Monday, September 1, 2014
Younger Dryas Impact Hypothesis Confirmation Work
This paper pretty clearly confirms the reality of the Younger Dryas Boundary layer and it certainly conforms to the expected results of a huge comet impact. We have the additional advantage of understanding that this comet impacted the Northern Ice Field to induce a crustal shift that ended the Ice Age. This is a critical step in proving that clear conjecture.
At least now there will be scant dispute over just what happened 12w900 years ago. No one else has woken up properly to the inevitable crustal shift and by simple application all the supporting direct evidence.
Once this is understood, we will have the Pleistocene and its ending properly sorted out and that the end was deliberately produced.
Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis
Firestone et al. sampled sedimentary sequences at many sites across North America, Europe, and Asia [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 106:16016–16021]. In sediments dated to the Younger Dryas onset or Boundary (YDB) approximately 12,900 calendar years ago, Firestone et al. reported discovery of markers, including nanodiamonds, aciniform soot, high-temperature melt-glass, and magnetic microspherules attributed to cosmic impacts/airbursts. The microspherules were explained as either cosmic material ablation or terrestrial ejecta from a hypothesized North American impact that initiated the abrupt Younger Dryas cooling, contributed to megafaunal extinctions, and triggered human cultural shifts and population declines. A number of independent groups have confirmed the presence of YDB spherules, but two have not. One of them [Surovell TA, et al. (2009) Proc Natl Acad Sci USA 104:18155–18158] collected and analyzed samples from seven YDB sites, purportedly using the same protocol as Firestone et al., but did not find a single spherule in YDB sediments at two previously reported sites. To examine this discrepancy, we conducted an independent blind investigation of two sites common to both studies, and a third site investigated only by Surovell et al. We found abundant YDB microspherules at all three widely separated sites consistent with the results of Firestone et al. and conclude that the analytical protocol employed by Surovell et al. deviated significantly from that of Firestone et al. Morphological and geochemical analyses of YDB spherules suggest they are not cosmic, volcanic, authigenic, or anthropogenic in origin. Instead, they appear to have formed from abrupt melting and quenching of terrestrial materials.
Firestone et al. (1) (hereafter Firestone et al.) proposed that a cosmic impact event occurred at the onset of the Younger Dryas (YD) cooling episode at about 12,900 calendar years Before Present (12.9 ka B.P.). Evidence cited to support their hypothesis includes elevated levels of iron- and silica-rich magnetic spherules, magnetic grains, iridium, and other materials such as nanodiamonds and high temperature melt glass, in association with proxies indicative of biomass burning (1⇓⇓⇓–5). Also reported were charcoal, carbon spherules, aciniform soot, and glass-like carbon. The group further proposed that the impact event may have contributed to the YD cooling episode, the near-contemporaneous extinction of approximately 36 species of megafauna, and triggered significant population declines among some surviving species including human regional populations. The absence of one or more associated impact craters remains problematic. Magnetic spherules, the focus of this paper, were reported by Firestone et al. at or proximal to: a thin sedimentary layer dating to 12.9 ka B.P. called the Younger Dryas Boundary (YDB), found at numerous widely distributed sites in a field extending from California’s Channel Islands in the west to Belgium in the east, and from Texas in the south to central Alberta, Canada in the north. Firestone et al. suggested that the spherules represented either ablated impactor material or terrestrial ejecta from one or multiple impacts over North America, centered on the Laurentide Ice Sheet and produced by an extraterrestrial (ET) object or objects whose origin and character remain indeterminate.
The discovery of YDB spherules (1) has produced two significant questions: what is their source and is their abundance and YDB enhancement real? Sedimentary iron- and silica-rich spherules are believed to form from the influx of ET material, directly by meteoritic ablation and indirectly during the sudden cooling of molten terrestrial ejecta from a cosmic impact (6⇓⇓⇓–10). They can also be formed by volcanism, anthropogenic processes, biogenesis, and diagenesis (11, 12) and, in the laboratory, by electrostatic discharge (13). Consequently, determining the nature and origin of observed spherules is essential. Attempts to replicate the spherule abundances and peaks observed by Firestone et al. have produced mixed results. Haynes et al. (14), Mahaney et al. (15), Fayek et al. (16), Israde et al. (4), and Pigati et al. (17) confirmed the presence of YDB spherules, while studies by Surovell et al. (18) and Pinter et al. (19) did not. Surovell et al. examined sediment sequences across the YDB at seven archeological sites in North America, using radiocarbon dates and/or diagnostic cultural artifacts to identify the stratum likely to be the YDB. In all cases, they observed far fewer YDB spherules than reported by Firestone et al. and found no significant abundance peak in spherules at the YDB. At the two sites common to the Firestone et al. study they reported finding not a single spherule in the YDB layer, although in some non-YDB strata, they reported finding extremely low spherule abundances. In producing those results, Surovell et al. asserted their adherence to the magnetic spherule extraction and analysis protocol, dated August 7, 2007, as originally developed by archeologist William Topping, subsequently improved by Allen West, and published in Firestone et al., hereafter referred to as the “protocol.” An updated and more detailed protocol was most recently published in Israde et al. (4).
We sought to understand the cause of the discrepant findings of Firestone et al. and Surovell et al. To do so, we limited our investigations to three archeological sites, two of which are common to the Firestone et al. and Surovell et al. studies and one site common to that of Surovell et al. Four lines of inquiry defined the scope of our inquiry: (i) determining spherule abundances and stratigraphic distribution; (ii) analyzing spherule surface morphology using a scanning electron microscope (SEM) photomicrographs and their composition by energy-dispersive x-ray spectroscopy (EDS); (iii) comparing the experimental methodologies used; and (iv) examining whether microspherule evidence might possess some archeological relevance to the human population decline predicted by Firestone et al. and Anderson et al. (20). To test the population decrease hypothesis, we assumed that if a major YDB spherule abundance peak was identified within or adjacent to strata associated with an apparent disruption in human activity or occupation, then that correspondence would suggest a potential connection.
Topper Site, South Carolina.
Sampling was conducted at the Clovis-aged Topper (TPR) quarry site near Allendale, SC where a complete stratigraphic profile was collected in June 2008 from the surface to 4 cm below a Clovis layer containing extensive debitage, or waste material resulting from stone tool production (SI Appendix, Fig. S1). At this site our analyses were limited to four samples of quartz-rich colluvium from 4–10 cm thick across a 31-cm-wide sequence ranging from 52–83 cmbs. These included a 4-cm-thick layer that was centered at 79 cm below the surface (cmbs) and previously accepted as the YDB layer by Firestone et al. and Surovell et al. The others included a contiguous 4 cm sample centered at 83 cmbs, a contiguous 10 cm sample centered at 72 cmbs, and a 10 cm non-contiguous sample centered at 52 cmbs (SI Appendix, Fig. S1). These samples were collected within a few meters laterally from those of Surovell et al. and approximately 80 m from those collected by Firestone et al.
At TPR, based on optically stimulated luminescence (OSL) dating (21) and cultural stratigraphy (22), the YDB layer dating to 12.9 ka B.P. is accepted by both Firestone et al. and Surovell et al. as being represented by a few-cm-thick layer containing highly abundant Clovis artifacts and debitage. For approximately 20 cm above the Clovis artifacts, there are extremely few human artifacts, indicating that the quarry experienced a multi-century hiatus in human activity, perhaps as long as 600–1200 y (20), after which the site was reoccupied, as evidenced by the presence of early Archaic Taylor-style points (21). We posited that the stratigraphic position of a peak in spherule abundance might indicate a potential temporal connection between the hypothesized YDB event and quarry dormancy.
Blackwater Draw, New Mexico.
From Blackwater Draw (BWD), the type location for the Clovis artifact interval, we acquired four sediment samples varying from 4.5–21 cm in thickness across an 88 cm interval. The samples were taken on January 18, 2006, by the site curator, Joanne Dickenson, from within a protected enclosure called the South Bank Interpretive Center (SI Appendix, Fig. S2). Sampled strata include the YDB layer, designated as the “D/C” interface, and identified by the curator as dating to the Clovis period at approximately 12.9 ka B.P., based on extensive radiocarbon dates and the biostratigraphic context of both Clovis artifacts and megafaunal fossils. The layer’s age, 12.9 ka B.P., was accepted by both Firestone et al. and Surovell et al. The 5-cm-thick YDB layer is centered at an elevation of 1,238.32 meters above sea level (masl). We also acquired one 21 cm non-contiguous sample below the YDB, centered at 1,237.90 masl, and two 5 cm non-contiguous samples above, one centered at 1,238.41 masl and the other at 1238.78 masl. All samples were collected within less than one meter of the location sampled for the Firestone et al. and Surovell et al. Investigations.
Paw Paw Cove, Maryland.
On August 8, 2008, we conducted field investigations at the southern end of Paw Paw Cove (PPC) at a proposed Clovis-age site on a westward-facing beach embankment on the Eastern Shore of Chesapeake Bay. We obtained a sample from a stratigraphic section represented by Darrin Lowery, the principal site archaeologist, as most likely to contain YDB proxies, based on his knowledge of the site (SI Appendix, Fig. S3). We estimate that our sample was collected within less than a few hundred meters of the site reported in Surovell et al. and includes sediment from the same stratum, as provided to them by Darrin Lowery. Surovell et al. assumed they were sampling the YDB layer, and we have not questioned that assumption; our goal is only to assess whether or not they detected any potential spherules present. One 15-cm-thick stratigraphic sample was extracted, centered at a depth of approximately one meter below the surface. The inferred YDB layer containing nearby Clovis artifacts was located immediately beneath a ubiquitous, orange-colored loess layer that lies atop a noticeably greyish colored stratum (23, 24).
To eliminate potential analytical bias, we participated in a blind test of the sediment samples taken from the two common sites, BWD and TPR. Blind testing was not applicable for the single sample collected from PPC. The eight unprocessed sediment samples, four from each site, were repackaged and distributed by a non-participating third party for blind processing by a member of our group (MAL). The packages were randomly numbered and labeled with the indicated source site, but not with depth or relationship to the YDB layer. At the conclusion of the blind test, we adopted the same chronostratigraphy used by the principal investigators for all of our sites, as well as by Firestone et al. and Surovell et al. YDB depths may vary between studies due to differences in the sampled locations.
Magnetic Grain Results.
Magnetic grains were extracted from a slurry of the bulk sediment sample, using a grade-52 NdB super-magnet (Methods). Overall, we observed some abundance enhancement of magnetic grains at or close to the YDB layer, but the peaks are not unique and are too small to be statistically significant with so few samples. Therefore, we cannot confirm or refute any peak abundance of bulk YDB magnetic grains at either BWD or TPR, as reported by Firestone et al. (Tables 1 and 2), although we did detect a small but consistent increase in the < 53-μm grain-size fraction at the YDB for both sites, as discussed below. No conclusions can be drawn from the single sample collected at PPC (Table 3). Whether an increase in bulk- or small-grain size observed in or immediately adjacent to the YDB layer is statistically significant requires analysis at more sites and samples taken at higher stratigraphic resolution.
Magnetic Spherule Results.
For spherule extraction, the Firestone et al. protocol specified sorting the entire magnetic fraction to at least < 150-μm, but our initial assessment revealed the outside diameters of nearly all spherules at BWD, PPC, and TPR to be less than 50-μm. Consequently, we separated the magnetic grains into three size fractions based on grain dimensions (dg): (i) dg > 229 μm with an ASTM #60 sieve; (ii) 229 μm > dg > 53 μm; and (iii) dg < 53 μm with an ASTM #270 sieve. We observed that this additional sorting greatly facilitated spherule detection and counting, as concluded by Israde et al. We determined by spherule counts from these three sites that almost all spherules are < 53 μm in diameter. Therefore, we report spherule abundances only from the < 53-μm magnetic grain fraction.
To determine spherule abundances in the samples, several portions of approximately 10 mg mass each and of dg < 53 μm were examined using a reflected-light 180-power optical microscope. The dg < 53-μm fraction represented from 20–30% of the total magnetic fraction, including grains of all sizes. Photomicrographs were obtained of each candidate spherule observed. Most were then placed manually upon an adhesive tab fixed to an aluminum stub for SEM imaging and EDS analysis to examine their surface morphology, minimize identification ambiguity, and to determine their composition.
Magnetic spherule surface morphology is distinctive when created by processes involving high temperatures followed by rapid cooling, such as during meteoritic atmospheric ablation and impacts (4⇓⇓⇓⇓⇓⇓⇓–12). For identification of spherule surficial morphology (microstructure), we followed the work of Zagurski et al. (25), who used SEM imaging to develop indices of magnetic particles in soil samples extracted from 33 Russian sites. A variety of spherule forms (16 subtypes) were found to be relatively ubiquitous in small concentrations throughout the soil column. The spherules were categorized according to their distinctive polygonal and dendritic surface patterning or microstructures (26) formed by rapid crystallization, indicating they were in a molten state and rapidly quenched (SI Appendix, Fig. S4).
Based on SEM analyses, the number of quench-melted spherules was then compared to the number of non-melted spherule candidates, including rounded magnetite grains and authigenic framboids, the latter of which display blocky crystals formed by slow crystallization (SI Appendix, Fig. S5). The ratio between the SEM-verified melted spherules and the total candidate spherules provided what we call a false-positive correction factor for each layer.
The numbers of spherules in each 10-mg portion were found to vary enough to prompt examination of multiple 10-mg aliquots to more accurately establish spherule abundances at each stratigraphic level. To assure statistical significance, we continued to search for spherules until a minimum of six candidates in each layer were confirmed as melted spherules. In Fig. 1, six confirmed melted spherules are shown each for TPR and PPC, with 30 confirmed spherules for BWD. These spherules exhibit the surface microstructures, which are consistent with formation by melting at high temperatures, followed by rapid quenching. We summed the total number of candidate-spherules per unit mass of magnetic grains in each layer and then multiplied that times the false-positive correction factor. The factor for both BWD and PPC is 0.8, meaning that 20% of candidates were rejected. For TPR, the factor was 0.25, meaning the 75% of candidates were rejected, many because they were apparently framboids. Last, we calculated total spherule abundance by normalizing the corrected value to yield the number of quench-melted spherules per kilogram of bulk sediment. SEM micrograph validation was only applied to those spherules found in or immediately above the YDB layer. For the stratigraphically highest and lowest samples, no SEM work was performed. The total number of uncorrected candidate-spherules is reported as an upper limit. The actual spherule values are almost certainly lower. In summary, we found that SEM analyses to determine microstructure and the application of a correction factor are essential, because true YDB spherules cannot be reliably identified by light microscopy alone as was attempted by Surovell et al., Pinter et al., and Pigati et al. (17).
Spherules picked from the YDB layer at each of the three sites. BWD also exhibited abundant spherules in the D stratum just above the YDB (D/C stratum). We found statistically significant numbers of spherules in the YDB at all three sites, whereas Surovell et al. found none. White numbers at bottom right represent diameter of spherules in microns (μm).
At the TPR Paleoindian quarry, we removed sediment lying directly on top of the layer of debitage (Fig. 2 and SI Appendix, Fig. S1), which delineated the level of highest Clovis usage of the quarry. Immediately above it there is almost no debitage. Next we lifted off selected pieces of Clovis debitage and carefully collected the sediment directly beneath each chert fragment.