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

Abstract

A B S T R A C TCarbon isotope compositions of dissolved inorganic carbon (DIC) and methane (CH 4 ) in porewater of marine sediments at seafloor temperatures show very large variation covering a δ 13 C range from −100‰ to +35‰. 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 δ 13 C 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, δ 13 C values approach a distribution where the offset of δ 13 C DIC from δ 13 C of total organic carbon (TOC) to more positive values is similar to the offset of δ 13 C CH4 to more negative values (δ 13 C DIC and δ 13 C CH4 approach a symmetric distribution relative to δ 13 C TOC ). The model never exceeds this symmetry of the DIC-CH 4 couple towards more positive values under steady-state conditions in a purely diffusive system.Our model shows that to reach an offset in δ 13 C between DIC and CH 4 in the order of 70‰, 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 δ 13 C CH4 values at the sulphate methane transition (SMT) as result of a reverse flux of carbon from DIC to CH 4 during AOM. Such a reverse AOM has no influence on the δ 13 C DIC at the SMT as methane is almost completely consumed. Only at high sedimentation rate combined with low porosity, δ 13 C DIC values significantly more negative than δ 13 C TOC occur at the SMT.

The Carbon-Isotope Record of the Sub-Seafloor Biosphere

“…Meister et al (2019b) [55] propose a model, where isotopic equilibration in fact occurs within the pathway, which would most likely be localized at this particular step.…”

“…Meister et al (2019b) [55] propose a model, where isotopic equilibration in fact occurs within the pathway, which would most likely be localized at this particular step. While differential reversibility could explain isotope fractionation for hydrogenotrophic methanogens, fractionation during acetoclastic methanogenesis ( Figure 1A) is very small and not anywhere near isotopic equilibrium.…”

“…Having a full reaction-diffusion model at hand, Meister et al (2019b) [55] discuss the sensitivity of carbon-isotope profiles to variations in organic matter burial and fractionation factors through the SMT and into the methanogenic zone. One observation is that fractionation factors determined in culture experiments with Methanosarcina barkeri under different conditions (Krzycki et al, 1987 [26]; Gelwicks et al, 1994 [27]; Londry, et al, 2008 [31]; etc.…”

“…Larger fractionation was observed in cultures using strains of Methanococcus, an obligate hydrogenotrophic methanogen, under substrate limitation (Botz et al, 1996) [28] (Figure 1). These larger fractionation factors would be necessary to reproduce the measured porewater profile (Meister et al, 2019b) [55], including the effect of CO 2 production by fermentation as shown by Equation (4).…”

“…In general, it is difficult to reproduce δ 13 C DIC more positive than about 10% and impossible to produce the 35% observed by Heuer et al (2009) [7]. factors would be necessary to reproduce the measured porewater profile (Meister et al, 2019b) [55], including the effect of CO2 production by fermentation as shown by Equation (4). Overall, methanogenesis results in 13 C-depleted CH4 and 13 C-enriched DIC, and, from a stoichiometric point of view, isotopic values of CH4 and DIC should be symmetrical with respect to the organic matter substrate from which all DIC and CH4 originates, independent of the pathway.…”