Net removal of dissolved organic carbon in the anoxic waters of the Black Sea
Margolin, A.R.; Gerringa, L.J.A.; Hansell, D.A.; Rijkenberg, M.J.A. (2016). Net removal of dissolved organic carbon in the anoxic waters of the Black Sea. Mar. Chem. 183: 13-24. dx.doi.org/10.1016/j.marchem.2016.05.003
In: Marine Chemistry. Elsevier: Amsterdam. ISSN 0304-4203; e-ISSN 1872-7581, meer
Dissolved organic carbon (DOC) concentrations in the deep Black Sea are ~2.5 times higher than found in the globalocean. The two major external sources of DOC are rivers and the Sea of Marmara, a transit point for waters from theMediterranean Sea. In addition, expansive phytoplankton blooms contribute autochthonous carbon to the BlackSea's ~800 Tg C DOC reservoir. Here, a basin-wide zonal section of DOC is explored using data from the 2013Dutch GEOTRACES GA04-N, cruise 64PE373. DOC distributions are interpreted with respect to well-described hydrographicand biogeochemical layers of the Black Sea. Observed DOC concentrations were N180 µmol kg-1 at thesurface, decreasing to ~125 µmol kg-1 at the base of the oxic layer and reaching a minimumof ~113 µmol kg-1 inthe upper anoxic layer between ~150 and 500 m. At greater depths the concentrations increased;maximum anoxiclayer concentrations of 122 µmol kg-1 were found in the homogeneous benthic bottomlayer (N1775 m). Concentrationsare then predicted based on conservationwith respect to salinity using linear end-membermixingmodels,and predictions are comparedwith observations to estimate net removal (i.e., deficits) and accumulation (i.e., surpluses).Amaximumsurplus of ~10 µmol kg-1 was identified at the surface, likely due to local primary production.DOC exported to depthwas non-conservative: up to ~34-41 µmol kg-1was removed from the basin's oxic layer inb5 years, and an additional 13±5 µmol kg-1 was removed fromthe anoxic layer during its ~300 to 600-year residencetime, given steady state. These deficits represent a removal of ~19% in the oxic water and a further removalof ~10% under anoxia, for a net removal of 48 µmol kg-1 (or ~29%) of allochthonous DOC,with respect to predictedconcentrations.We find no evidence for DOC accumulation (i.e., net production) in the anoxic Black Sea, and suggestthat concentrations are elevated relative to the ocean due to input of terrigenous DOC fromrivers;we estimatethat N50% of DOC in the deep Black Sea is terrigenous. The Black Sea's relatively elevated DOC pool may be analogousto a hypothesized anoxic Eocene ocean's elevated reservoir only if the Eocene ocean received a substantialamount of terrigenous DOC.
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