but coal and fine-grained sedimentary rocks such as shales of high organic and sulfide content are enriched in Hg with respect to other common rocks;and deposits of Hg ore have been concentrated in localized zones in the Earth’s crust, and at the surface,by processes associated with vulcanism, hot spring activity,fracturing,and faulting,especially in mo-bile belts along plate boundaries(Gavis and Ferguson1972;
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Jonasson and Boyle 1972;Painter et al.1994;Friske and Coker 1995).Much natural Hg also occurs in countless diffuse sec-ondary sources in continental regions and the ocean.Hg is continually introduced into the atmosphere from both primary and secondary natural sources.
Similarly,there are many point sources and diffuse sources of anthropogenic Hg.The largest single cause of atmospheric Hg pollution is the combustion of fossil fuels,notably coal,in electric power plants and elsewhere,although important Hg emissions have also arisen from activities such as incineration of municipal refuse,chloralkali production,and the smelting of nonferrous metals (D’Itri et al.1978;Lindberg 1987;Pacyna 1986;Nriagu and Pacyna 1988;Lindqvist et al.1991;Pacyna and Keeler 1995;Petersen et al.1995).Moreover,there is a wide variety of diffuse anthropogenic sources,such as discarded batteries,discarded fluorescent lights,discarded manometer Hg,latex paints,old industrial discharges dispersed in natural environments,gold mining wastes,and fungicides used in agriculture and in the manufacture of pulp and paper (Expert Panel on Mercury Atmospheric Processes 1994).
When the impact of Hg pollution is assessed,one important complication which must be kept clearly in mind is the fact that a large proportion of the airborne anthropogenic Hg de-posited in aquatic environments (mainly the ocean)and on land is reemitted into the atmosphere through natural processes such as microbial activities (Expert Panel on Mercury Atmos-pheric Processes 1994;Mason et al.1994;Hudson et al.1995;Vandal et al.1995)(Table 1;Fig.1).Failure to take this into account results in serious overestimation of natural emissions and corresponding underestimation of anthropogenic emissions.Estimates of direct anthropogenic input to the atmosphere should never be confused with the total anthropogenic input,
which includes the anthropogenic component of Hg recycled from secondary sources in natural environments as well as the direct emissions.
2.1.2.Forms and transformations of atmospheric Hg
More than 90%of the Hg in the atmosphere as a whole (regardless of whether it is natural or anthropogenic)is in the form of Hg(0)vapour,and the remainder consists of particle-bound Hg(II)species along with trace quantities of gaseous Hg(II)species (e.g.,HgCl 2,CH 3HgCl,and (CH 3)2Hg),dis-solved Hg(II)species and Hg(0)in water droplets,and Hg(0)sorbed to particles (Fitzgerald 1986;Brosset 1987;Lindberg 1986,1987;Schroeder and Jackson 1987;Brosset and Lord 1991;Lindberg et al.1991;Schroeder et al.1991;Vandal et al.1991;Fitzgerald et al.1994;Lamborg et al.1994;Mason et al.1994;Burke et al.1995;Hultberg et al.1995;Keeler et al.1995;Pleijel and Munthe 1995).(Note that from here on,methyl (i.e.,monomethyl)Hg will be represented as CH 3Hg +,if its anion is not specified.)Although particulate Hg(II)makes up only a small portion (<5%)of the overall atmos-pheric Hg burden,it is anomalously abundant in the air over cities and industrial centres (i.e.,near sources of pollution),where it may approach 50%of the total Hg concentration (Schroeder et al.1991;Lamborg et al.1994;Keeler et al.1994,1995;Pirrone et al.1996).
Different forms and oxidation states of Hg are interchange-able in atmospheric,aquatic,and terrestrial environments,and the proportions of the different species depend on the com-bined effect of numerous physicochemical and biological vari-ables.Accordingly,Hg is subject to a variety of abiotic and microbially mediated speciation reactions,as well as sorption and desorption by particulate matter,and the abundances of
the
Fig.1.Simplified diagram of the Hg cycle (from Mason et al.1994,reproduced with permission of Geochim.Cosmochim.Acta,Vol.58,pp.3191–3198,?1994Elsevier Science Ltd.).
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various forms of Hg display considerable spatial and temporal variation.Hg(0)is prone to oxidation in the presence of O 2and other oxidizing agents,but such reactions are sufficiently retarded by kinetic barriers to enable Hg(0)to persist for con-siderable lengths of time in the atmosphere (Fitzgerald et al.1994).Speciation reactions occurring in the aqueous phase of the atmosphere (e.g.,water droplets in clouds)include oxida-tion of Hg(0)to Hg(II)by (i )O 3or (ii )H 2O 2+H +,and reduction of Hg(II)to Hg(0)by (i )H 2O 2+OH –or (ii )reaction with SO 2to form HgSO 3,followed by photochemical de-composition with release of Hg(0)(Iverfeldt and Lindqvist 1986;Schroeder et al.1991;Pleijel and Munthe 1995).Such reactions strongly affect the partitioning of atmospheric Hg between the gaseous,liquid,and particulate phases,thereby influencing the rate of Hg deposition (see section 2.1.3).For instance,uptake of Hg vapour by the aqueous phase is en-hanced in the presence of O 3,probably owing to oxidation of Hg(0)to Hg(II)(Iverfeldt and Lindqvist 1986).Moreover,speciation reactions in natural waters and,to a lesser extent,in soil affect the reemission of Hg into the atmosphere;thus,microbial reduction of bioavailable (biochemically reactive)inorganic Hg(II)to Hg(0)in water promotes evasion of Hg into the atmosphere,besides tending to lower the rate of Hg(II)accumulation in bottom sediments and the rate at which bio-available inorganic Hg(II)species are converted to CH 3Hg +by methylating microbes (Fitzgerald et al.1994;Mason et al.1994;Vandal et al.1995).In brief,Hg speciation is of crucial impor-tance,because it controls the bioavailability of Hg and affects the movement of Hg from the atmosphere to terrestrial and aquatic sinks and back again (see section 2.1.3).
2.1.
3.Transport,deposition,and recycling of atmospheric Hg About half the anthropogenic Hg introduced into the air by direct emission is deposited locally (Mason et al.1994)(Table 1;Fig.1);the rest of it is entrained by the general atmospheric circulation and may be transported hundreds or even thousands of kilometers from its point of origin before being returned on the surface of the Earth.The distribution of the Hg depends largely on wind direction (Davies and Notcutt 1996;Pirrone et al.1996)but is also a function of many other factors that affect the speciation,partitioning,deposition,and reemission of the Hg.
It is mainly owing to the volatility of Hg(0)(a property which makes Hg unique among heavy metals),as well as to the comparative inertness of Hg(0)even in the presence of O 2,that Hg is prone to long-range transport and dispersal by at-mospheric circulation (Lindberg 1987).Hg(0)is by far the dominant Hg species involved in global atmospheric trans-port,but particulate Hg(II),though comprising only a small percentage of the total airborne Hg,may play an important part in the deposition of Hg in ecosystems and is thought to be the dominant form of airborne Hg introduced into aquatic environments (Fitzgerald et al.1994;Lamborg et al.1994).Deposition of Hg(0),however,is also very important and may be more rapid than the deposition of particulate Hg if the size range of the particles is such that they are less effi-ciently scavenged by precipitation than Hg(0)is (Lindberg 1986,1987).Even atmospheric CH 3Hg +,a minor component of airborne Hg (Fitzgerald 1986),may contribute a portion,at least,of the CH 3Hg +in some remote lakes,with microbial production of CH 3Hg +within the lake accounting for the rest (Fitzgerald et al.1994;Hultberg et al.1995).Careful measure-ment of the Hg content of environmental compartments (such as air,rain,snow,runoff,lake water,seston,fish,and sedi-ment)and calculation of Hg fluxes and mass balances have led to the conclusion that in many freshwater ecosystems depo-sition of airborne Hg is the primary mechanism of Hg input (Fitzgerald et al.1991,1994;Hultberg et al 1995).
Nonetheless,
Table 1.Published estimates of annual global emissions of natural and anthropogenic Hg into the atmosphere.
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much of the Hg deposited in lakes and oceans is returned to the atmosphere by evasion of Hg(0)from the surface water, largely owing to microbial reduction of bioavailable Hg(II)to Hg(0)(Fitzgerald et al.1994;Mason et al.1994;Vandal et al. 1995)and partly,no doubt,by revolatilization of Hg(0)taken up as such from the air;the rest may be transferred to deeper water,deposited on the bottom with sediments,and to a small but varying extent,converted to CH3Hg+or(CH3)2Hg(Hudson et al.1995;Vandal et al.1995).CH3Hg+is partitioned be-tween organisms,water,and sediments,readily accumulating in the aquatic biota and(unlike bioavailable inorganic Hg(II) species)undergoing biomagnification up the food chain;in contrast,(CH3)2Hg has a strong tendency to be lost through evaporation.But CH3Hg+species such as CH3HgCl,along with some inorganic Hg complexes such as HgCl2,may also be volatilized,and demethylation of CH3Hg+produces Hg(0), which may escape into the atmosphere.Owing to extensive loss of Hg from water by evasion of volatile species(mostly Hg(0)formed largely by reduction of inorganic Hg(II)), aquatic environments tend to trap and retain atmospheric Hg less effectively than terrestrial environments(Xiao et al.1991; Mason et al.1994).
Particulate,aqueous,and gaseous forms of Hg are removed from the atmosphere by wet and dry deposition(Pirrone et al. 1995).Thus,precipitation scavenges airborne Hg and acts as a vehicle for conveying it to the Earth’s surface(Lindberg 1986;1987;Fitzgerald et al.1994).However,the efficiency of this process depends on meteorological conditions and par-ticle size frequency(Lindberg1986;Pirrone et al.1995),and on various environmental factors that control the speciation of Hg,its solubility in water droplets,its volatility,and its ten-dency to be sorbed or desorbed by particles(see section2.1.2). An important point to bear in mind when designing pollution abatement schemes is that air pollutants other than Hg may interact with atmospheric Hg,altering its species composition and its partitioning between the liquid,solid,and vapour phases,thereby changing the kinetics of scavenging by pre-cipitation and the rates of wet and dry deposition(Lindberg 1986;Hudson et al.1995).Hudson et al.(1995)concluded that a decrease in the emission of particulate matter and other air pollutants in North America is responsible for the apparently paradoxical fact that Hg deposition has been declining since 1960,despite a concomitant increase in atmospheric Hg emis-sion.They also inferred that on a global scale the effects of anthropogenic SO2,oxidants,and particulate matter emitted during the period1850–1990have caused increased Hg depo-sition in continental areas in proportion to Hg deposition in the sea.Nevertheless,the net rate of Hg deposition may be a function of a complex assortment of other factors as well. Engstrom and Swain(1997)found evidence for a regional decline in Hg deposition in the north-central United States (as indicated by lake core data for eastern,but not western, Minnesota)since~1970,despite a continuing increase in global Hg emissions(as suggested by lake core data from southeast-ern Alaska).They attributed the regional effect to several dif-ferent causes,including reduced industrial utilization of Hg, incidental removal of Hg at sources of emission through ap-plication of pollution-control techniques,a shift from coal to natural gas as the fuel burned to heat buildings,more stringent control of waste incineration,and other factors,such as in-creased stack height,that favour long-range transport.
Furthermore,physicochemical and biological variables in natural environments control the rates of Hg reemission into the atmosphere.For instance,there are marked diurnal and seasonal changes in the fluxes of atmospheric Hg and the concentrations of Hg in atmospheric compartments,reflect-ing corresponding variations in environmental conditions (Fitzgerald1986;Lindberg1986;Lindberg et al.1991;Burke et al.1995;Keeler et al.1995;Lamborg et al.1995;Pirrone et al.1995).Factors affecting rates of Hg evasion to the atmosphere from natural waters and soils include the pH of lake water,activities of phytoplankton and other microbes in seawater and lake water,plant cover on contaminated soil,and ambient temperature(Fitzgerald1986;Lindberg1986;Vandal et al.1991,1995;Pirrone et al.1995).For example,the emis-sion of Hg vapour by lake water and soil increases with rising temperature,with the result that evasion of Hg from lakes is greater during the day than at night,and in warmer than in cooler weather(Lindberg1986;Pirrone et al.1995);more-over,the solubility of Hg(0)in seawater is higher at lower temperatures(Fitzgerald1986).Other noteworthy effects in-clude an increase in Hg(0)production with increasing primary productivity in lake water and a decline in Hg(0)pro-duction as the pH of the water decreases(Fitzgerald1986; Fitzgerald et al.1994;Vandal et al.1991,1995),suggesting that one of the several possible reasons for the elevated CH3Hg+content of fish in acidified lakes is that acidic condi-tions give an advantage to methylating microbes in their com-petition with Hg(0)-generating microbes for the limited sup-plies of bioavailable inorganic Hg(II)in the environment. 2.2.Estimates of total annual fluxes of natural and
anthropogenic Hg
Using calculations based on direct measurements,material balances,and models,a number of workers have estimated that human activities introduce several thousand tonnes of Hg into the Earth’s atmosphere every year through direct and in-direct emissions(Table1;Fig.1)(Weiss et al.1971;Lantzy and MacKenzie1979;Jaworowski et al.1981;Pacyna1986; Nriagu and Pacyna1988;D’Itri1991;Lindqvist et al.1991; Mason et al.1994;Hudson et al.1995;Pacyna and Keeler 1995).According to the most recent estimates of the global flux(Mason et al.1994;Hudson et al.1995),the anthropogenic emissions total~2000–5400t/year depending on whether the definition of anthropogenic flux is confined to direct,primary emissions or is expanded to include previously deposited an-thropogenic Hg reemitted from diffuse secondary sources in the environment,and on whether anthropogenic Hg deposited locally is counted(Table1).Direct emissions into the global atmosphere amount to~2000t/year,not counting an equal quan-tity that is deposited near its points of origin and is therefore not immediately available for long-range atmospheric trans-port;but if reemission of previously deposited anthropogenic Hg from the environment is counted,the total anthropogenic emission subject to long-range transport is~2700–3400t/year. If Hg deposited locally following direct emission into the at-mosphere is taken into account,the direct primary emissions actually amount to~4000t/year and the total anthropogenic output(the sum of the primary and secondary emissions)is ~4700–5400t/year.If the full environmental effect of anthro-pogenic Hg is to be assessed,it is necessary to combine the primary and secondary emissions;arbitrarily restricting the
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definition of anthropogenic emissions to direct primary emis-sions,while lumping the secondary emissions in with natural emissions merely because their immediate sources are in natu-ral environments,would be extremely misleading.Moreover, Hg deposited near its source should be included as well(even though its immediate impact is local),because it is subject to reemission into the atmosphere.Defined on this basis,the total anthropogenic contribution comes to~5000t/year(of which ~3000t is immediately available for long-range atmospheric transport),and the contribution from natural sources totals ~2000t/year(Table1).
Table1demonstrates that the more recent estimates of global Hg fluxes to the atmosphere(data published in the1980s and 1990s)agree well with each other but differ greatly from the older estimates(data published in the1970s),except for the anthropogenic Hg data of Weiss et al.(1971).Most of the numbers from the two oldest publications are one to four orders of magnitude higher than the more recent ones!In view of the technical improvements and the expansion and updating of scientific information which have been taking place over the years,it would be reasonable to assume,on general principles alone,that the most recent estimates tend to be the most reli-able ones.But Nriagu and Pacyna(1988)have given more explicit reasons for believing that the older estimates in general, and the estimates of Lantzy and Mackenzie(1979)in particu-lar(Table1),are inaccurate;for instance,they pointed out that Lantzy and Mackenzie(1979)made an incorrect assumption about the percentage of the heavy metal content of coal and oil that is released to the atmosphere on combustion.Another reason for doubting the reliability of the two oldest sets of numbers is that both are based in part on Greenland ice data (Weiss et al.1971)whose validity is questionable(see section 4.2.3).The discrepancies between the older and more recent data sets in the table probably reflect differences in the as-sumptions on which the models were based,in the choice of the parameters and geochemical pathways represented in the calculations,and in the estimated values of certain parameters (Nriagu and Pacyna1988;Rasmussen1994).Technical prob-lems connected with sample collection,handling,and analysis (e.g.,sample contamination,instrument sensitivity,etc.)could also have been contributing factors,although some workers suspect that dubious estimates of Hg fluxes have more to do with a lack of representative analytical data than with poor-quality data(Geological Society of Canada1995).One very important fact that may be relevant here is worth emphasizing: anthropogenic Hg deposited in natural environments is sub-ject to reemission by natural processes;consequently,much of the natural flux may actually consist of anthropogenic Hg un-dergoing recycling(Expert Panel on Mercury Atmospheric Processes1994;Mason et al.1994;Hudson et al.1995).Ac-cording to the estimates of Hudson et al.(1995),direct anthro-pogenic Hg emissions into the atmosphere amount to about 40%of the total Hg flux,but if reemission of previously de-posited anthropogenic Hg from terrestrial and aquatic(mainly marine)environments is taken into account,the overall an-thropogenic emission is closer to60%of the total(70–80%if anthropogenic Hg deposited near its point sources is taken into account)(Table1).
As may be suspected from the disparities in the estimates put forward by different workers,none of the calculated values of the global Hg emissions can be regarded as exact;at best they are rough approximations.Nevertheless,important and fundamental inferences may be drawn from these data.It must be emphasized that in spite of the wide differences between the absolute values of the Hg emissions as estimated by different investigators,all data sets are essentially in agreement to the extent that they support the following major conclusions: (i)several thousand tonnes of anthropogenic Hg are intro-duced into the Earth’s atmosphere every year;and(ii)the anthropogenic and natural emissions are of the same order of magnitude(not counting the data of Weiss et al.(1971)),the anthropogenic component being~30–80%of the total de-pending on how it is defined.From the standpoint of our in-quiry into the significance of long-range atmospheric trans-port of Hg and the relative importance of the anthropogenic and natural components of airborne Hg,the fundamental agree-ment between the various data sets is of greater interest and relevance than the differences.
Another matter related to the modelling of Hg emissions should be mentioned.As the reader may have noticed,the work of Jaworowski et al.(1981)is not represented in Table1. The reason for this omission is that their data do not include estimates of total natural and total anthropogenic Hg emis-sions.Jaworowski et al.(1981)estimated the total flux to the atmosphere as190000t/year(based on glacier ice for the years 1950–1978)and the total flux of particulate anthropogenic Hg as11000t/year,but such information alone is not sufficient for the purposes of this review.
It should also be noted that published estimates of the natural Hg emissions to the atmosphere have been reviewed by the Geological Survey of Canada(1995)(see Fig.3in their report).However,the following remarks have to be made about their representation of the data.(i)The global Hg emis-sion into the atmosphere according to Jaworowski et al.(1981) (190000t/year)is designated as the natural emission,whereas it is actually the total emission and therefore presumably in-cludes a substantial anthropogenic component.(ii)The natural Hg emission according to Weiss et al.(1971)is recorded as 150000t/year;but in fact,this is only the highest value in a wide range of possible values(25000–150000)calculated by Weiss et al.(1971).(iii)The value of the natural Hg emission that was adopted by Lindqvist et al.(1991)was treated as an independent estimate,but in truth,as clearly stated by Lindqvist et al.(1991)this value was derived directly from an estimate published by Nriagu(1989);the number calculated by Nriagu (1989)was2500and Lindqvist et al.(1991)rounded it off to3000.
3.Spatial variations in Hg concentrations
and fluxes
3.1.Systematic long-range variations in the
concentrations of Hg and associated airborne
pollutants,and measurements of Hg fluxes
3.1.1.Scandinavia
A number of different,independent sets of data representing various environmental compartments,including vegetation, show the existence of a distinct regional gradient in Scandinavia, with high Hg levels in the south and low levels in the north (Johansson1985;Brosset1987;Iverfeldt1991;Johansson et al. 1991;Lindqvist et al.1991;Steinnes and Andersson1991;
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Rognerud and Fjeld1993;Iverfeldt et al.1994;Steinnes1994). Moreover,measurements of the dynamics of atmospheric Hg demonstrate that winds are carrying Hg into the region from European sources to the south(Brosset1982,1987;Lindqvist et al.1991;Petersen et al.1995),and Hg in rain and air sam-ples is correlated with a number of well-known air pollutants (Brosset1982,1987;Iverfeldt1991;Lindqvist et al.1991). Scandinavian,especially Swedish,investigators have accu-mulated a large mass of evidence establishing the importance of long-range atmospheric transport of anthropogenic Hg to Scandinavia,and their achievement is all the more convincing because it demonstrates agreement between many different kinds of data.More specifically,the evidence is as follows.
Hg concentrations in precipitation and air vary gradation-ally from lower in the north to higher in the south(Brosset 1987;Lindqvist et al.1991).This tendency applies to Norway and Denmark,as well as to Sweden(where the most detailed Scandinavian research has been done)(Lindqvist et al.1991). Although the distribution of Hg in Sweden is roughly compa-rable to the distribution of point sources of Hg pollution within Sweden,detailed examination reveals that the correspondence is actually rather poor;and the occurrence of a north–south gradient throughout Scandinavia,not just in Sweden,demon-strates that the gradient is not primarily the result of local pollution.The fact that the prevailing winds in that part of the world blow from southwest to northeast,and the presence of major heavily industrialized,densely populated sources of air pollution in European countries to the south and southwest of Scandinavia,together with results of trajectory analysis which reveal south-to-north movement of airborne Hg over Sweden(Brosset1982,1987;Lindqvist et al.1991),strongly support the conclusion that most of the Hg was transported into Scandinavia by winds from sources of pollution in other European countries,such as East Germany and Czechoslovakia. Good agreement between empirical observations and model simulations(Petersen et al.1995)adds weight to this conclusion.
In Norway and Sweden the north–south gradient described above is reflected in Hg data for soil organic matter,moss,peat bogs,and lake sediment cores,as well as rain water(Johansson
1985;Iverfeldt1991;Johansson et al.1991;Lindqvist et al. 1991;Steinnes and Andersson1991;Rognerud and Fjeld1993; Iverfeldt et al.1994;Steinnes1994).In sediment cores from Swedish lakes Hg is generally highest at the top and decreases downward,but the proportion of surface Hg to the background Hg at deeper horizons is much higher in the south than in the north(Fig.2).High Hg levels in soil humus sampled in central Norway have been attributed to local natural(i.e.,geological) sources,and high levels in southeastern Norway have been blamed on local chloralkali plants;nevertheless,the general north-to-south gradational increase in Hg across Norway and Sweden is regarded as the result of long-range atmospheric transport.It is extremely unlikely that the simple pattern of north-to-south gradational increase in Hg levels observed in natural environments throughout Scandinavia could have been produced by geological sources of Hg in such a geologically diverse and complex region.One would expect natural Hg from geological formations to produce a more patchy pattern, with anomalously high environmental Hg levels(if any)oc-curring only in certain isolated,unevenly distributed localities (Evans1986;Painter et al.1994),as in central Norway,not a large-scale regional gradient such as the one described above.Moreover,there do not appear to be any grounds for believing that natural emissions upwind from Scandinavia caused such a gradient,whereas major sources of air pollution are known to be located in areas which,during the greater part of each year,are situated upwind from Scandinavia;and as discussed in the next paragraph,Hg in Swedish rain and air samples is known to correlate with other air pollutants typically generated by the burning of fossil fuels.
Hg levels in rain water collected in Sweden correlate with concentrations of other airborne pollutants,including soot, acids,SO42–,Cd,and Pb(Brosset1987;Lindqvist et al.1991). Similarly,covariance of Hg and soot in air samples from Sweden has been reported(Brosset1982).Two airborne Hg fractions were distinguished:diffuse background Hg(attributable to reemission from land and water)and another Hg fraction (deemed to have emanated directly from sources of pollution). The latter fraction,together with soot,with which it probably shares a common origin,is distributed in a manner indicating transport by high-level winds,mainly winds from the south, where major central European sources of Hg pollution occur. These observations constitute further evidence that the Hg introduced into Sweden by atmospheric circulation is
largely Fig.2.Hg profiles in sediment cores from lakes at different latitudes in Sweden,with a map showing the sampling sites(from Johansson1985,reproduced with permission of Verh.Int.Ver. Limnol.,Vol.22,pp.2359–2363,?1985E.Schweizerbart’sche Verlagsbuchhandlung).
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anthropogenic;and the soot,SO42–,and acids,in particular, suggest that combustion of fossil fuels(which is known to be a major contributor to atmospheric Hg pollution in general (Nriagu and Pacyna1988;Pacyna and Keeler1995))is the principal cause of the airborne Hg problem in Scandinavia.
3.1.2.Canada and the United States
Field studies conducted in widely separated parts of North America have yielded data that are mostly consistent with the body of evidence amassed in Scandinavia.
Nater and Grigal(1992)and Nater et al.(1992)reported that Hg concentrations in surface soil and forest litter tend to increase along a transect running from northwestern Minnesota to eastern Michigan,i.e.,toward sites of more intensive in-dustrialization and greater density of sources of pollution, such as Detroit.Accompanying the gradient in Hg levels were parallel gradients in wet SO42–deposition and Pb,Cd,and S concentrations.Furthermore,estimated levels of natural back-ground Hg from geological sources did not show this ten-dency,and subtraction of background Hg from the total Hg revealed an even more striking gradational west-to-east in-crease.Similarly,analyses of air and precipitation revealed much higher Hg levels in large urban and industrial centres (including Detroit and Chicago)situated in the southern part of the region than in rural areas further north,and mixed-layer back trajectories for precipitation events indicated aerial trans-port of Hg and other air pollutants,such as SO42–,NO3–,Pb, Cu,As,and Se,from various sources of pollution(e.g.,coal-burning industrial facilities)to the south,southeast,southwest, and west(Keeler et al.1994,1995;Hoyer et al.1995;Pirrone et al.1996).Chemical analysis and trajectory analysis of air in northern Wisconsin yielded comparable results(Lamborg et al. 1995).The observed regional trends were clearly caused by long-range atmospheric transport of Hg directly from urban and industrial sources of pollution.
In the Lake Champlain basin,as in the northern Midwest, the highest levels of Hg in precipitation resulted mainly from regional atmospheric transport from the south and(less fre-quently)from the west,southwest,and northwest,as deter-mined by a combination of chemical analysis and trajectory analysis(Burke et al.1995).Furthermore,Hg in precipitation gave a positive correlation with SO42–and somewhat weaker correlations with NO3–,H+,and Cl–.These observations imply contamination of the lake basin by wind-blown Hg released into the air mainly by combustion of fossil fuels in urban and industrial centres located in southern New England,the mid-Atlantic states,Ontario,and Québec.
Radiometrically dated cores from lakes in widely separated localities in Arctic and subarctic regions of Canada far from any industrial sources of pollution(from the east shore of Hudson Bay to the Yukon,and as far north as Cornwallis Island and as far south as Northern Ontario)show surface enrichment in Hg(see below)and decreasing concentrations with depth,suggesting atmospheric pollution from distant sources as a result of fossil fuel combustion which increased progressively over time(Lockhart et al.1993,1995).This in-terpretation is supported by the presence of a near-surface concentration of polycyclic aromatic hydrocarbons,which are combustion products.The influence of anthropogenic,as op-posed to natural,Hg emissions appears to be greater in eastern and central regions of northern Canada,whereas natural emis-sions predominate in the west(Lockhart et al.1995).This geo-graphic pattern is consistent with the direction of the prevailing westerly winds and the positions of industrial centres far to the south.It suggests that the northwestern sites are frequently exposed to relatively clean air from the Pacific Ocean and Alaska,whereas the central and eastern sites are often exposed to more heavily contaminated air from the urban and industrial areas in the central and eastern regions of North America. (Although airborne anthropogenic Hg is ubiquitous,its con-centrations and its abundance with respect to natural Hg are subject to considerable spatial variation,being dependent on many factors,including proximity to sources of emission,wind directions,and environmental factors that control Hg deposi-tion.)In a study of Hg in one of the lakes investigated by Lockhart et al.(1995)(Amituk Lake on Cornwallis Island (north-central Arctic)),Semkin et al.(1996)estimated that only 24%of the total Hg input was attributable to the weathering of local bedrock(Semkin et al.1996).The remaining76%, which was evidently transported to the lake by atmospheric circulation from outside the watershed,accumulated initially in the snowpack and was then released into the water during the summer,when the snow and ice melted.As with the other lakes,Hg concentrations in sediment cores were highest at the top and decreased progressively with depth,implying an in-crease in the deposition of airborne Hg over time.Similar Hg profiles have been recorded for lakes in southeastern Alaska, which,because of their frequent exposure to westerly winds blowing off the Pacific Ocean,probably show the overall effect of atmospheric Hg contamination in the Northern Hemi-sphere and reflect a continuing increase in global Hg emissions (Engstrom and Swain1997).Centres of intense industrial ac-tivity in the United States,southern Canada,Europe,and Asia are the likely sources of Hg and other air pollutants deposited in the North(Pacyna and Keeler1995).Approximately60–80t of atmospheric Hg from sources of pollution in Eurasia and North America are reportedly deposited in the Arctic every year(Pacyna and Keeler1995),but the trends seen in the Arctic lake cores are not universal throughout North America. Owing to a different set of circumstances peculiar to the north-central United States,Hg profiles in lakes of eastern Minnesota show a distinctly different pattern of variation char-acterized by a buried Hg peak reflecting a temporal rise in Hg contamination followed by a steady decline starting in the second half of the20th century(Engstrom and Swain1997). Thus,regional,and probably local,as well as global tendencies have to be considered.
In the Boreal Forest Zone of northern Québec,as in a comparable Swedish study discussed above,Lucotte et al.(1995) compared the Hg profiles in cores from lakes located at differ-ent latitudes.As would be expected,the uppermost sections of the cores showed Hg enrichment,suggesting a temporal in-crease in deposition(starting,in this case,about1940)because of atmospheric pollution.A gradational increase in degree of surface or near-surface enrichment in Hg from north to south, as in the Swedish cores,would also be expected,as the field area lies north of the major regional sources of anthropogenic Hg and downwind from many of them during much of the year. Surprisingly,however,the values for the Hg concentrations in the Hg-enriched surficial sediment(normalized with respect to background Hg in the deeper layers or with respect to organic C)showed no correlation with latitude;yet Pb data normalized
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with respect to organic C did show a gradational increase toward the south.The reason for the absence of systematic spatial variation is not yet known,and further research is needed to explain it.
Direct measurements of atmospheric transport of Hg to a remote lake,along with Hg budget calculations,have demon-strated the importance of airborne Hg in the pollution of such lakes,confirming the indirect evidence provided by sediment cores.As part of a research project on the dynamics and mass balance of Hg in a remote seepage lake in northern Wisconsin, Fitzgerald et al.(1992)determined the Hg content of air,rain, and snow collected over the lake surface or on the shore of the lake using ultraclean protocols.Their results showed that the Hg concentrations and atmospheric inputs were similar to those observed in the open ocean regions of the Northern Hemisphere. However,on computing the mass balance of Hg for the lake, they found that the measured total annual rate of atmospheric Hg deposition readily accounted for the total mass of Hg in the fish,water,and sediments of the lake.They concluded that airborne Hg makes up an important part of the Hg introduced into lakes and lacustrine organisms of the temperate zone.
In summary,the results of lake core analyses,soil and forest litter analyses,and direct measurement of atmospheric Hg con-centrations and fluxes,together with mass balance calcula-tions,all support the conclusion that long-range atmospheric transport of Hg has had a significant impact on lakes and terrestrial environments.Furthermore,this diverse body of evi-dence,including the association of Hg with other common air pollutants generated by combustion of fossil fuels,implies that air pollution accounts for much of the Hg input.In North
America,as in Scandinavia,the convergence of quite different, independent lines of evidence all pointing in the same direction makes a far stronger case than any one class of evidence by itself.
3.1.3.Long-range atmospheric transport of Hg to the ocean Mason et al.(1994)estimated that about90%of the Hg in the ocean has come from wet and dry atmospheric deposition.In a study of the distribution of gaseous Hg in the air overlying the Atlantic Ocean,Slemr and Langer(1992)found that the Hg levels were higher in the heavily industrialized Northern Hemisphere than in the less industrialized Southern Hemi-sphere(also see Nriagu1992),and Hg levels in the Northern Hemisphere increased from the equator to higher latitudes(from 0°to60°N)(Fig.3).Fitzgerald(1986,1995)reported a similar tendency for Hg in the air over the Pacific Ocean,but at a given latitude the Hg concentration was generally higher over the Atlantic,the disparity between the values for the two oceans being most extreme at the highest latitudes of the Northern Hemisphere(Fitzgerald1995)(Fig.3).These latitude-dependent, globe-spanning patterns of variation make sense only if they are assumed to be primarily a consequence of long-distance atmospheric transport of anthropogenic Hg.The only plausible reason for such a distribution would seem to be the anomalously high rate of anthropogenic Hg emission into the atmosphere in the temperate zone of the Northern Hemisphere(a reflection of the fact that most of the world’s industrial activity and urbani-zation are concentrated in that region)(Nriagu1992),together with predominantly northeastward dispersal of the Hg by the prevailing westerlies(winds that tend to blow in a southwest-to-northeast direction).The high Hg levels over the Atlantic Ocean with respect to the Pacific Ocean are readily explained,in large part,by greater proximity of midocean sampling sites to sources of Hg pollution on adjacent continents(the Atlantic being much narrower than the Pacific)(Fitzgerald1995).The particularly large difference between the two sets of data at high northern latitudes,however,suggests that the anthropogenic Hg of North American origin which is blown northeastward across the Atlantic by the westerlies exceeds the anthropogenic Hg of Far Eastern provenance that is blown across the Pacific. Nevertheless,the Far East may well be a major producer of airborne anthropogenic mercury,largely owing to the preva-lence of coal combustion in China(Florig1997).
4.Variations in Hg levels over time
4.1.Hg levels in the atmosphere
In some instances,changes in the atmospheric transport of Hg from distant sources of pollution have coincided with specific documented events that altered the emission of Hg from its points or suspected points of origin.Thus,a drop in the con-centrations of Hg in air and precipitation and a decrease in atmospheric Hg deposition,with a concomitant decline in at-mospheric sulfur levels,were observed in Sweden in the1980s and1990s owing to a decrease in emissions from sources of air pollution in central Europe(Iverfeldt et al.1994,1995; Munthe et al.1995).The progressive amelioration of the atmospheric Hg problem in Sweden has been attributed to the closing of Hg-emitting East German factories,especially chloralkali plants in the Halle-Leipzig-Bittefeld region,fol-lowing the reunification of Germany(Iverfeldt et al.1994). This documented link between pollution abatement in Germany and subsequent improvement of air quality in Sweden(which, it will be recalled,is downwind from the sources of pollution during much of the year)constitutes strong evidence
that Fig.3.Concentrations of Hg vapour in air over the Atlantic and Pacific Oceans at different latitudes in the Northern and Southern Hemispheres(from Fitzgerald1995,reproduced with permission
of Water Air Soil Pollut.,Vol.80,pp.245–254,?1995Kluwer Academic Publishers).Note that the data for the Atlantic Ocean were taken originally from Slemr and Langer(1992).
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considerable quantities of Hg introduced into the air from the East German factories had been contaminating environments in Sweden as a result of atmospheric transport.
Temporal trends in the Hg content of air in oceanic regions also testify to the importance of long-range atmospheric trans-port of anthropogenic Hg.The investigation of gaseous Hg in air over the Atlantic Ocean for the period1977–1990revealed a1.46%increase per year in the Northern Hemisphere and a 1.17%increase per year in the Southern Hemisphere,i.e.,rates of increase that are consistent with results of independent soil, peat bog,and lake sediment data(Slemr and Langer1992). These observations strongly suggest that anthropogenic rather than natural sources of Hg dominate the cycle of atmospheric Hg.As shown in lake cores and other data,they demonstrate that pollution has been increasing over time,and they also reflect the fact that industrial pollution has been more severe in the Northern than the Southern Hemisphere.
4.2.Vertical Hg profiles in sediment,peat,and ice deposits
https://www.doczj.com/doc/c5547496.html,ke sediment cores
A large body of literature stretching back many years docu-ments vertical variations in Hg concentration in cores of un-disturbed fine-grained lake sediments collected from lakes situated in widely separated regions of the Northern Hemi-sphere(e.g.,Scandinavia,Canada,and the United States)that have not been polluted by local point sources of Hg.Such cores typically show Hg enrichment in the uppermost(youngest) horizons,the Hg concentrations declining and finally levelling off with increasing depth(i.e.,age).Absolute dating by means of210Pb has usually revealed that the Hg content of the sedi-ment rose steadily and sharply after the onset of the Industrial Revolution(starting,in some cases,as late as the mid-20th century)and either continued to rise to the present day or peaked in the late20th century and then started to come back down.The generally accepted interpretation of these core pro-files is that they reflect temporal variations in Hg loading and that the Hg enrichment of the uppermost strata signifies atmos-pheric contamination originating in distant centres of industrial activity,whereas the relatively low Hg concentrations,which are found deeper in the core and which do not show any sig-nificant systematic variation as a function of age,represent the natural background Hg.Before inquiring into the validity of this interpretation,let us examine a few examples of core data on the assumption that the conventional interpretation is sound. In a section4.2.4,the case for concluding that the conventional explanation of the core profiles is indeed the correct one will be presented.In brief,the examples of lake core evidence are as follows.
A series of cores taken from forest lakes along a transect from southern Sweden to the northern extreme of the country show a greater or lesser degree of Hg enrichment at the top of the core(Johansson1985;Lindqvist et al.1991)(Fig.2).The degree of surface enrichment with respect to background Hg in deeper horizons is very high in the south and decreases gradationally toward the north,almost disappearing in the far north.These observations are in agreement with the results of a study conducted in Norway(Rognerud and Fjeld1993). From other kinds of data(see above),it is known that atmos-pheric Hg emanating from sources of pollution in European countries south of Scandinavia is transported across Sweden by the prevailing southwesterly winds,creating a north-to-south gradational increase in the severity of Hg contamination in the Swedish environment.There is little doubt,then,that the core data represent recent atmospheric Hg contamination; and there are no grounds for suggesting that the observed regional pattern seen in the cores is due to natural sources of Hg.
Lockhart et al.(1993,1995)collected cores from lakes in Arctic and subarctic regions of Canada,far from any known industrial source of Hg(see above).Core sections were ana-lysed for Hg and dated by means of210Pb and137Cs(the earliest210Pb date being1850A.D.);sections of two of the cores were also analysed for polycyclic aromatic hydrocarbons (PAHs).In all cases in which210Pb declined exponentially with depth(as would be expected for a continuous sediment sequence undisturbed by physical processes,such as biotur-bation,slumping,erosion,and turbidite deposition,or by post-depositional redistribution of metals),Hg content increased toward the top of the core(i.e.,increased with decreasing age). The PAHs,which are probably anthropogenic combustion prod-ucts of fossil fuels,showed similar profiles except that they commonly peaked below the sediment–water interface(in strata laid down in the mid-1900s),whereas Hg continued to increase right up to the top.The association of Hg enrichment with PAH enrichment adds weight to the conclusion that the elevated Hg content of the uppermost horizons of the cores resulted from air pollution caused mainly by the combustion of coal;the Hg and PAH emissions may well have come from the same sources (e.g.,coal-burning power plants)by the same pathways(long-distant atmospheric transport).The difference between the Hg and PAH profiles in the late20th century deposits may be explained by the fact that PAHs in the atmosphere are associ-ated with particulate matter,whereas the Hg is mostly Hg(0) vapour(Lockhart et al.1993).In recent years the emission of anthropogenic particulate matter into the atmosphere has been declining,whereas the emission of Hg has continued to in-crease(Hudson et al.1995);this could easily account for the difference between the Hg and PAH profiles in the uppermost sections of the cores.
Engstrom and Swain(1997)reported that Hg concentra-tions in210Pb-dated cores from three remote,virtually pristine lakes in a wilderness region of southeastern Alaska showed a tendency to increase from preindustrial times to the present (with no significant indication that the concentrations have peaked and are tapering off,as in eastern Minnesota).In all three lakes the Hg concentrations in surface sediments are about twice what they were at the onset of industrialization.As with the findings of Lockhart et al.(1993,1995),these results lead to the conclusion that the Hg profiles were produced by deposition of airborne Hg emitted by distant centres of pollu-tion in the temperate zone.As the prevailing winds in this region are westerlies blowing directly from the Pacific Ocean, the Hg profiles tend to reflect the net global or,at least,hemi-spheric variations in atmospheric Hg emissions,transport,and deposition over time rather than a regional or local continental effect,such as that,for instance,which characterizes the lakes of eastern Minnesota(Engstrom and Swain1997).
Munthe et al.(1995)found that sediment cores from two Swedish lakes known to be polluted with aerially transported Hg and related contaminants,including acids,were charac-terized by a large buried Hg maximum at a depth of~5cm
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(Fig.4).Hultberg et al.(1994)claimed that the sediment se-quences show parallel trends in soot and Pb content,together with acidity(as inferred from diatom fossils),although none of this important auxiliary information was presented or dis-cussed by Munthe et al.(1995).Unfortunately,radiometric dating seems not to have been done,and even the year of sample collection is not recorded.The authors maintain that the Hg levels peaked“at least a decade”before the time of sample collection,although they offer no evidence to substan-tiate this claim.But despite the lack of information about the geochronology of the sediment sequences,these results are of great interest for at least two reasons:(i)The buried maximum implies a sharp increase in Hg loading from the atmosphere over time,followed by a sudden and equally dramatic reduc-tion in Hg loading,starting at some time in the relatively recent past and continuing to the time of sample collection;
and(ii)the Hg profiles have essentially the same shape and peak at virtually the same depth,even though they represent two different lakes with distinctly different environments of deposition(one lake being severely acidified,whereas the other was neutralized by liming in1982and has been limed continu-ously ever since(Hultberg et al.1994)).Buried peaks such as these are readily attributable to systematic long-term changes in the rate of Hg deposition from the atmosphere over time; and the occurrence of almost identical maxima independently in two separate lakes is consistent with the occurrence of a widespread atmospheric phenomenon affecting both lakes at once,as opposed to a unique set of local conditions affecting only one lake.Munthe et al.(1995)concluded that the recent temporal decline in Hg concentrations is due to a decrease in Hg emissions from unspecified sources of pollution in“northern and central Europe,”citing literature describing similar trends for sulfur.Referring to the same two lakes,Hultberg et al. (1994)(echoing Iverfeldt et al.1994)ascribed the progressive decrease in Hg concentration to a sharp decline in Hg emissions from East Germany following the reunification of Germany. In view of the independent evidence in favour of it(Iverfeldt et al.1994;see above),this is a plausible interpretation,but another possible explanation is that the drop in Hg levels re-flects a steep decline in Hg emissions from industrial sources (notably chloralkali and waste treatment plants)within Sweden itself,which began around1960and continued until at least as recently as1980(Lindqvist et al.1991).Absolute dating by radiometric techniques is required to resolve this issue.
Dominik et al.(1991)recorded the results of a very exact, high-resolution analysis of Hg profiles in210Pb-and137Cs-dated cores from Lake Geneva,Switzerland.In this case the Hg was introduced mainly by fluvial transport,but the study is relevant to the issue of atmospheric Hg contamination in remote lakes,as it involves the same underlying principles concerning effects of Hg loading as distinct from possible effects of postdepositional migration of sedimentary Hg.As with the data of Lockhart et al.(1995),the210Pb dates show a smooth,exponential increase with depth and hardly any scatter as far back as the year1890,testifying to uniform conditions of sedimentation,absence of postdepositional physical distur-bance or migration of metals,and a relatively constant rate of sedimentation over a time scale on the order of100years; moreover,a137Cs maximum generated by nuclear bomb tests in1964strongly confirms the accuracy of the210Pb age scale. The Hg profiles reveal a gradual,progressive increase with decreasing age(i.e.,decreasing depth),starting at the end of the19th century and continuing until about1950,when the Hg concentration rose sharply to a prominent maximum,which is ascribed to a pollution event.Thereafter the Hg levels declined just as abruptly and tended to continue declining with decreas-ing age,but superimposed on this trend is a small secondary maximum with a210Pb date of1973,which represents a second pollution event that has actually been documented.As Rhone River sediments of the Alpine region have been subject to yearly monitoring since1970,the1973Hg peak in the Lake Geneva sediments can be attributed,with confidence,to an important episode of pollution that is known to have occurred during the period1971–1972(Dominik et al.1991).The Hg content of the sediment decreases with distance from the mouth of the Rhone River,providing a further indication that the river is the source of the Hg pollution.Also note that the two buried Hg peaks appear to be unique to Hg,as other heavy metals do not show this pattern of variation(Dominik et al.1991). Obviously,variation in the deposition of anthropogenic Hg, not postdepositional redistribution of Hg,accounts for the Hg profiles in the cores.
Other evidence for the deposition of airborne anthropo-genic Hg in lakes far from the sources of pollution,causing surficial sediments to have anomalously high Hg levels with respect to the natural background levels in deeper strata,in-clude(i)Hg profiles in radiometrically dated cores from the Turkey Lakes in Northern Ontario(Johnson et al.1986)and from lakes in northern and southern Québec(Ouellet and Jones1983;Lucotte et al.1995)and Minnesota(Engstrom and Swain1997);(ii)Hg profiles in undated cores from southern Ontario(Evans1986),northern and southern Québec (Louchouarn et al.1993),and northern Wisconsin(Rada et al. 1989);and(iii)both Hg and PAH profiles in radiometrically dated cores from lakes in the Adirondack Mountains of New York(Heit et al.1981).In these cases,a number of lakes show the typical decrease in Hg concentration with depth,although certain others vary rather erratically and show no consistent trend.Some of the Hg profiles show a buried Hg maximum, probably owing to a recent regional decrease in the rates of Hg loading(Ouellet and Jones1983;Engstrom and Swain1997). The reasons for the erratic variations are unknown,but they probably reflect peculiarities of local conditions;assuming that sample contamination and analytical error can be ruled out,possible explanations include postdepositional
mixing Fig.4.Hg profiles in sediment cores from two Swedish lakes
(G?rdsj?n and H?rsvatten)(from Munthe et al.1995,reproduced with permission of Water Air Soil Pollut.,Vol.85,pp.743–748,
?1995Kluwer Academic Publishers).
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(e.g.,by benthos),erosion by currents,and temporal variations in the composition or texture of the sediment or the rate of sedimentation(e.g.,because of occasional intrusion of tur-bidites into the stratigraphic sequence(Dominik et al.1991)). Temporal variations in atmospheric conditions that affect the rate of Hg deposition(e.g.,rainfall or air pollutants affecting Hg speciation and scavenging)may also alter the Hg profile (Hudson et al.1995).In any event,such inconsistencies serve to warn us that not all fine-grained stratigraphic sequences in lakes are suitable for the study of temporal variations in Hg loading.Only cores from sites where there have been no major changes in the nature and deposition rate of the sediment over time,and no major postdepositional disturbance,can be relied upon.Consequently,it is necessary to be discriminating and cautious in the use of lake core data as records of temporal variations in Hg loading and to be aware of possible compli-cations due to local conditions of sedimentation and post-depositional changes,which may obscure the effects of Hg loading from the atmosphere.A smooth exponential increase in210Pb-based age with increasing depth in the core is in-dicative of an undisturbed sequence laid down at a relatively constant rate,and the210Pb data inspire all the more confidence if they are confirmed by a sharp137Cs peak(a marker for the maximum bomb fallout of1963–1964)showing good agree-ment with the corresponding210Pb date(Dominik et al.1991; Lockhart et al.1993,1995).
4.2.2.Peat cores
Peat deposits in ombrotrophic bogs are regarded as excellent media for the preservation of records of atmospheric deposi-tion of heavy metals provided that they are situated above the water table(Damman1978;Madsen1981;Urban et al.1990; Jensen and Jensen1991;Steinnes and Andersson1991;Benoit et al.1994).Data for Hg are especially reliable for the study of pollution history,because Hg,unlike a number of other metals,is virtually immobile in peat,even when it is below the water table(Benoit et al.1994;Jensen and Jensen1991).Cores from peat bogs have revealed Hg profiles similar to those of sediment cores from lakes(see examples above);such profiles are thought to reflect variations in the loading of atmospheric Hg over time(Benoit et al.1994).As with lake cores,however, there have been important differences in the results from dif-ferent sampling sites,suggesting that local environmental and biological factors(e.g.,the growth of the plants from which the peat is formed)affect the vertical distribution of Hg and may mask,distort,or even obliterate any effects of temporal variations in Hg loading.Moreover,peat bogs are sensitive to regional variations in Hg deposition,which may obscure larger scale tendencies(Benoit et al.1994).As with lake cores,peat core data can be used to investigate temporal variations in atmospheric loading,but caution and discrimination are re-quired to avoid erroneous interpretations.It would be a serious mistake to assume that all Hg profiles in peat deposits are records of Hg loading,pure and simple,or that all of them reflect global or larger scale regional trends.Let us consider the results of three independent studies.
Madsen(1981)analysed a core from each of two widely separated ombrotrophic peat bogs in Denmark.The plants whose remains are the source materials of the organic deposits in such bogs are known to depend on atmospheric fallout for their nutrients,and in adapting themselves to the bog environment,they have evolved the ability to retain heavy metals strongly (Madsen1981;Jensen and Jensen1991).The ages represented by the cores ranged from the mid-18th century(i.e.,the begin-ning of the Industrial Revolution)to about1980according to 210Pb dates(confirmed by14C dating of a subsample from one
of the cores).The position of the water table was not recorded, but at both sites the210Pb age exhibited a smooth exponential increase with depth,suggesting that there has been no signifi-cant postdepositional redistribution of heavy metals nor any radical variation in the conditions of deposition and peat for-mation.The Hg deposition rate(based on measurements of Hg content,bulk density,and age)showed a long-term tendency to increase with decreasing age from about1800or the mid-19th century to1980,paralleling an increase in the estimated rate of Hg consumption in countries of the European Community based on data compiled independently by another worker (and,of course,paralleling the Hg profiles seen in many cores of lake sediments(see above)).Superimposed on the trend observed in each of the two cores are episodic short-term in-creases which could be due to natural events,most probably volcanic eruptions in Iceland and associated variations in meteorological conditions(e.g.,barometric pressure);but these fluctuations are not large enough to mask the overall temporal trend,which is attributable to air pollution(Madsen1981).
Jensen and Jensen(1991)estimated temporal variations in Hg deposition rates using210Pb-dated cores taken from six ombrotrophic peat bogs at widely separated localities in Norway and Sweden.The ages of the stratigraphic sequences represented by the cores fell in the range late18th or early19th century https://www.doczj.com/doc/c5547496.html,parison of cores from different sampling sites revealed widely differing patterns of temporal variation in Hg deposition rate.At one extreme there was a core from a bog at?verbygd in the far north of Norway,which gave a particularly close approximation of the general pattern of variation that is so frequently seen in lake cores:a sharp,steady increase from older(i.e.,deeper)horizons(starting,in this case, in the mid-1900s)to a large maximum in the youngest horizon (at the top of the section).There was also at least one minor late19th or early20th century peak.Despite differences in detail,the essential similarity of this profile to the profiles seen in Danish peat cores(Madsen1981)is obvious.
Diametrically opposed to the?verbygd core profile was a core profile representing a locality in southern Sweden,where the Hg deposition rate was lowest at the top and increased with depth(Jensen and Jensen1991).Between these two opposite extremes were profiles of intermediate or less well-defined character.In three of them,there was a tendency for the Hg deposition rate to increase toward the surface,as in the ?verbygd core,but it was weakly developed or partly over-shadowed by other variations superimposed on the trend (e.g.,by an anomalously high value near the bottom of the sequence in one of the cores).In one core there was no sys-tematic change whatsoever with depth.A possible explanation for the inconsistency of the results for different sites is that environmental and biological variables have obscured or oblit-erated any record of atmospheric Hg loading,except at certain favourable localities,or that local or smaller scale regional effects masked whatever larger scale tendencies existed.Regard-ing the latter possibility,it is noteworthy that the?verbygd bog was the most remote and northerly site,and therefore,least prone to the masking of global trends by local or regional
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variations in Hg deposition(Benoit et al.1994).As for possi-
ble effects of local geochemical conditions,note that Jensen
and Jensen(1991)recorded the depth of the water level at their sampling sites,providing some basis for determining whether
remobilization and loss beneath the water table had a sig-nificant effect on the Hg profiles.Their data suggest that there
has not been appreciable mobilization of Hg below the water
table.In each case,the Hg profile crosses the water table, and there is no indication of an abrupt change in the pattern on passing below that boundary.Thus,the trends cross the boundary unaltered.Except at?verbygd,the water table was 10–37cm below the surface,placing it below the stratigraphic interval in which the upward increase,if any,occurs.The ?verbygd core is exceptional,as the water table was only5cm below the surface,lying just below the horizons where the Hg deposition rates(and total Hg concentrations)were highest. Although this could be viewed as possible evidence for Hg depletion below the water table,the smooth downward de-crease in Hg deposition rate,which begins above the water table,continues without apparent interruption or change in slope as it crosses the water table,coming to an end far below it;there is no discontinuity at the water table.It is conceivable that there were systematic errors in dating owing to depletion
of210Pb below the water table(Damman1978;Urban et al. 1990),but there is no definite indication that the position of
the water level has had an appreciable effect on the Hg profiles
in the peat cores.Similarly,Benoit et al.(1994)found that Hg is virtually immobile in peat;unlike Fe,Mn,and Al,it shows
little or no effect of fluctuations in the water table or variations
in oxidation–reducton potential(E h).This is not surprising,as Hg is bound very strongly by humic matter(the main constitu-
ent of peat)and is commonly associated preferentially with the higher molecular weight humic fractions,which are much
less mobile than the lower molecular weight fractions(Jackson et al.1980;Jackson1989;Louchouarn et al.1993;Lucotte
et al.1995).Evidently Hg is subject to little or no postdeposi-
tional remobilization in peat bogs(Benoit et al.1994).
An example of a buried mid-20th century maximum in the rate of Hg deposition(comparable to examples seen in lake
cores)has been found in a study of cores from a peat bog in northern Minnesota(Benoit et al.1994).The cores,which
represent the years1750–1991,showed a smooth exponential
increase in210Pb-based age as a function of depth,and the 210Pb geochronology was verified by137Cs and pollen dating. The lowest values for Hg deposition rate represent the period
1750–1900and the highest represent the period1935–1960. Thus,the rate tended,at first,to increase progressively with decreasing age but peaked in the interval1935–1960and de-creased slightly but steadily thereafter.The late20th century decline in Hg deposition rate probably reflects a decrease in regional emissions and should not be regarded as an indication of hemispheric and global trends(Benoit et al.1994).
In summary,data for peat cores from ombrotrophic bogs
tend to corroborate lake core data,but as with lake sediments, Hg profiles of peat cores from different localities differ widely, probably owing to variations in local or regional conditions and related variations in plant growth.Fluctuations in the water table and remobilization of metals below it cause significant redistribution of certain metals in peat deposits(Damman1978; Urban et al.1990),but there are grounds for thinking that the position of the water table has had no appreciable effect on the vertical distribution of Hg.Whatever the causes of the site-specific variations,it would seem that some peat sequences are highly suitable for the measurement of larger scale temporal variations in Hg loading from the atmosphere,whereas others are not.In all cases one has to be alert to possible complica-tions due to environmental and biological factors unrelated to Hg fallout from the atmosphere,and to effects of variations in local or regional Hg emissions superimposed on the larger scale effects.
4.2.3.Ice deposits in Greenland and Antarctica Undisturbed ice and snow deposits of great age,such as the Greenland and Antarctic ice caps,should,in theory,be excel-lent media for the preservation of a long-term record of Hg deposition from the atmosphere.Hg sealed in subsurface snow, firn,or solid ice is not subject to the diagenetic processes that may cause remobilization in sediments(although Hg in surfi-cial snow is subject to redistribution by melting and wind erosion).Therefore,if ice cores and lake cores were found to have similar vertical Hg profiles,the ice data would tend to validate the theory that the vertical variations seen in lake cores reflect variations in loading rather than diagenetic remobi-lization.Unfortunately,however,research on Hg in ice-cap samples has yielded dubious results.A pioneering investiga-tion of Hg in210Pb-dated samples taken at different depths in the Greenland ice cap revealed a vertical distribution of Hg similar to the typical profiles seen in lake cores,suggesting effects of recent atmospheric pollution(Weiss et al.1971).But soon afterwards,Dickson(1972)and Carr and Wilkniss(1973) raised serious questions about the validity of the supposed temporal variation in Hg concentration,and subsequent re-search on samples of Greenland ice failed to confirm the origi-nal results,revealing no temporal trends whatsoever(Weiss et al.1975;Appelquist et al.1978).Research on Hg in Antarctic snow and ice has not been much more successful,as the results are considered suspect owing to the likelihood that samples were contaminated despite the use of refined ultraclean sam-pling techniques(Dick et al.1990;Sheppard et al.1991).Be-cause of the extremely low Hg concentrations in the Antarctic and Greenland ice caps,the problem of contamination looms very large(especially in Antarctica,since the impact of Hg pollution has been greater in the Northern Hemisphere than in the Southern Hemisphere);consequently,technical difficulties constitute a major barrier to progress in this area of research. The suitability of snow and ice deposits for this kind of study (at any rate,the suitability of particular sampling sites)may have to be considered as well.More research is needed to obtain reliable information on Hg in remote ice fields.Possibly further technical advances are also required.Nonetheless,a set of apparently excellent data obtained by Vandal et al.(1993) for natural Hg fallout encased in an Antarctic ice core spanning the period about3800–34000years B.P.implies that existing technology is equal to the task and holds out the hope of getting useful data from ice https://www.doczj.com/doc/c5547496.html,ing rigorous clean techniques,the authors not only attained a reasonable degree of precision but also revealed a highly systematic pattern of long-range temporal variation that closely matched the pattern predicted independently by calculations based on levels of marine sulfate.Unfortunately,however,their results do not extend into the era of industrial activity.
Jackson111
4.2.4.Interpretation of data from lake sediment cores
The following observations strongly support the conclusion that the typical Hg profiles seen in cores of lake sediments represent variations in the transport and deposition of anthro-pogenic Hg over time,not postdepositional redistribution of Hg as suggested by Rasmussen (1994).
A paper by Gobeil and Cossa (1993)on the geochemistry of mercury in the lower estuary of the https://www.doczj.com/doc/c5547496.html,wrence River has an important bearing on the lake core issue.Gobeil and Cossa (1993)analysed core slices not only for total Hg but also for dissolved Hg in pore water and Hg,Mn,and Fe extractable with “hydroxylamine”(presumably hydroxylamine hydrochlo-ride)and acetic acid (NH 2OH ?HCl–HAc),a reagent mixture of a type commonly used to extract “amorphous”Mn and Fe oxyhydroxides from sediments (although it may also solu-bilize small quantities of humic matter).The critical signifi-cance of this is that these sets of data make possible a direct test of the hypothesis that the vertical distribution of Hg is controlled by diagenesis,an hypothesis that cannot be prop-erly tested unless the composition of pore water is taken into account (J.Azcue,verbal communication).(Also note that al-though the data presented in this paper represent an estuarine environment,they demonstrate basic principles that are appli-cable to lakes.By the same token,it should be pointed out that although liquid industrial effluent rather than polluted air is to blame for the Hg pollution in this particular case,the same
bio-geochemical principles apply here as in lakes contami-nated with aerially transported Hg.Irrespective of how Hg is transported into a body of water,its behaviour is governed by the same physicochemical and biological phenomena once it enters the aquatic Hg cycle.)As the work of Gobeil and Cossa (1993)is worth examining in detail,four of their diagrams are reproduced here.These diagrams illustrate the profiles for total Hg,pore-water Hg,and NH 2OH ?HCl–HAc extractable Hg and Mn at one particular sampling site (Figs.5A–5D).Total Hg forms a well-defined buried peak (Fig.5A).Pore-water Hg (Fig.5B)shows a comparable distribution in the depth range of 6–32cm but a completely unrelated pattern of variation in the depth range of 0–6cm.NH 2OH ?HCl–HAc extractable Hg (Fig.5C)varies independently of total Hg but shows a strong positive correlation with NH 2OH ?HCl–HAc extractable Mn (Fig.5D),a much weaker correlation with NH 2OH ?HCl–HAc extractable Fe (not shown),and a marked inverse relationship with pore-water Hg (especially in the upper half of the core)(Fig.5B).Another important fact is that the total Hg is one to two orders of magnitude more abundant than the NH 2OH ?HCl–HAc extractable Hg and about four to five or-ders of magnitude more abundant than the Hg in the pore water.Also note that the range of total Hg values in the https://www.doczj.com/doc/c5547496.html,wrence estuary core studied by Gobeil and Cossa (1993)is similar to the range of values seen in some cores from remote lakes in both northern and southern Québec (Louchouarn et al.1993;Lucotte et al.1995).
The inverse correlation of NH 2OH ?HCl–HAc extractable Hg with pore-water Hg and its positive correlation with NH 2OH ?HCl–HAc extractable Mn demonstrate that some postdepositional redistribution of Hg has occurred as a result of dissolution of colloidal manganese oxyhdyroxide (MnOOH),with release of adsorbed or coprecipitated Hg into the pore water.However,comparison of the total Hg,pore-water Hg,and NH 2OH ?HCl–HAc extractable Hg and Mn (Figs.5A–5D)proves that diagenetic remobilization of Hg has affected only a very small fraction of the total sedimentary Hg.This conclu-sion is based on the following observations:(i )The vertical profile of total Hg is distinctly different from the vertical pro-files of the two Hg fractions (NH 2OH ?HCl–HAc extractable Hg and pore-water Hg)that demonstrate effects of diagenesis;(ii )neither NH 2OH ?HCl–HAc extractable Hg nor pore-water Hg amounts to more than a tiny fraction of the total Hg;and (iii )the total Hg maximum is ~10–15cm below the surface,indicating that the overall vertical distribution of Hg was not determined by diagenesis,whereas the NH 2OH ?HCl–HAc ex-tractable Hg fraction,which represents Hg diagenetically co-precipitated with MnOOH,rises to a maximum,as would be expected,right at the surface.At any rate,the buried total Hg peak can be explained by temporal variation in Hg loading,but it is about two to three times too deeply buried to be explainable by postdepositional redistribution of Hg.In brief,the four sets of data prove conclusively that at least one impor-tant mechanism of postdepositional alteration of sediments (i.e.,the reduction and dissolution of Mn and Fe oxyhydrox-ides)has had no appreciable effect on the vertical distribution of total Hg in the https://www.doczj.com/doc/c5547496.html,wrence estuary.Thus,the work of Gobeil and Cossa (1993)supports the view that Hg profiles in undisturbed cores primarily reflect temporal variations in Hg loading and that any alteration of the pattern of variation by diagenesis is negligible.More research along these
lines
Fig.5.Profiles of total Hg (A),pore-water Hg (B),and
NH 2OH ?HCl–HAc extractable Hg (C)and Mn (D)in a core from the https://www.doczj.com/doc/c5547496.html,wrence estuary,Canada (from Gobeil and Cossa 1993,reproduced with permission of Can.J.Fish Aquat.Sci.,Vol.50,pp.1794–1800?1993NRC Research Press).
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should now be carried out(in particular,comparative studies of lakes representing a wide range of environmental condi-tions and many different geographical areas)to determine how widely the conclusions based on the findings of Gobeil and Cossa(1993)are applicable to Hg in sediments.
Lindberg and Harriss(1974)and Bothner et al.(1980) examined total Hg and interstitial Hg profiles in cores of estu-arine sediment,although they did not take the oxyhydroxide-bound Hg fraction into account.Like Gobeil and Cossa(1993), they found that the relationship between total Hg and intersti-tial Hg could be highly variable even within a single core, indicating that the two parameters often vary independently of each other.Thus,there were examples of positive and negative correlations between total Hg and pore-water Hg occurring in different segments of the same core,with the result that there was no significant overall correlation.In other cores,however, the profiles of total Hg and pore-water Hg showed weakly to strongly developed parallel trends throughout the length of the core.As in the system studied by Gobeil and Cossa(1993),the vertical variation in total Hg content was probably determined almost entirely by the rate of Hg loading;apparently postde-positional remobilization,as represented by the interstitial Hg, affected only a very small fraction of the sediment-bound Hg. Moreover,Bothner et al.(1980)observed,as did Gobeil and Cossa(1993),that Hg was more soluble under anoxic condi-tions than in a well-oxygenated environment,probably owing to release of the soluble species HgS22–from anoxic sediment and the release of coprecipitated or sorbed Hg from FeOOH and MnOOH on reduction and dissolution of the oxyhy-droxides(Jackson1993a).But in the environments studied by Lindberg and Harriss(1974),the Hg in the pore water was in the form of dissolved organic(probably humic)complexes, and their data suggest that the molecular size range of the humic matter determined the relationship between interstitial Hg and total Hg.
One other study is to some extent comparable with the work of Gobeil and Cossa(1993).Louchouarn et al.(1993) examined vertical profiles of total Hg and citrate–dithionite–bicarbonate extractable ferric oxide and other sediment con-stituents in cores from four widely separated lakes in Québec, although they did not include data for interstitial Hg.From their results they concluded that there was no indication of a strong tendency for Hg to be associated with ferric oxide. (Actually,it would be more correct to say that there was no consistent tendency,as two of the lakes showed no apparent correlation between total Hg and ferric oxide,whereas in the other two lakes the core profiles revealed a definite tendency toward positive correlation.)The authors inferred that there was either a lack of postdepositional remobilization of Hg from ferric oxide or else remobilzation followed immediately by highly effective scavenging of the released Hg by other particulate binding agents,most probably organic matter. Analytical data for pore water would probably have helped to resolve the uncertainty regarding the processes controlling the distribution of Hg.
As discussed above,cores from two lakes in Sweden have revealed a major buried Hg maximum(Fig.4)(Munthe et al. 1995).This pattern of variation suggests a progressive increase in atmospheric Hg deposition over time,followed abruptly by a sharp decline in the rate of Hg deposition which is at-tributable to a large reduction in the emission of Hg by major industrial sources of atmospheric pollution in northern or cen-tral Europe(Munthe et al.1995).Either of two indpendently documented historical events could account for the pollution abatement recorded in the cores:(i)a reduction in emissions of Hg from known point sources in eastern Germany in the late 20th century(an event that was followed by a measured de-cline in atmospheric Hg levels over Sweden(Iverfeldt et al. 1994))or(ii)a decrease in Hg emissions from industrial cen-tres within Sweden starting in1960(Lindqvist et al.1991). Radiometric dating of the cores is required to establish the exact cause of the abrupt change seen in the core profiles, but there is every reason to believe that a marked decrease in regional emissions of anthropogenic Hg into the atmosphere is involved.
Equally persuasive evidence has been furnished by the previously mentioned study of high-resolution Hg profiles in very accurately dated cores that record the history of Hg pollution in Lake Geneva since the late19th century(Dominik et al.1991).The Hg profiles reveal two distinct buried Hg maxima attributable to pollution events in the Rhone River upstream from the lake.As the more recent of the two peaks was formed after annual monitoring of the Rhone had been initiated,it is possible to relate the peak to independently re-corded events occurring in the river.Accordingly,Dominik et al.(1991)were able to establish that the Hg maximum in the lake sediment was formed immediately after a recorded episode of Hg pollution in the Rhone River.(Furthermore, Dominik et al.(1991)claimed that there were no correspond-ing buried maxima in the profiles of other metals;this adds additional weight to the already compelling evidence that the Hg profiles in the core were shaped by temporal variations in Hg loading and not by diagenesis.)
Analogous examples of buried Hg maxima in cores have been reported for the Hg-polluted Wabigoon River system of Northwestern Ontario,in which a sharp reduction in the dis-charge of Hg from the point source of pollution(a chloralkali plant)led to a decline in the rate of Hg deposition in the river and a riverine lake.As a result,cores collected several years later showed Hg concentrations rising toward the top of the core,peaking,and then coming back down(Jackson and Woychuk1980;Rudd et al.1983).
Striking examples of a buried Hg maximum have also been observed in the west basin of Lake Ontario,and they have been attributed to a decrease in Hg loading from the Niagara River (Mudroch1983).The data show a progressive decrease in the intensity of the buried Hg maximum with distance northwest from the mouth of the river,but because of the pattern of currents in the lake(which are known independently from other studies),no gradational decrease in Hg away from the source of pollution is seen in cores taken north and northeast of the mouth of the river.Clearly the core profiles reflect both temporal and spatial variations in the rate of Hg deposition,not diagenesis.
Similarly,dated cores from two lakes in southern Québec show buried Hg maxima tentatively attributed to a decrease in the emission of particles from coal-fired power plants(Ouellet and Jones1983).In the same cores Pb increases steadily from deeper levels right up to the sediment–water interface,possibly owing to continuing pollution from motor vehicles(Ouellet and Jones1983).The marked difference between the Hg and Pb profiles casts doubt on the hypothesis that such core profiles
Jackson113
are controlled by diagenesis but is entirely consistent with the view that they are controlled by the rate of loading.
Dated cores from lakes in eastern Minnesota consistently show a buried Hg maximum,implying a regional decline in atmospheric Hg pollution since~1970,whereas cores from lakes in western Minnesota give no indication of a decline,the Hg levels tending to rise unabated up to the present time (Engstrom and Swain1997).This striking and consistent quali-tative difference between the Hg profiles of the two neigh-bouring geographical areas is attributable to regional differences in Hg loading over time,as suggested by related evidence reported independently by other workers.The cores from the eastern lakes reflect a number of factors,such as reduced in-dustrial utilization of Hg,effects of pollution control measures, and substitution of natural gas for coal as a fuel for heating buildings,which have decreased regional anthropogenic emis-sions in the late20th century,while the pattern of variation that characterizes the western lakes is probably linked to the fact that Hg levels in precipitation tend to be higher in the western than in the eastern part of the state,possibly owing to a greater abundance of wind-blown particulate Hg eroded from agricul-tural soil in the west(Engstrom and Swain1997).The results of the investigation are consistent with regional differences in Hg loading over time but are not compatible with the post-depositional redistribution hypothesis,especially since half the buried peaks(four out of eight)were at depths of11–15cm.
The suite of Swedish lake cores showing a north-to-south gradational increase in the degree of surface enrichment in Hg with respect to the background Hg deeper in the cores (Johansson1985;Lindqvist et al.1991)(Fig.2)strongly sup-ports the conclusion that the core profiles represent a geo-graphic trend in the severity of pollution.The core data are analogous to the previously mentioned series of Lake Ontario core data reported by Mudroch(1983),and there is every reason to suppose that the same principles apply.Furthermore, the inferences drawn from the Swedish core data have been confirmed independently by a wealth of other kinds of data, such as the geographical distribution of Hg,soot,acids,SO42–, and Pb in rain water,results of trajectory analysis,and Hg data for moss,peat,and soil organic matter(Lindqvist et al.1991; Steinnes and Andersson1991).Consequently,the weight of evidence overwhelmingly supports the conclusion that the core profiles represent a record of Hg loading and that the surface enrichment is a result of air pollution.
Hg profiles in cores are comparable to the profiles of associated substances that,like Hg itself,are known to be diagnostic of air pollution caused by the combustion of fossil fuels.Thus,in cores from certain lakes in Sweden,the ten-dency of Hg levels to increase toward the top correlated with similar trends in soot,Pb,and fossil diatoms indicative of acidic conditions(Hultberg et al.1994).Obviously,diagenesis cannot account for the distribution of soot and diatoms.Fur-thermore,the correspondence between the vertical variations of all these sediment constituents,as well as the fact that at-mospheric soot and strong acids and much of the Hg in the atmosphere have a common origin as products of fossil fuel combustion,strongly suggests that all the trends have a common cause.Besides,the recent decline in the Hg content of surface sediments in Swedish lakes parallels a decrease in atmospheric sulfur levels reflecting abatement of air pollu-tion in northern and central European(Munthe et al.1995).The only plausible conclusion seems to be that air pollution, not postdepositional redistribution,accounts for the observed trend in the Hg profiles of cores.By the same token,cores from remote areas(e.g.,the Arctic)have revealed a progres-sive increase in both Hg and PAHs(which,like Hg,are products of fossil fuel combustion)toward the top(Lockhart et al.1993),implying that both of these substances were de-posited as a result of long-range atmospheric contamination and that the core profiles represent an increase in the rate of loading over time.
Cores showing the typical surface enrichment in Hg and a progressive decrease in Hg with depth also show a parallel exponential decline in210Pb,demonstrating that there has not been appreciable postdepositional redistribution of trace metals by diagenesis,physical mixing,or erosion,besides indicating a nearly constant rate of sedimentation(Dominik et al.1991;Lockhart et al.1993,1995).
The vertical variations in Hg deposition rate recorded for some ombrotrophic peat bog cores strongly resemble the Hg profiles of lake cores that show a peak at or near the surface and a decrease with depth(Madsen1981;Jensen and Jensen 1991;Benoit et al.1994),suggesting a common mechanism for both.The question then arises whether Hg deposited in peat is subject to remobilization.Published data for certain metals other than Hg,including Pb and Zn,indicate that the metals are strongly immobilized in well-drained peat hummocks above the water table but are mobilized and leached to a considerable extent below the water table(Damman1978;Urban et al.1990). In contrast,the Hg profiles in the peat cores described by Jensen and Jensen(1991)cross the water level without show-ing any sign of a discontinuity,suggesting that Hg is not subject to appreciable solubilization below the water table. In view of the strong affinity of Hg for humic matter,espe-cially the relatively insoluble higher molecular weight hu-mic acid fractions(Jackson et al.1980;Jackson1989),it seems probable that Hg in peat is immobilized by humic acids,both above and below the water table(Benoit et al. 1994).The Hg profiles are best explained by temporal vari-ations in Hg loading,not by postdepositional redistribution of Hg(Benoit et al.1994).
Hg is not highly susceptible to postdepositional remobi-lization in lake sediments because it tends to be bound stongly by refractory organic matter such as humic substances (Jackson et al.1980;Jackson1989;Louchouarn et al.1993; Lucotte et al.1995).Hg is bound preferentially by the rela-tively insoluble higher molecular weight humic acids with respect to the more soluble lower molecular weight fulvic acids(Jackson et al.1980,1982).
5.Effects of airborne Hg on organisms in
remote ecosystems
As we have seen,a large and diverse body of research data has established that Hg introduced into the air from various sources of pollution has been spread throughout the world by the global atmospheric circulation,although it is concentrated chiefly in the middle to high latitude regions of the Northern Hemisphere,notably in certain subregions such as northeastern to north-central North America and southern Scandinavia. Significant quantities of this ubiquitous anthropogenic Hg, along with other air pollutants(some of which come from the
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same sources as the Hg),have been accumulating in remote and otherwise pristine aquatic and terrestrial ecosystems since the onset of the Industrial Revolution,and the contamination has continued up to the present day,although it has abated somewhat recently in certain regions such as Scandinavia. Now we must address a question of the utmost importance: What harmful effects,if any,is this diffuse,universal Hg pollution having on biological communities,including human societies?More research is needed to answer this question properly,but on the basis of the information currently avail-able,it is safe to say that the problem of long-range atmos-pheric transport of anthropogenic Hg to ecosystems is a cause for serious concern from an ecological and public health perspective for the following reasons.
Hg is a notoriously toxic element with a strong tendency to accumulate both in organisms and certain environmental compartments,such as fine sediments and soil organic matter, where it builds up persistent secondary sources potentially available for bioaccumulation;moreover,Hg,unlike a number of other heavy metals,including Cu and Zn,is not known to perform any beneficial biological functions.The highly toxic, bioavailable species CH3Hg+,which is formed from bioavail-able inorganic Hg(II)species by various free-living microbes in aquatic environments,is accumulated efficiently by organ-isms,because it is taken up easily and rapidly but excreted very slowly;and unlike inorganic Hg(II),it undergoes biomagnifi-cation up the food chain and is preferentially accumulated by the muscle tissue of fish,which may thereby be rendered un-safe to eat.Even if Hg in the ambient air and water is present only as a trace constituent,high concentrations of it may build up in organisms under suitable environmental condi-tions.Thus,although the Hg carried by the atmosphere is highly attenuated,it has a high potential to accumulate in organ-isms,sediments,and soils,even if a portion of it is returned to the atmosphere through volatilization.As demonstrated by Fitzgerald et al.(1991,1994),many freshwater environments receive the bulk of their Hg input from the atmosphere,and atmospheric deposition readily accounts for the total Hg bur-den of the water,sediments,and fish of remote seepage lakes in northern Wisconsin.Furthermore,mass balance calcula-tions suggest that small increases in atmospheric Hg loading lead directly to a rise in the Hg content of the fish(Fitzgerald et al.1991,1994).Similarly,Hultberg et al.(1995)concluded, after a3-year intensive investigation of the forested watershed of a lake in Sweden,that most of the Hg(both inorganic Hg and CH3Hg+)in the system was deposited there from the at-mosphere.Although soils were found to accumulate the bulk of the Hg deposited in the catchment basin,subsequent trans-port of Hg into the lake by runoff(not to mention Hg de-posited directly on the lake surface)was regarded as sufficient to increase the Hg content of fish appreciably(although ana-lytical data for fish were not included in the study).Extrapo-lation of their results led them to surmise that deposition of atmospheric Hg is responsible for elevated Hg concentrations in the tissues of fish in thousands of Swedish lakes.By the same token,Lindqvist(1994)inferred that accumulations of atmospheric Hg in forest top soil in Sweden have been respon-sible for increased bioaccumulation of Hg in lakes receiving drainage water from the soil.Accumulation of atmospheric Hg in marine food chains is suspected as well.A model developed by Rolfhus and Fitzgerald(1995)predicts that any increase in the deposition of anthropogenic airborne Hg in the sea(all other variables being constant)will cause a rise in the Hg content of the marine biota.
Although the total quantity of Hg deposited in a remote lake from the atmosphere may be small compared with the amount that might be introduced into a lake by the direct dis-charge of liquid effluents from an industrial establishment such as a chloralkali plant,the environmental degradation caused by the Hg could be disproportionately great,depending on the local environmental conditions.In a number of acidified re-mote lakes,for instance,Hg levels in fish are remarkably high even though Hg concentrations in the environment are low(see below).It is commonly observed that CH3Hg+production and the bioaccumulation of Hg in Hg-contaminated lakes and river systems depend largely on an assortment of environmental factors and microbial activities that control the speciation and hence the bioavailability of the Hg,whereas they are inde-pendent,or nearly independent,of the total quantity of Hg in the environment except on a very gross scale,and in some cases,are even inversely related to total Hg(Jackson1986, 1988,1991,1993a,1993b;Jackson et al.1993;Jackson and Woychuk1980,1981).Even in a relatively pristine lake where the only Hg present consists of trace quantities introduced from the atmosphere and local drainage basin,a serious Hg problem may result from an environmental change that stimu-lates the growth and activities of methylating bacteria,leading to increased rates of CH3Hg+accumulation in fish.A striking example of this phenomenon has been seen in certain Canadian reservoirs recently formed by enlargement of riverine lakes; owing to impoundment of the river system and consequent flooding of adjacent forested land,large amounts of labile organic matter were introduced into the aquatic environment, causing an upsurge of microbial methylating activity and lead-ing to marked increases in the Hg content of fish(Jackson 1988,1991).In short,“background”levels of Hg in the envi-ronment are high enough to cause a serious Hg problem if conditions are particularly favourable for the production and biological uptake of bioavailable Hg species.
Effects of acid precipitation on the interactions of aerially transported Hg with organisms in freshwater environments are of particular concern.As we have seen,the acids respon-sible for the phenomenon tend to accompany the Hg,because the Hg and the oxides of S and N,which are the precursors of the acids,are to a large extent emitted from the same sources of pollution(the most important ones being coal-burning power plants),and unfortunately,the acids aggravate the ill effects of Hg wherever the buffering capacity of the water is low.Therefore,the effects of atmospheric Hg pollution and possible methods of pollution abatement cannot be fully assessed without taking the synergistic role of the airborne acids(H2SO4and HNO3)into account.
Acidification of poorly buffered lake water by acid precipi-tation is generally accompanied by a marked rise in the Hg (specifically CH3Hg+)content of fish inhabiting poorly buff-ered lakes in remote areas(Johansson1985;Lindberg1987; Richman et al.1988;Winfrey and Rudd1990;Haines et al. 1995).Although empirical evidence for an inverse correlation between lake water pH and Hg concentrations in fish is well documented,the exact cause and effect mechanisms have not been established.Nevertheless,there are several prob-able explanations,any or all of which may be applicable:
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(i)the occurrence of Hg pollution along with the acid pollu-tion;(ii)greater availability of inorganic Hg(II)for conversion to CH3Hg+and greater bioavailability of the CH3Hg+at low pH owing to a higher degree of lipophilicity,and hence greater ability to pass through biological membranes(Haines et al. 1995;Mason et al.1995),as well as weaker sorption and com-plexing by metal-binding agents in the environment(Leckie and James1974);(iii)less production of Hg(0)from inorganic Hg(II)at low pH,making more inorganic Hg(II)available for transformation to CH3Hg+(Fitzgerald et al.1994;Mason et al. 1994;Vandal et al.1995);(iv)production of CH3Hg+at the expense of(CH3)2Hg at lower pH values,owing to the growth of CH3Hg+-synthesizing rather than(CH3)2Hg-producing microbes,together with spontaneous conversion of(CH3)2Hg to CH3Hg+,under acidic conditions(Wood1971;Fagerstr?m and Jernel?v1972;Gavis and Ferguson1972);and(v)the combined effect of pH and other physicochemical factors such as dissolved O2(Jackson and Woychuk1980,1981;Jackson 1987;Winfrey and Rudd1990;Matilainen et al.1991).
In conclusion,any process that releases Hg into the atmos-phere,such as the combustion of coal,incineration of wastes, or smelting of ores,increasing Hg levels in natural ecosystems, is environmentally harmful.The detrimental effect of the com-bustion of coal and other fossil fuels is twofold,as it releases both Hg and precursors of strong acids into the air.Hg or acid contamination alone is baneful enough to be a cause for serious concern,but the combination of Hg and acids is especially deleterious because of their interactions,which worsen the biological effects of the Hg.Elevated Hg concentrations in fish pose a potential health risk to humans,and possibly other animals,that consume the fish(the degree of risk depending on the Hg levels,the amounts of fish eaten in a given period of time,and factors such as the time of life or stage of devel-opment of the consumer(fetuses being especially vulnerable) (Clarkson1990;Wheatley and Paradis1995).In addition,Hg contamination in fish may pose an economic threat to people who make their living by fishing.Further research is needed to determine whether airborne Hg(or Hg in conjunction with other contaminants,such as acids)is having toxic effects on the biota of remote ecosystems.
6.Conclusions
The weight of evidence provided by many different kinds of data assembled independently by different investigators in dif-ferent parts of the world supports the following conclusions.
(1)Approximately5000t of Hg are introduced into the atmosphere every year as a direct or indirect result of various human activities,especially the combustion of fossil fuels (notably coal)and incineration of solid wastes.Direct,primary emissions from sources of pollution amount to about4000t, half of which is deposited near the sources of pollution,while the remaining2000t are entrained by the general atmospheric circulation.Reemission of anthropogenic Hg from diffuse secondary sources in the environment accounts for roughly 1000t.It is important to recognize that the flux of anthropo-genic Hg to the atmosphere comprises not only direct emis-sions from sources of pollution but also secondary emissions from diffuse sources in aquatic and terrestrial environments contaminated by previous deposition of anthropogenic Hg. Failure to take the secondary sources into account leads to underestimation of anthropogenic Hg emissions and correspond-ing overestimation of natural emissions.On this basis,truly natural emissions total roughly2000t/year.Thus,the total abundance of anthropogenic Hg in the atmosphere is some-what greater than the abundance of Hg from natural sources but is of the same order of magnitude.Natural sources of Hg must be taken into account in the study and modelling of Hg pollution and the biogeochemical cycling and environmental effects of Hg,as emphasized by Rasmussen(1994);but there is no need to overstress this point,as there is ample proof in the literature that environmental scientists have been doing this all along as a matter of course.
(2)From the estimates given above,it follows that about 3000t of anthropogenic Hg are subject to long-range(regional or global)atmospheric transport every year.Long-distance at-mospheric transport of this Hg results in measurable contami-nation of natural environments up to thousands of kilometers from the sources of the Hg emissions.
(3)The evidence available to date militates strongly against Rasmussen’s hypothesis that Hg profiles in lake sediment cores are determined primarily by diagenetic redistribution of the Hg(Rasmussen1994).A minute amount of postdeposi-tional redistribution does occur,but its effects on the overall distribution of Hg appear to be very slight,indeed negligible, although there is a need for more research in this area.Differ-ent lines of evidence support the conclusion that vertical Hg profiles in fine-grained,undisturbed lake sediments primarily reflect temporal variations in Hg loading,provided that the sediment texture and composition and the rate of deposition have not changed radically over time.
(4)Anthropogenic Hg is accompanied by other products of fossil fuel combustion,including the acids responsible for acid precipitation.It is important to bear this in mind when assess-ing the biological effects of the Hg and developing pollution abatement strategies,because the associated acids interact with Hg and organisms in poorly buffered freshwater environments, causing substantial increases in the concentrations of CH3Hg+in fish.Thus,effects of associated pollutants,as well as the Hg itself,must be taken into account in the investigation of the biological impact of Hg pollution.
(5)Many remote lakes and other remote ecosystems appar-ently receive most of their Hg input from the atmosphere,and there are grounds for believing that atmospheric Hg pollution has been causing appreciable increases in the Hg content of organisms in these lakes and also in the sea.
(6)Hg is an exceedingly toxic element with a strong ten-dency to accumulate in organisms and(in the form of CH3Hg+) to undergo biomagnification toward the upper ends of aquatic food chains;under environmental conditions particularly favourable for bioaccumulation,high concentrations of Hg may build up in aquatic biota,resulting in a severe contamination problem,even if Hg concentrations in associated water and sediments are relatively low(e.g.,at background levels).Ac-cordingly,contamination of the atmosphere with several thou-sand tonnes of Hg per year followed by long-range aerial transport of much of this Hg to remote ecosystems is a cause for serious concern,despite the fact that Hg dispersed in the atmosphere is highly attenuated.Contamination of the at-mosphere with Hg and associated contaminants such as acid precursors,with long-range atmospheric transport of these
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contaminants,is a major international environmental problem that should be dealt with by reducing or eliminating emissions.
7.Acknowledgments
I thank M.Alaee,R.J.Allan,J.Azcue,https://www.doczj.com/doc/c5547496.html,wson,A.Mudroch, W.M.J.Strachan,and L.Turner for contributing useful infor-mation and comments,and the authors and publishers who kindly permitted me to use their previously published dia-grams.This study was carried out with the financial support of Environment Canada.
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雅思口语Part2常考话题：有趣的地方 本文为大家收集整理了雅思口语Part2常考话题：有趣的地方。雅思口语Part2着重考察同学们的英语交流能力，同学们在备考阶段可以注意多积累素材，平时多模仿多练习，这样在考试中才不至于无话可说。 Describe an interesting place to visit Ok, I'd like to tell you about the temple of the Six Banyan Trees. It's located in the city center of Guangzhou and its not far away from Zhongshan 6 Road, near the People's Hospital. It's a good place to go to and climb to see the Flower Pagoda, which has eight sides to it. You can also go there to pray if you are a Buddhist. It is an active temple and is also the head of the Guangzhou Buddhist Association. You can also do a bit of shopping there if you like. Besides the souvenirs you can buy there, it also has a very lively fruit and meat market nearby, so if you just want to shop instead you can. The main feature of this place is the pagoda because it's the tallest in the city. From the outside it looks like it has only nine stories, but actually it has 17 inside. It's quite old, and was built some time in the 10th century. The temple itself is even older than that-I think it goes back to some time
2020雅思口语目标7分技巧 雅思考试中，听力和阅读部分拿到高分相对容易些，提高水平，在考试中细心一些即可，但是口语想要拿到7分以上却并不简单。下面就和大家分享雅思口语目标7分，希望能够帮助到大家，来欣赏一下吧。 雅思口语目标7分?口语评分标准告诉你该怎么备考 雅思口语评分标准告诉你“流利度”如何拿7分 很多雅思口语提升攻略里都会提到雅思口语评分标准的四个方面，但对其做细化分析的并不多，比如，为什么你的口语只能拿6分?口语6分与7分的差异究竟表现在哪里?其实这些在评分标准中都有具体的区分。雅思口语7分在“流利度”方面要求大家“讲话时表达详尽，没有明显的不连贯现象，偶尔会有犹豫、重复或者自我纠正，能够灵活使用连词或者有标志性的词汇”，而口语6分的评分标准要求要低很多，具体的要求是“想要讲的详细，但是会因为重复、自我纠正和犹豫失去连贯性，能在口语中使用连词，但并不是所有连词都用的恰当”。 通过对比，我们发现，其实想要在“流利度”方面达到7分并不是特别难，7分有一定的容错度，只要大家不超出界限即可，偶尔犹豫可以，但整体要给考官一种你在自如自信表达的感觉。
针对流利度，建议大家在平时多做实际交流练习，尽可能多找机会与“native speaker”交流，锻炼英式思维的同时，也能提升表达欲望。 雅思口语评分标准告诉你“词汇”如何拿7分 雅思口语评分标准第二项，词汇。口语词汇如何才能达到7分标准呢?口语词汇7分要求大家“灵活使用词汇，能用一些非常见词汇和习语，了解语体和词汇搭配，可偶尔出现使用不当的情况”。具体分析，我们发现7分的词汇要求中，有一个点大家经常忽略，那就是“习语”。其实高端词汇大家在备考中都会用到，但是习语的使用却相对较少，而非常见习语的使用更是少之又少。所以小站君建议大家在备考雅思口语的时候积累一些地道习语，用在口语表达中，不仅仅是Part2，Part1和Part3也可以根据语境使用。如果大家能在口语表达中灵活使用一些让考官大家赞许的习语，分数自然也会有不错地提升。 雅思口语评分标准告诉你“语法”如何拿7分 雅思口语在“语法”方面的要求主要是多样性和准确性，而7分是口语高分的一个分水岭，所以也是语法使用的一个分水岭。“语法”方面想要拿到7分，大家必须“灵活使用复杂结构”，但是容许有一些不影响理解的语法错误。所以口语语法想要拿到7分，必须要掌握复杂句式的使用方法，并能灵活运用。这一点需
雅思口语6-7.5分考前一个月必练词组，句子等180组 1名词词组： a few friends of mine an essential part There’s a great number of traffic accidents happened everyday. a good listener 一个很好的听众 all-round person 全方面人才 bright future 光明的未来 comprehensive skills 综合技能 cultural heritage 文化遗产 fitness center 健身中心 focus of attention 关注的焦点 internet café网吧 low salary/high salary 低薪水/高薪水 many aspects of life 生活的很多方面 modern society 现代社会 movie censorship电影审查制度 on-line shopping 网上购物 people of all ages各年龄段的人们 pleasant atmosphere 令人愉快的环境 pop song 流行歌曲 role model 偶像 scenic spot 景点 serious air pollution 严重的空气污染 the Imperial Palace/the Palace Museum 故宫 traffic jams in rush hours 高峰时期的塞车 valuable experiences 有价值的经历 well-rounded education 全面教育 2动词词组： 把我的思想从课业当中解脱出来：take my mind off my school work It’s located in… 确保某事：It can guarantee sth. to make sure this will not happen 给某人提供某物：provide sb. with sth. acquire a lot of knowledge one can not find in textbooks 学到书本上学不到的知识add to the opportunity/the success 增加机会/增加成功性 avoid health problems 避免亚健康/健康问题 be helpful/beneficial to 对…有利 be under too much pressure of school work 学校课业压力过重 become more aware of the importance of…越来越认识…的重要性 bring sb. up 把某人抚养长大 broaden one’s horizon 开阔眼界 do harm/good to children 对孩子有害/有利 do one’s best/utmost 尽某人最大力量 enforce laws 实施法律
雅思口语7分经验分享 雅思口语7分--既然选择出国，哪还在乎背一本机经,家带来了雅思口语7分经验分享，希望能够帮助到大家，下面就和大家分享，来欣赏一下吧。 雅思口语7分--既然选择出国，哪还在乎背一本机经 20XX-08-17 16:36新东方网整理分享到个人状况：大学勉强过四级，没考6级。工作后突然觉得学历不够，要继续读研，提高英语。于是断然辞职，用了将近半年多的时间复习，备考，战斗。 终于overall : 6.5 speaking:7 首先我想说的是，如果你英语不错，自认随便考考就可以考个7之类的朋友，建议你不要看此贴。因为你能力够好，不需要分享我的经验，我的每一步都是脚踏实地，没有运气，没有天赋，有的只是付出巨大的努力去换回应得的那一点点回报。 听力和阅读我就不多说了，基础不太好的同学，这两项应该都不会好到哪里去，所以我自认自己非常的努力，我的作文和口语从比这两项要考得好很多。
听力：我后来是每天坚持听BBC，并且听写下来，听力的单词一定要会拼写，我很遗憾，考了几次每次都有低级的单词拼写错误，所以一定要拼写下来。 阅读：我阅读一直没很大的提高，剑桥的书做了很多遍，有时做了7，有时6，考试还偶尔考个5.5，问题不知出在哪里，但多看总是没错的，我考完后因为读材料和自己申请，觉得阅读提高很多，现在再去考不知道会不会提高点。 重点我想说说口语和写作，这是我能分享的成功经验。我雅思先后考了4次，但庆幸每一次都有收获，没有浪费一分钱。没有之前的失败，不会有最后的成功，所以不管考几次，一定要自己认真总结。 但4次里面，我的口语都不低于6，之后是6.5，最后是7，稳步提升。我的经验就是我从不怯场，可能和工作有关，我的沟通能力较强，比较胆大，这可能也是我胜算的原因，还有一点就是：我努力。我准备了几乎所有的话题，每天背2——3个小时，也就是说我保持说口语的状态each day,并且说的都是和雅思考试相关的内容。 但这个很难坚持，我告诉很多人，他们都表示去死也不可能坚持每天说3小时口语。但，你如果不坚持，随便准备个十来个话题，我保证你必败，除非你就是运气好，再无其他。
雅思口语Part2参考答案之你去过的城市 一、参考范文 Sydney is my most favorite city among the cities I have visited. Sydney is the state capital of New South Wales of Australia. This city is situated in the bank of Tasman Sea and has around 4.6 million people. I have visited this city in 2008, after I finished my graduation and loved my stay there. Sydney had many attractive natural area, botanic garden, parks, and high rising buildings. This city has many heritage listed building that attracts the tourists and visitors. The Sydney Opera House is one of the most recognized landmarks in Australia and is a great place to visit. This city is known for the dynamic cultural hub and it has many famous museum, galleries and art galleries as well. Because of the great architecture, warm weather and hundreds of tourists attraction more than 11 million international and domestic tourists visit this city each year. I had been there for about 15 days and I really enjoyed everything about this city. I stayed at a 20 storied hotel that offered a really amusing sight views. The transportation system of the city is better than many other cities and I could have been maintaining the track and time of my schedules because of that. I loved being at open & wide spaces in the gardens and parks. People are welcoming and friendly there. A tourist can get plenty of helps both from people and the authority and can roam easily without any interruption. I saw 2/3 art museum and some cultural festivals and those were awesome.
雅思口语考试七规则，四要点 雅思口语考试七规则： 1. 心态 无论考什么试，心态都是很重要的。要有信心，不要怕。我见过很多学生，考口语的时候，出来说话都发抖。这有什么好怕?真是奇怪，就是进去说说话嘛。如果要工作也要面视嘛。不做作，给考官一个很好的第1印象。 2.当你进到考场里的时候，你坐下，然后考官说你不要紧张，等等。你不要说：“我一点也不紧张呀。”这个话说出来是很不礼貌的。反而，你说，谢谢，I feel morecomfortable now.也会收到以外的后果。 3. 不要去背一些机警的口语，这回限制你的发挥。看看是可以的，增加一下自己的思路，背的话，分就significantlydropped。希望大家记住这点。很重要。 4. 当考官问的问题，你不是很了解，或者不知道的时候，你要婉转的拒绝。For example, I can talksomething wot u know, and say: that wot i know about..., I have less experiencein these kind of .... . So could we change another one. I think I can talk umore. 等等类似的话。 5. 回答的答案不要大众化，要有自己的特色。外国人喜欢幽默和有个性的人。你如果一味的说谁都知道的事情，大家都会感到SLEEPYAND BORING。 6. 当你考完了以后，也不要去问考官关于自己的不足，等等。这个是违规的。 7. 大胆去说，如果发现不对，即时改更改，用i mean等词。 雅思口语考试四要点： 1. 流利度。这是最首要的，需要大家在雅思考试中说话时不要总是中断，嗯嗯啊啊的。这也是一个语言熟练度和思维的问题。如果你一句话说得很熟，就不会出现打梗;如果你思维开阔，就不会上句接不了下句。这需要大家以句子为单位操练(非单词)，同时看大量的英文材料扩展讲话素材。 2. 语音和语调。很多人发音不好似乎是先天的，且与当地的口音有关系。如果你普通话都讲不好，在雅思口语考试时发音也很难标准。语调方面，注意英语升降调的规律，说话尽量有升有降，抑扬顿挫，不要全部一个调调，听起来像机器人。在雅思口语准备过程中发音的训练靠多听多模仿，这方面的资源很多，不再赘述。 3. 词汇丰富度。这方面不需要过分夸大口语和书面语词汇的区别。但是雅思口语方面的词汇还是应该尽量积累的，尤其是习语和动词短语(不要告诉我你不知道是什么)。见过雅思的三个不同考官的经验告诉我，口语考试中，如果你能时常蹦出一个比较“高档”的词汇(如
初中英语单词教案 篇一：初中英语词汇课的教学设计】 初中英语词汇课的教学设计想方设法培养孩子的学习兴趣，让孩子们快乐地学习，快乐地成长。教师要授人以鱼，教会孩子们学习方法。而学会学习和发展是素质教育的要求,也是《英语》（新课准）的指导思想。 一、教学内容: 初中阶段的单词和词组。 二、教学目标: 1、提高、激发学生学习英语词汇的兴趣,培养学习的自觉性和合作精神。 2、会拼读单词, 掌握常见的使用方法。 3、掌握词汇的记忆方法, 以及各种变化方式。 4、培养词汇的听、说、读、写技能, 形成一定的语言运用能力。 5 、借助情感因素, 营造亲切、和谐、活泼的课堂气氛, 促进学习思维。 6 、拓展视野,培养爱国主义精神,形成健康的人生观。 三、教学重点:词汇的记忆和拓展。 四、教学难点:词汇的综合运用。 五、教学方法:启发式、讨论式、表演式、多媒体等。 六、教学用具:课件、实物、图片。 七、课时: 一节 八、学情分析: 1、有利条件: 大部分学生在小学时已经学习了英语,有了词汇学习的基础,具备了不少简单的词汇积累,在教师指导下能够完成学习任务。 2、不利条件: 由于初中阶段词汇量的增多,让学生增加了许多记忆上的困难。同时初中生受年龄特征的局限,很难在词汇的学习中有更多的付出,更不可能加以全面拓展。 九、教学过程: 1 、复习导入 ①老师让学生跟录音大声朗读上一节课教授的单词,以便复习巩固。 ②全班同学关上课本,由老师朗读要听写的单词（10 个）,学生听写（写出单词与意思）。 ③最后老师抽查一个小组,其它组用3 分钟检查并改正。
2 、教授词汇的方法 词汇是组成英语河流的源头,对学生来讲,掌握好学习英语词汇方法就像盖大楼打好地基一样重要,若不掌握好一定的词汇量,想学好、用好英语便是空想。在多年的教学实践当中,在词汇教学方面,我用下面方法有意识地培养学生记忆能力和教学设计。 ①利用字母和字母组合的读音规则，掌握单词的拼写形式。在词汇教学中， 要注意音和形的统一与结合，使学生把一定的音同可能对应的形联系起来，又把一定的形同可能对应的一定的音联系起来，通过反复练习在大脑中建立起来一类一类词的音、形模式联系，如让学生把长元音[i:] 和she,see,sea,piece 等单词中的元音字母及元音字母组合的形联系起来。 ②拓展记忆学习法。英语词汇学习中,有很多词有同音、近义、反义词和有与之相关联的词性,教学中可以拓展学习,以扩大学生词汇,并增强记忆。如large--huge （同义） short ——tall （反义） see ——sea（同音） dead9a.）——death（noun）——die （verb）③运用归类教学。如颜色类:red,black,white,green,yellow; 交通工具 类:bus,bike,car,train,ship 。 ④实物展示词汇教学。利用实物展示, 教学直观生动形象,学生乐于接受。教师指着实物问what'sthis/that? 帮助学生回答it 's a/an ??⑤利用多媒体教学。这种方法图、声、形具全,既省时又少力,且直观有趣。如教动词run 、sing 、hear 、jump 等。教happy 、angry 、sad 等。 6. 利用具体情景掌握单词的读音和语义。在具体情景中教单词、学单词，不但可以克服孤立记单词容易遗忘的缺陷，而且能培养学生灵活运用单词的能力，学会在交际中使用单词总之词汇教学方法多种多样,以上方法只要能结合实际加以运用,将会令你的教学事半功倍。 利 十、课堂练习设计（类型）: 1、看音标写单词: ① [si:] ②[veri] ③ [mi:t] 等, 2、补全单词: ① j--p ② h__l__ ③ l__ft 等, 3、根据句意和首字母, 写出所缺单词。 如it must to linda. 十一、布置作业:（复习、巩固、强化所学 内容,培养能力）。
雅思口语Part2常考话题：在学校里美好的一天本文收集整理了雅思口语Part2常考话题：在学校里美好的一天。雅思口语Part2着重考察同学们的英语交流能力，同学们在备考阶段可以注意多积累素材，平时多模仿多练习，这样在考试中才不至于无话可说。 I was trying to think of my best day. You know, my life at school seems so long ago. I can remember an occasion when I won first prize in a singing contest, and I think that had to have been my best day at school. The competition was held outside the school, and we had a stage set up for each of us to sing. The sound quality was not very good since it was an outdoor competition, but I suppose that might have acted in my favour, since I really don't think I am that good of a singer. But anyway, I can remember feeling on stage as nervous as anything, and just closing my eyes and letting myself sort of float in my imagination. I imagined this was my profession, and I thought of myself as a famous person-but I can't remember who it was, it was so long ago. Anyways, after the song was finished I remember people were giving me quite a good applause. When my name was announced as the winner I felt like flying again. You know,' it was something that meant so much to me just to be on stage, but to actually also land first prize was something else. Now that I
雅思口语7分对于part1的要求 如何提高雅思口语成绩?今天给大家带来了雅思口语7分对于part1的要求，希望能够帮助到大家，下面就和大家分享，来欣赏一下。 雅思口语7分对于part1的要求 众所周知，雅思口语考试一共分为3个部分，考试时间为11-14分钟，那么问题来了，如何在如此短的时间内“征服”你们的考官?不论他是面挂迷之微笑，亦或满目苍凉，还是摇头无奈，搞定他们分分钟不在话下。要问我口出狂言的底气在哪里，答案就在这篇*中。当童鞋们一听到口语7，不禁会纷纷惊厥，怎么做到的?其实就是一句话“漂亮的开头，华丽的结尾”。 在口语的三个部分中，part1无疑是大家留给考官们的first impression。虽说不能一语定乾坤，但是从印象分来说不得不引起大家的注意。从口语评判的四个维度分析来看，若想拿到7分，就得做到表达详尽，无明显困难，或不失连贯，具有一定灵活性地使用一系列连接词。灵活地使用词汇讨论各种话题，使用一些非常见的词汇及习语，对语体及词汇搭配有所认识，能够有效地进行改述;较灵活地使用一系列复杂的语法结构，虽然反复出现一些语法错误，但语句通常正确无误;发音表现出6分水平中所
有积极表现，但也表现出8分水平中部分积极表现。概括成一句话，就是“低错误率，高产出值”。在part1中，大家通常会被问到五大类题型：个人信息类、个人喜好类、休闲类、技能类、抽象类。若想一举将它们拿下，就要学会使用功能型结构词及万能语料。这就要求大家“先打扫干净屋子再请客人”，将脑海中固有的一些“口水话”全部摒弃，再给大脑注入新鲜的氧气和血液。譬如，当大家想表达因果关系时，不应一张嘴就是because，so，我们可以适当的使用for this reason，accordingly，as a consequence，on this account等诸如此类的口语表达;当想表达喜爱时，不能想当然就是I like，而应该多用用I’m fairly keen on、I’m really into、I simply adore、I am pretty fond of……，让表达不落入俗套。对于part1的问题，大多数还是关乎大家的个人信息以及个人喜好，所以回答起来也是驾轻就熟，相信答案的key words 大部分的鸭鸭们都是可以想到的，关键是如何将这些idea用两三句话完美无误的表达出来。需要注意几点：一是注意时态。明明问的是上周的活动，就不要再用一般现在时啦;二是注意句式的变化。不要一味依赖靠谱的简单句，认为祸从口出，就不敢多说，其实考官期待的是你主动与他们交流，尽可能多的描述事件的细节或者是给出相应的实例，增强答案的信服度，同时也可以增加答案的长度。譬如，in 20XX就可以变为in the year of 20XX;三是注意用词，避免cliché。尽可能使用一些高端大气上档次的词汇及词组，譬如，将nice替换成fabulous，terrific，marvelous
Street that you like to visit_where, how often, what do you like to do, why you like there I’d like to talk about a street ,a gathering place of non government organizations.,which is located in the function of zhongnan road and zhongshan road in wuhan city in china .There are more than 50 ngo established and ran on this street so you can always see many energetic college volunteers passing out leaflets about caring aids children here or some street performance which are aim to hencing public’s environmental awareness.i often go to this street, about once a week to take part in some voluntary activities or visit my friends who works in a foundation there, besides, some education salons also attract me a lot, it feel so free to sit in the coffee bar on the street with a group of persons who all pays more attention to chinese basic organization. I thinks , this is a place that reflect my dream because i do really want to be a professional person work on public service to help children in remote areas to receive basic education for my strong belief that........ What sport you would like to try for the first time What, how, equipment, why you want to learn this I’d like to talk about GAONAO, a kind of Chinese traditional sport i’d like to learn. I want to learn this because more than 80percent of students in my university can play it well and i do really admire them. Besides, my boyfriend is a crazy player of gaonao so in order to create our common interest and play it with him, i plan to learn it. At last, i think this sport is so interesting which probably could attract many foriners to play, so i want to recommend and teach this chinese traditional sports to others when i study abroad which is beneficial for me to make friends. The only equipment used is a small foot bag and the playing method is very easy, you have to throw foot bag very high and run to receive it with another person by leg within 1mins. And two other persons would try to stop both of you. If your competitor receive the foot bag first rather than you , you would fail.i will learn it form my patient and professional boyfriend. A photo of yourself Where you took it When you took How,\ How you felt about it I’d like to talk a photo of myself by a selfie stick in a special spot, bathroom in my dorm, recording my most frustrating but bravest moment in my university. My most
人物 —A good student A good student that I have studied with was Tom. Tom was my high school classmate and he was our class monitor. He always got top marks in our class. For example when most people scored a 60 on the test. Tom would always get 80 or 90. So his scores were always better than others. We studied together during the weekend at the library and during weekdays we studied together in the classroom. We were really good friends and we always help each other. I remember Tom would always help me with my English because it was my weakest subject. Through his help my English improved a lot. Besides studying, we also had many common interests such as doing sports and playing games. I think he was a good student because he was hardworking and diligent. In high school many students studied to 6pm and went home to play, but Tom would always study until 9 or 10. He puts in a lot more work than others so that’s why I think he is also a motivated person. Most importantly. I feel that he sets high standards fo r himself and that’s the reason for his success. --A good parent Someone I know who I think is a good parent is my mother. My mother is the matriarch of my family, she is a kind and understanding person. She has so much love in her that makes all the kids in my family love her. Although she is not the bread winner of the family, she makes many important decisions in my family. My father will often ask my mother’s opinion before he does something. To me my mom is like my friend, I can talk to her about everything, whether it’s about my relationship, my studies or my social circle. No one knows me better than my mom. My mother will always be there to give me encouragement and gives me constructive feedback. My mother is also a knowledgeable person, when I was young, whenever I had about my homework, she would always help me. She really sets a good example for me, I received lots of influences from her. I think she is the perfect example of a good parent. --A famous person A famous person who has impressed me was Steve Jobs. Steve jobs is the founder of Apple. I remembered the first time I knew about him was when I got my first apple product which was the Ipad. At that time I only knew him as the founder of Apple. As I started to use more and more apple products, I felt Jobs became closer to life. When he died, I read his biography and I started to admire him. He started as a college dropout with a dream and passion for greatness. He was an innovative genius who made products that would change our lives. His pursuit of perfection made Apple become a leader in the IT industry. What impresses me the most is that Jobs is a bold person. He is willing to do and try things that most people can only imagine. I think I learned a lot from Jobs and as a young person, I think we need to be like him. We often times only think but never take the initiative. So this is a famous person that has really impressed me. 地点
雅思口语乱说能得几分，雅思口语一般最低几分 雅思口语乱说能得几分 雅思口语考试扣分总结： 1、60%的考生，因表情被扣分； 2、70%的考生，因“舌头”被扣分； 3、90%的考生，因“单字不重音”被扣分； 4、90%的考生，只使用“单”词，而不使用“词串”； 5、99%的考生，因“不敢提问”被扣分； 6、99%的考生，因“哑巴思维”被严重扣分 了解了这些，就需要针对性，看看自己属于哪一类的，目前到达了什么程度，口语考试必须“说人话”。其实好多学生在考试的时候忘记了一个很重要的原则：口语只是为了munication。“Communication”，沟通表达的能力显得格外重要。 我之前考过两次雅思，口语每次都是4.5分。到底该如何准备口语，
自己一点想法也没有。之前上过某培训机构的面授课，有讲阅读听力写作的复习方法，可是感觉口语真的是一点用也没有，老师上课就问几个问题，点几个同学讲一下观点就完了，口语考试范围也没给，更不用说给素材。之前两次考试，Part1就只是简单回答一两句就没话说了，Part2更是支支吾吾说几句后等考官打断，Part3很没有逻辑，一通乱说，语法错得自己都边讲边羞愧。对于我来说，最大的问题首先就是考口语时没话说，其次就是语法问题。 很偶然的一次机会在微博上知道了Jolee老师，然后加了老师的微信，看她的朋友圈经常会分享前雅思考官的建议，还有一些口语素材和她学生的成功提分案例，所以非常相信老师。我报的是Jolee老师的雅思口语小班，从开课那天起，老师就跟我们强调评分标准分很重要，要从这四个方面去提高口语。上课的时候老师会把Part1和Part2的话题都串一遍，然后再让我们练习，纠正单词发音和语法问题，我都不知道学英语十多年了，连很简单的单词red的发音都是错的。另外课堂上会讲很多地道的表达，每节课一些核心词汇都会不停
雅思口语备考四大评分标准及得分要素 本文为大家带来雅思口语备考四大评分标准及得分要素，希望大家能够关注。雅思口语考官是严格按照评分标准来打分的，所以大家一定要对评分标准和得分要素进行详细的了解。 标准一：语言的流畅度与连贯性Fluency &Coherence 考官会听什么： (1)语言是否流畅。考生能否连续地说，还是需要放慢语速才能继续说下去?是否需要重复说过的话才能保证不停顿?能否自然地使用话语标记? (2)语言是否连贯。考生说的话是否言之有理?是否回答了问题?能够一直紧密围绕着话题讲吗? 得分要素：我们在书中编排了一些可改善语言流畅度和连贯性的练习。由于语言是一个有机的整体，丰富的词汇能使你的语言变得更通顺——你脑海中的词汇越多，你就越能自然流畅地表达自己的观点，也就能避免发言时卡壳和放慢语速，而这些正是将考生得分局限在5 分以下的问题。 标准二：语法知识的广度及准确度Grammatical Range &Accuracy 考官会听什么： (1)语法广度。考生是否能运用简单句和复合句?是否会使用各种时态? (2) 语法准确度。考生用的语法准确吗?是否用对了形容词比较级和名词复数形式?是否能意识到主谓一致?动词的形式是否正确?代词和介词的使用是否正确? 得分要素：考生需要向考官展示出自己能正确运用语法的能力，比如能正确使用介词和动词时态。但只是做到准确还不够，考生还要掌握各种语法结构。考官会看考生有没有正确使用话语标记?是否能使用复合句?介词用对了吗?会使用比较级吗?能运用几种动词时态?会使用条件句吗? 标准三：词汇Vocabulary 考官会听什么： 考生的词汇量有多大?所用词汇是否多数属于基本词汇，只是偶尔夹杂一两个不常用的词汇?词汇的使用是否正确?是否能够始终用丰富的词汇表达各种意思?
小站搬运 雅思口语基本话题答题实例讲解 4 magical steps to make a great long answer: 1. Answer the question directly 2. Explain why 3. Give an example 4. Explain the alternative / opposite By IELTS ex-examiner Simon
1.人物类话题 在雅思口语考试的part 2部分，人物类卡片题是必考的话题之一。首先，看看5-8月题库到底有哪些人物类话题： 尊敬的老人Describe an old person you respect. 道歉之人Describe a person who has apologized to you. 亲密朋友Describe one of your best friends. 敬仰的领导Describe a leader you admire 共度时光的人Describe a person that you like to spend time with 有创意的人Describe a creative person that you admire 一起住的人Describe a person who once moved to live with you 近水之地Describe a place near water (such as a river, a lake or the ocean) that you enjoyed visiting. 其实只要掌握了对付人物类话题的答题思路，遇到这类问题的时候应付起来其实不难！下面来看一道卡片题的范例。 Describe a family member you would like to work with. You should say, Who the person is; What kind of person he or she is; Whether you’ve worked together before; And explain why you would like to work with this person. 对于酱紫描述人的话题，烤鸭们可以往这个人物的品质和能力靠拢。一个人为人处世中的personalities或是abilities往往决定了这个人身上会发生的故事。因此，烤鸭在回答这类话题的时候，可以着重突出一下人物性格或者能力，然后谈论在这个人物身上发生的事情。 在描述或评价一个人物的时候，可以采取三个步骤，A. 直接说明此人的性格特点或能力，B. 补充解释，C. 举例说明。大家且看答题范例： ①I would very much like to work with my cousin, who is the son of my aunt on my father’s side. He’s two years older than me, and we basically grew up together. In most people’s eyes, he is a good student who can sail through all kinds of examinations. But for me, he is not merely a diligent student, but an intelligent and versatile young man. ②Even though we haven’t worked together on any business projects, I know for sure heis someone I can work with. 在这个范例的回答中，仅仅用了一段，便回答了题目中的“who this person is”、“what kind of person he or she is”以及“whether you’ve worked together before”这三个问题。不过cover 到了这3个小问还不够，这个回答中运用了哪些技巧呢？ 1. 在回答简单含义的句子的时候，可以尝试使用定语从句。如文中的句纸“I would very much like to work with my cousin, who is the son of my aunt on my father’s side”，就是将两个简单的意思连到了一句话中。 2. 可以使用让步状语从句强调自己要表达的意思。如范例中的Even though we haven’t worked together on any business projects, I know for sure he is someone I can work with. 虽然还木有共事，但是还是强调了自己想要和cousin共事的观点。