gre考试阅读真题解析(A)
<p> <span word="This">This</span> <span word="passage">passage</span> <span word="is">is</span> <span word="based">based</span> <span word="on">on</span> <span word="an">an</span> <span word="article">article</span> <span word="published">published</span> <span word="in">in</span> 1990.</p><p> <span word="Eight">Eight</span> <span word="times">times</span> <span word="within">within</span> <span word="the">the</span> <span word="past">past</span> <span word="million">million</span> <span word="years">years</span>, <span word="something">something</span> <span word="in">in</span> <span word="the">the</span> <span word="Earths">Earths</span> <span word="climatic">climatic</span> <span word="equation">equation</span> <span word="has">has</span> <span word="changed">changed</span>, <span word="allowing">allowing</span> <span word="snow">snow</span> <span word="in">in</span> <span word="the">the</span> <span word="mountains">mountains</span> <span word="and">and</span> <span word="the">the</span> <span word="northern">northern</span> <span word="latitudes">latitudes</span> <span word="to">to</span> <span word="accumulate">accumulate</span> <span word="from">from</span> <span word="one">one</span> <span word="season">season</span> <span word="to">to</span> <span word="the">the</span> <span word="next">next</span> <span word="instead">instead</span> <span word="of">of</span> <span word="melting">melting</span> <span word="away">away</span>. <span word="Each">Each</span> <span word="time">time</span>, <span word="the">the</span> <span word="enormous">enormous</span> <span word="ice">ice</span> <span word="sheets">sheets</span> <span word="resulting">resulting</span> <span word="from">from</span> <span word="this">this</span> <span word="continual">continual</span> <span word="buildup">buildup</span> <span word="lasted">lasted</span> <span word="tens">tens</span> <span word="of">of</span> <span word="thousands">thousands</span> <span word="of">of</span> <span word="years">years</span> <span word="until">until</span> <span word="the">the</span> <span word="end">end</span> <span word="of">of</span> <span word="each">each</span> <span word="particular">particular</span> <span word="glacial">glacial</span> <span word="cycle">cycle</span> <span word="brought">brought</span> <span word="a">a</span> <span word="warmer">warmer</span> <span word="climate">climate</span>. <span word="Scientists">Scientists</span> <span word="speculated">speculated</span> <span word="that">that</span> <span word="these">these</span> <span word="glacial">glacial</span> <span word="cycles">cycles</span> <span word="were">were</span> <span word="ultimately">ultimately</span> <span word="driven">driven</span> <span word="by">by</span> <span word="astronomical">astronomical</span> <span word="factors">factors</span>: <span word="slow">slow</span>, <span word="cyclic">cyclic</span> <span word="changes">changes</span> <span word="in">in</span> <span word="the">the</span> <span word="eccentricity">eccentricity</span> <span word="of">of</span> <span word="the">the</span> <span word="Earths">Earths</span> <span word="orbit">orbit</span> <span word="and">and</span> <span word="in">in</span> <span word="the">the</span> <span word="tilt">tilt</span> <span word="and">and</span> <span word="orientation">orientation</span> <span word="of">of</span> <span word="its">its</span> <span word="spin">spin</span> <span word="axis">axis</span>. <span word="But">But</span> <span word="up">up</span> <span word="until">until</span> <span word="around">around</span> 30 <span word="years">years</span> <span word="ago">ago</span>, <span word="the">the</span> <span word="lack">lack</span> <span word="of">of</span> <span word="an">an</span> <span word="independent">independent</span> <span word="record">record</span> <span word="of">of</span> <span word="ice">ice</span>-<span word="age">age</span> <span word="timing">timing</span> <span word="made">made</span> <span word="the">the</span> <span word="hypothesis">hypothesis</span> <span word="untestable">untestable</span>.</p><p> <span word="Then">Then</span> <span word="in">in</span> <span word="the">the</span> <span word="early">early</span> 1950<span word="s">s</span> <span word="Emiliani">Emiliani</span> <span word="produced">produced</span> <span word="the">the</span> <span word="first">first</span> <span word="complete">complete</span> <span word="record">record</span> <span word="of">of</span> <span word="the">the</span> <span word="waxings">waxings</span> <span word="and">and</span> <span word="wanings">wanings</span> <span word="of">of</span> <span word="past">past</span> <span word="glaciations">glaciations</span>. <span word="It">It</span> <span word="came">came</span> <span word="from">from</span> <span word="a">a</span> <span word="seemingly">seemingly</span> <span word="odd">odd</span> <span word="place">place</span>, <span word="the">the</span> <span word="seafloor">seafloor</span>. <span word="Single">Single</span>-<span word="cell">cell</span> <span word="marine">marine</span> <span word="organisms">organisms</span> <span word="called">called</span> <span word="foraminifera">foraminifera</span> <span word="house">house</span> <span word="themselves">themselves</span> <span word="in">in</span> <span word="shells">shells</span> <span word="made">made</span> <span word="from">from</span> <span word="calcium">calcium</span> <span word="carbonate">carbonate</span> . <span word="When">When</span> <span word="the">the</span> <span word="foraminifera">foraminifera</span> <span word="die">die</span>, <span word="sink">sink</span> <span word="to">to</span> <span word="the">the</span> <span word="bottom">bottom</span>, <span word="and">and</span> <span word="become">become</span> <span word="part">part</span> <span word="of">of</span> <span word="seafloor">seafloor</span> <span word="sediments">sediments</span>, <span word="the">the</span> <span word="carbonate">carbonate</span> <span word="of">of</span> <span word="their">their</span> <span word="shells">shells</span> <span word="preserves">preserves</span> <span word="certain">certain</span> <span word="characteristics">characteristics</span> <span word="of">of</span> <span word="the">the</span> <span word="seawater">seawater</span> <span word="they">they</span> <span word="inhabited">inhabited</span>. <span word="In">In</span> <span word="particular">particular</span>, <span word="the">the</span> <span word="ratio">ratio</span> <span word="of">of</span> <span word="a">a</span> <span word="heavy">heavy</span> <span word="isotope">isotope</span> <span word="of">of</span> <span word="oxygen">oxygen</span> <span word="to">to</span> <span word="ordinary">ordinary</span> <span word="oxygen">oxygen</span> <span word="in">in</span> <span word="the">the</span> <span word="carbonate">carbonate</span> <span word="preserves">preserves</span> <span word="the">the</span> <span word="ratio">ratio</span> <span word="of">of</span> <span word="the">the</span> <span word="two">two</span> <span word="oxygens">oxygens</span> <span word="in">in</span> <span word="water">water</span> <span word="molecules">molecules</span>.</p><p> <span word="It">It</span> <span word="is">is</span> <span word="now">now</span> <span word="understood">understood</span> <span word="that">that</span> <span word="the">the</span> <span word="ratio">ratio</span> <span word="of">of</span> <span word="oxygen">oxygen</span> <span word="isotopes">isotopes</span> <span word="in">in</span> <span word="seawater">seawater</span> <span word="closely">closely</span> <span word="reflects">reflects</span> <span word="the">the</span> <span word="proportion">proportion</span> <span word="of">of</span> <span word="the">the</span> <span word="worlds">worlds</span> <span word="water">water</span> <span word="locked">locked</span> <span word="up">up</span> <span word="in">in</span> <span word="glaciers">glaciers</span> <span word="and">and</span> <span word="ice">ice</span> <span word="sheets">sheets</span>. <span word="A">A</span> <span word="kind">kind</span> <span word="of">of</span> <span word="meteorological">meteorological</span> <span word="distillation">distillation</span> <span word="accounts">accounts</span> <span word="for">for</span> <span word="the">the</span> <span word="link">link</span>. <span word="Water">Water</span> <span word="molecules">molecules</span> <span word="containing">containing</span> <span word="the">the</span> <span word="heavier">heavier</span> <span word="isotope">isotope</span> <span word="tend">tend</span> <span word="to">to</span> <span word="condense">condense</span> <span word="and">and</span> <span word="fall">fall</span> <span word="as">as</span> <span word="precipitation">precipitation</span> <span word="slightly">slightly</span> <span word="sooner">sooner</span> <span word="than">than</span> <span word="molecules">molecules</span> <span word="containing">containing</span> <span word="the">the</span> <span word="lighter">lighter</span> <span word="isotope">isotope</span>. <span word="Hence">Hence</span>, <span word="as">as</span> <span word="water">water</span> <span word="vapor">vapor</span> <span word="evaporated">evaporated</span> <span word="from">from</span> <span word="warm">warm</span> <span word="oceans">oceans</span> <span word="moves">moves</span> <span word="away">away</span> <span word="from">from</span> <span word="its">its</span> <span word="source">source</span>, <span word="its">its</span> <span word="oxygen">oxygen</span>-18 <span word="returns">returns</span> <span word="more">more</span> <span word="quickly">quickly</span> <span word="to">to</span> <span word="the">the</span> <span word="oceans">oceans</span> <span word="than">than</span> <span word="does">does</span> <span word="its">its</span> <span word="oxygen">oxygen</span>-16. <span word="What">What</span> <span word="falls">falls</span> <span word="as">as</span> <span word="snow">snow</span> <span word="on">on</span> <span word="distant">distant</span> <span word="ice">ice</span> <span word="sheets">sheets</span> <span word="and">and</span> <span word="mountain">mountain</span> <span word="glaciers">glaciers</span> <span word="is">is</span> <span word="relatively">relatively</span> <span word="depleted">depleted</span> <span word="of">of</span> <span word="oxygen">oxygen</span>-18. <span word="As">As</span> <span word="the">the</span> <span word="oxygen">oxygen</span>-18-<span word="poor">poor</span> <span word="ice">ice</span> <span word="builds">builds</span> <span word="up">up</span>, <span word="the">the</span> <span word="oceans">oceans</span> <span word="become">become</span> <span word="relatively">relatively</span> <span word="enriched">enriched</span> <span word="in">in</span> <span word="the">the</span> <span word="isotope">isotope</span>. <span word="The">The</span> <span word="larger">larger</span> <span word="the">the</span> <span word="ice">ice</span> <span word="sheets">sheets</span> <span word="grow">grow</span>, <span word="the">the</span> <span word="higher">higher</span> <span word="the">the</span> <span word="proportion">proportion</span> <span word="of">of</span> <span word="oxygen">oxygen</span>-18 <span word="becomes">becomes</span> <span word="in">in</span> <span word="seawaterand">seawaterand</span> <span word="hence">hence</span> <span word="in">in</span> <span word="the">the</span> <span word="sediments">sediments</span>.</p>
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