Title:

Simulated Changes in the Deep Ocean Reservoir of Refractory Dissolved Organic Carbon (DOC) Mediated by Hydrothermal Fluid Fluxes

Of the 662 Pg marine dissolved organic carbon (DOC) reservoir, 95% is refractory DOC (RDOC), thought to be largely produced by the actions of the microbial carbon pump whereby bacterial remineralization of organic matter generates recalcitrant molecules that persist for thousands of years. It remains unclear to what extent the marine RDOC reservoir represents the accumulation of externally sourced organic matter, or how boundary fluxes contribute to the removal of RDOC. Here, we present modifications to the Marine Biogeochemistry Library (MARBL) within the Community Earth System Model to assess vent- associated RDOC fluxes. Seafloor crustal ages, fluid fluxes, and DOC concentrations measured at high temperature, diffuse, and off-axis hydrothermal vents were used to ascertain the global allochthonous supply of RDOC at 0-1 Myr seafloor crust and the removal of RDOC at 1-66.4 Myr seafloor crust in 3 distinct models (Vent , Vent , Vent ). Comparing Vent and Vent source sink off source off models indicates that high-temperature and diffuse vents contribute ~0.4 Tg C/yr, accumulating ~127.5 Tg (78% as RDOC) to the deep-ocean DOC reservoir over centennial timescales. Both RDOC and short-lived semi-labile DOC are discharged out of mid-ocean ridges (i.e., East Pacific Rise) and spread throughout the ocean, reaching lateral distances of up to ~3000 km for RDOC and ~500 km for semi-labile DOC, before undergoing photooxidation or microbial remineralization. In the Vent sink model, off-axis vents consume ~2 Tg/yr RDOC, primarily in the South Pacific, removing ~633 Tg from the deep ocean reservoir over similar timescales. Previous studies have suggested relatively little to no net change in deep-ocean DOC concentration between hydrothermal supply and removal, but vent-ocean exchange likely alters the RDOC reservoir size as well as the DOC lability, 13C-DOC, and DOC radiocarbon composition of the deep ocean over the centennial timescale of deep ocean ventilation.

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