Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline

In the context of the atmospheric CO2 14C/C ( Δ C atm 14 ) changes since the last ice age, two episodes of sharp Δ C atm 14 decline have been related to either the venting of deeply sequestered low-14C CO2 through the Southern Ocean surface or the abrupt onset of North Atlantic Deep Water (NADW) formation. In model simulations using an improved reconstruction of 14C production, Atlantic circulation change and Southern Ocean CO2 release both contribute to the overall deglacial Δ C atm 14 decline, but only the onset of NADW can reproduce the sharp Δ C atm 14 declines. To fully simulate Δ C atm 14 data requires an additional process that immediately precedes the onsets of NADW. We hypothesize that these “early” Δ C atm 14 declines record the thickening of the oceanʹs thermocline in response to reconstructed transient shutdown of NADW and/or changes in the southern hemisphere westerly winds. Such thermocline thickening may have played a role in triggering the NADW onsets.