Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago, Parts 1 and 2: A Discussion
Wolbach et al. published two papers on the Younger Dryas Impact Hypothesis (YDIH)—the paleoenvironmental effects of a purported cosmic impact at the beginning of the Younger Dryas Chronozone (YDC). Part 1 includes a selective summary of previous discussions of the YDIH but leaves out key reviews, uncritically accepts previous interpretations of purported impact, and fails to take into account abundant criticisms and contradictory data. A discussion of icecore evidence focuses only on the beginning of the YDC rather than on a longer interval that would allow the context of claimed impact indicators to be evaluated. The exceptionalism claimed for many of the key data points is the result of exaggerations, and the graphical analyses themselves are not reproducible. Part 2 presents data from sedimentary records. The authors assume that impacts triggered widespread fires, but the evidence for such a link between extraterrestrial impacts and wildfires is weak. The presence of charcoal at the beginning of the YDC (YDB) fails to unambiguously support the hypothesis of impact-related fires because there is also a large peak at the end of the YDC. Stratigraphic markers used to argue for widespread burning, such as the Usselo soil of northwest Europe and the black mat of the southwestern United States, were shown by their original investigators to have no plausible link to an impact event. Human population decline in North America is not supported by abundant published archaeological, geochronological, and stratigraphic evidence. Late Pleistocene megafauna extinctions varied in time for individual species across the Americas, Europe, and Asia and thus cannot be accounted for by a single impact. If there was some sort of extraterrestrial YDB-age event, it apparently had negligible terrestrial impact. The articles by Wolbach et al. (2018a, 2018b) are part of a long series of publications that began more than a decade ago dealing with the YDIH (Firestone et al. 2006, 2007). That hypothesis has generated considerable data and debate regarding the viability of the idea that at ∼12,900 cal BP (at the onset of the YDC), some sort of cosmic event (single impact, multiple impacts, and/or airbursts) affected North America, South America, western Europe, and western Asia (Wolbach et al. 2018a, p. 165). This purported event is claimed by the adherents of the YDIH to have brought on extraordinary environmental changes that “triggered an ‘impact winter’ and the subsequent YD climate episode, biomass burning, latePleistocenemegafaunal extinctions, and human cultural shifts and population declines” (Wolbach et al. 2018a, p. 165). A key criticism of the YDIH over the past decade is that the proponents do not consider, or perhaps misrepresent, the data and arguments that contradict the YDIH. The same problems that have plagued the evaluation of the YDIH are well displayed in the two articles. In this discussion we highlight these problems and alert readers to the considerable body of contradictory data. The first article (Wolbach et al. 2018a) focuses on evidence for burning and wildfires in ice cores from Greenland and includes appendixes that summarize data in support of the YDIH. Part 2 (Wolbach et al. 2018b) is a discussion of terrestrial evidence, largely from North America, claimed to support the ice-core data on burning and wildfires. Our discussion focuses on the background of the debate as summarized in part 1 of Wolbach et al. (2018a, tables A1 and A3), on the data from ice cores presented in part 1, and then on the data and discussion in part 2 (Wolbach et al. 2018b), an examination of the terrestrial evidence for burning and wildfires in North America and claimed “proxy indicators” for burning and an impact.