Southwest Atlantic water mass evolution during the last deglaciation

The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5-17.5kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean can liberate C-13-depleted carbon from the abyss, causing atmospheric CO2 to increase and the C-13 of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass 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 12 cores from the Brazil Margin. We show that records below 2300m remained C-13-depleted until 15kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic O-18 results are consistent with abyssal South Atlantic isolation until 15kyr B.P., in contrast to shallower sites. The depth dependent timing of the O-18 signal suggests that correcting O-18 for ice volume is problematic on glacial terminations. New data from 2700 to 3000m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth C-13 minima were most likely driven by an abrupt drop in C-13 of northern component water. Low C-13 at the Brazil Margin also coincided with an 80 decrease in C-14. 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.