Role of monsoonal changes in surface hydrography of the northwest Arabian Sea during the past 145 ka

Abstract

This paper is primarily based on planktic foraminiferal census data from the past ~ 145 ka in the northwest Arabian Sea (ODP Holes 722B and 723A) to better understand surface water hydrographic changes. The temporal distribution of faunal records largely reflects both glacial-interglacial and millennial-scale changes in surface productivity in response to the monsoonal variations. The distinctive planktic foraminiferal assemblages reflect higher surface productivity resulting from intense southwest (SW) monsoon upwelling during interglacial MIS 1 and MIS 5 in both- coastal and open-ocean waters of the northwest Arabian Sea. Whereas the effect of the SW monsoon during MIS 3 was limited and contributed to the upwelling and surface productivity only in the coastal region. We also recorded the millennial-scale climate variability in the northwest Arabian Sea, which is closely linked with the extreme cold Younger Dryas (YD) event and several North Atlantic Heinrich (H) events. This explains the strong teleconnection between North Atlantic cold events and the reduced intensity of the southwest monsoon, resulting in a characteristic decline in surface productivity of the Arabian Sea. Furthermore, the spectral analysis of some faunal records allowed us to identify a Milankovitch and three sub-Milankovitch periodicities, which are both precession-driven and the harmonic progression of the precessional cycle, indicating that the high-resolution variabilities in surface productivity due to changing monsoonal strength are primarily governed by orbital forcing. The increased fragmentation of foraminiferal tests during interglacial intervals at deeper hole 722B (below OMZ) reflects increased carbonate dissolution, which could be caused by additional CO₂ release during the remineralization of a higher amount of organic matter.