Addition of brackish water to tundra soils does not inhibit methane production: implications for Arctic coastal methane production

Abstract. In Arctic regions where coastal sediments contain permafrost, global climate change drives processes such as erosion and subsidence. The contribution of these processes to carbon emissions are still uncertain. Relative sea level rise can lead to more waterlogged environments, promoting anoxic degradation of organic matter but it can also lead to a greater exposure of coastal sediments to seawater. This could alter methane (CH₄) production dynamics, although the controls remain poorly understood. For instance, sulfates contained in seawater may have a tampering effect on methanogenesis through competitive inhibition but the increase in microbial abundance could enhance methanogenesis. In this study, we present CH₄ production rates alongside geochemical analyses in a rapidly evolving coastal landscape near the community of Tuktoyaktuk, NWT, Canada, which is located in the continuous permafrost zone. To better constrain CH₄ production dynamics along the land to ocean continuum, sediment profiles were collected from nearshore marine sediments, as well as from the active layer of the coastal (intertidal) zone and inland soils. Anoxic incubations were performed, amended with brackish water to simulate the effect of seawater on the breakdown of organic matter and the production of CH₄. We found marine sediments expectedly led to negligible CH₄ production rates, while the inland sites showed variable rates between null and 35 nmol cm^-3 d^-1. The coastal (intertidal) zone had the highest rates reaching 415 nmol cm^-3 d^-1. Interestingly, sulfate present in brackish water and sediments did not suppress methanogenesis in the incubations of the coastal and inland zones. Analyses of stable carbon isotopes from CH₄ produced in the incubation experiment indicated greater acetotrophy and higher organic matter lability in the coastal zone, possibly contributing to higher CH₄ production rates. This study highlights the potential for significant CH₄ emissions even with high sulfate concentrations which are classically thought to inhibit methanogenesis. This suggests that Arctic coastal microbial CH₄ production might be an understudied source to the atmosphere.