The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation
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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.  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.
Burke , A , Stewart , A L , Adkins , J F , Ferrari , R , Jansen , M F & Thompson , A F 2015 , ' The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation ' Paleoceanography , vol Early view . DOI: 10.1002/2015PA002778
© 2015. American Geophysical Union. All Rights Reserved. Reproduced in accordance with the publisher's institutional repository deposit policy. Originally published here: http://dx.doi.org/10.1002/2015PA002778
This work was supported by a Foster and Coco Stanback Postdoctoral Fellowship and a Marie Curie Career Integration grant (CIG14-631752) awarded to A.B., and an NSF grant awarded to A.F.T.