Abstract
Refractory dissolved organic carbon (RDOC) represents the second largest reservoir for ocean carbon storage, the bulk of which is held in the deep ocean, out of contact with the atmosphere on decadal to millennial timescales. Thus, understanding the mechanisms governing its production, delivery, and storage within the deep ocean is crucial for fully elucidating the oceanic carbon cycle and its impacts on global climate dynamics. Here we report observations of marine DOC across the Arctic, finding that the Eurasian Basin deep waters (>1,700 m) harbor the global maxima in deep water DOC concentrations. Given the basin's relatively long residence time (>150 years) and the absence of known RDOC delivery pathways into the ocean interior, we attempt to describe how the elevated Arctic Ocean deep water DOC is maintained. Using box model simulations, we find a significant role for brine rejection from continental shelf surface waters in delivering DOC to the abyss, which simultaneously ventilates Arctic Ocean deep waters. Comparison of kinetic loss rates for DOC consumption estimated as a function of subsurface temperatures demonstrates an elevated temperature sensitivity for Arctic RDOC relative to other ocean basins, possibly linked to its elevated terrigenous and/or “fresh” content, with the subzero temperatures of the Arctic currently suppressing DOC remineralization, helping to explain the deep water maxima. The Arctic Ocean currently stores ∼5.3 Pg C as DOC over the multi‐centennial scale residence times of its deep waters, which may be reduced by ∼1%–4% over the next century of warming.
The oceans act as a sink for atmospheric carbon dioxide, storing this greenhouse gas away from the atmosphere on decadal to millennial timescales. One of the important oceanic carbon reservoirs is the storage of dissolved organic carbon (DOC) molecules that are recalcitrant to mineralization by marine microbes, thus allowing them to persist for up to thousands of years. This study identifies the formation of dense waters from high salinity brines occurring over the Arctic Ocean continental shelves as one additional pathway that delivers recalcitrant DOC to ocean depths for storage. Currently, subzero temperatures suppress the slow breakdown of DOC, helping maintain its storage in the Arctic abyss; however, warming over the next century will likely reduce this climate service.
The Arctic Ocean harbors the global maxima in deep ocean dissolved organic carbon (DOC) concentrations Brine ventilation represents a newly uncovered export mechanism for delivery of fresh and terrigenous DOC to deep waters Arctic DOC exhibits a heightened temperature sensitivity to remineralization, currently suppressed by subzero temperatures