Abstract

Studies of Recent populations of the planktonic foraminifer, Globigerina pachyderma (Ehrenberg), show that more than 98% of Arctic and Antarctic specimens coil in a counter-clockwise direction (sinistral), whereas populations in temperate and tropical regions are mostly 98% clockwise (dextral) in manner of coiling. Basin cores off southern California reveal that modern dextral forms have persisted for about 11,000 years and were preceded by sinistral populations in the late Pleistocene, as determined by radiocarbon dating. Computations of sedimentation rates indicate that Pleistocene sediments were deposited from 25 to 500% faster than those in the Recent. Investigation of populations in Pliocene and Pleistocene strata of southern California show consistent sinistrality in the Pleistocene and in the middle part of the Pliocene. Assemblages from the upper and lower Pliocene are dominantly dextral. Thus, an additional stratigraphic tool is provided in improving the accuracy of late Cenozoic correlations of southern California.

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