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

Using high-precision ice core measurements of CO 2 , δ 13 C–CO 2 , CH 4 , and N 2 O, this study provides carbon isotope constraints on a sizeable, centennial-scale CO 2 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, CO 2 and CH 4 reach their highest levels (about 303 ppm and 800 ppb, respectively) over the past 800 ka prior to industrialization. The jump in CO 2 is not accompanied by a change in δ 13 C–CO 2, suggesting that multiple mechanisms contributed to the exceptionally elevated CO 2 values. Following the jump, a slow 0.2‰ enrichment in δ 13 C–CO 2 occurs. We propose that during the jump, the sudden resumption of deepwater formation in the North Atlantic (NA) triggered an amplified release of CO 2 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 δ 13 C–CO 2 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.

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