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Extraordinary Biomass-Burning Episode and Impact Winter Triggered
by the Younger Dryas Cosmic Impact ∼12,800 Years Ago.
1. Ice Cores and Glaciers
Wendy S. Wolbach,
1,
*Joanne P. Ballard,
2
Paul A. Mayewski,
3
Victor Adedeji,
4
Ted E. Bunch,
5
Richard B. Firestone,
6
Timothy A. French,
1
George A. Howard,
7
Isabel Israde-Alcántara,
8
John R. Johnson,
9
David Kimbel,
10
Charles R.
Kinzie,
1
Andrei Kurbatov,
3
Gunther Kletetschka,
11
Malcolm A.
LeCompte,
12
William C. Mahaney,
13
Adrian L. Melott,
14
Abigail
Maiorana-Boutilier,
15
Siddhartha Mitra,
15
Christopher R.
Moore,
16
William M. Napier,
17
Jennifer Parlier,
18
Kenneth B. Tankersley,
19
Brian C. Thomas,
20
James H. Wittke,
5
Allen West,
18,†
and James P. Kennett
21
ABSTRACT
The Younger Dryas boundary (YDB) cosmic-impact hypothesis is based on considerable evidence that Earth collided
with fragments of a disintegrating ≥100-km-diameter comet, the remnants of which persist within the inner solar
system ∼12,800 y later. Evidence suggests that the YDB cosmic impact triggered an “impact winter”and the subsequent
Younger Dryas (YD) climate episode, biomass burning, late Pleistocene megafaunal extinctions, and human cultural
shifts and population declines. The cosmic impact deposited anomalously high concentrations of platinum over much
of the Northern Hemisphere, as recorded at 26 YDB sites at the YD onset, including the Greenland Ice Sheet Project 2 ice
core, in which platinum deposition spans ∼21 y (∼12,836–12,815 cal BP). The YD onset also exhibits increased dust
concentrations, synchronous with the onset of a remarkably high peak in ammonium, a biomass-burning aerosol. In
four ice-core sequences from Greenland, Antarctica, and Russia, similar anomalous peaks in other combustion aerosols
occur, including nitrate, oxalate, acetate, and formate, reflecting one of the largest biomass-burning episodes in more
than 120,000 y. In support of widespread wildfires, the perturbations in CO
2
records from Taylor Glacier, Antarctica,
suggest that biomass burning at the YD onset may have consumed ∼10 million km
2
,or∼9% of Earth’s terrestrial bio-
mass. The ice record is consistent with YDB impact theory that extensive impact-related biomass burning triggered the
abrupt onset of an impact winter, which led, through climatic feedbacks, to the anomalous YD climate episode.
Online enhancements: appendix.
Introduction
Firestone et al. (2007) initially posited that exotic
materials found in the Younger Dryas boundary
(YDB) layer provide evidence of a major cosmic-
impact event, caused by Earth’s collision with a
cometary swarm ∼12,800 calendar years ago at the
onset of the Younger Dryas (YD) climate episode.
Studies of 140 sedimentary sequences distributed
across North and South America, western Europe,
and western Asia document peaks in exotic YDB
materials, including high-temperature iron-rich spher-
ules; silica-rich glassy spherules; meltglass; and nano-
diamonds, iridium, platinum (fig. A1; figs. A1 and A2
are available online), and osmium (tables A1, A2; ta-
bles A1–A4 are available online). Independent groups
have confirmed much of the YDB impact evidence,
but others have not or have offered alternate expla-
nations. It is not the intent of this contribution to
review and discuss previous publications concerning
the YDB impact proxies, and so a comprehensive bib-
liography is provided in table A3.
Many YDB sites exhibit evidence of a distinct
peak in biomass burning associated with the YDB
impact-related proxies. Part 1 of this contribution
Manuscript received September 11, 2017; accepted Sep-
tember 14, 2017; electronically published February 1, 2018.
* The authors’affiliations can be found at the end of the ar-
ticle.
†
Author for correspondence; e-mail: allen7633@aol.com.
000
[The Journal of Geology, 2018, volume 126, p. 000–000] q2018 by The University of Chicago.
All rights reserved. 0022-1376/2018/12602-0002$15.00. DOI: 10.1086/695703
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(this article) summarizes evidence of biomass burn-
ing in global ice-sheet records, and part 2 (Wolbach
et al. 2018) summarizes sedimentary records that
contain biomass-burning evidence, including acini-
form carbon (AC/soot), black carbon/soot, charcoal,
carbon spherules, and glass-like carbon. For a site map,
see figure 1; for descriptions and images of YDB
biomass-burning proxies, see Wolbach et al. (2018),
especially figures A1–A6 and “Biomass-Burning Prox-
ies Found at YDB Sites”in the appendix).
Peaks in impact-related biomass-burning proxies,
such as AC/soot, have been identified in at least
four previously documented cosmic-impact events:
Sudbury, Mjølnir, Manson, and Cretaceous-Tertiary
(K-Pg; “Biomass Burning in Previous Cosmic-Impact
Events”in the appendix, available online). Further-
more, it is widely accepted that biomass burning
during the K-Pg impact event initiated a severe cli-
mate change known as “impact winter”(“Biomass
Burning in Previous Cosmic-Impact Events”). The K-
Pg impact layer broadly contains peaks in diverse
biomass-burning proxies, including AC/soot (Wolbach
and Anders 1989), charcoal (Belcher et al. 2003; Rob-
ertson et al. 2004), and carbon spherules (Adatte
et al. 2005). As evidence of a high-temperature event,
these biomass-burning proxies are also associated
with peaks in high-temperature impact-related prox-
ies, including magnetic spherules (Adatte et al. 2005),
meltglass (Adatte et al. 2005), and nanodiamonds
(Carlisle and Braman 1991). Kaiho et al. (2016) pro-
posed that the K-Pg impact produced enough AC/
soot and dust to block sunlight and trigger a major
climate change (impact winter) that, in turn, de-
graded entire ecosystems and contributed to the
severe K-Pg mass extinctions.
The much more recent Tunguska impact event
in 1908 involved a meteorite/comet that detonated
∼15 km in the atmosphere over Siberia, toppled
80 million trees, and triggered biomass burning (Flo-
renskiy 1965). The discovery of high-temperature
meltglass near the epicenter of the airburst indicates
sufficiently high temperature to initiate biomass
burning (a minimum of 12007C; Kirova and Zas-
lavskaya 1966; Bunch et al. 2012).
Figure 1. Locations for ice cores and Younger Dryas boundary (YDB) sites with peak biomass-burning proxies. Black
diamonds represent 6 ice records that display chemical proxies in support of anomalously high YDB biomass burning.
Taylor Dome and Taylor Glacier are off the map in Antarctica. Circles represent 23 sites with a documented YDB
layer containing peaks in biomass-burning proxies.
000 W. S. WOLBACH ET AL.
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The onset of YD cooling at ∼12,800 cal BP rep-
resented an abrupt climatic event that was most
strongly expressed in the North Atlantic region,
where temperatures plunged ∼87C (Carlson 2013).
The commonly accepted explanation for YD cool-
ing is that iceberg calving and meltwater flooding
capped the North Atlantic, thereby shutting down
thermohaline circulation (Broecker 1997). Dimi-
nution of sunlight by dust loading and shifts in at-
mospheric circulation have been proposed as addi-
tional causes of YD climate change by Renssen et al.
(2015), whose numerical simulations indicate that
all three mechanisms were required to trigger YD
cooling. The YDB impact theory adds an additional
key element by suggesting that these climate-
changing mechanisms did not occur randomly but
rather were triggered by the YDB impact event.
After shutdown of the ocean conveyor, the YD ep-
isode persisted for another ∼1400 years, not because
of continued airburst/impacts but because, once cir-
culation stopped, feedback loops and inertia within
the ocean system maintained the changed state of
circulation until it reverted to its previous state
(Firestone et al. 2007; Kennett et al. 2018).
Independently published studies of biomass-burning
aerosols in three Greenland ice cores have previ-
ously shown that a large episode of biomass burning
closely coincided with the onset of YD cooling
(Legrand et al. 1992; Mayewski et al. 1993, 1997;
Fischer et al. 2015). Recognition of the magnitude
and timing of this event in the Greenland ice sheet
was a major factor in Firestone et al.’s (2007) infer-
ence that an episode of biomass burning was trig-
gered by an impact event at the YD onset. Later,
support for the original hypothesis came from the
discovery of peaks in impact-related proxies in YDB
ice samples collected at the margin of the Green-
land ice sheet (Kurbatov et al. 2010).
Because of excellent preservation and minimal
contamination, the Greenland ice sheet is an ideal
repository of aerosols, representing a detailed record
of wildfire activity in the Northern Hemisphere over
the past hundred thousand years (Mayewski et al.
1993). Examples of combustion aerosols detected in
ice cores and correlated with biomass-burning ac-
tivity include ammonium (NH
4
), nitrate (NO
3
), levo-
glucosan (Kehrwald et al. 2012), acetate, oxalate, and
formate (Legrand et al. 1992, 2016; Mayewski et al.
1993, 1996, 1997; Whitlow et al. 1994; Taylor et al.
1996; Fuhrer and Legrand 1997). Eichler et al. (2011)
correlated ice-core aerosols with known historical
wildfire episodes, including the one caused by the
Tunguska impact event, and found that NH
4
,NO
3
,
and potassium (K) are especially robust proxies of
biomass-burning episodes. Similarly, Fuhrer and Le-
grand (1997) found that wildfires account for up to
40% of Holocene NH
4
concentrations in Greenland
ice records, with the remainder derived mostly from
marine and continental sources, including plant and
microbial biomass.
Melott et al. (2010) proposed that some ice-core
NH
4
at the YD onset was produced by the high-
temperature and high-pressure passage of cometary
fragments through Earth’s atmosphere. Similarly,
they suggested that airborne impact ejecta could
have produced nitrates (NO
x
) that were deposited
in polar ice cores (Parkos et al. 2015; Melott et al.
2016). These two bolide-related hypotheses for the
formation of combustion aerosols are not mutually
exclusive with aerosol production by impact-related
wildfires.
In this contribution, we summarize evidence in
multihemispheric ice sequences in support of a
widespread peak in biomass-burning activity at or
close to the onset of the YD climate episode. We
then examine the potential temporal relationship
among the peaks in biomass burning, YD climate
change, and the YDB extraterrestrial-impact event,
as recorded in these ice sequences. The investiga-
tions of temporal relationships among these events
can determine whether the hypothesis of causality
(i.e., an impact trigger) is plausible or can be re-
jected.
Methods
The original ice-core data analyzed in this study
were compiled from a number of independent sources
(see “Sources of Ice-Core Data”in the appendix;
table A4); no new ice-core data are presented in this
contribution. The original data used multiple, incom-
patible age scales that prevent intercore comparisons
and hence require recalibration (“Age-Depth Issues
with Greenland Ice Cores”in the appendix). In ad-
dition, ice-core ages reported in years before AD 2000
(b2k) are incompatible with calibrated radiocarbon
ages reported in years before AD 1950 (cal BP), a dif-
ferenceof50y.Toaddresstheseissues,weconverted
all ice ages to the common Greenland Ice Core Chro-
nology 2005 (GICC05) timescale (b2k; Rasmussen
et al. 2008; Seierstad et al. 2014), with uncertainties of
5140y.Wethensubtracted50ytoconverttheagesto
cal BP (relative to AD 1950), to permit comparison of
the timescales of previously existing ice records here
in part 1 with those of sedimentary terrestrial records
in part 2 (Wolbach et al. 2018).
The compiled data are plotted with standard an-
alytical methods and are typically presented in both
raw and smoothed formats. For the latter, data points
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were lowess-smoothed with half-windows of ∼50–
1000 y, depending on data density in each ice core,
with fewer data points requiring larger smoothing
windows.
Results
Correlation of Greenland Ice Sheet Project 2 Plati-
num with Onset of the YD Climate Episode. An anom-
alously high peak in Pt concentrations recorded
in sediments and ice has been reported at or close
to the YD onset across North America, western Eu-
rope, and western Asia at 26 sites (fig. A1), including
an additional eight in this contribution. Petaev et al.
(2013) initially discovered this Pt peak spanning ∼21 y
that began precisely at the onset of the YD climate
episode in the Greenland Ice Sheet Project 2 (GISP2)
ice core (see fig. 1). They attributed this episode of
continuous Pt deposition to “multiple injections of
Pt-rich dust into the stratosphere”by an estimated
800-m-diameter iron asteroid (Petaev et al. 2013, p. 1).
This ice-core Pt-rich interval is especially signifi-
cant because the chronology is well resolved to ∼3y
per ice sample, as a result of higher accumulation
rates and subannual layering. This resolution is much
higher than nearly all terrestrial YDB stratigraphic
sequences. Pt-rich sediment and Pt-rich spherules
have also been reported in YDB-age sediment (An-
dronikov et al. 2014, 2015, 2016a, 2016b; Androni-
kov and Andronikova 2016; Moore et al. 2017; see
“Widespread YDB Platinum Deposition”in the ap-
pendix).
Quantifying Sea Salt and Continental Dust.To
test the hypothesis that peak Pt concentrations are
synchronous with the onset of YD climate change,
we analyzed GISP2 abundances of sea salt and
continental dust (Cl, Ca, Na, Mg, and K) that mark
the YD onset. Two of the key indicators of oceanic
sea-salt transport in Greenland ice are chlorine (Cl)
and sodium (Na), with ∼99% of the latter being of
marine origin (de Angelis et al. 1997). Concentra-
tions of these elements increased sharply at the YD
onset in response to an abrupt, major rise in wind
strength caused by the shift in atmospheric circu-
lation (Fuhrer and Legrand 1997; Mayewski et al.
1997). Dust from continental sources is dominated
by magnesium, sulfate, and calcium (Ca), the latter
of which is nearly 100% continental in origin (Fuhrer
and Legrand 1997; Mayewski et al. 1997). Elevated
abundances of these elements in Greenland ice
reflect the enhanced transport of continental dust
to the ice sheet, resulting from increases in wind
strength and/or terrestrial aridity. Because dust
transport typically increases at the onset of colder
intervals (Ram and Koenig 1997), the abundances of
these elements have been used to identify the YD
climate shift within the Greenland ice cores (Fuhrer
and Legrand 1997; Mayewski et al. 1997).
The results reveal that cumulative GISP2 con-
centrations of Cl, Ca, Na, Mg, and K rose abruptly
by a factor of ∼4 to a major peak at ∼12,836 cal BP
(fig. 2), marking the onset of a general trend in in-
creasing windiness beginning at the YD onset. This
increase began in the depth interval from 1712.70 to
1712.60 m, corresponding to the sudden increase of
Pt concentrations (1712.625–1712.5 m). Hence, the
record shows that the cumulative dust peak is es-
sentially synchronous with the initial rise in Pt de-
position (Petaev et al. 2013). Furthermore, this dust
peak at the onset of the YD is outstanding in the
entire ice-core record, being higher than 99.7% of
values in the previous 100,000 y (not plotted).
Correlation of YD Climate Change and Biomass Burn-
ing. To test the hypothesis that the YDB impact
event initiated widespread biomass burning and si-
multaneously triggered YD climate change, we com-
piled and analyzed 1106,000 independently acquired
data points for combustion-related aerosols (e.g., NH
4
,
Figure 2. Greenland Ice Sheet Project 2 (GISP2) plati-
num (Pt) record compared to GISP2 cumulative concen-
trations of sea salt and continental dust. Concentrations
of Cl, Ca, Na, Mg, and K are represented by stacked gray
lines. The dotted black line is a lowess curve (50-y half-
window) that shows data trends. Pt is shown on a semi-
log Y-axis for greater clarity. Results indicate that a peak
in impact-related Pt (asterisk) coincides precisely with a
peak at the onset of climate-related dustiness (plus sign),
rising to an anomalous dust peak that is higher than ∼99.7%
of values in the previous 100,000 y. Data are from Ma-
yewski et al. (1993) and Petaev et al. (2013).
000 W. S. WOLBACH ET AL.
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NO
3
, acetate, oxalate, and formate) taken from three
ice cores: the GISP2, the Greenland Ice Core Project
(GRIP), and the North Greenland Ice Core Project
(NGRIP; fig. 1). We also investigated climatic and
biomass-burning records from the Taylor Dome ice
core and Taylor Glacier in Antarctica and from the
Belukha ice core in Russia. These additional ice rec-
ords were selected because they have sufficiently
high chronological resolution to record detailed con-
centrations of multiple YD-age combustion aerosols.
GISP2 Ice Core. High-resolution data for NH
4
and d
18
O were independently acquired at a sam-
pling resolution of ∼3–5 y across the YD onset in-
terval (Mayewski et al. 1993). This record for NH
4
,a
biomass-burning proxy, displays one of the highest
peaksin the 120,000-y record in an interval dating to
12,830–12,828 cal BP (1712.3–1712.2 m; fig. 3A,
3C). This overlaps the Pt-rich interval dating to
12,836–12,815 cal BP (1712.250–1712.125 m) and
coincides with the YD onset.
Before the onset of impact-related Pt deposition,
the trendline of NH
4
values decreases, whereas after
the onset of Pt deposition, NH
4
values abruptly in-
crease and continue increasing for approximately a
century (fig. 3C). This unusual ramp-up appears to
indicate a long-term increase in wildfires, contain-
ing two of the highest NH
4
peaks in the entire rec-
ord (fig. 3C), but such a century-long increase in
biomass burning is not supported by the records for
lake charcoal and AC/soot (Wolbach et al. 2018). In
addition, it is unsupported by comparison to GISP2
Cl and Ca records, proxies for marine and conti-
nental dust, respectively. When NH
4
values are nor-
malized to values of these two elements, the ramp-
up nearly disappears (fig. A2), making its existence
questionable; all original peaks remain, but with
smaller amplitudes. The cause of this apparent ramp
is unknown but may simply be an artifact of the in-
creased windiness and/or other complex climatic
changes related to the YD onset.
Nearly a dozen earlier NH
4
peaks are higher than
that of the YD onset, as are several younger peaks
that are likely related to increases in anthropogenic
biomass burning (Cooke 1998; Power et al. 2008).
The GISP2 NH
4
peak that coincides with the Pt
anomaly is not the highest recorded, but it is greater
than 99.96% of 26,535 measured NH
4
values, making
it a standout in the entire pre-Holocene record.
Measurements for GISP2 d
18
O, a proxy for atmo-
spheric temperature, were at lower resolution (20-y
Figure 3. Greenland Ice Sheet Project 2 (GISP2) ice-core records. A,NH
4
record from 120,000 to 5000 cal BP. The
most dramatic rise, culminating in the most robust peak, coincides with the Pt peak (dashed line). B,Thed
18
Orecord
from 120,000 to 5000 cal BP (lower values represent colder temperatures), exhibiting a major drop at the Younger
Dryas (YD) onset. C, Pt anomaly from 12,836 to 12,815 cal BP (vertical gray bar); NH
4
measurements (solid black
curve) between 13,050 and 12,600 cal BP; and d
18
O record (stair-step plot with lower values representing colder
temperatures). Dotted lines are lowess curves, showing general trends in NH
4
values near the YD onset. A major NH
4
peak (asterisk) occurs at 12,828 cal BP, within the elevated-Pt interval. The shift in d
18
O values represent the onset of
YD climate change, coinciding with both Pt and NH
4
peaks. NH
4
peaks marked with triangles are the two highest
within the 120,000-y record; they occurred during an apparent ramp-up in biomass-burning activity beginning at the
onset of anomalous Pt deposition. Site location is shown in figure A1 and data sources in table A4.
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increments), but even so, the record clearly displays
a dramatic shift to colder temperatures, marking
the onset of the YD episode (fig. 3B,3C;Stuiver
et al. 1995). Most of this shift occurs across a single
ice sample from ∼1713.40 to ∼1711.69 m, dating to
12,855–12,814 cal BP and overlapping the Pt anomaly.
Thus, four separate lines of evidence in the GISP2
core demonstrate that a series of highly anomalous
changes co-occurred within a relatively thin ice sec-
tion (1712.625–1712.000 m), spanning a mere 15 y
between 12,836 and 12,821 cal BP (table 1). This
demonstrates that the Pt anomaly (marking the YDB
impact event; fig. 2) is synchronous with a peak in
NH
4
concentrations (marking a major episode of bio-
mass burning; fig. 3), an abrupt increase in dust con-
centrations (YD-related windiness reflecting a shift
in atmospheric circulation), and a drop in d
18
Ovalues
(onset of the YD climatic cooling). This supports a
strong interrelationship among all these processes.
NGRIP Ice Core. Fischer et al. (2015) indepen-
dently measured NH
4
concentrations at annual reso-
lution and attributed NH
4
peaks to biomass-burning
activity across North America. They identified a
single high NH
4
peak that begins at the YD onset,
reflecting the largest biomass-burning episode from
North American sources in the entire record. Fischer
et al. (2015) also collected biomass-burning events
into 201-y bins and found fewer apparent fires in
the bin that includes the YD onset. However, if the
YD cosmic impact produced just one major episode
of biomass burning, we infer that multiple separate
wildfires contributed to the overall episode, appear-
ing as a single high NH
4
peak and thus obscuring the
record.
The NGRIP ice interval from 120,000 to 5000 cal
BP displays only one anomalously high peak in NH
4
(fig. 4A), and this is contemporaneous with the
GISP2 Pt anomaly, as correlated with the GICC05
timescale (Rasmussen et al. 2008; Seierstad et al.
2014). This NH
4
peak occurs within a 3-cm interval
centered on 1524.75 m, dating to 12,819 cal BP. This
overlaps the GISP2 Pt anomaly’s age range of ∼12,836
to 12,815 cal BP. The amplitude of this peak is higher
than 99.6% of the ∼94,800 values measured in the
entire record (fig. 4A). The NH
4
peaks at the YD onset
in NGRIP correlate closely with those in GISP2, with
an apparent age difference of only ∼9 y, from 12,819
cal BP for NGRIP to 12,828 cal BP for GISP2. This
small difference presumably is due to ice layer count-
ing errors but is well within the GICC05 age uncer-
tainty of 5140 y, indicating statistical isochroneity.
The NGRIP NH
4
record also exhibits a ramp-up
in values beginning at the YD onset, which on face
value appears to mark a century-long episode of
steadily increasing biomass burning, similar to that
exhibited in the GISP2 record. At the end of the
ramp, there is an NH
4
peak that is higher than 99.5%
of all NH
4
values before the YD onset (fig. 4C), but, as
discussed in the GISP2 results, when NH
4
values are
normalized to Cl and Ca values, the existence of this
ramp-up is questionable.
We also compiled and analyzed NGRIP values for
d
18
O, a proxy representing atmospheric temperature
(fig. 4C; Steffensen et al. 2008). This proxy exhibits a
major shift to colder temperatures, marking the on-
set of the YD, which occurred in a year or less be-
tween ∼12,815 cal BP (1525.45 m) and ∼12,814 cal BP
(1525.42 m). This age span includes the end of the Pt
anomaly in GISP2 (12,836–12,815 cal BP), indicating
synchroneity between YD cooling in NGRIP and the
Pt anomaly in GISP2.
GRIP Ice Core. Although the GRIP ice record
extends to ∼386,000 y, the data up to ∼120,000 y old
are considered more reliably dated. We compiled
concentrations of the biomass-burning proxies NH
4
,
acetate, formate, and oxalate at relatively low chro-
nological resolution of ∼20–400 y/sample (fig. 5;
Legrand et al. 1992; Fuhrer and Legrand 1997). Le-
grand et al. (2016) correlated NH
4
and formate, in
particular, with known historical biomass burning
in Canada, confirming that these proxies appear ro-
bust in reflecting biomass-burning activity in North
America.
The GRIP concentrations of combustion aero-
sols began to increase sharply at ∼12,816 cal BP, in
the GICC05 cal BP timescale (bottom of sample:
1661.27 m), correlating with the GISP2 Pt anomaly
(12,836–12,815 cal BP). Values rose to peaks less
than 100 y later, at 12,710 cal BP (top of sample:
1657.725 m), closely correlating with the start of
the peak in the GISP2 record of sea salt and conti-
nental dust. At the YD onset, GRIP NH
4
concen-
Table 1. Information on GISP2 Peak Proxies Analyzed
Proxy Abbreviation Significance GISP2 depth (m) Ages (cal BP)
Platinum Pt Cosmic impact 1712.625–1712.000 12,836–12,815
Dust Cl, Ca, Na, Mg, K YD wind strength 1712.70–1712.00 12,837–12,817
Ammonium peak NH
4
Biomass burning 1712.30–1712.20 12,823–12,821
Oxygen isotope d
18
O YD temperature 1713.40–1711.69 12,855–12,814
Note. GISP2 pGreenland Ice Sheet Project 2 core; YD pYounger Dryas.
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trations rose to their fourth-highest value, greater
than 99.9% of all other values (fig. 5A). Simul-
taneously, concentrations of oxalate and formate
reached their highest known concentrations in the
∼386,000-y core (figs. 5C,5D), with acetate abun-
dances ranking among the highest in the entire core
(fig. 5B). Also, the ratio between the NH
4
and for-
mate at the YD onset in GRIP rose to its highest
value, nearly 3 times as high as the nearest other
value in the entire record (fig. 5E). These GRIP data
reveal that YDB biomass burning represents the
most anomalous episode of biomass burning in at
least 120,000 y and possibly in the past ∼386,000 y.
Antarctic and Siberian Ice Records. The Taylor
Dome, Antarctica, ice-core record exhibits a small
but distinct peak in NO
3
that closely correlates with
the YD onset (fig. 6A; Mayewski et al. 1996). How-
ever, this peak is subdued relative to other NO
3
val-
ues for the Holocene and the record before 15,000 cal
BP, but it is relatively much higher than that for the
ice interval from 15,000 to 11,500 cal BP across the
YD onset.
The base of the Belukha, Siberia, ice core exhibits
a major peak in NO
3
, indicating that a major episode
of biomass burning occurred at the YD onset. Aizen
et al. (2006) verified the link between NO
3
peaks in
the Belukha core and biomass burning, on the basis
of their observation of a NO
3
peak that is coeval
with Siberian fires produced by the Tunguska im-
pact in AD 1908.
Discussion
Significance of Combustion Aerosols. Well-dated
high-resolution ice-core sequences are crucially im-
portant for providing records of biomass burning dur-
ing the late Quaternary. Peaks in NH
4
are the most
widely employed proxy for biomass burning in ice
cores, strongly supported in some sequences by other
combustion aerosols (Legrand et al. 1992; Mayewski
et al. 1993). Several ice-core sequences (GISP2, NGRIP,
GRIP, Taylor Dome, and Belukha) have revealed
clear evidence that the onset of the YD is intimately
associated with one of the highest and most perva-
sive late Quaternary peaks in each of NH
4
,NO
3
,
formate, oxalate, and acetate. These peaks are effec-
tively coeval with abrupt cooling and other climatic
effects marking the onset of the YD episode.
Figure 4. North Greenland Ice Core Project (NGRIP) ice-core records. A,NH
4
record from 120,000 to 5000 cal BP.
One of the highest NH
4
peaks in the record coincides with the Greenland Ice Sheet Project 2 (GISP2) Pt peak (vertical
dashed line, shown for reference). B, The d
18
O record from 120,000 to 5000 cal BP shows a major decrease in tem-
perature at the Younger Dryas (YD) onset. C,ValuesforNH
4
(black curve) and d
18
O (gray stair-step curve) from 13,050
to 12,600 cal BP. The vertical gray bar denotes the GISP2 Pt anomaly interval, age-depth correlated between GISP2
and NGRIP with the Greenland Ice Core Chronology 2005 timescale. The thick dashed black lines are lowess curves
for NH
4
values that show steady to declining values before the YD onset, followed by an apparent century-long ramp-
up in NH
4
concentrations. A major NH
4
peak occurs at ∼12,819 cal BP, within the span of anomalous Pt deposition.
The d
18
O values show that the onset of YD cooling (marked with a plus sign) was contemporaneous with the NH
4
(asterisk) and Pt peaks. The two NH
4
peaks (asterisk and triangle) are higher than 99.6% of the values in the entire
record. Site location is shown in figure 1 and data sources in table A4.
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Another crucial factor involves atmospheric con-
centrations of CO
2
, which are largely controlled by
transfers between Earth’s major carbon reservoirs
(ocean and terrestrial biota), as well as carbon sinks
(e.g., soils). CO
2
is produced during biomass-burning
episodes that, if large enough, might be expected to
affect atmospheric CO
2
concentration, such as dur-
ing the biomass-burning peak at the YD onset. If so,
an increase in atmospheric CO
2
should be apparent in
polar ice. Although high-resolution ice-core records
of CO
2
are not available from the Northern Hemi-
sphere, Brook et al. (2015) reported atmospheric con-
centrations of CO
2
and d
13
C-CO
2
from an ice se-
quence across the YDB interval in Taylor Glacier,
Antarctica.
CO
2
values mostly increase before the YD onset,
after which they exhibit a small, but abrupt, rise,
followed by a gradual, long-term increase (fig. 7).
Although degassing of the ocean to the atmosphere
during deglaciation contributed to this generally
rising pattern, the abrupt rise at the YD onset is
consistent with a brief episode of major continental
biomass burning. In addition, d
13
C-CO
2
values de-
crease significantly and abruptly, beginning at the
YD onset, and continue declining through the first
∼500 y of the YD (fig. 7). This is consistent with a
major loss of terrestrial carbon due to climate change,
biomass burning during the early YD, and/or changes
in ocean circulation.
The sudden cooling at the YD onset would have
contributed to a major reduction in terrestrial bio-
mass, but if ocean circulation and related climate
changes were the only cause (Menviel et al. 2015),
then the isotopic values of atmospheric CO
2
would
not have rebounded as rapidly during the early YD.
Because near-glacial conditions during the YD and
related changes in ocean circulation persisted for
much longer than 300 y (i.e., ∼1300 y), it appears
unlikely that the decline in d
13
C-CO
2
was driven
solely by the effects of climate and ocean circula-
tion in reducing terrestrial biomass. In the absence
of YDB biomass burning, the record suggests that
YD cooling alone is unlikely to have accounted for
the abruptness, magnitude (0.2‰), and brevity of
the isotopic change.
The distinctive change in Antarctic d
13
C-CO
2
af-
ter the YD onset provides further evidence for major
transfer of organic carbon to the atmosphere. A num-
ber of processes could have caused this rise in d
13
C-
CO
2
, including climate-related plant die-offs resulting
in more fires as a result of increased fuel availability
and/or an impact-related increase in wildfires con-
suming substantially more biomass over a brief in-
terval. The sharp increase in CO
2
, synchronous with
the abrupt decrease in d
13
C-CO
2
, is limited to the ear-
liest YD and hence is consistent with a rare episodic
event.
The sudden rise in atmospheric CO
2
at the YD on-
set, recorded in Antarctic ice (fig.7),is2.44ppm.Be-
cause each part per million is equivalent to 7.805
gigatons (Gt) of CO
2
in the global atmosphere (O’Hara
1990), the recorded rise corresponds to the release of
19.04 Gt of CO
2
. Vegetation fires produce (∼1.95–
2.23) #10
4
kg/ha of CO
2
, not counting later reabsorp-
tion by new growth (Santín et al. 2016), so if the ob-
served jump in CO
2
were due only to vegetation fires,
it would be equivalent to the surface burning of (0.85–
0.94) #10
9
ha, or roughly 1 #10
7
km
2
. Because of
lower sea levels at the time of the YD onset, the total
land surface would have been somewhat larger than
Figure 5. Combustion aerosols from Greenland Ice Core
Project (GRIP) ice core. A–D, Coeval peaks in NH
4
,ace-
tate, oxalate, and formate, respectively, dating to or near
the Younger Dryas (YD) onset and representing the high-
est or near-highest episode of biomass burning of the past
120,000–386,000-y record. E,RatioofNH
4
to formate,
two strong indicators of biomass burning. The two high-
est ratio values in the record comprise the single highest
peak at the YD onset. Location of the GRIP site is shown
in figure 1, and data sources are in table A4.
000 W. S. WOLBACH ET AL.
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that today, at approximately 15 #10
7
km
2
,andsothe
burned percentage of global land surface would have
been (1 #10
7
km
2
)/(15 #10
7
km
2
), or approximately
6%. Still et al. (2003) estimated the area enclosing the
contemporary global biomass to be 106.2 #10
6
km
2
;if
that at the YD onset was similar, then ∼9.4% of Earth’s
vegetated surface burned. This compares closely to
calculations based on measured amounts of YDB AC/
soot, suggesting that 5.0%–9.6% of biomass burned at
the YD onset (Wolbach et al. 2018).
Significance of Widespread YDB Pt Peaks. The
anomalously high Pt peak recorded at the onset of
the YD in the GISP2 core (Petaev et al. 2013) and in
widely distributed sedimentary sequences over North
America (Moore et al. 2017) has been attributed to a
major cosmic impact. Additional evidence presented
here and in part 2 of this work (Wolbach et al. 2018)
suggests that this cosmic impact also triggered wide-
spread biomass burning that, in turn, contributed
to “impact winter”and abrupt YD climate change
(Firestone et al. 2007). On the other hand, Petaev et al.
(2013) inferred no temporal, and hence casual, linkage
between the YDB impact event and widespread bio-
mass burning identified in the GISP2 core by Ma-
yewski et al. (1993). However, that interpretation
relied on two incompatible GISP2 timescales: a pre-
liminary timescale in years before AD 1950 (Ma-
yewski et al. 1993) and the Meese timescale in years
before AD 2000 (Meese et al. 1997), a difference of 50 y.
When all data sets are calibrated to the GICC05
timescale (cal BP; before AD 1950), the Pt anomaly,
the biomass-burning peak, and the onset of YD cli-
mate change all co-occurred within the same 21-y
ice interval and thus appear synchronous within the
limits of ice-core dating.
Because anomalously high Pt concentrations also
can result from other processes, including volca-
nism, it is important to consider nonimpact mech-
anisms. Both Firestone et al. (2007) and Moore et al.
(2017) found no evidence of volcanic tephra in the
Pt-rich YDB layer and or other strata. Furthermore,
Moore et al. (2017) examined three samples of tephra
from the Laacher See eruption in Germany, which
occurred !200 y before the cosmic-impact event and
potentially could have contaminated the YDB layer
with volcanic Pt. However, they detected no mea-
surable concentrations of Pt or magnetic spherules in
that tephra.
Figure 6. NO
3
concentrations in ice sections from Antarctica and Belukha, Siberia. A, Taylor Dome, Antarctica, ice-
core abundances of NO
3
, a biomass-burning proxy (Brook et al. 2015), age-correlated with the Greenland Ice Sheet
Project 2 (GISP2) Pt peak and the Younger Dryas (YD) onset (vertical dashed line). B,NO
3
peak at the YD onset in the
Belukha, Siberia, ice core, correlated by age with the GISP2 record. Ice deposition in the core began at the YD onset
(dashed line). Locations are shown in figure 1 and data sources in table A4.
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Gabrielli et al. (2008) reported Pt concentrations
in Greenland ice cores deposited by major eruptions
from Mount Pinatubo, the largest in the twentieth
century, and from Hekla volcano in Iceland. The
maximum Pt enrichment they measured was 0.09 ppt;
the Greenland ice concentrations at the YD onset
found by Petaev et al. (2013) are nearly 1000 times
higher, and the maximum YDB value measured by
Andronikov et al. (2016a) is 5,000,000 times higher.
For more information on YDB Pt, see “Widespread
YDB Platinum Deposition”in the appendix.
In addition, GISP2 Pt concentrations (Petaev et al.
2013) can be compared with the GISP2 sulfate rec-
ord, a proxy for volcanic eruptions (Mayewski et al.
1993). This reveals that although three major vol-
canic eruptions occurred near the YD onset (fig. 8),
no significant Pt peaks are associated with any ep-
isode of volcanism recorded in this core. These var-
ious lines of evidence, therefore, suggest that volca-
nism is an unlikely source for the YDB Pt anomaly.
The average Pt/Ir ratio for the upper continental
crust is ∼1 (Rudnick and Gao 2003), whereas me-
teoritic ratios are typically much higher, averaging
42 (ratio range: 13,500–0.1; GERM Reservoir Data-
base 2016). In the GISP2 ice core, Petaev et al. (2013)
measured a maximum Pt/Ir ratio of ∼500, similar to
those of some iron meteorites, and thus concluded
that an iron meteorite was the likely source of YDB
Pt deposition. However, Andronikov et al. (2016a)
reported much lower average Pt/Ir ratios of 42 (range:
91–12) in YDB magnetic spherules that do not require
an iron meteorite as source.
To help determine the potential type of YDB im-
pactor, we investigated Pt-to-palladium (Pt/Pd) ra-
tios and Pt-to-iridium (Pt/Ir) ratios. Elemental abun-
dances of YDB Pt, Pd, and Ir are available from eight
sites (Firestone et al. 2007; Moore et al. 2017), ex-
cluding Greenland, where Pd measurements are not
available. The YDB Pt/Ir ratios average 7.8 (range:
Figure 7. CO
2
and d
13
C concentrations over a 2800-y
interval from Taylor Glacier, Antarctica. The dashed
vertical line represents the Greenland Ice Sheet Project 2
(GISP2) Pt peak and the onset of Younger Dryas (YD)
climate change as recorded in Greenland. CO
2
(upper
line) was increasing immediately before the YD onset
(A), rose sharply at the YD onset (B), and then increased
steadily during the YD (C) until ∼11,500 cal BP. The
d
13
C-CO
2
values (lower line) rose, gradually (D) peaking
at the YD onset (E), declined sharply afterward (E–F), and
suddenly rose again during the mid-YD, beginning at F.
The sharp decline in d
13
C-CO
2
in the earliest 300 y of the
YD is consistent with a significant decrease in terrestrial
carbon (organic degradation). Data are digitized from
Brook et al. (2015).
Figure 8. Greenland Ice Sheet Project 2 (GISP2) platinum
(Pt) and volcanic sulfate (Volc. SO
4
). Pt (ppt) deposition is
represented by the black curve, and sulfate (ppb) is rep-
resented by the dashed curve. The vertical gray bar repre-
sents the GISP2 Pt anomaly, dating from 12,836–12,815 cal
BP, with a high peak centered at 12,822 cal BP (dotted ver-
tical line). None of the four highest volcanic sulfate peaks
(asterisks) corresponds to the Pt anomaly.
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3.8–0.1),and YDB Pt/Pd ratios average 1.5 (range: 3.4 –
0.7), both ofwhich are higher than the average crustal
ratios. We compared YDB ratios to those from five
typesofmeteorites(fig. 9A; GERM Reservoir Data-
base 2016) and found that the ratios from three YDB
sites appear lower than meteoritic values. However,
ratios for the other five agree with the ratios of iron
meteorites (np10) but also overlap ratios from some
achondrites (np19), chondrites (np47), Martian
meteorites (np6), and/or ureilites (np23). These
data suggest that YDB Pt concentrations most likely
do not result from any single type of meteorite.
We also compared the same YDB ratios of Pt, Pd,
and Ir with those of three groups of impactites,
which are rocks that have been melted through
cosmic impact and thus contain small quantities of
impactor material. We used impactite data from the
GERM Reservoir Database (2016) for the 66 Ma K-
Pg impact event (np60), a 145 Ma impact (np8),
and a 2.55 Ga impact (np18). For three YDB sites,
Pt/Pd and Pt/Ir ratios are higher than those for these
three groups of impactites, and ratios from the other
five YDB sites overlap or are close to ratios of known
impactites (fig. 9B). These data suggest that YDB Pt
concentrations could result from the impact-related
mixing of meteoritic material and terrestrial target
rocks.
Although the above analyses indicate that the
YDB impactor could have been any one of several
types of meteorite, the wide range of YDB Pt/Ir ra-
tios (12–1264) is inconsistent with collision with a
single type of meteorite. Comets are a composi-
tionally variable mix of volatile ices, meteoritic ma-
terial, and presolar dust (Flynn et al. 2006), and al-
though available samples of cometary material are
too small to allow measurements of Pt, Pd, and Ir,
other elemental ratios are available from Comet
Wild 2 (Flynn et al. 2006). Iron-to-nickel (Fe/Ni) ra-
tios range widely from 7 to 286, and chromium-to-
copper (Cr/Cu) ratios range from 0.05 to 285 (Flynn
et al. 2006), similar to the wide range of YDB Pt/Ir
ratios (12–1264). These wide ranges of elemental
ratios confirm that cometary material is hetero-
geneous, similar to the YDB samples. Although the
type of YDB impactor remains unclear, the current
evidence does not support any specific meteoritic type
as source. Instead, the broad extent of biomass burn-
ing at the YD onset is more consistent with Earth’s
collision with a fragmented comet that in turn, trig-
gered widespread wildfires.
Astronomical Hypothesis for the YDB Impact Event.
Regarding the probability of a swarm of cometary
fragments hitting the Earth, Boslough et al. (2013)
claimed that the YDB event is “statistically and
physically impossible,”whereas Napier et al. (2013)
argued that such an encounter in the late Quater-
nary is a “reasonably probable event.”We outline
the latter hypothesis below; details and prime ref-
erences are given in Napier (2015).
With currently accepted impact rates, there is an
expectation of one extraterrestrial impact of energy
100–200 megatons over the past 20,000 y, which is
inadequate to produce the observed global trauma
(Bland and Artemieva 2006). However, near-Earth
Figure 9. Ratios of platinum to palladium (Pt/Pd) and platinum to iridium (Pt/Ir) for rocks, sediments, and me-
teorites. Younger Dryas boundary (YDB) sites are represented by filled circles. Dashed crosses represent upper crustal
abundances. A, Elemental ratios for eight YDB sites plotted against those for five types of meteorites. Five of the
8 sites overlap or show close correspondence to meteoritic compositions. B, Elemental ratios for eight YDB sites
plotted against those for impactites produced by three impact events. Five of the 8 YDB sites overlap or closely
correspond to impactite compositions.
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surveys of hazardous interplanetary objects are lim-
ited to the past ∼30 y, and extrapolation of con-
temporary impact rates to timescales beyond 10
4
y
cannot be justified without further investigation,
especially for comet populations.
Comets entering Earth-crossing orbits, which are
thus potential collision hazards, may be long-period
(LP) objects (200 ky ≲P≲4 My), Halley-type (HT;
20 y ≲P≲200 y), Jupiter family (JF)–type (4 y ≲P≲
20 y), or Encke-type (P≲4 y), the latter currently
with a population of 1. The LP system is spherical,
containing as many comets in retrograde orbits as in
prograde, and derives from the Oort cloud. The HT
system is spheroidal, with a preponderance of comets
in direct orbits, while the JF and Encke comets are in
direct orbits close to the plane of the ecliptic. There
may be ∼100 active comets with diameters over 2.3 km
in the HT population and ∼450 in the JF system,
although these numbers are very uncertain. Both
these populations are evanescent, with a typical JF
comet surviving for only 200–300 revolutions (∼3000 y)
before disintegration is complete. To maintain a steady
state, new comets must enter the JF system about
once a decade and the HT system about once every
century. The likely replacement reservoirs are the
Oort cloud and a trans-Neptunian population of icy
bodies on the fringes of the planetary system, the
latter of which was largely unknown 25 y ago. Its
properties are still being explored, but it is estimated
to contain 8 billion comets more than 1 km in diam-
eter; the Oort cloud may contain a trillion comets.
Dormant comets, called “centaurs,”have been
detected in transition from these reservoirs to the JF
population. They are in unstable orbits crossing
those of Jupiter, Saturn, and Uranus, becoming
more unstable as they move inward and becoming
active when they cross the water-snow line at ∼2.9
astronomical units (au) from the Sun (1 au pthe
mean Earth-Sun distance). The archetypal centaur,
Chiron, currently orbits between Saturn and Ura-
nus. Its half-life for ejection from the solar system is
about 1 My, and that for evolution into a Jupiter-
crossing orbit is 0.1–0.2 My (Hahn and Bailey 1990).
Population-balance arguments indicate that at any
given time there may be four to seven centaurs
larger than 240-km-wide Chiron inside 18 au and
about 30 that are 1100 km in diameter. Their orbits
are chaotic and can be followed only statistically,
because small changes in initial conditions induce
large subsequent variations (the butterfly effect).
Eventually, about half the centaurs in Chiron-like
orbits become Jupiter crossers at some point, and a
tenth become Earth crossers, moving in and out of
Earth-crossing epochs repeatedly (fig. 10).
The mass distribution of centaurs is top-heavy,
and the replenishment of the JF and comets in Encke-
like orbits is erratic, with occasional large injections
of mass into the inner planetary system. A 250-km
comet with typical density 0.5 g/cm
3
has 2000 times
the current mass of the JF and 1000 times that of the
entire current near-Earth asteroid system. In terms
of terrestrial interactions, its disintegration prod-
Figure 10. Computer-modeled orbital evolution of a Chiron clone. Simulation indicates that the clone eventually
becomes Earth crossing (gray boxed area below horizontal dashed “Earth”line). Light gray curves represent semimajor
axes (half of the longest orbital axis) and black curves the perihelion (the orbital point closest to the Sun). A,The
Chiron-clone centaur, orbiting originally in the Saturn/Uranus region, moves into Earth-crossing orbit (gray boxed
area below horizontal dashed “Earth”line) after 180 ka. B, Enlarged orbital history from 180 to 250 ka, showing that
once a centaur enters an Earth-crossing epoch (gray boxed area below horizontal dashed “Earth”line), it does so
repeatedly as its orbit fluctuates (black arrows mark Earth-crossing episodes). The Chiron clone is typically destroyed
in a cascading series of fragmentations, and its physical lifetime is likely to be much shorter than the dynamical
lifetimes indicated.
000 W. S. WOLBACH ET AL.
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ucts, during its active lifetime, will thus greatly dom-
inate over those of the near-Earth asteroids. For a
100-km comet, the factors are 128 and 64, respec-
tively. The timescale for an enhancement in mass
of the near-Earth environment is ∼0.5 My for a fac-
tor 1000 and ∼0.03–0.1 My for a factor 100.
The main modes of disintegration of a comet are
sublimation and fragmentation. The latter has two
prime modes: tidal splitting, in which the comet
breaks into fragments after a close planetary or solar
encounter, and spontaneous splitting, in which the
main nucleus stays intact while a number of short-
lived cometary fragments split off, often disappear-
ing from sight after a few weeks or days.
Fragmentation is the major mode of comet dis-
integration: the total mass lost by small fragments
repeatedly spalling from the nucleus may be com-
parable to the mass of the nucleus itself (Boehn-
hardt 2004). A 100-km comet of mass 2.5 #10
20
g,
with perihelion of ∼0.34 au, loses typically ∼10
16
g
of material through sublimation during each peri-
helion passage, but ∼10
17
–10
18
g during a splitting
event, equivalent in mass to ∼10
7
–10
8
Tunguska
bolides. Such an event may happen anywhere along
its orbit. For a comet in an Encke-like orbit, such
fragmentations are expected every third or fourth
orbit (di Sisto et al. 2009). These splittings may oc-
cur anywhere but have a tendency to occur near
perihelion. Debris from disintegrating comets readily
spreads out to cross sections much greater than
Earth’s diameter and so is encountered more fre-
quently, as can be seen by the prevalence of meteor
showers in the night sky, the products of cometary
decay. During dormant phases, inwhich sublimation
decreases, an active comet becomes more asteroidal
in appearance through acquiring a mantle of dust and
heavy organics. These phases may persist for up to
40% of the lifetime of the comet.
The structure of the meteoroid population in the
inner planetary system has been determined both
through numerous individual studies of meteors, go-
ing back to the 1950s, and from recent large-scale
radar and optical surveys. As many as 100 meteor
showers are accepted by the International Astronom-
ical Union, but a total of 230 showers and shower
components have now been identified from video-
based meteoroid orbit surveys (Wiegert et al. 2009;
Jenniskens et al. 2016). Some of these streams are
multicomponent, indicating that they result from
cascades of disruption of a parent body into subcom-
ponents.
A prominent feature of this orbiting material is an
interrelated system of meteoroidal material called
the Taurid Complex. At least 20 observed streams
are embedded within it, with the meteoroids mov-
ing in low-inclination, short-period, Earth-crossing
orbits. Many of these streams contain bodies of ki-
lometer and subkilometer dimensions, including
the 4.8-km-wide Comet Encke. A limitation of me-
teor surveys is that they detect only material that
hits Earth. It is therefore possible that the number of
meteoroid streams associated with the progenitor
of the Taurid Complex is greater than the ∼20 that
have been observed. A dust trail along the orbit of
Encke has also been detected, with a lower mass limit
of 7 million tons inferred from the Spitzer infrared
space telescope (Reach et al. 2007). This trail, which
is well away from Earth intersection, extends around
the entire orbit and would disperse in some revolu-
tions. Such trails, distinct from comet tails, are a
generic feature of short-period comets.
The Taurid Complex is best explained as debris
from the breakup of a large comet (∼100 km) in a
short-period, Earth-crossing orbit that recently ar-
rived from the centaur system (Clube and Napier
1984; Steel and Asher 1996). The orbits of the me-
teor streams and their associated bodies precess and
disperse because of the influence of Jupiter and
Saturn. From the observed dispersal of the Taurids,
Steel and Asher (1996) concluded that the progeni-
tor comet was at least 20,000–30,000 y old. Comet
Encke, part of the Taurid Complex, never approaches
closer than 26,000,000 km from Earth at present.
The fragments of a splitting event disperse as they
move away from the comet. Recently released com-
etary material forms an elongated, dense trail typi-
cally a few hundred Earth radii long and 10 radii wide
within an orbit (Napier 2015; Napier et al. 2015).
Taking account of orbital precession and nutation,
the recurrence time of encounters between Earth
and one such debris swarm in an Encke-like orbit is t
∼500/jMy, where the effective cross-sectional area
jof the swarm is in Earth radii. If there is an average
of one such swarm at any time, we expect passage
through it about once in 50,000 y, in the course of
which the Earth will encounter 10
17
/j∼10
13
–10
14
g
of material over a few hours, entering the Earth’s at-
mosphere at 30 km/s. The debris so encountered
will generally be a mixture of dust and larger frag-
ments, energetically equivalent to the impact of
∼1000–10,000 Tunguskas and with the potential to
create severe biotic and climatic disturbances (Hoyle
and Wickramasinghe 1978).
The presence of multiple meteor streams within
the Taurid Complex, including “asteroids”orbit-
ing within them, indicates that the original comet
evolved via a cascading hierarchy of fragmentations.
Over its active lifetime, the original comet and its
offspring may have generated several thousand trails
of mass ∼10
15
–(2 #10
17
) g, spreading and dispersing
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over time, containing subkilometer bodies that con-
tinued to generate meteoroid streams as they disin-
tegrated.
Asher and Clube (1993) demonstrated theoreti-
cally that the 7∶2 mean motion resonance with Ju-
piter plays an important role in shepherding Taurid
meteoroids, building up a swarm of enhanced den-
sity over a timescale of 1000 y. The existence of this
swarm has been confirmed through epochs of en-
hanced fireball activity going back ∼60 y (Asher and
Clube 1993; Dubietis and Arlt 2007; Reach et al.
2007) and possibly through Far Eastern historical
records going back a thousand years (Hasegawa 1992).
More meteoroids struck the Moon over five days in
1975, during a passage through the daytime Taurids,
than over the five years of the lunar seismic record.
Thus, over the late Quaternary lifetime of this ex-
ceptionally large short-period, Earth-crossing comet,
one or more encounters with a swarm of bolides
whose aggregate kinetic energy is comparable to
that of a nuclear war is a “reasonably probable event”
(Napier et al. 2013).
Firestorms from Encounters with Comet Swarms.
Although the blast damage from the Tunguska im-
pact covered 2000 km², the area of forest that was
ignited was only a tenth of that (Florenskiy 1965).
Conditions in the Siberian taiga of the twentieth
century may not, however, extrapolate reliably to
Earth’s Northern Hemisphere ∼12,800 y ago. The
radiant energy of a megaton-class nuclear explosion
unfolds over a few seconds, comparable to that from
a Tunguska-type event, and the effects of an airburst
are probably similar for both types of energy release.
Therefore, we may use studies of wildfires following
a nuclear war (e.g., Crutzen et al. 1984; National
Research Council 1985; Mills et al. 2014) as rough
guides to the consequences of multiple impacts. Crut-
zen et al. (1984) quote minimum forest fire areas of
500, 1000, and 2100 km² following 1-, 3-, and 10-
megaton explosions, respectively, while the maxi-
mum spread areas quoted by Hill (1961) are about a
factor of 10 higher.
The ignition threshold for wood is about 4 #
10
8
ergs/cm
2
, applied for 1–20 s. If the burn area
required to produce the increase in soot and char-
coal measured at the YDB is 10
7
km
2
(eq. [3] in
Wolbach et al. 2018), then an energy input of ∼4#
10
25
ergs is required, that is, about 10
3
megatons.
Assuming that three-quarters of the external input
does not directly affect flammable areas of Earth,
then approximately four times this energy is re-
quired to produce wildfires, possibly reduced by a
factor of a few times (Hill 1961). This equals the
kinetic energy of 10
13
g of material entering the
Earth’s atmosphere at the entry speed of Taurid
material (30 km/s).
The best explanation for the available evidence is
that Earth collided with a fragmented comet. If so,
aerial detonations or ground impacts by numerous
relatively small cometary fragments, widely dis-
persed across several continents, most likely ignited
the widespread biomass burning observed at the YD
onset.
Atmospheric Effects. The environmental effects
of (5 #10
12
)–10
14
g of smoke injected into the at-
mosphere have been extensively discussed in a
nuclear-winter context (Crutzen et al. 1984; Na-
tional Research Council 1985; Mills et al. 2014) and
include multidecadal cooling even with the lower
mass input. In the case of passage through a dense
cometary tail, there is the additional factor of 10
13
–
10
14
g of meteoric material trickling down through
the stratosphere, much of which would be converted
to submicron smoke with a high optical-scattering co-
efficient (Klekociuk et al. 2005) and a long residence
time, significantly darkening the sky, with probable
climatic effects (Hoyle and Wickramasinghe 1978;
Napier 2015). The cooling of the upper atmosphere
due to the diminution of vertical transport of water
might create supercooled crystals in the high atmo-
sphere. These have high reflectivity and could lead
to a self-sustaining, high-albedo layer of ice crystals
in the stratosphere (Hoyle 1981). Long-lived oceanic
cooling and the regrowth of sea ice and ice caps also
would be expected (Mills et al. 2014).
The onset of the YD was clearly an exceptional
event, and the contemporaneous Taurid progenitor
comet was also exceptional, in terms of both its size
and its Earth-crossing configuration. Its fragmen-
tation history over the late Quaternary could plau-
sibly have resulted in a brief, violent hurricane of
bolides capable of generating the extensive wildfires
and depositing impact-related proxies observed in the
YDB layer.
Impact-Related Ozone Depletion. Large impactors
are expected to cause depletion of stratospheric ozone
(O
3
), which, in turn, triggers an increase in solar ir-
radiance reaching Earth’s surface in the ultraviolet
spectrum (UV-B; 280–315-nm wavelength). Impact-
induced UV-B levels are predicted to far exceed
those currently experienced (Pierazzo et al. 2010).
Ultraviolet-B radiation has significant biological ef-
fects, including sunburn, skin cancer, cataracts for
humans and other animals, and reductions in growth
and productivity in plants (both terrestrial and
aquatic). Pierazzo et al. (2010) modeled the changes
in atmospheric chemistry associated with impac-
tors and found that a 1-km impactor, having 2.23 #
10
27
ergs of kinetic energy, would cause a 170% re-
duction in the column density of O
3
at mid-to-high
latitudes for 12 y. Melott et al. (2010) estimated an
impactor energy of 4 #10
29
ergs for the YDB, so we
000 W. S. WOLBACH ET AL.
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consider Pierazzo et al.’s (2010) result to be a lower
limit for YDB O
3
depletion. While Pierazzo et al.
(2010) discussed some potential impacts following
this level of O
3
depletion, they included only nu-
merical estimates of the UV index (a proxy for skin
damage in humans) and erythemal (sunburn) irra-
diance. Thomas et al. (2015) examined a wide range
of biological effects associated with O
3
depletion of
similar magnitude (∼70%) and found that it would
increase the risk of human skin cancer by a factor of
∼13 and inhibit the growth of oat seedlings fivefold.
In addition, changes in the productivity of aquatic
phytoplankton species would range from a few per-
cent (Neale and Thomas 2016) to more than a factor
of 5.
The likely presence of atmospheric particulates
and aerosols can reduce UV-B irradiance reaching
the ground. However, Thomas et al. (2016) found
that changes in O
3
column density were more im-
portant than increases in aerosol optical depth and
that high optical depth is expected to last only for
months, while significant O
3
depletion could per-
sist for several years. Therefore, it is reasonable to
conclude that depletion of O
3
after an impact would
have significant deleterious effects on ecosystems
through direct damage to primary plant producers,
humans, and many other animals.
Expanded YDB Impact Theory. On the basis of
evidence from the YDB, known impact events, and
nuclear detonations, here we update the YDB im-
pact theory.
Agiant,≥100-km-diameter comet entered an
Earth-crossing orbit in the inner solar system and
began a cascade of disintegrations (Napier 2010). Nu-
merous cometary fragments from the debris stream
entered Earth’satmosphere∼12,800 y ago and det-
onated above and/or collided with land, ice sheets,
and oceans across at least four continents in the
Northern and Southern Hemispheres (Firestone
et al. 2007; Napier 2010). Vaporization of cometary
materials and platinum-group element (PGE)–rich
target rocks injected Pt, Ir, Os, and other heavy
metals into the stratosphere (Petaev et al. 2013;
Wu et al. 2013; Moore et al. 2017), accompanied by
impact-related nanodiamonds (Kinzie et al. 2014),
meltglass (Bunch et al. 2012), and microspherules
(for all proxies, see table A3). The impact event de-
stabilized the ice-sheet margins, causing extensive
icebergcalvingintotheArcticandNorthAtlantic
Oceans (Bond and Lotti 1995; Kennett et al. 2018).
The airburst/impacts collapsed multiple ice dams
of proglacial lakes along the ice-sheet margins,
producing extensive meltwater flooding into the
Arctic and North Atlantic Oceans (Teller 2013; see
Kennett et al. 2018 for summary and references).
Destabilization of the ice sheet also may have trig-
gered extensive subglacial ice-sheet flooding, leav-
ing widespread, flood-related landforms across large
parts of Canada (Shaw 2002). The massive outflow
of proglacial lake waters, ice-sheet meltwater, and
icebergs into the Arctic and North Atlantic Oceans
caused rerouting of oceanic thermohaline circula-
tion. Through climatic feedbacks, this, in turn, led
to the YD cool episode (Broecker 1997; Teller 2013;
Kennett et al. 2018). Unlike for typical warm-to-
cold climate transitions, global sea levels rose up to
2–4 m within a few decades or less at the YD onset,
as recorded in coral reefs in the Atlantic and Pacific
Oceans (Bard et al. 2010; Kennett et al. 2018). Mul-
tiple impact-related drivers caused warm intergla-
cial temperatures to abruptly fall to cold, near-glacial
levels within less than a year (Steffensen et al. 2008),
possibly in as little as 3 mo (Manchester and Pat-
terson 2008). A rapid increase in wind strength across
Greenland at the YD onset deposited extensive dust,
sea salt, Pt-rich impact debris, and combustion aero-
sols into the ice sheet (this study). The radiant and
thermal energy from multiple explosions triggered
wildfires that burned ∼10% of the planet’s biomass,
producing charcoal peaks in lake/marine cores that
are among the highest in 368,000 y (Wolbach et al.
2018). This widespread biomass burning generated
large amounts of long-lived, persistent AC/soot
that blocked nearly all sunlight, rapidly triggering
impact winter that transitioned into the YD cool
episode (Wolbach et al. 2018). This widespread bio-
mass burning delivered combustion aerosols (e.g.,
NH
4
and NO
3
) to Greenland ice at some of the high-
est concentrations in ∼120,000–386,000 y (this study).
NH
4
,NO
3
, and other biomass-burning by-products
underwent chemical reactions in the atmosphere that
resulted in acid rain (Firestone et al. 2007; this study).
Climate change, wildfires, and related environmental
degradation contributed to the late Pleistocene mega-
faunal extinctions and human cultural shifts and
population declines (Firestone et al. 2007; Anderson
et al. 2011; Wolbach et al. 2018; this study).
Conclusions
From ice-sheet and ice-core records in Greenland,
Antarctica, and Russia, the evidence reported in this
study supports the co-occurrence of (1) the YDB im-
pact event, represented by elevated Pt concentrations
and many other impact-related proxies; (2) the YD
cool episode, represented by a temperature decrease
reflected by changes in d
18
O and elevated concentra-
tions of Cl, Ca, Na, Mg, and K, reflecting increased
atmospheric dustiness and changes in atmospheric
circulation; and (3) a major episode of biomass burn-
ing, represented by anomalously high coeval peaks
in NH
4
,NO
3
, acetate, oxalate, and formate. For the
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GISP2 core, all of these proxies overlap in the same
Pt-enriched, 21-y-long section of ice that peaks at
12,822 cal BP (range: 12,836–12,815 cal BP), a span
that overlaps the Bayesian-calculated age for the YDB
impact event of 12,835–12,735 cal BP (Kennett et al.
2015). Multiple lines of ice-core evidence appear syn-
chronous, and this synchroneity of multiple events
makes the YD interval one of the most unusual
climate episodes in the entire Quaternary record.
Because a cosmic impact is the only known event
capable of simultaneously producing the collective
evidence discussed in the two parts of this study, a
cause-and-effect relationship among these events is
supported, as proposed by YDB theory.
Author Affiliations
1. Department of Chemistry, DePaul University,
Chicago, Illinois 60614, USA; 2. Department of Geog-
raphy, University of Tennessee, Knoxville, Tennessee
37996-0925, USA; 3. Climate Change Institute, Uni-
versity of Maine, Orono, Maine 04469, USA; 4. De-
partment of Natural Sciences, Elizabeth City State
University, Elizabeth City, North Carolina 27909,
USA; 5. Geology Program, School of Earth Science
and Environmental Sustainability, Northern Ari-
zona University, Flagstaff, Arizona 86011, USA;
6. Department of Nuclear Engineering, University of
California, Berkeley, California 94720, USA; 7. Res-
toration Systems, Raleigh, North Carolina 27604,
USA; 8. Instituto de Investigaciones en Ciencias de la
Tierra, Universidad Michoacana de San Nicolás de
Hidalgo, Morelia, Michoacán, Mexico; 9. Santa Bar-
bara Museum of Natural History, Santa Barbara,
California 93105, USA; 10. Kimstar Research, Fay-
etteville, North Carolina 28312, USA; 11. Faculty of
Science, Charles University, Prague, Czech Repub-
lic; Institute of Geology, Czech Academy of Science
of the Czech Republic, Prague, Czech Republic; and
University of Alaska, 903 Koyukuk Drive, Fairbanks,
Alaska 99775, USA; 12. Center of Excellence in Re-
mote Sensing Education and Research, Elizabeth
City State University, Elizabeth City, North Caro-
lina 27909, USA; 13. Quaternary Surveys, 26 Thorn-
hill Avenue, Thornhill, Ontario L4J 1J4, Canada; 14.
Department of Physics and Astronomy, University
of Kansas, Lawrence, Kansas 66045, USA; 15. De-
partment of Geological Sciences, East Carolina Uni-
versity, Greenville, North Carolina 27858, USA; 16.
Savannah River Archaeological Research Program,
South Carolina Institute of Archaeology and An-
thropology, University of South Carolina, New El-
lenton, South Carolina 29809, USA; 17. Buckingham
Centre for Astrobiology, University of Bucking-
ham, Buckingham MK18 1EG, United Kingdom; 18.
Comet Research Group, Dewey, Arizona 86327,
USA; 19. Department of Anthropology and Depart-
ment of Geology, University of Cincinnati, Cincin-
nati, Ohio 45221, USA; 20. Department of Physics
and Astronomy, Washburn University, Topeka, Kan-
sas 66621, USA; 21. Department of Earth Science
and Marine Science Institute, University of Cali-
fornia, Santa Barbara, California 93106, USA.
ACKNOWLEDGMENTS
B. Culleton (Pennsylvania State University) pro-
vided essential assistance with
14
C calibration and
OxCal, along with C. Bronk Ramsey (University of
Oxford). S. Horn and M. Valente (University of Ten-
nessee) offered valuable suggestions for improving
the manuscript. G. Kletetschka was supported by
the Czech Science Foundation (GACR 17-05935S)
and institutional grant RVO 67985831. J. P. Ken-
nett appreciates long-term support from the US Na-
tional Science Foundation (Marine Geology and Geo-
physics) and also support from a Faculty Senate
grant of the University of California, Santa Bar-
bara. H. Kloosterman (now deceased) assisted with
European sites. P. Bobek and H. S. Svitavska as-
sisted at the Stara Jimka site in the Czech Repub-
lic. A. L. Melott and B. C. Thomas are grateful for
support from National Aeronautics and Space Ad-
ministration Exobiology and Evolutionary Biology
grant NNX14AK22G. We acknowledge the valu-
able contributions of coauthor David Kimbel, who
passed away during the writing of the manuscript.
Finally, we thank the editor of the Journal of Ge-
ology, several anonymous reviewers, and reviewer
J. Hagstrum, all of whom generously and thought-
fully contributed to improving the manuscript.
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