Linking Sediment Characteristics to Methane Emission Potential in Subarctic Lakes

Abstract:

High latitudes are experiencing warmer average annual temperatures, resulting in the thawing of permafrost, and possibly, the increased emission of methane (CH₄) from lakes and ponds. One potential impact of permafrost thaw is increased runoff of organic matter into streams, lakes and ponds. Warming can also potentially increase lake sediment temperatures, resulting in a lower methane (CH₄) storage capacity and increased CH₄ production. We focused our study on six lakes of varying size, location, and characteristics, located in the Stordalen Mire area in northernmost Sweden. We collected sediment cores in each lake and analyzed the dissolved CH₄ in the sediment at various depths in the core. The sediment CH₄ concentrations were compared to the grain sizes and compositions (total organic carbon (TOC), total sulfur (S), and total nitrogen) of the sediment at the corresponding depths. We also measured dissolved CH₄ concentrations in the lake water and compared them to those of the sediment. We found that on average, the CH₄ concentration was higher (16.7 μgCH₄ g_ds^-1 ± 16.4) in sediments with more TOC (31.4 wt % ± 10.8). There was also a strong positive correlation between sediment CH₄ and total S (r² = 0.37, p = 0.165), between sediment CH₄ and TOC (r² = 0.53, p = 0.101), and between TOC and total S (r² = 0.64, p = 0.066). This indicates in situ production of CH_4­ in the lake sediment at depths of peak CH₄ concentrations in at least two of the lakes. The peak of dissolved CH₄ was located deeper in the cores although this depth varied among the lakes. In four lakes, we found high CH₄ concentrations in the sediment, as well as high CH₄ concentrations in the water, indicating little oxidation in the water column or persistent CH₄ production in the sediment. This suggests that at least four of our studied lakes have great potential to release a substantial amount of their produced CH₄ to the atmosphere.