Carbon and oxygen isotopic records of benthic foraminifera from the Northeast Indian Ocean: implications on glacial-interglacial atmospheric CO 2 changes

Abstract

We present here continuous records of δ 18O and δ 13C in benthic foraminifera, extending well into the last ice age, in two piston cores from the Andaman Sea (sill depth ∼ 1.3 km) and the Bay of Bengal (3 km). These show that, contrary to the previous reports, the glacial to interglacial shift in δ 13C at mid-depths in the Northeast Indian Ocean was indistinguishable from the mean oceanic δ 13C change, negating a more vigorous renewal of intermediate waters globally during the glacial time. The corresponding δ 13C 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 CO 2 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 δ 13C 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 δ 13C 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.