Porewater <i>δ</i>¹³C_DOC indicates variable extent of degradation in different talik layers of coastal Alaskan thermokarst lakes

• Main Authors: O. H. Meisel, J. F. Dean, J. E. Vonk, L. Wacker, G.-J. Reichart, H. Dolman
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-04-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/18/2241/2021/bg-18-2241-2021.pdf
• ISSN: 1726-4170; 1726-4189

<p>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 <span class="inline-formula">28±3</span> wt % (<span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=23</span>) is considerably higher than in the underlying taberite soil with <span class="inline-formula">8±6</span> wt % (<span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=35</span>), but dissolved organic carbon (DOC) leaches from both layers in high concentrations: <span class="inline-formula">40±14</span> mg L<span class="inline-formula">⁻¹</span> (<span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=22</span>) and <span class="inline-formula">60±14</span> mg L<span class="inline-formula">⁻¹</span> (<span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=20</span>). Stable carbon isotope analysis of the porewater DOC (<span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_DOC</span>) showed a relatively wide range of values from <span class="inline-formula">−30.74</span> ‰ to <span class="inline-formula">−27.11</span> ‰ with a mean of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">28.57</mn><mo>±</mo><mn mathvariant="normal">0.92</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="cc170526505501bc8c89012fee9729bb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2241-2021-ie00001.svg" width="70pt" height="10pt" src="bg-18-2241-2021-ie00001.png"/></svg:svg></span></span> ‰ (<span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=21</span>) in the lake sediment, compared to a relatively narrow range of <span class="inline-formula">−27.58</span> ‰ to <span class="inline-formula">−26.76</span> ‰ with a mean of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">27.59</mn><mo>±</mo><mn mathvariant="normal">0.83</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="2725490854322eebaa3bb08f0a7a4852"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2241-2021-ie00002.svg" width="70pt" height="10pt" src="bg-18-2241-2021-ie00002.png"/></svg:svg></span></span> ‰ (<span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=21</span>) in the taberite soil (one outlier at <span class="inline-formula">−30.74</span> ‰). The opposite was observed in the soil organic carbon (SOC), with a narrow <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_SOC</span> range from <span class="inline-formula">−29.15</span> ‰ to <span class="inline-formula">−27.72</span> ‰ in the lake sediment (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">28.56</mn><mo>±</mo><mn mathvariant="normal">0.36</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="527ef9eb01d30114e7d3c9d9f23fd36c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2241-2021-ie00003.svg" width="70pt" height="10pt" src="bg-18-2241-2021-ie00003.png"/></svg:svg></span></span> ‰, <span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=23</span>) in comparison to a wider <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_SOC</span> range from <span class="inline-formula">−27.72</span> ‰ to <span class="inline-formula">−25.55</span> ‰ in the underlying taberite soil (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M43" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">26.84</mn><mo>±</mo><mn mathvariant="normal">0.81</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="c52a3b77fdd7a4d588069a90fa906142"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2241-2021-ie00004.svg" width="70pt" height="10pt" src="bg-18-2241-2021-ie00004.png"/></svg:svg></span></span> ‰, <span class="inline-formula">1<i>σ</i></span>, <span class="inline-formula"><i>n</i>=21</span>). The wider range of porewater <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_DOC</span> values in the lake sediment compared to the taberite soil, but narrower range of comparative <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_SOC</span>, along with the <span class="inline-formula"><i>δ</i>¹³</span>C-shift from <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_SOC</span> to <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_DOC</span> 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 <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_DOC</span> 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.</p>