Unveiling Permafrost Transformations: Investigating Organic Carbon Characteristics and Dynamics in Alaskan Lowland Landscapes

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

Organic Carbon Characteristics and Dynamics in Thermokarst Terrain on the Alaskan North Slope

Thermokarst processes have been accelerating since the 1950s in the Alaskan tundra (Chen et al., 2021; Jorgenson et al., 2006) which corresponds to warming permafrost temperatures (Biskaborn et al., 2019) and a disproportional warming climate of the Arctic region (Rantanen et al., 2022). On the Alaskan North Slope, thermokarst is steering the dynamics of thermokarst lakes and drained lake basins (DLBs; Jones et al., 2022), thereby thawing, mobilizing, and sequestering organic carbon. The consequences for the biogeochemical system, which holds significant amounts of organic carbon (Palmtag et al., 2022), remain understudied. In particular, the quality of organic carbon is an important factor for the mobilization potential and rates of release as greenhouse gases (Jongejans et al., 2021). In our study, we aim to investigate the soil organic carbon pool characteristics in a thermokarst terrain close to Utqiaġvik, Alaska

Microbial methane cycling in sediments of Arctic thermokarst lagoons

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

Thermokarst Lagoons - Carbon Pools and Panarctic Distribution

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

Spatial extent of thermokarst lagoons along pan-Arctic coasts - an upscaling approach

Along the ice rich pan-Arctic permafrost coasts thermokarst lagoons are a common landscape feature. These lagoons form when thermokarst lakes are inundated permanently or intermittently by the sea. This is the first estimation of the area of pan-Arctic thermokarst lagoons based on the mapping of 79 lagoons in 5 representative arctic regions: Mackenzie Delta (CA), Theshekpuk Lake coast (USA), Baldwin Peninsula (USA), Tiksi coast (RU), Lena Delta (RU). The extent of each of the lagoons was determined using the Global Surface Water dataset which is based on Landsat-5, -7, and -8 satellite images from 1984 to 2018 at 30m resolution (Pekel et al., 2016). Water bodies were defined by a water occurrence threshold of >75% over this time period. The raster dataset was vectorized and smaller geometric errors, which occurred during vectorization, were solved with the Fix Geometry function in QGIS3.6. The lagoon polygons were selected manually and these water bodies were split from the ocean by using the function “split by line”. The calculation of the polygon area is based on the re-projection in EPSG:32608, EPSG:26905, EPSG:32604, EPSG:32652 for Mackenzie Delta, Teshekpuk Lake coast, Baldwin Peninsula, Tiksi and Lena Delta coast respectively. The lagoon selection is based on the published dataset https://doi.org/10.1594/PANGAEA.934158. The dataset consists of a polygon shape file for the 79 extracted thermokarst lagoons, a point shape file with coordinates for all lagoons and a data sheet

How is permafrost carbon affected by seawater inundation? - Estimating greenhouse gas production in thermokarst lagoons of Bykovsky Peninsula, Siberia

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

Thermokarst primes subsea permafrost degradation and coastal change

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