Abstract. We conduct a model-data analysis of the ocean, atmosphere and terrestrial carbon system to understand their effects on atmospheric CO2 during the last glacial cycle. We use a carbon cycle box model SCP-M, combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and biological productivity across marine isotope stages spanning 130 thousand years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea ice cover and shallow water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimised in each marine isotope stage, against proxy data for atmospheric CO2, δ13C and ∆14C and deep ocean δ13C, ∆14C and carbonate ion. Our model-data results suggest that global overturning circulation weakened at marine isotope stage 5d, coincident with a ∼ 25 ppm fall in atmospheric CO2 from the penultimate interglacial level. This change was followed by a further slowdown in Atlantic meridional overturning circulation and enhanced Southern Ocean biological export productivity at marine isotope stage 4 (∼−30 ppm). There was also a transient slowdown in Atlantic meridional overturning circulation at MIS 5b. In this model, the last glacial maximum was characterised by relatively weak global ocean and Atlantic meridional overturning circulation, and increased Southern Ocean biological export productivity (∼−15–20 ppm during MIS 2–4). Ocean circulation and Southern Ocean biology rebounded to modern values by the Holocene period. The terrestrial biosphere decreased by ∼ 500 Pg C in the lead up to the last glacial maximum, followed by a period of intense regrowth during the Holocene (∼ 750 Pg C). Slowing ocean circulation, a cooler ocean and, to a lesser extent, shallow carbonate dissolution, contributed ∼−75 ppm to atmospheric CO2 in the ∼ 100 thousand-year lead-up to the last glacial maximum, with a further ∼−10 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological productivity was one of the ingredients required to achieve the last glacial maximum atmospheric CO2 level. The incorporation of longer-timescale data into quantitative ocean transport models, provides useful insights into the timing of changes in ocean processes, enhancing our understanding of the last glacial maximum and Holocene carbon cycle transition.
Meridional overturning circulation in the South Atlantic at the last glacial maximum
