Understanding the mechanisms behind high glacial productivity in the southern Brazilian margin

Abstract

This study explores the mechanisms behind the high glacial productivity in the southern Brazilian margin (SBM) during the last 70 kyr using planktonic foraminifera assemblage and subsurface temperature information derived using the modern analogue technique. We show that enhanced glacial productivity was driven by the synergy of two mechanisms operating in different seasons: (i) enhanced productivity in the upwelling region during short austral summer events; and (ii) the persistent presence of the Plata Plume Water (PPW) due to prolonged austral winter conditions. We suggest that the upwelling systems in the southern Brazilian margin were more productive during the last glacial period due to the enhanced Si supply for diatom production by high-Si thermocline waters preformed in the Southern Ocean. We hypothesize that orbital forcing did not have a major influence on changes in upwelling during the last glacial period. However, the more frequent northward intrusions of the Plata Plume Water were modulated by austral winter insolation at 60∘ S via changes in the strength of alongshore southwesterly winds. After the Last Glacial Maximum, the reduced Si content of thermocline waters decreased upwelling productivity, while lower austral winter insolation decreased the influence of the Plata Plume Water over the southern Brazilian margin, reducing regional productivity.