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

Thermokarst wetlands and ponds in the subarctic, which are located in land surface depressions resulting from permafrost melt, are strong sources of CH4, but little is known about respiration processes supporting these emissions. We determined CH4 fluxes and concentration profiles of dissolved gases and anions and some delta C-13 ratios of CO2 and CH4 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 CO2 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 CH4 gradients in the floating mat contributed < 2% to emissions. CH4 was apparently not oxidized deeper than 20 cm into the floating mat and the water body below. Anaerobic respiration required to reproduce nonsteady state CO2 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 CH4 fluxes. The results suggest that floating mat-covered thermokarst ponds located in northern Siberian bogs effectively convert recently fixed carbon into CH4 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.