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Abstract 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 CO₂concentration 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 CO₂concentration. 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. 

Abstract We use the transport matrices of a data‐constrained circulation model to efficiently compute the steady state distribution of the deep ocean dissolved organic carbon (DOC) at a 1° horizontal resolution by propagating the surface DOC boundary conditions into the ocean interior. An equivalent simulation in the traditional forward modeling approach would be prohibitively computationally expensive. Our model simulates the total DOC as the sum of two DOC pools, the refractory and the semi‐labile. The model is able to simulate the large‐scale features of the deep ocean DOC without local sources or sinks of DOC in the ocean interior. The deep ocean DOC in the model is sensitive to the preformed DOC concentrations in the formation sites of deep and bottom waters, where observations are lacking. Furthermore, our model experiments indicate that the deep Atlantic DOC gradient is sensitive to the mixing of deep waters with different concentrations of preformed refractory DOC, the transport of semi‐labile DOC from the surface North Atlantic, and the decay rate of semi‐labile DOC. These, combined with the observation that much of the deep ocean DOC gradient is in the Atlantic, suggests that the semi‐labile DOC may be an important component of the deep Atlantic DOC. Finally, we show that DOC export depends substantially on the depth level where it is evaluated.