首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
Eight times within the past million years, something in the Earth’s climatic equation has changed, allowing snow in the mountain
Eight times within the past million years, something in the Earth’s climatic equation has changed, allowing snow in the mountain
admin
2013-11-29
38
问题
Eight times within the past million years, something in the Earth’s climatic equation has changed, allowing snow in the mountains and the northern latitudes to accumulate from one season to the next instead of melting away each time. The enormous ice sheets resulting from this continual buildup lasted tens of thousands of years until the end of each particular glacial cycle brought a warmer climate. Scientists speculated that these glacial cycles were ultimately driven by astronomical factors: slow cyclic changes in the eccentricity of the Earth’s orbit and in the tilt and orientation of its spin axis. But up until around 30 years ago, the lack of an independent record of ice age timing made the hypothesis untestable.
Then in the early 1950s Emiliani produced the first complete record of the waxings and wanings of past glaciations. It came from a seemingly odd place. The seafloor single-cell marine organisms called "foraminifera" house themselves in shells made from calcium carbonate. When the foraminifera die, sink to the bottom, and become part of seafloor sediments, the carbonate of their shells preserves certain characteristics of the seawater they inhabited. In particular, the ratio of a heavy isotope of oxygen (oxygen-18) to ordinary oxygen (oxygen-16) in the carbonate preserves the ratio of the two oxygens in water molecules.
It is now understood that the ratio of oxygen isotopes in seawater closely reflects the proportion of the world’s water locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation slightly sooner than molecules containing the lighter isotope. Hence, as water vapor evaporated from warm oceans moves away from its source, its oxygen-18 returns more quickly to the oceans than does its oxygen-16. What falls as snow on distant ice sheets and mountain glaciers is relatively depleted of oxygen-18. As the oxygen-18-poor ice builds up the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen-I 8 becomes in seawater—and hence in the sediments.
Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the Earth’s astronomical cycles. Since that pioneering observation, oxygen-isotope measurements have been made on hundreds of cores. A chronology for the combined record enables scientists to show that the record contains the very same periodicities as the orbital processes. Over the past 800,000 years, the global ice volume has peaked every 100,000 years, matching the period of the orbital eccentricity variation. In addition, "wrinkles" superposed on each cycle-small-decrease or surge in ice volume have come at intervals of roughly 23,000 and 41, 000 years, in keeping with the precession and tilt frequencies of the Earth’s spin axis.
Which of the following did the scientists do who first constructed a coherent, continuous picture of past variations in marine-sediment isotope ratios according to the passage?
选项
A、Relied primarily on the data obtained from the analysis of Emiliani’s core samples.
B、Combined data derived from the analysis of many different core samples.
C、Matched the data obtained by geologists with that provided by astronomers.
D、Evaluated the isotope-ratio data obtained in several areas in order to eliminate all but the most reliable data.
答案
B
解析
转载请注明原文地址:https://kaotiyun.com/show/7uZO777K
0
专业英语八级
相关试题推荐
______istheoldestofthenationalparksandalsooneofthelargestintheUnitedStates.
Thephenomenonthatwordshavingdifferentmeaningshavethesameformiscalled
AreyourFacebookfriendsmoreinterestingthanthoseyouhaveinreallife?Hashigh-speedInternetmadeyouimpatientwithslo
Ihaveaplanthatwillraisewages,lowerprices,increasethenation’sstockofscientistsandengineers,andmaybeevencreat
RecyclingTheconceptofgreenconsumerismhasgainedmomentumoverthelastdecade,andthepublichasbeeninfluencedand
______ismanifestedbythewords,style,andgrammarusedbyspeakersandwritersinaparticularsituationorinaparticulart
还有一件更显著的事情,要算是牝鸡们的母爱。牝鸡孵化鸡雏的时候,平常是那么驯善的家禽,立刻要变成一些鸷鸟。它们保护自己的幼儿是一刻也不宜懈怠的。两只眼睛好像燃羞的两团烈火。脖子时宣要竖羞向四方倾们也要发出一种怪怪的叫声,飞走啄你。摄取盒物的时候:它们自己也决
Whatarethespeakersmainlydiscussing?
______isastudyofhowspeakersofalanguageusesentencestoeffectsuccessfulcommunication.
随机试题
在唐古拉山口道班的工人常用高压锅来煮面条,是因为那里沸点太高,不易煮开。 ()
Microsoftisnolongertheworld’sbiggestcompany【C1】______marketcapitalization.ThreeotherU.S.companieshaveovertaken【C
男,32岁,水肿,乏力,腹胀2个月,既往有血吸虫接触史。查体:全身水肿,腹壁静脉曲张,腹腔积液征(+)。鉴别肝性腹腔积液与下腔静脉阻塞所致腹水,下列哪项体征最有意义
甲公司为增值税一般纳税人,迨用的增值税税率为17%。20lo年4月5日,甲公司向乙公司销售一批商品,按价目表上标明的价格计算,其不舍增值税的售价总额为200000元。因属批量销售,甲公司同意给予乙公司10%的商业折扣;同时,为鼓励乙公司及早付清货款,甲公司
刘基刘基,字伯温,青田人。基幼颖异。元至顺间举进士,除高安丞,有廉直声。行省辟之,谢去。及太祖下金华,定括苍,闻基名,以币聘,基未应。总制孙炎再致书固邀之,基始出。既至,陈时务十八策,太祖大喜,筑礼贤馆以处基等,宠礼甚至。会陈友谅陷太平,谋东
有一些经济学家认为,加速城市化,以城市化为杠杆,撬动内需增长,不失为一个应对金融危机的积极选择。并提出可以借鉴“破窗理论”:当一扇窗户被打破以后,主人自然要购置新玻璃,这就间接促进了玻璃厂的扩大再生产,从而拉动了经济发展。因此说,城市化是加速经济发展的一项
关系模式规范化过程中,若要求分解保持函数依赖,那么模式分解一定可以达到3NF,但不一定能达到_____。
如果某事务成功完成执行,则该事务称为______事务。
EDI具有3个主要特点:第一,EDI是计算机应用系统之间的通信;第二,数据自动地投递和传输处理,应用程序对它自动响应;而EDI第三个重要特点是______。
InBritain,therearetwooccasionseachyearwhenpeopleusuallyreceivepresents:onChristmasDayandontheirbirthdays.In
最新回复
(
0
)