The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation

Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate that there is a mid-depth maximum in radiocarbon age during the last glacial maximum (LGM). This is in contrast to the modern ocean where intense mixing between water masses results in a relatively homogenous radiocarbon profile. Ferrari et al. [2014] suggested that the extended Antarctic sea ice cover during the LGM necessitated a shallower boundary between the upper and lower branches of the meridional overturning circulation (MOC). This shoaled boundary lay above major topographic features associated with strong diapycnal mixing, isolating dense southern-sourced water in the lower branch of the overturning circulation. This isolation would have allowed radiocarbon to decay, and thus provides a possible explanation for the mid-depth radiocarbon age bulge. We test this hypothesis using an idealized, 2D, residual-mean dynamical model of the global overturning circulation. Concentration distributions of a decaying tracer that is advected by the simulated overturning are compared to published radiocarbon data. We find that a 600 km (~5° of latitude) increase in sea ice extent shoals the boundary between the upper and lower branches of the overturning circulation at 45°S by 600 m, and shoals the depth of North Atlantic Deep Water (NADW) convection at 50°N by 2500 m. This change in circulation configuration alone decreases the radiocarbon content in the mid-depth South Atlantic at 45°S by 40‰, even without an increase in surface radiocarbon age in the source region of deep waters during the LGM.