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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

admin2012-10-23  35
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.
What can we safely infer from the text?
选项 A、Many a phenomenon might be caused by astronomical factors.
B、Any hypothesis should be abandoned unless supported by solid records.
C、Glaciers are the records keeping Earth’s astronomical cycles.
D、Oxygen isotopes are evenly distributed throughout the earth surface.
答案A
解析 题干问：“我们可以从文章得到的推论是……”。正确选项为A“许多现象也许都是由天文因素所造成的”。这是贯穿整篇文章的中心和要点。而选项B“任何假设除非有确凿的证据来支持，否则我们都应该抛弃”，选项C“冰川是地球天文周期的记录”和选项D“氧的同位素在地球表面是相当平均的”，其中有些信息是与原文的信息相反，有些信息不能从原文中得到推论。所谓推理即从个别走到一般的现象，这篇文章除了地球的冰川年代以外，还可有许多现象可能是由天文因素所造成的。
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