Means of Production

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Science  11 Jun 2004:
Vol. 304, Issue 5677, pp. 1567
DOI: 10.1126/science.304.5677.1567a

Millennial-scale changes in climate during the last glacial period, discovered in ice cores from Greenland, are also clearly recorded by laminated marine sediments from the Santa Barbara Basin. The variations seen in the sediments were thought to have been caused by changes in deep ocean convection originating in the North Pacific. Ortiz et al., using a 52,000-year-long sedimentary core from a location 1500 km to the south, off the coast of Baja California, Mexico, see the same millennial variations but ascribe them to an entirely different cause: surface ocean circulation changes driven by processes rooted in the tropics. Increases in the abundance of benthic forams as great as 100-fold occurred during warm periods, and the authors suggest that they resulted from more intense marine productivity and a concomitant increase in the delivery of organic carbon to the floor of the ocean. Greater productivity would have required more nutrients to upwell from deeper waters, which could have been caused by a shift in the mean state of the ocean toward more El Niño-like conditions, with a regionally deepened nutricline in place during cool climate periods and a shallower one during warm intervals. — HJS

Geology 32, 521 (2004).

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