Variations of ocean biogeochemistry in a transient deglacial simulation with MPI-ESM

Variations in ocean-atmosphere carbon exchange, in response to varying physical and biogeochemical ocean states, is one of the major causes of the glacial-interglacial atmospheric CO₂ changes. Most of the existing modelling studies use time-slice simulations with Earth System Models to quantify the proposed mechanisms, such as the impact of a weakened Southern Ocean westerlies and a massive discharge of freshwater from ice sheet melting on the deglacial atmospheric CO₂ rise. We present the variations of ocean biogeochemistry in a transient deglaciation (21 – 10 kB.P.) simulation using the Max Planck Institute Earth System Model. We force the model with reconstructions of atmospheric greenhouse gas concentrations, orbital parameters, ice sheet and dust deposition. In line with the physical ocean component, we account for the automatic adjustment of all marine biogeochemical tracers in response to changing bathymetry and coastlines that relate to deglacial melt water discharge and isostatic adjustment. We include a new representation of the stable carbon isotope (¹³C) in the ocean biogeochemical component to evaluate the simulation against δ¹³C records from sediment cores.

The model reproduces several proposed oceanic CO₂ outgassing mechanisms. First, the net primary production (NPP) in the North Atlantic Ocean dramatically decrease (by 40 – 80%) during the first melt water pulse (15 – 14 kB.P.) which is caused by the weakening in the strength of the Atlantic Meridional Overturning Circulation from 21 to 3 Sv. However, globally the oceanic NPP only slightly decreases by 8% as oceanic NPP in the South Hemisphere increases during the same period. Second, during the melt water pulse in the Southern Ocean the ventilation of intermediate waters, which has high DIC content and low alkalinity concentration, is slightly enhanced. Third, the surface alkalinity decreases due to dilution and due to episodic shifts between CaCO₃ production and opal production by phytoplankton. Lastly, CO₂ solubility decreases with increasing deglacial sea surface temperature. The increase of surface pCO₂ caused by the above mechanisms is, however, smaller than that of the prescribed atmospheric CO₂. Thus, the ocean is a weak carbon sink in this deglacial simulation.