Climate Science

Catching a Cold

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Science  04 Apr 2008:
Vol. 320, Issue 5872, pp. 21
DOI: 10.1126/science.320.5872.21b

Earth has wandered in and out of periods of extensive glaciation for hundreds of millions of years. The oxygen isotopic record of seawater indicates that the current Antarctic ice sheet began to form about 34 million years ago, across the Eocene-Oligocene transition. In contrast, extensive permanent ice sheets are thought not to have appeared in the Northern Hemisphere for another 25 million years. However, oxygen isotope ratios are affected by temperature as well as the isotopic composition of the water itself (which in turn is controlled mostly by the amount of ice that exists in the environment), so the cooling inferred from that record was not unequivocally established. Earlier work to construct a pure temperature record by measuring Mg/Ca ratios in foraminifera actually seemed to show that there was no appreciable ocean cooling 34 million years ago, implying that the drop in temperature believed to have occurred then was instead due to an earlier appearance of ice in the Northern Hemisphere. Now, Lear et al. report results from Mg/Ca measurements of exceptionally well-preserved samples that show a 2.5°C tropical sea surface temperature drop across the Eocene- Oligocene boundary. This finding at last provides direct evidence of the cooling that had previously been so difficult to demonstrate. — HJS

Geology 36, 251 (2008).

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