VARIATIONS IN HIGH-LATITUDE OCEANOGRAPHIC FRONTS IN THE SOUTHERN INDIAN OCEAN: AN ESTIMATION BASED ON FAUNAL CHANGES

A quantitative estimate of the magnitude of late Pleistocene/Holocene variations in the migration of siliceous microfauna is calculated from fossil assemblages preserved in marine sediments from piston cores located in the southeast Indian Ocean sector of the Subantarctic. Results of this modern analog technique indicate that fauna characteristic of colder waters today occupied surface and near-surface waters overlying the core sites for much of the last half million years. Warm-water fauna inhabited these waters for only brief time periods, usually during the early portion of interglacial isotopic stages. The estimated faunal migration pattern from the most southern sites is similar in most respects to that of atmospheric temperatures over Antarctica recorded in the Vostok ice core. A model of changes in the positions of the Subtropical Convergence and the Antarctic Polar Front, constructed from time series of faunal variations, indicates that although southward migrations of these oceanographic fronts in this region of the southern ocean were limited to approximately 4 degrees of latitude, northward displacements were of the order of 5 degrees to 7 degrees of latitude. Fauna at the Antarctic Polar Front react earlier than either the fauna at the Subtropical Convergence or the global ice volume signal to major global warming (terminations), providing further documentation of the early response of the high-latitude southern ocean relative to other components of the climate system to the various factors forcing Pleistocene climate.