Reconstructing dynamics of northern and southern sourced bottom waters during the last 200 ka using sortable silt records in the lower Bengal Fan.

Mean Sortable Silt (SS) size records in the hemipelagic deposits at Site U1452 (IODP 354) in the lower Bengal Fan are presented here to reconstruct the bottom water circulation strengths in the Bay of Bengal (BoB) during the last 200 ka (Marine Isotopic Stage 1 to 6). To eliminate possible amplitude shifts in SS size due to terrestrial sediment flux or turbiditic interventions, sedimentation history at the site is decomposed using End Member Analysis (EMA). SS size record in the BoB is mostly unimpacted by the terrestrial sediment flux and turbidity current deposition; hence the size sorting signature in SS records is best described to have arisen from benthic boundary layer current intensity. SS size record in the BoB indicates reduced bottom water flow speed during glacials (MIS 2 and 6), concomitant with the greater import of Antarctic/ Southern Ocean (SO) derived deep waters and increased speed during interglacials (MIS 1, 3 and 5) corresponding to an increased proportion of Northern Atlantic derived deep water mass in the northern Indian Ocean. Glacial and stadial decline in flow speed reflects on the density reversal between two deep water mass end members in the SO and a shoaled Atlantic meridional overturning circulation (AMOC). On the other hand, faster flow speed during the climate optima at MIS 5.5 and other warm substages of MIS 5 as well as during the Holocene present a deep and strong AMOC. Reduced bottom water circulation strength in the BoB during the MIS 5.5 to 5.4 transition might have contributed to atmospheric CO2 drawdown and global cooling during the early stages of glacial inception along with deep circulation changes in the SO. Also, millennial-scale variability in circulation strength in the BoB is linked to oceanic frontal shift in high northern latitudes and deepwater formation manifesting in reduced flow strength associated with local cold anomalies (C28-C23), successively culminating into glacial inception at MIS 5.4. Reduced flow speed is also observed during some of the Heinrich events indicating a strong teleconnection between deep North Atlantic and BoB. Productivity changes in the BoB might as well have been influenced by variable fluxes of northern and southern sourced water mass during the last 200 ka. [ABSTRACT FROM AUTHOR]