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During the last glacial–interglacial cycle, Arctic biotas experienced substantial climatic changes, yet the nature, extent and rate of their responses are not fully understood1–8. Here we report a large-scale environmental DNA metagenomic study of ancient plant and mammal communities, analysing 535 permafrost and lake sediment samples from across the Arctic spanning the past 50,000 years. Furthermore, we present 1,541 contemporary plant genome assemblies that were generated as reference sequences. Our study provides several insights into the long-term dynamics of the Arctic biota at the circumpolar and regional scales. Our key findings include: (1) a relatively homogeneous steppe–tundra flora dominated the Arctic during the Last Glacial Maximum, followed by regional divergence of vegetation during the Holocene epoch; (2) certain grazing animals consistently co-occurred in space and time; (3) humans appear to have been a minor factor in driving animal distributions; (4) higher effective precipitation, as well as an increase in the proportion of wetland plants, show negative effects on animal diversity; (5) the persistence of the steppe–tundra vegetation in northern Siberia enabled the late survival of several now-extinct megafauna species, including the woolly mammoth until 3.9 ± 0.2 thousand years ago (ka) and the woolly rhinoceros until 9.8 ± 0.2 ka; and (6) phylogenetic analysis of mammoth environmental DNA reveals a previously unsampled mitochondrial lineage. Our findings highlight the power of ancient environmental metagenomics analyses to advance understanding of population histories and long-term ecological dynamics.

Determining the sensitivity of the Greenland Ice Sheet (GrIS) to Holocene climate changes is a key prerequisite for understanding the future response of the ice sheet to global warming. In this study, we present new information on the Holocene glacial history of the GrIS in Inglefield Land, north Greenland. We use 10Be and in situ 14C exposure dating to constrain the timing of deglaciation in the area and radiocarbon dating of reworked molluscs and wood fragments to constrain when the ice sheet retreated behind its present-day extent. The 10Be ages are scattered ranging from ca. 92.7 to 6.8 ka, whereas the in situ 14C ages range from ca. 14.2 to 6.7 ka. Almost half of the apparent 10Be ages predate the Last Glacial Maximum and up to 89 % are to some degree affected by nuclide inheritance. Based on the few reliable 10Be ages, the in situ 14C ages and existing radiocarbon ages from Inglefield Land, we find that the deglaciation along the coast commenced at ca. 8.6–8.3 ka cal BP in the western part and ca. 7.9 ka in the central part, following the opening of Nares Strait and arrival of warm waters. The ice margin reached its present-day position at ca. 8.2 ka at the Humboldt Glacier and ca. 6.7 ka in the central part of Inglefield Land. Radiocarbon ages of reworked molluscs and wood fragments show that the ice margin was behind its present-day extent from ca. 5.8 to 0.5 ka cal BP. After 0.5 ka cal BP, the ice advanced towards its Little Ice Age position. Our results emphasize that the slowly eroding and possibly cold-based ice in north Greenland makes it difficult to constrain the deglaciation history based on 10Be ages alone unless they are paired with in situ 14C ages. Further, combining our findings with those of recently published studies reveals distinct differences between deglaciation patterns of northwest and north Greenland. Deglaciation of the land areas in northwest Greenland occurred earlier than in north Greenland, and periods of restricted ice extent were longer, spanning the Middle and Late Holocene. Overall, this highlights past ice sheet sensitivity to Holocene climate changes in an area where little information was available just a few years ago.

Observations show that the flow velocity of several marine-terminating glaciers and ice streams are strongly linked to tidal stage. Deformation of subglacial sediments often accounts for much of the flow, but the mechanical behavior of the sediment remains poorly understood. Measurements and laboratory experiments provide important constraints, but existing models have not been able to explain the internal processes driving transitions from stability to stick-slip within the sediment. In this presentation we use a coupled numerical model of grain and fluid dynamics to show that rapid rearrangements of load-bearing force chains within the granular sediments drive the mechanical transitions. Cyclic variations in driving stresses or pore-water pressure, caused by ocean tides, give rise to strain-rate dependent creeping motion at stress levels below the point of failure, while disruption of the force-chain network induces fast strain-rate independent flow above it. This finding contrasts previous descriptions of subglacial sediment mechanics, which either assume a rate-dependent rheology regardless of mechanical state or unconditional stability before the sediment is stressed to a yield point. Our new micro-mechanical computational approach is capable of reproducing important transitions between these two end-member models, and it can explain multimodal velocity patterns observed in marine-terminating glacier and ice stream systems.

The mechanical properties of subglacial sediments are known to directly influence the stability of ice streams and fast-moving glaciers, but existing models of granular sediment deformation are poorly constrained. In addition, upscaling to generalized mathematical models is difficult due to the mechanical nonlinearity of the sediment, internal porosity changes during deformation, and associated structural and kinematic phase transitions.In this presentation, we introduce the Discrete Element Method (DEM) for particle-scale granular simulation. The DEM is fully coupled with fluid dynamics. The numerical method is applied to better understand the mechanical properties of the subglacial sediment and its interaction with meltwater. The computational approach allows full experimental control and offers insights into the internal kinematics, stress distribution, and mechanical stability.During confined shear with variable pore-water pressure, the sediment changes mechanical behavior, from stick, to non-linear creep, and unconstrained failure during slip. These results are contrary to more conventional models of plastic or (non-)linear viscous subglacial soft-bed sliding. Advection of sediment downstream is pressure dependent, which is consistent with theories of unstable bed bump growth.Granular mechanics prove to significantly influence the geometry and hydraulic properties of meltwater channels incised into the subglacial bed. Current models assume that channel bed erosion is balanced by linear-viscous sediment movement. We demonstrate how channel flanks are stabilized by the sediment frictional strength. Additionally, sediment liquefaction proves to be a possible mechanism for causing large and episodic sediment transport by water flow.Though computationally intense, our coupled numerical method provides a framework for quantifying a wide range of subglacial sediment-water processes, which are a key unknown in our ability to model the future evolution of ice sheets.

The macroscopic behavior of granular materials is the result of the self-organizing complexity of the constituent grains. Granular materials are known for their ability to change phase, where each phase is characterized by distinct mechanical properties. This rich generic phenomenology has made it difficult to constrain generalized and adequate mathematical models for their mechanical behavior. Glaciers and ice streams often move by deformation of underlying melt-water saturated sediments. Glacier flow models including subglacial sediment deformation use simplified a priori assumptions for sediment rheology, which limit our ability to predict ice sheet dynamics in a changing climate.In this talk I will present the soft-body Discrete Element Method which is a Lagrangian method I use in order to simulate the unique and diverse nature of granular dynamics in the subglacial environment. However, the method imposes intense computational requirements on the computational time step. The majority of steps in the granular dynamics algorithm are massively parallel, which makes the DEM an obvious candidate for exploiting the capabilities of modern GPUs. The granular computations are coupled to a fluid-dynamics solver in order to include grain-fluid feedbacks, which prove to be important for stick-slip behavior of glaciers.All code is open source and freely licensed.

Observations show that the flow velocities of several marine-terminating glaciers and ice streams are strongly linked to variations in the subglacial water pressure and tidal stage. Deformation of subglacial sediments often accounts for much of the flow, but the mechanical behavior of these sediment remains poorly understood. Measurements and laboratory experiments provide important constraints, but existing models have not been able to explain the internal processes driving transitions from stability to stick-slip within the sediment.In this presentation we use a coupled numerical model of grain and fluid dynamics to show that rapid rearrangements of load-bearing force chains within the granular sediments drive mechanical transitions between stability and failure. Cyclic variations in driving stresses or pore-water pressure give rise to strain-rate dependent creeping motion at stress levels below the point of failure, while disruption of the force-chain network induces fast strain-rate independent flow above it. This finding contrasts previous descriptions of subglacial sediment mechanics, which either assume a rate-dependent rheology regardless of mechanical state or unconditional stability before the sediment is stressed to its yield point. Our new micro-mechanical computational approach is capable of reproducing important mechanical transitions, and it can explain multimodal velocity patterns observed in marine-terminating glacier and ice stream systems.

Cosmogenic nuclides are traditionally used to either determine the glaciation history or the denudation history of the most recent exposure period. A few studies use the cosmogenic nuclides to determine the cumulative exposure and burial durations of a sample. However, until now it has not been possible to resolve the complex pattern of exposure history under a fluctuating ice sheet. In this study, we quantify long-term erosion rates along with durations of multiple exposure periods in West Greenland by applying a novel Markov Chain Monte Carlo (MCMC) inversion approach to existing 10Be and 26Al.The new MCMC approach allows us to constrain the most likely landscape history based on comparisons between simulated and measured cosmogenic nuclide concentrations. It is a fundamental assumption of the model that the exposure history at the site/location can be divided into two distinct regimes: i) interglacial periods characterized by zero shielding due to overlying ice and a uniform interglacial erosion rate, and ii) glacial periods characterized by 100 % shielding and a uniform glacial erosion rate. We incorporate the exposure/burial history in the model framework by applying a threshold value to the global marine benthic d18O record and include the threshold value as a free model parameter, hereby taking into account global changes in climate. The other free parameters include the glacial and interglacial erosion rates as well as the timing of the Holocene deglaciation. The model essentially simulates numerous different landscape scenarios based on these four parameters and zooms in on the most plausible combination of model parameters.We apply the MCMC-model to the concentrations of 10Be and 26Al measured in previously published studies from Upernavik, Uummannaq and Sisimiut and quantify the most likely exposure/burial history integrated with erosion rates. Our results show a clear trend of decreasing erosion rate with increasing altitude, with a lowest total denudation rate of 1.7±0.2 m/Ma at 841 m a.s.l. and the highest rate of 25.6 ± 10.4 m/Ma at 150 m a.s.l. We find that the three areas have been exposed during 17 previous interglacials and quantify the most likely temporal extent of each of these, which enables us to reconstruct the ice margin fluctuation across the West Greenland fjord area throughout the last 1 million years. Our results contribute to understanding the landscape evolution under multiple glacial-interglacial cycles and provide a temporal framework for the sparse pre-LGM data that has been found, but have proven difficult to constrain in time.

Surface exposure dating with cosmogenic nuclides is a dating method under continuous development. It is particularly useful for dating ice-sheet fluctuations in glacial environments, which is essential to increase our understanding of past climate fluctuations and glacial dynamics. Constraining the landscape history in previously glaciated terrains may be difficult, however, due to unknown erosion rates and the presence of inherited nuclides. The potential use of cosmogenic nuclides in landscapes with a complex history of exposure and erosion is therefore often quite limited. In this study, we investigate the landscape history in eastern and western Greenland by applying a novel Markov Chain Monte Carlo (MCMC) inversion approach to the existing 10Be-26Al data from these regions. The new MCMC approach allows us to constrain the most likely landscape history based on comparisons between simulated and measured cosmogenic nuclide concentrations. It is a fundamental assumption of the model approach that the exposure history at the site/location canbe divided into two distinct regimes: i) interglacial periods characterized by zero shielding due to overlying ice and a uniform interglacial erosion rate, and ii) glacial periods characterized by 100 % shielding and a uniform glacial erosion rate. We incorporate the exposure/burial history in the model framework by applying a threshold value to the global marine benthic d18O record and include the threshold value as a free model parameter, hereby taking into account global changes in climate. The other free parameters include the glacial and interglacial erosion rates as well asthe timing of the Holocene deglaciation. The model essentially simulates numerous different landscape scenarios based on these four parameters and zooms in on the most plausible combination of model parameters.Here, we apply the MCMC-model to the concentrations of 10Be and 26Al measured in three previous studies of glacial fluctuations in Greenland (Upernavik, Sisimiut, and Jameson Land) and delineate the most likely exposure/burial history in these areas over the last 106 years. The results allow for comparison of glacial and interglacial erosion rates and exposure/burial history in similar landscape settings across different regions of Greenland.

We present 23 cosmogenic surface exposure ages from 10 localities in southern Sweden. The new Be ages allow a direct correlation between the east and west coasts of southern Sweden, based on the same dating technique, and provide new information about the deglaciation of the Fennoscandian Ice Sheet in the circum-Baltic area. In western Skåne, southernmost Sweden, a single cosmogenic surface exposure sample gave an age of 16.8±1.0ka, whereas two samples from the central part of Skåne gave ages of 17.0±0.9 and 14.1±0.8ka. Further northeast, in southern Småland, two localities gave ages ranging from 15.2±0.8 to 16.9±0.9ka (n=5) indicating a somewhat earlier deglaciation of the area than has previously been suggested. Our third locality, in S Småland, gave ages ranging from 10.2±0.5 to 18.4±1.6ka (n=3), which are probably not representative of the timing of deglaciation. In central Småland one locality was dated to 14.5±0.8ka (n=3), whereas our northernmost locality, situated in northern Småland, was dated to 13.8±0.8ka (n=3). Samples from the island of Gotland suggest deglaciation before 13ka ago. We combined the new Be ages with previously published deglaciation ages to constrain the deglaciation chronology of southern Sweden. The combined deglaciation chronology suggests a rather steady deglaciation in southern Sweden starting at c.17.9cal. ka BP in NW Skåne and reaching northern Småland, ∼200km further north, c.13.8ka ago. Overall the new deglaciation ages agree reasonably well with existing deglaciation chronologies, but suggest a somewhat earlier deglaciation in Småland.

Dådyrknoglerne fra sø-aflejringen i Hollerup dateret til Eem mellemistiden (130.000-115.000) blev gjort til nationale klenodier, da de af Møhl-Hansen (i 1955) blev tolket som efterladenskaberne af et måltid, hvor mindst ét dyr var blevet slagtet og knoglerne brudt op for at få adgang til den næringsrige marv. Dateringen taget i betragtning kunne de skyldige nemlig kun være repræsentanter af neandertalmennesket, og dermed var dådyrknoglerne fra Hollerup de første sikre spor, efter at der havde været mennesker i Danmark før den sidste istid. Eftersom denne konklusion blev draget for over 50 år siden, har vores undersøgelse til formål at analysere disse mærker på ny for at etablere, hvorvidt knoglerne og deres brud, med de tekniske forbedringer og komparative midler vi har i dag, endegyldigt kan knyttes til menneskelig adfærd. Resultatet af undersøgelsen viser, at bruddene ikke kan tolkes som værende menneskeskabte, og der sættes dermed spørgsmålstegn ved deres tilknytning til neandertalmennesket. Bliver spørgsmålet om tilstedeværelsen af neandertalere i de danske egne under den sidste mellemistid placeret i en bredere international kontekst, så er det – paradoksalt nok – netop denne periode, der fremstår som måske den mindst gunstige for denne arts udbredelse mod nord. Om dette resultat kan relateres til topografiske og klimatiske omstændigheder under Eem mellemistiden, vil til slut blive diskuteret med henblik på at belyse sammenhængen mellem biodiversitet, neandertalernes tilpasningsevner og et bæredygtigt subsistensgrundlag. Dådyrknoglerne fra sø-aflejringen i Hollerup dateret til Eem mellemistiden (130.000-115.000) blev gjort til nationale klenodier, da de af Møhl-Hansen (i 1955) blev tolket som efterladenskaberne af et måltid, hvor mindst ét dyr var blevet slagtet og knoglerne brudt op for at få adgang til den næringsrige marv. Dateringen taget i betragtning kunne de skyldige nemlig kun være repræsentanter af neandertalmennesket, og dermed var dådyrknoglerne fra Hollerup de første sikre spor, efter at der havde været mennesker i Danmark før den sidste istid. Eftersom denne konklusion blev draget for over 50 år siden, har vores undersøgelse til formål at analysere disse mærker på ny for at etablere, hvorvidt knoglerne og deres brud, med de tekniske forbedringer og komparative midler vi har i dag, endegyldigt kan knyttes til menneskelig adfærd. Resultatet af undersøgelsen viser, at bruddene ikke kan tolkes som værende menneskeskabte, og der sættes dermed spørgsmålstegn ved deres tilknytning til neandertalmennesket. Bliver spørgsmålet om tilstedeværelsen af neandertalere i de danske egne under den sidste mellemistid placeret i en bredere international kontekst, så er det – paradoksalt nok – netop denne periode, der fremstår som måske den mindst gunstige for denne arts udbredelse mod nord. Om dette resultat kan relateres til topografiske og klimatiske omstændigheder under Eem mellemistiden, vil til slut blive diskuteret med henblik på at belyse sammenhængen mellem biodiversitet, neandertalernes tilpasningsevner og et bæredygtigt subsistensgrundlag.