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-CO_2, 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.