Surface and deep water variations in the northeast Indian Ocean during 34–6 ka BP: Evidence from carbon and oxygen isotopes of fossil foraminifera

Abstract

δ13C and δ18O records from planktic (Globigerinoides ruber) and benthic foraminifera (Cibicides spp.) were obtained from a deep sea sediment core (SK157–18) collected in the central Bay of Bengal. The chronology of this gravity core (11°59′N; 90°01′E; water-depth 3069 m; core-length 230 cm) has been established using six radiocarbon ages. Large variations in carbon and oxygen isotopes are suggestive of significant changes in surface and deep water of the Bay of Bengal during 34–6 ka BP. Planktic δ18O values in core SK157–18 are significantly higher compared to cores in the western Bay of Bengal and the Andaman Sea. A marked δ18O decrease at 8–7 ka BP is suggestive of low-salinity surface water in the central Bay of Bengal, probably due to the intensification of the Indian southwest monsoon (ISM). An increasing δ13C trend in G. ruber is seen during ∼34–26 and 14–7 ka BP and decreasing δ13C trend between 26 and 15 ka BP intervals. However these δ13C shifts in planktic foraminifera are not coherent with the glacial–interglacial change and may have been caused by wind-induced variability in surface water δ13C.The δ18O and δ13C values in benthic foraminifera show considerable variations during the Holocene and glacial times. Last Glacial Maximum (LGM) to Holocene δ18O shift in benthic foraminifera (1.5‰) exceeds ice volume effect by ∼0.5‰, suggesting glacial deep water cooling of ∼2 °C, assuming no salinity change. This glacial deep water cooling may have been caused by the introduction of cooler Southern Ocean Deep Water (SODW). Large fluctuations in benthic δ13C are due to the changes in source water characteristics during the Holocene and last glaciation. Glacial benthic foraminifera exhibit pronounced decrease in δ13C suggesting reduced contribution from the North Atlantic Deep Water (NADW) and increased influx of SODW. In addition, oxidation of organic matter due to the reduced ventilation of deep water masses during the LGM may have contributed in the significant decrease in δ13C.