Sunday, January 6, 2019
Saturday, January 5, 2019
Scientists have unearthed ancient artifacts that are upending the history of mankind
Scientists have unearthed ancient artifacts that are upending the history of mankind
- Photos by Dan Conover
- Kelsey Meer, a University of Tennessee undergraduate intern, takes a turn on the expedition's Total Station, which records the coordinates for every artifact uncovered at the Topper Site.
MARTIN, S.C. – At a depth of about four feet, 13,000-year-old artifacts emerge from the floor of a hole known as HS-N207E66 in such dense profusion that they leave the volunteers little room to work.
Like others who've dug here since the 1980s, the crew assigned to HS-N207E66 has reached the Clovis layer at the Topper-Allendale archeological site. Excavations there tell different versions of the same story: Near the end of the last ice age, America's first great artisans came to this hillside to quarry a prized stone tool-making material called chert. The artifacts suggest the intense period of Clovis-era activity begins a few centuries before 11,000 B.C. and fades away roughly 500 years later.
Topper remains one of the most productive Clovis sites in North America, yet Clovis artifacts aren't the reason this place became famous. Just down the hill, below the chert outcropping that attracted the ancients, lies a deeper hole. And in 1988, archaeologists found something impossible there.
At the time, just about every respected archaeologist in the world knew that the mammoth-hunting Clovis people were the first human beings to reach the New World. They knew that the Clovis walked here from Siberia across a land bridge. And they knew there couldn't have been humans here before the Clovis arrived because the Americas were cut off from Europe by oceans before the Ice Age.
But then Dr. Al Goodyear III dug deeper and found simple stone tools buried in sediment far beneath the bottom of Topper's Clovis layer, and this inconvenient evidence pushed him into the midst of an already boisterous scientific revolution. The pre-Clovis rebels eventually prevailed, but for a generation of archaeologists, the price of that victory was paid in friendships, reputations and careers.
So it's only now — after millennia of silence, decades of stereotypes, and years of nasty professional infighting — that we're beginning to re-evaluate these people, the Clovis people. If they weren't the Siberian brutes we thought they were, then who were they?
And perhaps more importantly, was Clovis even "a people," or just the first great American idea?
Made In The South
Clovis culture is the most ancient and famous of the specific "cultural toolkits" archaeologists have identified in the American pre-historical past, and once upon a time, scientists thought they understood the Clovis story pretty well. Yet the more data modern researchers collect, the more mysterious the Clovis people become.
Unlike the more regional prehistoric cultures that followed, Clovis culture literally left its artifacts all over the map. Numerous finds over the past 85 years show that these wildly successful hunter-gatherers ranged widely across North and Central America and probed down into South America, too. But despite leaving so much evidence across so much dangerous wilderness real estate, the Clovis era was surprisingly short. The dates fluctuate as modern techniques for measuring the age of artifacts improve, but good estimates now suggest that Clovis culture probably emerged roughly 13,300 years ago, give or take a century or more, and faded out around the time of a sudden climate change that began about 12,800 years ago.
The late 20th century fight over Clovis-first orthodoxy split American archaeology from the mid-1970s to the late 2000s. The Old Guard, with their careers based on Clovis-first scholarship, challenged every new find down to its tiniest details. The rebels countered with more finds and took beating after beating before finally winning over a majority of their peers around 2008.
Because of that victory, there's an argument to be made today that the first great American invention was the Clovis Point, an oversized, symmetrical, double-edged spear tip with a particularly deft and risky design feature. In light of recent developments, it looks increasingly likely that this distinctive weapon — and the culture it defines — originated here in the Southeastern United States. Which would make the Clovis Point not only the first great American invention, but the South's first cultural export, pre-dating jazz and Elvis by at least 13,200 years.
"The technology is most likely home-grown," says expedition operations manager Tom Pertierra of Greenville, Fla., "because there are lanceolate points in North America that are older than Clovis and they demonstrate similar thinning technology. And the farther west you go in dating Clovis, the younger they are, not older."
In other words, Clovis didn't start in Siberia and migrate to the Carolinas. It started somewhere in South Carolina, Georgia, Florida, or Alabama, and after covering the Deep South, it moved west and north.
Connect The Dots
The idea that Clovis originated in the Southeast is only now entering the public discussion mainly because of the limits of human science. Archaeologists most likely didn't recognize the evidence because they were rational human beings who logically assumed they were looking for another answer.
To understand their dilemma, consider that the practice of archaeology is a bit like asking someone who has never driven a car to draw a picture of a 1975 AMC Pacer based on nothing more than a pitted chrome door handle and a lug nut. Whenever the past challenges us to connect its dots, much of what we see tends to resemble much of what we already assume, and so it has gone with the people we call the Clovis.
Named after the town in New Mexico where their artifacts were first identified in the 1930s, the Clovis people rose to the unassailed status as the First Americans in less than three decades. Along the way, the story of their pioneering ice-age migration became textbook dogma.
Since mainstream science in the mid-20th century assumed that reaching the New World by boat required nothing less than 15th-century sailing ships, migration across the ice-age land bridge seemed a logical assumption. With that idea in place, it was only reasonable that migration patterns in the Americas had to run from north to south and from west to east.
Other questionable assumptions followed. Because Clovis Points have been found embedded in butchered mammoth bones, writers extrapolated that the Clovis were macho super-hunters with an obsessive taste for big, dangerous animals. They must have been restless and aggressive, a male-dominated, bloodthirsty blitzkrieg that conquered a violent wilderness.
If they had a tragic flaw, it was excess awesomeness. The Clovis were so badass, the textbooks said, that they hounded the great North American megafauna to extinction in just a few gluttonous generations. Finally, with their conquest complete, the mighty Clovis retired, ceding the benefits of their hard-won glories to their less-ambitious descendants.
Most of the facts that shaped those ideas were derived by good science, but in retrospect, our mid-20th century theories about the Clovis people sound a bit too much like some middle-aged white guy at a VFW hall telling his life story to a bunch of ungrateful beatniks in 1966. When we were kids, we had to ambush a mastodon if we wanted breakfast, and sometimes it killed us. And we liked it.
But while that story aged poorly, it remained archaeology's default explanation for the peopling of the Americas until the late 2000s, when the increasing weight of three decades of evidence finally flipped Clovis-first orthodoxy on its collective head.
Today most scientists agree that there were people all over the Americas thousands of years before the Clovis appeared. Since that means at least some of those early arrivals probably got here by boat, our estimation of our Paleolithic ancestors is in the midst of an upgrade.
Yet with so much emphasis on who got here first, the Clovis themselves somehow faded into the background. It's only now, in the context of a more open-ended theory of our deep past, that the Clovis are re-emerging to renewed appreciation as one of the most fascinating, distinct, and innovative groups in American pre-history.
"You can tell by the skill levels back in the Clovis times, they were very skilled artisans in the points that they made," says Bill Lyles, a flint-knapper and retired pharmacist who has become one of the Topper site's top supervisors. "Clovis only lasted a few hundred years, and that skill level, as time passed, you begin to see it drop off. And it wasn't picked back up."
Instead of hyperactive Siberian nomads who over-hunted their way to cultural failure, what we call Clovis now looks more like a highly mobile group that managed to adapt to whatever the American landscape provided, or perhaps a successful idea that spread across existing trade networks. They almost certainly didn't exterminate the mammoths, and today it's considered far more likely that a 1,500-year cold snap called the Younger Dryas somehow led to the extinction of the great mammals and the demise of the Clovis era.
And finally, there's another theory — not mainstream, but promoted by one of the top researchers at the Smithsonian Institute — that contends that the emergence of Clovis in the Southeast offers evidence of a mind-boggling journey.
Even against a complex backdrop that looks more interesting than ever, Clovis culture stands out.
Past That Wasn't
As an academic field, pre-Columbian history marches across multiple culture-war battlefields these days. New evidence and old documents suggest that the Americas probably were far more populated in 1492 than history has properly recorded, with cities in Central and South America exceeding those of the Old World in terms of population, health, and grandeur. Even pre-Columbia North America — long viewed as an unspoiled Eden — looks more like a productively managed garden.
In this new rewrite of the American story, the rapid European settlement of the "New World" between the 15th and 18th centuries succeeded because it caused the most horrific series of plagues ever inflicted upon humanity. Serious scientists have proposed that waves of disease may have reduced the indigenous population by as much as 95 percent between 1492 and 1640.
But as controversial as the debates over those ideas have been, they're practically etiquette lessons compared to the hostility faced by the archaeologists who first asserted evidence of older-than-Clovis inhabitations here. Archaeologists used to speak in terms of a "Clovis Horizon," the idea that once a professional reached the bottom of the Clovis layer, there was no reason to dig deeper. Those who crossed that boundary were not only viewed as fools, but as heretics.
Al Goodyear, an associate research professor at the S.C. Institute of Archaeology and Anthropology at the University of South Carolina and the Topper site's primary investigator, crossed the boundary reluctantly. His Clovis finds at Topper had positioned the stout, affable Goodyear in the comfortable mainstream of Clovis-first American archaeology.
But that began to change in 1988 when — just to make sure — Goodyear decided to dig a test trench below Topper's 13,500-year-old Clovis Horizon. He says he didn't expect to find anything, and today speaks of the decision almost wistfully. "There were people who told me, 'If you ever find something beneath your Clovis layer, cover it back up and never speak of it again,'" he says. Then he shrugs.
What Goodyear found in that trench enlisted him in the uncomfortably small ranks of credentialed American archaeologists investigating older-than-Clovis evidence. When read from the bottom up, the layers suggest that unknown bands of ancient people who relied on simple micro-blade tools had quarried chert at Topper for thousands of years before disappearing around 16,000 years ago. The site's artifact record falls silent after that, perhaps for as long as 3,000 years, before revealing a burst of activity during the Clovis era. The area has been inhabited more-or-less continuously ever since.
Despite such then-controversial finds, the site attracted little public attention until 2004, when a rare chance to carbon-date Topper's very earliest artifacts determined they were older than carbon could reliably measure, making them anywhere from 40,000 to 70,000 years old. Suddenly Goodyear had the oldest archaeological site in North America, and his face was all over the cable news channels. He'd never been more famous, or looked more professionally vulnerable.
Yet the drumbeat of new finds and old dates continued, and as the months passed, Goodyear's radiocarbon date — though still ridiculously old — started looking less impossible. Clovis-first orthodoxy began to lose its unquestioned dominance at academic conferences and among the editors of peer-reviewed journals, and when the major figures in the debate arrived in Austin, Texas, for a 2008 Paleoamerican workshop organized by Tom Pertierra, the status quo finally shifted.
"Were there fistfights? No," Pertierra says. "Was there spirited discussion? Yes. But you know what we came out of there with? About an 80-20 turn [in favor of the older-than-Clovis evidence]."
Less than five years later, the idea of a Clovis Horizon looks quaint. New evidence for older-than-Clovis occupations has cropped up from Virginia to Texas, in caves in Oregon, and in mud dredged off the ocean floor near the mouth of the Chesapeake Bay. New data generated by scholars in related fields ranging from genetics to linguistics point to not one but multiple waves of pre-Columbian settlement, with several starting points and much earlier origins.
But it wasn't until 2012 that an archaeologist crossed the last boundary of mainstream acceptance. Eight years ago, Goodyear confided that he would never advise a graduate student to risk his or her career on a doctoral thesis about a pre-Clovis topic. Today, Eastern New Mexico University Ph.D-candidate Doug Sain — an eight-year supervisor at Topper who met his wife at the site — is writing his dissertation on the site's older-than-Clovis evidence.
Doug and Kristina Sain are part of the first generation of young archaeologists to enter professional life in the wake of the pre-Clovis revolution, and the new crop is well represented in the evening chow line at base camp. Joe Gingerich is a visiting researcher from the Smithsonian Institute. Chris Moore has been doing advanced geoarchaeology all over the Carolinas. Multiple graduate students come and go.
But the most easily recognizable of the young academics around camp is 32-year-old Derek T. Anderson, an archaeologist at the Cobb Institute of Archaeology and a lecturer at Mississippi State. A Topper supervisor since 2008, Anderson has carved out a reputation around camp as the quietly competent, understated authority the crew seeks out whenever tricky questions arise.
"I think that we have a better idea of what Clovis technology represents than people 25 years ago did," Anderson says when asked about the new multidisciplinary insights in American prehistory. "But there is still a lot that we don't know, particularly in the Southeast."
Though Goodyear retains the title of primary investigator, he functions more as a high-level supervisor during the annual spring dig and has worked in Columbia during much of the 2012 expedition. Pertierra runs the expedition's daily operations now, while Anderson, Sain, and the other young academics handle routine scientific duties.
Goodyear's generation broke through the Clovis Horizon, but as he and his colleagues near retirement, it's up to the next generation to make sense of what's been found there.
16,700-Year-Old Tools Found in Texas Change Known History of North America...AGAIN
16,700-Year-Old Tools Found in Texas Change Known History of North America
Archaeologists in Texas have found a set of 16,700-year-old tools which are among the oldest discovered in the West. Until now, it was believed that the culture that represented the continent’s first inhabitants was the Clovis culture. However, the discovery of the ancient tools now challenges that theory, providing evidence that human occupation precedes the arrival of the Clovis people by thousands of years.
According to the Western Digs , archeologists discovered the tools about half an hour north of Austin in Texas, at the site called Gault. They were located a meter deep in water-logged silty clay. The site contained more than 90 stone tools and some human remains including fragments of teeth.
Excavations being carried out at the Gault site, Texas. Credit: Archaeological Institute of America
The discovery changes everything people have been taught about the history of North America – that is, that the Clovis culture represented the first inhabitants of the continent. The results of the research were presented at the meeting of the Plains Anthropological Conference in 2015.
A hallmark of the toolkit associated with the Clovis culture is the distinctively shaped, fluted stone spear point, known as the Clovis point. These Clovis points were from the Rummells-Maske Cache Site, Iowa ( public domain ).
In the 1990s, at the same excavation site near Austin, archeologists unearthed tapered-oval spear heads dating back 13,000 years. Those times, they believed, belonged to the oldest widespread culture of the continent. However, the most recent discovery proves that the pre-Clovis inhabitants came to North America at least three millennia earlier.
Evidence of an early projectile point technology in North America at the Gault Site, Texas, USA
Evidence of an early projectile point technology in North America at the Gault Site, Texas, USA
Abstract
American archeology has long been polarized over the issue of a human presence in the Western Hemisphere earlier than Clovis. As evidence of early sites across North and South America continues to emerge, stone tool assemblages appear more geographically and temporally diverse than traditionally assumed. Within this new framework, the prevailing models of Clovis origins and the peopling of the Americas are being reevaluated. This paper presents age estimates from a series of alluvial sedimentary samples from the earliest cultural assemblage at the Gault Site, Central Texas. The optically stimulated luminescence age estimates (~16 to 20 thousand years ago) indicate an early human occupation in North America before at least ~16 thousand years ago. Significantly, this assemblage exhibits a previously unknown, early projectile point technology unrelated to Clovis. Within a wider context, this evidence suggests that Clovis technology spread across an already regionalized, indigenous population.
INTRODUCTION
Current research on the early human occupation of the Americas no longer recognizes Clovis as the expression of a founding population (1, 2). Increasing diversity, range, and time depths within the expanding database of sites predating Clovis attest to greater complexity in the early record (3) than previously thought. Archeological opinion on the nature, timing, arrival, and peopling scenarios remains divided (4–6). Despite this, there is increasing evidence to support a number of contemporaneous (7) and older (2, 8) cultural manifestations at least 2 thousand years (ka) before the appearance of Clovis (9). This includes the Western Stemmed Tradition (10), Beringian assemblages (11), and Eastern Seaboard sites (12–14) in North America alongside the El Jobo/Monte Verde and fishtail bifacial technologies and edge-trimmed traditions in South America (15–17). These technological patterns require careful and systematic evaluation to address the nature and timing of both the early occupation of the Americas and, subsequently, the origins of Clovis (Supplementary Materials).
Background
Initially identified in 2002, excavation at Area 15 of the Gault Site (Fig. 1) was undertaken to explore evidence of early cultures in Central Texas. Research focused on the manufacturing technologies, their relationship to Clovis, and the associated age of this assemblage. This report focuses on the optically stimulated luminescence (OSL) ages obtained from the lowest deposits in Area 15 that contain a material that predates Clovis, referred to as the Gault Assemblage.
The site occupies an upstream first- to second-order floodplain of the Buttermilk Creek valley, which has reliable springs and high-quality chert outcrops. The setting is an ecotone with mesic, riparian flora in the valley floor and xeric-adapted plants on the adjacent uplands. Regionally, the setting is in the Balcones Canyonlands, itself an ecotone, where resources of the limestone uplands (the Edwards Plateau) interface with the Blackland Prairie on the adjacent coastal plains.
The Area 15 excavation block, located within the valley floor, is a 56-m2 grid, which was stepped down in meter increments. Excavation was conducted in 1 m × 1 m squares within this grid in arbitrary levels of 5 cm deep from 93.00 m and below (based on an arbitrary site datum of 100.00 m). The upper ~1.8-m deposit is a midden, common to Central Texas (18). Below this is a ~1.2-m-thick silty clay deposit atop a ~0.20- to 0.50-m-thick fluvial gravel. The gravel rests on Cretaceous-age limestone bedrock of the Comanche Peak Formation (Supplementary Materials).
The sediments in Area 15 are well stratified with diagnostic projectile points and associated artifact complexes in chronological order (Fig. 2) and, in many cases, are separated from one another by a decrease in debitage counts. The midden deposit contains Archaic projectile points in good stratigraphic order. Within the midden, there is a vertically constrained distribution of small (<1 cm) diagnostic Andice notching flakes recovered between 94.90 and 94.00 m (fig. S5). Field observations of the orientation of artifacts suggest that shrink-swell movement of the soil has reoriented some materials and favored the downward migration of some of the smallest artifacts, but the absence of diagnostic artifacts like Bell-Andice notching flakes (Supplementary Materials) below the appropriate age deposit suggests that this movement has been limited. The silty clay beneath the Bell-Andice occupation contains an ~50-cm-thick sequence of Late Paleoindian components in a stratified order overlying an ~25-cm-thick Clovis component. Below this is an ~65- to 80-cm-thick deposit overlying undulating bedrock that consists of silty clay and fluvial gravel deposits, containing the Gault Assemblage. A further indicator of the stratigraphic integrity of the site is the separation between cultural components. Furthermore, bioturbation, such as animal or root disturbances, was identified and excavated separately in the field. Debitage counts indicate that decreased cultural material frequencies occur between the Gault, Clovis, and Paleoindian components. There is an ~10-cm-thick zone of decreased cultural material between the Clovis and Gault components. This suggests a reduction in site activity or possible occupational breaks between the three cultural depositions.
Diagnostic projectile points listed on the left were all found within the associated deposits (SMH, St. Mary’s Hall). Stratigraphic unit numbers are shown on the right, and the cultural horizons are highlighted in gray.
Stone tool assemblage
The stone tool assemblage recovered from the lowest, earliest deposits exhibits a small projectile point technology as well as both a biface and a blade-and-core tradition. The projectile points from the Gault Assemblage exhibit two stem morphologies: stemmed and lanceolate (Supplementary Materials). One stemmed projectile point (Fig. 3I) exhibits a slightly concave base, with concave lateral margins and short shoulders with beveled edges. In profile, this point is slightly curved, suggesting that it was manufactured on a flake. Two bifurcate stemmed points were also recovered (Fig. 3, H and J); both have a deep concave base, an expanding stem, and exhibit beveling. A small proximal tip with beveled edges was also recovered (Fig. 3K). These points were likely produced on flakes and predominantly manufactured using pressure flaking to shape and finish the points. These stemmed points are technologically and morphologically distinct from any later regional cultural manifestations. Superficially, they resemble point types within the regional Early Archaic yet differ in base treatment and blade bevel (Supplementary Materials). The two lanceolate projectile points (Fig. 3, X and Y) are similar in size, exhibit a concave base, and share similarities in the basal flaking and finishing. Only one point (Fig. 3X) is ground along the edges. Both points are snapped at the stem, but existing flaking pattern suggests comedial (midline) flaking. The lanceolate points superficially resemble Late Paleoindian types but do not fit any single point type from this period. A sixth point exhibits weak shoulders and a contracting stem (Fig. 3G). This point is made from smoky quartz and exhibits a central ridge produced from comedial flaking. Unlike the three points discussed above, the morphology of this point resembles Western Stemmed points, but its age places it outside of the known chronology for this type (7). All projectile points were recovered from undisturbed sediments within Area 15 with no evidence for the downward movement within their excavation units. This projectile point assemblage is unlike anything in the early archeological record of the Americas and indicates complex behavioral activities associated with a group or groups who colonized the New World.
(A to D, F, and L) Bifaces. (E) Blade core. (G) Quartz projectile point. (H and I) Projectile points. (K) Projectile point tip. (M, V, and W) Blade. (N) Unifacial tool. (O and T) Gravers. (P) Discoidal biface. (Q) End scraper. (R to U) Modified flake tools. (X and Y) Lanceolate projectile points. Descriptions are given in the Supplementary Materials.
Alongside these points, approximately 150,000 artifacts, consisting mainly of debitage, have been recovered from these lowest two units (see strata 1 and 2 in the Supplementary Materials). To date, 184 flaked stone artifacts have been analyzed (Supplementary Materials). These include the distinct stemmed projectile points, blades and blade cores, bifaces, and flake tools. The Gault Assemblage shares this generalized biface and blade-and-core lithic tradition with the overlying Clovis materials but differs significantly in the following ways.
The Gault biface assemblage exhibits the prevalent use of comedial (midline) flaking indicative of proportionally thinning a biface, which closely parallels the reductive technology used to create the Gault Assemblage projectiles. Clovis, however, demonstrates the use of full-face and overshot flaking to produce thinned bifaces. In addition, the flake striking platforms produced during manufacture are larger and less prepared than the Clovis flake platforms (19).
In contrast, the blade-and-core assemblage shares more commonalities with Clovis technology. Both technologies exhibit flat-backed blade cores that use a single blade face with an acute platform and unidirectional blade removals as well as conical cores that have blade removals around the circumference of the core. Evidence from the blade platforms indicates that the Gault Assemblage generally exhibits less preparation than Clovis platforms.
The similarities and differences suggest that there is no single linear trajectory toward Clovis technology within the Gault Assemblage. Instead, parts of the technological repertoire, like the blade-and-core tradition, appear to have continued in the Clovis levels at the Gault site, while the projectile points and the biface traditions underwent significant changes. In a broader context, the technologies present in the Gault Assemblage appear to represent a unique pattern within the early human occupations of the Americas that indicates a regional adaptation after the initial colonization of the New World.
MATERIALS AND METHODS
Four OSL samples were collected from the lowest cultural bearing deposits (see strata 1 and 2 in the Supplementary Materials) in Area 15 of the Gault Site (Fig. 4 and table S1). Samples were collected by horizontally hammering 4.5-cm-diameter steel tubes into clean open sections. After collection, the sample holes were slightly enlarged to obtain in situ gamma spectrometric measurements at all locations.
Stratigraphic unit numbers are shown on the left, and the cultural horizons are highlighted in gray. Clovis ages have been reported elsewhere (see text).
Samples were prepared at the Department of Geosciences at Murray State University, Murray, KY under low red fluorescent lighting fitted with Lee 106 filters. Pure quartz grains were obtained using standard OSL preparation methods, including treatment with 10% HCl and 30% H2O2, to remove carbonates and organic material, respectively. The samples were wet-sieved to obtain a 32- to 63-μm grain size fraction, and a heavy liquid separation using lithium polytungstate was performed to isolate quartz. No HF etching was applied.
Luminescence measurements were conducted at McMaster University, Hamilton, ON using a Risø OSL/TL-DA-15 reader fitted with a 7-mm-thick Hoya U-340 filter. Measurements were carried out using blue diodes (470 nm) operating at 90% power (~30 mW/cm2). Laboratory irradiations were performed using a calibrated 90Sr/90Y radioactive source attached to the Risø luminescence reader. Blue LED’s (light-emitting diode) multigrain aliquots were prepared on 9.8-mm-diameter stainless steel discs using a silicon spray and a 0.5-mm mask (~72 grains).
An initial equivalent dose (De) estimate was made by comparing the natural OSL signal of four aliquots to their OSL signal after a given dose. A second identical regenerative dose was applied to the same four aliquots, and the Infrared Stimulated Luminescence (IRSL) signal was measured as a check for feldspar contamination. All aliquots had an IRSL-to-OSL ratio of <1%, suggesting no significant feldspar contamination in the prepared samples. A dose recovery test was used to determine the preheat temperature (160°, 200°, 240°, and 260°C) at which a given dose could be best recovered. A preheat temperature of 200°C and a cut-heat temperature of 160°C produced a dose closest to the given dose and was used in all subsequent De measurements. Thermal transfer tests that were carried out to assess the possibility of charge transfer from light-insensitive shallow traps to light-sensitive OSL traps showed no significant thermal transfer (20). During this experiment, the OSL signal of several aliquots was first bleached by a 400-s exposure to blue LED’s. Apparent De’s were then calculated using the SAR protocol (21), but applying different preheat temperatures to different aliquots. Significant thermal transfer [>1 gray (Gy)] was only observed with the application of preheat temperatures more than 260°C.
Final De measurements were made on 48 aliquots for each measured sample. All measurements followed the SAR protocol (21, 22) on 0.5-mm multigrain aliquots of 32- to 63-μm quartz with a total stimulation time of 100 s. This fraction was targeted to isolate quartz-rich silt identified through petrography (23) that was presumed to be incorporated in the floodplain sediments through aeolian processes. The OSL signal was integrated from the first 0.4 s of the decay curve, and the subtracted background was integrated from the last 4 s. Aliquots were required to pass the following criteria for further analysis: <10% test dose error, <10% recycling ratio error, <10% recuperation, <10% palaeodose error, and a signal greater than 3σ above background. All De values incorporated an instrumental error of 1.5%.
Final De values used for age calculation were statistically modeled using the central age model [CAM; (24)] owing to low overdispersion (that is, <10%) and the normal distribution of De’s in each sample. A σb value of 0.045, calculated from dose recovery results, was added in quadrature to all De estimates to account for variability arising from the intrinsic luminescent properties (25).
External α and β dose rates were determined from the U, Th, and K concentrations of a small amount of sediment (~2 g) collected from each OSL sample and measured with neutron activation analysis and delayed neutron counting (conducted at the McMaster University Nuclear Reactor). Conversion of radioisotope concentrations was done using the data of Guérin et al. (26). Dose rates were calculated, assuming secular equilibrium in the U and Th decay chains. Dose rates were corrected for water content using laboratory-based measurements and for attenuation using factors from Brennan et al. (27) and Guérin et al. (28). External gamma dose rates were obtained in situ at all sample locations using a NaI(Tl) Harwell four-channel gamma spectrometer. Cosmic dose rates were calculated on the basis of the methods by Prescott and Hutton (29, 30) and calculated using a 2.0 g/cm3 of overburden density, assuming a linear sediment accumulation.
Ages were calculated by dividing the De modeled with the CAM by the corresponding total dose rate. In addition to the errors on the modeled De’s, the following systematic errors were incorporated into each age calculation: ±25% for moisture content and ±10% for cosmic dose rate. OSL age results are reported with 1σ errors in table S1. An average of the laboratory-measured moisture content from each sample of 20% was used for age calculation.
Disequilibrium measurements were not conducted on the samples at Gault (31). To investigate its potential impact on age estimations, Rn loss calculations were carried out using data tables provided by Guérin et al. (26). Ratios of 238U pre-Rn loss to 238U total U energy release were used to calculate multiplication factors to modify U concentration in the age calculations. Factors were calculated separately for alpha, beta, and gamma energy releases and then applied to find the effective U concentration for 100, 50, and 25% Rn loss. A U-only dose rate was obtained for each calculation and added back in to the total dose rate from Th, K, and cosmic rays (table S2).
RESULTS AND DISCUSSION
The OSL samples displayed favorable luminescence characteristics including low overdispersion (<10%) and normally distributed De distributions. Moreover, the fast component contributed more than 90% of the signal measured in the first 0.4 s of stimulation, suggesting that the OSL signals from these samples are fast component–dominant. A representative decay curve and growth curve are shown in fig. S1. All measured De’s were significantly lower than their corresponding D0 values (~65 Gy). Equivalent dose distributions are displayed as histograms and radial plots in fig. S2.
The OSL ages presented here establish the presence of a cultural component, stratified below Clovis, and associated with ages older than ~16 ka (Fig. 4 and table S1). These OSL ages range from 21.7 ± 1.4 ka to 16.7 ± 1.1 ka and, within error, are in the expected stratigraphic order (Fig. 4 and table S1). On the basis of the results of OSL dating presented here, we find a mean age for the Gault Assemblage (n = 4) of 18.5 ± 1.5 ka.
Ages associated with the temporal diagnostic artifacts above the Gault component are in excellent stratigraphic agreement (32). This includes four OSL ages of 11.9 ± 0.8 ka, 12.9 ± 0.6 ka, 13.2 ± 0.6 ka, and 13.6 ± 0.6 ka from the Clovis component (Fig. 4) (32). These dates agree with the known Clovis range of ~13.5 to 12.9 ka (33–35) and agree with the ages from other Clovis sites in Texas (36–38). These data emphasize the stratigraphic integrity of Area 15 and the agreement between the temporal diagnostic artifacts and OSL ages (Supplementary Materials).
The OSL ages for these early levels at the Gault Site are in good stratigraphic agreement with the known, younger, temporal diagnostic artifacts and age estimates indicating the reliability of this dating sequence (32). The significant reduction in artifact frequencies between the Clovis and Gault Assemblage confirms the presence of an older, isolated, assemblage below Clovis. Given the SDs for these OSL ages (Fig. 4 and table S1), the Gault Assemblage is dated to at least 16 ka, which is the youngest possible age for this occupation; however, the time span suggests that the inhabitation of the Gault site began ~1 to 2 ka before.
The Gault Site differs from other OSL-dated sites relevant to the early occupation of North America for two reasons. First, there is a well-dated (32) long stratigraphic sequence above the Gault Assemblage with distinct and well-separated occupational horizons. In addition, the OSL De distributions are normal and exhibit low (<10%) overdispersion, which provide more confidence for the modeled final De values used in age calculation (Supplementary Materials).
Disequilibrium in the U-series is commonly present in carbonate-rich environments and can potentially change the dose rate over time, leading to inaccurate age estimations (39). Although disequilibrium measurements were not carried out for the Gault samples, several inferences can be made about the potential impact that this would have on the ages we report. The samples collected at Gault have U contents that make up a relatively small (~20%) contribution to the total dose rate, so any influence of disequilibria on the resulting ages may not be very significant. For samples studied with a similar U content, and assuming a >50% disequilibrium in the U-series chain, OSL ages have been in 8% error from the true age (39). This generally falls close to or within the 1σ age errors that we report. Moreover, our Rn loss calculations suggest that for the Rn loss to have a statistically significant effect on age calculations relative to 0% Rn loss, for nearly all samples, >50% Rn loss would have had to have occurred (table S2).
In general, U concentrations in carbonate rocks are quite uniform at approximately 2 parts per million (ppm) (40). Phreatic cements have been less well studied; however, a U concentration of 1.80 ± 0.75 ppm was found for a series of freshwater phreatic cements by Chung and Swart (41). The total U concentration in our Gault dating samples ranges from 2.13 ± 0.1 ppm to 2.48 ± 0.1 ppm, indicating the possibility that much of the dose rate comes from carbonate elements. This is expected when examining the lithology of the units. The Gault Assemblage layers are the major host of phreatic carbonates at Gault.
Phreatic carbonates routinely form in situ as postburial phases and occur as surface coatings on grains and larger elements. If these had formed during the burial history and had reduced pore volume by their presence, then they may have added U to the source of dose rate to the quartz grains, changing the bulk dose rate over time. On the basis of earlier work of U concentrations in carbonate rocks and phreatic carbonates cited above, it would probably have added U to the pore space volume at a similar concentration as the surrounding host material. As this occurred over time, incremental increases in bulk dose rate would have occurred, and thus, the bulk dose rate over the burial history would have been lower than observed at the time of sampling. Thus, any effect from the crystallization of U-containing phreatic carbonate would make the ages older, in the same direction as any effects of Rn loss >50%.
The evidence from Area 15 at the Gault Site demonstrates the presence of a previously unknown projectile point technology in North America before ~16 ka. The physical and cultural stratigraphic evidence recovered from Area 15, as well as the associated OSL ages reported here and elsewhere (32), are consistent in showing a coherent sequence of the Gault Assemblage, Clovis, Late Paleoindian, Early Archaic, and Middle/Late Archaic occupations over an apparent span of more than 16,000 calendar years (Fig. 4). This sequence corresponds well with previous studies in Central Texas (42). The distinct technological differences between Clovis and Gault Assemblage, together with the stratigraphic separation between the cultural depositions, indicate a lack of continuity between the two complexes.
The Gault Assemblage at the Gault Site, specifically the projectile points, represents a regional manifestation within a number of possible contemporary patterns (Supplementary Materials). As evidence for the complexity in the early occupation of the Americas increases (1, 2), a more elaborate framework (9) for these early human occupations is required.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaar5954/DC1
Table S1. Sample OSL ages and dose rate data.
Table S2. Radon loss effects on each Gault OSL sample.
Table S3. Summary of the Gault Assemblage.
Table S4. Summary counts of the Gault biface assemblage.
Fig. S1. A representative decay curve (upper), growth curve (middle), and plot showing sensitivity changes through the SAR cycle (lower) for Gault 11-18.
Fig. S2. Equivalent dose (De) distributions for all samples displayed in a histogram (left) and radial plot (right).
Fig. S3. Location of the Area 15 excavation block and the excavation grid and profile.
Fig. S4. Relationship between the stratigraphy and cultural components in Area 15.
Fig. S5. Results of the geoarcheological analysis of the sediments in Area 15.
Fig. S6. Andice projectile point (left) with representative diagnostic notching flakes (right).
Fig. S7. Backplots of northing (blue) and easting (red) profiles showing the elevation of diagnostic Andice notching flakes and the cultural components discussed in the text.
Fig. S8. Limestone bedrock of Area 15, with three sets of flutes scoured into the limestone (discussed in the text).
Fig. S9. Pollen data from Boriack Bog and the NGRIP and GRIP ice core record as compared to stratigraphic units at Area 15.
Fig. S10. Gault Assemblage stone tool types and frequency (see table S2).
Fig. S11. Gault Assemblage projectile points.
Fig. S12. Principal components analysis of the Gault Assemblage stemmed projectile points and the Gower and Uvalde types.
Section S1. Area 15 stratigraphy
Section S2. Context of early dates in North America
Section S3. Gault Assemblage in Area 15
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