Pore water and solid-phase measurements on sediment cores from the Black Sea

The surface sediments in the Black Sea are underlain by extensive deposits of iron (Fe) oxide-rich lake sediments that were deposited prior to the inflow of marine Mediterranean Sea waters ca. 9000 years ago. The subsequent downward diffusion of marine sulfate into the methane-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ), including anaerobic oxidation of methane (AOM) with sulfate. While the sedimentary cycles of sulfur (S), methane and Fe in the SMTZ have been extensively studied, relatively little is known about the diagenetic alterations of the sediment record occurring below the SMTZ. Here we combine detailed geochemical analyses of the sediment and pore water with multicomponent diagenetic modeling to study the diagenetic alterations below the SMTZ at two sites in the western Black Sea. We focus on the dynamics of Fe, S and phosphorus (P) and demonstrate that diagenesis has strongly overprinted the sedimentary burial records of these elements. Our results show that sulfate-mediated AOM substantially enhances the downward diffusive flux of sulfide into the deep limnic deposits. During this downward sulfidization, Fe oxides, Fe carbonates and Fe phosphates (e.g. vivianite) are converted to sulfide phases, leading to an enrichment in solid phase S and the release of phosphate to the pore water. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe2+) lead to sequestration of downward diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front. […]