Geology

The Isthmus of Panama and the Ice Ages

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Science  07 Jan 2000:
Vol. 287, Issue 5450, pp. 13
DOI: 10.1126/science.287.5450.13b

The formation of the Isthmus of Panama about 3 million years ago (Ma), a fairly small event in terms of plate tectonics, had dramatic effects on evolution, ocean circulation, and Earth's climate. Previously isolated North and South American land faunas mixed, and the separation of Atlantic and Pacific waters imposed changes in ocean circulation. One hypothesis is that these changes in ocean circulation triggered the ice ages; the onset of severe Northern Hemisphere glaciation was about 2.5 Ma. Long-term changes in ocean circulation can be monitored using neodynium (Nd) and lead (Pb) isotopes, which reflect the regional geology of exposed continental rocks bordering ocean basins. Thus, different waters will have different isotopic compositions, so their mixing and circulation can be resolved.

Frank et al. and Reynolds et al. analyzed Nd and Pb isotopes from several ferromanganese crusts in the Atlantic and Pacific oceans and showed that the amount of water being exchanged through the Panama gateway waned noticeably before about 5 Ma, as the Isthmus began to form. This implies that the major effects on ocean circulation occurred considerably before glaciation increased. These records, along with new records of Nd isotopes from foraminifera preserved in sediments from near the Labrador Sea, reported by Vance and Burton, also show that formation of the ice sheets in North America and Eurasia increased erosion dramatically during the past 2 million years.—BH

Geology27, 1147 (1999); Earth Planet. Sci. Lett.173, 381 (1999); Earth Planet. Sci. Lett.173, 365 (1999).

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