More than Methane: Biogeochemical Processes in High Arctic Fjord Valley Infill Sediments

Rising air temperatures in the High Arctic are predicted to increase permafrost active layer depths, releasing previously frozen organic carbon and nutrients for microbial metabolism. The quantities of methane and carbon dioxide produced from decomposition of this organic carbon are strongly influenced by environmental variables, such as hydrology and redox status. We studied the active layer and permafrost of High Arctic low-centred polygons formed on fjord valley infill sediments in Adventdalen, Svalbard, to ascertain the dominant biogeochemical processes leading to greenhouse gas production. We obtained replicate cores from two contrasting sites within the valley, extracted the porewaters of the cores, and measured the aqueous and solid phase chemistry. Our results suggest that consistent water saturation and high organic carbon content at one site facilitated the formation of pyrite and siderite via the reduction of iron and sulphate. The low redox conditions in this saturated zone are conducive to methanogenesis, with concentrations of dissolved methane in the porewaters reaching almost 300 μmol L^-1. Conversely, at the other, well-drained site, which also has lower organic carbon content, the biogeochemistry is dominated by pyrite oxidation, leading to high concentrations and covariance of aqueous iron and sulphate in the cores. Methanogenesis at this site appears to be negligible. Carbon dioxide concentrations in cores from both sites reach >6000 μmol L^-1, indicative of microbial respiration. It is predicted that permafrost thaw will cause poorly-drained, low-centred polygons to degrade to well-drained, high-centred polygons. We hypothesise that the water-saturated, high organic carbon content sites are more vulnerable in the long-term to altered hydrology by polygon degradation. These polygon sites are likely to transition from methanogenesis to aerobic respiration, leading to rapid decomposition of organic carbon, increased carbon dioxide emissions and major changes in the redox chemistry of iron and sulfur.