No abstract

Analysis of micropaleontologic evidence in a deep-sea core in the south-west Indian Ocean indicates that the Agulhas Current was not a dominant feature of this region during glacial intervals. Interpretation of the fossil record, based on the analysis of modern faunal analogs, indicates that during glacial intervals the Agulhas Current was not the strong, year-round current that it is today. Evidence shows that during summer months a weak tropical current was present. During winter months the current was replaced by cool, high-salinity waters. This interpretation suggests that seasonal changes in circulation may have been more pronounced in the southwest Indian Ocean during glacial intervals than today.

A seasonal reconstruction of the Indian Ocean during the last glacial maximum (∼18,000 yr B.P.) reveals that its surface circulation and sea surface temperature patterns were significantly different from the modern Indian Ocean. This reconstruction is based on the planktonic foraminiferal biogeography and estimated sea surface temperatures in 42 Indian Ocean samples. Compared to modern conditions, the polar front was 5° to 10° latitude further north during the last glacial maximum; the Subtropical Convergence was 2° to 5° latitude further north. The West Australian Current was more intense as part of the West Wind Drift was deflected northward along the coast of Australia. The Agulhas Current was cooler and weaker during the summer and more saline and subtropical during the winter. In general, the low latitudes underwent little temperature change. The western Arabian Sea was warmer which implies less upwelling and a weaker Southwest Monsoon. On the average, the Indian Ocean was 1.9°C cooler in February and 1.7°C cooler in August during the last glacial maximum.

The distribution and abundance of planktonic Foraminifera from the Indian Ocean are used to illustrate geographic variations in faunal assemblages in the plankton and on the seabed caused by sedimentary and postdepositional processes and to analyze the effect of these variations on paleoecological reconstruction. Principal components analysis of these data describes the composition and distribution of faunal assemblages in plankton-tow samples, low-dissolution core-top samples, and high-dissolution core-top samples. Factor-comparison analysis describes the relationships among these three sets of assemblages: The species composition of low-dissolution faunal assemblages may be accurately described as a simple linear mixing of plankton assemblages. The geographical distributions of the faunal assemblages in the sediments, however, are often displaced equatorward of their counterparts in the plankton. Dissolution causes complex changes in the composition of faunal assemblages and produces an equatorward displacement of several high-dissolution assemblages relative to their counterparts in low-dissolution sediments. Three transfer functions, or equations, are derived using plankton, low-dissolution, and high-dissolution data. Numerical experiments indicate that transfer functions lose accuracy when applied to discordant data sets: The plankton transfer function often underestimates temperatures in core-top sediments, and the low-dissolution transfer function underestimates temperatures in high-dissolution sediments. These systematic differences in temperature estimates are illustrated by applying the three transfer functions to downcore samples representing conditions 18,000 years ago. Other experiments indicate that these distortions can be reduced by using larger size fractions and calibrating transfer functions with both low- and high-dissolution core-top samples.