PP23D-01
Presence of all Three Allotropes of Impact-Diamonds in the Younger Dryas Onset Layer (YDB) Across N America and NW Europe
We report the discovery of all three diamond allotropes (cubic diamond, lonsdaleite, and n-diamond) in an extraterrestrial (ET) impact layer (the YDB), dating to the Younger Dryas onset at 12.9 ka. YDB diamonds are distributed broadly across N America and NW Europe at 15 sites spanning 9,000 km or 23 percent of Earth's circumference. N-diamonds and lonsdaleite, or hexagonal diamond, do not co-occur with terrestrial diamonds, but are found in meteorites. Lonsdaleite is found on Earth only in association with known ET impacts, and thus, is a definitive impact indicator. The diamonds were identified by transmission electron microscopy (TEM) using selected area diffraction (SAED), which display reflections corresponding to the following lattice planar spacings definitive of diamond: (1) cubic: 2.06, 1.26, 1.07, and 0.89 A; (2) lonsdaleite: 2.184, 1.261, 1.092, and 0.826 A; and (3) n-diamond: 2.06, 1.26, 1.07, and 0.89 A, plus "forbidden" reflections of 1.78, 1.04, and 0.796 A. Nanodiamonds are rounded to highly angular, and range in size from 1 to 1700 nm with most between 1 and 50 nm. Concentrations are up to 3700 ppb, equaling more than 1 billion diamonds per cm3 of sediment (comparable to K/T levels of 3600 ppb). No diamonds were detected above or below the YDB layer at any site tested. These diamonds could not have formed from volcanic activity, because they combust at temperatures above 500° C in the presence of atmospheric levels of oxygen, and micrometeoritic diamonds are similarly destroyed. Also, the diamonds could not have accumulated from the constant rain of micrometeoritic debris, because multi-billions occur in YDB layer samples, but yet none have been found in non-YDB strata dating from 55,000 RCYBP to present. YDB diamonds are associated with abundance peaks in magnetic spherules, carbon spherules, soot, and iridium, which can peak in impact layers of known ET events, such as the K/T and the 1908 airburst at Tunguska, Siberia. Furthermore, a high proportion of the nanodiamonds are found deeply embedded within spherical particles of melted plant resins, a fact inexplicable by any normal terrestrial process. Altogether, this evidence strongly suggests that the widespread and abundant nanodiamonds constrained to the thin YDB layer resulted from a major ET impact/airburst at 12.9 ka.
PP23D-02
Late Pleistocene Megafaunal Extinction Consistent With YDB Impact Hypothesis at Younger Dryas Onset
At least 35 mammal and 19 bird genera became extinct across North America near the end of the Pleistocene. Modern increases in stratigraphic and dating resolution suggest that this extinction occurred relatively rapidly near 12.9 ka (11 radiocarbon kyrs). Within the context of a long-standing debate about its cause, Firestone et al., (2007) proposed that this extinction resulted from an extraterrestrial (ET) impact over North America at 12.9 ka. This hypothesis predicts that the extinction of most of these animals should have occurred abruptly at 12.9 ka. To test this hypothesis, we have critically examined radiocarbon ages and the extinction stratigraphy of these taxa. From a large data pool, we selected only radiocarbon dates with low error margins with a preference for directly dated biological materials (e.g., bone, dung, etc.) and modern chemical purification techniques. A relatively small number of acceptable dates indicate that at least 16 animal genera and several other species became extinct close to 12.9 ka. These taxa include the most common animals of the late Pleistocene such as horses, camels, and mammoths. Also, the remains of extinct taxa are reportedly found up to, but not above, the base of a widely distributed carbon-rich layer called the black mat. This stratum forms an abrupt, major biostratigraphic boundary at the Younger Dryas onset (12.9 ka), which also contains multiple ET markers comprising the impact layer (the YDB). Surviving animal populations were abruptly reduced at the YDB (e.g., Bison), with major range restrictions and apparent evolutionary bottlenecks. The abruptness of this major extinction is inconsistent with the hypotheses of human overkill and climatic change. We argue that extinction ages older than 12.9 ka for many less common species result from the Signor-Lipps effect, but the impact hypothesis predicts that as new dates are acquired, they will approach ever closer to 12.9 ka. The megafaunal extinction is strongly associated with abrupt and major vegetation changes, abrupt cooling, and widespread biomass burning at the onset of the Younger Dryas over North America. The stratigraphic and chronologic data are consistent with megafaunal extinction being caused by continental-scale ecosystem disruption due to an ET impact.
PP23D-03
Evaluating the Paleoindian Radiocarbon Record at the Onset of the Younger Dryas: Sensitivity Analyses and Bayesian Chronology-Building
The onset of the Younger Dryas (13.0-12.9 ka) in North America is marked in the archaeological record by the transition from Clovis to Folsom cultural assemblages, as well as the extinction of many megafauna species. The nature of the transition-gradual or abrupt, continuous or discontinuous, regionally uniform or variable - remains poorly understood because of: 1) low-precision and low-quality radiocarbon records; 2) concerns about the accuracy of the calibration curve before ca. 12.4 ka; and, 3) disagreement on the appropriate statistical models for chronology building. Here we evaluate two approaches to Paleoindian radiocarbon chronology, summed probability distributions and Bayesian phase/boundary models. Summed probability frequencies have been used as demographic proxies recently, but the effects of sample quality, density, and the variations in the calibration curve are largely unexplored. Sensitivity analyses were done by simulating radiocarbon ages at 10, 25, 50 and 100 cal yr intervals with varying measurement errors, which were calibrated and summed to obtain a probability distribution function for each run. We find that dense, high-precision radiocarbon records are necessary to detect gaps as small as 100 years in the record. Currently available radiocarbon databases for the Paleoindian period can at best be characterized as sparse and of low- to medium-precision, arguing against the use of summed probabilities as a proxy for human activity during that period. Bayesian statistical models incorporate a priori archaeological information (e.g., stratigraphic relationships, cultural assemblage) to constrain calibrated radiocarbon ages leading to more refined chronologies. Selected high-precision, reliable radiocarbon dates were used to build phase and boundary models for Clovis and post-Clovis periods, and to determine the likelihood of a gap between them consistent with depopulation consistent with an ET impact at the Younger Dryas boundary. Model results suggest the possibility of a hiatus, a hypothesis to be further tested with a comprehensive program of high-precision AMS 14C dating at stratified Paleoindian sites.
PP23D-04
Impact-Shocked diamonds, Abrupt Ecosystem Disruption, and Mammoth Extinction on California's Northern Channel Islands at the Allerod-Younger Dryas Boundary (13.0-12.9 ka)
Sedimentary records from California's Northern Channel Islands and the adjacent Santa Barbara Basin (SBB) indicate intense regional biomass burning (wildfire) near the Allerod-Younger Dryas boundary (YDB) at 13.0- 12.9 ka. Multiproxy records in SBB Ocean Drilling Project (ODP) Site 893 indicate that these wildfires coincided with the onset of Younger Dryas cooling and abrupt vegetational shift from closed montane forest to more open habitats. Here we report impact-shocked hexagonal diamonds (lonsdaleite) in an organic-rich, dark layer dating to the YDB and deeply buried in Arlington Canyon on Santa Rosa Island, California. Transmission electron microscopy (TEM) demonstrates that these diamonds are mono- and polycrystalline, and electron diffraction confirms the high-pressure hexagonal diamond polymorph. Lonsdaleite has never been found with mantle-derived diamond and is known on Earth only in meteorites and extraterrestrial impact craters. These crystals co-occur with high concentrations of other nanometer-sized diamond polymorphs (n- diamond; cubic). These discoveries provide strong supporting evidence for an extraterrestrial impact at the Allerod-Younger Dryas Boundary. The age of these impact diamonds coincides with the last known occurrence of pygmy mammoths (Mammuthus exilis) on the Northern Channel Islands (and contemporaneous with the extinction of other megafauna taxa on the continent), intense regional biomass burning, landscape transformation, vegetational shifts, and the beginning of an apparent 600-800 year gap in the archaeological record. Taken collectively these data are consistent with abrupt ecosystem disruption triggered by an extraterrestrial airburst or impact at 12.9 ka, one of many distributed across North America as proposed by the YDB impact hypothesis.