Fluctuations of Antarctic Bottom Water velocity in the Vema Channel during the last 160,000 years

Abstract

The Vema Channel is a gap in the Rio Grande Rise in the southwest Atlantic Ocean through which Antarctic Bottom Water (AABW) flows north from the Argentine Basin into the Brazil Basin. The bathymetry of the gap restricts the flow direction within narrow limits, and has possibly amplified velocity fluctuations. The history of three major fluctuations in bottom water velocity during the last 160,000 years has been determined by combining detailed stratigraphy and sedimentological analyses. Magnetostratigraphy, biostratigraphy, oxygen isotopic stratigraphy, and total carbonate fluctuations provide the chronology in eighteen deep-sea cores from the Vema Channel. Sedimentation rates show a pattern which is related to the present hydrography in the channel. Three cores near the present-day eastern margin of the AABW have the highest sedimentation rates, suggesting the existence of a stagnation zone where fine-grained sediment is deposited. The mean particle size of the carbonate-free silt fraction was selected as a sensitive, direct monitor of bottom water velocity fluctuations. Three major and one minor increase in bottom water velocity are indicated during the last 160,000 years. The relationship of these increases in bottom water velocity to paleoclimate is not clear and may require modification of available models relating paleoclimate to abyssal circulation. The paleovelocity of bottom water in the Vema Channel has been greater than at present both during glacial and interglacial periods for the last 160,000 years. Separate periods of similar paleoclimate may be quite different with respect to Southern Hemisphere bottom water production so that the trends in relative paleovelocity are more complex than predicted by the models of Newell (1974) and Weyl (1968).