A High‐resolution record of Late Quaternary upwelling along the Oman Margin, Arabian Sea based on planktonic foraminifera

Abstract

Planktonic foraminiferal abundances, fluxes, test sizes, and coiling properties are influenced in various ways by the southwest monsoon winds and associated upwelling in the western Arabian Sea. To determine the short‐term changes in the southwest monsoon, we have carried out a high‐resolution time‐series analysis of three upwelling indices (total flux of planktonic foraminiferal tests and flux and relative abundance of the planktonic foraminiferal species Globigerina bulloides) from Ocean Drilling Program (ODP) Site 723A (Oman Margin, western Arabian Sea) spanning the last 19 kyr. In addition, we have determined the relationships between upwelling intensity and the relative abundance, fluxes, and shell concentrations of various planktonic foraminiferal species. Upwelling indices suggest that from 19 to 16 ka (22 to 18.2 cal kyr B.P.) the SW monsoon was relatively strong compared to the period 15.8 to 12.5 ka (17.8 to 13.8 cal kyr B.P.). The intensification of the SW monsoon took place at 12 ka (13.1 cal kyr B.P.) and reached a peak between 10 and 5 ka (10.6 and 4.8 cal kyr B.P.). The high‐resolution data further demonstrate that the SW monsoon has started weakening from 5 ka (4.8 cal kyr B.P.) and the weakest phase was in place at 3.5 ka (3 cal kyr B.P.), which coincides with evidence of an arid climate in western Tibet. Fluxes and shell concentrations of many of the planktonic foraminiferal species increased between 12 and 5 ka in response to the intensification of the SW monsoon winds after the last glacial period. Globigerina bulloides shows a fivefold to tenfold increase in flux during this period of intense upwelling. The other species whose fluxes are influenced by this upwelling change are (in order from strongest to weakest response) Globigerinita glutinata, Globigerinoides ruber, Neogloboquadrina dutertrei, Globigerinella aequilateralis, Globigerina falconensis, and Globigerinoides sacculifer. The relative abundances of G. bulloides and G. ruber increased during intense upwelling, whereas the relative abundances of G. glutinata, N. dutertrei, G. falconensis, and G. sacculifer did not increase during this period, which might be due to differences in the productivity of various species in relation to upwelling change. Therefore the fluxes and shell concentrations provide better and more reliable information about the changes in the monsoon system in the Arabian Sea than relative abundance data.