Source Characterization and Temporal Variation of Methane Seepage from Thermokarst Lakes on the Alaska North Slope in Response to Arctic Climate Change

Abstract

The goals of this research were to characterize the source, magnitude and temporal variability of methane seepage from thermokarst lakes (TKL) within the Alaska North Slope gas hydrate province, assess the vulnerability of these areas to ongoing and future arctic climate change and determine if gas hydrate dissociation resulting from permafrost melting is contributing to the current lake emissions. Analyses were focused on four main lake locations referred to in this report: Lake Qalluuraq (referred to as Lake Q) and Lake Teshekpuk (both on Alaska's North Slope) and Lake Killarney and Goldstream Bill Lake (both in Alaska's interior). From analyses of gases coming from lakes in Alaska, we showed that ecological seeps are common in Alaska and they account for a larger source of atmospheric methane today than geologic subcap seeps. Emissions from the geologic source could increase with potential implications for climate warming feedbacks. Our analyses of TKL sites showing gas ebullition were complemented with geophysical surveys, providing important insight about the distribution of shallow gas in the sediments and the lake bottom manifestation of seepage (e.g., pockmarks). In Lake Q, Chirp data were limited in their capacity to image deeper sediments and did not capture the thaw bulb. The failure to capture the thaw bulb at Lake Q may in part be related to the fact that the present day lake is a remnant of an older, larger, and now-partially drained lake. These suggestions are consistent with our analyses of a dated core of sediment from the lake that shows that a wetland has been present at the site of Lake Q since approximately 12,000 thousand years ago. Chemical analyses of the core indicate that the availability of methane at the site has changed during the past and is correlated with past environmental changes (i.e. temperature and hydrology) in the Arctic. Discovery of methane seeps in Lake Teshekpuk in the northernmost part of the lake during 2009 reconnaissance surveys provided a strong impetus to visit this area in 2010. The seismic methods applied in Lake Teshekpuk were able to image pockmarks, widespread shallow gas in the sediments, and the relationship among different sediment packages on the lake's bottom, but even boomer seismics did not detect permafrost beneath the northern part of the lake. By characterizing the biogeochemistry of shallow TKL with methane seeps we showed that the radical seasonal shifts in ice cover and temperature. These seasonal environmental differences result in distinct consumption and production processes of biologically-relevant compounds. The combined effects of temperature, ice-volume and other lithological factors linked to seepage from the lake are manifest in the distribution of sedimentary methane in Lake Q during icecovered and ice-free conditions. The biogeochemistry results illustrated very active methanotrophy in TKLs. Substantial effort was subsequently made to characterize the nature of methanotrophic communities in TKLs. We applied stable isotope probing approaches to genetically characterize the methanotrophs most active in utilizing methane in TKLs. Our study is the first to identify methane oxidizing organisms active in arctic TKLs, and revealing that type I methanotrophs and type II methanotrophs are abundant and active in assimilating methane in TKLs. These organisms play an important role in limiting the flux of methane from these sites. Our investigations indicate that as temperatures increase in the Arctic, oxidation rates and active methanotrophic populations will also shift. Whether these changes can offset predicted increases in methanogenesis is an important question underlying models of future methane flux and resultant climate change. Overall our findings indicate that TKLs and their ability to act as both source and sink of methane are exceedingly sensitive to environmental change.