Vuggy carbonate crust formed by hydrocarbon seepage on the continental shelf of Baffin Island, northeast Canada

• Published in: GEOCHEMICAL JOURNAL Vol. 24; no. 3; pp. 143 - 158
• Main Author: Matsumoto, Ryo
• Format: Journal Article
• Language: English
• Published: Tokyo GEOCHEMICAL SOCIETY OF JAPAN 1990; Geochemical Society of Japan
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Marine and continental quaternary; Surficial geology; Hydrocarbon; C-13/C-12; Dolomite; Continental shelf; Atlantic Ocean; Calcrete; Labrador Sea; Fuel resources; Geodes; Calcite; Aragonite; Sedimentary rocks; North Atlantic; Organic residues
• ISSN: 0016-7002; 1880-5973
• DOI: 10.2343/geochemj.24.143

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.