FORAMINIFERAL ECOLOGY AND STABLE ISOTOPES AT METHANE SEEPS, SANTA BARBARA BASIN, CALIFORNIA: IMPLICATIONS FOR QUATERNARY CLIMATE CHANGE

Surface sediment samples from modern day methane seeps in Santa Barbara Basin, California were analyzed for benthic and planktonic foraminiferal assemblages as well as stable isotopic composition. Samples were investigated to test the following hypotheses, based on interpretations of late Quaternary sedimentary sequences of Santa Barbara Basin: 1) distinctive benthic foraminiferal assemblages are associated with environments containing high methane concentrations; 2) highly negative d¹³C signatures of benthic foraminifera may reflect high concentrations of environmental methane and interspecies gradients may reflect habitat differences; 3) highly negative d¹³C signatures of planktonic foraminifera indicate that methane reached surface waters and the atmosphere, and does not merely record post-depositional diagenesis or authigenic calcite precipitation.

Stable isotopic analysis of these samples show that both benthic and planktonic foraminifera record the highly negative d¹³C composition of methane. Analysis of oxygen isotopic composition and shell ultrastructure by SEM shows little evidence of post-depositional authigenic calcite precipitation or diagenesis of the foraminiferal shells. Thus, this evidence suggests that benthic and planktonic foraminifera can record the presence of methane throughout the water column and are not merely recording post-depositional changes in shell composition. Stable isotopic analysis of single benthic specimens yields information on the total range in carbon isotopic composition of pore waters near the seeps. In addition, unique benthic assemblages are associated with these samples, providing information on the types of benthic foraminifera that may be uniquely adapted to environments with high methane concentrations. These assemblages are marked by species previously observed only in late Quaternary intervals in Santa Barbara Basin, inferred to have been influenced by poor ventilation and high methane fluxes. Thus, the data from this study suggest that benthic and planktonic foraminifera can record catastrophic releases of methane into the water column, which is of potential importance for Quaternary climate change.