Carbon storage in the mid-depth Atlantic during millennial-scale climate events

Carbon isotope minima were a ubiquitous feature of the mid-depth Atlantic during Heinrich Stadial 1 (HS1, 14.5–17.5 kyr BP) and the Younger Dryas (YD, 11.5–12.9 kyr BP), yet their cause remains unclear. Recent evidence indicates that North Atlantic processes triggered the δ¹³C anomalies, with weakening of the Atlantic Meridional Overturning Circulation (AMOC) being the most likely driver. Model simulations suggest that slowing of the AMOC increases the residence time of mid-depth waters in the Atlantic, resulting in the accumulation of respired carbon. Here we assess ΣCO₂ variability in the South Atlantic using benthic foraminiferal B/Ca, a proxy for [CO₃ ²⁻]. Using replicated high-resolution B/Ca records from ~2 km water depth on the Brazil Margin, we show that [CO₃ ²⁻] decreased during HS1 and the YD, synchronous with apparent weakening of the AMOC. The [CO₃ ²⁻] response is smaller than in the tropical North Atlantic during HS1, indicating there was a north–south gradient in the [CO₃ ²⁻] signal similar to that for δ¹³C. The implied variability in ΣCO₂ is consistent with model results, suggesting that carbon is temporarily sequestered in the mid-depth Atlantic during millennial-scale stadial events. Using a carbon isotope mass balance, we estimate that approximately 75% of the HS1 δ¹³C signal at the Brazil Margin was driven by accumulation of remineralized carbon, highlighting the nonconservative behavior of δ¹³C during the last deglaciation.