Christopher R. Moore

Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

Significance We present detailed geochemical and morphological analyses of nearly 700 spherules from 18 sites in support of a major cosmic impact at the onset of the Younger Dryas episode (12.8 ka). The impact distributed ∼10 million tonnes of melted spherules over 50 million square kilometers on four continents. Origins of the spherules by volcanism, anthropogenesis, authigenesis, lightning, and meteoritic ablation are rejected on geochemical and morphological grounds. The spherules closely resemble known impact materials derived from surficial sediments melted at temperatures >2,200 °C. The spherules correlate with abundances of associated melt-glass, nanodiamonds, carbon spherules, aciniform carbon, charcoal, and iridium. Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.

Bayesian chronological analyses consistent with synchronous age of 12,835-12,735 Cal B.P. for Younger Dryas boundary on four continents.

Significance A cosmic impact event at ∼12,800 Cal B.P. formed the Younger Dryas boundary (YDB) layer, containing peak abundances in multiple, high-temperature, impact-related proxies, including spherules, melt glass, and nanodiamonds. Bayesian statistical analyses of 354 dates from 23 sedimentary sequences over four continents established a modeled YDB age range of 12,835 Cal B.P. to 12,735 Cal B.P., supporting synchroneity of the YDB layer at high probability (95%). This range overlaps that of a platinum peak recorded in the Greenland Ice Sheet and of the onset of the Younger Dryas climate episode in six key records, suggesting a causal connection between the impact event and the Younger Dryas. Due to its rarity and distinctive characteristics, the YDB layer is proposed as a widespread correlation datum. The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemisphere deposited the Younger Dryas boundary (YDB) layer, containing peak abundances in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, platinum, and osmium. Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835–12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas. Two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of the YDB layer within the limits of dating uncertainty (∼100 y). The widespread distribution of the YDB layer suggests that it may serve as a datum layer.

Early Hunter-Gatherer Tool Use and Animal Exploitation: Protein and Microwear Evidence from the Central Savannah River Valley

Abstract Results of protein residue and lithic microwear analyses are reported for Paleoindian and Early Archaic stone tools from a Carolina bay sand rim on the Aiken Plateau of South Carolina, USA. Protein residue analysis is performed using crossover Immunoelectrophoresis (CIEP), and indicates positive results for Bovidae, Cervidae, Galliformes, and Meleagris gallopavo. These results are complemented by a larger immunological study of 135 diagnostic hafted bifaces from South Carolina and Georgia. Among other species identified, bovid residue was found on multiple Paleoindian hafted bifaces, an Early Archaic hafted biface, and a Middle Archaic hafted biface. Results suggest continuity of species selection and availability across the Pleistocene/Holocene boundary and provide no support for the exploitation of extinct fauna. The data do provide compelling evidence for a demographic shift and/or regional extirpation of Bovidae possibly as late as the early mid-Holocene in the Southeast. In addition, microwear analysis of artifacts from Flamingo Bay indicate intensive hide scraping, antler boring, bone graving/planing/pointing, wood whittling, and hafting traces. Microwear data suggest intentional snap-fracture or bipolarization of exhausted or broken Clovis points for reuse as hide scrapers, and use of large bifacial knives and unifacial scrapers in intensive defleshing activities consistent with large animal butchery.

Widespread platinum anomaly documented at the Younger Dryas onset in North American sedimentary sequences

Previously, a large platinum (Pt) anomaly was reported in the Greenland ice sheet at the Younger Dryas boundary (YDB) (12,800 Cal B.P.). In order to evaluate its geographic extent, fire-assay and inductively coupled plasma mass spectrometry (FA and ICP-MS) elemental analyses were performed on 11 widely separated archaeological bulk sedimentary sequences. We document discovery of a distinct Pt anomaly spread widely across North America and dating to the Younger Dryas (YD) onset. The apparent synchroneity of this widespread YDB Pt anomaly is consistent with Greenland Ice Sheet Project 2 (GISP2) data that indicated atmospheric input of platinum-rich dust. We expect the Pt anomaly to serve as a widely-distributed time marker horizon (datum) for identification and correlation of the onset of the YD climatic episode at 12,800 Cal B.P. This Pt datum will facilitate the dating and correlating of archaeological, paleontological, and paleoenvironmental data between sequences, especially those with limited age control.

SIFTING THE SANDS OF TIME: GEOARCHAEOLOGY, CULTURE CHRONOLOGY, AND CLIMATE CHANGE AT SQUIRES RIDGE, NORTHEASTERN NORTH CAROLINA

Abstract Since 2000, East Carolina University has conducted archaeological research in the Tar River valley in the northern Coastal Plain of North Carolina designed to address poorly understood aspects of the region’s culture-history. In particular, survey and excavation along a portion of the Tar River have focused on problems related to Coastal Plain chronology, typology, and geoarchaeology. Here we provide an overview of testing done at one site, Squires Ridge, that contains stratified Woodland and Archaic period remains in a 1-m deposit of largely aeolian sandy soils. We suggest that the formation of the archaeologically stratified portions of sand ridges along the Tar River reflect millennial-scale climatic cyclicity representing regional manifestations of climate change during the Early to Middle Holocene. Sand ridges along the Tar River likely represent proxies of climate change, while the archaeology contained within them manifest human adaptations to such change.

Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments

Part 1 of this study investigated evidence of biomass burning in global ice records, and here we continue to test the hypothesis that an impact event at the Younger Dryas boundary (YDB) caused an anomalously intense episode of biomass burning at ∼12.8 ka on a multicontinental scale (North and South America, Europe, and Asia). Quantitative analyses of charcoal and soot records from 152 lakes, marine cores, and terrestrial sequences reveal a major peak in biomass burning at the Younger Dryas (YD) onset that appears to be the highest during the latest Quaternary. For the Cretaceous-Tertiary boundary (K-Pg) impact event, concentrations of soot were previously utilized to estimate the global amount of biomass burned, and similar measurements suggest that wildfires at the YD onset rapidly consumed ∼10 million km2 of Earth’s surface, or ∼9% of Earth’s biomass, considerably more than for the K-Pg impact. Bayesian analyses and age regressions demonstrate that ages for YDB peaks in charcoal and soot across four continents are synchronous with the ages of an abundance peak in platinum in the Greenland Ice Sheet Project 2 (GISP2) ice core and of the YDB impact event (12,835–12,735 cal BP). Thus, existing evidence indicates that the YDB impact event caused an anomalously large episode of biomass burning, resulting in extensive atmospheric soot/dust loading that triggered an “impact winter.” This, in turn, triggered abrupt YD cooling and other climate changes, reinforced by climatic feedback mechanisms, including Arctic sea ice expansion, rerouting of North American continental runoff, and subsequent ocean circulation changes.

Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 1. Ice Cores and Glaciers

The Younger Dryas boundary (YDB) cosmic-impact hypothesis is based on considerable evidence that Earth collided with fragments of a disintegrating ≥100-km-diameter comet, the remnants of which persist within the inner solar system ∼12,800 y later. Evidence suggests that the YDB cosmic impact triggered an “impact winter” and the subsequent Younger Dryas (YD) climate episode, biomass burning, late Pleistocene megafaunal extinctions, and human cultural shifts and population declines. The cosmic impact deposited anomalously high concentrations of platinum over much of the Northern Hemisphere, as recorded at 26 YDB sites at the YD onset, including the Greenland Ice Sheet Project 2 ice core, in which platinum deposition spans ∼21 y (∼12,836–12,815 cal BP). The YD onset also exhibits increased dust concentrations, synchronous with the onset of a remarkably high peak in ammonium, a biomass-burning aerosol. In four ice-core sequences from Greenland, Antarctica, and Russia, similar anomalous peaks in other combustion aerosols occur, including nitrate, oxalate, acetate, and formate, reflecting one of the largest biomass-burning episodes in more than 120,000 y. In support of widespread wildfires, the perturbations in CO2 records from Taylor Glacier, Antarctica, suggest that biomass burning at the YD onset may have consumed ∼10 million km2, or ∼9% of Earth’s terrestrial biomass. The ice record is consistent with YDB impact theory that extensive impact-related biomass burning triggered the abrupt onset of an impact winter, which led, through climatic feedbacks, to the anomalous YD climate episode.

PINE BARRENS AND POSSUM’S RATIONS: EARLY ARCHAIC SETTLEMENT IN THE NORTH CAROLINA SANDHILLS

Abstract Competing models of Early Archaic settlement in the Southeast propose broad-scale organization conditioned by either lithic raw material availability or seasonal exploitation of biotic resources and social interaction. We offer a view from beyond the quarries and away from the river with data from the North Carolina Sandhills, a unique physiographic zone of the interior Coastal Plain. Analysis of the distribution of Early Archaic sites at Fort Bragg, including posited upland base camps, raw material use, and application of GIS least-cost path analysis, suggests intensive interriverine settlement, with watershed divides serving as conveyance corridors between high-quality toolstone areas in the Piedmont and resource blooms in the Coastal Plain. Some settlement changes are evident within the Early Archaic sequence, including a gradual shift from logistical to residential mobility and infilling of the local landscape. In the proposed Sandhills model, biocultural needs, social interaction, and requirements for high-quality toolstone are identified at the local band level. Mobility and settlement are considered to be broadly structured by networks of interriverine trails between the major resource areas of the Piedmont and Coastal Plain. Aspects of both the bandmacroband model and Uwharrie-Allendale model apply to the Early Archaic record of the Sandhills, but with closer affinities to the latter.