Role of Lithology in Methane Flux from an Alaskan Thermokarst Lake

Thermokarst lakes are prevalent forms on the Alaska North Slope (ANS) that can emit significant quantities of the greenhouse gas methane to the atmosphere. These lakes form in permafrost regions and grow as the water amplifies subsurface heating and causes further degradation of the permafrost. Areas of thawed ground (thaw-bulbs, or taliks) can develop beneath them. When a talik intersects fractures or permeable zones linked to free-gas accumulations, coal beds, or permafrost-associated gas hydrate, methane can be released from the lake. In 2009, a four-core transect extending from an active gas seep in Qalluuraq Lake (Lake Q), an Alaskan thermokarst lake, was taken for lithological analysis to complement paleoecological and biogeochemical studies investigating the current and historical flux of methane from this lake. As part of this study, we completed sedimentalogical analyses to constrain the lithology of the system and evaluate potential near-lake floor geologic controls on methane seepage at this site. Visual descriptions were completed for each core. Grain size, x-ray diffraction (XRD), petrographic, elemental (C and N) and accelerator mass spectrometer (AMS) radiocarbon analyses were also conducted on subsamples from the cores. The major lithology of all four cores is a quartz-rich, chert-bearing, medium-grained sand. Organic material, including grasses, seed pods, and reworked coal fragments, was observed in some intervals as laminae or as dispersed material. XRD and petrographic analyses indicate quartz is the dominant mineral in every core, though some of the AMS dates on organic laminae in the cores fit with paleoecological reconstructions from the site relating to wetland succession. Though the modern Lake Q is a lacustrine environment, likely formed via thermokarst processes, its underlying sediment is fluvially reworked marine sand. Our analyses indicate that these sediments are part of the Gubik Formation, a Quaternary age sedimentary formation blanketing much of the ANS and representing a variety of depositional environments, including nearshore marine, fluvial, lacustrine, eolian, thermokarst, and glacial. Methane ebullition, resulting from wetlands initiation ~12,000 cal yr BP, appears to have winnowed fine-grained organic material from the active seep sediments. The resulting coarse-grained and moderately well-sorted seep sediments are an unrestricted conduit that facilitates the gas and fluid flux to the lake and atmosphere.