Late Circulation Response in the Deep Southwest Atlantic during Termination I

Abstract:

The rise in atmospheric CO₂ during Heinrich Stadial 1 (HS1; 14.5-17.5 kyr BP) may have been driven by release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean liberates ¹³C-depleted carbon from the abyss, causing atmospheric CO₂ to increase and the δ¹³C of CO₂ to decrease. One prediction of the Southern Ocean hypothesis is that watermass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of twelve cores from the Brazil Margin. We show that records below 2300 m remained ¹³C-depleted until 15 kyr BP or later, indicating the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ¹⁸O results are consistent with abyssal South Atlantic isolation until 15 kyr BP, in contrast to shallower sites. The depth dependent timing of the δ¹⁸O signal suggests that correcting δ¹⁸O for ice volume is ill advised on glacial terminations. New data from 2700-3000 m also show the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ¹³C minima are consistent with conservative behavior and were most likely driven by an abrupt decrease in the δ¹³C of northern component water. Low δ¹³C at the Brazil Margin coincided with a ~80‰ decrease in Δ¹⁴C. Our results are consistent with a weakening of the Atlantic Meridional Overturning Circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO₂ during HS1.