Potential impacts of sea level rise on methane production in a UK estuary

Methane (CH₄) is a potent greenhouse gas with a global warming potential far higher than that of carbon dioxide (CO₂). Near-shore marine ecosystems often emit less methane than freshwater wetlands due to higher sediment concentrations of sulfate (the second most abundant anion in seawater). Sulfate-reducing bacteria can outcompete methane-producing microorganisms and mediate the anaerobic oxidation of methane, curtailing methane emissions when sulfate is present. Sea level rise is one of the most significant global changes affecting estuaries. Although sea level rise poses a threat to their stability, estuaries may emit less methane and sequester more carbon as they experience greater sulfate availability through seawater incursion. To assess the impacts of increasing sea levels and salinity - aka sulfate - on methane production, we characterize the geochemistry (concentrations and stable isotopes of CH₄, CO₂, DIC, and SO₄^2-, anions, cations, alkalinity, [HS^-], and [Fe²⁺]) of four sites across a salinity gradient from marine to freshwater of the Medway Estuary in November 2021 and January 2022. We also manipulated salinity in sediment incubations with cores from the freshwater end of the estuary to characterize methane production when nominally freshwater sediments are exposed to higher sulfate concentrations. As hypothesized, freshwater sites (salinity 0.3 ppt) have the greatest concentrations of pore fluid dissolved methane (up to 1.5 mM), two orders of magnitude greater than brackish or marine sites (salinities 6 and 32 ppt). Lower δ¹³C_CH4 (< -65‰) characterizes freshwater and marine sites, while deeper in the brackish sites there is higher δ¹³C_CH4 (-18 to -30‰). We use the carbon isotopic composition of CO₂ and dissolved inorganic carbon (δ¹³C_CO2 and δ¹³C_DIC) to understand the depth distribution of methane production. These isotopic compositions increase with depth at the freshwater site, hinting at in situ methane production, but decrease at the other sites, possibly due to organic carbon or methane oxidation. Our freshwater endmember is dominated by iron reduction and methanogenesis, while the brackish sediments have greater rates of nitrate, iron, and sulfate reduction. The most seaward sediments have geochemical evidence of nitrate and iron reduction, with the sulfate reduction zone likely below 40 cm depth. Incubation results will be presented, illustrating how the addition of sulfate impacts methane production pathways in otherwise freshwater sediments.