Glacial Erosion Drives High Summer Mercury Exports from the Yukon River, Canada.

Mercury concentrations and yields in the Yukon River are the highest of the world's six largest panarctic drainages. Permafrost thaw has been implicated as the main driver of these high values. Alternative sources include mercury released from glacial melt and erosion, atmospheric mercury pollution, or surface mining. To determine the summer source and speciation of mercury across the Yukon River basin within Canada, we sampled water from 12 tributaries and the mainstem during July 2021. The total (unfiltered) mercury concentration in the glacier-fed White River was 57 ng/L, >10 times higher than all other sampled tributaries. The White River's high total mercury concentrations were driven by suspended sediment and persisted ∼300 km downstream of glacierized headwaters. Total mercury concentrations were lowest (typically <2 ng/L) in tributaries downstream of still-water landscape features (e.g., lakes and settling ponds), suggesting these features are effective sinks for sediment-bound mercury. Low total mercury concentrations (∼2 ng/L) were also observed in five tributaries across diverse thawing permafrost landscapes. These results suggest that glacial erosion and meltwater transport, not permafrost, drive enhanced exports of mercury with suspended sediment. Mercury exports may decline as glacial watersheds pass peak water. Other factors, including mercury released from permafrost thaw, are minor components at present.

The age of the opening of the Ice-Free Corridor and implications for the peopling of the Americas.

SignificanceThe Ice-Free Corridor (IFC) has long played a key role in hypotheses about the peopling of the Americas. Earlier assessments of its age suggested that the IFC was available for a Clovis-first migration, but subsequent developments now suggest a pre-Clovis occupation of the Americas that occurred before the opening of the IFC, thus supporting a Pacific coastal migration route instead. However, large uncertainties in existing ages from the IFC cannot preclude its availability as a route for the first migrations. Resolving this debate over migration route is important for addressing the questions of when and how the first Americans arrived. We report cosmogenic nuclide exposure ages that show that the final opening of the IFC occurred well after pre-Clovis occupation.

Increasing Pleistocene permafrost persistence and carbon cycle conundrums inferred from Canadian speleothems.

Permafrost carbon represents a potentially powerful amplifier of climate change, but little is known about permafrost sensitivity and associated carbon cycling during past warm intervals. We reconstruct permafrost history in western Canada during Pleistocene interglacials from 130 uranium-thorium ages on 72 speleothems, cave deposits that only accumulate with deep ground thaw. We infer that permafrost thaw extended to the high Arctic during one or more periods between ~1.5 million and 0.5 million years ago but has been limited to the sub-Arctic since 400,000 years ago. Our Canadian speleothem growth history closely parallels an analogous reconstruction from Siberia, suggesting that this shift toward more stable permafrost across the Pleistocene may have been Arctic-wide. In contrast, interglacial greenhouse gas concentrations were relatively stable throughout the Pleistocene, suggesting that either permafrost thaw did not trigger substantial carbon release to the atmosphere or it was offset by carbon uptake elsewhere on glacial-interglacial time scales.

Fossil and genomic evidence constrains the timing of bison arrival in North America.

The arrival of bison in North America marks one of the most successful large-mammal dispersals from Asia within the last million years, yet the timing and nature of this event remain poorly determined. Here, we used a combined paleontological and paleogenomic approach to provide a robust timeline for the entry and subsequent evolution of bison within North America. We characterized two fossil-rich localities in Canada's Yukon and identified the oldest well-constrained bison fossil in North America, a 130,000-y-old steppe bison, Bison cf. priscus We extracted and sequenced mitochondrial genomes from both this bison and from the remains of a recently discovered, ∼120,000-y-old giant long-horned bison, Bison latifrons, from Snowmass, Colorado. We analyzed these and 44 other bison mitogenomes with ages that span the Late Pleistocene, and identified two waves of bison dispersal into North America from Asia, the earliest of which occurred ∼195-135 thousand y ago and preceded the morphological diversification of North American bison, and the second of which occurred during the Late Pleistocene, ∼45-21 thousand y ago. This chronological arc establishes that bison first entered North America during the sea level lowstand accompanying marine isotope stage 6, rejecting earlier records of bison in North America. After their invasion, bison rapidly colonized North America during the last interglaciation, spreading from Alaska through continental North America; they have been continuously resident since then.

Genomic Data from Extinct North American Camelops Revise Camel Evolutionary History.

Recent advances in paleogenomic technologies have enabled an increasingly detailed understanding of the evolutionary relationships of now-extinct mammalian taxa. However, a number of enigmatic Quaternary species have never been characterized with molecular data, often because available fossils are rare or are found in environments that are not optimal for DNA preservation. Here, we analyze paleogenomic data extracted from bones attributed to the late Pleistocene western camel, Camelops cf. hesternus, a species that was distributed across central and western North America until its extinction approximately 13,000 years ago. Despite a modal sequence length of only around 35 base pairs, we reconstructed high-coverage complete mitochondrial genomes and low-coverage partial nuclear genomes for each specimen. We find that Camelops is sister to African and Asian bactrian and dromedary camels, to the exclusion of South American camelids (llamas, guanacos, alpacas, and vicuñas). These results contradict previous morphology-based phylogenetic models for Camelops, which suggest instead a closer relationship between Camelops and the South American camelids. The molecular data imply a Late Miocene divergence of the Camelops clade from lineages that separately gave rise to the extant camels of Eurasia. Our results demonstrate the increasing capacity of modern paleogenomic methods to resolve evolutionary relationships among distantly related lineages.

American mastodon extirpation in the Arctic and Subarctic predates human colonization and terminal Pleistocene climate change.

Existing radiocarbon ((14)C) dates on American mastodon (Mammut americanum) fossils from eastern Beringia (Alaska and Yukon) have been interpreted as evidence they inhabited the Arctic and Subarctic during Pleistocene full-glacial times (∼ 18,000 (14)C years B.P.). However, this chronology is inconsistent with inferred habitat preferences of mastodons and correlative paleoecological evidence. To establish a last appearance date (LAD) for M. americanum regionally, we obtained 53 new (14)C dates on 36 fossils, including specimens with previously published dates. Using collagen ultrafiltration and single amino acid (hydroxyproline) methods, these specimens consistently date to beyond or near the ∼ 50,000 y B.P. limit of (14)C dating. Some erroneously "young" (14)C dates are due to contamination by exogenous carbon from natural sources and conservation treatments used in museums. We suggest mastodons inhabited the high latitudes only during warm intervals, particularly the Last Interglacial [Marine Isotope Stage (MIS) 5] when boreal forests existed regionally. Our (14)C dataset suggests that mastodons were extirpated from eastern Beringia during the MIS 4 glacial interval (∼ 75,000 y ago), following the ecological shift from boreal forest to steppe tundra. Mastodons thereafter became restricted to areas south of the continental ice sheets, where they suffered complete extinction ∼ 10,000 (14)C years B.P. Mastodons were already absent from eastern Beringia several tens of millennia before the first humans crossed the Bering Isthmus or the onset of climate changes during the terminal Pleistocene. Local extirpations of mastodons and other megafaunal populations in eastern Beringia were asynchrononous and independent of their final extinction south of the continental ice sheets.

South Greenland ice-sheet collapse during Marine Isotope Stage 11.

Varying levels of boreal summer insolation and associated Earth system feedbacks led to differing climate and ice-sheet states during late-Quaternary interglaciations. In particular, Marine Isotope Stage (MIS) 11 was an exceptionally long interglaciation and potentially had a global mean sea level 6 to 13 metres above the present level around 410,000 to 400,000 years ago, implying substantial mass loss from the Greenland ice sheet (GIS). There are, however, no model simulations and only limited proxy data to constrain the magnitude of the GIS response to climate change during this 'super interglacial', thus confounding efforts to assess climate/ice-sheet threshold behaviour and associated sea-level rise. Here we show that the south GIS was drastically smaller during MIS 11 than it is now, with only a small residual ice dome over southernmost Greenland. We use the strontium-neodymium-lead isotopic composition of proglacial sediment discharged from south Greenland to constrain the provenance of terrigenous silt deposited on the Eirik Drift, a sedimentary deposit off the south Greenland margin. We identify a major reduction in sediment input derived from south Greenland's Precambrian bedrock terranes, probably reflecting the cessation of subglacial erosion and sediment transport as a result of near-complete deglaciation of south Greenland. Comparison with ice-sheet configurations from numerical models suggests that the GIS lost about 4.5 to 6 metres of sea-level-equivalent volume during MIS 11. This is evidence for late-Quaternary GIS collapse after it crossed a climate/ice-sheet stability threshold that may have been no more than several degrees above pre-industrial temperatures.

Pristine Early Eocene wood buried deeply in kimberlite from northern Canada.

We report exceptional preservation of fossil wood buried deeply in a kimberlite pipe that intruded northwestern Canada's Slave Province 53.3±0.6 million years ago (Ma), revealed during excavation of diamond source rock. The wood originated from forest surrounding the eruption zone and collapsed into the diatreme before resettling in volcaniclastic kimberlite to depths >300 m, where it was mummified in a sterile environment. Anatomy of the unpermineralized wood permits conclusive identification to the genus Metasequoia (Cupressaceae). The wood yields genuine cellulose and occluded amber, both of which have been characterized spectroscopically and isotopically. From cellulose δ(18)O and δ(2)H measurements, we infer that Early Eocene paleoclimates in the western Canadian subarctic were 12-17°C warmer and four times wetter than present. Canadian kimberlites offer Lagerstätte-quality preservation of wood from a region with limited alternate sources of paleobotanical information.

Sr-Nd-Pb isotope evidence for ice-sheet presence on southern Greenland during the Last Interglacial.

To ascertain the response of the southern Greenland Ice Sheet (GIS) to a boreal summer climate warmer than at present, we explored whether southern Greenland was deglaciated during the Last Interglacial (LIG), using the Sr-Nd-Pb isotope ratios of silt-sized sediment discharged from southern Greenland. Our isotope data indicate that no single southern Greenland geologic terrane was completely deglaciated during the LIG, similar to the Holocene. Differences in sediment sources during the LIG relative to the early Holocene denote, however, greater southern GIS retreat during the LIG. These results allow the evaluation of a suite of GIS models and are consistent with a GIS contribution of 1.6 to 2.2 meters to the ≥4-meter LIG sea-level highstand, requiring a significant sea-level contribution from the Antarctic Ice Sheet.

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH4.

Peatlands are a key component of the global carbon cycle. Chronologies of peatland initiation are typically based on compiled basal peat radiocarbon (14C) dates and frequency histograms of binned calibrated age ranges. However, such compilations are problematic because poor quality 14C dates are commonly included and because frequency histograms of binned age ranges introduce chronological artefacts that bias the record of peatland initiation. Using a published compilation of 274 basal 14C dates from Alaska as a case study, we show that nearly half the 14C dates are inappropriate for reconstructing peatland initiation, and that the temporal structure of peatland initiation is sensitive to sampling biases and treatment of calibrated 14C dates. We present revised chronologies of peatland initiation for Alaska and the circumpolar Arctic based on summed probability distributions of calibrated 14C dates. These revised chronologies reveal that northern peatland initiation lagged abrupt increases in atmospheric CH4 concentration at the start of the Bølling-Allerød interstadial (Termination 1A) and the end of the Younger Dryas chronozone (Termination 1B), suggesting that northern peatlands were not the primary drivers of the rapid increases in atmospheric CH4. Our results demonstrate that subtle methodological changes in the synthesis of basal 14C ages lead to substantially different interpretations of temporal trends in peatland initiation, with direct implications for the role of peatlands in the global carbon cycle.

Ancient DNA reveals late survival of mammoth and horse in interior Alaska.

Causes of late Quaternary extinctions of large mammals ("megafauna") continue to be debated, especially for continental losses, because spatial and temporal patterns of extinction are poorly known. Accurate latest appearance dates (LADs) for such taxa are critical for interpreting the process of extinction. The extinction of woolly mammoth and horse in northwestern North America is currently placed at 15,000-13,000 calendar years before present (yr BP), based on LADs from dating surveys of macrofossils (bones and teeth). Advantages of using macrofossils to estimate when a species became extinct are offset, however, by the improbability of finding and dating the remains of the last-surviving members of populations that were restricted in numbers or confined to refugia. Here we report an alternative approach to detect 'ghost ranges' of dwindling populations, based on recovery of ancient DNA from perennially frozen and securely dated sediments (sedaDNA). In such contexts, sedaDNA can reveal the molecular presence of species that appear absent in the macrofossil record. We show that woolly mammoth and horse persisted in interior Alaska until at least 10,500 yr BP, several thousands of years later than indicated from macrofossil surveys. These results contradict claims that Holocene survival of mammoths in Beringia was restricted to ecologically isolated high-latitude islands. More importantly, our finding that mammoth and horse overlapped with humans for several millennia in the region where people initially entered the Americas challenges theories that megafaunal extinction occurred within centuries of human arrival or were due to an extraterrestrial impact in the late Pleistocene.

Ancient permafrost and a future, warmer Arctic.

Climate models predict extensive and severe degradation of permafrost in response to global warming, with a potential for release of large volumes of stored carbon. However, the accuracy of these models is difficult to evaluate because little is known of the history of permafrost and its response to past warm intervals of climate. We report the presence of relict ground ice in subarctic Canada that is greater than 700,000 years old, with the implication that ground ice in this area has survived past interglaciations that were warmer and of longer duration than the present interglaciation.