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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.
[填空 ]冲分救命题 -9341
One of the virtues of the essay collection is that it presents_____ views side by side, thereby highlighting the contentious nature of many of the arguments within the discipline.
[阅读&逻辑 ]冲分救命题 -9340
The highlighted sentence serves primarily to
[阅读&逻辑 ]冲分救命题 -9339
The author would most likely agree with which of the following assertions about the environmental impact of hydroelectric power plants?
[阅读&逻辑 ]冲分救命题 -9338
The passage suggests that which of the following contributed to the erosion after the First World War of the progress previously realized by the labor movement in the United States?
[阅读&逻辑 ]冲分救命题 -9337
According to the passage, which of the following statements about the National War Labor Board is true?
[阅读&逻辑 ]冲分救命题 -9336
It can be inferred from the passage that which of the following was one of Local 36’s grievances against the Little Rock laundry owners?
[阅读&逻辑 ]冲分救命题 -9335
The passage is primarily concerned with discussing
[阅读&逻辑 ]冲分救命题 -9334
According to the passage, Appalachian square dancing differs from English country dancing in that Appalachian square dancing
[阅读&逻辑 ]冲分救命题 -9333
The passage suggests that which of the following was true of
[阅读&逻辑 ]冲分救命题 -9332
The statements given are structured to lead to which of the following conclusions?
[阅读&逻辑 ]冲分救命题 -9331
Consider each of the choices separately and select all that apply. It can be inferred that historical demographers generally believe which of the following about communities of fewer than 10,000 people in early-modern Germany?
[阅读&逻辑 ]冲分救命题 -9330
The author suggests that according to historical demographers, the birth rate in early-modem German cities
[阅读&逻辑 ]冲分救命题 -9329
Which of the following most logically completes the argument?
[阅读&逻辑 ]冲分救命题 -9328
The highlighted portion of the passage serves primarily to
[阅读&逻辑 ]冲分救命题 -9327
According to the passage, certain African literatures have
[阅读&逻辑 ]冲分救命题 -9326
It can be inferred from the passage that the Tea Cess Committee reorganized as the Indian Tea Market Expansion Board in order to
[阅读&逻辑 ]冲分救命题 -9325
The passage suggests that "small packets of leaf fragments" were
[阅读&逻辑 ]冲分救命题 -9324
In the argument in the editorial, the part that is highlighted plays which of the following roles?
[阅读&逻辑 ]冲分救命题 -9323
Consider each of the choices separately and select all that apply.Which of the following can be inferred from the passage about the"period of relative warmth"
[阅读&逻辑 ]冲分救命题 -9322
Which of the following best describes the organization of the passage?