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

  • Author(s): Throckmorton, HM
  • Heikoop, JM
  • Newman, BD
  • Altmann, GL
  • Conrad, MS
  • Muss, JD
  • Perkins, GB
  • Smith, LJ
  • Torn, MS
  • Wullschleger, SD
  • Wilson, CJ
  • et al.
Abstract

©2015. American Geophysical Union. All Rights Reserved. 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 CH4and CO2production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4in 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 CH4cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4and DIC balance by estimating mechanisms of CH4and 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 CH4production mechanism. The majority of subsurface CH4was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH4oxidation. Thus, surprisingly, CH4oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4produced 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 CH4production at the watershed scale and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH4and CO2production.

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