Factors controlling the carbon isotope composition of dissolved inorganic carbon and methane in marine porewater: An evaluation by reaction-transport modelling

Carbon isotope compositions of dissolved inorganic carbon (DIC) and methane (CH4) in porewater of marine sediments at seafloor temperatures show very large variation covering a delta C-13 range from -100 parts per thousand to +35 parts per thousand. These extreme values are the result of isotope fractionation during microbial carbon metabolism, but the combined effect of all factors controlling the isotope distributions is still not completely understood. We used a model approach to evaluate the effects of reaction and transport on carbon isotope distributions in modern sediment porewater under steady state. Simulated delta C-13(DIC) profiles typically show negative values in the sulphate reduction zone and more positive values in the methanogenic zone. With increasing depth in the methanogenic zone, delta C-13 values approach a distribution where the offset of delta C-13(DIC) from delta C-13 of total organic carbon (TOC) to more positive values is similar to the offset of delta C-13(CH4) to more negative values (delta C-13(DIC) and delta C-13(CH4) approach a symmetric distribution relative to delta C-13(TOC)). The model never exceeds this symmetry of the DIC-CH4 couple towards more positive values under steady-state conditions in a purely diffusive system. Our model shows that to reach an offset in delta C-13 between DIC and CH4 in the order of 70 parts per thousand, as frequently observed in methanogenic zones, a larger fractionation than reported from culture experiments with acetoclastic or autotrophic methanogens would be required. In fact, the observed isotope offset in natural systems would be consistent with the known inorganic equilibrium fractionation factor at in-situ temperature, which may suggest isotope exchange via a microbial pathway, during methanogenesis. Furthermore, the model reproduces strongly negative delta C-13(CH4) values at the sulphate methane-transition (SMT) as result of a reverse flux of carbon from DIC to CH4 during AOM. Such a reverse AOM has no influence on the delta C-13(DIC) at the SMT as methane is almost completely consumed. Only at high sedimentation rate combined with low porosity, delta C-13(DIC) values significantly more negative than delta C-13(TOC) occur at the SMT.