Modeling CO2 chemistry, δ13C, and oxidation of organic carbon and methane in sediment porewater: Implications for paleo-proxies in benthic foraminifera

I present a numerical diffusion–advection-reaction model to simulate CO2 chemistry, δ13C, and oxidation of organic carbon and methane in sediment porewater. The model takes into account detailed reaction kinetics of dissolved CO2 compounds, H2O, H+, OH−, boron and sulfide compounds. These reactions are usually assumed to be in local equilibrium, which is shown to be a good approximation in most cases. The model also includes a diffusive boundary layer across which chemical species are transported between bottom water and the sediment–water interface. While chemical concentrations and δ13CTCO2δ13CTCO2 at these locations are frequently assumed equal, I demonstrate that they can be quite different. In this case, shells of benthic foraminifera do not reflect the desired properties of bottom water, even for species living at the sediment–water interface (z = 0 cm). Environmental conditions recorded in their shells are strongly influenced by processes occurring within the sediment. The model is then applied to settings in the Santa Barbara Basin and at Hydrate Ridge (Cascadia Margin), locations of strong organic carbon and methane oxidation. In contrast to earlier studies, I show that a limited contribution of methane-derived carbon to porewater TCO2 in the Santa Barbara Basin cannot be ruled out. Simulation of methane venting shows that at oxidation rates greater than FCH4∼50μmolcm-2y-1, the δ13C of porewater TCO2 at z > 1 cm is depleted by more than 15‰ relative to bottom water. Depletions of this magnitude have not been observed in living benthic foraminifera, even at methane vents with much higher oxidation rates. This suggests that foraminifera at these sites either calcify at very shallow sediment depth or during times when oxidation rates are much lower than ∼50 μmol cm−2 y−1.