A snapshot of CO₂ and CH₄ evolution in a thermokarst pond near Igarka, northern Siberia

Abstract

Thermokarst wetlands and ponds in the subarctic, which are located in land surface depressions resulting from permafrost melt, are strong sources of CH 4 , but little is known about respiration processes supporting these emissions. We determined CH 4 fluxes and concentration profiles of dissolved gases and anions and some δ 13 C ratios of CO 2 and CH 4 in a thermokarst pond and adjacent smaller thermokarst depressions in the forest tundra near Igarka, northern Siberia in August 2006. Methane was emitted at 110–170 mg m −2 d −1 and produced mostly by CO 2 reduction, which also provided high Gibbs free energies on the order of 50–70 KJ mol −1 H 2 due to high H 2 concentrations. The diffusive flux calculated from CH 4 gradients in the floating mat contributed <2% to emissions. CH 4 was apparently not oxidized deeper than 20 cm into the floating mat and the water body below. Anaerobic respiration required to reproduce nonsteady state CO 2 concentration maxima in the floating mat above the water body was 30–80 nmol cm −3 d −1 or 250 mg m −2 d −1 and thus on a similar order of magnitude as CH 4 fluxes. The results suggest that floating mat‐covered thermokarst ponds located in northern Siberian bogs effectively convert recently fixed carbon into CH 4 and thus allow for emissions independently from the finite, bog‐derived carbon source. The relative contribution of recently fixed and old bog‐derived carbon to C fluxes requires further investigation, however.