Sortable silt Endmember model results for IODP Hole 354-U1452C@en

Mean Sortable Silt (S͞S) size records in the hemipelagic deposits at Site U1452 (IODP 354) in the lower Bengal Fan is 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 S͞S size due to terrestrial sediment flux or turbiditic interventions, sedimentation history at the site is decomposed using End Member Analysis (EMA). S͞S 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. S͞S size record in the BoB indicates reduced bottom water flow speed during glacials (MIS 2 and 6), concomitant with greater import of Antarctic/Southern Ocean (SO) derived deep waters and increased speed during interglacials (MIS 1, 3 and 5) corresponding to increased proportion of Northern Atlantic derived deep watermass in the northern Indian Ocean. Glacial and stadial decline in flow speed reflects on the density reversal between two deep watermass 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 are 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 watermass during the last 200 ka.

本文呈现了孟加拉扇下段U1452站位（国际大洋发现计划[International Ocean Discovery Program，简称IODP]354航次）半远洋沉积中的平均可分选粉砂（Mean Sortable Silt，记为S͞S）粒径记录，用于重建过去20万年（海洋同位素阶段1至6，简称MIS）孟加拉湾（Bay of Bengal，简称BoB）的底层水循环强度。为消除陆源沉积物通量或浊流作用对S͞S粒径振幅偏移的潜在影响，本研究采用端元分析（End Member Analysis，简称EMA）对该站位的沉积演化历史进行分解。孟加拉湾的S͞S粒径记录基本不受陆源沉积物通量与浊流沉积的干扰，因此其粒径分选特征可归因于底栖边界层的流强变化。孟加拉湾的S͞S粒径记录显示，冰期（MIS 2、6）期间底层水流速度降低，此时南极/南大洋（Antarctic/Southern Ocean，简称SO）来源的深层水输入量显著增加；间冰期（MIS 1、3、5）期间水流速度升高，对应北印度洋区域北大西洋来源深层水占比提升。冰期与冰阶的水流速度下降，反映了南大洋两处深层水团端元间的密度反转，以及大西洋经向翻转环流（Atlantic Meridional Overturning Circulation，简称AMOC）的减弱。与之相反，在MIS 5.5的气候最适期、MIS 5的其余暖亚期以及全新世期间，水流速度更快，对应更强盛的深层环流与更深厚的大西洋经向翻转环流。在MIS 5.5至5.4的转换期，孟加拉湾底层水循环强度降低，这一变化可能促成了冰川起始早期的大气CO₂分压降低与全球降温，同时伴随南大洋深层环流的调整。此外，孟加拉湾水循环强度的千年尺度变率，与北极高纬度海域的海洋锋面位移及深水形成作用密切相关：当出现局地冷异常（C28-C23）时，水流强度降低，该过程逐步演变为MIS 5.4的冰川起始事件。部分海因里希事件（Heinrich Events）期间也观测到水流速度下降，表明北大西洋深层水与孟加拉湾底层环流之间存在显著的遥相关关系。过去20万年来，孟加拉湾的生产力变化可能同样受到南北源水团通量变化的调控。