Description: Subsea permafrost forms when sea level rise from deglaciation or coastal erosion results in inundation of terrestrial permafrost. The response of permafrost to flooding in these settings will be determined by both ice-rich Pleistocene deposits and the thermokarst basins that thawed out during the Holocene. Thermokarst processes lower ground ice content, create partially drained and refrozen depressions (Alases) and thaw bulbs (taliks) beneath them, warm the ground, and can thaw the ground below sea level. We hypothesize that inundated Alases offshore with relatively lower ice content and higher porewater salinities in their sediments (possibly resulting from lagoon interaction) thaw faster than Yedoma terrain. To test this hypothesis, we estimated permafrost thaw rates offshore of the Bykovsky Peninsula in Tiksi Bay, northeastern Siberia using geoelectric surveys with floating electrodes. The surveys traversed a former undrained lagoon, drained and refrozen Alas deposits, and undisturbed Yedoma terrain at varying distances from shore. A continuous Yedoma-Alas-beach-lagoon survey was also carried out to obtain an indication of pre-inundation subsurface electrical resistivity. While the estimated degradation rates of the submerged Yedoma lies in the range of similar sites, and slows with increasing distance offshore, the Alas rates were more diverse and at least twice as fast within 125 m of the coastline. The latter is possibly due to saline lagoon water that infiltrated the Alas while it was still unfrozen. The ice-bearing permafrost depths of the former lagoon were generally the deepest of the terrain units, but displayed poor correlation with distance offshore. We attribute this to heterogeneous talik thickness upon the lagoon to sea transition, as well as permafrost aggradation processes beneath the spit. Given the prevalence of thermokarst basins and lakes along parts of the Arctic coastline, their effect on subsea permafrost degradation must be similarly prevalent. Remote sensing analyses suggest that 40% of lagoons wider than 500 m originated in thermokarst basins along the pan-Arctic coast. The more rapid degradation rates shown here suggest that low-ice content conduits for fluid flow may be more common than currently thought based on thermal modelling of subsea permafrost distribution.

Description: Greenhouse gas (GHG) emissions from abrupt thaw beneath thermokarst lakes were projected to at least double radiative forcing from circumpolar permafrost-soil carbon fluxes by the end of this century, primarily through the release of methane, a much stronger GHG than CO2. Thermokarst lagoons represent the first stage of a thermokarst lake transition to a marine setting with so far neglected consequences for GHG production and release. We expected that along the transition from a thermokarst lake to a thermokarst lagoon, sediment concentrations of terminal electron acceptors like sulfate increase with an associated drop in methanogenic activity, a shift towards non-competitive methylotrophic methanogenesis, and the occurrence of sulfate-driven anaerobic methane oxidation (AOM). To explore this, we targeted a variety of geochemical and microbial parameters including sediment methane and CO2 concentrations, gaseous carbon isotopic signatures, hydrochemistry, GHG production rates, ratios of CH4/CO2, and occurrence of methane-cycling microbial taxa in sediments of two thermokarst lakes and a thermokarst lagoon on the Bykovsky Peninsula located in northeastern Siberia adjacent to Tiksi Bay. We found multiple lines of evidence that AOM in sediment layers influenced by Tiksi Bay water (i.e. the lagoon) functions as effective microbial methane filter. Annually, the lagoon is decoupled from Tiksi Bay for more than six months, resulting in more saline conditions below the ice cover compared to Tiksi Bay. Despite sub-zero near-surface sediment temperatures for approximately nine months per year, we show that, at least in early spring, AOM led to near-surface sediment methane concentrations approximating only about 1% of those measured in near-surface thermokarst lake sediments. Structural equation modelling stresses pore-water chemistry and increases in anaerobic methanotrophic abundance as main controls for the drop of in-situ methane concentrations and the corresponding increase in carbon isotopic signature. Shallow sediment layers (i.e. younger carbon) corresponded with higher rates of potential methane production, especially in the non-lagoon settings but even in the lagoon, potential methane production rates in the surface sediment layers were relatively unaffected by the marine influence. We propose that this reflects the overall dominance of non-competitive methylotrophic methanogenesis independent of pore-water chemistry and sediment depth. Overall, our study suggests that thermokarst lake to lagoon transitions have the potential to offset atmospheric methane fluxes from abrupt thaw lake structures long before thermokarst lakes fully transgress onto the Arctic shelf.

Description: Thermokarst lagoons, forming when thermokarst lakes are inundated by the sea, are an transition stage where terrestrial permafrost is introduced into the subsea realm. Here, permafrost and lacustrine carbon pools are transformed along Arctic coasts. During thaw previously frozen organic carbon can be converted into the greenhouse gases (GHG) carbon dioxide (CO2) and methane by microorganisms and leading to further climate warming. Especially for transition ecosystems like thermokarst lagoons it is largely unknown how GHG release is changing and whether thermokarst lagoons are a carbon source or sink. For getting a first glimpse of the consequences of saltwater inundation, we mimic the inundation of coastal permafrost in an experiment by incubating permafrost and thermokarst samples with artificial sea water under controlled conditions (4°C, dark, anaerobic) for 12 month. We used terrestrial samples from a 2.5 m high Yedoma outcrop, a thermokarst lake core, as well as samples from two neighboring thermokarst lagoons (a nearly-closed and a semi-closed) from the Bykovsky Peninsula, Northeast Siberia. By applying two different scenarios we aim to estimate (1) future GHG releases from newly formed Arctic lagoons by adding artificial seawater with a constant concentration and (2) the impact of increasing salinity on GHG production by incubating the samples under freshwater, brackish and marine conditions. Here we present (1) total organic carbon and dissolved organic carbon content for deep-drilled sediment cores (~ 30m) and (2) preliminary results on GHG production (methane and CO2) rates measured over 6 month. First results show that (1) GHG production is higher for inundated terrestrial sediments than for inundated lagoon sediments and (2) increasing salinity is favoring carbon dioxide production while methane production is low. In conclusion newly formed thermokarst lagoons, if upscaled to the thermokarst affected shorelines, are likely produce a significant amount of GHG under our experiment set-up.

Description: The permafrost carbon (C) pool is a major storage component of the terrestrial C cycle and it is vulnerable in a warming climate. Permafrost carbon is mobilized by different processes of thaw and erosion, including thermokarst and thermo-erosion. For example, thermokarst lagoons in the Arctic form along ice-rich permafrost coasts of Siberia, Alaska, and Canada by thaw subsidence, lake formation, and subsequent breaching by coastal erosion and marine inundation of lakes or drained lake basins. Thermokarst lagoon formation is an important step in the process of mobilizing terrestrial permafrost C pools along rapidly changing Arctic coasts. In addition, they affect the temperature and salinity of former thermokarst lake taliks during their transition to the marine environment. During current and future climate change in the Arctic, sea-level rise, accelerated permafrost thaw, intensified coastal erosion and changing sea ice regimes likely will increase the rate of thermokarst lagoon formation. Given the potentially increasing frequency of thermokarst lagoon formation and their rapid effect on permafrost degradation during the transition from a terrestrial to a marine system, it is important to understand how sedimentation regime, permafrost warming, and organic C stocks are affected during this transition. The objective of this master thesis is to asses (1) the sediment and pore water characteristics, (2) the C inventory, and (3) the spatial coverage of such thermokarst lagoon features with a multidisciplinary approach using sedimentological, hydrochemical, biogeochemical and remote sensing techniques. Samples of 30 m long sediment cores from two thermokarst lagoons on the Bykovsky Peninsula (Laptev Sea, Siberia) were analysed to characterise and quantify the C-pools as well as the sediment and pore water properties. The lagoons are examples for two different lagoon systems, an open and a semi-closed lagoon system. GIS and remote sensing tools were used to identify, map, and characterise thermokarst lagoons on a panarctic context along coasts of Siberia, Alaska, and NW Canada. The results showed that salt intrusion into sediments is higher in the open lagoon with electrical conductivities of up to 108mS/cm leading to cryotic talik formation. The total organic C density varies between 2 and 85 kg/m3 for the chosen sites, with higher values found in the class “open system lagoons”. To evaluate the larger-scale spatial relevance of this data, I identified eight lagoons along the southern Laptev Sea coast covering an area of about 18.2 x 106m2 and extrapolated my measured data on C storage to a regional level. I measured 16.5 kg/m3 as the mean for lagoon C density, which is well within the range of the terrestrial yedoma (8 kg/m³) and thermokarst (24 kg/m3) deposits in the yedoma region. Using this lagoon C density mean and the spatial coverage, I calculated 9.4 Mt C in the first 30m of southern Laptev Sea lagoon sediments, which makes it a substantial inventory of formerly frozen but now unfrozen C that has become available for microbial degradation. Along the pan-arctic coast between Taimyr Peninsula in North Siberia and Tuktoyaktuk Peninsula in Northwest Canada I mapped 690 lagoons of which 292 (42%) originated from thermokarst basins indicating the broader relevance of my findings to many regions along the Arctic coast. Thermokarst lagoons along the southern Laptev Sea coast were on average five times larger than non-thermokarst lagoons. The case study on Bykovsky provides an initial estimate of the potential contribution of this highly dynamic degradation process that combines permafrost degradation in both the marine and the terrestrial system.

Description: Thermokarst lagoons represent the transition state from a freshwater lacustrine to a marine environment, and receive little attention regarding their role for greenhouse gas production and release in Arctic permafrost landscapes. We studied the fate of methane (CH4) in sediments of a thermokarst lagoon in comparison to two thermokarst lakes on the Bykovsky Peninsula in northeastern Siberia through the analysis of sediment CH4 concentrations and isotopic signature, methane-cycling microbial taxa, sediment geochemistry, lipid biomarkers, and network analysis. We assessed how differences in geochemistry between thermokarst lakes and thermokarst lagoons, caused by the infiltration of sulfate-rich marine water, altered the microbial methane-cycling community. Anaerobic sulfate-reducing ANME-2a/2b methanotrophs dominated the sulfate-rich sediments of the lagoon despite its known seasonal alternation between brackish and freshwater inflow and low sulfate concentrations compared to the usual marine ANME habitat. Non-competitive methylotrophic methanogens dominated the methanogenic community of the lakes and the lagoon, independent of differences in porewater chemistry and depth. This potentially contributed to the high CH4 concentrations observed in all sulfate-poor sediments. CH4 concentrations in the freshwater-influenced sediments averaged 1.34 ± 0.98 μmol g−1, with highly depleted δ13C-CH4 values ranging from −89‰ to −70‰. In contrast, the sulfate-affected upper 300 cm of the lagoon exhibited low average CH4 concentrations of 0.011 ± 0.005 μmol g−1 with comparatively enriched δ13C-CH4 values of −54‰ to −37‰ pointing to substantial methane oxidation. Our study shows that lagoon formation specifically supports methane oxidizers and methane oxidation through changes in pore water chemistry, especially sulfate, while methanogens are similar to lake conditions.

Description: Lowland permafrost landscapes are experiencing dramatic changes as the climate in the Arctic has been warming almost four times the rate of the global average in the past four decades. On the Alaskan North Slope, extensive thermokarst processes are steering the dynamics of lakes and drained lake basins (DLBs). With progressing climate change, re-aggradation of permafrost in DLBs becomes potentially impeded. Additionally, along the Beaufort Sea coast, thaw-induced destabilization is causing substantial erosion, exposing previously frozen terrestrial deposits to the marine environment. The consequences for the biogeochemical system, which holds significant amounts of organic carbon, remain understudied. Therefore, we aim to investigate the carbon pool characteristics in thermokarst terrain close to Utqiaġvik. Sediment cores were sampled in 2022 and include two thermokarst lakes, one DLB and one undisturbed upland core. While West Twin Lake has freshwater conditions, East Twin Lake exhibits brackish water. The up to 2 m long sediment cores are investigated with a multidisciplinary approach. Bio- and hydrochemical analyses offer a detailed understanding of the current carbon pool properties. Additionally, n-alkane biomarker analyses, accompanied by carbon isotopy and the C/N ratio, serve as proxies to characterize the degradation state of organic carbon and its changes post permafrost thaw. Initial findings on carbon quantity and quality are presented, along with preliminary results from a 12-month-long incubation experiment. In this experiment, carbon dioxide and methane production rates are measured at ten depths along the sediment cores. The outcomes of this study contribute to a more comprehensive understanding of organic carbon degradation and its implications for the future carbon pool at a landform-specific level.

Description: The response of permafrost to submergence can vary between ice-rich late Pleistocene deposits and the thermokarst basins that thawed out during the Holocene. We hypothesize that inundated Alases offshore thaw faster than submerged Yedoma. To test this hypothesis, we estimated depths to the top of ice-bearing permafrost offshore of the Bykovsky Peninsula in northeast Siberia using electrical resistivity surveys. The surveys traversed submerged lagoon deposits, drained and refrozen Alas deposits, and undisturbed Yedoma from the coastline to 373 m offshore. While the permafrost degradation rates of the submerged Yedoma were in the range of similar sites, the submerged Alas permafrost degradation rates were up to 170% faster. Given the abundance of thermokarst basins and lakes along parts of the Arctic coastline, its effect on subsea permafrost degradation must be similarly prevalent. Remote sensing analyses suggest that 54% of lagoons wider than 500 m originated in thermokarst basins.

Description: Permafrost region subsurface organic carbon (OC) pools are a major component of the terrestrial carbon cycle and vulnerable to a warming climate. Thermokarst lagoons are an important transition stage with complex depositional histories during which permafrost and lacustrine carbon pools are transformed along eroding Arctic coasts. The effects of temperature and salinity changes during thermokarst lake to lagoon transitions on thaw history and lagoon deposits are understudied. We analyzed two 30-m-long sediment cores from two thermokarst lagoons on the Bykovsky Peninsula, Northeast Siberia, using sedimentological, geochronological, hydrochemical, and biogeochemical techniques. Using remote sensing we distinguished between a semi-closed and a nearly closed lagoon. We (1) characterized the depositional history, (2) studied the impact of marine inundation on ice-bearing permafrost and taliks, and (3) quantified the OC pools for different stages of thermokarst lagoons. Fluvial and former Yedoma deposits were found at depth between 30 and 8.5 m, while lake and lagoon deposits formed the upper layers. The electrical conductivity of the pore water indicated hypersaline conditions for the semi-closed lagoon (max: 108 mS/cm), while fresh to brackish conditions were observed beneath a 5 m-thick surface saline layer at the nearly closed lagoon. The deposits had a mean OC content of 15 ± 2 kg/m3, with higher values in the semi-closed lagoon. Based on the cores we estimated a total OC pool of 5.7 Mt-C for the first 30 m of sediment below five mapped lagoons on the Bykovsky Peninsula. Our results suggest that paleo river branches shaped the middle Pleistocene landscape followed by late Pleistocene Yedoma permafrost accumulation and early Holocene lake development. Afterward, lake drainage, marine flooding, and bedfast ice formation caused the saline enrichment of pore water, which led to cryotic talik development. We find that the OC-pool of Arctic lagoons may comprise a substantial inventory of partially thawed and partially refrozen OC, which is available for microbial degradation processes at the Arctic terrestrial-marine interface. Climate change in the Arctic leading to sea level rise, permafrost thaw, coastal erosion, and sea ice loss may increase the rate of thermokarst lagoon formation and thus increase the importance of lagoons as biogeochemical processors of former permafrost OC.