科研之友
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摘要
A unique chimney-shaped carbonate build-up was produced by microbially mediated anaerobic oxidation of methane at a deep-sea cold seep. The build-up was sampled from 1600 m water depth in the area of the Lomonosov Rise (NW Black Sea, Crimean shelf slope). The carbonate chimney grew free into the anoxic water column, with its base attached to a steep slope composed of plagiogranite and void of sediments. The perfectly preserved 1.5 m high chimney stores reliable records of the diversity of mineralogy, geochemistry, and stable isotope composition of deep-sea methane-related carbonates never studied before. The build-up consists of Mg-calcite (MgCO3 = 9-13 mol%) with minor aragonite. The carbonate matrix encloses organic matter, and Emiliania huxleyi coccoliths, as well as minor framboidal pyrite, gypsum, barite, and diatomite fragments. The contribution of detrital silicate material is negligible. Micritic Mg-calcite in the inner zone of the chimney forms obtuse rhombohedrons clustered in hemispherical aggregates (clots) and hosts isolated prismatic aragonite (<< 1 vol.%). In the outer zone, Mg-calcite exists as foliated crystals and spherulites, while aragonite spherulites are restricted to the top surface. The carbon isotope compositions of carbonates (delta C-13 = 46.5 to -33.0 parts per thousand VPDB) and remnant bacterial mats (delta C-13 = -76.9 to -81.6%. VPDB) provide evidence of a biogenic methane source for the build-up formation. Oxygen came mainly from sea water, judging by a narrow range of delta O-18 values in carbonates (+ 0.2 to 1.3 parts per thousand. VPDB). The prevalence of Mg-calcite throughout the build-up indicates an ANME-2/SBR consortium being the main component of the bacterial mats living at the seep. Aragonite grew mostly late in the seep history, which hints to environmental changes and the ensuing predominance of the ANME-1/SBR consortium. The observed relationship between the prokaryotic communities and the main carbonate phases is common to the microbial mats of the Black Sea, but is actually opposed to the pattern of AOM-community and AOM-related carbonates found in sediments. Different morphologies of carbonates in the outer and inner zones of the build-up point to variations in the methane flux gradient, which controlled the biomass density of the AOM consortia. The REE + Y patterns of Mg-calcite from the inner zone are typical of carbonates crystallised in a freshened sea-water environment, while both Mg-calcite and aragonite from the outer zone show distinct sea-water signatures. Inasmuch as the time span of build-up growth (radiocarbon ages from 9000 +/- 180 to 7500 +/- 180 year BP) overlaps the period when the Black Sea 'Lake' reconnected to the Mediterranean Sea, the progressive arrival of sulphate-rich waters can be inferred to be another possible control of REE + Y patterns in seep carbonates besides the methane flux gradient and related activity of AOM consortia.
- 出版日期2015-5-1
