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
Abstract
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).
Blind Study.
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.
Table 1.
Table 2.
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).
Fig. 1.
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).
Topper Spherules.
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.
Fig.
2.
Topper
site, SC. Tariq Ghaffar, Public Broadcasting System’s Time Team
America archeologist, excavating pedestals (lower arrow) capped by
debitage (upper arrow) at the TPR SC Clovis quarry Archeological
site. YDB layer indicated by dotted line. Samples were collected
from immediately above and immediately below selected pieces of
debitage. Photo from Al Goodyear and Meg Galliard.
For
the sediment atop the debitage, two aliquots were examined of the
small magnetic grain fraction < 53 μm), which
represented 20% of the total magnetic aliquot that weighed
approximately 65 mg. One aliquot of 13 mg yielded 14
spherule candidates and a second aliquot of 30 mg yielded 10
candidates. SEM examination indicated that many of these were
framboids so were excluded. The other spherules exhibited surface
microstructures indicative of rapid melting and quenching as
described by previous researchers (25⇓–27).
Spherule sizes at TPR range from 25 to 45 μm, averaging
approximately 30 μm, and they are geochemically and
morphologically similar to YDB spherules extracted from the other
two sites.
Spherule
concentrations exhibit a significant peak of approximately
260 spherules/kg in the 4-cm-thick YDB layer immediately above
the debitage (Fig. 3, Table 1).
For the 4-cm-thick sediment layer immediately underlying or shadowed
by fragments of chert débitage and centered 2 cm below the
debitage layer there is a significant decrease to about 30
candidate-spherules/kg (false-positive correction yields
8 spherules/kg). In the 5-cm-thick sample centered at 9 cm
above the debitage layer there are 115 spherules/kg and the
layer approximately 29 cm above the debitage strata contained
an estimated 31 non-corrected candidate-spherules/kg. Table 1 shows
that the lowest spherule abundance occurs just beneath the YDB layer
and coincides with inferred high quarry usage during Clovis times.
The highest spherule abundance appears in the YDB layer,
contemporary with the inferred abandonment of the quarry, consistent
with evidence for a population decline posited by Firestone et al.
and Anderson et al. (20).
Fig.
3.
Blackwater Draw Spherules.
At
BWD the D/C stratum was accepted by both Firestone et al. and
Surovell et al. as the YDB layer. A sample collected from that
stratum produced approximately 25 spherules from 34 mg of
the dg < 53-μm
magnetic fraction representing 28% of the total 120-mg magnetic
aliquot. SEM-confirmed spherules totaled about 624 spherules/kg
(Fig. 4, Table 2).
This compares to approximately 36 spherules from 17 mg in the D
stratum centered 10 cm above the D/C stratum for a total of
1,318 spherules/kg. Spherule sizes in the two layers varied
between 17–50 μm, averaging 30 μm. In contrast to TPR,
where our results appear to support the existence of a distinct and
relatively narrow abundance peak in strata at the YDB, BWD spherule
abundances appear to exhibit a broader abundance peak of about 20 cm
width just above the YDB stratum. At -40 cm, the D/C stratum
spherule abundance is distinctly lower at 18 candidate-spherules/kg,
while at +40 cm, candidate-spherule abundance is at
approximately 314 spherules/kg. Enhanced spherule abundance of
the layers overlying the YDB layer may be due to enrichment by
fluvial action as suggested by Haynes et al. or slightly later
deposition.
Fig.
4.
Paw Paw Cove Spherules.
The
size range of spherules extracted from the single PPC sample of YDB
age ranged from 20–49 μm, averaging 30 μm. Total
abundance was 317 spherules/kg. Seven spherules were extracted from
32 mg of the dg < 53-μm
fraction, representing 27% of the total magnetic aliquot weighing
approximately 289 mg. Examination by both optical microscopy
and SEM revealed the spherules to be generally similar to those
extracted from the BWD and TPR sites. Nearly all spherules examined
were found to be rich in titanium, whereas such spherules were found
to be rarer at the other two sites, amounting to approximately 20%
of total spherules. One spherule was found to be enriched in iron
and the rare earth elements (REEs), cerium (22%), and lanthanum
(10%), along with praseodymium and neodymium as discussed below.
Because the sample was extracted from a cleaned cutbank at depth,
anthropogenic sources are considered unlikely but cannot be
completely ruled out.
Spherule Geochemical Analyses.
Geochemical
analyses of many spherule candidates from each site were performed
at North Carolina State University’s Analytical Instrumentation
Facility using a Hitachi S3200 Variable Pressure SEM with EDS
capability. Tables summarizing the elemental and oxide abundances of
these spherules are presented in SI
Appendix, Tables S1–S3.
Spectrographic and major oxide abundances for representative
magnetic spherules are shown below for each site. Each figure also
combines an optical photomicrograph at 180-power magnification with
an associated SEM micrograph.
For
TPR, 11 EDS analyses were performed on spherules yielding averages
of 61 wt % FeO, 12 wt % Al2O3, 17 wt % SiO2, and 8 wt
% TiO2; all other oxides were < 2 wt%. These values are
consistent with average percentages for sediment at the Earth’s
surface, indicating that these spherules formed from melted
terrestrial surficial sediments (SI
Appendix, Table S1).
Of the 11 EDS analyses, seven showed enrichments in iron (> 53 wt%),
two were titanium-rich (> 34 wt%), and three were
silica-rich (> 27 wt%) (SI
Appendix, Table S1).
The TPR spherule shown in the SEM image in Fig. 5 exhibits
a relatively dull finish due to its rough faceting, suggestive of a
Zagurski index identifier “polygonal granular” (Spgr) (25).
The dull, somewhat flattened surface shown in the optical
photomicrograph is likely due to a perforated surface around a
hollow interior. Departures from perfect sphericity are common, thus
the only reliable approach of verifying that a candidate is a true
spherule is by using SEM-EDS analysis to examine its surface
morphology and geochemistry.
Fig.
5.
For
BWD, 52 EDS analyses were performed on YDB spherules yielding
averages of 85 wt % FeO, 2 wt % Al2O3, 3 wt % SiO2,
and 7 wt % TiO2; all other oxides were < 2 wt%.
These values are similar to average percentages for sediments at the
Earth’s surface, indicating that these spherules formed from
melted terrestrial surficial sediments (SI
Appendix, Table S2).
Of the 52 EDS analyses, 45 showed enrichments in iron (> 55 wt%),
seven were titanium-rich (> 31 wt%), and none were
silica-rich (> 50 wt%) (SI
Appendix, Table S2).
The BWD microspherule shown in Fig. 6 exhibits
a dendritic quench-textured microstructure suggestive of the
Zagurski index identifier “grooved” (Sg) (25).
The predominately iron oxide composition of this spherule, shown
in Fig. 6,
is similar to that of the TPR spherule in Fig. 5.
Essentially all of the BWD spherules display surface features
indicative of rapid melting and quenching that produced distinctive
microstructures. Most appear similar to Zagurski index identifiers
“grooved” (Sg) and “ordered granular” (Sogr) (25).
Approximately 5% have surface features similar to either Zagurski
index identifier “polygonal granular” (Spgr) or “sinuous
patterned” (Ss) (25).
Fig.
6.
For
PPC, eight EDS analyses were performed on six YDB spherules yielding
averages of 40% FeO, 5% Al2O3, 2% MgO, 3% SiO2, 2% Na2O, 30% TiO2,
and 2% Mn2O7; all other oxides were < 2%. These values
reasonably match average percentages for sediments at the Earth’s
surface indicating that these spherules formed from melted
terrestrial surficial sediments (SI
Appendix, Table S3).
Of the eight EDS analyses, one was enriched in iron (> 51 wt%),
five were titanium-rich (> 39 wt%), and none were
silica-rich (> 4 wt%) (SI
Appendix, Table S2).
Five of the six spherules investigated (83% of the total) were found
to be titanium-rich, a much higher percentage than those from both
TPR and BWD where values of 20% total were more typical.Fig. 7 shows
an SEM and optical microscopic image of a PPC titanium-rich
spherule.
Fig.
7.
PPC
50-μm-diameter spherule (PPC-YDB-Sph-4-50mic-100707) extracted from
sediment at a depth accepted as coeval with the YDB. (A)
SEM image; (B)
optical photomicrograph; and (C)
EDS spectrum and elemental oxide abundances.
Another
spherule (Fig. 8)
exhibits a relatively smooth surface that is enriched in the rare
earth elements (REEs) cerium and lanthanum. This PPC spherule’s
shape is not perfectly spherical. Its surface may have experienced
either volatile eruptive out-gassing or accretionary collision of
multiple spherules causing numerous hemispherical mounds of various
sizes. One of these mounds appears as an inverted cup-shaped
structure. It and other surface mounds are enriched in REEs
praseodymium and neodymium. The surficial cup-shaped structure may
be indicative of a partially collapsed bubble, perhaps due to
out-gassing and deflation of the spherule while molten.
Fig.
8.
Focused
ion beam (FIB) sectioning of the REE-rich spherule from PPC revealed
a 10-μm-diameter spherical void with its center displaced toward a
prominent bulge (Figs. 8 and 9).
The chemical compositions of the inner crust and interior are
similar to that of the exterior. Differences in iron abundances
account for the dark and light portions of the patterned interior.
EDS analysis revealed an elemental composition rich in REEs
including lanthanum (10%), cerium (20%) with trace amounts of
praseodymium and neodymium, which are not typical elemental
components of surficial rocks in the Maryland region (27).
Although this unusual spherule may be an atypical anthropogenic
contaminant, REEs are also well-known constituents of cosmic
material, especially chondritic meteorites (28⇓–30).
Previous studies have reported elevated levels of REEs also in
association with the YDB in North America and Europe* (1).
Fig.
9.
Framboidal Spherules.
Pinter
et al. examined YDB sediment at a site not previously tested for
spherules and suggested that Firestone et al. misidentified
framboids as spherules. Framboids are formed by slow crystal growth
and so do not possess the distinctive surface microstructures found
on YDB magnetic microspherules and are therefore easily
differentiated. A typical spherical framboid is shown in SI
Appendix, Fig. S5.
Framboids were observed but perceived to be rare at both the BWD and
PPC sites where no attempt to quantify their abundance was made. At
TPR, framboids are very abundant in the YDB layer, approximately
≥1000 per kg, decreasing above and below. It is unclear
why both spherules and framboids exhibit highest abundances in the
YDB at this site.
Comparison of Protocols Used by Surovell et al. and Firestone et al.
Surovell
et al. purportedly used the same protocol as Firestone et al. yet
were unable to find a single spherule in YDB sediments at three
previously reported sites. They concluded that the “discrepancy
between the two studies is particularly troublesome.” Our
investigation reveals the abundant presence of YDB spherules at all
three widely separated sites, consistent with the results by
Firestone et al. Because of this difference we now examine the
methodology of Surovell et al. who reported their methods in detail.
Comparing the methodology of each, we find Surovell et al. deviated
substantially in several critical aspects, and we suggest that this
departure resulted in their finding no YDB spherules at these three
sites. A summary comparison of the three protocols is in SI
Appendix, Table S4.
We
have identified five major deficiencies that contributed to the
negative results reported by Surovell et al.
- Deficiency: YDB Layer Sample Thickness. Source: Firestone et al. “we found a thin, sedimentary layer (usually < 5 cm).” Source: Surovell et al. “SI Appendix, Table S1 displays seven sites at which the YDB layer ranges from 5–28 cm, averaging 11 cm.”
Firestone
et al. identified the YDB as being a thin layer containing increased
abundances of markers and collected samples at seven main sites with
YDB thicknesses ranging from 0.5–5 cm, averaging 2.3 cm.
Surovell et al. collected thicker samples ranging from 5–28 cm
thick, averaging 11 cm, which are nearly five times as thick.
Consequently spherule abundance would be diluted making them more
difficult to detect. For comparison our YDB sample at TPR was
collected as a 4-cm-thick sample while Surovell et al.’s was
10-cm-thick. In five of seven instances Surovell et al. collected
samples of ≥10 cm thickness. Although this is thicker than
recommended we do not consider this to represent a major flaw by
itself. However, it becomes of greater potential importance when
combined with other deviations from the protocol of Firestone et al.
- Deficiency: Inadequate Aliquot Size. Source: Firestone et al. Protocol: they analyzed “one or more 100–200 mg aliquots…. Microspherules are usually rare, often making it necessary to inspect the entire magnetic fraction.” Source: Surovell et al. “examined 10–40 mg…per sample,” and did not investigate the entire magnetic fraction of any sample.
The
amount of magnetic grains that Surovell et al. examined was
inadequate to be statistically significant, invalidating any
conclusions regarding spherule abundances. Surovell et al. examined
from 20–100 times smaller aliquots of the magnetic fraction than
did Firestone et al. with the result that they found no spherules in
five-out-of-seven YDB layers. This deficiency is a major contributor
to their reported lack of spherules, because the aliquots analyzed
by Surovell et al. were of insufficient size to visually detect even
a single spherule. To illustrate the consequences of that
deficiency, consider our results from TPR, where we estimated 260
spherules in 5.40 grams (or 5,400 mg) of magnetic grains.
This amounts to approximately two spherules in every 40 mg, the
maximum amount that Surovell et al. analyzed. Without some means of
amplifying the detectability of those two spherules, it seems
unlikely that Surovell et al. would have detected even a single
spherule, and, indeed, they reported finding none.
Size-sorting
of the extracted magnetic grains is essential to overcome the
difficulty in detecting rare spherules among the far more numerous,
non-spherical magnetic grains. To illustrate that difficulty,
consider again our results from TPR, where we estimate 260 spherules
in 5.40 grams of magnetic grains for every kilogram of
sediment. Assuming a linear grain size distribution and spherical
grains for simplicity, we estimate there may be roughly 2.5 million
discrete magnetic particles in those 5.40 grams, resulting in a
ratio of spherules to grains of approximately 1∶10,000. Our
spherule counts indicate that the portion containing the smallest
grains (< 53 μm) accounts for 90% of the total
spherule abundance. Thus, eliminating larger grains greatly reduces
the probability of these obscuring small spherules and also enhances
spherule prevalence, making spherules easier to detect. Although
careful counting might overcome this problem, it is better to follow
the prescribed protocol to increase accuracy of counts.
Size-sorting
also addresses a more serious problem, which is to overcome the
downward migration of microspherules, also known as “downward
fining.” This phenomenon, well-known in sedimentology, is the
process by which agitation of sediment results in fine particles
preferentially migrating downward through the voids between larger
grains, thereby concentrating larger grains at the top and smaller
grains at the bottom of a container (31).
This phenomenon is particularly applicable to spherule counting,
because spherules tend to be much smaller on average than
non-spherulitic magnetic grains. Size-sorting counters downward
fining. We also found that it is vitally important to thoroughly mix
and evenly split each size-sorted portion on a smooth impervious
surface as a last step prior to selecting an aliquot for spherule
counting. Even so, there may yet be normal but acceptable variation
between aliquots. Neglecting to size-sort, an investigator might
extract a small aliquot of unsorted, spherule-depleted material from
the top of a container. The end result is the mistaken conclusion
that spherules are absent from the entire magnetic sample, whereas
they are only absent from the top of the container. Based on the
published methodology adopted by Surovell et al., we suggest that
their study may have suffered from this effect because size-sorting
was not conducted.
Finding
a single spherule among 10,000 magnetic grains is such a tedious
endeavor that some have claimed that any results may be highly
subjective and therefore unreliable (14).
In light of the above difficulties, we considered whether magnetic
spherule counting is a quantitative science, or rather, a subjective
art as suggested by Haynes et al. (14).
Cognitive neuroscience might reveal whether limitations in human
perception affect the ability of an operator to discriminate
spherules among numerous non-spherulitic distractors. Several
factors are known to contribute to successful target detection
amongst numerous distractors, including distractor abundance,
heterogeneity, target prevalence, and discriminability (32, 33).
Each of these factors may play some role in accurately measuring the
number of target-spherules. For example, visual search for
infrequent targets (≤ 1% prevalence) is known to be highly
error-prone, resulting in > 30% of targets being missed
altogether (34).
Because spherule prevalence in magnetic fractions is often extremely
low (0.006%–0.125%), visual searches are likely to be error prone
with target misses common. Also, magnetic distractor grains tend to
be sub-rounded with surface finish and color very similar to the
often silvery-black, glossy surfaced, spherules. This similarity,
coupled with the rarity of spherules, leads to a demanding visual
search that could plausibly produce a severe underestimate of target
numbers.
To
help counter these inherent search difficulties, size-sorting
eliminates the largest grains in the search set and improves visual
search performance in two fundamental ways. First, it decreases the
number of distractors. For example, if the relative abundance of the
smaller grains is low (20% of the total grain-mass), then
size-sorting removes many distractors (80% of grain-mass),
significantly improving target prevalence. Second, size-sorting
minimizes the size disparity between targets and distractors.
Although it may appear counterintuitive, distractors that are highly
dissimilar to one another generally produces a slow and effortful
search (e.g., finding a red target amongst blue, yellow, purple, and
green distractors). By contrast, when distractors are more similar
to each other, search becomes dramatically easier. Targets tend to
pop out from the homogeneous set of distractors (e.g., finding red
targets amongst many purple distractors) (32, 35).
This effect becomes more evident as size homogeneity increases.
Consequently, the exclusion of the large grains is likely to greatly
reduce the perceptual demands of the search task, leading to a
substantially more accurate measurement of spherule abundances.
Cognitive
neuroscience thus indicates there are significant difficulties
inherent in spherule detection that require mitigation to reduce
process subjectivity. Even after minimizing difficulties, it is
reasonable to expect variation in results (e.g., from operator
error), even amongst researchers using the same protocol. Indeed, we
found that analyzing successive aliquots from the same magnetic
fraction produced variations up to ± 50%. However, researchers
adhering to a rigorous protocol that includes size-sorting have a
greater chance of overcoming the inherent difficulties of such a
search. Our results are sufficiently similar to those of Firestone
et al. to suggest that spherule detection is replicable to at least
within ± 50%, and therefore, refuting the speculation that
spherule detection is strictly subjective. Researchers routinely
quantify spherules in ocean sediments and in ice cores with
reproducible results (26).
It
is noteworthy that when we began this investigation, we
inadvertently failed to size-sort the magnetic fraction from some of
these sites. As a result, we initially found no spherules at all,
consistent with the results of Surovell et al. However, once we
implemented rigorous size-sorting, we observed spherules in large
numbers consistent with Firestone et al.
- Deficiency: Perfect Sphericity. Source: In Fig. 2 of Firestone et al., two of the four spherules in that figure are clearly somewhat oval-shaped and non-spherical. Source: Surovell et al., they decided to “eliminate a number of particles that at first glance appeared to be highly spherical but were not.”
These
quotes demonstrate that Surovell et al. deviated from the Firestone
et al. protocol by independently devising a more restrictive optical
criteria for selecting spherule candidates. Surovell et al. limited
the spherule count to only those particles matching an extreme
degree of sphericity, even though both the protocol and spherule
images published by Firestone et al. and others indicate that
spherules commonly deviate from perfect sphericity (36⇓⇓⇓–40).
By incorporating such a condition into their protocol, Surovell et
al. would most likely have rejected candidates such as the rough
spherules shown inFigs. 5 and 7,
as well as the lumpy spherule rich in REEs found at PPC (Fig. 8).
Many spherules have conspicuous punctures in their shell-like
surface as revealed in SEM images, as well as other flaws that
reduce their apparent sphericity and reflectivity. Even so, each
exhibits the characteristic quench-melted dendritic texture that is
characteristic of YDB spherules and that differentiates them from
framboids or detrital grains.
We
include a mosaic of SEM images of representative YDB spherules
(Fig. 1)
to verify that spherules we counted were consistent with the
original Firestone et al. protocol under which Surovell et al.
operated. Our spherule abundances are based upon the SEM
confirmation of spherule candidates. We used a conservative
identification protocol to select spherule candidates similar to but
not quite as restrictive as that employed by Surovell et al. This
included high-surface reflectivity and approximate sphericity based
on overhead illumination. Nevertheless, we found it essential to use
the SEM to verify a candidate spherule’s true nature. Based on our
results from light microscopy and SEM, we find that perfect
sphericity is not necessarily a defining characteristic of YDB
spherules, and so, because Surovell et al. utilized this additional
step, it is possible they eliminated a significant percentage of YDB
spherules present.
Without
SEM imagery to examine their surface microstructure, it is
impossible to differentiate YDB magnetic spherules from those
created by other natural or anthropogenic sources including
framboids or rounded detrital magnetite, which, in our experience,
often appear identical to YDB spherules under an optical microscope.
Firestone et al. examined selected YDB magnetic spherules with SEM
imaging but, unfortunately, published only optical micrographs and
EDS analyses. SEM imagery shows magnetic spherule surface morphology
due to meteoritic ablation, terrestrial impact or anthropogenic
ejecta to be essentially identical (38⇓–40).
EDS analyses provides information to distinguish among them. It is
impossible to infer the origin of cosmic, volcanic, anthropogenic,
or impact spherules by visual identification alone†.
It is essential to conduct geochemical analyses and compare the
results with that from known spherule populations.
For
example, Pigati et al. (17)
reported candidate YDB spherules at Murray Springs, AZ, confirming
the abundance results of Haynes et al. (14)
and Firestone et al., but also reported abundant non-YDB spherules
in Chile. However, they performed no SEM or EDS analyses to
determine the morphological and geochemical characteristics of any
spherules within and outside the YDB layer. Since their non-YDB
sampling sites in Chile are within a few kilometers of dozens of
active volcanoes, it is almost certain that they observed numerous
volcanic spherules and possibly none due to impact. Because those
authors did not perform SEM imaging and EDS analyses, it is
impossible for them to reach reliable conclusions about what they
found.
Surovell
et al. did not perform SEM imaging or geochemical analyses, and yet,
like Pigati et al., asserted that all magnetic spherules are cosmic
in origin. Pinter et al. and Haynes et al. did not report the
results of their spherule SEM analyses and likewise concluded
spherules were of cosmic origin without supporting data. Lacking SEM
imaging and/or EDS analyses, the accuracy of their spherule counts
and speculations about origin are highly suspect. As an example of
this, Pinter et al. reported observing large numbers of framboids
and detrital magnetite well outside the YDB and then speculated that
most YDB spherules are simply these other particles. Our results and
images indicate their claim to be unfounded. There are fundamental
and easily observed differences between quench-melted spherules,
unmelted detrital magnetite, and authigenic framboids.
The
EDS results of our study are consistent with those of Firestone et
al. that YDB spherules do not appear to be of cosmic origin.
Spherule composition is shown plotted on a ternary diagram
(Fig. 10A)
that compares normalized molar percentages of magnesium, iron, and
titanium oxides observed in 276 cosmic microspherules collected in
Antarctic ice (41)
with 85 spherules from TPR, BWD, and PPC. The results demonstrate
that cosmic spherules typically are enriched in MgO, as confirmed by
Taylor et al. (41),
in contrast to YDB spherules that are depleted. Also, cosmic
spherules typically are depleted in TiO2, while YDB spherules are
TiO2 enriched. Thus, YDB spherules are geochemically
distinctive and dissimilar to cosmic spherules. Regarding other
potential origins, the spherules we observed are unlikely to be of
anthropogenic origin because: (i)
they were buried at sufficient depths to preclude downward
contamination of modern spherules; and (ii)
they exhibit abundances peaking at approximately 12.9 ka. Also,
a volcanic source is unlikely because we detected no volcanic
materials such as ash and tephra in any YDB layer examined. It seems
unlikely that abundant volcanic spherules would persist in sediment
without abundant ash or tephra.
Fig.
10.
Ternary
diagrams comparing the geochemistry of spherules collected from TPR
(red diamonds), BWD (blue circles) and PPC (green squares) with (A)
cosmic spherules (black triangles) (source: Taylor et al.,) and (B)
terrestrial undifferentiated metamorphic rocks (source: USGS, 2001).
All but a few spherules from TPR are similar to metamorphic
minerals. In summary, YDB spherules do not resemble cosmic
spherules, but instead appear consistent with formation by a
high-energy source requiring the melting of terrestrial sediment.
Firestone
et al. proposed that some YDB spherules most likely formed from the
melting of terrestrial sediments. We investigated that hypothesis by
comparing the geochemistry of YDB spherules with terrestrial
metamorphic rocks. The results are shown in an aluminum-calcium-iron
ternary diagram (Fig. 10B)
plotting the normalized molar percentages of calcium, iron, and
aluminum oxides resulting from 85 EDS analyses of spherules
collected from TPR, BWD, and PPC. These data are compared to 115
samples of undifferentiated metamorphic rocks, including schist and
shales from 12 U.S. states, mostly in the Northeast and Midwest, as
compiled by the USGS in their PLUTO database (42).
These results demonstrate geochemical similarity between YDB
spherules and metamorphic rocks, supporting the hypothesis of
Firestone et al. that YDB spherules likely formed from melted
terrestrial rocks. Furthermore, the spherules were originally in a
molten state, as confirmed by our SEM imagery exhibiting surface
textures characteristic of quench-textured molten droplets (25).
Various YDB spherules observed are comprised of FeO at a maximum of
99% or SiO2 at 64% or TiO2 at 58% (SI
Appendix, Tables S1–S3).
These percentages imply a range of formation temperatures that
includes 1550 °C (the melting point of FeO), 1650 °C (the
melting point of TiO2), and 1730 °C (the melting point of SiO2).
The high temperatures inferred from the YDB spherules’
microstructures are consistent with the melting and rapid cooling of
an energetic and transient process. The spherule’s wide spatial
distribution might be due to a small number of very energetic
event(s) or very many dispersed events of lesser magnitude. Based on
spherule stratigraphy, we speculate that spherule deposition
occurred over a brief period, temporally proximate to the YD onset.
At some sites, spherule abundance may have experienced subsequent
enhancement due to environmental processes (e.g., deflation).
Human Population Decline.
Firestone
et al. proposed that the YDB impact triggered a decline in Clovis
populations at 12.9 ka B.P. and, subsequently, Anderson et al.
(20)
provided three lines of evidence in support of a human decline at
multiple sites in North America, including the TPR site, as well as
Europe and elsewhere. That hypothesis led us to investigate whether
YDB microspherule results at the sites we examined could shed light
on this issue.
First,
the Clovis-age YDB layer at BWD has been clearly identified by the
distribution of megafaunal remains and Clovis artifacts, and a
hand-dug Clovis well (43).
At the sample collection site, Folsom artifacts lie above an
archeological gap zone that is 10–20 cm thick and lying
immediately above the Clovis layer. This culturally dead zone, which
extends discontinuously across the 157-acre site, has no known
Folsom artifacts in contact with Clovis artifacts indicating a
post-Clovis occupational hiatus of 200–600 y (19).
The presence of this archeological gap is consistent with the
proposed population decline.
At
PPC, on the Delmarva Peninsula, Befell et al. (44)
presented data indicating that beginning at the YD onset at 12.9 ka
B.P. and continuing for 1,400 y the Potomac River-Chesapeake
Bay area was abandoned by humans. This also appears to be the case
at PPC, where the Clovis horizon is overlain by loess-derived strata
tens of centimeters thick that was deposited after the YD onset and
contains no reported human artifacts. As at BWD, these results are
consistent with a post-YDB population decline.
At
TPR, low abundances of candidate spherules (≤ 30/kg) coincide
with the debitage layer that was formed during the occupation of
Clovis peoples (Fig. 3 and Table 1).
This was followed by a significant increase in YDB spherule
abundance (260/kg) just above the debitage layer. We call this the
“chert shadow effect.” It coincided with the almost complete
disappearance of Clovis artifacts, suggesting human abandonment of
the quarry (Fig. 11)
that is roughly coeval with spherule deposition. The observed
archeological gap lasted for approximately 600 y or longer
(19).
Because the quarry was a regionally important sources of
high-quality chert, it seems unlikely that the quarry would have
become dormant if the local population had remained unchanged. Thus,
our data appears consistent with a population decline. The time of
quarry dormancy appears to coincide with whatever phenomena created
and deposited the spherules. However, Buchanan et al. (45)
and Holiday et al. (2010) (46)
find no evidence to support a population decline.
Fig.
11.
Graph
depicting distribution of total-known Clovis artifacts (n = 453
of projectile points, preforms, blades, and cores) compared to
post-Clovis (n = 1 point), a decline in quarry usage of
> 99% that persisted for approximately 600 y (SI
Appendix, Table S5;
data from Anderson et al. 2011).
Correlation of Three Test Sites.
For
magnetic grains we found densities equal to or higher than the
values reported by Firestone et al. and higher than those of
Surovell et al. at all three sites sampled. For confirmed YDB
spherules the comparison abundances are summarized for all three
sites in Table 4.
Our data are consistent with those of Firestone et al. and
inconsistent with those of Surovell et al., who found no magnetic
spherules in YDB strata at the three sites. However, the relatively
high concentrations we report at all sites could have resulted from
our use of a 25% stronger magnet (N52) and thinner plastic bags
(2–3 mil), leading to increased recovery rates. In our
opinion these differences in methodology are insufficient to account
for the complete absence of magnetic spherules reported by Surovell
et al. From their published protocol it is clear that they did not
sufficiently adhere to the protocol of Firestone et al. for
extraction of magnetic spherules. Nevertheless, we find in agreement
with Surovell et al. that magnetic-grain concentrations alone are
not a reliable proxy for accurate identification of the YDB layer.
At BWD the magnetic grain abundance peaks slightly above the YDB
layer and, while grains do peak at the YDB at TPR, concentrations
are similar to that found in overlying strata. However, if the
magnetic grains are size-sorted and weighed, the abundances of the
smallest grains seem to show a weak correlation, although two sites
are an insignificant number and any causal link to the proposed
impact event is unclear.
Table 4.
Our
comparative study has revealed a number of deficiencies in the
protocol employed by Surovell et al. that are cumulative in their
effect. The probability of successful spherule detection by that
group was consequently reduced up to several orders of magnitude. In
turn, this invalidates any conclusions by Surovell et al. about the
three sites studied in this paper, and, most likely, the other four
of their sites not addressed here. One exception may be their
results from Agate Basin, WY where Surovell et al. reported
significant coeval peaks in magnetic grains and spherules (< 255/kg)
in a layer they determined to be just above the YDB. For the Agate
Basin site their protocol, although flawed, may still have been
sensitive enough to detect larger spherules in high concentrations.
Agate Basin has the largest spherule abundance reported by Surovell
et al., which is located farther north than the other sites they
studied. If accurate, their discovery of a spherule peak proximate
to the YDB layer at the Agate Basin site may be an important
contribution to YDB research, but more research is needed to confirm
their results. We speculate that if one or more ET objects impacted
near or on the Laurentide Ice Sheet as proposed, then spherule size
and abundance should increase with proximity to the hypothesized
impact site. This trend may be reflected in the results from Agate
Basin. Alternately, it may simply be an enhancement due to local
environmental processes.
Conclusions
Regarding
the Surovell et al. study, our analyses indicate that YDB spherule
abundances at three sites examined are consistent with those
reported by Firestone et al. and inconsistent with those reported by
Surovell et al. We conclude that any numerical differences between
our results and those of Firestone et al. are within normal
variation. The prescribed protocol produces reliably quantifiable
results.
Our
work indicates that Surovell et al., did not follow three of the
most critical elements of the Firestone et al. protocol: (i)
size-sorting of the magnetic fraction; (ii)
the examination of sufficient amounts of magnetic material; and
(iii)
examination of candidate spherules by SEM and EDS. The result of
these omissions was a methodology whose sensitivity was inadequate
to detect significant numbers of spherules in any stratum, except
perhaps those with abundant large spherules as may be the case in
Agate Basin. We emphasize that future independent investigators
testing for the presence or absence of YDB magnetic spherules
include as standard procedure rigorous size-sorting as well as
SEM/EDS analyses. At all three sites tested, grain size-sorting
using a 53-μm screen greatly improved the ratio of spherules to
distractor-grains, profoundly reducing the difficulties in
searching, identifying, and accurately counting these small objects.
For
magnetic grains, peaks in the total magnetic grain density at the
three sites are not reliably correlated with the peaks in spherules
or the onset of the Younger Dryas. This is consistent with the
results reported by Surovell et al. However, a peak in the smallest
(< 53 μm) size portion of the magnetic grains does
appear to have a weak correlation with YDB strata at TPR and BWD,
perhaps indicative of some as-yet-unexplained depositional process
operating at that time.
Microspherules
at all three sites are morphologically and geochemically similar,
averaging about 30 μm. Their chemical composition varies from
aluminosilicate glass to magnetite to titanomagnetite. With the
exception of one magnetic spherule that is highly enriched in rare
earth elements, the compositions of YDB spherules are similar to
terrestrial metamorphic rocks and differ significantly from those
formed by cosmic or authigenic processes. Volcanic and anthropogenic
sources are considered unlikely.
Regarding
a potential human population decline, our results from the TPR
quarry site demonstrate that the spherule-enriched YDB layer
approximately coincides with the start of multi-century hiatus in
activity of this Clovis quarry. These results, combined with those
from BWD and PPC, are consistent with a population decline that was
coeval with the YDB event as previously proposed.
The
scope of our study was limited to considering only the
identification, occurrence, and nature of YDB magnetic spherules and
the possible implications. Our results are consistent with, but do
not prove, that a previously proposed cosmic impact occurred at
12.9 ka B.P. (the YDB impact hypothesis). The ultimate source
of the magnetic microspherules in YDB sediment remains a mystery
warranting further investigation.
Methods
The
protocol we used here for extracting the magnetic fraction and the
separation and identification of magnetic spherules was essentially
the same as that used by Firestone et al. and improved by
Israde-Alcantara et al. (4 and SI
Appendix therein).
Initial detection of candidate microspherules was made using an
optical microscope to scan aliquots of approximately 10 mg
scattered uniformly across a white microscope slide at a
magnification of 130–150 power. There were some modifications as
follows: (i)
we used the strongest recommended NdFeB magnet (grade 52); (ii)
we captured any grains small enough to be suspended on the surface
of the liquid in which the magnetic fraction was dispensed by
decanting the liquid through multiple 20-μm mesh filters and
recovered the trapped residue in each; (iii)
we found it useful to, and highly recommend, examining sectioned
spherules to compare interior and exterior compositions; and (iv)
we used a smaller-than-recommended screen mesh size (53-μm grid
spacing) because it became evident that spherule detection and
counting would be greatly facilitated by additional size-sorting
(extra screens) to isolate smaller grain sizes. By using the smaller
mesh size we were able to obtain spherule results that were
consistent with those of Firestone et al. while using similar sized
aliquots as Surovell et al.
All
EDS data were collected by an Oxford Si(Li) X-ray detector with a
4Pi Universal Spectral Engine pulse processor. Since samples of this
type that had been previously analyzed by other authors using more
sensitive techniques (1, 5, 36, 38)
and had been found to contain only metallic oxides, our analyses
assume that all of the metallic elements are oxidized. The
quantities of the various elements were determined by standardless
EDS analysis and then converted to oxides by stoichiometry.
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