Porewater δ13C DOC indicates variable extent of degradation in different talik layers of coastal Alaskan thermokarst lakes

Meisel, Ove H.; Dean, Joshua F.; Vonk, Jorien E.; Wacker, Lukas; Reichart, Gert-Jan; Dolman, Han

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 inline-formula28±3 wt % (inline-formula1σ, inline-formulan=23) is considerably higher than in the underlying taberite soil with inline-formula8±6 wt % (inline-formula1σ, inline-formulan=35), but dissolved organic carbon (DOC) leaches from both layers in high concentrations: inline-formula40±14 mg Linline-formula−1 (inline-formula1σ, inline-formulan=22) and inline-formula60±14 mg Linline-formula−1 (inline-formula1σ, inline-formulan=20). Stable carbon isotope analysis of the porewater DOC (inline-formulaδ13Cinline-formulaDOC) showed a relatively wide range of values from inline-formula−30.74 ‰ to inline-formula−27.11 ‰ with a mean of inline-formula M23inlinescrollmathml - normal 28.57 ± normal 0.92 70pt10ptsvg-formulamathimgcc170526505501bc8c89012fee9729bb bg-18-2241-2021-ie00001.svg70pt10ptbg-18-2241-2021-ie00001.png  ‰ (inline-formula1σ, inline-formulan=21) in the lake sediment, compared to a relatively narrow range of inline-formula−27.58 ‰ to inline-formula−26.76 ‰ with a mean of inline-formula M28inlinescrollmathml - normal 27.59 ± normal 0.83 70pt10ptsvg-formulamathimg2725490854322eebaa3bb08f0a7a4852 bg-18-2241-2021-ie00002.svg70pt10ptbg-18-2241-2021-ie00002.png  ‰ (inline-formula1σ, inline-formulan=21) in the taberite soil (one outlier at inline-formula−30.74 ‰). The opposite was observed in the soil organic carbon (SOC), with a narrow inline-formulaδ13Cinline-formulaSOC range from inline-formula−29.15 ‰ to inline-formula−27.72 ‰ in the lake sediment (inline-formula M36inlinescrollmathml - normal 28.56 ± normal 0.36 70pt10ptsvg-formulamathimg527ef9eb01d30114e7d3c9d9f23fd36c bg-18-2241-2021-ie00003.svg70pt10ptbg-18-2241-2021-ie00003.png  ‰, inline-formula1σ, inline-formulan=23) in comparison to a wider inline-formulaδ13Cinline-formulaSOC range from inline-formula−27.72 ‰ to inline-formula−25.55 ‰ in the underlying taberite soil (inline-formula M43inlinescrollmathml - normal 26.84 ± normal 0.81 70pt10ptsvg-formulamathimgc52a3b77fdd7a4d588069a90fa906142 bg-18-2241-2021-ie00004.svg70pt10ptbg-18-2241-2021-ie00004.png  ‰, inline-formula1σ, inline-formulan=21). The wider range of porewater inline-formulaδ13Cinline-formulaDOC values in the lake sediment compared to the taberite soil, but narrower range of comparative inline-formulaδ13Cinline-formulaSOC, along with the inline-formulaδ13C-shift from inline-formulaδ13Cinline-formulaSOC to inline-formulaδ13Cinline-formulaDOC 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 inline-formulaδ13Cinline-formulaDOC 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.

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Meisel, Ove H. / Dean, Joshua F. / Vonk, Jorien E. / et al: Porewater δ13CDOC indicates variable extent of degradation in different talik layers of coastal Alaskan thermokarst lakes. 2021. Copernicus Publications.

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