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Abstract Thermokarst lakes cause abrupt and sustained permafrost degradation and have the potential to release large quantities of ancient carbon to the atmosphere. Despite concerns about how lakes will affect the permafrost carbon feedback, the magnitude of carbon dioxide and methane emissions from deep permafrost soils remains poorly understood. Here we incubated a very deep sediment core (20 m) to constrain the potential productivity of thawed Yedoma and underlying Quaternary sand and gravel deposits. Through radiocarbon dating, sediment incubations and sediment facies classifications, we show that extensive permafrost thaw can occur beneath lakes on timescales of decades to centuries. Although it has been assumed that shallow, aerobic carbon dioxide production will dominate the climate impact of permafrost thaw, we found that anaerobic carbon dioxide and methane production from deep sediments was commensurate with aerobic production on a per gram carbon basis, and had double the global warming potential at warmer temperatures. Carbon release from deep Arctic sediments may thus have a more substantial impact on a changing climate than currently anticipated. These environments are presently overlooked in estimates of the permafrost carbon feedback. 

The rapid climate warming is affecting the Arctic which is rich in aquatic systems. As a result of permafrost thaw, thermokarst lakes and ponds are either shrinking due to lake drainage or expanding due to lake shore erosion. This process in turn mobilizes organic carbon, which is released by permafrost deposits and active layer material that slips into the lake. In this study, we combine hydrochemical measurements and remote sensing data to analyze the influence of lake change processes, especially lake growth, on lake hydrochemical parameters such as DOC, EC, pH as well as stable oxygen and hydrogen isotopes in the Arctic Coastal Plain. For our entire dataset of 97 water samples from 82 water bodies, we found significantly higher CH4 concentrations in lakes with a floating-ice regime and significantly higher DOC concentrations in lakes with a bedfast-ice regime. We show significantly lower CH4 concentrations in lagoons compared to lakes as a result of an effective CH4 oxidation that increased with a seawater connection. For our detailed lake sampling of two thermokarst lakes, we found a significant positive correlation for lake shore erosion and DOC for one of the lakes. Our detailed lake sampling approach indicates that the generally shallow thermokarst lakes are overall well mixed and that single hydrochemical samples are representative for the entire lake. Finally, our study confirms that DOC concentrations correlates with lake size, ecoregion type and underlying deposits. 

Abstract Wetlands in Arctic drained lake basins (DLBs) have a high potential for carbon storage in vegetation and peat as well as for elevated greenhouse gas emissions. However, the evolution of vegetation and organic matter is rarely studied in DLBs, making these abundant wetlands especially uncertain elements of the permafrost carbon budget. We surveyed multiple DLB generations in northern Alaska with the goal to assess vegetation, microtopography, and organic matter in surface sediment and pond water in DLBs and to provide the first high-resolution land cover classification for a DLB system focussing on moisture-related vegetation classes for the Teshekpuk Lake region. We associated sediment properties and methane concentrations along a post-drainage succession gradient with remote sensing-derived land cover classes. Our study distinguished five eco-hydrological classes using statistical clustering of vegetation data, which corresponded to the land cover classes. We identified surface wetness and time since drainage as predictors of vegetation composition. Microtopographic complexity increased after drainage. Organic carbon and nitrogen contents in sediment, and dissolved organic carbon (DOC) and dissolved nitrogen (DN) in ponds were high throughout, indicating high organic matter availability and decomposition. We confirmed wetness as a predictor of sediment methane concentrations. Our findings suggest moderate to high methane concentrations independent of drainage age, with particularly high concentrations beneath submerged patches (up to 200μmol l⁻¹) and in pond water (up to 22μmol l⁻¹). In our DLB system, wet and shallow submerged patches with high methane concentrations occupied 54% of the area, and ponds with high DOC, DN and methane occupied another 11%. In conclusion, we demonstrate that DLB wetlands are highly productive regarding organic matter decomposition and methane production. Machine learning-aided land cover classification using high-resolution multispectral satellite imagery provides a useful tool for future upscaling of sediment properties and methane emission potentials from Arctic DLBs. 

Abstract. Lakes in permafrost regions are dynamic landscapecomponents and play an important role for climate change feedbacks. Lakeprocesses such as mineralization and flocculation of dissolved organiccarbon (DOC), one of the main carbon fractions in lakes, contribute to thegreenhouse effect and are part of the global carbon cycle. These processesare in the focus of climate research, but studies so far are limited to specificstudy regions. In our synthesis, we analyzed 2167 water samples from 1833lakes across the Arctic in permafrost regions of Alaska, Canada, Greenland,and Siberia to provide first pan-Arctic insights for linkages between DOCconcentrations and the environment. Using published data and unpublisheddatasets from the author team, we report regional DOC differences linked tolatitude, permafrost zones, ecoregions, geology, near-surface soil organiccarbon contents, and ground ice classification of each lake region. The lakeDOC concentrations in our dataset range from 0 to1130 mg L−1 (10.8 mg L−1 median DOC concentration). Regarding thepermafrost regions of our synthesis, we found median lake DOC concentrationsof 12.4 mg L−1 (Siberia), 12.3 mg L−1 (Alaska),10.3 mg L−1 (Greenland), and 4.5 mg L−1 (Canada). Our synthesisshows a significant relationship between lake DOC concentration and lakeecoregion. We found higher lake DOC concentrations at boreal permafrostsites compared to tundra sites. We found significantly higher DOCconcentrations in lakes in regions with ice-rich syngenetic permafrostdeposits (yedoma) compared to non-yedoma lakes and a weak but significantrelationship between soil organic carbon content and lake DOC concentrationas well as between ground ice content and lake DOC. Our pan-Arctic datasetshows that the DOC concentration of a lake depends on its environmentalproperties, especially on permafrost extent and ecoregion, as well asvegetation, which is the most important driver of lake DOC in this study.This new dataset will be fundamental to quantify a pan-Arctic lake DOC poolfor estimations of the impact of lake DOC on the global carbon cycle andclimate change. 

Abstract. In September 2019, the researchicebreaker Polarstern started the largest multidisciplinary Arctic expedition to date,the MOSAiC (Multidisciplinary drifting Observatory for the Study of ArcticClimate) drift experiment. Being moored to an ice floe for a whole year,thus including the winter season, the declared goal of the expedition is tobetter understand and quantify relevant processes within theatmosphere–ice–ocean system that impact the sea ice mass and energy budget,ultimately leading to much improved climate models. Satellite observations,atmospheric reanalysis data, and readings from a nearby meteorologicalstation indicate that the interplay of high ice export in late winter andexceptionally high air temperatures resulted in the longest ice-free summerperiod since reliable instrumental records began. We show, using aLagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiCfloe carrying the Central Observatory (CO) formed in a polynya event northof the New Siberian Islands at the beginning of December 2018. The resultsfurther indicate that sea ice in the vicinity of the CO (<40 kmdistance) was younger and 36 % thinner than the surrounding ice withpotential consequences for ice dynamics and momentum and heat transferbetween ocean and atmosphere. Sea ice surveys carried out on variousreference floes in autumn 2019 verify this gradient in ice thickness, andsediments discovered in ice cores (so-called dirty sea ice) around the COconfirm contact with shallow waters in an early phase of growth, consistentwith the tracking analysis. Since less and less ice from the Siberianshelves survives its first summer (Krumpen et al., 2019), the MOSAiCexperiment provides the unique opportunity to study the role of sea ice as atransport medium for gases, macronutrients, iron, organic matter,sediments and pollutants from shelf areas to the central Arctic Ocean andbeyond. Compared to data for the past 26 years, the sea ice encountered atthe end of September 2019 can already be classified as exceptionally thin,and further predicted changes towards a seasonally ice-free ocean willlikely cut off the long-range transport of ice-rafted materials by theTranspolar Drift in the future. A reduced long-range transport of sea icewould have strong implications for the redistribution of biogeochemicalmatter in the central Arctic Ocean, with consequences for the balance ofclimate-relevant trace gases, primary production and biodiversity in theArctic Ocean.