Paleoproductivity in the Equatorial Indian Ocean for the last 260,000 yr: A transfer function based on planktonic foraminifera

Abstract

A Principal Component Analysis (PCA) of planktonic foraminifera in surface sediment of the Northern Equatorial Indian Ocean defines two major assemblages. The first one dominated by Globigerinoides sacculifer, G. ruber, Globigerinella aequilateralis and Globoquadrina conglomerata characterizes non-upwelling areas. The second dominated by Globigerina bulloides, G. falconensis and Globigerinita glutinata characterizes upwelling areas. The distribution of planktonic foraminiferal species in the area is mainly related to Primary Production (PP). The PCA applied to Core MD 900963 samples defines an alternation of the two assemblages through time, a non-upwelling one similar to that present in the core tops and an upwelling assemblage which contains in addition Neogloboquadrina dutertrei, Globorotalia menardii and G. crassaformis. Equatorial Indian Ocean core-top samples are introduced as additional rows in the PCA calculated on Core MD 900963 samples and a transfer function is constructed correlating factorial coordinates of core tops within the range of changes occurring in Core MD 900963 with estimated PP values. Applied to the last 260,000 yr, the transfer function indicates that the precession is the major orbital forcing on equatorial productivity changes and that annual PP increases by about 85% from interglacial to glacial interstadials. These results are in agreement with an independent productivity index based on coccoliths. Direct impact of insolation on equatorial productivity could be explained by the link between Southern Oscillation and intensity of zonal winds which seem to lead the productivity of surface waters at the location of Core MD 900963.