gre考试阅读真题解析(A)_GRE阅读

gre考试阅读真题解析(A)

　　Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the Earths 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 cyclesmall decreases or surges in ice volumehave come at intervals of roughly 23,000 and 41,000 years, in keeping with the precession and tilt frequencies of the Earths spin axis.

　　17. Which of the following best expresses the main idea of the passage?

　　 Marine sediments have allowed scientists to amass evidence tending to confirm that astronomical cycles drive the Earths glacial cycles.

　　 The ratio between two different isotopes of oxygen in seawater correlates closely with the size of the Earths ice sheets.

　　 Surprisingly, single-cell marine organisms provide a record of the Earths ice ages.

　　 The Earths astronomical cycles have recently been revealed to have an unexpectedly large impact on the Earths climate.

　　 The earth has experienced eight periods of intense glaciation in the past million years, primarily as a result of substantial changes in its orbit.

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