Cold-seeps and authigenic carbonate formation in Monterey Bay, California

Authigenic carbonate associated with modern 'cold seep' biological communities and their extinct analogues exhibit a broad range in stable isotope and mineral composition within the limited geographic area of Monterey Bay. Although such variations in ancient samples have been used to infer differing tectonic settings, these carbonates all formed within a faulted continental margin environment and chemical variations reflect local differences in the sources and flux of carbon to sediment pore fluids. The slow seepage of fluid, and with it dissolved carbon, along the transform-faulted continental margin results in discrete areas of enhanced microbial sulfate reduction, oxidation of methane from both biogenic and thermogenic origins, as well as the active precipitation of both high-Mg calcite (HMC) and dolomite. The authigenic carbonates include semicontinuous pavements of shallow cemented sediments surrounding benthic communities; circular or pipe-like 'chimneys' interpreted as cemented conduits formed as a result of methane gas expulsion; centimeter- to meter-scale rings, doughnuts or slabs winnowed from variable depths within sediments; and carbonate veins (ankerite or calcite) or cements in faulted basement rocks draped with bacterial mat. Abundant pyrite framboids, preferentially filling the tests of the benthic foraminifer Uvigerina peregrina and characteristic of the HMC-bearing samples, are products of a zone of shallow microbial sulfate reduction, a process fundamental to the nourishment of the chemosynthetic cold seep communities. Sites on a sedimented ridge west of the San Gregorio Fault Zone have carbon isotopic values between -35 and -56 parts per thousand that are strongly influenced by carbon derived from methane. The higher values of delta(18)O (more enriched in O-18) are found closer to the active fault zone. Carbonates from sites within the San Gregorio Fault Zone or from Monterey Canyon floor exposures of Miocene sediments have carbon isotope values between -7 and -26 parts per thousand that are mixtures of sedimentary organic carbon and microbially oxidized hydrocarbons from organic-rich, petroleum source rocks. Dolomite-bearing samples are more enriched in both O-18 and C-13. Bulk carbonate samples with heavier O-18 (delta(18)O > 3.5 parts per thousand) result from the abundance of authigenic dolomite and low temperatures of formation. Local destabilization of clathrates might also create pore fluids that are more enriched in both C-13 and O-18. (C) 1999 Elsevier Science B.V. All rights reserved.