Extremely negative δ13C-DOC signatures of Siberian and Alaskan thermokarst lake pore water as an indicator of methane oxidation

To get a better understanding of the methane production in Arctic thermokarst lakes we focused on dissolved organic carbon (DOC) in pore water samples from thermokarst lake sediment cores in northeast Siberia. The measured DOC concentrations and δ13-DOC of these pore waters from two lakes in the Kytalyk Nature Reserve, Russia, show a distinctive difference between two Arctic lake types. We found that the bedfast-ice lake, which freezes all the way to the lake bed in winter, contains up to three times more DOC in its sediment pore waters (300 ppm) than the floating-ice lake (100 ppm), which maintains an unfrozen water body throughout winter. Thus it appears that carbon stored in bedfast-ice lakes is better preserved. This finding has implications for future greenhouse gas emissions. In an increasingly warming Arctic landscape with longer summers and less extreme winters more and more bedfast-ice lakes will shift to floating-ice lakes (Arp et al., 2012, Geophys. Res. Lett.). Previously well-preserved carbon in the lake beds might then become available for increasing microbial decomposition. This relation is important with regard to the potential of rising methane production from thermokarst lake systems. The analysis of DOC and its accompanying isotopic δ13-DOC signature of sediment pore water from highlatitude permafrost lakes are a novel approach in trying to better understand the carbon and methane history of these Arctic water bodies. They play an important role in the Arctic carbon cycling due to their high abundance in Arctic lowlands and their potential for high methane emissions. Recent estimates suggest methane from such northern lakes could be increasing up to 20-54% by the end of this century if the lakes and ponds remain ice-free for only an additional 20 days per season (Wik et al., 2016, Nat. Geoscience).