Mid-Pleistocene transition altered upper water column structure in the Bay of Bengal

Abstract

The unique water column structure and seasonal change in the hydrodynamics make the Bay of Bengal an interesting area to understand the effect of monsoon on the water column stratification. We have reconstructed a 1.45 Myr record of planktic foraminifera assemblages and a 1.16 Myr (MIS 47- MIS 8) record of the difference in the stable oxygen isotopic ratio (Δδ18O) between the mixed layer dwelling Globigerinoides ruber and thermocline dweller Neogloboquadrina dutertrei from the IODP Site U1446 to understand the change in water column stratification and its relationship with the monsoon. From the glacial-interglacial shift in Δδ18O and mixed layer and thermocline assemblages, we infer a stronger stratification during the warm interstadials and comparatively weaker stratification during the cold stadials. The strengthened summer monsoon induced high direct rainfall and riverine influx inhibited mixing in the upper layer and created a shallow thermocline during the interstadials. We report a large variability in Δδ18O prior to the mid-Pleistocene transition (MPT). Interestingly, the glacial-interglacial shift in Δδ18O significantly decreased post MPT. A stronger upwelling and deeper mixed layer during the stadial periods reduced the glacial-interglacial shift in Δδ18O post mid-Pleistocene transition. The strengthened summer monsoon during the post MPT period increased the stratification in the Bay of Bengal