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1.3 km) and the Bay of Bengal (3 km). These show that, contrary to the previous reports, the glacial to interglacial shift in δ13C at mid-depths in the Northeast Indian Ocean was indistinguishable from the mean oceanic δ13C change, negating a more vigorous renewal of intermediate waters globally during the glacial time. The corresponding δ13C shift in deep waters is estimated to be about 50% larger than that reported previously. Jointly with some recent data from the Pacific, our results indicate a modest glacial-Holocene shift in the intermediate to deep water chemical gradients in the Indo-Pacific as a whole, implying that it was perhaps not the dominant mechanism for the glacial-interglacial atmospheric CO2 variations. Also, in conflict with previous work, our measurements suggest significant cooling of both the intermediate and deep waters during the glacial time. The high-resolution records from the Andaman Sea help reconstruct paleoenvironmental changes at intermediate depths during the last deglaciation. Rapid increases in δ13C occurring in two stages during the early deglaciation appear to have been caused by the fluctuations in the North Atlantic Deep Water production. A negative excursion in δ13C during the mid-deglaciation is ascribed to enhanced nutrient regeneration at mid-depths associated with the greatly intensified summer monsoon around the Pleistocene-Holocene boundary.
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doi:10.1016/j.palaeo.2008.03.007 
Copyright © 2008 Elsevier B.V. All rights reserved.
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
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S. Masood Ahmad
, a, 
, G. Anil Babu1, a, Vathachira M. Padmakumaria and Waseem Razaa
Received 1 August 2007;
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 δ18O 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 δ18O (1.64‰) is less than documented earlier from the Bay of Bengal cores. Two prominent negative δ18O 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 δ18O of G. ruber during the Holocene suggest variability in riverine input. Planktonic δ18O values show a combined effect of increased sea surface salinity and decreased sea surface temperature (SST) during the LGM. In contrast, the planktonic δ13C values are not linked to the glacial-to-Holocene transition.
Comparison of the benthic δ18O and δ13C time series with those of a Pacific core (RC13-110) suggests a similar glacial deep water evolution. The LGM to Holocene δ18O 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 δ13C in the glacial northeast Indian Ocean (NEIO) are most likely the result of deep ocean circulation changes. The glacial deep NEIO δ13C 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 δ13C. A positive shift in δ13C during the early deglaciation is consistent with other records from this region. Deglacial δ13C fluctuations appear to have been caused by the switch ‘on’ and ‘off’ of NADW production.
Keywords: Foraminifera; Stable isotopes; Northeast Indian Ocean; Last Glacial Maximum; Holocene
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Corresponding author. Fax: +91 40 27171564.1 Present address: Department of Mines & Geology, Government of Andhra Pradesh, India.
