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The ocean's ability to store large quantities of carbon, combined with themillennial longevity over which this reservoir is overturned, has implicatedthe ocean as a key driver of glacial–interglacial climates. However, thecombination of processes that cause an accumulation of carbon within theocean during glacial periods is still under debate. Here we presentsimulations of the Last Glacial Maximum (LGM) using the CSIRO Mk3L-COAL(Carbon–Ocean–Atmosphere–Land) earth system model to test the contributionof physical and biogeochemical processes to ocean carbon storage. For the LGMsimulation, we find a significant global cooling of the surface ocean(3.2 °C) and the expansion of both minimum and maximum sea ice coverbroadly consistent with proxy reconstructions. The glacial ocean stores anadditional 267 Pg C in the deep ocean relative to the pre-industrial (PI)simulation due to stronger Antarctic Bottom Water formation. However,889 Pg C is lost from the upper ocean via equilibration with a loweratmospheric CO2 concentration and a global decrease in export production,causing a net loss of carbon relative to the PI ocean. The LGM deep oceanalso experiences an oxygenation ( >  100 mmol O2 m−3) anddeepening of the calcite saturation horizon (exceeds the ocean bottom) atodds with proxy reconstructions. With modifications to key biogeochemicalprocesses, which include an increased export of organic matter due to asimulated release from iron limitation, a deepening of remineralisation anddecreased inorganic carbon export driven by cooler temperatures, we find thatthe carbon content of the glacial ocean can be sufficiently increased(317 Pg C) to explain the reduction in atmospheric and terrestrial carbonat the LGM (194 ± 2 and 330 ± 400 Pg C, respectively). Assumingan LGM–PI difference of 95 ppm pCO2, we find that 55 ppm can beattributed to the biological pump, 28 ppm to circulation changes and theremaining 12 ppm to solubility. The biogeochemical modifications alsoimprove model–proxy agreement in export production, carbonate chemistry anddissolved oxygen fields. Thus, we find strong evidence that variations in theoceanic biological pump exert a primary control on the climate

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle