Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane?

Abstract

[1] The sulfate-methane transition (SMT), a biogeochemical zone where sulfate and methane are metabolized, is commonly observed at shallow depths (1-30 mbsf) in methane-bearing marine sediments. Two processes consume sulfate at and above the SMT, anaerobic oxidation of methane (AOM) and organoclastic sulfate reduction (OSR). Differentiating the relative contribution of each process is critical to estimate methane flux into the SMT, which, in turn, is necessary to predict deeper occurrences of gas hydrates in continental margin sediments. To evaluate the relative importance of these two sulfate reduction pathways, we developed a diagenetic model to compute the pore water concentrations of sulfate, methane, and dissolved inorganic carbon (DIC). By separately tracking DIC containing 12 C and 13 C, the model also computes d 13 C-DIC values. The model reproduces common observations from methane-rich sediments: a welldefined SMT with no methane above and no sulfate below and a d 13 C-DIC minimum at the SMT. The model also highlights the role of upward diffusing 13 C-enriched DIC in contributing to the carbon isotope mass balance of DIC. A combination of OSR and AOM, each consuming similar amounts of sulfate, matches observations from Site U1325 (Integrated Ocean Drilling Program Expedition 311, northern Cascadia margin). Without AOM, methane diffuses above the SMT, which contradicts existing field data. The modeling results are generalized with a dimensional analysis to the range of SMT depths and sedimentation rates typical of continental margins. The modeling shows that AOM must be active to establish an SMT wherein methane is quantitatively consumed and the d 13 C-DIC minimum occurs. The presence of an SMT generally requires active AOM.

Observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data

“…As the occurrence of gas hydrate is closely linked to the site-specific shape of the methane solubility curve with depth and available methane in the subsurface to form gas hydrate (either from in situ microbial production or in combination with advection from fluids, where methane was generated at greater depth, including thermogenic sources), an initial thought has been, that the depth of the SMTZ may be an indicator for predicting the top of the first gas hydrate occurrence as it may reflect the in situ methane fluxes [Bhatnagar et al, 2008[Bhatnagar et al, , 2011Kastner et al, 2008;Dickens and Snyder, 2009;Chatterjee et al, 2009;Malinverno and Pohlman, 2011]. An additional suggestion had been that there is a 1:10 relationship between the depths of the SMTZ and TGHOZ [Paull et al, 2005].…”

Pockmark activity inferred from pore water geochemistry in shallow sediments of the pockmark field in southwestern Xisha Uplift, northwestern South China Sea

“…With easily degradable organic matter and sulfate being consumed, the rate of sulfate reduction declines, thereby resulting in less steep slopes for sulfate concentrations below 1 mbsf. Differentiating between the relative contributions of OSR and AOM is critical for estimating the methane flux into the SMI, which in turn, is necessary to predict deeper occurrences and distribution of gas hydrates in continental margin sediments (Malinverno and Pohlman, 2011). As shown in the equations for OSR and AOM, OSR produces 2 mol of bicarbonate per mole of sulfate reduced, whereas AOM produces 1 mol of bicarbonate per mole of sulfate reduced.…”

First discovery and characterizations of late Cretaceous seep carbonates from Xigaze in Tibet, China

“…However, modern cold seep examples illustrate that most seep carbonates have relatively heavy carbon isotopes compared to the seepage hydrocarbon, with the difference of   13 C values even more than 50‰ [14,42]. This phenomenon suggests an inevitable 13 C-enriched carbon source incorporated in cold seep carbonates, possibly the normal marine dissolved inorganic carbon or deep sourced dissolved inorganic carbon [43]. Mixing of these carbon sources in different proportion, due to changes of methane fluxes and flow rates, causes the large variations of   13 C values of seep carbonates.…”

“…In addition, the organoclastic sulfate reduction is modeled to contribute to the dissolved inorganic carbon in pore water at the methane-sulfate transition in the cold seep environment [43]. Thus, in situ oxidized organic matters could offer carbon to cold seep carbonates, as well.…”