Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

Atmospheric carbon dioxide concentrations (pCO₂) have increases by approximately  from the Last Glacial Maximum (LGM) to the late Holocene (last deglaciation). These changes in atmospheric greenhouse gases are recognized as climate system responses to gradual changes in insolation. Previous modeling studies have suggested that the deglacial increases in atmospheric pCO₂ are primarily attributed to the release of CO₂ from the ocean. In addition, it has been suggested that abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO₂. However, there is still limited understanding in oceanic circulation changes, factors responsible for changes in chemical tracers in the ocean of the last deglaciation and its impact on atmospheric pCO₂.

In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes as in reconstructions. We validate the simulated ocean carbon cycle changes and discuss possible biases and missing or underestimated processes in the model by comparing simulated carbon isotope ratios with sediment core data.

The qualitative changes in atmospheric pCO₂ are consistent with ice core records: during Heinrich Stadial 1 (HS1), atmospheric  increases by . This is followed by a decrease of  during the Bølling–Allerød (BA) and an increase of  during the Younger Dryas (YD). However, the model underestimates the changes in atmospheric  during these events compared to ice core data. Radiocarbon and stable isotope signatures ( and ) indicate that the model underestimates the activated deep ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean, and active ventilation in the North Pacific Intermediate Water during HS1. The relatively small changes in simulated atmospheric  changes during HS1 may be attributed to these underestimations of ocean circulation changes. The changes in  associated with strengthening and weakening in the AMOC during the BA and YD are generally consistent with the sediment core record. On the other hand, while the data show a continuous  increase in the deep ocean throughout the YD, the model shows the opposite trend. This suggests that the model simulates excessive weakening of the AMOC during the YD, or limited representations in geochemical processes in the model including marine ecosystem responses and terrestrial carbon storage.

Decomposing the factors behind changes in ocean  reveals that changes in temperature and alkalinity have the main effects on atmospheric  changes. The compensation of the effects of temperature and alkalinity suggests the AMOC changes and associated bipolar climate changes contribute to a slight decrease or increase in atmospheric  during the BA and YD periods, respectively.