Surface and deep water changes in the northeast Indian Ocean during the last 60 ka inferred from carbon and oxygen isotopes of planktonic and benthic foraminifera

Abstract

Stable carbon and oxygen isotopic records of planktonic ( Globigerinoides ruber) and benthic foraminifera (mostly Cibicidoides wuellerstorfi) from a deep-sea core in the northeast Indian Ocean are used to infer surface and deep water characteristics for the last ~ 60 kyr. The gravity core (SK-157-14) studied here was retrieved from the Ninetyeast Ridge (5°11′N; 90°05′E) at a water-depth of 3306 m. Chronology of the core was established using nine radiocarbon dates and oxygen isotope stratigraphy. Significant variations in δ 18O during the last 2–60 kyr BP are suggestive of large changes in monsoonal precipitation over the Indian sub-continent. The last glacial maximum (LGM) to Holocene shift in planktonic foraminifera δ 18O (1.64‰) is less than documented earlier from the Bay of Bengal cores. Two prominent negative δ 18O excursions at ~ 8–7 and ~ 20–18 kyr BP are attributed to the sudden influx of freshwater as a result of intensified monsoonal precipitation. Large fluctuations in δ 18O of G. ruber during the Holocene suggest variability in riverine input. Planktonic δ 18O values show a combined effect of increased sea surface salinity and decreased sea surface temperature (SST) during the LGM. In contrast, the planktonic δ 13C values are not linked to the glacial-to-Holocene transition. Comparison of the benthic δ 18O and δ 13C time series with those of a Pacific core (RC13-110) suggests a similar glacial deep water evolution. The LGM to Holocene δ 18O shift in benthic foraminifera (mostly C. wuellerstorfi) exceeds the ice volume effect by ~ 0.5‰, indicating a glacial deep water cooling of ~ 2 °C, assuming no salinity change. Variations in the distribution of δ 13C in the glacial northeast Indian Ocean (NEIO) are most likely the result of deep ocean circulation changes. The glacial deep NEIO δ 13C characteristics clearly point to reduced North Atlantic Deep Water (NADW) input. Consequently the contribution from the Southern Ocean deep water may have increased resulting in low δ 13C. A positive shift in δ 13C during the early deglaciation is consistent with other records from this region. Deglacial δ 13C fluctuations appear to have been caused by the switch ‘on’ and ‘off’ of NADW production.