North Atlantic reservoir ages linked to high Younger Dryas atmospheric radiocarbon concentrations

Records of changing atmospheric radiocarbon concentration (Delta C-14(atm)) from the last deglaciation suggest that an anomaly during the Younger Dryas (YD) cold phase is the largest of the last 15,000 y. However, the relative influences of the variable C-14 production rate and changes in the flux and storage of the carbon cycle on Delta C-14(atm) during the YD is uncertain. The latter is strongly influenced by carbon exchange between the atmosphere and other reservoirs, such as the deep ocean. In particular, reorganisation of the North Atlantic's overturning circulation, widely associated with intervals of abrupt climate change such as the YD, may have a significant effect on ocean-atmosphere carbon exchange. Here we reconstruct apparent surface water C-14 ages (reservoir ages (R(t)) in the Atlantic Ocean north of 50 degrees N through the YD interval. Within less than 500 calendar years of the start of the YD cold phase, R(t) increased dramatically, reaching values of up to 1000 y. After 12,300 y BP, R(t) gradually decreased, approaching modern North Atlantic surface ocean values of 400 y by the end of the YD. The method employed here to reconstruct North Atlantic surface ocean R(t) depends on a number of assumptions, most significant of which is the synchronicity of the events in the NGRIP ice chronology and the St Kilda isotope record which were used for correlation. Despite these uncertainties, the calculations of R(t) in this study are in good agreement with previously reported marine-terrestrial C-14 data linked by the widespread YD Vedde Ash isochrone. The C-14 concentration of the surface North Atlantic changed in opposition to Delta C-14(atm), throughout the YD, suggesting that extensive sea-ice cover limited air-sea exchange and that a direct link exists between the strength of Atlantic overturning circulation and the C-14 ventilation rate of the deep ocean on sub-centennial timescales. (C) 2011 Elsevier B.V. All rights reserved.