Porewater redox species and processes in the Black Sea sediments

Abstract

During the 2003 KNORR and the 2005 ENDEAVOR cruises to the Black Sea, voltammetric solid-state microelectrode data were collected for the vertical distribution of redox species in porewaters from the upper 200 to 300 mm layer of sediments with mm vertical resolution. We discuss vertical distributions of dissolved sulfide, manganese and iron species in sediments from the oxic, suboxic, anoxic/sulfidic parts of the Black Sea. Results of voltammetric profiling demonstrate many processes governing the porewater chemistry of oxic shelf sediments, suboxic shelf slope sediments, deep anoxic sediments (Unit I), and turbidites in deep anoxic sediments. Sediments from the oxic shelf areas appear to be spatially similar in their porewater chemistry revealing the same sequence of redox species, as elsewhere in the Ocean, except for the lack of manganese. This lack of manganese is explained by its diagenetic remobilization and accumulation in anoxic waters and suboxic sediments. Sediments from the suboxic part of the shelf slope reveal high concentrations of manganese (II) and iron (II) in porewaters. Still, the flux of Mn(II) from these sediments comprises only a part of the Mn(IV) that precipitates from the water column and can be diagenetically recycled in these sediments. These sediments appear to be the major net sink of manganese as it is not accumulated in sediments of either the oxic or anoxic parts of the Sea. The porewater chemistry of deep anoxic cores showed major spatial variations in the magnitude and even direction of the flux of sulfide. The sediments from the eastern central Black Sea show an expected Unit 1 source of sulfide to the overlying water and have high sulfide content, up to 1600 μM. In the anoxic western and near coastal cores, turbidites cause significant loss of porewater sulfide down to nondetectable sulfide (< 0.2 μM). The latter can be explained by the origin and oxidized Mn and Fe content of turbidites.