Organic carbon, mercury, and sediment characteristics along a land–shore transect in Arctic Alaska

<p>Climate warming in the Arctic results in thawing permafrost and associated processes like thermokarst, especially in ice-rich permafrost regions. Since permafrost soils are one of the largest organic carbon reservoirs of the world, their thawing leads to the release of greenhouse gases due to increasing microbial activity with rising soil temperature, further exacerbating climate warming. To enhance the predictions of potential future impacts of permafrost thaw, a detailed assessment of changes in soil characteristics in response to thermokarst processes in permafrost landscapes is needed, which we investigated in this study in an Arctic coastal lowland. We analysed six sediment cores from the Arctic Coastal Plain of northern Alaska, each representing a different landscape feature along a gradient from upland to thermokarst lake and drained basin to thermokarst lagoon in various development stages. For the analysis, a multiproxy approach was used, including sedimentological (grain size, bulk density, ice content), biogeochemical (total organic carbon (TOC), TOC density (<span class="inline-formula">TOC_vol</span>), total nitrogen (TN), stable carbon isotopes (<span class="inline-formula"><i>δ</i>¹³C</span>), TOC<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e653eaf840568ee76bb20ba3bf368ae0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2871-2025-ie00001.svg" width="8pt" height="14pt" src="bg-22-2871-2025-ie00001.png"/></svg:svg></span></span>TN ratio, mercury (Hg)), and lipid biomarker (<span class="inline-formula"><i>n</i></span>-alkanes, <span class="inline-formula"><i>n</i></span>-alkanols, and their ratios) parameters. We found that a semi-drained state of thermokarst lakes features the lowest OC content, and TOC and TN are generally higher in unfrozen deposits, hinting at a more intact state of organic matter. Indicated by the average chain length (ACL), <span class="inline-formula"><i>δ</i>¹³C</span>, <span class="inline-formula"><i>P</i>_aq</span>, and <span class="inline-formula"><i>P</i>_wax</span>, we found a stronger influence of aquatic organic matter (OM) in the OM composition in the soils covered by water compared to those not covered by water. Moreover, the results of the <span class="inline-formula"><i>δ</i>¹³C</span>, TOC<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3af55808dad7e355d8e0b0b2a0272ce7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2871-2025-ie00002.svg" width="8pt" height="14pt" src="bg-22-2871-2025-ie00002.png"/></svg:svg></span></span>TN ratio, and CPI indicate that the saline deposits contain stronger degraded OM than the deposits not influenced by saltwater. Additionally, we found positive correlations between the TOC and <span class="inline-formula">TOC_vol</span> and the Hg content in the deposits. The results indicate that thermokarst-influenced deposits tend to accumulate Hg during thawed periods and thus contain more Hg than the upland permafrost deposits that have not been impacted by lake formation. Our findings offer valuable insights into the dynamics of carbon storage and vulnerability to decomposition in coastal permafrost landscapes, reflecting the interplay of environmental factors, landform characteristics, and climate change impacts on Arctic permafrost environments.</p>