According to the passage. the large ice sheets typical of glacial cycles are most directly caused by答案解析-雷哥GRE

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This passage is based on an article published in 1990. Eight times within the pat million years, some- thing in the Earth`s climatic equation has changed. allowing snow in the mountains and the northern Line 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 pre- cession and tilt frequencies of the Earth`s spin axis.

According to the passage. the large ice sheets typical of glacial cycles are most directly caused by

changes in the average temperatures in the tropics and over open oceans prolonged increases in the rate at which water evaporates from the oceans extreme seasonal variations in temperature in northern latitudes and in mountainous areas steadily increasing precipitation rates in northern latitudes and in mountainous areas the continual failure of snow to melt completely during the warmer seasons in northern latitudes and in mountainous areas

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309969cs

这套模考出得实在烂，数学太简单，填空只有单选，双选，六选二，阅读几乎都是长阅读，还没有逻辑题

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哈士奇和鸡阿姨

我靠，小妖精让我一顿好找，gre能不能稍微体谅一下我们这些秃头的，题目稍微按顺序来一下不行吗，尼玛这么跳跃，上一道题是倒数第三段，我心想下一题怎么的也是最后两段里了，找到死都找不出来，结果在第一段。。我能说脏话吗 some- thing in the Earth`s climatic equation has changed. allowing snow in the mountains and the northern Line 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.

1楼 | 2020-08-19 17:27:54 回复

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