Deep Pacific ventilation ages during the last deglaciation: Evaluating the influence of diffusive mixing and source region reservoir age

Enhanced ventilation of the deep ocean during the last deglaciation may have caused the rise in atmospheric carbon dioxide that drove Earthʼs climate from a glacial to interglacial state. Recent results based on the projection age method, however, suggest the ventilation rate of the deep Pacific slowed during the deglaciation, opposite the expected pattern (Lund et al., 2011). Because the projection age method does not account for tracer diffusion (Adkins and Boyle, 1997) it can yield spurious results and therefore requires validation with alternative techniques. Here ventilation ages are determined using the transit-time equilibration-time distribution (TTD–ETD) method which explicitly accounts for diffusive mixing in the ocean interior (DeVries and Primeau, 2010). The overall time history of deep Pacific TTD–ETD and projection ages is very similar; both show a 1000-yr increase in ventilation age during Heinrich Stadial 1 (HS1; 14.5–17.5 kyr BP) and a 500-yr increase during the Younger Dryas (YD). The similarity is due in part to the use of projection age error estimates that take into account uncertainty in both calendar age and benthic 14C age. Centennial-scale offsets between the TTD–ETD and projection ages are due primarily to the different approaches used to estimate surface ocean radiocarbon content. Both the TTD–ETD and projection age results imply that the ventilation rate of the deep Pacific decreased during the deglaciation, opposite the pattern expected if Southern Ocean upwelling and enhanced meridional overturning drove outgassing of CO2 from the abyss. Variations in surface water reservoir age could cause an apparent shift in deep Pacific ventilation age but existing proxy records from the Southern Ocean appear to be inconsistent with such a driver.