Sunday, November 29, 2015

Academic Bun Fight: the Pleistocene/Holocene Boundary and the Younger Dryas Impact Hypothesis

Bayesian chronological analyses consistent with synchronous age of 12,835–12,735 Cal B.P. for Younger Dryas boundary on four continents


Kennett et al


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.

That was really bad science

Problematic dating of claimed Younger Dryas boundary impact proxies




The PNAS paper by Kennett et al. (1) uses statistical methods in an attempt to improve the geochronological control for purported Younger Dryas boundary (YDB) impact proxies. The underpinning data for these analyses are problematic, however, as discussed by Meltzer et al. (2) and Holliday et al. (3). Several examples illustrate the problems. At Barber Creek the YDB zone is at ∼100 cm below the surface, but in situ wood charcoal dated to 10,500 ± 50 14C y B.P. (∼12.5 k cal yrs) is documented below 100 cm (3). The large SD for the modeled age of the YDB here (1) (12,865 ± 535 cal yrs) easily accommodates the high-precision date on the charcoal from below the spherule zone. At Blackville the sediments dated by optically stimulated luminescence are mixed and thus the dates cannot be considered reliable (3). The supposed impact proxies at Bull Creek are from 307- to 312-cm depth (3). The radiocarbon date of ∼12,960 cal yrs is from 298 to 307 cm and is a bulk sample on soil organic matter, thus representing a mean residence time for the soil carbon. Impact proxies are, therefore, older than ∼12,960 y by some unknown amount; they are also found in abundance in strata less than 3,000 y old. The Usselo soil in northwest Europe spans ∼1,400 y based on ∼50 radiocarbon ages, dating primarily to the Allerød and into the YD (2).


Reply to Holliday and Boslough et al.: Synchroneity of widespread Bayesian-modeled ages supports Younger Dryas impact hypothesis


Kennett et al


Holliday (1) rejects age-depth models for the Younger Dryas boundary layer (YDB) in Kennett et al. (2), claiming that they are incorrect for several reasons, including age reversals, high age uncertainties, and use of optically stimulated luminescence (OSL) dating. These same claims previously were presented in Meltzer et al. (3) and were discussed and refuted in Kennett et al. (2). These criticisms apply to nearly all dated archaeological and geological sequences, including the Odessa meteorite impact crater, where paradoxically, Holliday et al. (4) modeled an impact age using OSL dating (greater than 70% of dates used) with large uncertainties (to greater than 6,000 y) and age reversals (greatre than 40% of dates are reversals). Thus, Holliday (1) argues against a practice that he and many other researchers have used and continue to use today. In an ideal world, all dates would be in perfect chronological order with high accuracy and certainty, but such scenarios are rarely possible (2). It is because of such dating difficulties that Bayesian analysis is a powerful chronological tool, and is rapidly becoming the archaeological standard.

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