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Independent Evaluation of Conflicting Microspherule Results from Different Investigations of the Impact Hypothesis for the Younger Dryas

Independent Evaluation of Conflicting Microspherule Results from Different Investigations of the Impact Hypothesis for the Younger Dryas

Sedimentary sequences from many sites across North America, Europe, and Asia were sampled by Firestone et al. (Firestone et al., 2007). The sediments were dated to the onset or border of the Younger Dryas approximately 12,900 calendar years BP, and Firestone reported the discovery of markers, that included Nanodiamonds, aciniform soot, high temperature melt-glass, and magnetic microspherules that were attributed to cosmic impacts/airbursts. Firestone et al. explained the microspherules as either cosmic material ablation or ejecta from a hypothesised Impact in North America that abruptly initiated the Younger Dryas cooling period, and contributed to the extinction of the megafauna, as well as triggering human cultural shifts and population declines. The presence of spherules from the Younger Dryas border has been confirmed by a number of independent groups, though 2 did not confirm the presence. Of these 1 (Surovell et al., 2009) collected and analysed samples from 7 Younger Dryas sites, purportedly using the same protocol as Firestone et al. without finding a single spherule in the sediments from the Younger Dryas border sediments from 2 sites that had previously been reported. In order to examine this discrepancy, LeCompte et al. conducted an independent blind investigation of 2 sites that were common to both studies, as well as a 3rd site had been investigated only by Surovell et al. They recovered abundant microspherules in the samples from all 3 sites that were separated widely that dated to the Younger Dryas border which is consistent with the results obtained by Firestone et al. and they concluded that the analytical protocol employed by Surovell et al. deviated significantly from that used by Firestone et al. It was suggested by morphological and geochemical analyses of spherules from the Younger Dryas border that they are not cosmic, volcanic, or anthropogenic in origin. They actually appear to have formed from abrupt melting and quenching of terrestrial materials.

It was proposed by Firestone et al. (Firestone et al., 2007) that at the onset of the Younger Dryas cooling episode at about 12,900 calendar years BP (12.9 ka) a cosmic event occurred. Included in the evidence in support of their hypothesis are elevated iron levels, magnetic spherules that are rich in silica, magnetic grains, iridium, as well as other minerals such as Nanodiamonds, and high temperature melt glass, which are in association with proxies that are indicative of the burning of biomass (Firestone et al., 2007; Kennett et al., 2009; Kennett et al., 2009; Israde-Alcántara, et al. 2012; Bunch et al., 2012). Charcoal, Carbon spherules, aciniform soot, and carbon that is glass-like. It was proposed further (Firestone et al., 2007) that the impact event may have contributed to the cooling episode of the Younger Dryas, the extinction that was near contemporaneous, of 36 species of megafauna, as well as triggering significant populations declines among some species that survived, which included human regional populations. It is problematic that there are not 1 or more impact craters. The focus of this paper, magnetic spherules, were reported by Firestone et al. at or proximal to: a thin sedimentary layer that dated to 12.9 ka, the Younger Dryas boundary (YDB), that has been found at many sites that are distributed widely in a field extending from the Channel Islands of California in the west to Belgium to the east, and from Texas in the south to central Alberta in the north. It was suggested by Firestone et al. that spherules represent either ablated material from an impactor or terrestrial ejector from 1 or more multiple impacts over North America, which would have been cantered on the Laurentide Ice Sheet and produced by an extraterrestrial (ET) object or objects the origin and character of which remain indeterminate.

There are 2 significant questions that have been raised by the discovery of spherules in the Younger Dryas boundary (Firestone et al., 2007):

1)    What is their source? and

2)    Is their abundance and YDB enhancement real?

It is believed that sedimentary spherules rich in iron and silica form the influx of extraterrestrial material, directly by ablation form meteorites and indirectly by the sudden cooling of ejecta from a molten cosmic impact (Puffer, Russell & Rampino, 1980; Brownlee, 1981; Koeberl, 1990; Genge, 2008; Simonson & Glass, 2004). There are a number of ways they can be formed:

·        Volcanism,

·        Anthropogenic processes,

·        Biogenesis, and

·        Diagenesis (Hodge & Wright, 1964; Puffer, 1974), and

·        By electrostatic discharges in the lab (Poppe, Gūttler & Springborn, 2010)

Consequentially, it is essential to determine the nature and origin of the observed spherules. Mixed results have been produced by attempts to replicate the abundances and peaks of spherules that were observed by Firestone et al. The presence of YDB spherules has been confirmed by a number of studies (Mahaney et al., 15; Fayek et al., Fayek et al., 2012; Israde-Alcántara et al., 2012); Pigati et al., 2012), while studies by others (Surovell et al., 2009; Pinter et al., 2011) did not find any spherules. Radiocarbon dates and/or diagnostic cultural artefacts were used by Surovell et al. to examine sediment sequences across the YDB from 7 archaeological sites across North America to identify the stratum that they believed was likely to be the YDB. In all the samples studied they observed many fewer YDB spherules than was reported by Firestone et al., and at the YDB they found no significant peaks of abundance in spherules at the YDB. At 2 sites common to the Firestone et al., study Surovell at al. reported finding no spherules in the YDB layer, though they reported extremely low numbers of spherules in some non-YDB strata. Based on the results obtained by Surovell et al. they asserted their adherence to the magnetic extraction and analysis protocol, dated 7 August 2007, as was developed originally by archaeologist William Topping, which had been subsequently improved by Allen West, and published in Firestone et al., which is referred to in this paper as the “protocol.” Most recently Israde et al. 2012) published an updated protocol that was more detailed.

LeCompte et al. aimed their study at finding the reason for the discrepant findings of Firestone et al. and Surovell et al. In order to do this they limited their study to investigating 3 archaeological sites, 2 of which are common to studies by Firestone et al. and Surovell et al. and 1 site common to Surovell et al. The scope of their study was defined by 4 lines of inquiry:

i)       determining the abundance of spherules and stratigraphic distribution;

ii)      analysing the surface morphology of spherules with a scanning electron microscope (SEM) photomicrographs and their composition by energy dispersive x-ray spectroscopy (EDS);

iii)      comparing the experimental methodologies that were used; and

iv)     examining whether some archaeological relevance to the decline of the human population that was predicted by Firestone et al. and Anderson et al. (20) might be possessed by microspherule evidence.

It was assumed by LeCompte et al. in order to test the decrease in population hypothesis, that if the major peak in spherule abundance was identified within or adjacent to strata that were associated with an apparent disruption in human activity or occupation, then a potential connection would be suggested by that correspondence.

Topper site, South Carolina – YDB sampling site information

The Topper (TPR) quarry site near Allendale, South Carolina, of Clovis age,   was sampled where a complete stratigraphic profile was collected in June 2008 which extended from the surface to 4 cm below a Clovis layer that contained extensive debitage, or waste material from the production of stone tools. The analyses of LeCompte et al. were limited to 4 samples of colluvium rich in quartz that were from a 4-10 cm thick across a sequence that was 31 cm wide that ranged from 52-83 cm below the surface (cmbs). Among these there was a layer that was 4 cm thick that had previously been accepted as the Younger Dryas border layer by Firestone et al. and Surovell et al. Included among the others there was a 4 cm sample that was centred at 83 cm below the surface, a contiguous 10 cm sample centred at 72 cm below the surface, and a 10 cm non-contiguous sample that was centred at 52 cm below the surface. These samples were collected within a few centimetres laterally of those that had been collected by Surovell et al. and about 80 cm from the Firestone et al. samples.

At the Topper Quarry the Younger Dryas layer dating to 12.9 ka BP, based on optically stimulated luminescence (OSL) (Waters, Forman, Stafford & Foss, 2009) and cultural stratigraphy (Goodyear & Steffy, 2003), has been accepted by both Firestone et al. and Surovell et al., as being represented by a layer that contained many Clovis artefacts and debitage. There are extremely few human artefacts in the approximately 20 cm above the Clovis artefacts, which indicates there was a hiatus of several hundred years in human activity at the site, which was possibly 600-1,200 years (Anderson, Goodyear, Kennett & West, 2011), after which the site was reoccupied, as evidenced by the presence of early Archaic Taylor-style points (Waters, Forman, Stafford & Foss, 2009). It was postulated by LeCompte et al. that the stratigraphic position of a peak in the abundance of spherules might indicate a temporal connection between the Younger Dryas border event that had been hypothesised and the dormancy at the quarry.

Blackwater Draw, New Mexico

The type location for the Clovis artefact interval, Blackwater Draw, LeCompte et al. provided 4 samples of sediment that varied in thickness of between 4.5 and 21 cm across the thickness of 88 cm. The samples, from within the protected enclosure called the South Bank Interpretative Centre, were taken on 18 June 2006 by Joanne Dickenson, the curator of the site. The Younger Dryas border layer was among the strata that were sampled, which was designated the “D/C” interface, which was identified by the curator as dating to the Clovis period of approximately 12.9 ka based on extensive radiocarbon dates as well as the biostratigraphic context of both Clovis artefacts and fossils of megafauna. The age of the layer, 12.9 ka, was accepted by Firestone et al. and Surovell et al. The Younger Dryas border layer, that is 5 cm thick, is centred at an elevation of 1,238.32 m above sea level (masl). A single 21 cm sample that was non-contiguous was acquired by LeCompte et al. from below the Younger Dryas border, which was centred at 1,237.90 m above sea level, and 2 5 cm non-contiguous samples from above, 1 of which was centred at 1,238.41 m above sea level and another at 1,238.78 m above sea level. All of the samples were from a part of the site that was less than 1 m from the site of the Firestone et al. samples and those of the Surovell et al. investigations.

Paw Paw Cove, Maryland

Field investigations were conducted at a site that was proposed as being of Clovis age at the southern end of Paw Paw Cove on a beach embankment that faced to the west on the Eastern Shore of Chesapeake Bay. A sample was obtained from a stratigraphic section that was represented by Darrin Lowery, the principal archaeologist of the site, as being the most likely to contain proxies dating to the Younger Dryas border, based on his knowledge of the site. It is estimated by LeCompte et al. that their sample was collected less than a few hundred metres of the site that had been reported by Surovell et al. and it includes sediment from the same stratum as that provided to them by Darrin Lowery. It had been assumed by Surovell et al. that they were sampling the Younger Dryas border, and that assumption has not been questioned by LeCompte et al., the goal of LeCompte et al. was to assess whether or not any potential spherules had been detected. A single stratigraphic sample had been extracted which was centred at a depth of approximately 1 m below the surface. The inferred Younger Dryas border layer that contained Clovis artefacts nearby was located immediately below a ubiquitous loess layer that was orange coloured that lies on top of a stratum that was noticeably greyish coloured (Lowery, 1989; Lowery et al., 2010).

Blind study

LeCompte et al. took part in a blind test of the sediment samples that had been obtained from 2 common sites, Blackwater Draw and Topper, in order to eliminate any analytical bias. Blind testing was not applicable from the single samples that had been obtained from Paw Paw Cove. The 8 sediment samples that had not been processed, 4 from each site, were repackaged and distributed by a 3rd party that did not participate in the blind processing tests by a member of their group (MAL). The packages were randomly numbered and labelled with the indicated source, though not with depth or relationship to the Younger Dryas border layer. When the blind test concluded LeCompte et al. adopted the same chronostratigraphy that had been used by the principal investigators for all of the tests, as well as by Firestone et al. and Surovell et al. Depths of the Younger Dryas border layer may vary between studies as the result of differences between locations that had been sampled.


The analysis by LeCompte et al. suggests that with regard to the study by Surovell et al. the spherule abundance in the Younger Dryas border layer at 3 sites that were examined are consistent with those reported by Firestone et al. and therefore not consistent with those reported by Surovell et al. LeCompte et al., concluded that any differences there are between their results and those of Firestone et al. are within normal variation. The prescribed protocol produces results that are reliably quantifiable.

It is indicated by the work of LeCompte et al. that the analysis of Surovell et al. did not follow 3 of the most critical elements of the protocol of Firestone et al.:

i)                   Sorting of the sizes of the magnetic fraction,

ii)                The examination of amounts of magnetic material that are sufficient, and

iii)              Examination of candidate spherules by SEM and EDS.

These omissions resulted in a methodology that was of inadequate sensitivity to detect significant numbers of spherules in any stratum, with the possible exception of those that had abundant large spherules as may be the case in Agate Basin. It is emphasised by LeCompte et al. that in the future investigators that are testing for the presence or absence of spherules in the Younger Dryas border stratum include rigorous size sorting as a standard procedure as well as examination by SEM/EDS analyses. Grain size sorting by the use of a 53 μm screen improves greatly the ratio of spherules to distractor grains, which reduces profoundly the difficulties of searching, identifying and counting accurately these small objects.

According to LeCompte et al. total magnetic grain density peaks at 3 sites are not correlated reliably with the peaks in spherules or the onset of the Younger Dryas. This is consistent with reported results from the study by Surovell et al. A peak in the smallest size portion, <53 μm, of the magnetic grains does, however, appear to correlate weakly with strata from the Younger Dryas border at the Topper site and the Blackwater Draw site, possibly indicating some depositional process that is yet to be discovered that was operating at that time.

At all 3 sites microspherules are similar, morphologically and geochemically, averaging about 3 μm. The chemical composition of these microspherules varies from aluminosilicate glass to magnetite to titanomagnetite. The compositions of spherules from the Younger Dryas border layer are similar to terrestrial metamorphic rocks, differing significantly from those that are formed by cosmic or authigenic processes, with the exception of a single magnetic spherule that is highly enriched in rare earth elements. It is considered to be highly unlikely that the sources are of volcanic origin or anthropogenic origin.

With regard to a potential decline in human population, the results of the study by LeCompte et al. at TPR quarry site demonstrate that the Younger Dryas border layer that is highly enriched in spherules coincides approximately with the onset of a multi-century hiatus in activity at the Clovis quarry. These results are consistent with a decline of population that was coeval with the Younger Dryas border event as was proposed previously, when these results are combined with those from Blackwater Draw and Paw Paw Cove.

The scope of the study by LeCompte et al. was limited to considering only the identification, occurrence, and nature of Younger Dryas border magnetic spherules and the possible implications. Their results are consistent with, though do not prove, that a cosmic impact that was proposed previously (the Younger Dryas border impact hypothesis) occurred 12.9 Ka. It remains a mystery what the ultimate source of the magnetic spherules present in the Younger Dryas border sediment is.

Sources & Further reading

  1. LeCompte, M. A., et al. (2012). "Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis." Proceedings of the National Academy of Sciences 109(44): E2960-E2969.


Author: M.H.Monroe
Last updated: 18/12/2017
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