Ancient plant DNA reveals High Arctic greening during the Last Interglacial.

Summer warming is driving a greening trend across the Arctic, with the potential for large-scale amplification of climate change due to vegetation-related feedbacks [Pearson et al., Nat. Clim. Chang. (3), 673-677 (2013)]. Because observational records are sparse and temporally limited, past episodes of Arctic warming can help elucidate the magnitude of vegetation response to temperature change. The Last Interglacial ([LIG], 129,000 to 116,000 y ago) was the most recent episode of Arctic warming on par with predicted 21st century temperature change [Otto-Bliesner et al., Philos. Trans. A Math. Phys. Eng. Sci. (371), 20130097 (2013) and Post et al., SciAdv (5), eaaw9883 (2019)]. However, high-latitude terrestrial records from this period are rare, so LIG vegetation distributions are incompletely known. Pollen-based vegetation reconstructions can be biased by long-distance pollen transport, further obscuring the paleoenvironmental record. Here, we present a LIG vegetation record based on ancient DNA in lake sediment and compare it with fossil pollen. Comprehensive plant community reconstructions through the last and current interglacial (the Holocene) on Baffin Island, Arctic Canada, reveal coherent climate-driven community shifts across both interglacials. Peak LIG warmth featured a ∼400-km northward range shift of dwarf birch, a key woody shrub that is again expanding northward. Greening of the High Arctic-documented here by multiple proxies-likely represented a strong positive feedback on high-latitude LIG warming. Authenticated ancient DNA from this lake sediment also extends the useful preservation window for the technique and highlights the utility of combining traditional and molecular approaches for gleaning paleoenvironmental insights to better anticipate a warmer future.

Arctic shrub colonization lagged peak postglacial warmth: Molecular evidence in lake sediment from Arctic Canada.

Arctic shrubification is an observable consequence of climate change, already resulting in ecological shifts and global-scale climate feedbacks including changes in land surface albedo and enhanced evapotranspiration. However, the rate at which shrubs can colonize previously glaciated terrain in a warming world is largely unknown. Reconstructions of past vegetation dynamics in conjunction with climate records can provide critical insights into shrubification rates and controls on plant migration, but paleoenvironmental reconstructions based on pollen may be biased by the influx of exotic pollen to tundra settings. Here, we reconstruct past plant communities using sedimentary ancient DNA (sedaDNA), which has a more local source area than pollen. We additionally reconstruct past temperature variability using bacterial cell membrane lipids (branched glycerol dialkyl glycerol tetraethers) and an aquatic productivity indicator (biogenic silica) to evaluate the relative timing of postglacial ecological and climate changes at a lake on southern Baffin Island, Arctic Canada. The sedaDNA record tightly constrains the colonization of dwarf birch (Betula, a thermophilous shrub) to 5.9 ± 0.1 ka, ~3 ka after local deglaciation as determined by cosmogenic 10 Be moraine dating and >2 ka later than Betula pollen is recorded in nearby lake sediment. We then assess the paleovegetation history within the context of summer temperature and find that paleotemperatures were highest prior to 6.3 ka, followed by cooling in the centuries preceding Betula establishment. Together, these molecular proxies reveal that Betula colonization lagged peak summer temperatures, suggesting that inefficient dispersal, rather than climate, may have limited Arctic shrub migration in this region. In addition, these data suggest that pollen-based climate reconstructions from high latitudes, which rely heavily on the presence and abundance of pollen from thermophilous taxa like Betula, can be compromised by both exotic pollen fluxes and vegetation migration lags.

Delayed postglacial colonization of Betula in Iceland and the circum North Atlantic.

As the Arctic continues to warm, woody shrubs are expected to expand northward. This process, known as 'shrubification,' has important implications for regional biodiversity, food web structure, and high-latitude temperature amplification. While the future rate of shrubification remains poorly constrained, past records of plant immigration to newly deglaciated landscapes in the Arctic may serve as useful analogs. We provide one new postglacial Holocene sedimentary ancient DNA (sedaDNA) record of vascular plants from Iceland and place a second Iceland postglacial sedaDNA record on an improved geochronology; both show Salicaceae present shortly after deglaciation, whereas Betulaceae first appears more than 1000 y later. We find a similar pattern of delayed Betulaceae colonization in eight previously published postglacial sedaDNA records from across the glaciated circum North Atlantic. In nearly all cases, we find that Salicaceae colonizes earlier than Betulaceae and that Betulaceae colonization is increasingly delayed for locations farther from glacial-age woody plant refugia. These trends in Salicaceae and Betulaceae colonization are consistent with the plant families' environmental tolerances, species diversity, reproductive strategies, seed sizes, and soil preferences. As these reconstructions capture the efficiency of postglacial vascular plant migration during a past period of high-latitude warming, a similarly slow response of some woody shrubs to current warming in glaciated regions, and possibly non-glaciated tundra, may delay Arctic shrubification and future changes in the structure of tundra ecosystems and temperature amplification.

Molecular exploration of fossil eggshell uncovers hidden lineage of giant extinct bird.

The systematics of Madagascar's extinct elephant birds remains controversial due to large gaps in the fossil record and poor biomolecular preservation of skeletal specimens. Here, a molecular analysis of 1000-year-old fossil eggshells provides the first description of elephant bird phylogeography and offers insight into the ecology and evolution of these flightless giants. Mitochondrial genomes from across Madagascar reveal genetic variation that is correlated with eggshell morphology, stable isotope composition, and geographic distribution. The elephant bird crown is dated to ca. 30 Mya, when Madagascar is estimated to have become less arid as it moved northward. High levels of between-clade genetic variation support reclassifying Mullerornis into a separate family. Low levels of within-clade genetic variation suggest there were only two elephant bird genera existing in southern Madagascar during the Holocene. However, we find an eggshell collection from Madagascar's far north that represents a unique lineage of Aepyornis. Furthermore, divergence within Aepyornis coincides with the aridification of Madagascar during the early Pleistocene ca. 1.5 Ma, and is consistent with the fragmentation of populations in the highlands driving diversification and the evolution of extreme gigantism over shorts timescales. We advocate for a revision of their taxonomy that integrates palaeogenomic and palaeoecological perspectives.

Recent changes in a remote Arctic lake are unique within the past 200,000 years.

The Arctic is currently undergoing dramatic environmental transformations, but it remains largely unknown how these changes compare with long-term natural variability. Here we present a lake sediment sequence from the Canadian Arctic that records warm periods of the past 200,000 years, including the 20th century. This record provides a perspective on recent changes in the Arctic and predates by approximately 80,000 years the oldest stratigraphically intact ice core recovered from the Greenland Ice Sheet. The early Holocene and the warmest part of the Last Interglacial (Marine Isotope Stage or MIS 5e) were the only periods of the past 200,000 years with summer temperatures comparable to or exceeding today's at this site. Paleoecological and geochemical data indicate that the past three interglacial periods were characterized by similar trajectories in temperature, lake biology, and lakewater pH, all of which tracked orbitally-driven solar insolation. In recent decades, however, the study site has deviated from this recurring natural pattern and has entered an environmental regime that is unique within the past 200 millennia.

Near-universal trends in brGDGT lipid distributions in nature.

Bacterial brGDGT lipids are a prevalent tool in studies of terrestrial paleoclimate. Their distributions correlate empirically with environmental temperature and pH, and their ubiquity in terrestrial, freshwater, and marine environments gives them wide applicability. Whether correlations with temperature and pH emerge due to a physiological response of source organisms and/or a shift in bacterial community composition remains an open question with important implications for proxy development and application. We applied a newly described technique for grouping brGDGTs to a globally compiled dataset (n = 3129) consisting of all modern sample media known to host brGDGTs. We found strong resemblances in the relationships between brGDGT fractional abundances and both temperature and pH across nearly all sample types examined. We also found near-universal connections between the brGDGTs themselves. Given the markedly different bacterial communities expected to inhabit these settings, these widespread relationships may suggest physiological and/or biochemical bases for observed brGDGT distributions.

Terrestrial evidence for ocean forcing of Heinrich events and subglacial hydrologic connectivity of the Laurentide Ice Sheet.

During the last glacial period, the Laurentide Ice Sheet (LIS) underwent episodes of rapid iceberg discharge, recorded in ocean sediments as "Heinrich events" (HEs). Two competing models attempt to describe the stimulus for HEs via either internal ice sheet oscillations or external ocean-climate system forcing. We present a terrestrial record of HEs from the northeastern LIS that strongly supports ocean-climate forcing. Subglacial carbonate precipitates from Baffin Island record episodes of subglacial melting coincident with the three most recent HEs, resulting from acceleration of nearby marine-terminating ice streams. Synchronized ice stream acceleration over Baffin Island and Hudson Strait is inconsistent with internal ice sheet oscillations alone and indicates a shared ocean-climate stimulus to coordinate these different glaciological systems. Isotopic compositions of these precipitates record widespread subglacial groundwater connectivity beneath the LIS. Extensive basal melting and flushing of these aquifers during the last HE may have been a harbinger for terminal deglaciation.

Quaternary record of aridity and mean annual precipitation based on δ15N in ratite and dromornithid eggshells from Lake Eyre, Australia.

The cause(s) of the late Pleistocene megafauna extinction on the Australian continent remains largely unresolved. Unraveling climatic forcing mechanisms from direct or indirect human agents of ecosystem alteration has proven to be extremely difficult in Australia due to the lack of (1) well-dated vertebrate fossils and (2) paleo-environmental and -ecological records spanning the past approximately 100 ka when regional climatic conditions are known to have significantly varied. We have examined the nitrogen isotope composition (δ(15)N) of modern emu (Dromaius novaehollandiae) eggshells collected along a precipitation gradient in Australia, along with modern climatological data and dietary δ(15)N values. We then used modern patterns to interpret an approximately 130-ka record of δ(15)N values in extant Dromaius and extinct Genyornis newtoni eggshells from Lake Eyre to obtain a novel mean annual precipitation (MAP) record for central Australia spanning the extinction interval. Our data also provide the first detailed information on the trophic ecology and environmental preferences of two closely related taxa, one extant and one extinct. Dromaius eggshell δ(15)N values show a significant shift to higher values during the Last Glacial Maximum and Holocene, which we interpret to indicate more frequent arid conditions (<200 mm MAP), relative to δ(15)N from samples just prior to the megafauna extinction. Genyornis eggshells had δ(15)N values reflecting wetter nesting conditions overall relative to those of coeval Dromaius, perhaps indicating that Genyornis was more reliant on mesic conditions. Lastly, the Dromaius eggshell record shows a significant decrease in δ(13)C values prior to the extinction, whereas the Genyornis record does not. Neither species showed a concomitant change in δ(15)N prior to the extinction, which suggests that a significant change in vegetation surrounding Lake Eyre occurred prior to an increase in local aridity.

Fossil avian eggshell preserves ancient DNA.

Owing to exceptional biomolecule preservation, fossil avian eggshell has been used extensively in geochronology and palaeodietary studies. Here, we show, to our knowledge, for the first time that fossil eggshell is a previously unrecognized source of ancient DNA (aDNA). We describe the successful isolation and amplification of DNA from fossil eggshell up to 19 ka old. aDNA was successfully characterized from eggshell obtained from New Zealand (extinct moa and ducks), Madagascar (extinct elephant birds) and Australia (emu and owl). Our data demonstrate excellent preservation of the nucleic acids, evidenced by retrieval of both mitochondrial and nuclear DNA from many of the samples. Using confocal microscopy and quantitative PCR, this study critically evaluates approaches to maximize DNA recovery from powdered eggshell. Our quantitative PCR experiments also demonstrate that moa eggshell has approximately 125 times lower bacterial load than bone, making it a highly suitable substrate for high-throughput sequencing approaches. Importantly, the preservation of DNA in Pleistocene eggshell from Australia and Holocene deposits from Madagascar indicates that eggshell is an excellent substrate for the long-term preservation of DNA in warmer climates. The successful recovery of DNA from this substrate has implications in a number of scientific disciplines; most notably archaeology and palaeontology, where genotypes and/or DNA-based species identifications can add significantly to our understanding of diets, environments, past biodiversity and evolutionary processes.

Rapidly receding Arctic Canada glaciers revealing landscapes continuously ice-covered for more than 40,000 years.

Arctic temperatures are increasing faster than the Northern Hemisphere average due to strong positive feedbacks unique to polar regions. However, the degree to which recent Arctic warming is unprecedented remains debated. Ages of entombed plants in growth position preserved by now receding ice caps in Arctic Canada help to address this issue by placing recent conditions in a multi-millennial context. Here we show that pre-Holocene radiocarbon dates on plants collected at the margins of 30 ice caps in Arctic Canada suggest those locations were continuously ice covered for > 40 kyr, but are now ice-free. We use in situ 14C inventories in rocks from nine locations to explore the possibility of brief exposure during the warm early Holocene. Modeling the evolution of in situ 14C confirms that Holocene exposure is unlikely at all but one of the sites. Viewed in the context of temperature records from Greenland ice cores, our results suggest that summer warmth of the past century exceeds now any century in ~115,000 years.

Humans rather than climate the primary cause of Pleistocene megafaunal extinction in Australia.

Environmental histories that span the last full glacial cycle and are representative of regional change in Australia are scarce, hampering assessment of environmental change preceding and concurrent with human dispersal on the continent ca. 47,000 years ago. Here we present a continuous 150,000-year record offshore south-western Australia and identify the timing of two critical late Pleistocene events: wide-scale ecosystem change and regional megafaunal population collapse. We establish that substantial changes in vegetation and fire regime occurred ∼70,000 years ago under a climate much drier than today. We record high levels of the dung fungus Sporormiella, a proxy for herbivore biomass, from 150,000 to 45,000 years ago, then a marked decline indicating megafaunal population collapse, from 45,000 to 43,100 years ago, placing the extinctions within 4,000 years of human dispersal across Australia. These findings rule out climate change, and implicate humans, as the primary extinction cause.

A comprehensive database of quality-rated fossil ages for Sahul's Quaternary vertebrates.

The study of palaeo-chronologies using fossil data provides evidence for past ecological and evolutionary processes, and is therefore useful for predicting patterns and impacts of future environmental change. However, the robustness of inferences made from fossil ages relies heavily on both the quantity and quality of available data. We compiled Quaternary non-human vertebrate fossil ages from Sahul published up to 2013. This, the FosSahul database, includes 9,302 fossil records from 363 deposits, for a total of 478 species within 215 genera, of which 27 are from extinct and extant megafaunal species (2,559 records). We also provide a rating of reliability of individual absolute age based on the dating protocols and association between the dated materials and the fossil remains. Our proposed rating system identified 2,422 records with high-quality ages (i.e., a reduction of 74%). There are many applications of the database, including disentangling the confounding influences of hypothetical extinction drivers, better spatial distribution estimates of species relative to palaeo-climates, and potentially identifying new areas for fossil discovery.

Climate change not to blame for late Quaternary megafauna extinctions in Australia.

Late Quaternary megafauna extinctions impoverished mammalian diversity worldwide. The causes of these extinctions in Australia are most controversial but essential to resolve, because this continent-wide event presaged similar losses that occurred thousands of years later on other continents. Here we apply a rigorous metadata analysis and new ensemble-hindcasting approach to 659 Australian megafauna fossil ages. When coupled with analysis of several high-resolution climate records, we show that megafaunal extinctions were broadly synchronous among genera and independent of climate aridity and variability in Australia over the last 120,000 years. Our results reject climate change as the primary driver of megafauna extinctions in the world's most controversial context, and instead estimate that the megafauna disappeared Australia-wide ∼13,500 years after human arrival, with shorter periods of coexistence in some regions. This is the first comprehensive approach to incorporate uncertainty in fossil ages, extinction timing and climatology, to quantify mechanisms of prehistorical extinctions.

Recent warming reverses long-term arctic cooling.

The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60 degrees N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Simulating Arctic climate warmth and icefield retreat in the last interglaciation.

In the future, Arctic warming and the melting of polar glaciers will be considerable, but the magnitude of both is uncertain. We used a global climate model, a dynamic ice sheet model, and paleoclimatic data to evaluate Northern Hemisphere high-latitude warming and its impact on Arctic icefields during the Last Interglaciation. Our simulated climate matches paleoclimatic observations of past warming, and the combination of physically based climate and ice-sheet modeling with ice-core constraints indicate that the Greenland Ice Sheet and other circum-Arctic ice fields likely contributed 2.2 to 3.4 meters of sea-level rise during the Last Interglaciation.

Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise.

Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.

Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction.

Most of Australia's largest mammals became extinct 50,000 to 45,000 years ago, shortly after humans colonized the continent. Without exceptional climate change at that time, a human cause is inferred, but a mechanism remains elusive. A 140,000-year record of dietary delta(13)C documents a permanent reduction in food sources available to the Australian emu, beginning about the time of human colonization; a change replicated at three widely separated sites and in the marsupial wombat. We speculate that human firing of landscapes rapidly converted a drought-adapted mosaic of trees, shrubs, and nutritious grasslands to the modern fire-adapted desert scrub. Animals that could adapt survived; those that could not, became extinct.