Multiple sources of atmospheric CO2 activated by AMOC recovery at the onset of interglacial MIS 9.

Using high-precision ice core measurements of CO2, δ13C-CO2, CH4, and N2O, this study provides carbon isotope constraints on a sizeable, centennial-scale CO2 jump at the onset of Marine Isotope Stage 9 (MIS 9). The very end of the Heinrich stadial (HS) characterizing Termination IV (T-IV, ca. 343 to 333 ka ago) shows a 250-y-long jump in greenhouse gas concentrations, followed by a 1.3 ka gradual decline back to the initial concentration. During this so-called overshoot, CO2 and CH4 reach their highest levels (about 303 ppm and 800 ppb, respectively) over the past 800 ka prior to industrialization. The jump in CO2 is not accompanied by a change in δ13C-CO2, suggesting that multiple mechanisms contributed to the exceptionally elevated CO2 values. Following the jump, a slow 0.2‰ enrichment in δ13C-CO2 occurs. We propose that during the jump, the sudden resumption of deepwater formation in the North Atlantic (NA) triggered an amplified release of CO2 from the Southern Ocean (SO) by a northward shift of the Intertropical Convergence Zone (ITCZ) and the SO westerlies, potentially in combination with a rapid land carbon release. The latter is expected from temporally enhanced wildfire activity related to higher fuel load and regionally changing weather conditions in connection to the ITCZ shift. A combination of marine proxy records and box model simulation suggests that the δ13C-CO2 decrease expected from these processes is compensated by a net temperature increase in global sea surface temperature (SST) at the time of the AMOC resumption.