NAEHR, THOMAS H., and DEBRA S. STAKES
Monterey Bay Aquarium Research Institute, Moss Landing, CA

Abstract: Fluid Flow and Carbonate Precipitation in the Eel River Basin and Monterey Bay - Two examples from the California Margin

Fluid venting along the Californian continental margin occurs in a variety of tectonic settings and geochemical environments. Fluid flow along the active transform margin of the Monterey Bay area is controlled by a combination of compression, strike-slip movement and the exposure of organic-rich Miocene sediments in the deeper parts of Monterey Canyon. In the Eel River Basin, compressional tectonics and the presence of gas hydrates in near surface sediments seem to control the seepage of methane-rich fluids.

Authigenic carbonates associated with cold seep chemosynthetic communities from both areas exhibit a wide range of mineral and stable isotopic compositions. The mineralogy of the authigenic precipitates varies from dolomite and high-Mg-calcite to aragonite, the latter being present only at Eel River Basin seep sites. The carbon isotopic composition of carbonates from both sites shows an unusually wide variation, ranging from +9.7 ‰ to -55.9 ‰, which indicates a complex carbon source from both a ¹³C-depleted and a residual, ¹³C-enriched, carbon pool. The d¹⁸O values of the Eel River Basin carbonates vary between +3.2 ‰ and +6.6 ‰, pointing toward a heavy, ¹⁸O-enriched oxygen source, possibly related to the decomposition of gas hydrate. In Monterey Bay, d¹⁸O values range from +6.8 ‰ to -8.5 ‰ indicating the local presence of meteoric water. Although no gas hydrates have been detected in Monterey Bay sediments, heavy oxygen isotope values may indicate their presence in the past. The wide range of isotopic and mineralogical variation in authigenic carbonates from both Monterey Bay and Eel River Basin demonstrates that these parameters alone are not indicative for certain tectonic settings. Rather, these variations reflect local controls on the flux of carbon, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the carbon reservoir.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California