Ensemble simulation of the Last Glacial Maximum marine biogeochemistry and atmospheric pCO₂ drawdown due to the soft-tissue biological carbon pump

Abstract. During the Last Glacial Maximum (LGM), atmospheric pCO₂ was approximately 90 ppm lower than in the pre-industrial era. Several hypotheses have been proposed to explain this difference, including changes in nutrient supply, increased iron input to the ocean, and variations in overturning circulation strength driven by differences in wind stress and atmospheric moisture diffusivity. Current modeling approaches that simulate LGM marine biogeochemistry typically use parameter sets calibrated under pre-industrial conditions, assuming that these parameter values are generic and independent of environmental conditions. This could introduce uncertainty due to the imperfect knowledge of the values that should be assigned to the parameters for the LGM environment. The extent to which this uncertainty affects the simulated LGM marine biogeochemistry remains unclear. In this study, we employ an optimality-based variable stoichiometry plankton ecosystem model coupled with a 3D Earth system model to simulate LGM conditions. We conduct sensitivity analyses with 24 combinations of marine biogeochemical (reduced benthic denitrification rate, decreased sedimentary iron input, a higher PO₄³⁻ inventory, and increased atmospheric iron deposition) and physical boundary conditions (changes in wind stress pattern and increased meridional moisture diffusivity over the Southern Ocean). For each combination, we perform 20 simulations using 20 biogeochemical parameter sets selected out of 600 – each calibrated against present-day observations and representing pre-industrial biogeochemistry about equally well – resulting in a total of 480 simulations. Our results show that changes in iron input exert the most profound influence on marine biogeochemistry, but reduced sedimentary input counteracts the contribution of enhanced atmospheric deposition to pCO₂ drawdown. Changes in macro-nutrient alone have limited effects, owing to co-limitation effects and the variable stoichiometry in our model. The impact of physical conditions on biogeochemical tracers varies, depending on the specific biogeochemical settings. We found that the changes in carbon to nutrient ratios in particulate organic matter are positively correlated with the changes in Fe supply, and could amplify the effect of Fe availability on changes in the atmospheric pCO₂. Compared to pre-industrial reference conditions, atmospheric pCO₂ under full LGM conditions decreases by 36 to 58 ppm across the 20 simulations. The difference between the maximum and minimum glacial pCO₂ decreases amounts to 50 % of the 43 ppm average decrease. These findings highlight that although the 20 parameter sets similarly reproduce pre-industrial marine biogeochemistry, significant variance remains in the marine biogeochemical and atmospheric pCO₂ responses to LGM forcings.