LORENSON, THOMAS D., U.S. Geological Survey, Menlo Park, CA; and THOMAS H. NAEHR, Monterey Bay Aquarium Research Institute, Moss Landing, CA; and DANIEL L. ORANGE, U.C. Santa Cruz, CA
Abstract: Gas Seeps and Associated Gas Hydrate of the Okhotsk Sea Near Sakhalin Island, Russia, and Eel River Basin, Northern California - A Comparison of Gas and Carbonate Geochemistry
Similarities can be drawn between gas hydrate localities in the Okhotsk Sea near Sakhalin Island and those of the Eel River basin. Both localities occur near onshore oil and gas production, gas is actively seeping, and authigenic carbonates associated with chemosynthetic communities are present. A fundamental question is what source(s) drive these systems? Despite the proximity of thermogenic hydrocarbons to these localities, carbon in methane and carbonate are largely from microbial sources. Methane carbon isotopic composition of gas hydrate at both sites ranges from -58 ‰ to -69 ‰, suggesting a microbial source for methane. Seep gases from the Eel River basin likely have mixed sources of hydrocarbons; they are nearly 100% methane (Cl) with Cl/(C2+C3) ratios over 3500. However trace amounts of thermogenic gases are present and the methane carbon isotopic composition ranges from 42.6 ‰ to -49.5 ‰ suggesting a link with thermal sources. Seep gases were not sampled in the Okhotsk Sea. The mineralogy of carbonate precipitates from the Eel River basin varies from aragonite to highMg-calcite and dolomite. Their carbon isotopic composition shows an unusually wide range, varying from +9 ‰ to -40 ‰ , indicating a complex carbon source from both a 13C-depleted (oxidation of microbial methane) and a residual or thermally-sourced, 13C-enriched carbon pool. The d18O values vary between +3.2 ‰ and +6.6 ‰, suggesting a heavy, 18O-enriched oxygen source, possibly related to the decomposition of gas hydrate. Authigenic carbonate nodules from the Okhotsk Sea are high-Mg-calcite with d13C values ranging from 25.8 ‰ to -45.7 ‰}, consistent with oxidation of microbial methane. The d18O values of 3.5 ‰ to 3.9 ‰ suggest precipitation in equilibrium with regional bottom water.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California