ALBERT

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: As air temperatures rise and sea ice cover declines in the Arctic, permafrost coastal cliffs thaw more rapidly and wave energy rises. Thus, as the open water season continues to lengthen, climate change triggers a large part of the Arctic shoreline to become increasingly vulnerable to erosion. Arctic erosion supplies nutrient-laden and carbon-rich sediment into nearshore ecosystems. A retreating coastline also has consequences for residential, cultural, and industrial infrastructure. Despite its importance, erosion is currently neglected in global climate models, and existing physics-based numerical models of Arctic shoreline erosion are too complex and regionally-focused to be applied on a pan-Arctic scale. Here, we apply our simplified numerical erosion model, ArcticBeach v1.0, to the entire Arctic coastline. ArcticBeach v1.0 has previously been shown to simulate retreat rates at two sites that differ substantially in their main mechanisms of retreat (sub-aerial erosion/thaw slumping versus notch/block erosion). The model uses heat and sediment volume balances in order to predict horizontal cliff retreat and vertical erosion of a fronting beach. It contains an erosion module that uses empirical equations to estimate cross-shore sediment transport, coupled to a storm surge module forced by wind. We present Arctic maps of regional variation in trends in 2-meter air temperature, sea ice concentration, and wind speed.

Description: Climate warming is particularly pronounced in the Arctic with temperatures rising twice as much as in the rest of the world. It seems natural that this warming has profound effects on the speed of erosion of Arctic coasts, since the majority consists of permafrost, composed of unlithified material and hold together by ice. Permafrost stores approximately 1307 Gt of carbon, which is almost 60 % more than currently being contained in the atmosphere. Understanding the main drivers and dynamics of permafrost coastal erosion is of global relevance, especially since floods and erosion are both projected to intensify. However, the assessment of the impacts of climate warming on Arctic coasts is impaired by little data availability. We reviewed relevant scientific literature on changing dynamics of Arctic coast, potential drivers of these changes and the impacts on the human and natural environment. We provide a comprehensive overview over the state of the art and share our thoughts on how we envision potential pathways of future Arctic coastal research. We found that the overwhelming majority of all studied Arctic coasts is erosive and that in most cases erosion rates per year are increasing, threatening coastal settlements, infrastructure, cultural sites and archaeological remains. The impacts on the natural environment are also manifold and reach from changing sediment fluxes which limit light availability in the water column to a higher input of carbon and nutrients into the nearshore zone with the potential to influence food chains.

Description: The thermal regime under lakes, ponds, and shallow near shore zones in permafrost zones in the Arctic is predominantly determined by the temperature of the overlying water body throughout the year. Where the temperatures of the water are warmer than the air, unfrozen zones within the permafrost, called taliks, can form below the water bodies. However, the presence of bottom-fast ice can decrease the mean annual bed temperature in shallow water bodies and significantly slow down the thawing or even refreeze the lake or sea bed in winter. Small changes in water level have the potential to drastically alter the sub-bed thermal regime between permafrost-thawing and permafrost-forming. The temperature regime of lake sediments is a determining factor in the microbial activity that makes their taliks hot spots of methane gas emission. Measurements of the sediment temperature below shallow water bodies are scarce, and single temperature-chains in boreholes are not sufficient to map spatial variability. We present a new device to measure in-situ temperature-depth profiles in saturated soils or sediments, adapting the functionality of classic Lister-type heat flow probes to the special requirements of the Arctic. The measurement setup consists of 30 equally spaced (5cm) digital temperature sensors housed in a 1.5 m stainless steel lance. The lance is portable and can be pushed into the sediment by hand either from a wading position, a small boat or through a hole in the ice during the winter. Measurements are taken continuously and 15 minutes in the sediment are sufficient to acquire in-situ temperatures within the accuracy of the sensors (0.01K after calibration at 0°C). The spacing of the sensors yield a detailed temperature-depth-profile of the near-surface sediments, where small-scale changes in the bottom water changes dominate the temperature field of the sediment. The short time needed for a single measurement allows for fine-meshed surveys of the sediment in areas of interest, such as the transition zone from bottom-fast to free water. Test campaigns in the Canadian Arctic and on Svalbard have proven the device to be robust in a range of environments. We present data acquired during winter and summer, covering non-permafrost, thermokarst lake and offshore measurements.

Description: The thermal regime under lakes, ponds, and shallow near shore zones in permafrost zones in the Arctic is predominantly determined by the temperature of the overlying water body throughout the year. Where the temperatures of the water are warmer than the air, unfrozen zones within the permafrost, called taliks, can form below the water bodies. However, the presence of bottom-fast ice can decrease the mean annual bed temperature in shallow water bodies and significantly slow down the thawing or even refreeze the lake or sea bed in winter. Small changes in water level have the potential to drastically alter the sub-bed thermal regime between permafrost-thawing and permafrost-forming. The temperature regime of lake sediments is a determining factor in the microbial activity that makes their taliks hot spots of methane gas emission. Measurements of the sediment temperature below shallow water bodies are scarce, and single temperature-chains in boreholes are not sufficient to map spatial variability. We present a new device to measure in-situ temperature-depth profiles in saturated soils or sediments, adapting the functionality of classic Lister-type heat flow probes to the special requirements of the Arctic. The measurement setup consists of 30 equally spaced (5cm) digital temperature sensors housed in a 1.5 m stainless steel lance. The lance is portable and can be pushed into the sediment by hand either from a wading position, a small boat or through a hole in the ice during the winter. Measurements are taken continuously and 15 minutes in the sediment are sufficient to acquire in-situ temperatures within the accuracy of the sensors (0.01K after calibration at 0°C). The spacing of the sensors yield a detailed temperature-depth-profile of the near-surface sediments, where small-scale changes in the bottom water changes dominate the temperature field of the sediment. The short time needed for a single measurement allows for fine-meshed surveys of the sediment in areas of interest, such as the transition zone from bottom-fast to free water. Test campaigns in the Canadian Arctic and on Svalbard have proven the device to be robust in a range of environments. We present data acquired during winter and summer, covering non-permafrost, thermokarst lake and offshore measurements.

Description: Organic carbon (OC) stored in Arctic continental shelf sediment is a climate-sensitive but poorly quantified component of the global carbon cycle. The current interglacial period means that most shelf permafrost, along with its OC, is currently warmer than -2 °C, and therefore susceptible to small additional warming in the near future. Estimating how much OC is potentially stored in subsea permafrost is thus key to a quantitative understanding of potential impacts of permafrost thaw on carbon mobilization in a warming Arctic. We developed a process-based model of permafrost distribution and organic matter (OM) sedimentation and decomposition to estimate the contribution of submarine permafrost to Arctic shelf organic carbon stocks. Driven by Earth System Model forcing, our model calculates 1D heat flow below the earth surface, ice caps and sea bed, and uses a reactivity continuum model of OM decomposition. We restrict our modeling to sediment that was buried within the last four glacial cycles (450 kyr), and therefore neglect OC stocks deeper than about 100 m, including any gas hydrates. Restricting OM decomposition to the liquid habitat for microbial activity in the sediment, we estimated that permafrost below the Arctic Shelf stores at least as much OC as the terrestrial counterpart at pre-industrial time, and probably in the range of twice to three times as much OC. We compared the effect of varying the OC sedimentation rates and OC reactivity. Higher reactivity in marine sediments combined with lower ice contents to increase the rate of OM decomposition, relative to sediment deposited in terrestrial settings. As a result, permafrost in our model preserved a greater proportion of marine OM from decomposition while having little effect (〈 5%) on the amount of recalcitrant terrestrial OC. These differences in sedimentation rate and reactivity influence the distribution of OC preservation on the Arctic shelf. Our modeling shows that subsea permafrost is a relevant OC stock and that more research is needed to understand microbial OM decomposition in cold but not necessarily frozen sediments. Given that deeper deposits and gas hydrates are not included, we provide conservative estimates of Arctic shelf OC stocks and suggest that the shelves have acted as long-term carbon sinks over multiple glacial--interglacial cycles.