recent debate on Younger Dryas impact hypothesis, many references on Wikipedia: Rich Murray 2016.02.10
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recent debate on Younger Dryas impact hypothesis, many references on Wikipedia: Rich Murray 2016.02.10
 
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recent debate on Younger Dryas impact hypothesis, many references on Wikipedia: Rich Murray 2016.02.10
Recent debates[edit] [ the last section of a detailed discussion ] Recently new studies were published in the matter of the YDB impact hypothesis, criticizing the methodology and pointing to inconsistencies regarding the chronological data.[57][58][59] This new research, which analyzed sediments claimed, by the hypothesis proponents, to be deposits resulting from a bolide impact were, in fact, dated from much later or much earlier time periods than the proposed date of the cosmic impact. The researchers examined 29 sites that are commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only three of the sites actually date from that time. According to the researchers, the Younger Dryas impact event evidence "fails the critical chronological test of an isochronous event at the Younger Dryas onset, which, coupled with the many published concerns about the extraterrestrial origin of the purported impact markers, renders the Younger Dryas impact hypothesis unsupported. There is no reason or compelling evidence to accept the claim that a cosmic impact occurred about 12,800 years ago and caused the Younger Dryas."[57] These same studies were addressed and replied by Kennett and his colleagues, still advocating the validity of the YDB impact hypothesis.[60] 57 Meltzer DJ, Holliday VT, Cannon MD, Miller DS (May 2014). "Chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12,800 years ago". Proc. Natl. Acad. Sci. U.S.A. 111 (21): E2162–71. doi:10.1073/pnas.1401150111. PMC 4040610. PMID 24821789. 58 Holliday, Vance T. (2015-12-08). "Problematic dating of claimed Younger Dryas boundary impact proxies". Proceedings of the National Academy of Sciences 112 (49): E6721–E6721. doi:10.1073/pnas.1518945112. ISSN 0027-8424. PMC 4679064. PMID 26604317. 59 Boslough, Mark; Nicoll, Kathleen; Daulton, Tyrone L.; Scott, Andrew C.; Claeys, Philippe; Gill, Jacquelyn L.; Marlon, Jennifer R.; Bartlein, Patrick J. (2015-12-08). "Incomplete Bayesian model rejects contradictory radiocarbon data for being contradictory". Proceedings of the National Academy of Sciences 112 (49): E6722–E6722. doi:10.1073/pnas.1519917112. ISSN 0027-8424. PMC 4679022. PMID 26604316. 60 Kennett, James P.; Kennett, Douglas J.; Culleton, Brendan J.; Tortosa, J. Emili Aura; Bunch, Ted E.; Erlandson, Jon M.; Johnson, John R.; Pardo, Jesús F. Jordá; LeCompte, Malcome A. (2015-12-08). "Reply to Holliday and Boslough et al.: Synchroneity of widespread Bayesian-modeled ages supports Younger Dryas impact hypothesis". Proceedings of the National Academy of Sciences 112 (49): E6723–E6724. doi:10.1073/pnas.1520411112. ISSN 0027-8424. PMC 4679043. PMID 26604309. http://www.pnas.org/content/112/49/E6723.full free full text 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 (>70% of dates used) with large uncertainties (to >6,000 y) and age reversals (>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. Holliday (1) also claims to “provide evidence for multiple horizons with ‘impact proxies’ at times other than the YDB.” Those claims have been refuted in detail (2, 5–7). In every case, those contradictory studies have serious flaws, including: (i) correct protocols were not followed, and (ii) the evidence was not analyzed using electron microscopy, an essential requirement. Independent workers who followed the correct procedures (e.g., ref. 5) confirmed the presence of YDB impact proxies at multiple sites, with few to no proxies above and below. Contrary to Holliday’s (1) claims, no interval other than the YDB layer in 23 widely separated stratigraphic profiles, spanning up to 50,000 y, contains the same broad assemblage of proxies (2). Boslough et al. (8) question why Kennett et al. (2) did not create a Bayesian age-depth model for the Gainey site in Michigan. As previously explained (2), Bayesian analysis is most robust when the available dataset meets certain criteria, including having deeply stratified deposits with numerous dates bracketing the stratigraphic level of interest. Gainey, a site with near-surface, bioturbated deposits, does not meet those criteria, and so it was not modeled. Most importantly, all available dates are on a single stratum, making it impossible to create an age-depth model. Even so, the Gainey YDB layer contains thousands of high-temperature magnetic spherules, glassy spherules, and nanodiamonds, intermixed with thousands of Paleoindian lithics having a widely accepted age of ∼12,800 Cal B.P. (2, 7, 9). Previous studies concluded that the proxy-rich, lithics-rich stratum at Gainey is consistent with the YDB layer (7). We continue to support that conclusion. Boslough et al. (8) also claim that their single young 14C date (calibrated to 207 ± 87 Cal B.P.) proves that Gainey does not contain the YDB stratum. Because this young date was from carbon intermixed in the same stratum with Paleoindian lithics dating to ∼12,800 Cal B.P., the two ages are mutually exclusive, and one must be rejected. In this case, the 12,800-y-old lithics are indisputably in situ, making it certain that the younger 14C date Boslough et al. (8) mention is on carbon that intruded from younger surficial deposits. Out-of-sequence 14C dates are a common dating problem that is solved by discounting outlying young dates. Because Paleoindians were certainly not living at Gainey ∼200 y ago, this younger date cannot reasonably be used to reject Gainey as a YDB site. We reaffirm the validity of the Bayesian statistical analyses in Kennett et al. (2) demonstrating that the age of the YDB layer on four continents is synchronous within an age range of 12,835–12,735 Cal B.P., within the confines of dating uncertainties (95% confidence interval). Only the YDB layer in stratigraphic sections at 23 sites contains abundance peaks in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, and osmium (e.g., refs. 2, 5⇓–7, 9). The Bayesian-modeled YDB age range also overlaps that of an extraterrestrial platinum peak, independently identified in the Greenland ice sheet (2) that coincides unequivocally with the onset of the Younger Dryas cooling episode, supporting a causal connection between the Younger Dryas impact event and major climate change (2). Next Section Footnotes 1 To whom correspondence should be addressed. Email: [hidden email]. Published online before print November 24, 2015, doi: 10.1073/pnas.1520411112 PNAS December 8, 2015 vol. 112 no. 49 E6723-E6724 Classifications Letter Physical Sciences Earth, Atmospheric, and Planetary Sciences synchronous age of 12,835–12,735 Cal BP for Younger Dryas boundary on four continents -- my findings from Santa Fe to San Diego: Rich Murray 2015.08.02 ============================================================ FRIAM Applied Complexity Group listserv Meets Fridays 9a-11:30 at cafe at St. John's College to unsubscribe http://redfish.com/mailman/listinfo/friam_redfish.com |
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