δ¹³C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2

Abstract. Abrupt millennial-scale climate change events of the last deglaciation (i.e. Heinrich Stadial 1 and the Younger Dryas) were accompanied by marked increases in atmospheric CO₂ (CO_2atm) and decreases in its stable carbon isotopic ratios (δ¹³C), i.e. δ¹³CO_2atm, presumably due to outgassing from the ocean. However, information on the preceding Heinrich Stadials during the last glacial period is scarce. Here we present δ¹³C records from two species of planktonic foraminifera from the western South Atlantic that reveal major decreases (up to 1 ‰) during Heinrich Stadials 3 and 2. These δ¹³C decreases are most likely related to millennial-scale periods of weakening of the Atlantic meridional overturning circulation and the consequent increase (decrease) in CO_2atm (δ¹³CO_2atm). We hypothesise two mechanisms that could account for the decreases observed in our records, namely strengthening of Southern Ocean deep-water ventilation and weakening of the biological pump. Additionally, we suggest that air–sea gas exchange could have contributed to the observed δ¹³C decreases. Together with other lines of evidence, our data are consistent with the hypothesis that the CO₂ added to the atmosphere during abrupt millennial-scale climate change events of the last glacial period also originated in the ocean and reached the atmosphere by outgassing. The temporal evolution of δ¹³C during Heinrich Stadials 3 and 2 in our records is characterized by two relative minima separated by a relative maximum. This w structure is also found in North Atlantic and South American records, further suggesting that such a structure is a pervasive feature of Heinrich Stadial 2 and, possibly, also Heinrich Stadial 3.