A Proxy for Reconstructing Pore Water TCO2 Gradients and Carbon Oxidation Histories in the Northeast Pacific Using the Carbon Isotopic Composition of Benthic Foraminifera

The tests of modern benthic foraminifera inhabiting the dysoxic, laminated sediments of the Northeastern Pacific margin record del13C values that reflect the isotopic composition of overlying (epifaunal species) and pore water TCO2 (infaunal species). Pore water TCO2 gradients indicate the rate of organic carbon oxidation occurring during early diagenesis, i.e. the fate of most Corg rain to the sea floor. Hence, reconstructing paleo-pore water TCO2 gradients can shed light on paleo-carbon rain rates as well as provide constraints on intermediate water ventilation histories in the North Pacific. In a previous study in Santa Monica Basin, CA, we showed that B. argentea secretes CaCO3 with a del13C value consistent with bottom water and the infaunal species (B. tenuata) had a del13C value equal to the del13C value of pore water TCO2 at 5 mm sediment depth. We used this relationship to reconstruct paleo-TCO2 gradients in Santa Monica Basin. We have now examined and will report upon the distribution of living infaunal forams, their isotopic values, pore water TCO2 gradients, pore water del13CO2 profiles and the amount of methane in pore waters at 2 additional Santa Monica Basin sites, one site in Santa Barbara Basin and 4 sites on the western continental margin of Mexico. The foraminifera living within the upper 1 cm of dysoxic, laminated sediments show depth distributions that consistently have B. argentea living near the sediment water interface, B. tenuata living deepest and B. subadvena living at intermediate depths. The stratification in species abundances is reflected their respective del13C values; B. argentea consistently reflects pore water del13C values near the sediment water interface, B. tenuata is the isotopically lightest, and B. subadvena del13C values fall between the other two species. Each species appears to secrete calcite at a preferred depth based upon the consistent isotopic value displayed by all Rose Bengal stained individuals. This observation is further verified by applying a diffusion/reaction model to the pore water TCO2 profiles. Using this model, we predict a del13CO2 pore water gradient for each site. From modeled del13CO2 distributions and measured foram del13C values, the depth at which a foram has precipitated its test is predicted. This predicted depth, in most instances, agrees with the depth of maximum abundance.