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冲分救命题 -6647【hard】 00:00:00 关闭计时
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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 1950’s 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-18 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 decreases or surges 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.

The passage suggests that the scientists mentioned in line 8 considered their reconstruction of past astronomical cycles to be

正确答案: B 耗时:
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樱花桑塔纳

通篇看,作者对对一段学者提出的speculation的总体基调是赞同的,唯一问题是没有证据untestable,所以表示反义的AD不能选。而E又说结论都是确定的,但实际没有确定所以E排除。C的问题是,文中是从海洋里找的证据,而非在astronomical 找证据,所以错。B难点在于指代,the latter代指的就是glacier cycle,关于它的record如果找到就完美了。后文就是在围绕这个record是怎么找到的在阐述。
2楼  | 2021-05-01 20:25:53 回复

181139t

心急误选了C,感觉是不仔细看原文最容易造成误解的了,文中讲的是关于ice age的数据不足而不是天文方面的数据
1楼  | 2021-05-01 17:49:27 回复
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