Radiocarbon and stable carbon isotopes to discern age and migration pathways of methane (CH4) from lakes in the Mackenzie River Delta, Northwest Territories, Canada

Hadley McIntosh Marcek, University of Maryland (UMCES CBL), Solomons, MD, United States, Ann P McNichol, Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA, United States, Scott Dallimore, Geological Survey of Canada Pacific, Sidney, BC, Canada, Lance Lesack, Simon Fraser University, Burnaby, BC, Canada, Beth Orcutt, Bigelow Laboratory for Ocean Sciences, East Boothbay, United States, Charles Geoffrey Wheat, Moss Landing Marine Laboratories, Moss Landing, United States, Jeff Chanton, Florida State University, Tallahassee, United States and Laura Lapham, UMCES, Chesapeake Bio. Lab, Solomons, United States
Abstract:
Arctic lakes emit large amounts of methane (CH4) to the atmosphere and are important in the global methane cycle. Discerning the sources of CH4 (permafrost thaw, thermogenic processes, vs. recent organic matter degradation) and understanding how it migrates to eventually be released to the atmosphere is critical. Stable carbon isotopes provide some level of information but if CH4 has undergone microbial oxidation, the results will be obfuscated. By combining radiocarbon and stable isotope measurements, ancient and modern sources can be deciphered even if some level of oxidation has occurred. In addition, applying a dual isotope approach to different pools of CH4 (dissolved and gas bubbles) provides a whole-lake look at the transport of CH4 which has rarely been done in arctic lakes. We apply this unique whole-lake approach to lakes in the Mackenzie River Delta (Northwest Territories, Canada), an excellent location to contrast the effects of geology and permafrost cover. The Mackenzie River Delta is a productive, lake-rich region with discontinuous permafrost and the outer delta overlies natural gas and oil reserves. We present radiocarbon (Δ14C-CH4) and stable isotope carbon (δ13C-CH4) values for dissolved CH4 from surface water (8 lakes) and CH4 captured in bubbles (3 lakes) to test the hypothesis that CH4 bubbling out of the lakes (ebullition) is formed from older carbon sources and CH4 dissolved in surface waters comes from the degradation of recent organic matter. To make the connection between the sediments and the surface waters, in one lake, we present a two-year continuous time-series of dissolved CH4 concentrations and δ13C-CH4 spanning both open-water and ice-cover in bottom water. Data generated thus far supports the hypothesis that CH4 diffusing out of the lakes is near-modern in age from microbial decomposition of recent organic matter, while CH4 in bubbles have significantly older precursor carbon sources or are thermogenic (radiocarbon-dead). We also find that CH4 in one of the outer delta lakes has a thermogenic source and during winter the dissolution of bubbles trapped under the ice increases CH4 concentrations in the lake water. Taken together, results from this study expand our knowledge of CH4 source and migration pathways within an important Arctic delta.
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