Jack H. Ray

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.

Isotopic evidence for Younger Dryas aridity in the North American midcontinent

Determining the impact of the Younger Dryas (YD) climate event on the unglaciated North American midcontinent has proved difficult due to a scarcity of suitable paleoclimate proxies. Here we present a well-dated carbon isotope (δ13C) record from a buried soil sequence in southwestern Missouri, which reveals a large isotopic excursion during the YD chronozone. In this region of the modern prairie-forest border, the δ13C signature of soil organic matter is a reliable indicator of past climatic change because δ13C values are controlled primarily by the relative abundance of C3 and C4 plants, which is tied to the environmental setting. Between ca. 13,200 and 11,900 yr ago, the abundance of C4 grasses increased by upwards of 50% of the total biomass, indicating expansion of grassland most likely driven by increased aridity during this period. Environmental gradients in the midcontinent must have been very steep, because at the same time that a C4-rich prairie existed in southwestern Missouri, spruce forests grew in Iowa, Illinois, and Ohio.

Trampling experiments in the search for the earliest americans

Experiments were undertaken to evaluate a natural versus cultural origin for a set of modified pebbles and cobbles found in pre-Clovis-age contexts at the Big Eddy site (23CE426) in southwest Missouri, U.S.A. Two experiments involving Asian elephants and American bison provided evidence that pre-Clovis-age modified cobbles, pebbles, and flakes probably were produced by the trampling of large mammals traversing alluvial gravel bars in search of food and water in a riparian environment. The production of “zoofacts” and behavioral aspects of elephants are described with respect to research on the earliest Americans.

The Big Eddy Site: A Multicomponent Paleoindian Site on the Ozark Border, Southwest Missouri

Relatively few deep, stratified, multicompanent Paleoindian sites have been found in alluvial contexts in North America. Recent excavations at the Big Eddy site in southwest Missouri, however, reveal the presence of unique, stratified Paleoindian horizons containing Clovis, Dalton, and San Patrice components. Limited excavations below the Clovis stratum also provide strong evidence for the presence of one or more pre-Clovis horizons. Results of the work at Big Eddy, including discussions of geomorphic contexts, prehistoric components, features, artifacts, and radiocarbon assays, are provided.

The 1997 Excavations at the Big Eddy Site (23CE426) in Southwest Missouri

Abstract : Extensive archaeological and geoarchaeological investigations at the Big Eddy site (23CE426) in the Downstream Stockton easement have documented stratified archaeological deposits extending to at least 4.0 m below surface in the central part of the site. This locality was utilized throughout prehistory, but perhaps most intensively during Late Paleoindian and middle Late Archaic times. The site also contains significant Early Archaic, Early/Middle Paleoindian, and possible pre-Clovis components, and has yielded the first reliable dates associated with a fluted point component in this portion of the midcontinent. Reliable dates also have been obtained for the Late Paleoindian horizon, which lies directly above the fluted-point horizon and contains abundant lithic debris produced by Dalton and San Patrice peoples. Based on sedimentological data, thin-section analysis, refitting studies, microdebitage distribution, and the intact nature of numerous lithic features, the early deposits at the Big Eddy site have very good integrity, Analyses focus on site formation processes and dating, carbon isotopes, an extensive lithic (principally chipped-stone) assemblage, a modest archaeobotanical data set, and limited faunal remains. Given the sites great significance and continued erosion, mitigation should be undertaken as soon as possible.

Heat Treatment, Ozarks Cherts, and Prehistoric Toolstone Use in Southwest Missouri

Burlington and Jefferson City cherts often dominate Ozarks lithic assemblages, and this record contains ample evidence for the heat treatment of both. In this paper, we use a technological investment model to understand why prehistoric knappers may have invested in heat treatment at the Big Eddy Site in southwest Missouri. Tech investment models offer one way to evaluate the cost-benefit relationships of various technologies and, consequently, the conditions under which a manufacturing strategy might be adopted. We conduct a heat treatment experiment to measure the untreated utilities of the two materials, the approximate time needed to heat each, and the resultant gains in utility a knapper acquires by spending the time to do so. In the Big Eddy case, the tech investment model suggests that the two toolstones were heat-treated differentially in response to differences in utility gains and availability on the landscape.

Ear spools, ceramics, and burial mounds from southwest Missouri: Caddoan and Spiro connections on the Northern Frontier

ABSTRACT A Caddoan presence in the extreme southwest portion of Missouri was firmly established after investigations were conducted in the upper White River valley in the late 1950s. Later, James Brown exposed the myth that the southwestern Ozarks was a cultural enclave that lagged behind Mississippian developments in other parts of the Trans-Mississippi South. Recent discoveries and reanalysis of artifacts from sites located across much of southwest Missouri suggest that a substantial presence and influence of Caddoan peoples extended far beyond the upper White River valley. This area, referred to here as the Northern Frontier, extends from the lower James River valley on the south to the Osage River on the north. Multiple lines of evidence are presented that suggest this portion of the western Ozarks was within the Caddoan orbit and sphere of influence.

Geoarchaeological Investigations in the Upper North Fork River Valley in Southern Missouri

Abstract The first geoarchaeological investigations in the upper North Fork River valley in 1993 and 1994 revealed the presence of multiple terraces and associated sediment assemblages that contain archaeological deposits. Most of the field work, however, focused on the two lowest and youngest sediment assemblages. These include an aggrading flood plain (F-1) and a low terrace (T-1). The flood plain sediment assemblage is late Holocene in age, probably less than 2,000 years old. Although not common, Late Woodland and Mississippian artifacts are deeply buried in this unit. The T-1 appears to be late Pleistocene in age. Artifacts dating from Late Paleoindian to Mississippian times are very common on T-1 landforms, but they appear to be restricted to the upper 1 m of the sediment assemblage. Alluvial deposits similar to the F-1 sediment assemblage appear to be widespread in the Ozarks, whereas chronological and sedimentological discrepancies exist between the T-1 sediment assemblage in the North Fork River valley and those located elsewhere in the Ozarks, especially north of the Ozarks Divide.