JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E8, 10.1029/2001JE001532, 2002
Trajectories and distribution of material ejected from the Chicxulub
impact crater: Implications for postimpact wildfires
David A. Kring
Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Daniel D. Durda
Southwest Research Institute, Boulder, Colorado, USA
Received 26 June 2001; revised 18 January 2002; accepted 24 January 2002; published 30 August 2002.
[1] The trajectories of low- and high-energy ejecta from the Chicxulub impact crater have
been computed using a numerical code and compared with analyses of debris deposited in
K/T boundary sections. These calculations indicate that low-energy ejecta produces
centimeter- to meter-thick deposits within 4000 km of the point of impact, although its
azimuthal distribution depends on the incident angle of the projectile with the surface of
the Earth. The high-energy ejecta or vapor plume rises through the atmosphere and
isotropically expands at the top of the atmosphere. The velocity of this ejecta increases as
it rises. Approximately 12% of the high-energy ejecta is lost because it reaches escape
velocities, while 25% of the material reaccretes within 2 hours, 55% reaccretes within
8 hours, and 85% reaccretes within 72 hours. This ejecta is distributed globally, but it is
concentrated around the Chicxulub impact site and at the antipode (corresponding to
India and the Indian Ocean 65 million years ago); it is also slightly smeared in a
longitudinal direction because of Earth’s rotation. Because this debris does not reaccrete
all at once, the collisional shock heating of the atmosphere will be drawn out over a
period of a few days and will occur in pulses. This could be an important factor when
calculating postimpact chemical reactions in the stratosphere. The calculations also
indicate the ejecta would have ignited wildfires on several continents around the world,
although the distribution of those fires depends partly on the trajectory of the
INDEX TERMS: 1630 Global Change: Impact phenomena; 5420 Planetology: Solid Surface
projectile.
Planets: Impact phenomena (includes cratering); 6022 Planetology: Comets and Small Bodies: Impact
phenomena; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology
and Atmospheric Dynamics: Paleoclimatology; KEYWORDS: Fire, Cretaceous-Tertiary boundary, K/T
boundary, Chicxulub, impact cratering, impact ejecta
1. Introduction
[2] The Cretaceous-Tertiary (K/T) boundary sequence of
impact ejecta is composed of two macroscopic layers in
North America and a single layer in Europe, northern Africa,
New Zealand, the south Atlantic Ocean, and the Pacific
Ocean. Following Orth et al. [1987], we and others have
been interpreting the lower layer in and adjacent to North
America to be relatively low-energy proximal ejecta deposited from a ballistically emplaced ejecta curtain and the
upper layer to be higher energy ejecta carried in a vapor-rich
plume that rose far above Earth’s atmosphere and distributed
material globally [Hildebrand et al., 1990; Hildebrand and
Boynton, 1990; Vickery et al., 1992; Kring, 1993; Pollastro
and Bohor, 1993; Kring et al., 1994; Kring, 1995]. Based on
mineralogical and geochemical properties, it appears this
latter, upper layer in North America corresponds to the single
layer of impact ejecta found in Earth’s other hemisphere.
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2001JE001532$09.00
[3] This simple stratigraphic division has been useful
because it has facilitated the correlation of K/T boundary
sequences throughout the world, even before it was realized
that the impact occurred on the Yucatán Peninsula of Mexico
[Hildebrand et al., 1991]. However, there are subtle variations in the compositions of these layers that presumably
reflect details in the emplacement of debris from the ejecta
curtain and from the vapor plume. These details are becoming
more important as we try to resolve the climatic and environmental perturbations induced by the impact event, because
most of these effects are the direct or indirect result of the
ejected debris as it rains through the atmosphere [e.g., Toon et
al., 1997; Kring, 2000]. These effects include shock-heating
of the atmosphere as material reaccretes to Earth, affecting
nitrogen chemistry, producing nitric acid rain [Lewis et al.,
1982; Prinn and Fegley, 1987; Zahnle, 1990], and leading to
the spontaneous combustion of vegetation [Melosh et al.,
1990]; an enhancement of atmospheric opacity, causing surface temperatures to fall and preventing sunlight from reaching the surface where it is needed for photosynthesis [Alvarez
et al., 1980; Toon et al., 1982; Pollack et al., 1983; Covey et
6-1
6-2
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Table 1. Shocked Quartz in K/T Boundary Ejectaa
Proto-Caribbean
Haiti
North America
Berwind Canyon, CO
Clear Creek North, CO
Raton Basin, CO/NM
Brownie Butte, MT
Teapot Dome, WY
Dogie Creek, WY
Sussex, WY
Pyramid Butte, ND
Morgan Creek, Sask.
Frenchman River, Sask.
Red Deer Valley, Alta.
Atlantic Ocean
DSDP 603B
Pacific Ocean
LL44-GPC3
ODP 886
DSDP 465
DSDP 576
DSDP 577
DSDP 596
ODP 803
Europe, marine sediments
Caravaca, Spain
Agost, Spain
Stevns Klint, Denmark
Petriccio, Italy
Nye Klov, Denmark
Petriccio, Italy
Pontedazzo, Italy
Turkmenia
New Zealand,
marine sediments
Woodside Creek
Distance,
km
Mean Size,
mm
Maximum Size,
mm
Fluence,
g/cm3
Referencesb
860 – 910
150
1250
0.022
1, 5; 3
2200 – 2300
2200 – 2300
2200 – 2300
2980 – 3180
2640 – 2740
2640 – 2740
?
?
?
3350 – 3550
3840 – 3940
200
160
150
140
260
640
500
500
560
500
250
530
480
550
0.013
-
1
2; 3
2; 3
2; 3
2, 4; 7
2; 3
2; 3
2; 7
2; 7
2; 7
2; 7
2; 3
2540 – 2640
-
300
-
2; 3
5800
6450
7900
9100
9600
9700
11000
50
50
50
50
50
50
50
320
225
212
190
90
150
125
-
6;
6;
6;
6;
6;
6;
6;
6800 – 7000
?
7520 – 7720
8070 – 8270
?
8070 – 8270
8460 – 8660
?
90
90c
110c
120c
120c
-
190
150
180
190
120
150
-
2; 7
2;
2, 4; 7
2; 7
4;
2, 4; 7
4; 7
2; 7
13900 – 14100
-
110
-
4; 7
6
6
6
6
6
6
6
a
These data should be used with caution. Mean sizes and maximum grain dimensions can be influenced by laboratory
protocols and the amount of bulk sediment available for processing. For example, the material available from Pacific
Ocean sites is much less than that available from continental sites.
b
References for shocked quartz data are listed first and references for reconstructed distances from Chicxulub impact
site are listed after the semi-colon in the table. 1, Kring et al. [1994]; 2, Izett [1990]; 3, Kring [1995]; 4, Bohor and Izett
[1986]; 5, Hildebrand [1992]; 6, Bostwick and Kyte [1996]; 7, this study.
c
Considered tentative, as population of grains examined was small.
al., 1990, 1994]; enhancement of stratospheric sulfuric acid
aerosols which also depresses surface temperatures and
eventually leads to sulfuric acid rain [Brett, 1992; Sigurdsson
et al., 1992; Pope et al., 1994, 1997; Ivanov et al., 1996;
Kring et al., 1996; Pierazzo et al., 1998; Yang and Ahrens,
1998]; enhancement of stratospheric CO2 which could
potentially cause greenhouse warming effects [O’Keefe and
Ahrens, 1989; Takata and Ahrens, 1994; Pope et al., 1994],
although those effects may have been smaller than initially
estimated [Pierazzo et al., 1998].
[4] Preliminary studies of these environmental effects
have often assumed that the material entrained in the
vapor-rich plume was distributed homogeneously over the
surface of the globe. However, measurements of the debris
in K/T boundary sediments suggest otherwise. For example,
the fluence of Ir deposited in K/T boundary sediments
ranges from 3 to 340 ng/cm2 [Alvarez et al., 1984; Kyte et
al., 1985]. Unfortunately, it is difficult to determine any
geographic pattern from these analyses, because some of the
variation could have been produced by chemical fractionation in the vapor plume, secondary enrichment due to lateral
transport of sediments after deposition, secondary chemical
diffusion into stratigraphic layers above or below the
boundary, or drilling disturbances when sampling the material [e.g., Kyte et al., 1985; Evans et al., 1993].
[5] A more reliable indicator is the distribution of shockmetamorphosed quartz, because it is not susceptible to
secondary geochemical mobilization. Shocked quartz was
produced from target lithologies during the initial stages of
the impact event. It was then ejected from the crater, mixed
with the vaporized Ir-rich material from the projectile, and
distributed globally (or nearly so). Previous studies have
demonstrated that the fluence of shocked quartz in Haiti
(860 to 910 km SE of impact site 65 Ma; Table 1) is about
an order of magnitude greater than it is in the Raton Basin
of Colorado and New Mexico (2200 to 2300 km NW of
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
impact site 65 Ma) [Kring et al., 1994], and about two
orders of magnitude greater than the fluence in the Pacific
basin (5,800 to 11,000 km W, SW, and NW of the impact
site 65 Ma) [Bostwick and Kyte, 1996]. These data suggest
more vapor plume material may have been deposited near
the impact site than at greater distances. This, in turn,
implies that there may have been more atmospheric heating
over the proto-Caribbean Sea than over the Pacific Ocean.
The distribution and, thus, consequences of aerosols and
other climatically active chemical constituents that are
entrained in the vapor plume may be similarly affected.
[6] Several attempts have previously been made to model
the trajectories and distribution of ejected debris, but they
have produced contradictory results. Argyle [1989] calculated the ballistic trajectories of material in a hypothetical
vapor plume (i.e., before Chicxulub had been located),
assuming material was launched uniformly over all elevation
angles and azimuths. His results suggested the ejecta should
be thickest around the impact site and at the antipode. Melosh
et al. [1990] did a similar calculation and, while different in a
few details, also concluded that the ejecta should be concentrated near the impact site and at the antipode.
[7] In a more recent set of studies, the trajectories and
distribution of material seemed to grow less clear. Alvarez et
al. [1995] outlined a model in which material in the upper
layer (e.g., shocked quartz) is carried in a vapor-rich plume.
They assume, however, that the material in the vapor-rich
plume was only launched at angles >50 from the horizon.
In sharp contrast to the work of Argyle [1989] and Melosh et
al. [1990], their results suggested there should be a forbidden zone near the antipodal position in which no high
energy ejecta was deposited. On the other hand, Alvarez
[1996] described a model in which it is the low-energy
ejecta (e.g., without shocked quartz), transported as an
ejecta curtain, that cannot reach the antipodal region. In
that study, shocked quartz is carried with high-energy ejecta
in a vapor plume that rises above the atmosphere, launching
ejecta on ballistic trajectories. This vapor-rich plume of
material, unlike the vapor-rich plume of material of Alvarez
et al. [1995], could potentially reach the forbidden zone he
defined for the low-energy ejecta. The model suggests the
distribution of material could be variable across the surface
of the Earth, depending on the relative amount of material
launched at different angles and speeds, but no conclusions
are drawn regarding the distribution of this material. Following this work, it was unclear whether there was or was
not a concentration of material deposited at the antipode and
whether shocked quartz (which is deposited in the highenergy ejecta layer) is carried in a vapor-rich plume that
does or does not reach the antipode.
[8] To resolve these discrepancies and correlate the results
of these types of model calculations with field observations of
K/T boundary strata, we will first review the evidence in the
geologic record pertaining to the mechanisms of ejection,
transport, and deposition. We will then explore the assumptions in previous model calculations; use a computer simulation of the launch and deposition of both the ejecta curtain
and vapor plume material to determine when and where it reenters the Earth’s atmosphere; calculate the time needed for
different types of ejected components to settle through the
atmosphere and oceanic water columns; and compare and
contrast these model results with the observed distribution of
6-3
different types of impact ejecta found in K/T boundary sediments. The model is global, so only global predictions are
made, without reference to local turbidity in the atmosphere,
ocean currents, local topography, etc., so comparisons with
data from individual K/T boundary sites will be made within
this context. We will then explore the implications these
results have for the postimpact environment, particularly
those factors that may have been linked to the mass extinction
of Earth’s plant and animal species at the close of the
Mesozoic Era. Preliminary results of the more detailed
calculations presented in this paper were previously published
in abstract form [Durda et al., 1997; Kring and Durda, 2001].
2. Cretaceous-Tertiary Boundary Impact Ejecta
2.1. Ballistically Emplaced Ejecta Curtain
[9] The debris in K/T boundary sections that we will be
modeling has previously been linked to the Chicxulub
impact crater on the basis of stratigraphic, mineralogic,
geochemical, and isotopic criteria [e.g., Hildebrand et al.,
1991; Kring and Boynton, 1992; Swisher et al., 1992;
Sharpton et al., 1992; Krogh et al., 1993]. This debris
was excavated from a transient cavity that was approximately 100 km in diameter [Morgan et al., 1997; Kring,
1995]. Based on scaling relationships [e.g., Melosh, 1989],
the volume of material displaced in the transient cavity is
equivalent to (3p/10) (RTC)3 or 1 105 km3, where
RTC is the radius of the transient cavity. Again, based on
scaling relationships, we estimate that the fraction of the
displaced material that was ejected to produce the K/T
boundary deposits was >104 km3 (>25 trillion metric tons).
Hydrocode simulations of the impact event suggest that 2.6
to 8.4 103 km3 of this ejecta consisted of vapor and 2.9 to
4.9 104 km3 consisted of melt [Pierazzo et al., 1998],
while the remainder was solid debris.
[10] This material was ejected from the transient cavity as
a consequence of the interaction between a shock wave and
rarefaction wave, both of which propagated into the Earth’s
crust and were attenuated as their energy was diluted by an
expanding volume of rock. After both the shock and
rarefaction waves had passed, a large fraction of the material
they encountered was left with a residual particle velocity,
which produced a flow of material that began at the center
of the impact event, was initially directed downward and
then moved upward, producing a roughly bowl shaped
cavity of excavated material.
[11] This flow pattern has two consequences. First, the
flow lines are oblique to the concentric zones of target
material that has been shocked to different degrees
(Figure 1). Consequently, highly shocked and relatively
lightly shocked material was mixed within the ejected
material. Second, the ejected material was launched from
the crater at a typical angle of 45 (perhaps ranging from
35 to 50 [Cintala et al., 1999]) on a series of ballistic
trajectories that carried material from the impact site to
various distances from the crater. This material formed an
ejecta curtain (or inverted cone) of debris that radiated
rapidly from the impact crater [Oberbeck, 1975]. Material
at the top of the curtain approached the ground as the
curtain moved farther from the crater. Material at the base of
the curtain hit the ground at velocities that were comparable
to their launch velocities (hundreds of meters to kilometers
6-4
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 1. Schematic diagram showing the volume of excavated material and the paths of motion of
ejected material. The paths cross regions that have been shocked to different states and, thus, mix material
that has been shocked to different degrees.
per second). There was a tremendous horizontal component
to the depositional velocity, which caused the debris to flow
across the ground before coming to rest. Even millimetersized or smaller debris, which was subject to atmospheric
deceleration, had depositional velocities that caused it to
mix with surface sediments at the landing site (Figure 2)
[see also Pollastro and Bohor, 1993].
[12] In small impact events, the entire ballistic trajectory
occurs within Earth’s atmosphere. However, in the case of
large impact events like the K/T boundary Chicxulub
impact event, a large fraction of this ejected material is
launched above the Earth’s atmosphere. In general, debris
that is deposited ^400 km from where it was launched was
carried above the Earth’s atmosphere.
Figure 2. Photograph of a section of K/T boundary sediments deposited in the Raton Basin, Colorado.
The lowermost unit of the K/T boundary sequence is turbid, indicating mixing between the underlying
Cretaceous carbonaceous shale and the overlying layer of impact ejecta. This sample is from the Madrid
Road locality. The thickness of the specimen, including the Cretaceous shale below the K/T boundary
sediments, is 2.2 cm.
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
[13] Material ejected from regions close to the center of
the transient crater was launched first, had the highest
velocity, and traveled farther. Debris ejected from the outer
margin of the transient crater (which, in the case of the
Chicxulub crater, is 50 km from the center of the transient
crater) was launched later, had a lower velocity, and was
deposited much closer to the rim of the crater. There was
much more lower-velocity material than higher-velocity
material, so the thickness of the ejecta blanket is greater
near the crater and decreases in thickness in an exponential
pattern [McGetchin et al., 1973]. In Haiti, for example,
which was 860 to 910 km from the impact site, the thickness of the K/T boundary ejecta is approximately 46 cm,
while it is only 1 to 2 cm thick in the Raton Basin about
2200 to 2300 km from the impact site [Hildebrand and
Stansberry, 1992; Vickery et al., 1992; Kring, 1995].
[14] This particular layer of impact ejecta is found in
North America and other localities relatively close to the
Chicxulub impact site. Where best preserved, it is composed
of abundant spherules. These spherules have largely been
altered by diagenetic processes, but relict glass in the K/T
boundary deposit in Haiti indicate these spherules are
impact melt spherules [Sigurdsson et al., 1991; Kring and
Boynton, 1991; Izett, 1991]. This ‘‘spherule’’ population
actually consists of oblate spheroids, dumbbells, teardrops,
and irregularly shaped particles in addition to truly spherical
particles. In Haiti the melt spherules have lengths up to 10
mm. The truly spherical particles tend to have smaller
dimensions than the non-spherical forms and have a mean
diameter of 1.25 ± 0.49 mm [Izett, 1991]. They were
deposited in a marine limestone sequence and have largely
been altered to smectitic clays. With increasing distance, the
size of the spherules decrease. At Deep Sea Drilling Project
hole 603B off the coast of North Carolina (2540 to 2640 km
from the impact site), the spherules are typically 0.5 to 1
mm in diameter, but spherules up to 2 mm diameter have
been found [Klaver et al., 1987]. Like those in Haiti, these
spherules were deposited in a marine environment and
altered to smectitic clays. In the Raton Basin (2200 to
2300 km from the impact site), spherules with diameters
of 0.05 to 1 mm were deposited in shallow water continental
sites and altered to kaolinite [Pollastro and Pillmore, 1987;
cf. Izett, 1991] and goyazite [Pollastro and Bohor, 1993].
Similar diameter spherules are found in Wyoming (2640 to
2740 km from the impact site) where they are altered to
goyazite [Bohor et al., 1987]. They range in diameter from
0.04 to 1.4 mm and have a mean diameter of 0.90 ± 0.17
mm [Izett, 1991]. The diameters of the spherules are
consistent with a 1/Ra distribution, where R is the radial
distance from impact site and a is 2.0 to 2.5 [Vervack and
Melosh, 1992].
[15] The amount of impact melted material in the lower
layer, relative to the amount of solid ejecta, should also
increase with distance from the impact site [Vickery et al.,
1992]. Analyses of three samples of the Haitian sediments
indicates that at least 44%, 42%, and 42% are composed
of pseudomorphosed impact melt spherules, indicating that
at least 40% of the ejecta curtain deposited about 900 km
from the impact site was composed of molten material
(D. A. Kring, unpublished data, 1992). Sigurdsson et al.
[1991] estimated that about 50% of the Haitian deposit
may have once been glassy impact melt spherules. At the
6-5
more distant Dogie Creek and Teapot Dome, Wyoming,
sites, the goyazite spherules represent up to 30% and 10%,
respectively, of the material in the lower layer of ejecta
[Izett, 1991]. These values are less than the >40 and 50%
values determined at the Haiti site, opposite the trend
expected [Vickery et al., 1992], but this may be the result
of poor preservation.
2.2. Ballistically Distributed Vapor Plume Material
[16] The bulk of the impacting asteroid or comet, as well
as a substantial amount of material from the target lithologies, were vaporized and ejected in a plume of material that
rose far above the Earth’s atmosphere before reaccreting to
the Earth. The iridium anomaly in K/T boundary sediments
that originally led to the suggestion of an impact event was
produced when this projectile-rich plume of material was
deposited. The plume also entrained and deposited shocked
quartz and other minerals from the impact target lithologies
[e.g., Bohor et al., 1984, 1993], diamonds [Carlisle and
Braman, 1991; Gilmour et al., 1992; Hough et al., 1997],
microscopic melt spherules that may have condensed from
the vapor plume of mixed target and projectile material
[e.g., Smit and Klaver, 1981; Montanari et al., 1983; Smit
and Kyte, 1984; Montanari, 1991; Kyte and Bohor, 1995],
fullerenes [Heymann et al., 1994], and possibly amino acids
[Zhao and Bada, 1989]. These materials are also associated
with soot, which is believed to have been produced by
postimpact wildfires [e.g., Wolbach et al., 1985, 1988].
[17] This diverse assemblage of material, entrained in an
expanding vapor-rich plume and deposited globally, indicates that a heterogeneous mixture of material was ejected
from the impact site and that the heterogeneity was preserved as the vapor plume expanded and remixed with the
atmosphere. This heterogeneity is reflected in particles of
different sizes, different thermal stabilities, and different
chemistry. It is also clear from studies of the material in K/T
boundary deposits that the mineral and melt components
entrained in the vapor plume were not distributed homogeneously over the surface of the Earth, reflecting the mechanics of the vapor plume expansion process and factors that
are a function of each particle’s diameter. As discussed later
in the paper, these variations also influence the environmental effects of the impact event.
[18] Several of the components entrained in the vapor
plume are similar in size to the mineral fragments and glass
shards entrained in volcanic ash plumes. The spherules and
shocked quartz grains are the largest components. In Europe
and northern Africa spinel-bearing spherule diameters range
from 150 to 500 mm [Bohor and Betterton, 1990] and 30 to
300 mm [Robin et al., 1991]. In the Pacific the spinel-bearing
spherules are <200 mm in diameter, although other clinopyroxene-bearing spherules can be slightly larger [Kyte et al.,
1996]. (A few larger diagenetically altered spherules, up to
1.5 mm in diameter, have also been found in the Pacific [Kyte
et al., 1995, 1996]. These are interpreted to be similar to the
impact melt spherules in the lower boundary layer in continental sites in the United States, although the indivisible
nature of the sediments in the Pacific do not make it possible
to differentiate material carried in the lower-energy ejecta
curtain and higher-energy vapor plume. Likewise, it is not
clear in which regime a highly altered fragment of meteoritic
6-6
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 3. Distribution of (a) low-energy ejecta following a vertical impact event and (b) low-energy
ejecta following an oblique impact from the northwest to the southeast with an inclination of 25 above the
surface. We did not attempt to simulate the instabilities in the low-energy ejecta that can produce hummocky
distribution of material or rays. The paleogeography is adopted from the work of DeConto et al. [2000].
material [Kyte, 1998], which is possibly a macroscopic
remnant of the impacting asteroid or comet, was carried.)
[19] Shocked quartz grains range up to 1.25 mm in size,
although most are <300 mm in size and, in the Pacific
Ocean, are dominantly <50 mm in size [e.g., Bohor and
Izett, 1986; Kring et al., 1994; Bostwick and Kyte, 1996].
Among shocked quartz grains, there appears to be a
decrease in the largest grain at any site as a function of
increasing distance, although the modal size of the population of grains does not necessarily decrease as a function of
increasing distance (Table 1). However, this material makes
up only a small fraction of the boundary sediment. Less than
1% of the material in the layer of high-energy ejecta is
identifiable as shocked minerals, diamonds, spinel, and Ir
[Kring et al., 1994]. The original state of the remainder of
the material has, until recently, been unknown due to alteration processes, either because it was much finer grained
than the surviving components and/or had a chemical
composition or physical state (e.g., glass) that was susceptible to alteration. Recent work by Smit [1999], who found a
few relic textures in outcrops in Spain and Russia, suggests
that the bulk of the high-energy ejecta was composed of
glassy to microcrystalline spherules with an average diameter of 250 mm.
[20] There are several different types of spherules, some
of which probably reflect real differences at the time of
deposition, whereas others are probably produced by diagenesis. Smit [1999] suggests that the differences between
the goethite spherules at Stevns Klint, Denmark, As-rich
pyrite spherules at Zumaya, K-feldspar spherules at Caravaca, Spain, and the glauconite spherules at Fonte d’Olio,
Italy are largely diagenetic. However, Smit [1999] also
points out that several types of spherules can be found in
the same site, in which case he suggests they probably reflect
original differences in spherule compositions. This latter
group of spherules includes Ni-rich magnesioferrite bearing
spherules with a smectitic matrix and what look like ghosts
of olivine crystals, K-feldspar spherules with what look like
skeletal pseudomorphs after clinopyroxene and plagioclase,
and glauconitic spherules with skeletal K-feldspar. These
differences were probably a function of the chemical composition of the portion of the plume in which they were
generated. Compositional heterogeneities in the plume material (or in the mixture of plume material and atmosphere
during reaccretion—see discussion below) are also suggested by the range of spinel group minerals in the spherules.
[21] Some of the smallest mineralogic components in the
ejecta are diamonds. Hough et al. [1997] found diamonds
up to 30 mm in size in K/T boundary sediments at Arroyo El
Mimbral and Arroyo El Peñon (Mexico). Polycrystalline
grain aggregates range from 1 to 30 mm and individual
crystals in these aggregates are 0.1 to 1 mm in size. Gilmour
et al. [1992] found diamonds from Brownie Butte and
Berwind Canyon (United States) are 6 nm in size. The
carbon isotopic signature indicates they are not extraterrestrial, but were rather created in the impact event from a
mixture of carbon sources and condensation from the vapor
plume [Gilmour et al., 1992; Hough et al., 1997].
[22] Hydrocode simulations [Pierazzo et al., 1998] indicate the material carried in this plume is generated within the
first 20 seconds following the impact event. The vapor plume
is characterized by a high-temperature plasma with temperatures in its core that are initially in excess of 10,000 K, but
which cooled to a few hundred to about one thousand Kelvins
in about a minute [Pierazzo et al., 1998]. Material at the
edges of the plume are cooler, but are still in excess of 1000 K
during the first minute following impact. Thermal heterogeneities in the plume and/or brief interaction times help
allowed quartz, chromite, and zircon to survive in the plume.
These grains also survived reentry when the plume material
shock-heated the Earth’s atmosphere [Zahnle, 1990; Melosh
et al., 1990], in part because those temperatures were not
sufficient to melt or vaporize these phases. In addition, many
of the grains were too small to be significantly heated by
friction with the Earth’s atmosphere. Studies of accreting
interplanetary dust particles suggest that very few particles
<50 mm are melted and even particles as large as 200 mm can
survive atmospheric reentry [Brownlee, 1985]. If there were
regions of the atmosphere where grains were destroyed, they
may have been filled by lateral mixing of shocked quartz and
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
6-7
Figure 4. Distribution of high-energy ejecta following a vertical impact with ev = 5 (a) and ev = 2
(b) and of high-energy ejecta following an oblique impact with a trajectory from the northwest to the
southeast at an angle of 25 above the surface with ev = 5 (c) and ev = 2 (d). The paleogeography is
adopted from the work of DeConto et al. [2000].
other particles while the grains settled to the ground (or
seafloor).
[23] Model calculations by Pierazzo et al. [1998] using
detailed numerical techniques indicate that vaporized sediments and water are entrained in the same vapor plume
(although not well-mixed), not in a second vapor plume as
suggested by Alvarez et al. [1995]. However, the simulations by Pierazzo et al. [1998] do not follow the trajectory
of material far beyond the first minute after impact. To
follow the material and map out its distribution, we employ
a second set of computer simulations.
3. Outline of Computer Simulations
[24] We reexamined the launch and deposition of both
low- and high-energy ejecta with new computer simulations.
We began with a code designed to numerically model the
ballistic trajectories and reaccretion of ejecta produced by
impacts on rapidly rotating, irregularly shaped asteroids
[Geissler et al., 1996] and modified it to reflect Earth’s
gravity, shape, and rotation. Ejecta was launched from the
paleogeographic point of Chicxulub on the Earth’s surface,
25.56 N and 62.26 W.
[25] In the simulations, low-energy ejecta forms a cone
with a concentrated mass distribution angled 45 from the
surface of the Earth or, in the case of an oblique impact, a cone
tilted 15 from vertical in the downrange direction [Gault
and Wedekind, 1978]. Ejecta speeds are drawn at random
according to a power law distribution between Vo and Vesc.
The exponent of the speed distribution for this simulation is
ev = 1.2, which reproduces a low-energy ejecta blanket with a
thickness that matches that observed for the Chicxulub crater
[Kring, 1995] and that varies with the distance from the point
of impact and azimuthal direction according to functions that
6-8
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 4. (continued)
depend on whether the impact was vertical or oblique
[McGetchin et al., 1973; Gault and Wedekind, 1978].
[26] High-energy ejecta rises through the atmosphere in
a column which hydrocode calculations [Pierazzo et al.,
1998] indicate is ]2 times the diameter of the transient
crater. The particle speeds in this vapor plume are initially
low but rapidly accelerate as the plume rises through the
atmosphere and isotropically expands at the top of the
atmosphere. As in the low-energy case, ejecta speeds are
randomly distributed between Vo = 2 km/s and Vesc =
11.2 km/s according to power law distributions, although
for the high-energy case there exists a great deal of
uncertainty as to the appropriate value for the speed
exponent, ev. Until refinements to existing hydrocode
models lead to a clearer understanding of the velocity
evolution of plume material, we chose to examine a range
of possibilities with two different speed distributions with
exponents ev = 2 and 5 (a speed distribution with
mass distributed uniformly across all speeds [Melosh et
al., 1990] has ev = 0).
[27] We also considered impact events with different
incident angles, because the trajectory of the projectile that
produced the Chicxulub crater is still uncertain. Several
different trajectories have previously been proposed. Kyte
and Bostwick [1995] suggested the trajectory was east to
west at an incident angle <45 to explain variations in spinel
chemistry in K/T boundary sediments. It is argued the
projectile was moving from the southeast to the northwest,
impacting with an incident angle of 20 to 30 above the
horizon, based on gravity data which have been interpreted
to indicate asymmetries in the structural uplift, in the
position of the structural uplift within the crater, and other
geophysical features [Schultz and D’Hondt, 1996]. In
another case, it is argued the projectile was moving from
the southwest to the northeast, impacting with an incident
angle of 60, based on gravity and seismic data which have
been interpreted to indicate asymmetries in the structural
uplift, collapsed transient cavity wall, and peak ring; possible asymmetry in the crater rim uplift; and thrusting in the
zone of collapse modification within the crater [Hildebrand
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
6-9
Figure 5. Representative ballistic trajectories of high-energy ejecta carried in a vapor plume which
expands isotropically above the atmosphere. At the scale needed to show these trajectories, the Earth (and
the Chicxulub impact site) are essentially a dot at the intersection of the trajectories.
et al., 1998]. The uncertainty of these determinations is
perhaps best indicated by the fact that both of the latter two
studies noted the asymmetry of the structural uplift and used
it to infer two completely different trajectories. There are
other issues still unresolved. For example, with regard to the
20 to 30 trajectory from the southeast, it is no longer clear
there is a relationship between the position of the structural
uplift and trajectory [Ekholm and Melosh, 2001]. With
regard to the 60 trajectory from the southwest, crater
asymmetries of the type described are generally not expected
for incident angles that high, only at much lower incident
angles [e.g., Gault and Wedekind, 1978].
[28] While it is unclear which of the proposed trajectories
(if any) is correct, we agree that a non-vertical impact is likely.
As demonstrated by Shoemaker [1962], the most probable
incident angle of impact is 45. However, this impact angle
will have little effect on the crater morphology or the
distribution of ejecta. It is only when impact angles get very
low (]30) that a crater and/or its ejecta is modified to
produce asymmetries [Gault and Wedekind, 1978]. For that
reason, our calculation of a vertical impact is sufficient to
illustrate the distribution of low- and high-energy ejecta for
the most probable trajectory and trajectories with steeper
incident angles. To illustrate how a shallower incident angle
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KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 6. Number of particles reaccreting to the top of the atmosphere as a function of time at several
geographic localities: (a) Colorado, (b) central Europe, (c) India, and (d) Amazonia. The total number of
particles in this calculation is 100,000 and ev = 5.
can affect ejecta, we have chosen to simulate a northwest to
southeast trajectory, with an incident angle of 25. In an
oblique impact of this type, there are three components to the
vapor-rich ejecta [Schultz, 1996]. A small fraction of the
material is initially ‘‘channeled’’ along the corridor cut
through the atmosphere by the incoming projectile and then
rises upwards. However, most of the material rises upward
(similar to a the case of a vertical impact) or is ejected
downrange. The distribution of material between the rising
and downrange plumes of material is uncertain. It was not
determined in laboratory experiments [Schultz, 1996], nor in
3D hydrocode simulations because they have not been run
sufficiently long yet. Until these simulations have been run
long enough, we will model the distribution of vapor-rich
ejecta over the full range from a completely vertical rising
plume of material to a completely downrange plume of
material. In the case of the latter, when the plume reaches
the top of the atmosphere it expands non-isotropically with a
particle trajectory density weighted toward the entry direction
by a cos2 function. Particle speeds are distributed as described
above.
4. Model Trajectories
4.1. Distribution of Debris From the Ballistically
Emplaced Ejecta Curtain
[29] In Figure 3a, the distribution of low-energy ejecta
following a vertical impact is illustrated based on 20,000
tracer particles. The low-energy ejecta is symmetrically
distributed around the impact site. It is concentrated within
1000 km of Chicxulub, but forms a blanket of material
extending 4000 km to 45N and 0S. This simulation
suggests debris should have covered a large part of North
America (what is now Belize, Guatemala, Mexico, the
United States, and southern Canada), but only the northernmost edge of South America. Debris is also expected
throughout the proto-Caribbean Sea and Gulf of Mexico
and in portions of the Atlantic Ocean (along what is now the
U.S. coast) and Pacific Ocean (along Mexico’s coast, which
has since been largely destroyed by subduction). The
amount of material in the ejecta blanket decreases while
the distance from the point of impact increases. Any of the
debris that lands >400 km from the crater reached heights
above the atmosphere before falling back to the ground.
[30] In Figure 3b, the distribution of low-energy ejecta is
shown for a 25 oblique impact from the northwest. In this
case, the low-energy ejecta blanket is slightly asymmetrical,
but it is, again, concentrated within 1000 km of Chicxulub. Less material covers western North America and the
Pacific Ocean than in the vertical case, while more lands in
the Atlantic Ocean.
4.2. Distribution of Debris From the Impact Vapor
Plume
[31] In Figures 4a and 4b, the distribution of high-energy
ejecta following a vertical impact is illustrated, for particle
speed distributions with ev = 2 and 5 based on the
reaccretion locations of 100,000 tracer particles. The ejecta
is distributed globally, although it is concentrated to varying
degrees near the antipode, similar to the results of Argyle
[1989], with some smearing to the west caused by the
rotation of the Earth. Unlike Argyle [1989] (and also Melosh
et al. [1990]), where the vapor plume speeds were uniformly distributed in mass, emphasizing the number of lowspeed particles landing near the impact site, our results, with
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
ejection speeds weighted toward Vesc, indicate a less pronounced concentration of plume material near the source
crater.
[32] These results show that while the distribution of
particle speeds affects the spatial distribution of reaccreted
ejecta in a quantitative way, the qualitative aspects of the
antipode concentration and rotational smearing remain the
same. Potentially, detailed observations of the fluence of
shocked quartz and iridium as a function of geographic
location may allow future researchers to discern between
various plume particle speed distributions.
[33] In Figures 4c and 4d, the distribution of high-energy
ejecta is shown for a 25 oblique impact from the northwest
in which all of the vapor-rich material is ejected downrange.
As in the vertical impact case, high-energy ejecta is distributed globally and concentrated near the antipode,
although there are subtle differences when the results are
compared with those from a vertical impact. Plume material
is slightly concentrated in the downrange path of the ejected
material, resulting in a secondary concentration of particles
to the southeast of Chicxulub. As described in Section 3, the
distribution of vapor-rich material between vertically rising
and downrange carried plumes is uncertain in an oblique
impact. So the real distribution of material in an oblique
impact likely lies between 4a,b and 4c,d.
[34] The concentration of high-energy ejecta near the
impact site and near the antipode was seen in several other
simulations involving both vertical impacts and oblique
impacts with a variety of trajectories, so it appears to be a
very robust result. In most cases, most of the high-energy
ejecta stays within 50,000 km of Earth, with several percent
reaching 100,000 km or more, before reentering the atmosphere. That is, some material travels along trajectories that
carry it nearly halfway to the Moon (Figure 5). (The
gravitational influence of the Moon was included in a test
case and, although the trajectories of some particles
launched to great distances from Earth were affected
slightly by the Moon’s gravity, the overall final distribution
of material reaccreted around the Earth was indistinguishable from a model without the Moon. An additional
perturbation on the smallest particles lofted farthest from
the Earth is the force of radiation pressure from sunlight.
This effect has not been included here, although its influence should be investigated in future models.)
[35] Approximately 25% of the material reaccretes within
2 hours, 55% within 8 hours, and 85% within 72 hours.
However, the distribution of arrival times for material at any
particular site around the globe is variable, because of
different distances to those sites and the velocity distribution
of material traveling to those sites. To illustrate this point,
we have plotted the number of particles reaccreting to
several parts of the globe within the first 200 minutes after
a vertical impact with a speed exponent of 5 (Figures 6a –
6d). These include the western interior of North America
(represented by Colorado), central Europe, the antipodal
region (represented by India), and a point in the southern
hemisphere (represented by Amazonia). Material first
arrives at these sites in order of their distance from the
crater. Also, in the case of Colorado, a large amount of
material arrives in a substantial pulse. Near the antipode,
debris also arrives in a concentrated fashion, but the pattern
is more spread out over time. In the case of Amazonia, there
6 - 11
Figure 7. High-energy (vapor plume) material accreting to
the top of the atmosphere as a function of longitude and
time in the latitude between 20 and 30 (a), which is
the latitude of the antipode, and 5 to 5 (b), which is the
equatorial region. At any particular longitude, the material
reaccretes continuously, although there are distinct pulses of
more intense activity. These pulses are particularly evident
at the antipode (a) and much less so in the equatorial region
(b). Most of the material reaccretes within 4 days.
is a small pulse of material, but it largely arrives with a
relatively constant flux.
[36] Over longer periods of time after the impact event, as
ejecta continues to reaccrete to the Earth, one sees there are
repeated pulses (Figure 7). These pulses are generated when
a location on the Earth rotates beneath the concentrated
stream of material produced by the nearly instantaneous
ejection of material from Chicxulub. As discussed in section
5.1, this pulsing effect influenced the environmental effects
of the impact event.
[37] After all the ejecta has reaccreted, the overall distribution of material (or fluence at any particular point) forms
an uneven pattern around the globe (Figure 4). There is a
concentration of material above the impact site and in the
antipodal region. Because Earth is rotating beneath the
stream of debris between the time the ejecta is launched
and it reaccretes, the material in the antipodal region is
smeared out longitudinally. A small fraction (12%) of
material ejected from Chicxulub escapes Earth, even when
we make the very conservative assumption that the speeds of
6 - 12
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 8. A contour map showing the distribution of power delivered to the atmosphere by re-entering
high-energy ejecta when ev = 5. The power delivered in some areas of the atmosphere is in excess of
100 kW/m2, with a peak of 350 kW/m2.
materials in the vapor plume are 11.2 km/s (Earth’s escape
speed). This is due to the effect of Earth’s rotation, which
adds a V of 0.4– 0.5 km/s to material launched in the same
direction as Earth’s rotation.
5. Sedimentation of Debris
[38] The above model illustrates how ejecta was distributed when it reaccreted to the top of the atmosphere. From
that point, the material collides with the atmosphere, probably reacted with the atmosphere, and settled to the ground
or seafloor. The initial stages of this sedimentation process
are violent, as 1013 metric tons of material slams into the
atmosphere at hypervelocities and deposits 4 1023 J to 1 1024 J of energy (or 10 billion times more energy than that in
either the Nagasaki or Hiroshima nuclear explosions). The
shock event produced when the two gas-rich reservoirs
collide and subsequent cooling of the gas will determine
the molecular nature of gas species in the atmosphere and the
compositions of any liquid or solid condensates. Many of the
condensates produced in the expanding vapor plume may
have been remelted in this collisional event and had to
undergo a new generation of quenching and/or crystallization after the collision and partial to complete mixing with
the atmosphere occurred. Mixing with the oxygen-rich
terrestrial atmosphere may be the reason why some of the
spherule components, like spinel, are so oxidized [Kring et
al., 1996]. Spinel grains in ejecta deposits, thus, are likely to
reflect postimpact conditions in the stratosphere rather than
in the rising vapor plume [cf. Ebel and Grossman, 1999;
Siret et al., 2000]. One estimate, which is based on the
compositions of spinel-bearing spherules, suggests local
temperatures exceeded 2300C [Kyte and Bohor, 1995].
5.1. Sedimentation of Impact Ejecta Through the
Atmosphere and Thermal Effects
[39] The interaction of the re-entering ejecta and the
atmosphere dramatically influenced the environmental
effects of the impact event. The re-entering debris carries
climatically active gases, causes chemical reactions that
produce additional climatically active gases, deposits dust
which affects the passage of sunlight and catalyzed chemical reactions, and can substantially heat the atmosphere.
This latter effect has previously been suggested as the cause
for postimpact wildfires [Melosh et al., 1990]. We have reevaluated that suggestion in light of our new model results,
which differ from those of Melosh et al. in that our model
includes the effects of the rotation of the Earth, assumes a
slightly different ejecta speed distribution, and allows evaluation of the heating effects of re-entering debris at specific
locations rather than azimuthal zones at various distances
from the impact site.
[40] Using our model results, we calculated the distribution of mass and power delivered to the atmosphere as a
function of time around the globe. We assigned mass to the
particles in the model simulations based on previous hydrocode simulations that explored the size of the crater as a
function of different impact parameters [Pierazzo et al.,
1998]. Three hydrocode simulations with different impact
parameters produced suitably sized craters that fit the
parameters of the Chicxulub crater. Using these hydrocode
results, we calculate that the ejected mass entrained in the
vapor plume ranged from 1 1016 kg to 2.5 1016 kg.
Integrating this mass with the distribution of particles in our
model calculations, we then calculated the power deposited
in the atmosphere. Because the surface mass density of
material delivered to different parts of the world varies by
up to a factor of 15 and because the velocity of that
reaccreting material also varies between 3 and 11 km/s,
the power is variable. To illustrate the variation, a contour
map showing the power delivered around the world is shown
(Figure 8) and we have plotted the power delivered in 5
specific locations around the world as a function of time
(Figures 9 and 10, for ev = 5 and 2, respectively). The
power delivered is greatest near the impact site and at the
antipode.
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
6 - 13
Figure 9. The power delivered to the atmosphere above specific geographic locations as a function of
time: (a) the antipode, (b) the antipode at a different scale, (c) India, (d) Colorado, (e) Amazonia, and (f )
the South Pole. The diagrams (a, c –f ) are plotted to the same scale. This diagram assumes a velocity
distribution in which the exponent is 5.
[41] The total amount of power distributed around the
world is independent of the details of the velocity distribution, but the magnitude of the power delivered to any
particular point on the Earth’s surface ( particularly the
antipode), relative to the global average, can be greatly
affected by the velocity distribution assumed in the model.
However, in all cases, the power deposited in the atmosphere and the subsequent heating is significant.
[42] As discussed by Melosh et al. [1990], about half of the
power deposited is radiated into space and another fraction is
absorbed in the atmosphere by water vapor and carbon
dioxide. In accord with that study, we therefore assume about
1/3 of the total radiation reaches the surface and any power
delivered in excess of 12.5 kW/m2 for >20 minutes can ignite
vegetation, even if it is initially wet. This implies that the
vegetation in India, Colorado, Amazonia, and nearby areas
were consumed by fire. Thus, fires occurred at several
locations worldwide and were not limited to the region
immediately around the impact site [cf. Shuvalov, 2000]. A
global map showing the areas where the minimum surface
conditions of 12.5 kW/m2 for >20 min occurred is shown in
Figure 11.
[43] The amount of power delivered to different parts of
the atmosphere is sensitive to the velocity distribution of
material in the vapor plume. As outlined above, we considered two cases where the exponent controlling the velocity
6 - 14
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 10. The power delivered to the atmosphere above specific geographic locations as a function of
time: (a) the antipode, (b) India, (c) Colorado, (d) Amazonia, and (e) the South Pole. This diagram
assumes a velocity distribution in which the exponent is 2.
distribution of ejecta speeds was 2 or 5. The results
(Figures 9 and 10) suggest that in the 2 case the heating in
Colorado is much greater than in India, while in the 5 case
the heating in India is much greater than that over Colorado.
This is because in the 2 case the ejecta is slower and does
not travel as far.
[44] An important result of this work is the pulsing effect
created in the degree of atmospheric heating at some points
around the globe, particularly near the antipode. The ejected
material does not reaccrete to the top of the atmosphere in a
single event, nor does it reaccrete homogeneously over
time. The impact event and the ejection of debris occurs
at a point in time and inertial space; that material exists
largely as a dense clump as it rises in the vapor-rich plume,
although the clump of material spreads out somewhat due to
the distribution of particle velocities. As this plume material
travels in ballistic trajectories around the planet, the simple
geometric effect of ‘polar crowding’ concentrates the reaccretion of material at the point antipodal to the location, in
inertial space, of the impact. As the Earth rotates below the
continually reaccreting material, however, locations near the
antipode see a daily ‘pulse’ in the amount of reaccreted
material and atmospheric heating. This means that the
atmosphere near the antipode is not heated in a single
event, but in about 3 different, gradually diminishing,
events over a 3 day period (e.g., Figure 7a). A similar
though somewhat less pronounced effect occurs near the
impact site itself.
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 11. Global maps showing the locations (orange) where the power radiated to the ground is
sufficient (12.5 kW/m2 for >20 min) to ignite vegetation, even if it was initially wet, for ev = 5: (a)
vertical impact, (b – e) oblique impact from northwest to southeast with 25% (b), 50% (c), 75% (d), and
100% (e) of ejecta distributed in downrange plume. A similar set of maps for ev = 2 is shown in (f )
through ( j).
6 - 15
6 - 16
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
Figure 11. (continued)
[45] After the material was decelerated in the atmosphere,
solid particulates (shocked quartz, shocked zircon, shocked
chromite, diamonds, liquid condensates, etc.) settled to the
surface of the Earth. The velocity with which they settle will
depend strongly on particle size and slightly on particle
shape. If we assume the particles are spherical, have a
density of 3 g/cm3, and fall in a Stokes regime, then settling
times will be more than a year for 1 mm diameter particles
and an hour for millimeter diameter particles from an
altitude of 85 km (Figure 12). These are maximum times
scales, because it does not take into account the effects of
coagulation of material into larger, faster-falling particles. In
addition, once particles settle into the troposphere, they can
be rained out relatively quickly. In the modern atmosphere,
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
6 - 17
Figure 12. Time needed for particles of different dimensions to settle through the atmosphere (H. J.
Melosh, personal communication). A density of 3 g/cm3 and an initial altitude of 85 km were assumed.
The dimensions of several types of particles (soot, diamond, shocked quartz, and spinel-bearing
spherules) found in K/T boundary sediments are indicated along the bottom of the diagram. Quartz and
spherules settle out of the atmosphere within hours to a few days. It is not clear how high soot from fires
will rise in the atmosphere, so sedimentation times in excess of a year are an upper limit.
this is at a height of 15 km above the surface, which may
or may not have been the same 65 million years ago,
depending, in part, on the effects of the impact event on
the atmosphere.
[46] As Section 2.2 outlined, there is a range of particle
sizes in the high-energy ejecta. Shocked mineral grains from
the target range from <30 to 1250 mm, although most are
<300 mm in size. Spherules, which appear to be condensates, range from 30 to 500 mm. As illustrated in Figure
12, these particles took about an hour to 10 days to settle
to the surface of the Earth. However, there was additional
material in the form of diamonds, soot, and possibly some
kind of fine-grained glassy component. The amount of
material in the submicron fraction is particularly important,
because it can cause significant darkening at the surface of
the Earth. As indicated in Figure 12, the diamonds can
remain in the stratosphere for periods approaching a few
years. Calculations exploring the consequences of such finegrained material [Toon et al., 1982; Covey et al., 1990],
suggest a significant amount of submicron dust could have
made it too dark to see for 1 to 6 months and too dark for
photosynthesis for 2 months to 1 year. Shorter timescales
will occur if coagulation of particles in the atmosphere is
significant. Sedimentation timescales on the order of years
is consistent with a <60 ± 12 yr estimate based on extra-
terrestrial 3He concentrations in the marine sedimentary
sequences in Italy [Mukhopadhyay et al., 2001].
[47] Recently, some paleontologic evidence suggests the
material took at least 3 months to settle to the ground. Sweet
and Lerbekmo [1999] found that a fern spore spike, which is a
measure of postimpact recovery [e.g., Tschudy et al., 1984],
is distributed throughout the upper layer of K/T boundary
ejecta. Taken at face value, this implies the dust settled out of
the atmosphere slow enough for ferns to have recolonized the
surface, which Art Sweet (personal communication, 1999)
estimates requires at least 3 months. This implies that
significant submicron component in the debris deposited in
the atmosphere and that the amount of sunlight reaching the
Earth’s surface was reduced, adding to the effect of sulphate
aerosols [e.g., Pierazzo et al., 1998; Pierazzo, 2001].
[48] Soot produced by the wildfires generated in the
areas identified in Figure 11 may have taken a similarly
long time to settle to the surface because of their small
dimensions (0.1 mm spherules or 1 mm clusters of
spherules [Wolbach et al., 1990]). It has been observed in
modern boreal forest fires that the altitude particles reach
(Hi, m) is a function of the intensity of the fire (I, kW/m):
Hi = 0.23 I [Lavoué et al., 2000]. In relatively low intensity
ground and surface fires, altitudes may be limited to a few
kilometers. However, intense fires, like crown fires, can
6 - 18
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
generate plumes that rise to the tropopause or into the lower
stratosphere. Recent satellite observations show boreal
forest fires, which are a mix of surface and crown fires
[Lavoué et al., 2000], are injecting particulates 5 km above
the tropopause [Fromm et al., 2000]. Postimpact wildfires
were probably comparable to or more intense than the
hottest modern fires, so material should have risen at least
to the tropopause and probably into the stratosphere,
particularly if the tropopause was temporarily lost because
of the postimpact atmospheric disturbance. Consequently,
timescales for settling of soot may be months to times
comparable to that of the diamonds, depending partly on
the amount of turbulence in the postimpact atmosphere.
Measurements of smoke aerosols lofted into the stratosphere from modern fires indicate slow decay over a 2
month interval [Fromm et al., 2000]. This is significant,
because soot or smoke aerosols are very good absorbers of
sunlight, causing a radiative effect of at least several tens of
W/m2 [Li and Kou, 1998], compounding the effects of the
fine-grain ejecta and sulphate aerosols. Also, since the soot
was carried to high altitudes in the troposphere where it
could be distributed longitudinally, and probably into the
stratosphere where it could be distributed globally, its
distribution in K/T boundary sediments cannot be used to
map where fires occurred.
5.2. Sedimentation of Impact Ejecta Through Water
Columns
[49] Since a large fraction of the Earth’s surface is
covered by water, most of the debris had to settle through
a water column after settling through the atmosphere, either
in a freshwater system or a marine system. In the case of
oceanic regions where the water depth may be several
kilometers, these settling times were sometimes substantial.
[50] Settling velocities are a function of particle density,
particle shape, water salinity, and water temperature. For
example, particles fall faster through warm water than cold
water. In a marine system (34% salinity), a 1 mm particle
will fall 10% faster through 30C than through 0C water
and a 100 mm particle will fall 90% faster through 30C than
through 0C water [Gibbs et al., 1971]. Particles of any size
fall 5.5% slower through marine water (34% salinity) than
through fresh water [Gibbs et al., 1971]. Based on the
experiments of Gibbs et al. [1971], we calculated the
settling velocities of particles deposited from the highenergy vapor-rich plume. We assumed an average particle
density of 2.65 g/cm3, since the debris consisted largely of
shocked quartz and glassy mafic to ultramafic spherules.
The largest amount of experimental data is also for particles
with this density, rather than the 3 g/cm3 used in section
5.1. As a measure of the uncertainty associated with this
assumption, 1 mm to 100 mm particles will fall 10 to 20%
faster if their densities are 3 g/cm3 rather than 2.65 g/cm3
and 20 to 30% faster if their densities are 3 g/cm3 rather
than 2.5 g/cm3.
[51] Paleotemperatures for these systems, particularly the
marine system, vary with latitude and depth and are also
highly uncertain [Barrera, 1994; Barrera et al., 1987;
Huber et al., 1995; Wilson and Opdyke, 1996; Mitchell
et al., 1997]. For our purposes, we have assumed an
average temperature of our marine system to be 15C,
which is an approximate mean between expected surface
Figure 13. Time needed for particles to settle through (a)
freshwater and (b) marine environments. The curves are for
different size particles (from top to bottom): 1, 10, 20, 50,
100, 200, 500, and 1000 mm. In the case of freshwater a
temperature of 22C was assumed. In the case of the marine
system an average salinity of 34 % and average temperature
of 15C were assumed. The horizontal axis of (a) represents
depths from 0 to 100 m, while that of (b) represents depths
from 0 to 5000 m.
temperatures (19 to 32C) in the tropics, bottom
water temperatures in the tropics (9 to 14C), and water
temperatures in the high latitudes (4 to 12.5C). For our
freshwater systems, we have assumed 22C, approximately
room temperature.
[52] Our results are also for spherical grains, because so
much of the debris appears to have been spherical grains.
Solid quartz grains with the same maximum dimension will
fall only 25% slower [Fisher, 1965]. If there were highly
porous or vesicular, like bubble-wall volcanic shards, then
the settling velocities could be several times slower [Fisher,
1965; Oehmig and Wallrabe-Adams, 1993], but this is not
likely based on the spherule-rich character of the relatively
unaltered deposits described by Smit [1999].
[53] The settling times for different size ejecta particles as
a function of water depth are plotted for a marine system
and for a freshwater system (Figure 13). For reference, we
have also indicated the paleowater depths represented by
several well-known K/T boundary sites. The 250 mm
particles at Caravaca, (Spain) which represents 800 to
1000 m water depth [Smit, 1999], took 12 hours to fall to
the seafloor. The 200 mm shocked quartz at Beloc (Haiti)
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
and the 500 mm particles at Gubbio (Italy), both of which
represent 2 km water depth [Smit, 1999], took 25 and 8
hours, respectively, to fall to the seafloor. And the 100 mm
spherules at the GPC-3 site (Pacific Ocean), which represents 5 km water depth [Smit, 1999], took nearly 9 days
to fall to the seafloor. These are representative settling
times. Each of these sites are composed of ejecta particles
of different sizes, so the settling occurred over diverse
timescales and promoted sorting, which can be seen in
several stratigraphic sections.
6. Discussion and Implications for Postimpact
Environmental Effects
[54] As suspected from field evidence, our model results
indicate high-energy ejecta was deposited globally, albeit
unevenly. As first suggested by Argyle [1989], there is a
concentration of debris deposited at the impact antipode,
which, in the case of the Chicxulub impact event 65
million years ago, is over the Indian sub-continent and
the proto-Indian Ocean. The forbidden zone of Alvarez et
al. [1995] in which no debris is deposited in that part of the
world, is an artifact of the assumed elevation angles and
choice of maximum ejection speeds less than Vescape; i.e.,
they did not let the vapor plume expand at the top of the
atmosphere.
[55] These heterogeneities had a dramatic influence on
the ignition of postimpact wildfires. While fires are generated near the impact site and the antipode regardless of
trajectory, the distribution of fires elsewhere in the world is
dependent on the trajectory and, thus, distribution of
resulting impact ejecta. If the projectile trajectory was
vertical (Figures 11a and 11f ), then fires were ignited in
southern North America, central and northwestern South
America, southern Africa, India, and potentially southern
Asia, Australia, and parts of Antarctica. Fires could have
spread to other parts of these continents, but it appears
Europe would be spared in this case. In contrast, if the
projectile was from the northwest, which would have
ejected a lot more debris to the southeast of Chicxulub
and into the equatorial region, then fires could have been
ignited throughout the equatorial region and southern hemisphere, while leaving the northern hemisphere largely
unscathed (Figures 11e and 11j). These outcomes do not
depend strongly on the velocity distribution in the expanding vapor plume (compare Figures 11a – 11e with Figures
11f – 11j) although the exact areas on the continents where
fires are initially ignited might vary.
[56] Interestingly, the different distribution of these
effects can potentially be used to determine the trajectory
of the projectile which, at the moment, is still uncertain. If
charcoal is found at K/T boundary sites throughout the
southern hemisphere and not in Canada, for example, then a
trajectory from the southwest, south, or southeast rather
than other directions (including vertical) would be indicated. On the other hand, if charcoal is found in European
K/T boundary sections (assuming a nearshore or continental
site is located), then a trajectory from the northeast is more
likely. We emphasize that charcoal rather than soot is the
better indicator of the location of wildfire, because soot
becomes airborne and will be distributed in latitudinal bands
around the Earth if not globally.
6 - 19
[57] Alternatively, if charcoal cannot be located, then one
may be able to determine the projectile’s trajectories based
on the postimpact recovery pattern among plants. If a
postfire recovery pattern is seen, then the ignition of wildfires can be assumed. This test requires either additional
continental K/T boundary sequences in places other than
North America, or a very close examination of the recovery
patterns in North America as a function of distance from
Chicxulub (compare Figures 11a and 11c). We note that if
fires did not spread across the North American continent,
then an impact trajectory from the northwest may not be
likely. Model results (Figure 11c) indicate fires would not
occur in what is now the western United States, but floral
recovery patterns [e.g., Tschudy et al., 1984] strongly
suggest fires did occur in the Raton Basin.
[58] As described in Section 2.1, debris has been found in
sediments from North America, the Gulf of Mexico, the
proto-Caribbean, and Atlantic Ocean. One could potentially
also use the distribution of this debris to determine the
projectile’s trajectory. If a slight increase in the amount of
high-energy ejecta is found in a particular direction around
the impact site, that might indicate the downrange path
because the vapor plume carries material asymmetrically in
that direction. Also, in the case of an oblique impact, the
ballistic sedimentation speed of low-energy ejecta (when it
lands) will be greater in the downstream direction than it is in
the upstream direction. Consequently, if the projectile came
from the southeast, the sedimentation speeds would be
greater in North America than in South America. This might
be evident in the scouring of the sedimentation surface or
mixing ejecta with material in the preexisting surface in
stratigraphic sections across the boundary. But both of these
effects may be too subtle to recover from the geologic record.
[59] With our model results, we have been able to
estimate the timescale needed for the sedimentation of K/T
boundary sequences on land. For example, in Colorado, the
low-energy ejecta would arrive within 15 min, assuming
the debris was not slowed significantly by the atmosphere.
The high-energy ejecta would have begun reentering the
atmosphere within 2 to 3 minutes of the impact event,
producing fires in the area a few minutes later as the bulk
of the debris hit. The high-energy ejecta begins hitting the
atmosphere above Amazonia, Europe, India, and the antipode 13 to 14 min, 18 to 20 min, 41 to 42 min, and 46 min,
respectively, after the impact event, although the bulk of it
will hit slightly later as illustrated in Figure 6. In regions near
the crater, like Colorado, we still need to evaluate the
interacting effects the two types of ejecta, since the lowenergy ejecta was still traveling through the atmosphere
when the high-energy ejecta began hitting the top of the
atmosphere. About an hour after the impact event in Colorado, the larger particulate material entrained in the vaporplume (e.g., shocked quartz) would have settled to the
ground unless fire-driven plumes kept it lofted. Over the
next several months, the finer micron to submicron particulate material would have settled to the ground, including the
soot generated by the fires. Thus, the K/T boundary
sequence records several different geologic events, in addition to the biologic events (extinction, fern spore spike, etc.).
Similar chronologies can be established for any area of the
world where the ejecta was not mixed while settling or
subsequently reworked.
6 - 20
KRING AND DURDA: MATERIAL EJECTED FROM THE CHICXULUB IMPACT CRATER
7. Conclusions
[60] Model calculations that simulate the trajectories of
material in the low-energy ejecta curtain and the highenergy vapor plume produced by the Chicxulub impact
event reveal how sediment in the K/T boundary units was
transported and deposited, have implications for the postimpact environment, and can potentially be used to determine the trajectory of the projectile that produced the
Chicxulub impact crater.
[61] The high-energy ejecta rose far above the Earth’s
atmosphere, some of it carried on trajectories that extended
halfway to the Moon, before reaccreting to the Earth. The
material enveloped the entire planet and was distributed
globally, but was concentrated near the impact site and at
the antipode. This debris deposited a tremendous amount of
energy into the atmosphere. Power levels in excess of 37.5
kW/m2 for more than 20 minutes in the atmosphere produced ground-level power levels in excess of 12.5 kW/m2,
which was sufficient to ignite wildfires on several continents. Fires likely burned in the equatorial region and on the
Indian subcontinent, but the distribution of wildfires elsewhere in the world would have depended on the trajectory
of the projectile, which is still unknown.
[62] Because the impact ejecta and the wildfires they
generated was not distributed homogeneously around the
world, detailed studies of ejecta and indicators of wildfires
in K/T boundary sediments can potentially be used to
determine the trajectory of the asteroid or comet that
produced the Chicxulub impact crater.
[63] Acknowledgments. We thank Jay Melosh and Elisabetta Pierazzo very much for discussing several issues with us, particularly when we
initiated the project, and for their review of an early draft of the paper. We
thank two anonymous reviewers for their help with the manuscript and the
editor, Paul Lucey, for handling it. This work was partially supported by
The University of Arizona (DAK) and a NASA Planetary Geology and
Geophysics grant NAG5-3631 (DDD).
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D. D. Durda, Southwest Research Institute, 1050 Walnut Street, Suite
400, Boulder, CO 80302, USA.
D. A. Kring, Lunar and Planetary Laboratory, University of Arizona,
1629 East University Boulevard, Tucson, AZ 85721, USA. (kring@
LPL.arizona.edu)