Distribution of planktonic foraminifera in the northern Bay of Bengal

Abstract

The composition, content and distribution of planktonic foraminifera from 65 samples taken by piston and gravity cores from the continental shelf and slope areas of the northern Bay of Bengal and the middle Bengal Fan have been attempted. From a total of 12,000 planktonic foraminifera, 26 different species have been identified. Most of the species are tropical to subtropical, except for Globigerinita uvula, which was found in the Pleistocene sediments of the core 115 KL. Globigerinoides bulliodes, a typical species of the subpolar region, is present in the shelf area—indicating upwelling activity in the region. Globoquadrina conglomerata and Globorotaloides hexagona, known to have disappeared from the Atlantic Ocean 50,000–100,000 years ago, show their presence mostly in the sediments from the outer shelf and slope areas. Surficial distribution of planktonic foraminifera in the Bay of Bengal shows three biofacies in the shelf and slope areas. Biofacies 1 is dominated by benthic with 10–20% of planktonic foraminifera, biofacies 2 by 20–50% planktonics and, especially, by the species Globigerinoides ruber (white). Biofacies 3 contains 60–95% planktonics and is dominated by the species Neogloboquadrina dutertrei. Downcore distribution indicates that only 4–5 species, namely, G. ruber, N. dutertrei, Globigerinoides sacculifer, Globigerinella aequilateralis and Globigerinoides quadrilobatus constitute 65–90% of the total planktonic foraminifera composition. Specific prevalence of Globigerina bulloides in the inner shelf, Orbulina universa in the slope and in the Fan and Globorotalia (Menardella) menardii in the outer shelf and beyond are noted. Oxygen isotope analysis indicates that the core 66 SL from the shallow eastern shelf is totally composed of Recent sediments with very low sedimentation rate, and the core 114 SL from the upper continental slope consists totally of Pleistocene sediments.