Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes

Abstract

Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem‐scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) toward hydrogenotropic (September) methanogenesis at sites located toward the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape‐scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant‐mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly, CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to <0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e‐ acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale and suggests broad scale investigations are required to build better regional or pan‐Arctic representations of CH4 and CO2 production.

Stable Carbon Isotopes Reveal Soil‐Stream DIC Linkages in Contrasting Headwater Catchments

“…We estimated that CO 2 production by acetoclastic methanogenesis could potentially contribute up to 6% of the groundwater DIC production (section 2.5). Similar rates have been reported in groundwater of arctic tundra catchments (Throckmorton et al, ). Riparian and near stream zones have been recognized as methane‐rich areas and potential hot spots of methanogenesis (Hope et al, ; Jones & Mulholland, ), a pathway which also appears to plays a significant role in the regulation of catchment scale cycling of DIC.…”

Impacts of temperature and soil characteristics on methane production and oxidation in Arctic polygonal tundra

“…Acetate was most abundant and exhibited the most dynamic concentration changes among individual organic 10 acids measured from the anaerobic microcosms, which is consistent with previous studies on LCP and HCP carbon decomposition (Yang et al, 2016). The consumption of acetate correlated with increases of CH 4 and CO 2 concentrations in previous incubation experiments suggested either acetoclastic methanogenesis or syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis, which are both consistent with isotopic analyses of CH 4 from the site (Throckmorton et al, 2015;Vaughn et al, 2016). Using reaction stoichiometry for acetoclastic methanogenesis and anaerobic respiration 15 through iron reduction (Istok et al, 2010), we estimated the amount of acetate being consumed by these parallel processes in the transition zone and permafrost (Fig.…”

The Ephemeral Signature of Permafrost Carbon in an Arctic Fluvial Network

“…Ambient δ 13 C‐DIC values from this study (−15.9 to −7.6‰) compared well to values reported from other Arctic watersheds in Alaska (approx. −20 to −8‰; Throckmorton et al, ). Results from this study show that ambient δ 13 C‐DIC was universally enriched relative to respiratory δ 13 C‐DIC (Figure a).…”