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MilankovitchproposedintheearlytwentiethcenturythattheiceageswerecausedbyvariationsintheEarth’sorbitaroundtheSun.Forsometimethistheorywasconsidereduntestable,largelybecausetherewasnosufficientlyprecisechronologyoftheiceageswithwhichtheorbitalvariationscouldbematched.

ToestablishsuchachronologyitisnecessarytodeterminetherelativeamountsoflandicethatexistedatvarioustimesintheEarth’spast.Arecentdiscoverymakessuchadeterminationpossible:relativeland-icevolumeforagivenperiodcanbededucedfromtheratiooftwooxygenisotopes,16and18,foundinoceansediments.Almostalltheoxygeninwaterisoxygen16,butafewmoleculesoutofeverythousandincorporatetheheavierisotope18.Whenaniceagebegins,thecontinentalicesheetsgrow,steadilyreducingtheamountofwaterevaporatedfromtheoceanthatwilleventuallyreturntoit.Becauseheavierisotopestendtobeleftbehindwhenwaterevaporatesfromtheoceansurfaces,theremainingoceanwaterbecomesprogressivelyenrichedinoxygen18.Thedegreeofenrichmentcanbedeterminedbyanalyzingoceansedimentsoftheperiod,becausethesesedimentsarecomposedofcalciumcarbonate(calciumcarbonate:n.[化]碳酸钙)shellsofmarineorganisms,shellsthatwereconstructedwithoxygenatomsdrawnfromthesurroundingocean.Thehighertheratioofoxygen18tooxygen16inasedimentaryspecimen,themorelandicetherewaswhenthesedimentwaslaiddown.

AsanindicatorofshiftsintheEarth’sclimate,theisotoperecordhastwoadvantages.First,itisaglobalrecord:thereisremarkablylittlevariationinisotoperatiosinsedimentaryspecimenstakenfromdifferentcontinentallocations.Second,itisamorecontinuousrecordthanthattakenfromrocksonland.Becauseoftheseadvantages,sedimentaryevidencecanbedatedwithsufficientaccuracybyradiometricmethodstoestablishaprecisechronologyoftheiceages.Thedatedisotoperecordshowsthatthefluctuationsinglobalicevolumeoverthepastseveralhundredthousandyearshaveapattern:aniceageoccursroughlyonceevery100,000years.ThesedatahaveestablishedastrongconnectionbetweenvariationsintheEarth’sorbitandtheperiodicityoftheiceages.

However,itisimportanttonotethatotherfactors,suchasvolcanicparticulatesorvariationsintheamountofsunlightreceivedbytheEarth,couldpotentiallyhaveaffectedtheclimate.TheadvantageoftheMilankovitchtheoryisthatitistestable:changesintheEarth’sorbitcanbecalculatedanddatedbyapplyingNewton’slawsofgravitytoprogressivelyearlierconfigurationsofthebodiesinthesolarsystem.Yetthelackofinformationaboutotherpossiblefactorsaffectingglobalclimatedoesnotmakethemunimportant.

1.Inthepassage,theauthorisprimarilyinterestedin

(A)suggestinganalternativetoanoutdatedresearchmethod

(B)introducinganewresearchmethodthatcallsanacceptedtheoryintoquestion

(C)emphasizingtheinstabilityofdatagatheredfromtheapplicationofanewscientificmethod

(D)presentingatheoryanddescribinganewmethodtotestthattheory

(E)initiatingadebateaboutawidelyacceptedtheory

2.TheauthorofthepassagewouldbemostlikelytoagreewithwhichofthefollowingstatementsabouttheMilankovitchtheory?

(A)Itistheonlypossibleexplanationfortheiceages.

(B)Itistoolimitedtoprovideaplausibleexplanationfortheiceages,despiterecentresearchfindings.

(C)Itcannotbetestedandconfirmeduntilfurtherresearchonvolcanicactivityisdone.

(D)Itisoneplausibleexplanation,thoughnottheonlyone,fortheiceages.

(E)Itisnotaplausibleexplanationfortheiceages,althoughithasopeneduppromisingpossibilitiesforfutureresearch.

3.Itcanbeinferredfromthepassagethattheisotoperecordtakenfromoceansedimentswouldbelessusefultoresearchersifwhichofthefollowingweretrue?

(A)Itindicatedthatlighterisotopesofoxygenpredominatedatcertaintimes.

(B)Ithadfarmoregapsinitssequencethantherecordtakenfromrocksonland.

(C)Itindicatedthatclimateshiftsdidnotoccurevery100,000years.

(D)Itindicatedthattheratiosofoxygen16andoxygen18inoceanwaterwerenotconsistentwiththosefoundinfreshwater.

(E)Itstretchedbackforonlyamillionyears.

4.Accordingtothepassage,whichofthefollowingistrueoftheratiosofoxygenisotopesinoceansediments?

(A)Theyindicatethatsedimentsfoundduringaniceagecontainmorecalciumcarbonatethansedimentsformedatothertimes.

(B)Theyarelessreliablethantheevidencefromrocksonlandindeterminingthevolumeoflandice.

(C)Theycanbeusedtodeducetherelativevolumeoflandicethatwaspresentwhenthesedimentwaslaiddown.

(D)Theyaremoreunpredictableduringaniceagethaninotherclimaticconditions.

(E)Theycanbeusedtodetermineatmosphericconditionsatvarioustimesinthepast.

5.Itcanbeinferredfromthepassagethatprecipitationformedfromevaporatedoceanwaterhas

(A)thesameisotopicratioasoceanwater

(B)lessoxygen18thandoesoceanwater

(C)lessoxygen18thanhastheicecontainedincontinentalicesheets

(D)adifferentisotopiccompositionthanhasprecipitationformedfromwateronland

(E)moreoxygen16thanhasprecipitationformedfromfreshwater

6.Accordingtothepassage,whichofthefollowingis(are)trueoftheiceages?

I.Thelasticeageoccurredabout25,000yearsago.

II.Iceageshavelastedabout10,000yearsforatleastthelastseveralhundredthousandyears.

III.Iceageshaveoccurredaboutevery100,000yearsforatleastthelastseveralhundredthousandyears.

(A)Ionly

(B)IIonly

(C)IIIonly

(D)Iandonly

(E)I,IIandIII

7.Itcanbeinferredfromthepassagethatcalciumcarbonateshells

(A)arenotassusceptibletodeteriorationasrocks

(B)arelesscommoninsedimentsformedduringaniceage

(C)arefoundonlyinareasthatwereoncecoveredbylandice

(D)containradioactivematerialthatcanbeusedtodetermineasediment’sisotopiccomposition

(E)reflecttheisotopiccompositionofthewateratthetimetheshellswereformed

8.Thepurposeofthelastparagraphofthepassageisto

(A)offeranoteofcaution

(B)introducenewevidence

(C)presenttworecentdiscoveries

(D)summarizematerialintheprecedingparagraphs

(E)offertwo