Abstract. The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial-interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate. Here we present simulations of the Last Glacial Maximum (LGM) using the CSIRO Mk3L-COAL Earth System Model to test the contribution of physical and biogeochemical processes to ocean carbon storage. For the LGM simulation, we find a significant global cooling of the surface ocean (3.2 °C) and the expansion of both minimum (Northern Hemisphere: 105 %; Southern Hemisphere: 225 %) and maximum (Northern Hemisphere: 145 %; Southern Hemisphere: 120 %) sea ice cover broadly consistent with proxy reconstructions. Within the ocean, a significant reorganisation of the large-scale circulation and biogeochemical fields occurs. The LGM simulation stores an additional 322 Pg C in the deep ocean relative to the Pre-Industrial (PI) simulation, particularly due to a strengthening in Antarctic Bottom Water circulation. However, 839 Pg C is lost from the upper ocean via equilibration with a lower atmospheric CO2 concentration, causing a net loss of 517 Pg C relative to the PI simulation. The LGM deep ocean also experiences an oxygenation (> 100 mmol O2 m−3) and deepening of the aragonite saturation depth (> 2000 m deeper) at odds with proxy reconstructions. Hence, physical changes cannot in isolation produce plausible biogeochemistry nor the required drawdown of atmospheric CO2 of 80–100 ppm at the LGM. With modifications to key biogeochemical processes, which include an increased export of organic matter due to a simulated release from iron limitation, a deepening of remineralisation and decreased inorganic carbon export driven by cooler temperatures, we find that the carbon content in the glacial oceanic reservoir can be increased (326 Pg C) to a level that is sufficient to explain the reduction in atmospheric and terrestrial carbon at the LGM (520 ± 00 Pg C). These modifications also go some way to reconcile simulated export production, aragonite saturation state and oxygen fields with those that have been reconstructed by proxy measurements, thereby implicating changes in ocean biogeochemistry as an essential driver of the climate system.
Southern Hemisphere westerlies as a driver of the early deglacial atmospheric CO2 rise
