首页
登录
职称英语
Eight times within the past million years, something in the Earth’s climatic
Eight times within the past million years, something in the Earth’s climatic
游客
2024-12-14
29
管理
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. [br] Which of the following best expresses the main idea of the passage?
选项
A、Marine sediments have allowed scientists to amass evidence tending to confirm that astronomical cycles drive the Earth’s glacial cycles.
B、The ratio between two different isotopes of oxygen in seawater correlates closely with the size of the Earth’s ice sheets.
C、Surprisingly, single-cell marine organisms provide a record of the Earth’s ice ages.
D、The Earth’s astronomical cycles have recently been revealed to have an unexpectedly large impact on the Earth’s climate.
答案
A
解析
转载请注明原文地址:https://tihaiku.com/zcyy/3874043.html
相关试题推荐
InEnglish,"vis"bearsthemeaningof"tolookatsomething"andcanbecombine
2050andImmortalityIsWithinOurGraspAeroplaneswillbe
2050andImmortalityIsWithinOurGraspAeroplaneswillbe
2050andImmortalityIsWithinOurGraspAeroplaneswillbe
2050andImmortalityIsWithinOurGraspAeroplaneswillbe
WithinEUprimaryeducation,aclearmajorityofpupilschoosetostudyEngl
WithinEUprimaryeducation,aclearmajorityofpupilschoosetostudyEngl
WithinEUprimaryeducation,aclearmajorityofpupilschoosetostudyEngl
Atsometimeinyourlifeyoumayhaveastrongdesiretodosomethingstra
Atsometimeinyourlifeyoumayhaveastrongdesiretodosomethingstra
随机试题
Accordingtothenewsitem,whichofthefollowingwillHLNfocusoninpolitica
Georgehadstolensomemoney,butthepolicehadcaughthimandhehadbeenp
鸦片战争后,提出“师夷长技以制夷”思想的是A.林则徐 B.王韬 C.龚自珍
在以下指标中,()与土的工程性质的关系更密切,规律性更强。A.液限 B.塑限
下列命题不正确的是()。A.有理数对于乘法运算封闭 B.有理数可以比较大小
入党誓词的内容是什么?
治疗胃痛寒凝气滞证,宜选用的方剂是A.良附丸 B.丹栀逍遥散 C.柴胡疏肝散
有关均衡价格的说法,不正确的是( )。 A.由于供给和需求力量的相互作用,市
某矿施工网络图见图2G320030—1,其中B工作的机动时间为()d。
最新回复
(
0
)