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Deglaciation of the northwestern Laurentide Ice Sheet in the central Mackenzie Valley opened the northern portion of the deglacial Ice-Free Corridor between the Laurentide and Cordilleran ice sheets and a drainage route to the Arctic Ocean. In addition, ice sheet saddle collapse in this section of the Laurentide Ice Sheet has been implicated as a mechanism for delivering substantial freshwater influx into the Arctic Ocean on centennial timescales. However, there is little empirical data to constrain the deglaciation chronology in the central Mackenzie Valley where the northern slopes of the ice saddle were located. Here, we present 30 new 10Be cosmogenic nuclide exposure dates across six sites, including two elevation transects, which constrain the timing and rate of thinning and retreat of the Laurentide Ice Sheet in the area. Our new 10Be dates indicate that the initial deglaciation of the eastern summits of the central Mackenzie Mountains began at ∼15.8 ka (17.1–14.6 ka), ∼1000 years earlier than in previous reconstructions. The main phase of ice saddle collapse occurred between ∼14.9 and 13.6 ka, consistent with numerical modelling simulations, placing this event within the Bølling–Allerød interval (14.6–12.9 ka). Our new dates require a revision of ice margin retreat dynamics, with ice retreating more easterly rather than southward along the Mackenzie Valley. In addition, we quantify a total sea level rise contribution from the Cordilleran–Laurentide ice saddle region of ∼11.2 m between 16 and 13 ka.

Permafrost cores provide physical samples that can be used to measure the characteristics of frozen ground. Measurement of core physical properties, however, are typically destructive and time intensive. In this study, multi-sensor core logging (MSCL) is used to provide a rapid, high-resolution, non-destructive method to image and collect the physical properties of permafrost cores, allowing the visualization of cryostructures, measurement of bulk density and magnetic susceptibility, and estimation of volumetric ice content. Six permafrost cores with differing properties were analyzed using MSCL and compared with established destructive analyses to assess the potential of this instrument both in terms of accuracy and relative rate of data acquisition. A calibration procedure is presented for gamma ray attenuation from a 137Cs source that is specific to frozen core materials. This accurately measures bulk density over the wide range of material densities found in permafrost. MSCL bulk density data show strong agreement with destructive analyses based on discrete sample measurements. Bulk density data from the gamma attenuation, along with soil density measurements for different sediment types, are used to estimate volumetric ice content. The results show strong agreement with destructive analyses, illustrating that MSCL can provide accurate non-destructive estimates of ice contents and provide a digital archive of permafrost cores for future applications.

The lower Athabasca River basin in northeastern Alberta contains one of the highest known concentrations of prehistoric archaeological sites in the boreal forests of western Canada. This is due to the combination of readily available sources of lithic raw material stone near a major travel corridor, and extensive archaeological survey conducted in advance of oil sands mining. Typological studies have proposed immediate post-glacial occupations that were contemporaneous with, or immediately followed, the catastrophic glacial Lake Agassiz flood through the area at the end of the Pleistocene. Here, we complement the typology age estimates by using stratigraphic relations and infrared stimulated luminescence (IRSL) dating of aeolian material to determine the age of initial human occupation, and reconstruct the environment encountered by early inhabitants of the region. We find that the first occupations in our study area took place near the Pleistocene-Holocene boundary (ca. 11.3 ± 0.8 ka BP), shortly after catastrophic flooding from Lake Agassiz. The post-flood environment was dominated by cold climatic conditions that supported permafrost, presumably during the late Pleistocene, and underwent significant aeolian deposition. Our results indicate that this area represents a portion of the eastern edge of the deglacial corridor into which plants, animals, and humans dispersed following retreat of the Laurentide Ice Sheet.

Deglaciation of the northwestern Laurentide Ice Sheet in the central Mackenzie Valley opened the northern portion of the deglacial Ice-Free Corridor between the Laurentide and Cordilleran ice sheets and a drainage route to the Arctic Ocean. In addition, ice-sheet saddle collapse in this section of the Laurentide Ice Sheet has been implicated as a mechanism for delivering substantial freshwater influx into the Arctic Ocean on centennial timescales. However, there is little empirical data to constrain the deglaciation chronology in the central Mackenzie Valley where the northern slopes of the ice saddle were located. Here, we present 30 new 10Be cosmogenic nuclide exposure dates across six sites, including two elevation transects, which constrain the timing and rate of thinning of the Laurentide Ice Sheet from the area. Our new 10Be dates indicate that the initial deglaciation of the eastern summits of the central Mackenzie Mountains began at ~15.8 ka (17.1 – 14.6 ka), ~1,000 years earlier than previous reconstructions. The main phase of ice-saddle collapse occurred between ~14.9 and 13.2 ka, consistent with numerical modelling simulations, placing this event within the Bølling–Allerød interval (14.6 – 12.9 ka). Our new dates require a revision of ice margin retreat dynamics, with ice retreating more easterly rather than southward along the Mackenzie Valley. In addition, we quantify a total sea-level rise contribution from the Cordilleran-Laurentide ice saddle region of ~11.2 m between 16 ka and 13 ka.

Significance Megafauna strongly influence vegetation structure, and population declines can alter ecosystem functioning. Overhunting of grazing megafauna is argued to have driven the collapse of widespread, northern steppe-tundra and its replacement by woody vegetation at the end of the ice age. However, in Alaska and Yukon, mammoth and horse became extinct around the time that steppe-tundra was replaced by shrub tundra, leaving it unclear whether this vegetation change caused, or was caused by, reduced megafauna populations. Comparison of accurately dated pollen records with a radiocarbon-dated bone chronology shows that shrubs began expanding before grazer populations declined. This indicates that climate was the primary control of steppe-tundra persistence and that climate-driven vegetation change may pose threats to faunal diversity in the future.

Traditionally, paleontologists have relied on the morphological features of bones and teeth to reconstruct the evolutionary relationships of extinct animals.

The Arctic is currently experiencing dramatic ecosystem changes with immediate effects on biodiversity. Sedimentary ancient DNA is a unique and valuable source of information on ecosystem changes over a long temporal scale. Understanding these past changes may help predict the relative impacts of climate change, herbivory, and anthropogenic effects on present ecosystems. In the BiodivERsA project “Future ArcTic Ecosystems” (FATE), we aim to assess changes in past herbivore abundance over large spatial (circumarctic) and temporal (Last Glacial Maximum until today) scales using three (semi-)quantitative methods on sedimentary ancient DNA of plants, herbivores, and herbivore proxies (i.e. coprophilous fungi and parasites) – metabarcoding, hybridization capture enrichment, and droplet digital PCR (ddPCR).Metabarcoding was applied to DNA of plants and also of coprophilous fungi as proxies of herbivore abundance. This approach is an established and important tool for assessing biodiversity from recent environmental DNA; however, quantification of specific taxa may be complicated due to inherent methodological biases (e.g. amplification efficiency due to primer bias), and our current understanding of the factors affecting potential quantification by metabarcoding is still limited. Moreover, ancient DNA is highly fragmented, which may prevent PCR amplification altogether. As an alternative, target enrichment by hybridization capture is a method that does not depend on target PCR amplification and is typically not affected by DNA fragmentation. Furthermore, hybridization capture can be used to target numerous genetic markers of a vast range of highly diverse taxa. We are using hybridization capture to enrich DNA of a range of herbivore species and numerous proxy organisms. Metabarcoding and hybridization capture can be applied to a vast taxonomic range and may be used quantitatively based on relative sequencing read abundance; however, the respective read abundance may be confounded by random and systematic errors and other biases. We are therefore using an additional quantification method – ddPCR – on several selected taxa, which is taxon-specific but facilitates highly accurate quantification of template DNA molecules in a given sample. The combined taxonomic and quantitative results of these three approaches are used to generate highly resolved datasets on past vegetation and herbivores, which allows us to reconstruct past vegetation changes over large spatial (circumarctic) and temporal (Last Glacial Maximum until today) scales.Detailed inferences on herbivore abundance and reconstructing past ecological conditions may be important for ecosystem management and conservation in the face of accelerating changes in Arctic ecosystems due to global climate change.