Carbonate precipitation as the result of fluid venting in the Cenozoic Eel River Basin

In 1997, an R/V Pt. Lobos/Ventana expedition to the Eel River Basin offshore Northern California discovered several sites of active gas venting, characterized by degassing sediments, living chemosynthetic communities, and extensive carbonate precipitates. Although gas hydrates were not observed during the 1997 expedition, they had previously been recovered at these sites (Brooks et al.; Marine Geology, 96 (1991), 103-109). Whereas observations of gas hydrates or free gas provide insight into the short-term variability in areas of fluid venting, carbonate precipitates are good candidates to provide a long-term record of changing processes at fluid expulsion sites. In my talk I will focus on the mineralogy and isotope geochemistry of the carbonate precipitates from these vent sites and what they can tell us about changing geochemical conditions through time.

The slab- and chimney-shaped precipitates recovered from the seafloor are composed of carbonate-cemented clay and siltstones. Based on detailed XRD and electron microprobe analyses, their mineralogy varies from aragonite to high-Mg-calcite and dolomite. At one site, repeated changes in the geochemical conditions (pore water Mg/Ca ratio, pH, P_CO2, phosphate and sulfate concentrations) during carbonate precipitation seem to have caused the formation of alternating aragonite and Mg-calcite layers.

The carbon isotopic composition of the carbonates shows an unusually wide range, varying from +9 ‰ to –40 ‰ (PDB), which indicates a complex carbon source from both a ¹³C-depleted and a residual, ¹³C-enriched, carbon pool. The d¹⁸O values vary between +3.2 ‰ and +6.6 ‰ (SMOW), pointing toward a heavy, ¹⁸O-enriched oxygen source. Different carbonate minerals exhibit significant differences in their oxygen isotopic fractionation. I will discuss the importance of considering the mineralogy of authigenic precipitates when estimating conditions, i.e. the presence or absence of gas hydrates, during carbonate formation.

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