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In the Indian monsoon realm, spatial homogeneity of hydroclimate response to glacial abrupt climate events in the North Atlantic Ocean and its underlying mechanism are contentious. Here we reconstruct sea surface temperature (SST) and seawater oxygen isotope (δ18O sw) in the central Andaman Sea, which span over the past 26,000 years, to investigate the potential driving mechanism of millennial-scale climate changes in tropical Indian Ocean. Our results demonstrate that millennial-scale SST and δ18O sw changes during the last deglaciation are associated with changes in Asian monsoon system which follows North Atlantic climate changes. In particular, during the transition from BØlling/AllerØd (B/A) to Younger Dryas (YD), the strengthening of East Asian winter monsoon (EAWM) promotes southward invasion of northern cold air masses to Indo-China Peninsula, cooling the Andaman Sea. Meanwhile, the enriching δ18O sw is a consequence of reducing local rainfall amount and Irrawaddy River inflow due to a weakening Indian summer monsoon (ISM). With aid of an isotope-enabled climate model, our results further demonstrate that the changes in EAWM and ISM during B/A-YD transition are closely coupled with variations of Atlantic meridional overturning circulation (AMOC), corroborating its pivotal role in millennial-scale changes in tropical Indian Ocean hydroclimate during the last deglaciation. • Andaman Sea characterizes abrupt changes in sea surface temperature and δ18O of seawater during the last deglaciation. • δ18O sw and precipitation are characterized by spatial homogeneity of response to abrupt climate change in North Atlantic. • Millennial-scale temperature changes in Andaman Sea show a local feature controlled by East Asian winter monsoon. [ABSTRACT FROM AUTHOR]

The ocean heat content (OHC) and associated surface stratification in tropical oceans are ideal variables to monitor global climate change. However, the variations in surface stratification of the tropical Indian Ocean at the millennial time scale have been less understood. In this context, the present study aimed at reconstructing the sea surface temperature (SST) and thermocline water temperature (TWT) using Mg/Ca of planktonic foraminifera in the sediments of the Andaman Sea, which is one of the hot centers in the Indo-Pacific warm pool (IPWP) over the past 42 ka. The study results confirm the SST changes are consistent in response to the high-latitude temperature records with strong sub-orbital and millennial variations, while the TWT patterns trend remarkably different from the millennial-scale SST and show the variations correspond to the natural summer insolation that triggers the latitudinal movements of the Intertropical Convergence Zone (ITCZ). Furthermore, salinity influences the thermal differences (ΔT=SST-TWT) by varying the vertical temperature and stratification. Analysis of available data shows that the mean northward position of ITCZ would input more freshwater into the Andaman Sea. Freshwater lids can reduce mixing by acting as effective barriers and preventing heat from transferring from the surface to the subsurface, thereby causing subsurface cooling and increased stratification. In contrast, more southward ITCZ (boreal winter monsoon) supplies less freshwater into the Andaman Sea. A highly saline surface water and air cooling promotes mixing and subsurface warming, resulting in decreased stratification. The study results reveal a millennial-scale vertical thermal seesaw synchronizing in the tropical Indian Ocean with abrupt Atlantic meridional overturning circulation (AMOC) changes. During the Bolling-Allerod (B/A) warming period, the dynamic AMOC maintains the surface water stratification by controlling the subsurface cooling processes with increased precipitation and freshwater brought by the northward ITCZ. In contrast, Younger Dryas (YD) and Heinrich events (H) are characterized by reduced summer monsoon precipitation associated with the sluggish AMOC and southward ITCZ, resulting in less stratification and subsurface warming in the Andaman Sea. The thermal proxy records adopted in the study afford robust evidence on the millennial-scale variability in surface stratification driven synchronously by AMOC and regional evaporation vs. precipitation changes in the tropical Indian Ocean, offering new insights into the impacts of the IPWP OHC on abrupt climate changes. • Millennial-scale climate variability was reconstructed by SST and TWT in the Andaman Sea. • Salinity influences the thermal differences (ΔT) by varying the vertical temperature and stratification. • Millennial scale variability of surface stratification was driven synchronously by AMOC and tropical dynamic changes. [ABSTRACT FROM AUTHOR]

The Bengal Fan, as the largest submarine fan in the world, receives a large amount of sediments discharged from the Himalayas through the Ganges-Brahmaputra (G-B) river system. However, previous studies of this system seldom focused on the role of channels, which are widely developed in the Bengal Fan, in sediment transportation over the last glacial cycle. Here, we discuss the coupled sedimentary archive in two gravity cores along the Active Channel in the Bengal Fan. Our findings are based on grain size, clay minerals, and Sr and Nd isotope compositions of these two sediment cores. End-member modeling of grain-size data reveals that the intermediate end-member represents the flux of distal fluvial particles from the G-B river system, the fine end-member denotes regional sediment supply of weathered volcanic materials, especially from the eastern Indian Peninsula, and the coarse end-member probably reflects nearby terrigenous input, aeolian input and/or volcanic glass. Sediment provenance analysis based on clay minerals, and Sr and Nd isotopes confirmed sediment supply from the G-B river system which was characterized by high illite percentage, high 87Sr/86Sr ratios and low ε Nd values, especially during five stages (50–45 ka, 42–37 ka, 31–28.5 ka, 24–20 ka and 14–9 ka). During these stages, heavy fine-grained sediment supply from the G-B river system was discharged into the study area under the influence of climate change rather than sea-level fluctuation, additionally emphasizing the importance of channels in the sediment transport process. Moreover, we find a significant alteration of sediment sources at ∼26 ka, which may be related to conversion of the deep-water currents in the Bay of Bengal. • There is staged sediment supply from the G-B Rivers into the Bay of Bengal. • Heavy sediment supply of the G-B Rivers was affected by climate change. • The channels developed on the Bengal Fan are important in the sediment transport process. • Sediment source change at ∼26 ka may be related to conversion of deep-water currents. [ABSTRACT FROM AUTHOR]

The Holocene evolution of the Indian Ocean Monsoon (IOM), especially the spatiotemporal nature of its maximum precipitation period, remains controversial. Here we reconstruct high-resolution local δ18O sw and sea surface temperature (SST) values by pairing Mg/Ca values and the δ18O data from planktonic foraminifera Globigerinoides ruber (white) in the southern Andaman Sea in order to understand IOM rainfall variability during the Holocene. Local δ18O sw values show that local maximum rainfall occurred between 10.6 ka and 8.6 ka in the southern Andaman Sea, before declining markedly during the 8.6–5.0 ka period, and then decreasing slightly after 5.0 ka. The Mg/Ca-SST was relatively low during the 10.3–8.6 ka period, before increasing slightly between 8.6 ka and 4.0 ka; it declined thereafter. But the sedimentation rate at the site of Core ADM-C1 relating to monsoonal precipitation of the source region of sediments exhibited its highest value during the 8.6–5.5 ka period. After making further comparisons with other published records for this region, we would propose that maximum IOM precipitation during the early Holocene has exhibited a clearly meridional migration: it was located in the tropics around 7°N during the 10.6–8.6 ka period, migrated northward to the subtropics during the 8.6–5.5 ka period, before migrating southward again after 5.0 ka. We would suggest that the northward shift of maximum precipitation during the Early Holocene in the IOM region was in response to the progressively northward migration of the Intertropical Convergence Zone (ITCZ) and the change of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) activities. • Local precipitation is estimated by paired Mg/Ca and oxygen isotope of G. ruber. • Holocene high-resolution monsoon rainfall is reconstructed in the southern Andaman Sea. • Peak IOM precipitation show a meridional south-to-north migration during the Early Holocene. [ABSTRACT FROM AUTHOR]