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

Authigenic carbonate associated with modern 'cold seep' biological communit ies and their extinct analogues exhibit a broad range in stable isotope and mineral composition within the limited geographic area of Monterey Bay. Al though such variations in ancient samples have been used to infer differing tectonic settings, these carbonates all formed within a faulted continenta l margin environment and chemical variations reflect local differences in t he sources and flux of carbon to sediment pore fluids. The slow seepage of fluid, and with it dissolved carbon, along the transform-faulted continenta l margin results in discrete areas of enhanced microbial sulfate reduction, oxidation of methane from both biogenic and thermogenic origins, as well a s 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 expulsi on; centimeter- to meter-scale rings, doughnuts or slabs winnowed from vari able depths within sediments; and carbonate veins (ankerite or calcite) or cements in faulted basement rocks draped with bacterial mat. Abundant pyrit e framboids, preferentially filling the tests of the benthic foraminifer Uv igerina peregrina and characteristic of the HMC-bearing samples, are produc ts 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 v alues between -35 and -56 parts per thousand that are strongly influenced b y carbon derived from methane. The higher values of delta(18)O (more enrich ed in O-18) are found closer to the active fault zone. Carbonates from site s within the San Gregorio Fault Zone or from Monterey Canyon floor exposure s of Miocene sediments have carbon isotope values between -7 and -26 parts per thousand that are mixtures of sedimentary organic carbon and microbiall y oxidized hydrocarbons from organic-rich, petroleum source rocks. Dolomite -bearing samples are more enriched in both O-18 and C-13. Bulk carbonate sa mples with heavier O-18 (delta(18)O > 3.5 parts per thousand) result from t he abundance of authigenic dolomite and low temperatures of formation. Loca l 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.