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

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.% MgC03) with subor-dinate amounts of aragonite (-9200 ppm Sr) and calcian dolomite (51.5 to 53.2 mol.% CaC03). Magne-sian calcite occurs as rnicrocrystailine cement, and aragonite forms botryoidal aggregates in cavities and solution pits of the crust. Dolomite occurs as isolated euhedral grains with characteristic corroded sur-faces. Calcite and aragonite have the similar isotopic signatures, ranging in <5I3C between —26.1 and — 33.2%c PDB and in <5lsO between 4.4 and 7.8%o PDB. Relative to calcite and aragonite, dolomite is enriched in 13C (-8.9 to - 11.6%o PDB ^,3C) and depleted in ,sO (-1.2 to - 1.7%o PDB, 5lsO). Combined petrographic and geochemical evidence suggests that dolomite was formed in the sulfate reduction zone prior to the formation of carbonate crust under the strong influence of ,sO-depleted freshwater, which presumably was derived from the melting of glacial ice. Then high magnesian calcite precipitated to form a dense carbonate layer in the near surface sediments caused by anaerobic oxida-tion of hydrate-derived nC-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 cur-rent, the crust would be exposed to a zone of aerobic methane oxidation at the sediment/water inter-face. This led to dissolution of the upper surface of the crust and subsequent precipitation of botryoidal aragonite within solution vugs and cavities. © 1990, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.