Evidence of glacial warming and monsoon induced productivity changes in the surface mixed and thermocline layers from the western Arabian Sea during late Quaternary

The surface mixed layer hydrography in the Western Arabian Sea (WAS) is primarily governed by seasonal wind reversals. The stronger wind speed accompanied by stronger southwest monsoon (SWM) causes thickening of mixed layer, driven by turbulent kinetic energy and mixing, hence weaker surface stratification and deepening of thermocline occur. Five radiocarbon 14C dates and LR04 established the core (VM3504-PC) chronology. The core covers the last ~172 kyr with an average sedimentation rate of 5.3 cm/kyr. To understand the variation in palaeoproductivity, upper ocean water column stratification, surface mixed and thermocline layer variability, the micropaleontological (planktonic foraminiferal assemblages; PFA) and geochemical analyses (δ18OG. ruber and δ18ON. dutertrei) were used. Our result shows an increased relative abundance of eutrophic mixed layer species (Globigerina bulloides, Globigerinita glutinata, Globoturborotalita rubescens), suggesting enhanced upwelling during interglacial periods (MIS5 and MIS1) caused by the strong SWM.

Interestingly, mid-MIS4 indicates enhanced upwelling induced productivity based on the increased relative abundance of the mixed layer eutrophic species and decreased in the relative abundance of mixed layer oligotrophic species (G. ruber, Globigerinoides sacculifer and Globigerinella siphonifera) as well as upper thermocline eutrophic species N. dutertrei. This mid-MIS4 warming result is supported by both PFA and geochemical proxies, which suggests the stronger SWM related variation in the salinity or changes in the relative influence of different water masses in the study area. A moderate decrease (increase) in the relative abundance of mixed layer oligotrophic species (in the relative abundance of mixed layer eutrophic species), as well as N. dutertrei indicates moderate winter convection mixing during MIS2. The highest degree of stratification occurs at surface mixed layer during MIS6 as compare to MIS2, and shallow winter convection mixing was present at upper thermocline layer as evident by N. dutertrei. Regarding surface and thermocline layer variability, the deepening of thermocline and thickening of the mixed layer during interglacial periods based on Δδ18OG. ruber- N. dutertrei and mixed layer species percentage (MLS%) were noticed.