Net removal of dissolved organic carbon in the subsurface Black Sea

Abstract:

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 Mediterranean, while 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 “MedBlack” cruise 64PE373. DOC distributions are interpreted with respect to well-described hydrographic and biogeochemical layers of the Black Sea. 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. Maximum anoxic layer concentrations of 122 µmol kg^-1 were found in the homogeneous benthic bottom layer (>1775 m). Determined from the relationship of DOC with salinity, we found that ~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 ~300-600 years residence time, given steady state. We find no evidence for DOC accumulation in the anoxic Black Sea, and suggest that concentrations are elevated relative to the ocean due to input of terrigenous DOC from rivers. The Black Sea’s relatively elevated DOC pool may be analogous to the previously hypothesized anoxic Eocene ocean’s elevated reservoir if the Eocene ocean received a substantial amount of terrigenous DOC from rivers.