Methane production and bubble emissions from arctic lakes: Isotopic implications for source pathways and ages

Abstract

[1] This study reports an atmospheric methane (CH 4 ) source term previously uncharacterized regarding strength and isotopic composition. Methane emissions from 14 Siberian lakes and 9 Alaskan lakes were characterized using stable isotopes ( 13 C and D) and radiocarbon ( 14 C) analyses. We classified ebullition (bubbling) into three categories (background, point sources, and hot spots) on the basis of fluxes, major gas concentrations, and isotopic composition. Point sources and hot spots had a strong association with thermokarst (thaw) erosion because permafrost degradation along lake margins releases ancient organic matter into anaerobic lake bottoms, fueling methanogenesis. With increasing ebullition rate, we observed increasing CH 4 concentration of greater radiocarbon age, depletion of 13 C CH4 , and decreasing bubble N 2 content. Microbial oxidation of methane was observed in bubbles that became trapped below and later within winter lake ice; however, oxidation appeared insignificant in bubbles sampled immediately after release from sediments. Methanogenic pathways differed among the bubble sources: CO 2 reduction supported point source and hot spot ebullition to a large degree, while acetate fermentation appeared to contribute to background bubbling. To provide annual whole-lake and regional CH 4 isofluxes for the Siberian lakes, we combined maps of bubble source distributions with long-term, continuous flux measurements and isotopic composition. In contrast to typical values used in inverse models of atmospheric CH 4 for northern wetland sources (d 13 C CH4 = À58%, 14 C age modern), which have not included northern lake ebullition as a source, we show that this large, new source of high-latitude CH 4 from lakes is isotopically distinct (d 13 C CH4 = À70%, 14 C age 16,500 years, for North Siberian lakes).Citation: Walter, K. M., J. P. Chanton, F. S. Chapin III, E. A. G. Schuur, and S. A. Zimov (2008), Methane production and bubble emissions from arctic lakes: Isotopic implications for source pathways and ages,

Methane transport from the active layer to lakes in the Arctic using Toolik Lake, Alaska, as a case study

“…We did not observe bottom water enrichments in methane or 222 Rn in water depth profile sampling conducted during our campaigns (in 2011, surface water 222 Rn was 9.2 × 10 3 dpm·m −3 with methane concentrations of 93 nM, while bottom water 222 Rn was 9.0 × 10 3 dpm·m −3 with methane concentrations of 87 nM; in 2012, water 222 Rn was 5.8 × 10 3 dpm·m −3 with methane concentrations of 132 nM, while bottom water 222 Rn was 3.1 × 10 3 dpm·m −3 with methane concentrations of 73 nM), indicating that sediments in Toolik Lake are not a major source of these gases, at least during the sampling period; Ra was also not enriched at depth (Table S1). Indeed, Cornwell and Kipphut (25) found very low organic matter sedimentation rates and low dissolved oxygen consumption rates within Toolik Lake sediments, suggesting that the methane in Toolik Lake may be from a different origin. Similar spatial distribution patterns of methane, showing highest concentrations close to the lakeshore, have been seen in other Arctic lakes (9,26,27).…”

Modeling the impediment of methane ebullition bubbles by seasonal lake ice

“…Methane involved in this "ice-bubble storage" (IBS) is later released during spring thaw. The CH 4 content of bubbles decreases as they are encapsulated, which suggests CH 4 dissolution into the water column (Walter et al, 2008). Dissolved CH 4 accumulates in many lakes during the ice-cover period due to the slowdown or inactivity of methanotrophs in the cold, often anoxic water column (Michmerhuizen et al, 1996;Phelps et al, 1998;Boereboom et al, 2012), so dissolved CH 4 from bubbles may not be immediately subject to oxidation.…”

“…Gas was collected by displacement into inverted, water-filled glass serum bottles, which were then sealed with butyl rubber stoppers and aluminum crimp caps until later analysis. Additionally, bubbles from 246 ebullition events in Goldstream Lake were collected from submerged bubble traps above ebullition seeps from 2008 to 2011 following methods described by Walter et al (2008). These "fresh" samples enabled us to calculate the CH 4 composition of bubbles after they ascend through the water column but before they interact with lake ice, allowing for the conversion of measured volumetric fluxes (ml gas seep −1 d −1 ) to molar fluxes (mol CH 4 seep −1 d −1 ).…”

“…To increase the sample size, we also compared modeled concentrations to measurements of 30 encapsulated bubbles from four other thermokarst lakes in interior Alaska, the northern Seward Peninsula in Alaska, and northern Siberia (Walter et al, 2008;Sepulveda-Jauregui et al, 2014b). It was often impossible to classify ebullition sites beneath white ice during the spring ice-melt period as A, B, or C, so measurements from all ebullition classes were pooled and adjusted to account for observed differences in fresh bubble CH 4 concentrations among lakes (SepulvedaJauregui et al, 2014b).…”

Microbes in thawing permafrost: the unknown variable in the climate change equation

“…16S rRNA gene sequence data have identified both hydrogenotrophic and acetotrophic (methylotrophic) methanogen phylotypes in Arctic tundra samples, at substantial abundance (Wagner and Liebner, 2010). The two groups of methanogens differ in their substrates, syntrophic associations and isotopic fractionation of carbon: it is important to distinguish between the methanogenic pathways to predict the proportions of CH 4 and CO 2 , as well as fluxes (Walter et al, 2008). Changes in methanogen abundance could also confuse estimates of the temperature and pH response factors.…”