Grain Size Constraints on Glacial Circulation in the Southwest Atlantic

Abstract

AbstractKnowledge of past deep‐ocean current speeds has the potential to inform our understanding of changes in the climate system on glacial‐interglacial timescales, because they may be used to help constrain changes in deep‐ocean circulation rates and pathways. Of particular interest is the paleo‐flow speed of southern‐sourced deep water, which may have acted as a carbon store during the last glacial period. A location of importance in the northward transport of southern‐sourced bottom water is the Vema Channel, which divides the Argentine and Brazil basins in the South Atlantic. We revisit previous studies of paleo‐flow in Vema Channel using updated techniques in grain size analysis (i.e., mean sortable silt grain size), in Vema Channel cores and cores from the Brazil margin. Furthermore, we update the interpretation of the previous grain size studies in the light of many years further research into the glacial circulation of the deep Atlantic. Our results are broadly consistent with the existing data and suggest that during the last glacial period there was slightly more vigorous intermediate to middepth (shallower than 2,600 m) circulation in the South Atlantic Ocean than during the Holocene, whereas around 3,500 m the circulation was generally more sluggish. Increased glacial flow speed on the eastern side of the Vema Channel was likely related to an increase in northward velocity of AABW in the channel. An increase in Antarctic Bottom Water flow through the Vema Channel may have helped to sustain the large volume of southern‐sourced deep water in the Atlantic during the glacial period.