The research team employed an optimality-based plankton ecosystem model that allows for variable carbon, nitrogen, and phosphorus stoichiometry to investigate the effects of increased atmospheric dust deposition, reduced sedimentary iron supply due to continental shelf exposure, changes in nutrient inventories, and altered ocean circulation during glacial conditions.
The simulations revealed that changes in iron supply exert a particularly strong influence on marine biological productivity and atmospheric CO₂ concentrations. Enhanced atmospheric dust deposition during glacial periods can increase iron availability, stimulating phytoplankton growth and organic carbon export. Conversely, lower sea levels expose parts of the continental shelves, potentially reducing sedimentary iron inputs to the ocean and weakening the biological carbon pump (Fig. 1a, b).
The study also demonstrated that phytoplankton can adjust their cellular carbon, nitrogen, and phosphorus ratios in response to environmental conditions. Consequently, their responses to changes in nutrient availability do not necessarily follow fixed elemental ratios. Such physiological flexibility may further amplify the effects of changes in iron supply on atmospheric CO₂ concentrations.
Reproducing the Modern Ocean Does Not Guarantee Accurate Glacial Climate Simulations
Another important finding of this study concerns uncertainties in Earth system models.
The research team initially tested 600 combinations of marine biogeochemical parameters and selected the 20 parameter sets that best reproduced modern ocean observations. These were subsequently combined with 24 different sets of glacial physical and biogeochemical conditions, resulting in a total of 480 Earth system simulations.
The results showed that although all 20 selected parameter sets could reasonably reproduce pre-industrial ocean conditions, the simulated reductions in atmospheric CO₂ under full glacial conditions ranged from 36 to 58 ppm, indicating substantial differences among the simulations.
These findings demonstrate that even when different model parameter sets reproduce available ocean observations equally well, they do not necessarily respond similarly to changing climatic conditions. The study therefore emphasizes that reconstructions of past climates and projections of future ocean changes should account for parameter uncertainty rather than relying solely on a single optimal parameter set.
Implications for Future Climate Research
This study highlights that understanding past changes in atmospheric CO₂ concentrations requires consideration not only of ocean circulation and nutrient availability but also of phytoplankton physiological flexibility and marine iron cycling.
The findings also provide insights into the potential limitations of marine carbon dioxide removal strategies. For example, the effectiveness of enhancing phytoplankton growth through increased ocean iron supply may depend on ecosystem responses and uncertainties in model predictions.
By employing ensemble simulations across multiple parameter sets and environmental conditions, this study provides a more comprehensive framework for evaluating past and future changes in the marine carbon cycle.
Publication Information
Chien, C.-T., Pahlow, M., Somes, C. J., Schartau, M., & Oschlies, A. (2026). Ensemble simulation of the Last Glacial Maximum marine biogeochemistry and atmospheric pCO₂ drawdown due to the soft-tissue biological carbon pump. Earth System Dynamics, 17, 1277–1297. https://doi.org/10.5194/esd-17-1277-2026
Figure 1. Changes in (a) net primary production (NPP) and surface dissolved iron concentration (dFe), and (b) particulate organic carbon (POC) export and the carbon-to-nitrogen ratio of particulate organic matter (pC:N), relative to the default pre-industrial biogeochemical conditions (PIallbgc). Colors represent atmospheric CO₂ partial pressure (pCO₂), while different symbols indicate different biogeochemical configurations. LGMFedep represents the application of atmospheric iron deposition fluxes during the Last Glacial Maximum (LGM); LGMFesed represents reduced sedimentary iron supply due to continental shelf exposure; LGMPO4 represents a 15% increase in the global ocean phosphate inventory; LGMbdeni represents reduced benthic denitrification due to continental shelf exposure; and LGMallbgc combines all the aforementioned LGM biogeochemical conditions. The cluster of points showing relatively small deviations from the reference simulations (PIallbgc) primarily corresponds to the LGMPO4 and LGMbdeni configurations.
