ABSTRACT

Vuggy carbonate crust 10 cm in thickness has been discovered associated with hydrocarbon seepage on the continental shelf of Baffin Island, northeast Canada, at a water depth of about 400 m. The carbonate crust is principally composed of high magnesian calcite (14.1 to 18.1 mol.% MgCO₃) with subordinate amounts of aragonite (∼9200 ppm Sr) and calcian dolomite (51.5 to 53.2 mol.% CaCO₃). Magnesian calcite occurs as microcrystalline cement, and aragonite forms botryoidal aggregates in cavities and solution pits of the crust. Dolomite occurs as isolated euhedral grains with characteristic corroded surfaces. Calcite and aragonite have the similar isotopic signatures, ranging in δ¹³C between -26.1 and -33.2‰ PDB and in δ¹⁸O between 4.4 and 7.8‰ PDB. Relative to calcite and aragonite, dolomite is enriched in ¹³C (-8.9 to -11.6‰ PDB δ¹³C) and depleted in ¹⁸O (-1.2 to -1.7‰ PDB δ¹⁸O). Combined petrographic and gochemical evidence suggests that dolomite was formed in the sulfate reduction zone prior to the formation of carbonate crust under the strong influence of ¹⁸O-depleted freshwater, which presumably was derived from the melting of glacial ice. Then high magnesian calcite precipitated to forma dense carbonate layer in the near surface sediments caused by anaerobic oxidation of hydrate-derived ¹³C-depleted methane and increased alkalinity. When the sediments on the crust were winnowed and blown away by an expulsion of methane from the sea bed or a strong bottom current, the crust would be exposed to a zone of aerobic methane oxidation at the sediment/water interface. This led to dissolution of the upper surface of the crust and subsequent precipitation of botryoidal aragonite within solution vugs and cavities.