Modeling Shallow Pore Water Chemistry As a Proxy for Methane Hydrate Abundance In Marine Sediments

Methane hydrates have received considerable attention over the last two decades because they may constitute a future energy resource, a subsea geohazard, and a large component of the global carbon cycle. Behind much of the current interest lie two related questions: how and why are gas hydrates distributed with respect to sedimentary depth at a given location? We develop a one-dimensional (1-D) numerical model to simulate pore water chemistry above marine gas hydrate systems to account for carbon cycling processes across a shallow biogeochemical horizon known as the sulfate-methane transition (SMT). However, there are two competing reaction pathways that can potentially form the SMT. Moreover, the amount and distribution of marine gas hydrate impacts the chemistry of several dissolved pore water species such as the SO₄^2- and the dissolved inorganic carbon (DIC). Furthermore, the pore water profiles across the SMT in shallow sediment show broad variability leading to different interpretations for how carbon, including CH₄, cycles within gas-charged sediment sequences over time.

Our model accounts for downhole changes in pore water constituents (e.g., dissolved CH₄, SO₄^2-, DIC, Ca²⁺, d¹³C of DIC, gas hydrate and free gas) to interpret carbon cycling processes in shallow sediment below the seafloor. Transient and steady-state pore water profiles are generated for three distinct hydrate settings with differing carbon chemistry when the model is tested with site-specific parameters. The simulated pore water profiles resemble those measured at the sites, and the model explains the similarities and differences in pore water chemistry. The model explains how an upward flux of CH₄ consumes most SO₄^2- at a shallow SMT implying that anaerobic oxidation of methane (AOM) is the dominant SO₄^2- reduction pathway, and how a flux of ¹³C-enriched DIC enters the SMT from depth impacting chemical changes across the SMT. This leads to elevated concentrations of pore water DIC with a d¹³C much greater than that of CH₄ (>-60 ‰), even though AOM causes the SMT. The proportion of DIC flux from depth determines the concentration and d¹³C of DIC at the SMT. Crucially, neither the DIC concentration nor its d¹³C at the SMT can be used to discriminate between the competing SO₄^2- reduction pathways causing the SMT. A 1:1 flux balance between SO₄^2- and DIC across the SMT confirms that AOM is the dominant cause for SMT in these systems. Thus, the SMT depth can be used as a direct proxy to evaluate the upward methane flux and the average hydrate saturation in marine gas hydrate systems.