Does the oxidation of methane leave an isotopic fingerprint in the geologic record?

Abstract

Pore water ΣCO2 and δ13C data are presented for the dysoxic Santa Barbara and Santa Monica Basins in an attempt to constrain the influence of methane oxidation on the pore water gradients and benthic foraminiferal δ13C. Although these two basins are adjacent to one another Santa Barbara Basin has steeper pore water ΣCO2 and δ13C gradients. Methane oxidation could be contributing to the higher gradients in this basin. The isotope mass balance for pore water ΣCO2 indicates, however, that the pore water gradients in the upper 2 cm of both basins are controlled only by the rate of photosynthate carbon oxidation and the rate of diffusion. Methane derived CO2 has no discernable influence on pore water or carbonate δ13C values at these sediment depths in either basin. Benthic foraminiferal species inhabiting different depth horizons in the top centimeter incorporate these steep isotopic gradients in their calcite tests and can be used therefore to reconstruct the history of carbon oxidation within the basins. On the basis of these modern geochemical constraints, a more likely explanation for the apparent δ13C excursions recorded in the late Pleistocene sediments from Santa Barbara Basin involves enhanced carbon rain and carbon oxidation in the basin sediments rather than clathrate destabilization.