Ifthesalinity(含盐量)ofoceanwatersisanalyzed,itisfoundtovaryonlyslightlyfromplacetoplace.Nevertheless,someofthesesmallchangesareimportant.Therearethreebasicprocessesthatcauseachangeinoceanicsalinity.Oneoftheseisthesubtractionofwaterfromtheoceanbymeansofevaporation--conversionofliquidwatertowatervapor.Inthismanner,thesalinityisincreased,sincethesaltsstaybehind.Ifthisiscarriedtotheextreme,ofcourse,whitecrystalsofsaltwouldbeleftbehind.
Theoppositeofevaporationisprecipitation(降水),suchasrain,bywhichwaterisaddedtotheocean.Heretheoceanisbeingdiluted(稀释)sothatthesalinityisdecreased.Thismayoccurinareasofhighrainfallorincoastalregionswhereriversflowintotheocean.Thussalinitymaybeincreasedbythesubtractionofwaterbyevaporation,ordecreasedbytheadditionoffreshwaterbyprecipitationorrunoff(形成地表水部分的降水).
Normally,intropicalregionswherethesunisverystrong,theoceansalinityissomewhathigherthanitisinotherpartsoftheworldwherethereisnotasmuchevaporation.Similarly,incoastalregionswhereriversdilutethesea,salinityissomewhatlowerthaninotheroceanicareas.
Athirdprocessbywhichsalinitymaybealteredisassociatedwiththeformationandmeltingofseaice.Whenseawaterisfrozen,thedissolvedmaterialsareleftbehind.Inthismanner,seawaterdirectlybeneathfreshlyformedseaicehasahighersalinitythanitdidbeforetheiceappeared.Ofcourse,whenthisicemelts,itwilltendtodecreasethesalinityofthesurroundingwater.
IntheWeddellSea,offAntarctica,thedensestwaterintheoceansisformedasaresultofthisfreezingprocess,whichincreasesthesalinityofcoldwater.Thisheavywatersinksandisfoundinthedeeperportionsoftheoceansoftheworld.
练习题：
Choosecorrectanswerstothequestion:
1.Thebesttitleofthepassagecanbe“_____”.
A.TypicalOceansandTheirRespectiveFeatures
B.TheCausesoftheChangesinSalinityofOceanWater
C.DifferentOceansHaveDifferentSalinity
D.ThePrecipitationandEvaporationofOceans
2.Whichofthefollowingprocesseswillincreasesalinityofoceanwaters
A.Evaporation.
B.Precipitation.
C.Melting.
D.Dilution.
3.Accordingtothispassage,thesea_islikelytohavethelowestsalinity.
A.intropicalareas
B.offAntarctica
C.ofhighrainfall
D.withabundant
4.TheWeddellSea_____.
A.isagoodexampleofincreasedsalinityinfreezingseawater
B.ismuchlargerinareathantheArcticoceans
C.hasamuchlowersalinitynowthanever
D.hasthedenserwaterinitsupperparts
5.Coastalregionsarementionedascaseswhere_____.
A.seawaterislesssaltybecausefreshwaterjoinsin
B.riverscarryindustrialexhaustintosea
C.seaicetendstomeltmorequicklythaninthecenterofoceans
D.heavywatersinkstothedeeperportionsoftheoceans
参考答案及解析
1.[B]主旨大意题。本文第1段第3句是全文的主题句，告诉读者有三种原因致使海洋盐分变化并逐一介绍这三种原因，选项B比较全面地归纳了本文的主题。选项A和C中心词为oceans,不妥，选项D只提及了前面两种原因，不全面。
2.[A]亊实细节题。只要明白第1段第5句中的inthismanner是指第4句中的evaporation,不难做出正确选择。选项B、C会使含盐量减少,很容易排除。具体的反证分别出现在第2段首句、第3段尾句和第4段首句。
3.[C]推理判断题。综合3种情况，造成海洋含盐量低的原因有：降雨多、日晒少、远离热带和海冰融化，由此可推断选项C最符合。A和D是使海水含盐量增加的，因此可排除。B是否一定海水含盐低，文中并未提到。