Carbon isotope constraints on glacial Atlantic meridional overturning: Strength vs depth

Abstract Despite its importance for climate and the carbon cycle, the deep ocean circulation during the Last Glacial Maximum (LGM) remains poorly understood. Whereas most studies suggest a shallower Atlantic Meridional Overturning Circulation (AMOC) than at present day, there is disagreement about its transport rate, with estimates ranging from stronger to weaker than today. Older deep ocean radiocarbon ages have been suggested to imply a more sluggish circulation. Here we use a global isotope-enabled ocean-climate model to systematically explore the different effects of AMOC depth and strength on carbon isotope (13C and radiocarbon) distributions and constraints provided by sediment data. We find that existing data constrain the AMOC depth well, favoring simulations with a shallower-than-present LGM AMOC reaching 2000 − 2500 m of depth. However, they provide weaker constraints on AMOC strength. Comparisons with two high vertical resolution LGM δ 13 C profiles suggest LGM AMOC strength between 11 and 18 Sv, but more data are needed to refine this estimate. Contrary to past conjectures, we find radiocarbon age to be only weakly related with deep water transport rates, but strongly dependent on Southern Ocean surface reservoir ages, which are highly correlated with AMOC depth. In addition, upon changes of deep transport rates and/or water mass geometry, variations in modeled δ 13 C and radiocarbon age are highly correlated, suggesting that they do not act as independent traces for physical ocean processes.