Thermokarst Processes Research Articles

Temperature sensitivity of the mineral permafrost feedback at the continental scale.

Oxidative weathering of sulfide minerals in sedimentary rocks releases carbon dioxide (CO2) into the atmosphere. In permafrost zones, this could be a positive feedback on climate change if it increases with warming, yet sulfide oxidation rates and their temperature response remain unknown over large spatial and temporal scales. We analyze a 60-year sulfate concentration dataset from catchments across the Mackenzie River Basin. Sulfate fluxes increased by 45% in the mainstem with 2.3°C of warming, and the temperature sensitivity suggests that continental-scale CO2 fluxes could double by 2100. The largest increases occur in catchments with geomorphic settings which act to rapidly expose rocks through physical weathering and thermokarst processes. Comparisons with a weathering model suggest that warming can increase reaction rates, and changes in the exposure of minerals with warming are also required. Future warming across vast Arctic landscapes could further increase sulfide oxidation rates and affect regional carbon cycle budgets.

Thermokarst landscape exhibits large nitrous oxide emissions in Alaska’s coastal polygonal tundra

Global atmospheric concentrations of nitrous oxide have been increasing over previous decades with emerging research suggesting the Arctic as a notable contributor. Thermokarst processes, increasing temperature, and changes in drainage can cause degradation of polygonal tundra landscape features resulting in elevated, well-drained, unvegetated soil surfaces that exhibit large nitrous oxide emissions. Here, we outline the magnitude and some of the dominant factors controlling variability in emissions for these thermokarst landscape features in the North Slope of Alaska. We measured strong nitrous oxide emissions during the growing season from unvegetated high centered polygons (median (mean) = 104.7 (187.7) µg N2O-N m−2 h−1), substantially higher than mean rates associated with Arctic tundra wetlands and of similar magnitude to unvegetated hotspots in peat plateaus and palsa mires. In the absence of vegetation, isotopic enrichment of 15N in these thermokarst features indicates a greater influence of microbial processes, (denitrification and nitrification) from barren soil. Findings reveal that the thermokarst features discussed here (~1.5% of the study area) are likely a notable source of nitrous oxide emissions, as inferred from chamber-based estimates. Growing season emissions, estimated at 16 (28) mg N2O-N ha−1 h−1, may be large enough to affect landscape-level greenhouse gas budgets.

A new inventory and future projections of thermokarst lakes in the permafrost regions of the Qilian Mountains, northeastern Qinghai-Tibet Plateau, China

Thermokarst lakes—lacustrine basins formed by thermokarst processes—are key components of permafrost ecosystems that significantly influence the geomorphology, hydrology, ecology, and the stability of infrastructure, while also contributing to greenhouse gas emissions. High-resolution mapping of thermokarst lakes' current and future distribution presents a considerable challenge. In this study, we leverage a deep learning approach to delineate the boundaries of thermokarst lakes in the permafrost regions of the Qilian Mountains at the northeastern margin of the Qinghai-Tibet Plateau (QTP) using Sentinel-2 imagery, followed by an estimation of future susceptibility to thermokarst lake formation by employing a machine learning method. We identified a total of 2795 thermokarst lakes covering an area of approximately 51.6 km2. Based on future climate scenarios, there will be a significant reduction in the proportion of high-susceptibility areas. Specifically, under the most extreme SSP5–8.5 scenario, the proportion of areas highly susceptible to thermokarst lake formation decreases from the current 6.7 % to 0.01 % in the period 2081–2100. Our comprehensive inventory provides essential data for research on permafrost hydrology and the evaluation and management of water resources, and enhances our understanding of thermokarst lake formation in relation to environmental factors.

International monitoring of permafrost

The international community has taken upon itself the responsibility for the condition of the environment as a human habitat. International conventions draw attention of governments to the pollution and the deterioration of the environment as a result of economic and other human activity. Particular attention is paid to issues of climate change and sustainable development, emissions of greenhouse gases into the atmosphere are taken under government control. According to research data, at the present stage there is an onset of a thawing process of not just Arctic ice, but also of permafrost. The thawing of permafrost on one hand is a result of climate warming, on the other hand it will inevitably lead to emission of greenhouse gases into the atmosphere and become a source (cause) of climate change. Permafrost covers the territories of northern parts of America, Europe, Asia, islands of the Arctic Ocean, and the Antarctica. The paper argues for the necessity of research on the thermokarst processes on the basis of international cooperation, the development of an international permafrost monitoring network. Establishing the principles of organization of international cooperation, the funding and the creation of a permafrost monitoring network in international agreements will become an another step towards ensuring the environmental security of our planet. Research and reproduction of scientific knowledge were conducted along the spectrum of related fields of international and environmental law and areas of legal regulation, and also at the crossing of law and fields of permafrost studies, geology, ecology, biology, environmental protection.

О ПРОИСХОЖДЕНИИ ГРИВНО-ЛОЖБИННОГО РЕЛЬЕФА ЮГА ЗАПАДНОЙ СИБИРИ

In the south of Western Siberia, predominantly in the territories of the Ishim Plain and Baraba Lowland, significant areas areoccupied by complexes of ridges and inter-ridge depressions elongated in a north-easterly direction. In the literature such forms of relief are called‘griva’ (ridges), and the type of relief – ridge-hollow.The key question in the study of these geomorphological complexes is related to their genesis. Among the numerous views on the originof these forms, there are two most popular groups of hypotheses, opposite in terms of the prevailing forming process they indicate, — aeolian andaqueous. Proponents of the first one insist on the decisive role of wind accumulation; the second one, omitting the variability of paleogeographicalconditions, considers all possible factors associated with the impact of water: accumulation, erosion, their combined effect, etc. It is worth notingthat many hypotheses either lack palaeogeographic references or are outdated. By analyzing the accumulated materials about this problem, as well as by comparing the ridge complexes with morphologically similarforms in other regions, we revealed discrepancies in the lithological description and morphology of ridge-hollow complexes within different hypotheses. The differences in a number of characteristics of individual ridge complexes have also been pointed out.Basing on the modern ideas about the paleogeographical conditions of the region, the identified differentiation of the ridge-hollow complexes, and taking into account the revealed discrepancies, we propose the polygenetic hypothesis to solve the problem of the genesis of the ridges,according to which the formation of the loessoid deposits, which make up the ridges, begins in the Middle Sartanian time (about 19 thousand yearsago) and occurs in the cryo-arid conditions. An arid climate leads to the development of active aeolian activity within drained lakes and shallowriver valleys; in addition, active cryogenic weathering leads to the grinding of material to the silty fraction, characteristic of the loessoids in thesouth of Western Siberia. The formation of dune relief is a consequence of these processes. The created ridge forms continue to be influenced bypermafrost processes. A further tendency toward climate mitigation, consisting primarily in increased wetness, causes the development of thermokarst processes and waterlogging of ridges located in lake hollows and flood plains of large river valleys. These transformations have a modelingeffect, smoothing out the shapes of the ridges and gradually bringing them closer to their modern outlines. The result is ridge-hollow complexes.

The first 10 m resolution thermokarst lake and pond dataset for the Lena Basin in the 2020 thawing season

ABSTRACT Climate warming rates in the Arctic are far greater than the global average, exerting stronger impacts on permafrost degradation and thermokarst landform development. Thermokarst lakes and ponds (TLPs), which are widely distributed in the Lena Basin in the Russian Arctic, play a vital role in altering local ecosystem. However, the detailed distribution of TLPs in the Lena Basin still remains poorly known. In this study, we built the first 10 m resolution TLP dataset for the Lena Basin in the 2020 thawing season by utilizing 4902 Sentinel-2 images. A robust mapping workflow was developed and implemented in the Google Earth Engine (GEE) platform. The accuracy assessment demonstrates a satisfactory accuracy (93.63%), and our results exhibit a better consistency with real TLPs than global water body products. A total of 380,477 TLPs (~0.53% of the total surface area of the Lena Basin) were identified, showing an uneven distribution in the five sub-basins. The TLPs were found to be mainly located within plain areas, with an active layer thickness in the range of 80–100 cm. The higher ground ice content and mean annual ground temperature were favorable for TLP development. This dataset will be valuable for investigating the complex interaction between TLPs and permafrost. It will also serve as a baseline product for better incorporating thermokarst processes into permafrost-climate models.

Differentiation of cognate bacterial communities in thermokarst landscapes: implications for ecological consequences of permafrost degradation

Abstract. Thermokarst processes likely result in new habitats harboring novel bacterial communities in degraded permafrost soil (PB), thermokarst lake sediments (SB), and lake water (WB). Our study aimed to investigate the paired PB, SB, and WB across the Qinghai–Tibet Plateau (QTP) by assessing the spatial pattern of diversity as well as assembly mechanisms of these bacterial communities. Each habitat had distinct bacterial assemblages, with lower α diversity and higher β diversity in WB than in SB and PB. However, up to 41 % of the operational taxonomic units (OTUs) were shared by PB, SB, and WB, suggesting that many taxa originate from the same sources via dispersal. SB and WB had reciprocal dispersal effects, and both were correlated with PB. Dispersal limitation was the most dominant assembly process shaping PB and SB, while homogeneous selection was the most dominant for WB. Bacterial communities of the three habitats correlated differently with environmental variables, but latitude, mean annual precipitation, and pH were the common factors associated with their β diversity, while total phosphorus was the common factor associated with their assembly processes. Our results imply that thermokarst processes result in diverse habitats that have distinct bacterial communities that differ in diversity, assembly mechanisms, and environmental drivers.

Thermokarst Lake Susceptibility Assessment Induced by Permafrost Degradation in the Qinghai–Tibet Plateau Using Machine Learning Methods

The rapidly warming climate on the Qinghai–Tibet Plateau (QTP) leads to permafrost degradation, and the thawing of ice-rich permafrost induces land subsidence to facilitate the development of thermokarst lakes. Thermokarst lakes exacerbate the instability of permafrost, which significantly alters regional geomorphology and hydrology, affecting biogeochemical cycles. However, the spatial distribution and future changes in thermokarst lakes have rarely been assessed at large scales. In this study, we combined various conditioning factors and an inventory of thermokarst lakes to assess the spatial distribution of susceptibility maps using machine-learning algorithms. The results showed that the extremely randomized trees (EXT) performed the best in the susceptibility modeling process, followed by random forest (RF) and logistic regression (LR). According to the assessment based on EXT, the high- and very high-susceptibility area of the present (2000–2016) susceptibility map was 196,222 km2, covering 19.67% of the permafrost region of the QTP. In the future (the 2070s), the area of the susceptibility map was predicted to shrink significantly under various representative concentration pathway scenarios (RCPs). The susceptibility map area would be reduced to 37.06% of the present area in RCP 8.5. This paper also performed correlation and importance analysis on the conditioning factors and thermokarst lakes, which indicated that thermokarst lakes tended to form in areas with flat topography and high soil moisture. The uncertainty of the susceptibility map was further assessed by the coefficient of variation (CV). Our results demonstrate a way to study the spatial distribution of thermokarst lakes at the QTP scale and provide a scientific basis for understanding thermokarst processes in response to climate change.

First Quantification of the Permafrost Heat Sink in the Earth's Climate System

AbstractDue to an imbalance between incoming and outgoing radiation at the top of the atmosphere, excess heat has accumulated in Earth's climate system in recent decades, driving global warming and climatic changes. To date, it has not been quantified how much of this excess heat is used to melt ground ice in permafrost. Here, we diagnose changes in sensible and latent ground heat contents in the northern terrestrial permafrost region from ensemble‐simulations of a tailored land surface model. We find that between 1980 and 2018, about ZJ of heat, of which ZJ (44%) were used to melt ground ice, were absorbed by permafrost. Our estimate, which does not yet account for the potentially increased heat uptake due to thermokarst processes in ice‐rich terrain, suggests that permafrost is a persistent heat sink comparable in magnitude to other components of the cryosphere and must be explicitly considered when assessing Earth's energy imbalance.

The role of dead ice in transforming glacier forelands under the rapid climate warming of recent decades, OscarIILand, Svalbard

AbstractThe rapid climate changes of recent decades are causing rapid glacier recession in the high Arctic and the loss of large patches and blocks of dead ice. Their temporally differentiated melting is very significantly transforming the relief of marginal zones, creating dead‐ice landscapes. The article focuses on dead‐ice degradation processes in the forelands of two glaciers (Erikkabreen and Haakenbreen) on Oscar II Land in Spitsbergen. Detailed in‐field geomorphological mappings have been conducted twice (in 1989 and 2022) in the two glaciers' forelands. These were combined with analysis of available photogrammetry and Digital Elevation Models, allowing the authors to describe the dynamics of dead‐ice melting processes within various landforms and the effect of those processes on the forms' ultimate morphology. This provided a broad view of the trajectory of melt processes depending on local geomorphological and hydrological conditions. Particular attention was paid to the interactions between the evolution of permafrost and the course of melting of dead ice. In light of the highly dynamic thermokarst processes in the forelands of Svalbard glaciers, the term ‘dead‐ice‐rich permafrost’ is proposed. It was found that global climate changes are increasing the role of dead ice in transforming the relief of glacier forelands, thereby increasing the occurrence of dead‐ice landscapes. The conclusions confirmed those for other contemporary glaciation areas and may significantly help in interpreting the morphogenesis of glacial relief formed during past continental glaciations.

Developing the Model of the Morphological Pattern for Thermokarst Plains with Fluvial Erosion Involving Remote Sensing Data

The paper reports a new variant of the model of the morphological pattern for thermokarst plains with fluvial erosion involving numerous remote sensing data. This variant of the model takes into account different trends of the thermokarst processes for watershed plains and lowered bottoms of khasyreis (drained thermokarst lakes). The given model is tested at eight key sites located in different physiography, climate and geocryology. Testing was based on the high-resolution remote sensing data. The developed model is verified with a statistical analysis of lake area distributions within both watershed plain surface and lowered bottoms of khasyreis, demonstrating a state of dynamic balance in the course of emergence, growth and drainage of the thermokarst lakes. The analysis shows that the morphological pattern of the thermokarst plains with fluvial erosion changes evolutionarily, with reducing the area of the main watershed surface according to a dependence close to exponential; the specific type of the dependence results primarily from the ratio of the lake generation density and the location density of the fluvial sources.

Bedfast and floating-ice dynamics of thermokarst lakes using a temporal deep-learning mapping approach: case study of the Old Crow Flats, Yukon, Canada

Abstract. In light of the recent climate warming, monitoring of lake ice in Arctic and subarctic regions is becoming increasingly important. Many shallow Arctic lakes and ponds of thermokarst origin freeze to the bed in the winter months, maintaining the underlying permafrost in its frozen state. However, as air temperatures rise and precipitation increases, fewer lakes are expected to develop bedfast ice. In this work, we propose a novel temporal deep-learning approach to lake ice regime mapping from synthetic aperture radar (SAR) and employ it to study lake ice dynamics in the Old Crow Flats (OCF), Yukon, Canada, over the 1992/1993 to 2020/2021 period. We utilized a combination of Sentinel-1, ERS-1 and ERS-2, and RADARSAT-1 to create an extensive annotated dataset of SAR time series labeled as either bedfast ice, floating ice, or land, which was used to train a temporal convolutional neural network (TempCNN). The trained TempCNN, in turn, allowed us to automatically map lake ice regimes. The classified maps aligned well with the available field measurements and ice thickness simulations obtained with a thermodynamic lake ice model. Reaching a mean overall classification accuracy of 95 %, the TempCNN was determined to be suitable for automated lake ice regime classification. The fraction of bedfast ice in the OCF increased by 11 % over the 29-year period of analysis. Findings suggest that the OCF lake ice dynamics are dominated by lake drainage events, brought on by thermokarst processes accelerated by climate warming, and fluctuations in water level and winter snowfall. Catastrophic drainage and lowered water levels cause surface water area and lake depth to decrease and lake ice to often transition from floating to bedfast ice, while a reduction in snowfall allows for the growth of thicker ice. The proposed lake ice regime mapping approach allowed us to assess the combined impacts of warming, drainage, and changing precipitation patterns on transitions between bedfast and floating-ice regimes, which is crucial to understanding evolving permafrost dynamics beneath shallow lakes and drained basins in thermokarst lowlands such as the OCF.

Strontium isotopes trace the dissolution and precipitation of mineral organic carbon interactions in thawing permafrost

Interactions between minerals and organic carbon (OC) in soils are key to stabilize OC and mitigate greenhouse gas emissions upon permafrost thaw. However, changes in soil water pathways upon permafrost thaw are likely to affect the stability of mineral OC interactions by inducing their dissolution and precipitation. This study aims to assess and quantify how mineral OC interactions are affected by dissolution and precipitation in thawed relative to unthawed layers. We hypothesize that a change in the radiogenic strontium (Sr) isotopic ratio (87Sr/86Sr) involved in mineral OC interactions upon changing water saturation conditions implies a destabilization of the mineral OC interaction. We quantified mineral OC interactions using selective extractions in soils facing gradual thaw (Eight Mile Lake, AK, USA) and in sediments with a thawing history of abrupt thaw (Duvanny Yar, Russia), and we measured the 87Sr/86Sr ratio of the selective extracts targeting the Sr associated to mineral OC interactions. Firstly, for water saturated layers with a higher proportion of mineral OC interactions, we found a difference in the 87Sr/86Sr ratio relative to the surrounding layers, and this supports the preservation of a Sr “stable” pool in these mineral OC interactions. We estimated that a portion of these mineral OC interactions have remained undissociated since their formation (between 4% and 64% by Sr isotope mass balance). Secondly, we found no difference in 87Sr/86Sr ratio between layers accumulating Fe oxides at redox interfaces regularly affected by water table changes (or upon thermokarst processes) relative to surrounding layers. This supports the dominance of a Sr “labile” pool inherited from processes of dissolution and precipitation of the mineral OC interactions. Thirdly, our estimations based on a Sr isotope mass balance support that, as a consequence of permafrost thaw, a larger proportion of Sr from primary mineral weathering (>80%) controls the Sr in mineral OC interactions in the saturated zone of deeply thawed soils relative to poorly thawed soils (∼50%). In conclusion, we found that the radiogenic Sr isotope method, applied for the first time in this context, is promising to trace dissolution-precipitation processes of mineral OC interaction in thawing permafrost.

Automated Identification of Thermokarst Lakes Using Machine Learning in the Ice-Rich Permafrost Landscape of Central Yakutia (Eastern Siberia)

The current rate and magnitude of temperature rise in the Arctic are disproportionately high compared to global averages. Along with other natural and anthropogenic disturbances, this warming has caused widespread permafrost degradation and soil subsidence, resulting in the formation of thermokarst (thaw) lakes in areas of ice-rich permafrost. These lakes are hotspots of greenhouse gas emissions (CO2 and CH4), but with substantial spatial and temporal heterogeneity across Arctic and sub-Arctic regions. In Central Yakutia (Eastern Siberia, Russia), nearly half of the landscape has been affected by thermokarst processes since the early Holocene, resulting in the formation of more than 10,000 partly drained lake depressions (alas lakes). It is not yet clear how recent changes in temperature and precipitation will affect existing lakes and the formation of new thermokarst lakes. A multi-decadal remote sensing analysis of lake formation and development was conducted for two large study areas (~1200 km2 each) in Central Yakutia. Mask Region-Based Convolutional Neural Networks (R-CNN) instance segmentation was used to semi-automate lake detection in Satellite pour l’Observation de la Terre (SPOT) and declassified US military (CORONA) images (1967–2019). Using these techniques, we quantified changes in lake surface area for three different lake types (unconnected alas lake, connected alas lake, and recent thermokarst lake) since the 1960s. Our results indicate that unconnected alas lakes are the dominant lake type, both in the number of lakes and total surface area coverage. Unconnected alas lakes appear to be more susceptible to changes in precipitation compared to the other two lake types. The majority of recent thermokarst lakes form within 1 km of observable human disturbance and their surface area is directly related to air temperature increases. These results suggest that climate change and human disturbances are having a strong impact on the landscape and hydrology of Central Yakutia. This will likely affect regional and global carbon cycles, with implications for positive feedback scenarios in a continued climate warming situation.

Transient Freeze‐Thaw Deformation Responses to the 2018 and 2019 Fires Near Batagaika Megaslump, Northeast Siberia

AbstractWildfires in Arctic regions impact landforms via permafrost degradation and subsequent deformation that can last for many years. However, it remains uncertain on if and how much deformations occur, and what controls their magnitude, particularly during the first couple of years. Here, we examine the transient post‐fire deformation responses near the Batagaika megaslump, which is the world's largest retrogressive thaw slump at Batagay, Sakha Republic. There were wildfires in the summers of 2018 and 2019 on the same slope, which could trigger the formation of another megaslump; many fires occurred nearby in 2019. We use interferometric synthetic aperture radar (InSAR) to measure surface displacements, including both post‐fire and span‐fire images. We also perform onsite measurements of temperature and thaw depth around the two scars near Batagaika megaslump in 2019, 2020, and 2021 and around the 2014 scar in 2019. At the three fire scars formed in 2018 and 2019, we demonstrate year‐to‐year and location‐specific changes in the amplitude of subsidence, heave, and duration. The 2018 scar shows cumulative subsidences of up to 10 cm by March 2021, more clearly than the nearby 2019 scar. On the other hand, another 2019 scar adjacent to the 2014 scar shows up to 13 cm net subsidence during the first span‐fire year, although the subsiding area is limited. These diverse transient post‐fire responses demonstrate that under the yedoma area the spatial heterogeneities of the active layer depth and the timing of fires will control subsequent thermokarst processes.

Lithostratigraphy and Chronology of Vistulian Periglacial Deposits in Józefów (Central Poland) Based on Luminescence Dating

Abstract Sediments in Józefów were deposited in a small lake in the Eemian Interglacial and Early Vistulian (MIS 5). They were then covered by periglacial mineral sediments during the Upper Pleniglacial and the Late Vistulian (MIS 2). These sediments were deformed by gelifluction and thermokarst processes. There is no traces of deposition from MIS 4 and MIS 3. Optically stimulated luminescence (OSL) dating confirmed the age of the sediments and enabled the reconstruction of the deposition phases and deformation of the lake infilling.

From permafrost soil to thermokarst lake sediment: A view from C:N:P stoichiometry

Thermokarst lakes are formed as a result of thawing ice-rich permafrost, transforming vast permafrost soil into lake sediment and changing the biogeochemistry of carbon (C), nitrogen (N), and phosphorus (P). Degraded permafrost soil and thermokarst lake sediment are two distinct fates of pristine permafrost in the thermokarst processes. However, we do not clearly understand the differences and relationships between degraded permafrost soil and thermokarst lake sediment from a stoichiometric perspective. In this study, 44 thermokarst lakes across the Qinghai-Tibet Plateau were investigated to collect lake sediment and surrounding degraded permafrost soil. In general, C, N, and P concentrations as well as C:N, C:P, and N:P ratios in soil and sediment decreased with increasing latitude, while increased with increasing mean annual precipitation. The degraded permafrost soil had much higher C, N, and P concentrations and C:N:P stoichiometric ratios than the lake sediment, particularly for C. Moreover, the concentrations of C, N, and P, as well as the ratios of C:P and N:P in sediment showed significant positive relationships with their corresponding components in soil but with different slopes. Standard major axis regression showed allometric scaling relationships between C, N, and P. The C:N:P ratio was 269:18:1 in degraded permafrost soil and 178:15:1 in lake sediment. The results suggest that the process from pristine permafrost to lake sediment releases more C, N, and P than the process from pristine permafrost to degraded permafrost soil, and that C changes more profoundly than N and P. Moreover, thermokarst processes substantially change the elemental balance and decouple the C:N:P relationship between degraded permafrost soil and lake sediment, suggesting that the further transformation from degraded permafrost soil to lake sediment will lose more C, which can be intensified by increasing precipitation. The results enriched our understanding of the variations in C, N, and P biogeochemistry during thermokarst processes.

Thermokarst processes increase the supply of stabilizing surfaces and elements (Fe, Mn, Al, and Ca) for mineral–organic carbon interactions

AbstractThe stabilizing properties of mineral–organic carbon (OC) interactions have been studied in many soil environments (temperate soils, podzol lateritic soils, and paddy soils). Recently, interest in their role in permafrost regions is increasing as permafrost was identified as a hotspot of change. In thawing ice‐rich permafrost regions, such as the Yedoma domain, 327–466 Gt of frozen OC is buried in deep sediments. Interactions between minerals and OC are important because OC is located very near the mineral matrix. Mineral surfaces and elements could mitigate recent and future greenhouse gas emissions through physical and/or physicochemical protection of OC. The dynamic changes in redox and pH conditions associated with thermokarst lake formation and drainage trigger metal‐oxide dissolution and precipitation, likely influencing OC stabilization and microbial mineralization. However, the influence of thermokarst processes on mineral–OC interactions remains poorly constrained. In this study, we aim to characterize Fe, Mn, Al, and Ca minerals and their potential protective role for OC. Total and selective extractions were used to assess the crystalline and amorphous oxides or complexed metal pools as well as the organic acids found within these pools. We analyzed four sediment cores from an ice‐rich permafrost area in Central Yakutia, which were drilled (i) in undisturbed Yedoma uplands, (ii) beneath a recent lake formed within Yedoma deposits, (iii) in a drained thermokarst lake basin, and (iv) beneath a mature thermokarst lake from the early Holocene period. We find a decrease in the amount of reactive Fe, Mn, Al, and Ca in the deposits on lake formation (promoting reduction reactions), and this was largely balanced by an increase in the amount of reactive metals in the deposits on lake drainage (promoting oxidation reactions). We demonstrate an increase in the metal to C molar ratio on thermokarst process, which may indicate an increase in metal–C bindings and could provide a higher protective role against microbial mineralization of organic matter. Finally, we find that an increase in mineral–OC interactions corresponded to a decrease in CO2 and CH4 gas emissions on thermokarst process. Mineral–OC interactions could mitigate greenhouse gas production from permafrost thaw as soon as lake drainage occurs.

Evidence for thermokarst depressions on comet 67P/Churyumov-Gerasimenko: An interplanetary comparison

Aims. The nucleus of comet 67P/Churyumov-Gerasimenko (67P) is ice rich and shows evidence of morphologies driven by sublimation processes. In particular, the fine-particle deposits (FPD) that cover its surface reveal depressions of many tens of meters. A detailed analysis of these morphologies and of the properties of the fine-particle deposits could help to constrain the amount of water ice in the subsurface. The aim of this paper is to characterize these depressions and constrain their formation and evolution by a detailed quantitative study of their morphometry and thermal environment, and through a comparison with similar morphologies on Earth and Mars. Methods. To study the cometary depressions, we constructed a database of morphometrical parameters for 131 depressions on 67P, 50 alases on Earth, and 200 scalloped depressions on Mars. We measured these parameters manually with the ArcGIS software. We used the images of the Narrow Angle Camera for 67P (down to a resolution of 1 m pixel−1), of the USGC-Digital Orthophoto Quadrangle for the Earth (5 m pixel−1), and of the High Resolution Imaging Science Experiment for Mars (25 cm pixel−1). We also used the digital elevation models associated with these images. In addition, we performed a study of the thermal environment of each depression on 67P to estimate the maximum temperature, the total erosion, and the total energy received from the Sun during one revolution. The morphometrical and thermal environment parameters were then used together for a comparative study of the depressions on 67P, the Earth, and Mars. Results. Our results show that depressions on 67P are geological analogs to alases on Earth and scalloped depressions on Mars; we call them cometary thermokarst depressions. They have been formed by a thermokarst process (i.e., permafrost thawing that causes a subsidence) triggered by water-ice sublimation. The cometary thermokarst depressions on 67P are distributed throughout the nucleus, without preferential distribution for a particular hemisphere or region. They are exclusively located in the fine-particle deposit layer, which is a few meters thick and is thought to have to contain a volume fraction of water ice of about 50% for the subsidence to occur. The formation and evolution of the depressions is driven by the Sun; the side facing the dominant insulation is the preferential direction of erosion. Our results contribute to a better understanding of the periglacial system on comet 67P and of the erosion processes on the nucleus.

Chronostratigraphic and geomorphologic challenges of last glacial loess in Poland in the light of new luminescence ages

Abstract. The aim of this study is to check the validity of luminescence ages obtained from last glacial–interglacial Polish loess palaeosol sequences (LPSs) by several established current protocols, with respect to sound geomorphological and chronostratigraphic interpretations. We report 38 new optically stimulated luminescence (OSL) ages from fine-grained (4–11 µm) quartz separates extracted from four loess palaeosol sequences in Poland, measured in the Bayreuth Luminescence Laboratory, Germany. The investigated sections are situated in Lower Silesia in the southwest (Zaprężyn, Trzebnica Hills, and Biały Kościół, Strzelin Hills), the Sandomierz Upland (Złota) in central Poland, and the Volhynian Upland (Tyszowce) in the east, allowing for regional comparison. From one Silesian section (Biały Kościół) 12 new post-infrared infrared stimulated luminescence (pIRIR) ages are presented in addition to the quartz ages of identical sample material. The obtained ages are compared to already published independently elaborated middle-grain (45–63 µm) and coarse-grain (90–125 µm) quartz ages and pIRIR ages from fine grains produced in the Gliwice Luminescence Laboratory (Poland). This comparison shows that in many cases the middle- and coarse-grain quartz ages underestimate the fine-grain quartz ages, but a general rule has not been able to be established so far, likely due to different geological origin of the quartz grains. Even fine-grain quartz ages ≥∼ 50 ka may be underestimated with respect to lithostratigraphic expectations. For pIRIR ages, however, no evidence for age underestimates has been found in the studied sections, but they are more easily prone to age overestimates due to unknown residual doses at deposition in a periglacial environment. Basic agreement between the luminescence-based chronologies elaborated in the two involved laboratories can be stated for the first time in contrast to other previous studies. The observed age differences are, however, critical for the accurate time bracketing of geomorphologic and pedostratigraphic features such as ice wedging, thermokarst erosion events, and interstadial soil formations and for their attribution to marine isotope stages. Alternative interpretations are discussed including possible periglacial mirroring of pre-LGM ice advances (Ristinge and Klintholm advances) in the southwestern Baltic Sea area. The uncertainty in luminescence ages from pre-Holocene loess due to fossil ice during permafrost conditions is the major systematic error source which will be addressed but at present is far from an unambiguous solution. The present study focuses on a complex of interstadial soils now labelled L1SS1 and on harsh periglacial climate afterwards and before, yielding some unexpected results for the timing of ice wedging and thermokarst processes. In order not to leave the users alone with the decision about the most credible dating, the suggested way forwards is to simultaneously apply various luminescence dating protocols including different quartz grain sizes and pIRIR from fine polymineral grains, as an honest approach to reliable time bracketing of geomorphological processes and stratigraphic events under debate. A refinement of this approach remains challenging as far as the sole reliable dating protocol is not ensured.