Dynamics of the Marine Dissolved Organic Carbon Reservoir in Glacial Climate Simulations: The Importance of Biological Production.

The marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation. Plain Language Summary: There is a significant reservoir of dissolved organic carbon (DOC) in the ocean originating from the production and decay of microbes, nearly equal to the amount of carbon contained as carbon dioxide (CO2) in the atmosphere. Studies have suggested that the total amount of DOC in the ocean may respond to climate effects on ocean temperatures and circulation, and that releases of DOC from the ocean as CO2 to the atmosphere may have contributed to some climate change events in Earth's history. To date, however, little modeling work has been done to understand this suggestion. In this study, we used a computer model of the earth's ocean and atmosphere to simulate the response of marine DOC to climate conditions representing the height of Earth's last glacial period. We found that the total amount of marine DOC simulated in our model could vary in response to the specific climate conditions applied and the modeled impacts on ocean biological production. Importantly, we found that these changes in the DOC reservoir are sufficiently large that without it, changes in atmospheric CO2 levels cannot be fully understood. Our results suggest that understanding marine DOC is crucial in understanding global climate changes throughout Earth's history. Key Points: The marine dissolved organic carbon (DOC) reservoir is mobile in response to glacial climate forcing and rapid changes in the meridional overturning circulationChanges in the size of the DOC reservoir are primarily driven by changes in DOC production rather than DOC degradationMarine DOC cycling is directly related to atmospheric pCO2 over simulated climate change events [ABSTRACT FROM AUTHOR]