Methane-derived carbonates and authigenic pyrite from the northwestern Black Sea

Methane seeps in the northwestern Black Sea are accompanied by carbonate and pyrite precipitates. Sediments were sampled at locations on the Romanian (120 m depth) and Ukrainian (180–200 m depth) shelf and slope. Layered carbonate crusts are formed of (i) carbonate-cemented siliciclastic sediment containing dreissenoid bivalves, (ii) microcrystalline high-Mg-calcite or aragonite, and (iii) aragonitic cement. The Dreissena sediment is subrecent and was deposited during the freshwater-phase of the Black Sea. It has been affected by seepage of methane-rich fluids, which induced intergranular precipitation of authigenic carbonates. The microcrystalline carbonates exhibit an intense autofluorescence. High-Mg-calcite contains 11–14 mol% MgCO 3. The aragonitic cement (8300–9500 ppm Sr) forms either isopachous layers or botryoids. Microbial filaments about 10–20 μm in diameter and up to 900 μm in length are preserved within and on carbonate crusts. The carbonates are depleted in 13C. Microcrystalline carbonate ranges from −27 to −41‰ PDB, and botryoidal aragonite ranges from −26 to −38‰ PDB. The 13C depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane. Carbonate deposits do not project up into the oxic water column. They are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation. 14C contents give apparent radiocarbon ages of 20,640±180 a BP for a sample of botryoidal aragonite and 19,110±180 a BP for a sample of microcrystalline carbonate, compatible with minimum ages of carbon derived from a fossil hydrocarbon source. The δ 18O values of methane-derived carbonates show a narrow range from +1.2 to +0.2‰ PDB. 87Sr/ 86Sr ratios of microcrystalline carbonate (mean 0.70927) and aragonitic cement (mean 0.70918) are indistinguishable from ambient seawater (mean 0.70917) and thus indicate a shallow Sr source. Higher 87Sr/ 86Sr ratios of the Dreissena sediment (mean 0.71005) are probably caused by Sr derived from detrital mica. Carbonates are accompanied by blocks and crusts composed of pyrite. The framboidal sulphide exhibits a palisade-like fabric with framboids arranged to parallel pillars. Sulphur isotopic ratios (δ 34S) ranging from +16.8 to +19.7‰ CDT indicate that the sulphur derives not from the 34S-depleted H 2S of the water column or the uppermost sediment layers. Most likely, pyrite formed in the lacustrine sediments afte