Abstract
Abstract. Thermokarst lakes play an important role in permafrost environments by warming and insulating the underlying permafrost. As a result, thaw bulbs of unfrozen ground (taliks) are formed. Since these taliks remain perennially thawed, they are zones of increased degradation where microbial activity and geochemical processes can lead to increased greenhouse gas emissions from thermokarst lakes. It is not well understood though to what extent the organic carbon (OC) in different talik layers below thermokarst lakes is affected by degradation. Here, we present two transects of short sediment cores from two thermokarst lakes on the Arctic Coastal Plain of Alaska. Based on their physiochemical properties, two main talik layers were identified. A “lake sediment” is identified at the top with low density, sand, and silicon content but high porosity. Underneath, a “taberite” (former permafrost soil) of high sediment density and rich in sand but with lower porosity is identified. Loss on ignition (LOI) measurements show that the organic matter (OM) content in the lake sediment of 28±3 wt % (1σ, n=23) is considerably higher than in the underlying taberite soil with 8±6 wt % (1σ, n=35), but dissolved organic carbon (DOC) leaches from both layers in high concentrations: 40±14 mg L−1 (1σ, n=22) and 60±14 mg L−1 (1σ, n=20). Stable carbon isotope analysis of the porewater DOC (δ13CDOC) showed a relatively wide range of values from −30.74 ‰ to −27.11 ‰ with a mean of -28.57±0.92 ‰ (1σ, n=21) in the lake sediment, compared to a relatively narrow range of −27.58 ‰ to −26.76 ‰ with a mean of -27.59±0.83 ‰ (1σ, n=21) in the taberite soil (one outlier at −30.74 ‰). The opposite was observed in the soil organic carbon (SOC), with a narrow δ13CSOC range from −29.15 ‰ to −27.72 ‰ in the lake sediment (-28.56±0.36 ‰, 1σ, n=23) in comparison to a wider δ13CSOC range from −27.72 ‰ to −25.55 ‰ in the underlying taberite soil (-26.84±0.81 ‰, 1σ, n=21). The wider range of porewater δ13CDOC values in the lake sediment compared to the taberite soil, but narrower range of comparative δ13CSOC, along with the δ13C-shift from δ13CSOC to δ13CDOC indicates increased stable carbon isotope fractionation due to ongoing processes in the lake sediment. Increased degradation of the OC in the lake sediment relative to the underlying taberite is the most likely explanation for these differences in δ13CDOC values. As thermokarst lakes can be important greenhouse gas sources in the Arctic, it is important to better understand the degree of degradation in the individual talik layers as an indicator for their potential in greenhouse gas release, especially, as predicted warming of the Arctic in the coming decades will likely increase the number and extent (horizontal and vertical) of thermokarst lake taliks.
Highlights
Thermokarst lakes are common in permafrost landscapes of Siberia, Canada, and Alaska where they regionally cover more than 40 % of the land area (Lehner and Döll, 2004)
We found the lake sediment has a soil organic carbon (SOC) source pool with a narrow δ13C range and a concurrent δ13CDOC range that is considerably wider
The taberite facies have a broad range in δ13CSOC values but a narrow δ13CDOC value range
Summary
Thermokarst lakes are common in permafrost landscapes of Siberia, Canada, and Alaska where they regionally cover more than 40 % of the land area (Lehner and Döll, 2004). The name thermokarst lake refers to the process of lake formation through thaw-induced permafrost degradation resulting in the formation of topographic depressions in previously stable ground. This process is especially effective when the permafrost soils contain high proportions of ground ice (West and Plug, 2008). Along the Beaufort Sea coast of northern Alaska, permafrost soils locally contain up to 50 % ice in soil volume (Kanevskiy et al, 2013) Upon thaw, these soils undergo substantial volume loss, causing soil collapse and large-scale subsidence. Sudden processes like lake drainage, and flooding of drained lake basins, can abruptly lead to the formation of new thermokarst lakes within days or weeks (van Huissteden et al, 2011), subjecting the permafrost landscape to constant change
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
