Search

Surface seawater stratification shows a sensitive response to solar insolation and large-scale ocean current, further playing an important role in the air-sea interactions at different time scales. However, the variation of surface seawater structure and its driving mechanism in the tropical Indian Ocean is poorly understood. We present a new record of thermocline temperature (TWT) obtained using oxygen isotope and Mg/Ca-temperature of the planktonic foraminifer Pulleniatina obliquiloculata (P. obliquiloculata) from core BoB-24 in the central Bay of Bengal (BoB). The results indicate that TWT rose rapidly from the Younger Dryas (YD) and persisted at a peak temperature value of 22.5 °C between 10 and 7 cal ka BP, and subsequently reduced by 1.5 °C in the middle Holocene period. The temporal variability of TWT occurred following the boreal summer insolation, associated with tropical climate changes such as the movement of the Inter-tropical Convergence Zone (ITCZ) as well as relevant changes of seasonal Indian monsoons. During the YD period, the southward movement of ITCZ and strengthened Indian winter monsoon (IWM) influenced the deepening of pycnocline, which caused subsurface warming and decreased stratification. During the middle Holocene, the northward return of ITCZ and strengthening of Indian summer monsoon (ISM) led to a stronger surface water stratification and present cooling trend of optimum TWT (10–7 cal ka BP). Furthermore, the millennial TWT changes presented in this study revealed a close link with North Atlantic climate changes, indicating a dynamic action of Atlantic meridional overturning circulation (AMOC). • Variabilities of thermocline temperature and oxygen isotope over the last 15 kyr were reconstructed in the Bay of Bengal. • Monsoon-driven salinity changes influence the thermal stratification by tropical dynamic changes. • Millennial scale variability of surface stratification was driven by Atlantic meridional overturning circulation. [ABSTRACT FROM AUTHOR]

Studying marine sedimentary archives from the Indian Ocean have provided great opportunities to assess the forcing/response relationship between the Indian monsoons, millennial-scale climate variability as revealed in Greenland ice cores, and possible roles of the Atlantic meridional overturning circulation (AMOC) changes that are most likely responsible for driving both. Here we present new evidence of sea surface temperature (SST) estimates and δ18O of seawater (δ18O sw , presumably salinity-driven) from the paired data of Mg/Ca and δ18O measured from planktonic foraminifer Globigerinoides ruber (white) in core BoB-24 collected from the central Bay of Bengal (BoB), tropical Indian Ocean. Eleven accelerator mass spectrometry (AMS) radiocarbon dates on planktonic foraminifera Neogloboquedrina dutertrei from the core provide a reliable age model. Our results indicate a gradual surface warming with several short-lived fluctuations since ~42 kyr ago, and a ~3 °C cooling during the Last Glacial Maximum (LGM) and a ~3 °C warming from the last glacial to the Holocene. Our δ18O sw revealed an increased salinity during the Marine Isotope Stage (MIS) 2, which is likely attributable to a weakening of Indian Summer Monsoon (ISM) precipitation and associated river discharge in the BoB. Our record also indicates clearly a phase of strengthening ISM during the mid-MIS 3 (37.5–32 ka) as evidenced by low foraminiferal δ18O sw values of ~0.2‰. Lower than present δ18O sw values during the Bølling/Allerød (B/A) event (14.5–12.6 ka) and the early Holocene (10–5 ka) in core BoB-24 indicate lower salinity that in turn suggest enhanced ISM precipitation and increased freshwater output from the Ganges-Meghna-Brahmaputra-Irrawaddy (GMBI) rivers together with peninsular rivers and Irrawaddy-Salween rivers from Myanmar. This study brings data evidence that implies weakened ISM was linked with Northern Hemisphere cooling events, especially during the MIS 2. Furthermore, the millennial-scale hydroclimate changes in the BoB presented in this study support the dynamic role of AMOC in driving a teleconnection between the North Atlantic and Indian Ocean monsoon regions. • Millennial-scale climate variability reconstructed by SST and δ18O sw in the BoB. • Weakened ISM have been proved linked with Northern Hemisphere cooling events. • A teleconnection between the North Atlantic and Indian Ocean by dynamic role of AMOC. [ABSTRACT FROM AUTHOR]

The long-term evolution of deep-sea turbidity current activity is closely related to climate and sea level changes; however, the impact of abrupt climate change (e.g. millennial-scale events in high northern latitudes) is poorly known. In this paper, deep-sea turbidite records in the western part of the lower Bengal Fan were identified since the Last Glacial Maximum (LGM) to investigate the effects of abrupt climate change on the development of the Earth's largest deep-sea fan. Our records show a termination of turbidite activity in the western lower fan at 16.6 ka–15.4 ka almost contemporaneous with Heinrich 1 event (a cooling stadial in high northern latitudes). The trigger of this termination in turbidite activity may have been a sharp decrease in erosion rates during a time of relatively stable sea level and rapid source-sink processes. The subsequent episodes of the Bolling/Allerod warm interstadial and Younger Dryas cold stadial and the rapid sea level rise event that followed possibly induced adjustment of turbidity current activities in the Bengal Fan, as indicated by other published records. In summary, we show that close links exist between abrupt climate changes in the high-latitude Northern Hemisphere and deep-sea turbidity current activity in the low-latitude South Asia. • Deep-sea turbidite records in the Bengal Fan since LGM were recovered. • Termination of turbidite in the western lower fan was triggered by HS1 event. • Links between high latitudes climate and low latitudes sedimentation were revealed. [ABSTRACT FROM AUTHOR]

Abstract Major and trace elements of sediments from two gravity cores (10YDY09 and 12I712) in the northeastern Indian Ocean have been analyzed for sediment provenance and the relationship of the sediments with climate change since 50 ka. The rare earth element (REE) compositions were analyzed, and the sediment contributions were estimated for the major potential sources, including the Ganges-Brahmaputra (G-B), Godavari-Krishna (G-K) and Mahanadi Rivers. Our results showed that the sediments in the study area were a mixture of sediments discharged from the above three rivers. The sediment contribution of the G-K Rivers increased notably during the Holocene when the rainfall increased, which may be caused by the strengthening of the East Indian Coastal Current (EICC) and the resulting large amounts of sediments supplied by the G-K Rivers. The higher sediment contribution from the G-B Rivers demonstrated the role of the Active Channel in transporting sediments to the distal Bengal Fan. During the last glacial maximum (LGM), especially in the late period, the Indian summer monsoon weakened, but the winter monsoon strengthened, resulting in a decrease in chemical weathering and elemental enrichment in the northeastern Indian Ocean. Conversely, when the rainfall was low, the chemical weathering strengthened and K 2 O/TiO 2 ratios were lowered due to the longer weathering time and resulting sediment potassium loss. However, when the sea level reached a certain height during the Holocene, the fluvial sediments could not easily enter deep-water environments through submarine channels, which were well developed in the Bengal Fan. Highlights • The major and trace elements of two cores were analyzed for sediment provenance. • The role of the Active Channel in the formation of the Bengal Fan was discussed. • The influence of the sea level fluctuations and rainfall changes over the past 50 ka was evaluated. • Coastal currents driven by the monsoon winds are key forces in sediment transport. [ABSTRACT FROM AUTHOR]

Based on clay mineralogy, siliciclastic grain sizes and geochemical records from deep-sea sediments in the central Bay of Bengal (BoB), the evolutionary history of the Indian summer monsoon and associated weathering process in the Himalayan system since 37 ka were reconstructed. Clay minerals and Th La elements indicate a mixture of sediments originating from the Ganges-Brahmaputra (G-B) Rivers, Indo-Burman ranges/Irrawaddy River and Godavari-Krishna (G-K) Rivers. The Th La quantitative reconstruction revealed that sediment supply from the G-B Rivers was reduced during a series of millennial-scale cold climate events (i.e., Younger Dryas and Heinrich Events) and the Last Glacial Maximum. By removing the control of sea-level factors on the percentage of clay minerals in the sediments, we suggests that the marked decrease in the illite chemistry index and the increase in the (illite+chlorite)/smectite ratio during cold climate periods reflect enhanced physical weathering and weakened chemical weathering in the Himalayan system. Comparing the contribution from the G-B Rivers and the sedimentation rates in different regions of the BoB, it was inferred that the sediments of the G-B Rivers were mostly deposited in the upper and middle Bengal Fan. During the Holocene, the Active Channel played a crucial role in delivering sediments from the G-B Rivers to the lower Bengal Fan. The decreased chemical index of alteration (CIA) and increased K₂O/TiO₂ ratio indicated the weakening of the Indian summer monsoon. At that moment, the Himalayan system was involved in cold climatic conditions accompanied by glacial extension, which led to enhanced physical weathering and increased sediment contributions from the highlands. • Core sediments are originated from the G-B Rivers, Indo-Burman ranges/Irrawaddy River and G-K Rivers. • Sediment supply from the G-B Rivers was reduced during a series of millennial-scale cold climate events. • During cold climate periods, physical weathering enhanced and chemical weathering weakened in the Himalayan system. • Sediment of G-B Rivers was mainly stored in the upper and middle BoB over the past 37 ka. • The Active Channel played a crucial role in Holocene delivering sediment from the G-B Rivers to the lower Bengal Fan. [ABSTRACT FROM AUTHOR]