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

• Published in: Paleoceanography
• Main Authors: Burke, Andrea, Stewart, Andrew L., Adkins, Jess F., Ferrari, Raffaele, Jansen, Mate F., Thompson, Andrew F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2015
• Subjects: Radiocarbon; Overturning circulation; Last Glacial Maximum; Antarctic; Southern Ocean; Antarc*; NADW; North Atlantic Deep Water; North Atlantic; Sea ice
• Online Access: https://research-portal.st-andrews.ac.uk/en/researchoutput/the-glacial-middepth-radiocarbon-bulge-and-its-implications-for-the-overturning-circulation(2d6fd527-fc89-4086-b792-e9234ef5fb01).html; https://doi.org/10.1002/2015PA002778; https://research-repository.st-andrews.ac.uk/bitstream/10023/8088/1/Burke_2015_Paleo_Glacial.pdf; http://onlinelibrary.wiley.com/doi/10.1002/2015PA002778/full#footer-support-info

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.