Net removal of dissolved organic carbon in the anoxic waters of the Black Sea

Dissolved organic carbon (DOC) concentrations in the deep Black Sea are ~2.5 times higher than found in the global ocean. The two major external sources of DOC are rivers and the Sea of Marmara, a transit point for waters from the Mediterranean Sea. In addition, expansive phytoplankton blooms contribute autochthonous carbon to the Black Sea's ~800 Tg C DOC reservoir. Here, a basin-wide zonal section of DOC is explored using data from the 2013 Dutch GEOTRACES GA04-N, cruise 64PE373. DOC distributions are interpreted with respect to well-described hydrographic and biogeochemical layers of the Black Sea. Observed DOC concentrations were >180 μmol kg^-1 at the surface, decreasing to ~125 μmol kg^-1 at the base of the oxic layer and reaching a minimum of ~113 μmol kg^-1 in the upper anoxic layer between ~150 and 500 m. At greater depths the concentrations increased; maximum anoxic layer concentrations of 122 μmol kg^-1 were found in the homogeneous benthic bottom layer (>1775 m). Concentrations are then predicted based on conservation with respect to salinity using linear end-member mixing models, and predictions are compared with observations to estimate net removal (i.e., deficits) and accumulation (i.e., surpluses). A maximum surplus of ~10 μmol kg^-1 was identified at the surface, likely due to local primary production. DOC exported to depth was non-conservative: up to ~34-41 μmol kg^-1 was removed from the basin's oxic layer in <5 years, and an additional 13 ± 5 μmol kg^-1 was removed from the anoxic layer during its ~300 to 600-year residence time, given steady state. These deficits represent a removal of ~19% in the oxic water and a further removal of ~10% under anoxia, for a net removal of 48 μmol kg^-1 (or ~29%) of allochthonous DOC, with respect to predicted concentrations. We find no evidence for DOC accumulation (i.e., net production) in the anoxic Black Sea, and suggest that concentrations are elevated relative to the ocean due to input of terrigenous DOC from rivers; we estimate that >50% of DOC in the deep Black Sea is terrigenous. The Black Sea's relatively elevated DOC pool may be analogous to a hypothesized anoxic Eocene ocean's elevated reservoir only if the Eocene ocean received a substantial amount of terrigenous DOC.