Radiocarbon and stable carbon isotopes of methane and dissolved inorganic carbon reveal mixing and dissolution of thermogenic methane from bubbles seeping into an Arctic lake under ice through winter
Abstract
Lakes can act as conduits for methane (CH4) and carbon dioxide gas seepage from subsurface hydrocarbon reservoirs buried deep within the Earth's surface. Within the Arctic, the release of thermogenically produced gases is evident where the permafrost is thinner (<50 m) and lakes form taliks. Thereby, lake sediments can allow gases to be rapidly released into overlying lakes and eventually to the atmosphere. Yet, when the lakes ice over in winter, gas can collect under the ice and dissolve into the surrounding lake. Here, we wanted to capture such physical processes and compare to microbial, biogeochemical processes occurring in lake sediments that also produce these gases. Our study lake lies in the Mackenzie Delta (Canada) region where high fluxes of CH4 to the atmosphere have been measured and has known bubble seeps. To determine if the seep gas dissolves into the water when the lake is ice covered during winter, we deployed autonomous instrument packages adjacent to a bubble seep and away from the seep. Packages were outfitted with OsmoSamplers, which are osmotically powered pumps that continuously collect water over time, and sensors, e.g., temperature and dissolved oxygen (DO). Deployments resulted in a two-year continuous time-series of dissolved CH4 concentrations, dissolved inorganic carbon (DIC) concentrations, δ13C-CH4, and δ13C-DIC spanning open-water and ice-cover in bottom water. We also report radiocarbon (Δ14C) ages of dissolved and gaseous CH4 to decipher ancient and modern sources within the different pools. Dissolved CH4 concentrations were strongly linked to DO presence, and during ice-cover dissolved CH4 composition shifted from a microbial, diffusive source with 13C-depleted CH4 to a thermogenic source with 13C-enriched CH4 from bubble dissolution. DIC concentrations steadily increased during ice-cover, while δ13C-DIC values decreased. Data generated thus far support our initial hypothesis that CH4 diffusing out of the lakes is near-modern in age from microbial decomposition of recent organic matter, while CH4 in bubbles is ancient and formed from thermogenic processes. Taken together, results from this study will expand our knowledge of gas migration pathways within an Arctic region affected by heterogeneous thermogenic gas release to the atmosphere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMOS0430001M
- Keywords:
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- 3002 Continental shelf and slope processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3045 Seafloor morphology;
- geology;
- and geophysics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3050 Ocean observatories and experiments;
- MARINE GEOLOGY AND GEOPHYSICS