In the context of the atmospheric CO2 14C/C (δCatm14) changes since the last ice age, two episodes of sharp δCatm14 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 δCatm14 decline, but only the onset of NADW can reproduce the sharp δCatm14 declines. To fully simulate δCatm14 data requires an additional process that immediately precedes the onsets of NADW. We hypothesize that these "early" δCatm14 declines record the thickening of the ocean s 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. © 2014 Elsevier B.V.