Abstract

In surface sediments of the subantarctic and cooler subtropical areas of the southeastern Indian Ocean, coiling ratios of Neogloboquadrina pachyderma are more dextral in coarser size fractions than in finer size fractions. A sample from near lat 45°S. contained 14 percent (95 percent confidence limits 8 to 22 percent) dextral tests in the 0.124 to 0.175-mm fraction, and 67 percent (57 to 76 percent) in the size fraction >0.175 mm. The difference decreases toward the north where dextral coiling eventually becomes dominant in all size fractions, and toward the south where sinistral coiling becomes dominant in all size fractions. This phenomenon is thought to be due to the existence of at least two races or taxa in the range of forms usually classed as N. pachyderma. To these, the names N. pachyderma pachyderma (Ehrenberg) and N. pachyderma incompta (Cifelli) are applied provisionally.

Whether the taxonomic interpretation is correct or not, the empirically determined differences in coiling ratios of the different size fractions provide a previously unsuspected latitude-dependent variable in planktonic foraminiferal faunas. When applied to the analysis of fossil samples, they enhance the value of coiling ratios as paleotemperature indices and can be used to detect north-south movements of ocean surface hydrologic features.

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