Role of Riverine Dissolved Organic and Inorganic Carbon and Nutrients in Global-ocean Air-sea CO2 Fluxes

• Main Authors: Savelli, Raphael, Carroll, Dustin, Menemenlis, Dimitris, Dutkiewicz, Stephanie, Manizza, Manfredi, Bloom, A. Anthony, Castro-Morales, Karel, Miller, Charles E., Simard, Marc, Bowman, Kevin W., Zhang, Hong
• Format: Other/Unknown Material
• Language: unknown
• Published: Authorea, Inc. 2024
• Subjects: Arctic; Arctic Ocean; Phytoplankton
• Online Access: http://dx.doi.org/10.22541/essoar.171322743.32265483/v1

While the preindustrial ocean was assumed to be in equilibrium with the atmosphere, the modern ocean is a carbon sink, resulting from natural variability and anthropogenic perturbations, such as fossil fuel emissions and changes in riverine exports over the past two centuries. Here we use a suite of sensitivity experiments based on the ECCO-Darwin global-ocean biogeochemistry model to evaluate the response of air-sea CO2 flux and carbon cycling to present-day lateral fluxes of carbon, nitrogen, and silica. We generate a daily export product by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral fluxes from 5171 watersheds worldwide. From 2000 to 2019, carbon exports increase CO2 outgassing by 0.22 Pg C yr-1 via the solubility pump, while nitrogen exports increase the ocean sink by 0.17 Pg C yr-1 due to phytoplankton fertilization. On regional scales, exports to the Tropical Atlantic and Arctic Ocean are dominated by organic carbon, which originates from terrestrial vegetation and peats and increases CO2 outgassing (+10 and +20%, respectively). In contrast, Southeast Asia is dominated by nitrogen from anthropogenic sources, such as agriculture and pollution, leading to increased CO2 uptake (+7%). Our results demonstrate that the magnitude and composition of riverine exports, which are determined in part from upstream watersheds and anthropogenic perturbations, substantially impact present-day regional-to-global-ocean carbon cycling. Ultimately, this work stresses that lateral fluxes must be included in ocean biogeochemistry and Earth System Models to better constrain the transport of carbon, nutrients, and metals across the land-ocean-aquatic-continuum.