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  3. 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

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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 factor: 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 first 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 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, oxygenisotope measurements have been made on hundreds of cores. The combined record enables scientists to show that the re-cord contains the very periodicities as the orbital processes. Over the past 800, 000 years, the global ice volume 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.
Single-cell marine organisms referred to as "foraminifera" as mentioned in the text might serve as ______
选项 A、a proof against the existence of oxygen-16.
B、a testimony to sediment formation processes.
C、a valid record justifying glacial periodic cycles.
D、an indicator of the ratio of the two oxygens.
答案C
解析 题干问：“在文中提到被称为‘有孔虫’的单细胞海洋有机物的目的是……”。正确选项为C“为解释冰川的周期性提供有力的证据”。选项A“证明了氧16不存在”，选项B“证明海底沉淀物的形成过程”和选项D“显示了两种氧的同位素的比例”都不是文章举这个例子的主要目的。
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