Matching Milankovitch

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Science  01 Mar 2002:
Vol. 295, Issue 5560, pp. 1603
DOI: 10.1126/science.295.5560.1603d

It is generally agreed that changes in the amount of incident solar energy, arising from variations of Earth's orbit around the Sun, are the ultimate cause of our glacial cycles, but how well the timing of these cycles agrees with the pattern of insolation remains controversial. For example, although the start of the last deglaciation coincides with the increase in insolation at that time, the penultimate deglaciation appears to have begun thousands of years earlier than would have been expected on the basis of orbital forcing. In order to understand better the timing of glacial cycles, more glacial terminations need to be examined.

Using the same precise U-Th dating method but on entirely different samples, Bard et al. and Robinson et al. probe the timing and structure of marine isotope stage (MIS) 7, which is the interglacial period that lasted from approximately 240 to 190 thousand years ago (ka). They find substantial agreement with predictions based on orbital forcing patterns. Bard et al. present the growth record of an Italian stalagmite and show that the MIS 7.1 high stand of sea level occurred between 202 and 190 ka and was between 9 and 18 meters below present sea level, in good agreement with standard astronomical theory. Robinson et al. measure the oxygen isotope composition of aragonite sediments from the Bahamas to investigate sea level changes over the duration of MIS 7, finding that the deglaciation at the beginning of MIS 7 occurred when it should have (and not before), based on astronomical theory. These records suggest that MIS 7 was more like MIS 1, the current interglacial period, than the preceding warm interval MIS 5, in the sense that its chronology matches the pattern of orbital forcing of that time. — HJS

Earth Planet. Sci. Lett., in press.

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