Tundra vegetation productivity and composition are responding rapidly to climatic changes in the Arctic. These changes can, in turn, mitigate or amplify permafrost thaw. In this Review, we synthesize remotely-sensed and field-observed vegetation change across the tundra biome, and outline how these shifts could influence permafrost thaw. Permafrost ice content appears to be an important control on local vegetation changes; woody vegetation generally increases in ice-poor uplands, whereas replacement of woody vegetation by (aquatic) graminoids following abrupt permafrost thaw is more frequent in ice-rich Arctic lowlands. These locally observed vegetation changes contribute to regional satellite-observed greening trends, although the interpretation of greening and browning is complicated. Increases in vegetation cover and height generally mitigate permafrost thaw in summer, yet increase annual soil temperatures through snow-related winter soil warming effects. Strong vegetation-soil feedbacks currently alleviate the consequences of thaw-related disturbances. However, if the increasing scale and frequency of disturbances in a warming Arctic exceeds the capacity for vegetation and permafrost recovery, changes to Arctic ecosystems could be irreversible. To better disentangle vegetation-soil-permafrost interactions, ecological field studies remain crucial, but require better integration with geophysical assessments. [H1] IntroductionArctic tundra is changing rapidly, with a pervasive trend toward more abundant and taller vegetation as shrubs and trees expand northward 1 . Field and satellite observations suggest that tundra vegetation has become more productive, a phenomenon known as tundra greening. Such increases in the biomass and stature of Arctic tundra vegetation can alter the thermal properties of the ground surface. Canopies can mediate the effect of increasing summer air temperatures on soil temperatures 2-4 and contribute to insulation of soils in winter through trapping of snow [5][6][7][8] .Vegetation and soil characteristics also influence surface energy partitioning and the thermal diffusivity of the soil 9,10 . Permafrost (permanently frozen ground) underlies soil and vegetation, and is the foundation of Arctic tundra ecosystems. In turn, vegetation and near-surface soils insulate permafrost 11 , regulating the effects of atmospheric conditions. However, the Arctic is warming more than twice as fast as the global average, amplified by loss of sea ice cover 1 . Even if Arctic temperatures were to stabilize at 2°C of warming, as aimed for with the Paris Agreement, approximately 40% of near-surface permafrost is still projected to thaw 12 . Permafrost-dominated ecosystems are thus at risk 13 , even under modest CO2 emission scenarios 1 , with consequences for Arctic inhabitants 14 .

Abstract. Permafrost ground is one of the largest repositories of terrestrial organic carbon and might become or already is a carbon source in response to ongoing global warming. With this study of syngenetically frozen, ice-rich and organic carbon (OC)-bearing Yedoma and associated alas deposits in central Yakutia (Republic of Sakha), we aimed to assess the local sediment deposition regime and its impact on permafrost carbon storage. For this purpose, we investigated the Yukechi alas area (61.76495∘ N, 130.46664∘ E), which is a thermokarst landscape degrading into Yedoma in central Yakutia. We retrieved two sediment cores (Yedoma upland, 22.35 m deep, and alas basin, 19.80 m deep) in 2015 and analyzed the biogeochemistry, sedimentology, radiocarbon dates and stable isotope geochemistry. The laboratory analyses of both cores revealed very low total OC (TOC) contents (<0.1 wt %) for a 12 m section in each core, whereas the remaining sections ranged from 0.1 wt % to 2.4 wt % TOC. The core sections holding very little to no detectable OC consisted of coarser sandy material were estimated to be between 39 000 and 18 000 BP (years before present) in age. For this period, we assume the deposition of organic-poor material. Pore water stable isotope data from the Yedoma core indicated a continuously frozen state except for the surface sample, thereby ruling out Holocene reworking. In consequence, we see evidence that no strong organic matter (OM) decomposition took place in the sediments of the Yedoma core until today. The alas core from an adjacent thermokarst basin was strongly disturbed by lake development and permafrost thaw. Similar to the Yedoma core, some sections of the alas core were also OC poor (<0.1 wt %) in 17 out of 28 samples. The Yedoma deposition was likely influenced by fluvial regimes in nearby streams and the Lena River shifting with climate. With its coarse sediments with low OC content (OC mean of 5.27 kg m−3), the Yedoma deposits in the Yukechi area differ from other Yedoma sites in North Yakutia that were generally characterized by silty sediments with higher OC contents (OC mean of 19 kg m−3 for the non-ice wedge sediment). Therefore, we conclude that sedimentary composition and deposition regimes of Yedoma may differ considerably within the Yedoma domain. The resulting heterogeneity should be taken into account for future upscaling approaches on the Yedoma carbon stock. The alas core, strongly affected by extensive thawing processes during the Holocene, indicates a possible future pathway of ground subsidence and further OC decomposition for thawing central Yakutian Yedoma deposits.

Ice-rich permafrost in the circum-Arctic and sub-Arctic (hereafter pan-Arctic), such as late Pleistocene Yedoma, are especially prone to degradation due to climate change or human activity. When Yedoma deposits thaw, large amounts of frozen organic matter and biogeochemically relevant elements return into current biogeochemical cycles. This mobilization of elements has local and global implications: increased thaw in thermokarst or thermal erosion settings enhances greenhouse gas fluxes from permafrost regions. In addition, this ice-rich ground is of special concern for infrastructure stability as the terrain surface settles along with thawing. Finally, understanding the distribution of the Yedoma domain area provides a window into the Pleistocene past and allows reconstruction of Ice Age environmental conditions and past mammoth-steppe landscapes. Therefore, a detailed assessment of the current pan-Arctic Yedoma coverage is of importance to estimate its potential contribution to permafrost-climate feedbacks, assess infrastructure vulnerabilities, and understand past environmental and permafrost dynamics. Building on previous mapping efforts, the objective of this paper is to compile the first digital pan-Arctic Yedoma map and spatial database of Yedoma coverage. Therefore, we 1) synthesized, analyzed, and digitized geological and stratigraphical maps allowing identification of Yedoma occurrence at all available scales, and 2) compiled field data and expert knowledge for creating Yedoma map confidence classes. We used GIS-techniques to vectorize maps and harmonize site information based on expert knowledge. We included a range of attributes for Yedoma areas based on lithological and stratigraphic information from the source maps and assigned three different confidence levels of the presence of Yedoma (confirmed, likely, or uncertain). Using a spatial buffer of 20 km around mapped Yedoma occurrences, we derived an extent of the Yedoma domain. Our result is a vector-based map of the current pan-Arctic Yedoma domain that covers approximately 2,587,000 km2, whereas Yedoma deposits are found within 480,000 km2 of this region. We estimate that 35% of the total Yedoma area today is located in the tundra zone, and 65% in the taiga zone. With this Yedoma mapping, we outlined the substantial spatial extent of late Pleistocene Yedoma deposits and created a unique pan-Arctic dataset including confidence estimates.

Highlights 16-Small thermokarst lakes and basins grew rapidly during the Holocene Thermal Maximum 17 -Short-term phases of forcing climate lead to very active thermokarst processes 18 -Endmember analysis reveals different depositional environments in growing lakes 19 -Distal and proximal depositional and post-sedimentary conditions are differentiated 20 -Sedimentological and biogeochemical characteristics are weakly correlated 21 22 Abstract: 23Differentiating thermokarst basin sediments with respect to the involved processes and 24 environmental conditions is an important tool to understand permafrost landscape dynamics 25 and scenarios and future trajectories in a warming Arctic and Subarctic. Thermokarst basin 26 deposits have complex sedimentary structures due to the variability of Yedoma source 27 sediments, reworking during the Late Glacial to Holocene climate changes, and different stages 28 of thermokarst history. 29Here we reconstruct the dynamic growth of thermokarst lakes and basins and related changes 30 of depositional conditions preserved in sediment sequences using a combination of 31 biogeochemical data and robust grain-size endmember analysis (rEMMA). This multi-proxy 32 approach is used on 10 sediment cores (each 300-400 cm deep) from two key thermokarst sites 33 to distinguish four time slices that describe the Holocene thermokarst (lake) basin evolution in 34Central Yakutia (CY). Biogeochemical proxies and rEMMA reveal fine-grained sedimentation 35 with rather high lake levels and/or reducing conditions, and coarse-grained sedimentation with 36 rather shallow lake levels and/or oxidizing (i.e. terrestrial) conditions in relation to distal and 37 proximal depositional and post-sedimentary conditions. Statistical analysis suggests that the 38 biogeochemical parameters are almost independent of thermokarst deposit sedimentology. 39Thus, the biogeochemical parameters are considered as signals of secondary (post-sedimentary) 40reworking. The rEMMA results are clearly reflecting grain-size variations and depositional 41 conditions. This indicates small-scale varying depositional environments, frequently changing 42 lake levels, and predominantly lateral expansion at the edges of rapidly growing small 43 thermokarst lakes and basins. These small bodies finally coalesced, forming the large 44 thermokarst basins we see today in CY. 45Considering previous paleoenvironmental reconstructions in Siberia, we show the initiation of 46 thaw and subsidence during the Late Glacial to Holocene transition between about 11 and 9 cal 47 kyrs BP, intensive and extensive thermokarst activity for the Holocene Thermal Maximum 48(HTM) at about 7 to 5 cal kyrs BP, severely fluctuating water levels and further lateral basin 49 growth between 3.5 cal kyrs BP and 1.5 cal kyrs BP, and the cessation of thermokarst activity 50 and extensive frost-induced processes (i.e. permafrost aggradation) after about 1.5 cal kyrs BP. 51However, gradual permafrost warming over recent decades, in addition to human impacts, has 52 led to ren...