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

Methane seeps in the northwestern Black Sea are accompanied by carbonate an d pyrite precipitates. Sediments were sampled at locations on the Romanian (120 m depth) and Ukrainian (180-200 m depth) shelf and slope. Layered carb onate crusts are formed of (i) carbonate-cemented siliciclastic sediment co ntaining dreissenoid bivalves, (ii) microcrystalline high-Mg-calcite or ara gonite, and (iii) aragonitic cement. The Dreissena sediment is subrecent an d was deposited during the freshwater-phase of the Black Sea. It has been a ffected by seepage of methane-rich fluids, which induced intergranular prec ipitation of authigenic carbonates. The microcrystalline carbonates exhibit an intense autofluorescence. High-Mg-calcite contains 11-14 mol% MgCO3. Th e aragonitic cement (8300-9500 ppm Sr) forms either isopachous layers or bo tryoids. Microbial filaments about 10-20 mum in diameter and up to 900 mum in length are preserved within and on carbonate crusts. The carbonates an d epleted in C-13. Microcrystalline carbonate ranges from -27 to -41 parts pe r thousand PDB, and botryoidal aragonite ranges from -26 to -38 parts per t housand PDB. The C-13 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 wa ter column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation. C-14 contents give apparent radiocarbon ages of 20,640 +/- 180 a BP for a sample of botryoidal aragonit e and 19,110 +/- 180 a BP for a sample of microcrystalline carbonate, compa tible with minimum ages of carbon derived from a fossil hydrocarbon source. The delta O-18 values of methane-derived carbonates show a narrow range fr om +1.2 to +0.2 parts per thousand PDB. Sr-87/Sr-86 ratios of microcrystall ine carbonate (mean 0.70927) and aragonitic cement (mean 0.70918) are indis tinguishable from ambient seawater (mean 0.70917) and thus indicate a shall ow Sr source. Higher 87Sr/86Sr ratios: of the Dreissena sediment (mean 0.71 005) are probably caused by Sr derived from detrital mica. Carbonates are a ccompanied by blocks and crusts composed of pyrite. The framboidal sulphide exhibits a palisade-like fabric with framboids arranged to parallel pillar s. Sulphur isotopic ratios (delta S-34) ranging from +16.8 to + 19.7 parts per thousand CDT indicate that the sulphur derives not from the S-34-deplet ed H2S of the water column or the uppermost sediment layers. Most likely, p yrite formed in the lacustrine sediments after the first incursion of Medit erranean seawater. (C) 2001 Elsevier Science B.V. All rights reserved.