Why do numerical models simulate contrasting primary productivity responses in the North Pacific during the Last Glacial Maximum?

Abstract

Primary productivity (PP) of the ocean is a crucial factor controlling of the carbon cycle of the Earth. The Last Glacial Maximum (LGM) provides a valuable end-member climate state for evaluating its control. Numerical models, however, disagree on how PP in the North Pacific responded to LGM climate, limiting mechanistic interpretation of proxy records. We compare LGM and pre-industrial simulations from MIROC-ES2L and MITgcm-REcoM2 with proxy reconstructions, and use a high-resolution ROMS-CoSiNE simulation to diagnose interactions among iron supply, mixing, circulation, and macronutrient limitation. MIROC-ES2L simulates widespread LGM PP enhancement and a pronounced increase in subarctic surface nitrate, whereas MITgcm-REcoM2 simulates slightly lower subarctic PP and higher subtropical PP, in closer agreement with the reconstructed spatial contrast. The models also differ strongly in mixed-layer depth, background nitrate, and dissolved iron, indicating that their productivity responses cannot be explained by dust deposition alone. Seasonal diagnostics suggest that local vertical mixing controls nutrient seasonality, while contrasting background nutrient inventories and large-scale redistribution shape the intermodel differences. ROMS-CoSiNE further shows that enhanced iron availability can shift the subarctic Pacific from iron limitation toward nitrogen or phosphorus limitation. Productivity increases where physical processes replenish macronutrients, including the western subarctic Pacific and Kuroshio Extension, but decreases in parts of the northeastern Pacific where intensified nutrient consumption produces macronutrient depletion. These results reconcile the apparent paradox of greater glacial iron supply but lower productivity at some subarctic sites: the response depends on the coupled balance among iron fertilization, macronutrient resupply, and upper-ocean physics. Accurate glacial productivity simulations therefore require realistic initial nutrient fields, circulation, mixing, and ecosystem limitation, not dust forcing alone.