Variations in the northeastern indian ocean : a high-resolution record of the past 800 , 000 years from site 758 1

We present a high-resolution (6 k.y. sample interval) record of planktonic foraminifer faunal variations for the past 800 k.y. from ODP Site 758 in the northeastern Indian Ocean. The record is examined within the context of a coarse fraction stratigraphy which is a lithologic index of CaCθ3 preservation, and an oxygen isotope stratigraphy which provides a chronostratigraphy and a record of climate change. Variations in the relative abundance of 27 planktonic foraminifer species primarily reflect fluctuations in the intensity of CaCθ3 dissolution. CaCθ3 dissolution covaries with climate fluctuations at a cyclicity of about 100 k.y. Glacial-aged sediments are generally well preserved, as indicated by faunal and lithologic indices. Interglacial-aged sediments show poorer preservation. The -100-k.y. cycles are superimposed upon the long-term Brunhes Dissolution Cycle. This cycle is characterized by an interval of poor preservation centered between 400 and 550 ka and is bounded by good preservation events at 25 and 750 ka. Ecological factors also control variations in the foraminifer fauna. Changes in ecology are inferred from downcore fluctuations in the relative abundances of foraminifer species that have a similar level of resistance to dissolution. We focus on three species (Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii) with a relatively high resistance to dissolution. The long-term increase in N. dutertrei since at least 800 ka is interpreted as either a gradual decrease in the sea-surface salinity, or an increase in the biogenic productivity of the surface waters. These factors may be controlled by the strength of the monsoon climate. Extremely high abundances of P. obliquiloculata are observed in certain downcore intervals and are interpreted as times when the surface water conditions in the northeast Indian Ocean were similar to those in the modern western tropical Pacific, but not the modern Indian Ocean.