Did a Bolide Impact Trigger the Younger Dryas (2)

We present arguments and evidence against the hypothesis that a large impact or airburst caused a significant abrupt climate change, extinction event, and termination of the Clovis culture at 12.9 ka. It should be noted that there is not one single Younger Dryas (YD) impact hypothesis but several that conflict with one another regarding many significant details. Fragmentation and explosion mechanisms proposed for some of the versions do not conserve energy or momentum, no physics-based model has been presented to support the various concepts, and existing physical models contradict them. In addition, the a priori odds of the impact of a >4 km comet in the prescribed configuration on the Laurentide Ice Sheet during the specified time period are infinitesimal, about one in 10 15 . There are three broad classes of counterarguments. First, evidence for an impact is lacking. No impact craters of the appropriate size and age are known, and no unambiguously shocked material or other features diagnostic of impact have been found in YD sediments. Second, the climatological, paleontological, and archeological events that the YD impact proponents are attempting to explain are not unique, are arguably misinterpreted by the proponents, have large chronological uncertainties, are not necessarily coupled, and do not require an impact. Third, we believe that proponents have misinterpreted some of the evidence used to argue for an impact, and several independent researchers have been unable to reproduce reported results. This is compounded by the observation of contamination in a purported YD sample with modern carbon.

Quaternary Science Reviews, 2014

The Younger Dryas impact hypothesis suggests that multiple extraterrestrial airbursts or impacts resulted in the Younger Dryas cooling, extensive wildfires, megafaunal extinctions and changes in human population. After the hypothesis was first published in 2007, it gained much criticism, as the evidence presented was either not indicative of an extraterrestrial impact or not reproducible by other groups. Only three years after the hypothesis had been presented, a requiem paper was published. Despite this, the controversy continues. New evidence, both in favour and against the hypothesis, continues to be published. In this review we briefly summarize the earlier debate and critically analyse the most recent reported evidence, including magnetic microspherules, nanodiamonds, and iridium, shocked quartz, scoria-like objects and lechatelierite. The subsequent events proposed to be triggered by the impact event, as well as the nature of the event itself, are also briefly discussed. In addition we address the timing of the Younger Dryas impact, a topic which, despite its importance, has not gained much attention thus far. We show that there are three challenges related to the timing of the event: accurate age control for some of the sites that are reported to provide evidence for the impact, linking these sites to the onset of the Younger Dryas and, most importantly, an apparent age discrepancy of up to two centuries between different sites associated with the proposed impact event. We would like to stress that if the markers at different locations have been deposited at different points in time, they cannot be related to the same event. Although convincing evidence for the hypothesis that multiple synchronous impacts resulted in massive environmental changes at w12,900 yrs ago remains debatable, we conclude that some evidence used to support the Younger Dryas impact hypothesis cannot fully be explained at this point in time.

2010

We have uncovered a thin layer of magnetic grains and microspherules, carbon spherules, and glass-like carbon at nine sites across North America, a site in Belgium, and throughout the rims of 16 Carolina Bays. It is consistent with the ejecta layer from an impact event and has been dated to 12.9 ka BP coinciding with the onset of Younger Dryas

Elements, 2012

The Younger Dryas Impact Hypothesis (YDIH) states that North America was devastated by some sort of extraterrestrial event ~12,800 calendar years before present. Two fundamental questions persist in the debate over the YDIH: Can the results of analyses for purported impact indicators be reproduced? And are the indicators unique to the lower YD boundary (YDB), i.e., ~12.8k cal yrs BP? A test reported here presents the results of analyses that address these questions. Two different labs analyzed identical splits of samples collected at, above, and below the ~12.8ka zone at the Lubbock Lake archaeological site (LL) in northwest Texas. Both labs reported similar variation in levels of magnetic micro-grains (>300 mg/kg >12.8ka and <11.5ka, but <150 mg/kg 12.8ka to 11.5ka). Analysis for magnetic microspheres in one split, reported elsewhere, produced very low to nonexistent levels throughout the section. In the other split, reported here, the levels of magnetic micro-spherules and nanodiamonds are low or nonexistent at, below, and above the YDB with the notable exception of a sample <11,500 cal years old. In that sample the claimed impact proxies were recovered at abundances two to four orders of magnitude above that from the other samples. Reproducibility of at least some analyses are problematic. In particular, no standard criteria exist for identification of magnetic spheres. Moreover, the purported impact proxies are not unique to the YDB.

2018

On June 2, 2016 at 10h56m UTC, a −18.9 ± 0.5 magnitude superbolide was observed over Arizona. We present analysis of this event based on 6 cameras and a multispectral sensor observations by the SkySentinel continuous fireball-monitoring camera network, supplemented by a dash cam footage and a fragmentation model. The bolide began its luminous flight at an altitude of 104.8 ± 0.5 km at coordinates φ = 34.59 ± 0.04°N planetographic latitude, λ = 110.45 ± 0.04°W longitude, and it had a preatmospheric velocity of 17.6 ± 0.5 km/s. The calculated orbital parameters indicate that the meteoroid did not belong to any presently known asteroid family. From our calculations, the impacting object had an initial mass of 11.4± 0.5 metric tonnes with an estimated initial size of 1.89 ± 0.07 m.

Proceedings of the National Academy of Sciences, 2012

Proceedings of the Boston Area Colloquium in Ancient Philosophy, 2007

Aristotle argues often and at length against Plato's paradigms, the transcendent forms. This paper argues that Aristotle endorses his own, somewhat different paradigms and that they are central to his philosophy. Aristotle's paradigm is one or more species in a genus that serve as standards through which the genus' other species are known. Characteristically, the differentiae of other species in the genus are composed of the differentia of the paradigm species and its privation. Aristotle develops the theory in Metaphysics I. In the context of the Metaphysics, paradigmatism takes a back seat to the pros hen priority of primary substance; however, this paper argues that paradigmatism is central in Aristotle's other scientific works. The first part of the paper argues that Metaphysics I advances the doctrine of paradigmatism, and it disables objections that might arise from other parts of the Metaphysics. The second part of the paper shows the significance of paradigmatism by exploring four example from the biological works. They become readily intelligible through paradigmatism and would be difficult or impossible to understand without it. The papers's third part shows that the doctrine plays important, but slightly different roles in Aristotle's productive and practical science. Specifically, it argues that paradigmatism explains the relation of epic to tragedy in the Poetics and the relation of the polity to both the best states and the lesser states in the Politics. 1 This paper is dedicated to the memory of my teacher, Father Joseph Owens, who passed away shortly before it was delivered. Father Owens was himself the paradigm of an Aristotelian scholar. I owe more to his guidance and his example than I can express. 4 J. Cook Wilson, "On the Platonic Doctrine of the ἀσύμβλατοι ἀριθμοί," Classical Review 18 (1904): 247-60, was concerned primarily with understanding the doctrine of form numbers that Aristotle ascribes to Plato in Metaphysics M. Aristotle claims at EN I 6, 1096a17-19 that Platonists did not make forms of classes like number that contain a prior and a posterior. Cook Wilson identifies this with Aristotle's claim in Metaph. B 3, 999a6-12 that no genus exists apart from species when the latter contain a prior and a posterior (pp. 247-48, 256). Hence, he effectively denies that there can be genera of ordered series. A. C. Lloyd, "Genus, Species, and Ordered Series in Aristotle," Phronesis 7 (1962): 68-69, rejects Cook Wilson's "logical" interpretation for what he terms a metaphysical one, namely, that Aristotle means to deny that the universal predicated of a series is one of the secondary substances of the Categories. My contention here is that Aristotle holds the species of all genera to be ordered.

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