Modeling ocean–atmosphere carbon budgets during the Last Glacial Maximum–Heinrich 1 meltwater event–Bølling transition

Abstract

Benthic carbon isotope data indicate that the rate of North Atlantic Deep Water (NADW) formation and the mode of oceanic thermohaline circulation (THC) varied considerably across the transition from the Last Glacial Maximum (LGM) to the Heinrich 1 meltwater event (MWE) and, subsequently, to the Bolling warm period. We simulate changes in the Ocean-atmosphere carbon cycle induced by and linked to these oceanic fluctuations by means of a carbon cycle box model which resolves the major oceanic basins. The output from an ocean general circulation model (OGCM), which is forced by observed or reconstructed boundary conditions at its surface, serves to constrain the physical parameters of the carbon cycle model. The OGCM depicts three modes of Atlantic THC: an interglacial mode with vigorous NADW formation; a glacial mode with active, although weaker (-65%) NADW formation; and an MWE mode characterized by the complete lack of NADW formation. The carbon cycle model is forced from the LGM scenario into the MWE and finally into the Bolling interstadial. The glacial circulation mode accounts for approximately half (i.e., 37 +/-3 mu atm, depending on parameterization of biological productivity) of the observed glacial reduction in atmospheric CO2 partial pressure (pCO(2)). Approximately 70% of this pCO(2) decline is linked to changes in sea-surface temperature and salinity. The MWE circulation mode has only a small effect on atmospheric pCO(2) (+/-1 mu atm) but goes along with a massive redistribution of carbon from the Indo-Pacific and Southern oceans to the Atlantic Ocean, which stores 85 +/-8 Gt (gigatons) excess carbon during the MWE. The onset of NADW formation after a meltwater event, has the potential to release 81 +/-6 Gt carbon from the model ocean to the atmosphere, corresponding to an atmospheric pCO(2) increase by 38 +/-3 mu atm, equivalent to approximately half of the modern, man-made pCO(2) load.