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New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico

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In the 1990s the Chicxulub impact was linked to the K-T boundary by impact spherules at the base of a sandstone complex that was interpreted as impact-generated tsunami deposit. Since that time a preponderance of evidence failed to support this interpretation, revealing long-term deposition of the sandstone complex, the K-T boundary above it and the primary impact spherule ejecta interbedded in Late Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub impact predates the K-T boundary. Impact-tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic disturbance, though no such evidence has been presented. The decades old controversy over the cause of the K-T mass extinction will never achieve consensus, but careful documentation of results that are reproducible and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important step in that direction by showing (1) that the stratigraphically older spherule layer from El Peñon, NE Mexico, represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that this impact caused no species extinctions.
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Journal of the Geological Society
doi: 10.1144/0016-76492008-116
2009; v. 166; p. 393-411Journal of the Geological Society
Gerta Keller, Thierry Adatte, Alfonso Pardo Juez, et al.
NE Mexico
New evidence concerning the age and biotic effects of the Chicxulub impact in
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© The Geological Society of London
Journal of the Geological Society, London, Vol. 166, 2009, pp. 393–411. doi: 10.1144/0016-76492008-116.
393
New evidence concerning the age and biotic effects of the Chicxulub impact in
NE Mexico
GERTA KELLER
1
*, THIERRY ADATTE
2
, ALFONSO PARDO JUEZ
3
& JOSE G. LOPEZ-OLIVA
4
1
Department of Geosciences, Princeton University, Princ eton NJ 08540, USA
2
Geological and Palaeontological Institute, University of Lausanne, Anthropole, CH-1015 Lausanne, Switzerland
3
CES Fundacion San Valero, c/Violeta Parra 9, E-50015-Zaragoza, Spain
4
Department of Geology, University of Nuevo Leon, Linares, Mexico
*Corresponding author (e-mail: gkeller@princeton.edu)
Abstract: In the 1990s the Chicxulub impact was linked to the KT boundary by impact spherules at the
base of a sandstone complex that was interpreted as an impact-generated tsunami deposit. Since that time a
preponderance of evidence has failed to support this interpretation, revealing long-term deposition of the
sandstone complex, the KT boundary above it and the primary impact spherule ejecta interbedded in Late
Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub
impact predates the KT boundary. Impact-tsunami proponents have challenged this evidence largely on the
basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic
disturbance, although no such evidence has been presented. The decades-old controversy over the cause of the
KT mass extinction will never achieve consensus, but careful documentation of results that are reproducible
and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important
step in that direction by showing (1) that the stratigraphically older spherule layer from El Pen
˜
on, NE Mexico,
represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that
this impact caused no species extinctions.
Of the five major mass extinctions in Earth’s history, only the
CretaceousTertiary (KT) mass extinction has been positively
linked to an asteroid impact, based primarily on the presence of
a global iridium anomaly coincident with the mass extinction of
planktic foraminifers (Alvarez et al. 1980), the discovery of the
Chicxulub crater in northern Yucatan (Hildebrand et al. 1991)
and impact glass spherules at the base of a sandstone complex
below the KT boundary in NE Mexico (Smit et al. 1992;
Stinnesbeck et al. 1993). Nevertheless, the cause for this mass
extinction has remained contentious, mainly because strati-
graphic data from NE Mexico, the Chicxulub crater on Yucatan
and Texas indicate that the Chicxulub impact predates the KT
boundary by about 300 ka (Keller et al. 2003, 2004a,b, 2007).
Chicxulub impact spherules were originally discovered at the
base of a thick sandstone complex, which infills submarine
channels below the KT boundary and Ir anomaly at El Mimbral
in NE Mexico. To tie the widely separated Ir anomaly to the
Chicxulub impact spherule layer, the intervening sandstone
complex was attributed to an impact-generated tsunami (Smit et
al. 1992, 1996; Smit 1999). In this scenario, the spherules rained
from the sky within hours of the impact, followed by the tsunami
waves, and the iridium settled during the subsequent weeks. This
scenario became popular, but several problems arose from the
very beginning.
For example, (1) at El Mimbral, El Pen
˜
on and other sections
the impact spherules at the base of the sandstone complex are
separated by a 1520 cm thick limestone with occasional
burrows that are infilled with spherules (Keller et al. 1997,
2003). This indicates that spherule deposition occurred in two
phases separated by the considerable time it took to form the
limestone layer. (2) The spherule layers contain a matrix of
clastic grains, shallow-water foraminifers, plants and wood
debris, which indicate erosion and transport from nearshore areas
some time after the initial spherule deposition (Keller et al.
1994a,b; Alegret et al. 2001). (3) Several burrowed horizons
were discovered in the fine-grained layers of the upper part of
the sandstone complex, which indicate repeated colonization of
the sea floor during deposition (Ekdale & Stinnesbeck 1998). (4)
Mineralogical analysis revealed two zeolite-enriched layers that
can be correlated throughout NE Mexico and indicate times of
volcanic influx (Adatte et al. 1996).
Each of these discoveries reveals long-term deposition that is
inconsistent with a tsunami interpretation. Adatte et al. (1996)
and Stinnesbeck et al. (1996) proposed deposition during a sea-
level lowstand with erosion from nearshore areas and transport
into deeper waters via submarine channels along with occasional
gravity slumps along the slope of the Gulf of Mexico (see recent
reviews by Keller 2005, 2008a,b). Although the controversy over
impact-generated deposits v. long-term deposition is still continu-
ing, the evidence listed above in favour of long-term deposition
and a pre-KT age for the Chicxulub impact remains solid and
has gained strong additional support from KT sequences along
the Brazos River in Texas (Yancey 1996; Gale 2006; Keller et al.
2007, 2008a, 2009). Opponents have argued that the Chicxulub
impact marks the KT boundary, which therefore must be placed
coincident with the spherules at the base of the sandstone
complex (Arenillas et al. 2006; Smit et al. 2004; Schulte et al.
2006, 2008). This is an ideological argument that also results in
circular reasoning: Chicxulub is KT age, therefore impact
spherules define the KT boundary (see Keller et al. 2008a).
None of the impact-independent KT defining criteria (e.g. mass
extinction, evolution of first Danian species, ä
13
C shift, boundary
clay) or even the iridium and other PGE anomalies are present at
the base of the sandstone complex (see review by Keller 2008a).
The evidence for long-term deposition of the sandstone
complex in Mexico and Texas implied that the original Chicxu-
lub impact spherule layer should be present in older marine
sediments and that the spherule layers at the base of the
sandstone are the results of subsequent reworking and redeposi-
tion. After an intensive search below the sandstone complex
throughout northeastern Mexico, numerous outcrops were found
with impact spherule layers in planktic foraminiferal zone CF1
(range of Plummerita hantkeninoides, Pardo et al. 1996), which
spans the last 300 ka of the Maastrichtian. The most significant
of these are at Mesa Juan Perez, Loma Cerca and El Pen
˜
on,
where 12 m thick spherule layers were discovered in upper
Maastrichtian sediments at 2 m, 9 m and 4 m below the sand-
stone complex, respectively (Keller et al. 2002, 2003, 2009;
Schulte et al. 2003; Keller 2008a). Some workers interpreted this
stratigraphically older spherule layer as slump, citing a small
(60 cm) fold within the reworked spherule layer near the base of
the sandstone complex at Loma Cerca (Soria et al. 2001; Schulte
et al. 2003; but see Keller & Stinnesbeck 2002). The recent
discovery in Texas of an older, primary Chicxulub spherule layer
(now altered to cheto smectite) below the sandstone complex
with up to three reworked spherule layers has lent new support to
the hypothesis that the Chicxulub impact predates the KT mass
extinction (Keller et al. 2007, 2008a, 2009).
The controversy over the cause of the end-Cretaceous mass
extinction has raged on for nearly three decades, supported by
the popular consensus that the Chicxulub impact caused the mass
extinction. Any evidence to the contrary is generally greeted with
disbelief, citing the lack of consensus. However, any decades-old
controversy will never achieve consensus, nor is consensus a
precondition to advance science and unravel truth. What is
necessary is careful documentation of results that are reproduci-
ble and verifiable. However, convincing scientists that a long-
held belief in the impact theory is wrong will demand extra-
ordinary documentation of verifiable evidence.
This study takes an important step in that direction by
presenting new outcrops from the Maastrichtian below the sand-
stone complex along the hillside of El Pen
˜
on where we detail the
physical stratigraphy, outcrop architecture and faunal turnover
across the stratigraphically oldest Chicxulub spherule layer.
Specifically, we (1) document the stratigraphy and lateral extent
of the spherule layer that is 45 m below the sandstone complex;
(2) detail the physical characteristics of the Chicxulub spherule
deposit and contrast these with the reworked spherule layers at
the base of the sandstone complex; (3) correlate El Pen
˜
on with
the Loma Cerca and Mesa Juan Perez sections; (4) evaluate the
biotic effects of the Chicxulub impact only c. 600 km from the
impact crater on Yucatan based on planktic foraminifers; if this
impact was as destructive as commonly assumed (i.e. caused the
KT mass extinction), then biotic effects in such close proximity
should have been catastrophic; (5) for comparison, we illustrate
the KT faunal turnover and mass extinction at the stratigraphi-
cally higher La Parida and La Sierrita sections. Planktic
foraminifers are highly sensitive to environmental changes and
the only group for which about two-thirds of the species were
extinct by the KT boundary, with all but one of the remaining
species disappearing within the first 200 ka of the Danian.
Location and palaeogeographical setting
El Pen
˜
on is located 40 km east of Linares, Nuevo Leon
(24858’N, 99812.5’W, Fig. 1). At this locality, as elsewhere
throughout northeastern Mexico, upper Maastrichtian marls of
the Mendez Formation form low-lying hills, which are capped by
a thick sandstone complex with reworked Chicxulub impact
spherules at the base (see reviews by Smit 1999; Keller et al.
2003; Keller 2008a,b). In this study we concentrate on El Pen
˜
on
and localities between 25 and 35 km to the NW, including La
Parida, Loma Cerca, La Sierrita and Mesa Juan Perez (Fig. 1).
The sandstone complex generally forms lenticular bodies that
infill scoured submarine channels, and the KT interval and
younger sediments are eroded, except at La Parida and La
Sierrita. To the south El Mulato, La Lajilla and El Mimbral also
preserve good KT intervals (Keller et al. 1994b, 1997; Lopez-
Oliva & Keller 1996; Alegret et al. 2001). For this study we
illustrate the KT sequences at La Parida and La Sierrita. La
Parida is located near the hamlet of La Parida c.25kmNWof
El Pen
˜
on (25812.5’N, 99831.1’W). About 100 m north of La
Parida creek, the sandstone complex is 80 cm thick, and it thins
out to the west over a distance of 50 m and disappears leaving a
Fig. 1. (a) Location of localities studied
that contain Chicxulub impact ejecta in
Mexico, Guatemala, Belize, Cuba and
Texas. (b ) Locations of NE Mexico sections
discussed in this study.
G. KELLER ET AL.394
conformable contact between the Maastrichtian Mendez and
Tertiary Velasco Formations (Stinnesbeck et al. 1996). Similarly,
the La Sierrita section located 5 km south of La Parida is about
20 m beyond the channel infilling sandstone complex.
Upper Maastrichtian sediments in the El Pen
˜
on to Mesa Juan
Perez area were deposited at .500 m depth (Alegret et al. 2001)
along the continental slope of the Gulf of NE Mexico, which
was cut by numerous submarine channels related to the uplift of
the Sierra Madre Oriental (Galloway et al. 1991; Sohl et al.
1991). Sediments eroded from the Sierra Madre Oriental and
nearshore areas around the Gulf of Mexico were deposited into
these submarine channels, forming the lenticular bodies of the
sandstone complex that are commonly found in NE Mexico.
Deposition occurred during the latest Maastrichtian sea-level
lowstand, which exposed nearshore areas to erosion and seaward
transport into deeper waters (Adatte et al. 1996; Keller et al.
2003). The KT boundary event occurred during the subsequent
sea-level rise and is characterized by the mass extinction of
tropical planktic foraminifers, the immediate first appearance of
Danian species, and iridium anomaly, brownred clay layer and
13
C shift (for recent review see Keller 2008a).
Methods
The classic El Pen
˜
on outcrop with the sandstone complex,
labelled El Pen
˜
on 1 in this study, was first described in the field
guide by Keller et al. (1994a; see also Smit et al. 1996;
Stinnesbeck et al. 1996; Keller et al. 1997). At El Pen
˜
on 1 only
about 1 m of the underlying marls is accessible by excavation
and we sampled it at 10 cm intervals. About 80 m SW along the
hillside, a 4080 cm deep trench was dug from below the base
of the sandstone complex to 9 m down the hillside, to clear
debris and expose fresh rocks. A 2 m thick Chicxulub impact
spherule layer was discovered about 4 m below the sandstone
complex. This locality is labelled El Pen
˜
on 1B (Fig. 2). Subse-
quent fieldwork traced this spherule layer intermittently and with
variable thickness over 50 m towards the El Pen
˜
on 1 outcrop.
About 510 m from its disappearance we collected a sequence
of horizontally bedded upper Maastrichtian marls (labelled El
Pen
˜
on 1A, Fig. 2). At all three localities (El Pen
˜
on-1, 1A, 1B)
sediments were examined for changes in lithology, bedding,
bioturbation, structural disturbance and slumping. Samples were
collected at an average of 1520 cm intervals, and at 10 cm
Fig. 2. El Pen
˜
on hill showing the base of the sandstone complex with reworked spherules (dashed line) dipping to the NE where the classic El Pen
˜
on 1
outcrop is located (circle). Parallel to the sandstone complex and 45 m below is the primary Chicxulub impact spherule ejecta layer (continuous line)
interbedded in late Maastrichtian marls. Locations BF mark exposures of the spherule layer, F and G show large clasts. Location A marks horizontally
bedded marls above the primary spherule layer.
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 395
intervals above and below the 2 m thick spherule layer El Pen
˜
on
1B. Loma Cerca and Mesa Juan Perez sections were also
sampled at 1520 cm intervals. At La Sierrita and La Parida,
samples were collected across the KT boundary at 5 cm and
10 cm intervals.
Planktic foraminifers were processed by standard methods
(Keller et al. 1995) and analysed quantitatively for small and
large size fractions (3863 ìm, 63150 ìm, .150 ìm) for El
Pen
˜
on and .63 ìm for Mesa Juan Perez. Quantitative counts
were based on 300 specimens. All specimens were picked,
mounted on microslides for a permanent record, and identified.
The remaining sample residues were searched for rare species,
which were included in the species census data. The planktic
foraminiferal fauna is very rich and abundant at El Pen
˜
on and
NE Mexico in general. Preservation is good, but calcite shells are
recrystallized and therefore not useful for stable isotope analysis.
Lithostratigraphy of El Pen
˜
on
El Pen
˜
on consists of a series of low-lying hills that are topped
by the sandstone complex (Fig. 1; Keller et al. 1997). The main
hill is about 200 m long and strata dip 88 to the NE. The classic
El Pen
˜
on 1 outcrop is located near the northeastern end where
the sandstone complex overlies ground level (Fig. 2). In the up-
dip direction to the SW, late Maastrichtian sediments are
exposed beneath the sandstone complex (dashed line in Fig. 2).
However, exposure is poor because the hillside is overgrown by
cacti and shrubs, and strewn by large blocks and debris from
the collapsing sandstone complex. Nevertheless, exposures of
the Mendez marl Formation can be observed intermittently,
including a spherule layer 4 to 5 m below the sandstone
complex (continuous line, Fig. 2). This spherule layer was
observed at five locations (BF) and is of variable thickness
ranging from a few centimetres to 2 m and absent in some
intervals (A, G, Fig. 2). Vegetation cover and landslides prevent
continuous tracing. Over about 80 m all exposures are at a
constant 45 m below and parallel to the sandstone complex
(continuous line, Fig. 2). No slumps or significant faults were
observed, although recent rock slides from the overlying sand-
stone complex are common. The stratigraphic continuity and
absence of slump features demonstrates that the spherule layer
within the late Maastrichtian marls cannot be interpreted as the
result of slumped sediments from the spherule layer at the base
of the sandstone complex. Such an interpretation requires the
sandstone complex to be part of the ‘slump’.
El Pen˜on 1
The classic outcrop at El Pen
˜
on 1 is most notable for its c.8m
thick sandstone complex, which has been described previously
(Keller et al. 1994a,b, 1997, 2003; Smit et al. 1996; Stinnesbeck
et al. 1996). The lithology consists of three main units (Fig. 3).
At the base, unit 1 is about 1 m thick and consists of the impact
spherule layers separated by a 20 cm thick sandy limestone with
occasional J-shaped and spherule-filled burrows that are trun-
cated by erosion. In the middle, unit 2 consists of a 45 m thick
massive sandstone with several disconformities and also occa-
sional truncated J-shaped and spherule-filled burrows near the
base. At the top, unit 3 consists of 23 m of alternating sand and
laminated fine silt or shale layers that are burrowed by Chon-
drites, Thalassinoides, and Zoophycos (Keller et al. 1997, 2003;
Ekdale & Stinnesbeck 1998). Two zeolite-enriched (volcanic)
layers are present and can be correlated throughout NE Mexico
(Adatte et al. 1996). These characteristics indicate times of
Fig. 3. The classic El Pen
˜
on 1 outcrop showing the reworked spherule-rich unit 1 at the base of the sandstone complex with two spherule layers separated
by a sandy limestone layer. J-shaped burrows infilled with spherules and truncated at the top are present in this limestone layer and near the base of the
sandstone unit 2. This marks two spherule depositional events separated by the considerable time it took to form the limestone layer.
G. KELLER ET AL.396
volcanic influx and normal sedimentation with the ocean floor
colonized by invertebrates alternating with times of rapid influx
of sand (coarse-grained layers devoid of fossils).
Originally, the sandstone complex was interpreted as the
Chicxulub impact-generated mega-tsunami deposit on the basis
of the spherule unit 1 at the base and an Ir anomaly at the top at
El Mimbral (e.g. Smit et al. 1992, 1996; Smit 1999). This view
is still prevalent, but difficult to maintain given the sedimentary
characteristics, trace fossils and zeolite layers of the sandstone
complex, all of which reflect long-term deposition in a slope
environment where rapid influx of clastic material (gravity flows,
slumps) alternated with periods of normal sedimentation during
the latest Maastrichtian sea-level fall (Adatte et al . 1996; Keller
& Stinnesbeck 1996).
El Pen˜on 1A
El Pen
˜
on 1A has two outcrops, labelled A and B (Fig. 2), which
are 10 m apart and about 20 m and 30 m from El Pen
˜
on 1,
respectively. These two outcrops combined have good exposures
of Late Maastrichtian marls of the Mendez Formation and the
interbedded spherule layer (Fig. 2, outcrops A and B). In outcrop
A exposure of the marl sequence between the sandstone complex
and spherule layer is about 3.5 m. Vegetation and a recent
rockslide obscure the uppermost metre and grading for road
access covers the the lower part. The exposed part of the
sequence clearly shows horizontally bedded marls and two marly
limestone layers parallel to the sandstone complex at the top of
the hill (Fig. 4). There is no evidence of structural disturbance or
slumping. Chondrites burrows are common throughout the marl
and marly limestone sequence and attest to normal marine
deposition. Two thin (c. 1 cm) rust-coloured layers are present
40 cm apart (Fig. 4, layers B and C). Each layer overlies a
strongly burrowed omission surface, which represents an interval
of highly condensed sedimentation.
Rust-coloured layer B (sample Pe-11, Table 1) is mineralogi-
cally very similar to the marls above and below, which also
contain unusually high iron hydroxide (goethite) contents. This
layer represents a hardground with reduced sedimentation. Rust-
coloured layer C differs from the marls by very high plagioclase
(33%) and significant smectite and zeolite contents, but lower
phyllosilicate contents. This mineralogical composition suggests
a volcaniclastic origin. Such bentonite layers are commonly
found in the Maastrichtian Mendez Formation of NE Mexico,
including in the sandstone complex (Adatte et al. 1996).
The second El Pen
˜
on 1A outcrop (exposure B, Fig. 2) is less
than 10 m from the marl sequence A, and reveals a 40 cm thick
spherule layer exposed over 5 m (Fig. 5). The continuation of
this spherule layer to the left is obscured by a recent rockslide
and to the right by vegetation. The exposed 5 m long spherule
layer forms a coherent unit parallel to the sandstone complex that
is 45 m above and is easily recognized in the field (Fig. 5,
locations AC). The lower 510 cm of the spherule unit consists
of dense, resistant melt rock and spherules. There is no evidence
of slumping or faulting.
Lithostratigraphy of the two El Pen
˜
on 1A outcrops thus reveals
a normal depositional environment during the late Maastrichtian,
including a hardground and volcaniclastic influx all parallel to
the sandstone complex 45 m above. This rules out any inter-
pretation that the spherule layer could be the result of chaotic
Fig. 4. Late Maastrichtian sequence at El Pen
˜
on 1A, exposed about 20 m from El Pen
˜
on 1, consists of horizontally bedded marl, two marly limestone
layers (A) and two thin red layers (B and C). Red layer B marks condensed sedimentation; red layer C marks volcaniclastic influx. The stratigraphic
layering is parallel to the overlying sandstone complex and shows normal marine sedimentation with abundant burrows. There is no evidence of tectonic
disturbance.
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 397
slumping, tectonic disturbance, or tsunami deposition related to
the Chicxulub impact. Deposition of marls and marly limestones
occurred in a relatively deep (.500 m) upper slope environment
inhabited by abundant Chondrites. Sedimentation was interrupted
by rapid influx of spherules (Fig. 5), after which normal marl
sedimentation resumed up to the hardground (rust-coloured layer
B, Fig. 4). Above the hardground, normal marl sedimentation
resumed, followed by volcaniclastic influx forming the second
rust-coloured layer C.
El Pen˜on 1B
Between El Pen
˜
on 1A and 1B heavy vegetation and large
boulders of the sandstone complex cover the upper Maastrichtian
sequence (Fig. 2). Approximately 50 m from the El Pen
˜
on 1A
outcrop we trenched the El Pen
˜
on 1B section (outcrop D in Fig.
2) to expose fresh rocks and examine the stratigraphic sequence
(Fig. 6, layers AC). Large blocks of the overlying sandstone
complex cover the topmost 0.51.0 m. This interval was there-
Table 1. Mineralogical data from the two red layers at El Pen˜on 1A
Sample Phyllosilicates Quartz K-feldspar Plagioclase Calcite Goethite
Pe-9 22.53 21.61 0 7.34 48.52 0.00
Pe-10 22.58 23.59 0 6.17 47.67 0.00
Pe-11 (B) 27.97 25.60 0.00 2.89 36.78 6.76
Pe-12 20.89 13.87 0 4.79 60.45 0.00
Pe-13 23.48 19.45 0 6.65 50.43 0.00
Pe-14 27.56 20.99 0 4.52 46.94 0.00
Pe-15 (C) 14.78 17.47 0 33.56 34.19 0.00
Pe-16 21.30 17.75 0 4.43 56.52 0.00
Fig. 5. El Pen
˜
on 1A spherule layer exposed over 5 m (locations AC) about 10 m from the upper Maastrichtian marl and marly limestone sequence shown
in Figure 4. The spherule layer is interbedded in the horizontally bedded marls and parallel to the sandstone complex 45 m above.
G. KELLER ET AL.398
fore excavated below spherule unit 1 at El Pen
˜
on 1 (Fig. 3) and a
composite sequence is shown in Figure 6. The trench exposed
4 m of horizontally bedded marls with two marly limestone
layers marked by more resistant beds and higher calcite content
(c. 60%), similar to El Pen
˜
on 1A. Chondrites burrows are
common throughout the sequence. Spherule clasts are present
about 1.1 m above the spherule layer. Bulk-rock composition of
marls and marly limestones is in the range of 4060% calcite,
1520% quartz, c. 5% plagioclase and 2035% phyllosilicates
(Fig. 6).
A 2 m thick Chicxulub impact spherule unit was discovered
between 4 and 6 m below the sandstone complex (Fig. 6, layer
C). The spherule unit overlies an erosional surface with angular
and rounded (25 cm, occasionally 10 cm) rip-up clasts from the
underlying marls. The rip-up clasts decrease in abundance and
size upsection. Four layers with upward decreasing spherule
abundance make up the spherule unit. More densely packed
impact glass at the base of each layer forms more resistant beds
in outcrops (Fig. 6, layer B). Thin sections of these 510 cm
thick resistant layers reveal impact melt glass and compressed or
welded spherules with convexconcave contacts in a calcite
matrix of 8090% (Fig. 7). Foraminifers are absent, though rare
foraminiferal shells can be seen encased in melt rock glass, as
also observed in the late Maastrichtian spherule layer at Loma
Cerca (Keller et al. 2002). In the upper parts of each layer
spherules are generally isolated in a marly matrix (Fig. 8e and f)
and decrease in abundance and size towards the top (Fig. 8c and
d). Above the spherule unit, the contact to the overlying marls is
gradational and diverse foraminiferal assemblages are present
within bioturbated sediments (Fig. 8a and b). Marls below the
spherule unit are similar to those above it, except for slightly
higher quartz (2530%), plagioclase (7%) and feldspar contents
and lower calcite (Fig. 6). Platinum group element analysis (Ir,
Pb, Pt, Ru, Rh) revealed no anomalous concentrations in the
spherule layers or marls (Stu
¨
ben et al. 2005).
Mineralogical values of the marls and abundant burrows thus
reflect normal pelagic sedimentation above and below the 2 m
thick impact spherule unit. The impact spherule unit differs from
the reworked spherules at the base of the sandstone complex by
distinct layers of welded glass, calcite cement, absence of
detritus and, in the upper part, gradational contact with spherules
in a marl matrix. These characteristics indicate marine sedimen-
tation rather than slumps or chaotic deposition. The welded glass
(Fig. 6, layer B and Fig. 7) suggests rapid deposition while still
hot, possibly by accumulation as rafts on the sea surface before
sinking rapidly. Absence of shallow-water debris and exotic
lithologies indicate locally derived sediments, which strongly
contrasts with the two reworked spherule layers at the base of the
Fig. 6. Composite section of the sandstone complex at El Pen
˜
on 1 and underlying trenched sequence (layers AC) at El Penon 1B about 80 m SW. El
Pen
˜
on 1B consists of horizontally bedded marls and two marly limestone layers with a 2 m thick Chicxulub impact spherule unit (B) about 4 m below the
reworked spherule layers at the base of the sandstone complex. The 2 m thick spherule unit consists of four upward fining layers with more resistant bases
(B). Mineralogical composition reveals normal pelagic marl deposition with high calcite at the base of each layer and increasing marls and quartz in the
upper parts.
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 399
Fig. 7. Chicxulub impact spherules, glass shards and melt rock in a calcite matrix from the primary spherule deposit near the base of uppermost
Maastrichtian zone CF1 at El Pen
˜
on 1B. Spherules range from 2 mm to 5 mm in size. (ac) vesicular spherules; (df) compressed vesicular spherules;
(gi) dumbbell (g) and compressed vesicular spherules with concaveconvex contact (i); (km) vesicular glass shards; (n, o) melt rock glass of welded,
amalgamated spherules; original spherules can rarely be recognized (n). (p) foraminifer in melt rock. Deposition of the Chicxulub impact spherule layer
occurred rapidly, possibly by raft-like accumulation of hot spherules at the sea surface and rapid sinking. This is suggested by the calcite matrix,
compressed spherules, melt rock, and the absence of clastic grains, clasts or other reworked components.
G. KELLER ET AL.400
sandstone complex (Keller et al. 1994b, 2003; Alegret et al.
2001). These characteristics suggest that the late Maastrichtian
spherule unit represents the original Chicxulub impact ejecta
layer. If this is correct, then the stratigraphic position near the
base of zone CF1 records the time of the Chicxulub impact about
300 ka before the KT mass extinction, as earlier documented
from NE Mexico, the Chicxulub impact crater and Texas (Keller
et al. 2003, 2004a,b, 2007).
The maximum lateral extent of the late Maastrichtian spherule
layer at El Pen
˜
on is still unknown because of rockslides and
Fig. 8. Chicxulub impact spherules in marly matrix from the upper part of the primary impact spherule layer near the base of zone CF1 at El Pen
˜
on 1B.
From bottom to top: (e, f) spherules in marly matrix; (c, d) gradational contact between spherules and overlying upper Maastrichtian marls; (a, b) upper
Maastrichtian planktic foraminifers in marls above spherule layer; marls show the same mineralogical composition as in the marly matrix of the spherule
unit.
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 401
vegetation cover. However, within about 10 m to the right of the
trench, the spherule unit thins to about 2540 cm and occasional
large (1015 cm) rounded marl clasts are surrounded by spher-
ules (Fig. 2, outcrops E and F). At 20 m from the trench the
spherule layer disappears, although occasional clasts are present
(Fig. 2, outcrop G). To the left of the trench, the spherule unit
remains .1 m thick for at least 1020 m, as visible among the
vegetation (Fig. 2, outcrop C).
Correlation of spherule layers
El Pen˜on 1
El Pen
˜
on 1 outcrops (AG) reveal intermittent exposure of the
late Maastrichtian spherule layer over a distance of about 80 m
parallel to the sandstone complex that marks the top of the hill.
The maximum lateral extent of this spherule layer is still
unknown because of rockslides and vegetation cover. However,
within about 10 m to the right of the trench (Fig. 2, outcrop D),
the spherule unit thins to about 2540 cm and occasional large
(1015 cm) rounded marl clasts are surrounded by spherules
(Fig. 2, outcrops E and F). At 20 m from the trench the spherule
layer disappears, though occasional clasts are present (Fig. 2,
outcrop G). To the left of the trench, the spherule unit remains
.1 m thick for at least 1020 m, as visible among the vegetation
(Fig. 2, outcrop C).
The stratigraphic sections at El Pen
˜
on 1A and 1B illustrate the
similarity in marine sedimentation between the late Maastrichtian
spherule layer and sandstone complex (Fig. 9). Above and below
this spherule layer both sections consist of horizontally bedded
light grey marls with abundant burrows (Chondrites). Two marly
limestone layers (c. 20% higher calcite contents) are present,
with the lower one 4050 cm above the spherule layer and the
second layer 1 m and 1.5 m above the first layer at El Pen
˜
on 1A
and 1B, respectively. The two 1 cm thick rust-coloured layers of
El Pen
˜
on 1A were not observed in El Pen
˜
on 1B. The lower rust-
coloured layer is a hardground and may correspond to the
spherule clasts in El Pen
˜
on 1B above the lower marly limestone.
The shorter interval between the two marly limestone layers in
El Pen
˜
on 1B and the spherule clasts suggests erosion. Above the
upper marly limestone approximately the same marl intervals are
exposed, with the uppermost part covered by large broken
sandstone blocks and vegetation. In places where in situ sand-
stone could be accessed, rare spherules were observed. The
massive sandstone unit is continuously present.
The major difference between El Pen
˜
on 1A and 1B is the
thickness of the spherule layer, which is 2 m thick at El Pen
˜
on
1B and narrows to 1 m thick within 1020 m and to 40 cm
within 50 m (El Pen
˜
on 1A). Rapid narrowing is also observed
within 20 m to the right (outcrops E, F and G, Fig. 2). This
lenticular shape of the spherule layer is commonly observed
throughout NE Mexico and marks deposition in submarine
channels (Adatte et al. 1996; Keller et al. 2003; Schulte et al.
2003).
El Pen
˜
on 1 has excellent exposure of the sandstone complex
with two spherule layers at the base, but only 1 m of the
underlying marls is accessible near the ground level (Fig. 3). El
Pen
˜
on 1 differs from 1A and 1B mainly by the two reworked
spherule layers separated by a sandy limestone below the
sandstone (Fig. 9). The reworked spherule layers narrow laterally
Fig. 9. Correlation of El Pen
˜
on 1 outcrops showing similar horizontal deposition of marls, marly limestone and primary impact spherule layer, which was
deposited in a submarine channel centred at El Pen
˜
on 1B. El Pen
˜
on can be correlated to Loma Cerca and Mesa Juan Perez sections at 25 km and 35 km to
the north, respectively. Variable erosion in submarine channels below the reworked spherule unit at the top accounts for the reduced marl layer at Loma
Cerca A and Mesa Juan Perez. The KT boundary is exposed at La Sierrita and contains no evidence of Chicxulub impact spherules.
G. KELLER ET AL.402
and mark a channel deposit, similar to the lower spherule layer
in El Pen
˜
on 1B c. 80 m distant.
Loma CercaMesa Juan Perez
The spherule layer embedded in Maastrichtian marls at El Pen
˜
on
is not an isolated occurrence; similar deposits have been
documented in many outcrops through northeastern Mexico
(Affolter 2000; Schilli 2000; Keller et al. 2002, 2003; Schulte et
al. 2003). Here we show the correlation with Loma Cerca and
Mesa Juan Perez sections between 25 and 35 km to the NW (Fig.
9). At Loma Cerca B, only a thin spherule layer is present at the
base of the sandstone complex, whereas at Loma Cerca A and
Mesa Juan Perez this spherule layer is 50 cm thick. Interbedded
in late Maastrichtian marls at Loma Cerca B are two spherule
layers at 6.5 and 10 m below the sandstone complex (Keller et
al. 2002). We correlate these spherule layers to El Pen
˜
on 1B, but
note that the upper layer (7.07.8 m) is probably reworked as
suggested by peak abundance of benthic foraminifers (Keller et
al. 2002). Another difference is the absence of marly limestone
layers at this location, which is probably due to regional
variations in calcite deposition. Loma Cerca A has a nearly
2.4 m thick deposit of spherules mixed with marl about 1.9 m
below the reworked spherules at the base of the sandstone
complex. At Mesa Juan Perez about 8 km to the north the late
Maastrichtian spherule layer is about 70 cm thick and separated
from the reworked spherule layer by about 1.8 m (Fig. 9). The
reduced marl thickness, as compared with Loma Cerca B, is
probably due to greater erosion and downcutting of the submar-
ine channel at these locations.
Schulte et al. (2003) described five additional outcrops in the
Mesa Juan Perez area over a lateral distance of about 250 m. At
these locations, spherules are present in variable abundances
ranging from a few centimetres to 1 m at the base of the
sandstone complex and occasionally contain a sandy limestone
layer, similar to El Pen
˜
on 1. Additional spherule deposits are
observed in late Maastrichtian marls 23 m below. These
spherule deposits are described as devoid of marl clasts, with
sharp upper and lower contacts, but discontinuously exposed or
‘lens-like’. A small overturned fold (60 cm) with large marl
clasts at the centre was observed in one section (see also Soria et
al. 2001). Other outcrops reveal spherules distributions over 2 m
dispersed within marls, similar to Loma Cerca A (Fig. 9; Schulte
et al. 2003, p. 121, fig. 4).
Although Schulte’s analysis concentrated on the reworked
spherule deposits at the base of the sandstone complex and did
not differentiate between the two stratigraphically separated
spherule layers, there are some similarities to our observations at
El Pen
˜
on. For example, Schulte et al. (2003) noted that the
spherule layer embedded in Maastrichtian marls contains no
detritus, in contrast to the abundant shallow-water detritus in the
spherule deposits at the base of the sandstone complex. This is
consistent with our observations. Schulte et al. observed clusters
of welded and amalgamated melt rock and clasts of spherules in
a marl matrix in the reworked spherule layer. These clasts and
clusters probably originated from erosion of the primary spherule
layer interbedded in Maastrichtian marls where we observed
distinct sublayers with welded, amalgamated melt rock (Fig. 7)
and upward grading of spherules in marly matrix (Fig. 8).
Schulte et al. (2003) interpreted the spherule deposits at the
base of the sandstone complex as the result of reworking,
redeposition, slumps and turbidity currents with deposition in
submarine channels. This is consistent with previous interpreta-
tions. In contrast, the spherule deposits in late Maastrichtian
marls are explained as derived from these deposits via a complex
interplay of slumps, folding and liquefaction that redistributed
and embedded the spherules into the marls up to 6 m below. In
view of the regional distribution and new data from El Pen
˜
on,
including the strong differences in the composition of the
spherule layers, this interpretation is very unlikely. More consis-
tent with the data is the interpretation that the lower spherule
layer is the original ejecta fallout and was subsequently
reworked, with particularly strong reworking and redeposition
from nearshore areas, at the base of the sandstone complex.
Biotic effects of the Chicxulub impact
El Pen˜on
Biostratigraphy places the spherule layers interbedded in marls at
El Pen
˜
on, Loma Cerca and Mesa Juan Perez near the base of
zone CF1, the range of Plummerita hantkeninoides, a species
that evolved in magnetochron C29r about 300 ka earlier than the
KT boundary (Pardo et al. 1996; Keller et al. 2002, 2003). The
same age for the Chicxulub impact spherules was determined
from spherule deposits in Texas and from the Chicxulub crater
on Yucatan (Keller et al. 2004a,b, 2007). The biotic effects of
this impact can be evaluated based on species richness and the
relative abundances of single species populations, two commonly
used proxies to assess environmental changes. Both proxies were
analysed at El Pen
˜
on in two size fractions to evaluate the
response of small (63150 ìm) and larger (.150 ìm) species.
Larger species comprise a very diverse group of generally
complex, ornamented and highly specialized K-strategists (Abra-
movich & Keller 2003; Abramovich et al. 2003) that thrived in
tropical and subtropical environments, but were intolerant of
environmental changes and hence prone to extinction (Begon et
al. 1998; Keller & Pardo 2004; Keller & Abramovich 2009). All
of these species (two-thirds of the species assemblage) went
extinct at the KT boundary. Small species are less diverse,
ecologic generalists, or r-strategists, and generally more tolerant
of environmental perturbations, including variations in tempera-
ture, salinity, oxygen and nutrients (Koutsoukos 1996; Keller
2001; Keller & Abramovich 2009). Some of these species
responded to environmental catastrophes by opportunistic
blooms, such as observed for Heterohelix and Guembelitria
species (Pardo & Keller 2008).
A total of 52 species are present in the .150 ìm size fraction
at El Pen
˜
on during the late Maastrichtian. Of these 75% (39
species) are K-strategists and 25% (13 species) are r-strategists
(Fig. 10). Across the Chicxulub impact spherule layer species
richness remained unchanged: none of the 52 species extant
below this level went extinct. About 2 m above the spherule layer
is a gradual decrease to 4244 species, rising slightly at the
unconformity at the base of the sandstone complex as a result of
reworking. The variability in species richness is due to the rare
and sporadic occurrences of nine (K-strategy) species, or 17% of
the total assemblage. Their increasingly rare and sporadic
occurrences may be due to increasing biotic stress (no change
was observed in preservation). Most species (83%) are continu-
ously present. These data indicate that the species richness
decrease cannot be assigned to the biotic effects of the Chicxulub
impact because (1) it occurs much later, (2) the rare species are
already endangered prior to deposition of the impact spherule
layer, and (3) all rare species are known to have survived to the
KT boundary at the stratotype section in Tunisia and elsewhere
(e.g. Keller et al . 2002; Luciani 2002; Molina et al. 2006; Keller
et al. 2008b).
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 403
Relative abundance changes in single species populations are
more sensitive indicators of environmental change than the
presence or absence of species. During the late Maastrichtian, K-
strategy species (.150 ìm) show normal diversity and abun-
dances. Nearly half of the K-strategists are common, with the
assemblages dominated (1020%) by Pseudoguembelina costu-
lata, Rugoglobigerina rugosa and R. scotti (Fig. 10). Also
common are pseudotextularids, other rugoglobigerinids and
globotruncanids (e.g. arca, aegyptiaca, rosetta, orientalis,
stuarti). Among r-strategists, the larger morphotype of Hetero-
helix globulosa is common in this assemblage. Relative species
abundance variations above and below the spherule unit are
within normal fluctuations of the section with no significant
changes, except for a decrease in H. globulosa and increase in
Pseudotextularia deformis and Globotruncana stuarti in the
upper 2 m of the section. No specific biotic effects in K-
strategists can be attributed to the Chicxulub impact.
Species richness in the smaller (63150 ìm) size fraction
totals 39 species, of which 64% (25 species) are K-strategists and
36% (14 species) are r-strategists (Fig. 11). Similar to larger
species, diversity remained unchanged across the spherule layer
and throughout the section, with variability between 34 and 36
species, rising to 38 species at the unconformity at the base of
the reworked spherule layers. Variability is due to five K-strategy
species, which are rare and sporadically present.
Species abundances in the smaller size fraction (63150 ìm)
are dominated by the small biserial r-strategist Heterohelix
navarroensis, which varies between 40 and 50% across the
spherule layer and decreases in the upper part to an average of
40% (Fig. 11). Other r-strategists vary between 5 and 15% and
consist of small heterohelicids, globigerinellids, and hedbergel-
lids. The disaster opportunist Guembelitria is a minor component
(,5%). In the same size fraction, K-strategists are dominated by
Pseudoguembelina costulata and P. costellifera. All other K-
species are rare (,1%). There are no significant abundance
variations across the spherule unit, except for P. costellifera,
which decreases 8% concurrent with an increase in H. navar-
roensis. This may reflect a change in the watermass stratification,
although whether this was related to the Chicxulub impact cannot
be determined.
Mesa Juan Perez
Quantitative analysis of planktic foraminifers is generally carried
out on the .63 ìm size fraction, which consists mainly of the
most common smaller and larger species (Fig. 12). This size
fraction was earlier analysed at Loma Cerca (Keller et al. 2002)
and now also at Mesa Juan Perez. Both sections show similar
results, with a total of 41 species and species richness per sample
varying between 30 and 39 species (Fig. 12). As at El Pen
˜
on the
Fig. 10. Relative species abundances of planktic foraminifers (.150 ìm) across the 2 m thick Chicxulub spherule unit near the base of the upper
Maastrichtian zone CF1 at El Pen
˜
on 1B. No species went extinct and there are no significant species population changes as a result of this impact. Two
spherule layers, separated by a burrowed limestone at the base of the sandstone complex, are reworked from shallow nearshore areas.
G. KELLER ET AL.404
variability is due to 17% rare and sporadically occurring species.
Among small species heterohelicids (r-strategists) are common.
Among larger species (k-strategists) rugoglobigerinids and pseu-
doguembelinids (P. costulata, P. costellifera) dominate (Fig. 12).
No species extinctions occurred across the late Maastrichtian
Chicxulub impact spherule layer and no significant variations are
observed in species abundances at Mesa Juan Perez (Fig. 12).
The same observations were earlier reported at Loma Cerca B
(Keller et al. 2002, fig. 6, p. 151). The results from three sections
are thus consistent and show no significant environmental
changes or species extinctions at the time of the Chicxulub
impact about 300 ka before the KT mass extinction.
Biotic effects of the KT boundary event
The KT interval and younger sediments are eroded at El Pen
˜
on,
as well as many other localities throughout NE Mexico where the
sandstone complex forms flat-topped hills. However, the KT
boundary transition and Ir anomaly are present at several
localities outside the submarine channels (e.g. El Mimbral, La
Sierrita (Fig. 9), La Parida, El Mulato, La Lajilla, Lopez-Oliva &
Keller 1996; Rocchia et al. 1996; Keller et al. 2003; Stu
¨
ben et
al. 2005). Several of these sections show a 510 cm thick
Maastrichtian marl layer between the KT boundary and rem-
nant sandstone complex (Lopez-Oliva & Keller 1996). This
suggests that the sandstone complex predates the KT boundary,
as recently observed along the Brazos River, Texas, where there
is at least 0.8 m of Maastrichtian claystone between the top of
the sandstone complex and the KT boundary (Gale 2006;
Keller et al. 2007). Schulte et al. (2006, 2008) reported this
interval as 1.6 m thick in an old core, but argued that the
reworked spherule layer at the base of the sandstone complex
should define the KT boundary (however, see Keller et al.
2009).
For this study we review the La Parida and La Sierrita
sections, which show relatively complete KT transitions (Fig.
1). At La Parida the sandstone complex is 80 cm thick and thins
out to the west to a 510 cm thick sand layer over a distance of
50 m (Stinnesbeck et al. 1996). The section was sampled at the
point where the sandstone layer is only 5 cm thick (Fig. 13). A
510 cm thick marl layer overlies the sandstone layer (Lopez-
Oliva & Keller 1996). Marls below and above it contain upper-
most Maastrichtian planktic foraminiferal assemblages indicative
of zone CF1. The overlying grey shale contains early Danian
assemblages characteristic of the lowermost Danian zone P1a
(Subzone P1a(1), Fig. 14). The very thin zone P0 that marks the
KT clay or red layer was not observed at La Parida. Except for
the boundary red clay, the KT section appears continuous, as
Fig. 11. Relative species abundances of planktic foraminifers (63150 ìm) across the 2 m thick Chicxulub spherule unit at El Pen
˜
on 1B. No species went
extinct and there are no significant species population changes as a result of this impact. (See Fig. 10 for complete caption.)
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 405
indicated by the abrupt dominance of the disaster opportunist
Guembelitria cretacea, followed by abundant Parvularugoglobi-
gerina eugubina. The biotic effects across the KT event can
thus be evaluated.
A total of 44 Cretaceous species were identified at La Parida
and 31 (69%) went extinct at the KT boundary, all of them
specialized K-strategists (Fig. 14). The presence of most of these
species in early Danian sediments is probably due to reworking.
Ten of the species (22%), all r-strategists, are known to have
survived the catastrophe for at least some time. One species, the
disaster opportunist Guembelitria cretacea, thrived in the im-
mediate aftermath of the catastrophe globally. The evolution of
new species (r-strategists) began almost immediately after the
mass extinction in zones P0 and P1a, although zone P0 is
missing at La Parida. These mass extinction and evolution
patterns are characteristic in planktic foraminifers throughout the
Tethys, although species abundances may vary depending on
regional conditions (Koutsoukos 1996; Luciani 2002; Keller &
Pardo 2004; Molina et al. 2006; Fornaciari et al. 2007).
In the La Sierrita area sediments above the thick sandstone
complex on the hills are eroded, but the KT boundary can be
recovered in the valleys between the hills. One such locality is
La Sierrita A (also called Los dos Plebes, Stu
¨
ben et al. 2005),
located between Loma Cerca and Mesa Juan Perez (Fig. 9).
Similar to La Parida, the sandstone complex thins out laterally
over a distance of 20 m and disappears. This is where the KT
boundary can be recovered, including the thin brownred clay
layer, small Ir anomaly (0.3 ppb) and ä
13
C shift (Stu
¨
ben et al.
2005) that marks the KT boundary along with the mass
extinction of Cretaceous species and evolution of early Danian
Fig. 12. Relative species abundances of planktic foraminifers (.63 ìm) across the primary impact spherule layer at Mesa Juan Perez. No species went
extinct and there are no significant species population changes as a result of this impact. Differences in the relative abundances of some species as
compared with El Pen
˜
on 1B are due to the fact that the .63 ìm size fraction analysed includes both large and small species, which were separately
analysed for El Pen
˜
on 1B.
Fig. 13. La Parida outcrop showing the thin sandstone layer that
underlies a 10 cm thick marl followed by Danian shale.
G. KELLER ET AL.406
species (Figs 15 and 16). There is no evidence of Chicxulub
impact spherules. The first Parvularugoglobigerina eugubina,
index species of zone P1a, were found 10 cm above the KT
boundary. Marls below the sandstone contain zone CF1 assem-
blages.
Discussion
Dating two closely spaced events, such as the Chicxulub impact
and the KT mass extinction, requires high sediment accumula-
tion rates that physically separate the events in space and time.
Continental shelves and upper slopes provide such environments
because of their high biological productivity coupled with high
terrigenous influx. The KT sections in NE Mexico are excel-
lent. Located on the outer continental shelf to upper slope of the
Gulf of Mexico they provide expanded sedimentation records,
including the sandstone complex of the submarine channels and
thick Chicxulub spherule units at 45 m below in undisturbed
sediments at El Pen
˜
on 1A and 1B, as well as at Loma Cerca and
Mesa Juan Perez (Fig. 1). Such temporal and spatial separation
between the KT and Chicxulub events is difficult to observe in
deep-sea environments because of the highly reduced sedimenta-
tion rate coupled with periods of intensified bottom current
activity leading to erosion of older sediments and redeposition.
As a result, Chicxulub spherules in deep-sea sites (e.g. Blake
Nose, Demarara Rise) tend to be in disturbed sediments, but
close to the KT boundary, which has been interpreted as
evidence in support of the KT age for this impact and tsunami
disturbance (Norris et al. 1999, 2000; MacLeod et al. 2006; but
see Keller 2008a,b). Previous studies of NE Mexico localities
concentrated exclusively on the sandstone complex and inter-
preted the reworked spherule layers as evidence of KT age and
Chicxulub tsunami disturbance (Smit et al. 1992, 1996; Smit
1999; Soria et al. 2001; Schulte et al. 2003; Arenillas et al.
2006).
The new data presented here for El Pen
˜
on and Mesa Juan
Perez, and earlier observed at Loma Cerca, the Chicxulub crater
core Yaxcopoil-1 and in Texas (Keller et al. 2002, 2003,
2004a,b, 2007) demonstrate the pre-KT age of the Chicxulub
impact. A depositional scenario for NE Mexico consistent with
current evidence is shown in Figure 17 along with the climate
curve for the South Atlantic deep-sea Site 525 (Li & Keller
Fig. 14. The KT mass extinction at La Parida shows two-thirds of the species extinct, all of them K-strategists, similar to the mass extinction pattern
globally. A 10 cm thick Maastrichtian marl above the sandstone layer indicates that the top of the sandstone is not synchronous with the KT boundary.
The absence of the boundary clay suggests a short hiatus or condensed interval. The presence of many Cretaceous species above the KT boundary is
probably due to reworking.
Fig. 15. KT boundary at La Sierrita shows the characteristic KT clay
and redbrown layer with an Ir anomaly that marks the most continuous
sections worldwide.
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 407
1998). The Chicxulub impact occurred during the late Maastrich-
tian warm event, about 300 ka before the KT boundary, and
probably caused catastrophic destruction as a result of impact-
induced earthquakes and giant tsunamis, and probably altered
climate and environmental conditions for decades. However, in
the geological time scale the record of this immediate destruction
is not preserved and no credible evidence of tsunami deposits or
massive slumps have been found to date.
Fig. 16. Planktic foraminiferal species ranges, along with Ir anomaly and ä
13
C shift (from Stu
¨
ben et al. 2005) across the KT boundary at La Sierrita. In
contrast to La Parida, few Cretaceous species are present above the KT boundary, which suggests absence of reworking.
Fig. 17. Summary diagram of La Parida and El Pen
˜
on shows the temporal and spatial sequence of the Chicxulub impact, sandstone complex and the KT
boundary. A sea-level fall coincides with deposition of the sandstone complex and correlates with the global cooling following the short warm event in
zone CF1. The KT mass extinction is unrelated to the Chicxulub impact and may have been caused by massive volcanic eruptions.
G. KELLER ET AL.408
What has been preserved are the impact melt rock spherules
that rained down throughout the region and settled rapidly
while still hot to form mats of agglutinated and compressed
glass spherules. At El Pen
˜
on the presence of spherules in four
upward fining layers followed by gradual return to normal marl
deposition suggests current activity. Despite the probably
immediate massive environmental destruction caused by the
Chicxulub impact, marine biota show no long-term effects. Of
the 52 planktic foraminiferal species extant prior to the impact,
none went extinct and none underwent major changes in
species populations. Over the next c. 150200 ka normal
marine sedimentation prevailed during warm climatic condi-
tions.
The sandstone complex formed in submarine channels during
the latest Maastrichtian cooling and sea-level fall estimated
around 100150 ka before the KT boundary. This global
cooling may have been caused by the massive Deccan volcanic
eruptions and SO
2
release (Ravizza & Peucker-Ehrenbrink 2003;
Chenet et al. 2007; Self et al. 2008). The sea-level fall exposed
the spherule deposits in nearshore areas and transported them
and shallow-water debris seaward, infilling submarine channels
and forming the sandstone complex (Fig. 17). During the low sea
level erosion and redeposition occurred intermittently, alternating
between rapid influx (e.g. clastic debris, sand, shallow-water
foraminifers, plant fragments) and periods of normal sedimenta-
tion, which permitted colonization by invertebrates (e.g. lime-
stone, marl, shale). As sea level rose, normal marine
sedimentation resumed during the last 100 ka before the KT
mass extinction, as suggested by the Late Maastrichtian marl
layer at La Parida, La Lajilla, El Mulatto and other NE Mexico
localities, limestone in the Chicxulub crater, and clay and
mudstones in Texas (Keller et al. 2003, 2004a,b, 2007). The
KT mass extinction, which occurred well after the Chicxulub
impact and sandstone deposition, resulted in the rapid extinction
of two-thirds of the planktic foraminiferal species; this extinction
rate is comparable with the mass extinction globally.
The absence of any recognizable biotic effects as a result of
the Chicxulub impact comes as a surprise mainly because we
have assumed that this impact caused the KT mass extinction.
A survey of impact craters and mass extinctions over the past
500 Ma reveals that apart from the KT boundary none of the
five major mass extinctions are associated with an impact
(Courtillot 1999; Wignall 2001; Keller 2005). The Chicxulub
crater with a maximum diameter of 180 km is the largest known
impact. Other well-studied impacts that show no extinctions or
significant other biotic effects include the 90100 km diameter
late Eocene Chesapeake Bay and Popigai craters, and the 100
120 km diameter late Triassic Manicouagan and late Devonian
Alamo and Woodleigh craters (Montanari & Koeberl 2000;
Wignall 2001; Poag et al. 2002; Keller 2005).
If not the Chicxulub impact, what caused the KT mass
extinction? We have previously suggested another larger impact
based on the prevailing view that the KT Ir anomaly is of
cosmic origin (Keller et al. 2003; Stu
¨
ben et al. 2005). However,
the absence of any biotic effects attributable to the Chicxulub
impact suggests that even a larger impact alone would probably
not have been sufficient to cause the KT mass extinction. In
addition, there is currently no credible evidence of a second
larger impact at KT time. Another problem is that the KT Ir
anomaly is frequently not just a single anomaly as commonly
reported, but multiple anomalies of diverse origins (e.g. impact,
volcanic, redox conditions, Graup & Spettel 1989; Grachev et al.
2005, 2007; Stu
¨
ben et al. 2005; Keller 2008a) that have yet to be
fully understood. The Ir anomaly can thus no longer be consid-
ered sufficient credible evidence for a large impact at the KT
boundary.
A likely although often overlooked cause for the KT
catastrophe is the Deccan Trap volcanic eruptions, as has long
been advocated by McLean (1985) and Courtillot et al. (1986,
1988). Deccan Trap eruptions were long thought to have oc-
curred over several million years, but recent studies suggest that
the main phase (80%) of eruptions may have been very rapid,
over a period of ,100 ka (Chenet et al. 2007), and ended at the
KT mass extinction (Keller et al. 2008b). These new results
suggest that Deccan volcanism and associated climate and
environmental effects may have triggered the KT catastrophe
and that the Chicxulub impact was an early contributor, but not
the main cause.
Conclusions
The Chicxulub impact, long thought to be the cause for the KT
mass extinction, is revealed as both being pre-KT age and
having caused no species extinctions. This is indicated by
evidence from the Brazos River area of Texas (Keller et al. 2007,
2009), the La Sierrita area (Mesa Juan Perez and Loma Cerca)
and the classic El Pen
˜
on area of NE Mexico.
(1) The original Chixculub impact spherule layer at El Pen
˜
on
is about 45 m below two reworked spherule layers that are
separated by a 20 cm thick sandy limestone at the base of the
sandstone complex, which was originally interpreted as an
impact-generated mega-tsunami deposit.
(2) The original impact spherule layer at El Pen
˜
on is 2 m
thick, thins out laterally to c. 40 cm over about 50 m, and is
parallel to the sandstone complex above. There is no significant
tectonic disturbance or slumps, although fallen blocks of the
sandstone complex sometimes obscure the original spherule layer
in the outcrops.
(3) Spherules were deposited rapidly, as evident by aggluti-
nated spherules with convexconcave contacts and a layer of
melt rock that indicates rapid settling. No transported shallow-
water debris is present.
(4) Sediments between the sandstone complex and the Chicxu-
lub impact spherule layer consist of horizontally bedded and
bioturbated marls and marly limestones with two thin rust-
coloured layers representing condensed sedimentation. Volcanic
influx is prevalent in one rust-coloured layer. This indicates that
normal sedimentation resumed after the Chicxulub impact.
(5) The age of the original Chixulub spherule layer is late
Maastrichtian near the beginning of zone CF1, or about 300 ka
prior to the KT boundary, consistent with earlier observations
in the Chicxulub crater core and Texas (Keller et al. 2003,
2004a,b, 2007).
(6) Planktic foraminifers, which underwent extinction of two-
thirds of all species at the KT boundary, reveal no significant
biotic effects across the Chicxulub impact ejecta layer at El
Pen
˜
on, Mesa Juan Perez and Loma Cerca (Keller et al. 2002) or
in Texas (Keller et al. 2009). Not a single species went extinct
and there are no significant changes in species abundances.
(7) The KT boundary interval is eroded at El Pen
˜
on, but the
biotic effects of the KT event can be assessed in the nearby La
Parida and La Sierrita sections. The KT mass extinction at
these two localities is marked by the global KT defining
criteria, which include extinction of about two-thirds of the
species, the evolution of the first Danian species immediately
after the KT boundary, the ä
13
C shift, clay layer and Ir
anomaly.
(8) These observations indicate that the Chicxulub impact can
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 409
no longer be considered of KT age and the primary cause for
the KT mass extinction. Deccan volcanism and associated
climate and environmental effects may have triggered the KT
catastrophe with the Chicxulub impact an early contributor.
We gratefully acknowledge H. Falcon-Lang for suggestions that greatly
improved this paper. We thank N. MacLeod for a constructive review,
and D. Stu
¨
ben and Z. Berner for geochemical analysis. The material of
this study is based upon work supported by the US National Science
Foundation through the Continental Dynamics Program and Sedimentary
Geology and Paleobiology Program under NSF Grants EAR-0207407 and
EAR-0447171, and the Swiss National Fund No. 21-67702.02/1.
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Received 10 September 2008; revised typescript accepted 14 January 2009.
Scientific editing by Howard Falcon-Lang.
AGE AND BIOTIC EFFECTS OF CHIXULUB IMPACT 411
... However, some KPB specialists hold that the Chicxulub impact predates the KPB by ~200-300 k.y. (Keller et al., , 2002a(Keller et al., , 2009Stinnesbeck et al., 2001). The stratigraphic distribution of impact spherules and iridium anomalies in the Gulf of Mexico and the Caribbean led Keller et al. (2003) to propose multiple bolide impacts through the KPB: an older Maastrichtian impact (66.3 Ma), which for them is the Chicxulub impact; another larger impact at 66 Ma that distributed the iridium at the KPB; and a third impact in the early Danian (65.9 Ma). ...
... According to Keller's group, the SLL of El Peñón presents occasional J-shaped, spherule-filled burrows that are truncated by erosion, which reflects invertebrate colonization on the seafloor and long-term deposition (Keller et al., 2002a(Keller et al., , 2009Keller, 2014). Previously, these structures had been interpreted as water-escape structures resulting from the soft-sediment deformation of unconsolidated sediments due to the rapid deposition of the massive sandstones of Unit II (Smit et al., 1996). ...
... For example, Keller et al. (2002a) reported that the two ichnofossils shown in their figures 4B-4C come from the base of Unit II at El Peñón and Rancho Canales, respectively. Later, these same two ichnofossils were illustrated by Keller et al. (2009;their fig. 3), this time asserting that they both come from El Peñón (in the SLL and near the base of Unit II, respectively). ...
Chapter
Full-text available
This volume pays tribute to the great career and extensive and varied scientific accomplishments of Walter Alvarez, on the occasion of his 80th birthday in 2020, with a series of papers related to the many topics he covered in the past 60 years: Tectonics of microplates, structural geology, paleomagnetics, Apennine sedimentary sequences, geoarchaeology and Roman volcanics, Big History, and most famously the discovery of evidence for a large asteroidal impact event at the Cretaceous–Tertiary (now Cretaceous–Paleogene) boundary site in Gubbio, Italy, 40 years ago, which started a debate about the connection between meteorite impact and mass extinction. The manuscripts in this special volume were written by many of Walter’s close collaborators and friends, who have worked with him over the years and participated in many projects he carried out. The papers highlight specific aspects of the research and/or provide a summary of the current advances in the field.
... Ever since the discovery of the Chicxulub crater, impact spherules at or near the Cretaceous-Paleogene boundary in Mexico, the Caribbean, Central America, the United States, and the North Atlantic have become major players in solving the age of the impact relative to the mass extinction based on their stratigraphic positions. However, consensus on the age of the Chicxulub impact has remained controversial, primarily because of differing interpretations regarding these impact deposits, with one side arguing that the impact occurred precisely at the Cretaceous-Paleogene boundary, thus coinciding with the mass extinction (review in Schulte et al., 2010), and the other side arguing that the impact predated the Cretaceous-Paleogene boundary (Keller et al., 2004a(Keller et al., , 2009areview in Keller, 2011). ...
... (5) Bermúdez et al. (2016) argued that the presence of concave-convex contacts and agglutinated spherules indicates settling while still hot and therefore supports their interpretation that the Gorgonilla spherules represent a primary deposit. This argument has several flaws: (a) A majority of the spherules are perfectly spherical, (b) rare concave-convex contacts and amalgamated spherules are poorly developed compared with such features in the primary deposits of NE Mexico (see next section; Fig. 3A; Keller et al., 2009a), and (c) such rare occurrences can be explained as reworked material from a primary deposit and/or the result of postdepositional compaction. ...
... Since the discovery of the Chicxulub impact structure in the Yucatan Peninsula (Hildebrand et al., 1991;Pope et al., 1991), many impact spherule-rich deposits, both primary and reworked, have been documented at or near the Cretaceous-Paleogene boundary in the Gulf of Mexico (e.g., Alvarez et al., 1992;Keller et al., 1994aKeller et al., , 2009aLopez-Oliva and Keller, 1996;Schulte et al., 2003;Smit, 1999;Smit et al., 1992Smit et al., , 1996Stinnesbeck et al., 1993), Texas (e.g., Adatte et al., 2011;Keller et al., 2007Keller et al., , 2011bSchulte et al., 2006;Yancey, 1996), Carib bean Sea, Haiti, Belize, and Guatemala (e.g., Izett et al., 1990;Maurrasse and Sen, 1991;Sigurdsson et al., 1991;Smit et al., 1996;Stinnesbeck et al., 1997;Keller et al., 2001Keller et al., , 2003bAlegret et al., 2005), and the North Atlantic Ocean (e.g., Olsson et al., 1997;Norris et al., 1999;MacLeod et al., 2007;Keller et al., 2013;Fig. 10). ...
Article
The end-Cretaceous mass extinction (66 Ma) has long been associated with the Chicxulub impact on the Yucatan Peninsula. However, consensus on the age of this impact has remained controversial because of differing interpretations on the stratigraphic position of Chicxulub impact spherules relative to the mass extinction horizon. One side argues that the impact occurred precisely at the Cretaceous-Paleogene boundary, thus coinciding with the mass extinction; the other side argues that the impact predated the Cretaceous-Paleogene boundary, based on the discovery of primary impact spherules deposits in NE Mexico and Texas near the base of planktic foraminiferal zone CF1, dated at 170 k.y. before the Cretaceous-Paleogene boundary. A recent study of the most pristine Chicxulub impact spherules discovered on Gorgonilla Island, Colombia, suggested that they represent a primary impact deposit with an absolute age indistinguishable from the Cretaceous-Paleogene boundary. Here, we report on the Gorgonilla section with the main objective of evaluating the nature of deposition and age of the spherule-rich layer relative to the Cretaceous-Paleogene boundary. The Gorgonilla section consists of light gray-yellow calcareous siliceous mudstones (pelagic deposits) alternating with dark olive-brown litharenites (turbidites). A 3-cm-thick dark olive-green spherule-rich layer overlies an erosional surface separating Maastrichtian and Danian sediments. This layer consists of a clast-supported, normally graded litharenite, with abundant Chicxulub impact glass spherules, lithics (mostly volcanic), and Maastrichtian as well as Danian microfossils, which transitions to a calcareous mudstone as particle size decreases. Mineralogical analysis shows that this layer is dominated by phyllosilicates, similar to the litharenites (turbidites) that characterize the section. Based on these results, the spherule-rich layer is interpreted as a reworked early Danian deposit associated with turbiditic currents. A major hiatus (>250 k.y.) spanning the Cretaceous-Paleogene boundary and the earliest Danian is recorded at the base of the spherule-rich layer, based on planktic foraminiferal and radiolarian biostratigraphy and carbon stable isotopes. Erosion across the Cretaceous-Paleogene boundary has been recorded worldwide and is generally attributed to rapid climate changes, enhanced bottom-water circulation during global cooling, sea-level fluctuations, and/or intensified tectonic activity. Chicxulub impact spherules are commonly reworked and redeposited into younger sediments overlying a Cretaceous-Paleogene boundary hiatus of variable extent in the Caribbean, Central America, and North Atlantic, while primary deposits are rare and only known from NE Mexico and Texas. Because of their reworked nature, Gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred.
... T HE Cretaceous-Paleogene (K-Pg) boundary mass extinction event (65.68 Ma) is one of the most fascinating event in the geological history and has been studied globally by the several workers (Alvarez et al., 1980;MacLean, 1985;Courtillot et al., 1986Courtillot et al., , 1988Duncan & Pyle, 1988;Pope et al., 1991;Smit et al., 1996;Keller et al., 2003Keller et al., , 2007Keller et al., , 2009aKeller, 2010;Schulte et al., 2010). During this period, planktonic protists thriving in the ocean were almost completely vanished. ...
... For the past 30 years, Chicxulub impact and Deccan volcanism has been believed as potential cause for the K-Pg boundary catastrophe (MacLean, 1985;Courtillot et al., 1986Courtillot et al., , 1988Duncan & Pyle, 1988;Pope et al., 1991;Smit et al., 1996;Schulte et al., 2010). But the recent studies by Keller and group (Keller et al., 2003(Keller et al., , 2007(Keller et al., , 2009aKeller, 2010) suggested a pre-KT age for the Chicxulub impact. Deccan volcanism also believed to have occurred over about one million year prior to the mass extinction leaving sufficient time for recovery between eruptions. ...
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A significant latest Maastrichtian calcareous nannofossil assemblage is recorded from the exposed section near Syndai Village, Meghalaya. A total of twenty two samples from sedimentary succession consisting of shales and sandy shales, calcareous at places, were studied; out of which ten samples were found productive in terms of calcareous nannofossils recovery. The presence of Micula prinsii in all the productive samples along with the other latest Maastrichtian nanno taxa suggests that the assemblage belongs to Micula prinsii Zone and well correlates with the CC26b Zone of Perch Nielsen and UC20dTP Zone of Burnett which are an amalgamation of old and new biozonation schemes from a range of palaeolatitudes and biogeographic provinces from both oceanic and shelf palaeoenvironments. Micula prinsii Perch–Nielsen, the latest Maastrichtian marker all over the globe, is recorded from both deep–sea sections and shelf areas. It is most evolved form of the genus Micula and got extinct just before K– Pg boundary. The Micula prinsii Zone is marked by the first occurrence of Micula prinsii to the last occurrence of unreworked, non–survivor Cretaceous taxa. In the present study, cluster analysis envisaged the palaeodepositional environmental changes within the Micula prinsii Zone in northeastern India. In the lower part of the section, the abundance of Micula concava and Micula staurophora with the increased numbers of Watznaueria barnesiae indicates environmentally stressful conditions with low productivity in surface water. However, in the upper part the increased numbers of Calculites obscurus with the decrease in Micula concava and Micula staurophora abundance indicates relatively increased productivity in surface water in marginal marine depositional environment.
... Many scenarios and hypotheses were proposed to explain the origin and causes of these catastrophic events and the sudden extinction of so many animals and plant species. These hypotheses include: an extraterrestrial bolide impact, volcanism, carbon dioxide poisoning, and environmental effects such as sea-level fluctuations and climatic changes (see Keller and Von Salis Perch-Nielsen, 1995;Obaidalla, 2005;Keller et al., 2009; more references therein). ...
Article
Calcareous nannofossil data, δ¹³C and δ¹⁸O values, and carbonate contents of the uppermost Maastrichtian–lower Paleocene succession cropping out at the Misheiti section (East Central Sinai, Egypt) have been used to denote and reveal the changes across the Cretaceous/Paleogene (K/Pg) boundary. The study interval belongs to the uppermost Sudr Formation and the Dakhla Formation. Four calcareous nannofossil zones (Micula prinsii, NP1, NP2/3, and NP4) were recognized. The δ¹³C profile and the carbonate content show significant decreases across the K/Pg boundary. A hiatus at the K/Pg boundary is indicated by the absence of the basal part of Zone NP1. Geochemical results and calcareous nannofossil assemblages reflect fluctuations of the paleotemperature during the latest Maastrichtian–early Paleocene.
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In this chapter, we discuss the global paleoclimatic and paleoenvironmental changes focusing on their causes and consequences. Two main processes can yield the climatic changes; the earth’s internal processes and the extraterrestrial impacts. Both of them have a strong effect on the earth’s system. The paleoclimatic change is well preserved in the earth’s sedimentary record and can be reviled by using multidisciplinary studies including mineralogy, geochemistry, and the fossil contents. Egypt is a key area of one of the most pronounced climatic changes that occurred in the earth’s geologic history; the Paleocene Eocene thermal maximum (PETM) that used recently as analog for the current warming.
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The end of the Cretaceous is a dramatic period in the Earth's history, when a catastrophic event known as the Cretaceous-Paleogene mass extinction event took place (66.043±0.011 Ma). During the early Danian a minor hyperthermal event, known as Dan-C2 event, was raised and caused a minor perturbation in the carbon cycle. These two events were coincident with the second and the last phases of Deccan eruption in India. In El-Beida section, Quseir area, Red Sea, Egypt, the (K/Pg) is placed in the middle part of Dakhla Formation on the contact between the Hamama and Beida Member and is determined based on the high occurrence HO of Nerphrolithus frequens (CC26), the abrupt shift in δ13Corg from -23.5‰ to -24.3‰, and Hg enrichment was observed just below KPB; the Hg reaches 63.5 ppb, the earliest Danian zones (NP1, lower part of NP2, P0 and P1a) are missing due to the KPB hiatus. Dan-C2 is placed at the top of the Cretaceous-Paleogene boundary hiatus, and coincident to the base of NP2 nannofossil biozone. In general, only the second carbon isotope spike of the Dan-C2 is present and is characterized by 1.5‰ negative shift in organic carbon isotope, a sudden drop in TOC content, and mercury enrichment coincident with the δ13Corg onset that may be linked to the Deccan volcanic eruption (phase-3).
Article
Major geological chronostratigraphical boundaries are marked by dramatic changes in the geological record, including biological extinctions, sea-level fluctuations, and changes of the chemical composition of the atmosphere and of sedimentary rocks. Volcanism has been suggested as one of the primary causes of intense biological and geological crises, augmenting the interest in their research on geochemical proxies. This work reviews the use of mercury (Hg) as a tracer of volcanic activity and extreme environmental crises, with focus on the Cetaeous?Paleogee tasitio K/Pg, he ost of the plaet's dinosaurs perished. Understanding this issue deepens the understanding of the paleoenvironments in different periods of Earth's history and how volcanic processes relate to mass extinctions. © 2018 Secretaria Regional do Rio de Janeiro da Sociedade Brasileira de Quimica. All rights reserved.
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The Cretaceous-Paleogene boundary (KPB) mass extinction (~ 66.02 Ma) and the Paleocene-Eocene Thermal Maximum (PETM) (~ 55.8 Ma) are two remarkable climatic and faunal events in Earth's history that have implications for the current Anthropocene global warming and rapid diversity loss. Here we evaluate these two events at the stratotype localities in Tunisia and Egypt based on climate warming and environmental responses recorded in faunal and geochemical proxies. The KPB mass extinction is commonly attributed to the Chicxulub impact, but Deccan volcanism appears as a major culprit. New mercury analysis reveals that major Deccan eruptions accelerated during the last 10 ky and reached the tipping point leading up to the mass extinction. During the PETM, climate warmed rapidly by ~ 5 °C, which is mainly attributed to methane degassing from seafloor sediments during global warming linked to the North Atlantic Igneous Province (NAIP). Biological effects were transient, marked by temporary absence of most planktic foraminifera due to ocean acidification followed by the return of the pre-PETM fauna and diversification. In contrast, the current rapid rise in atmospheric CO2 and climate warming are magnitudes faster than at the KPB or PETM events leading to predictions of a PETM-like response as best case scenario and rapidly approaching sixth mass extinction as worst-case scenario.
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The end-Cretaceous biological crisis is represented by the demise of the non-avian dinosaurs. However, most crucial biologically was the elimination of the photosynthesizing marine phyto- and zooplankton forming the base of the marine food chain. Their abrupt demise attests to sunlight screening darkening the atmosphere for a few years. Alvarez et al. (Science 208:1095–1108, 1980. doi:10.1126/science.208.44) noticed in deep marine end-Cretaceous sediments an anomalous rise in the chemical element iridium (Ir), which is rare on planet Earth and thus suggests an extraterrestrial origin through an impact of a large asteroid. This impact would have ejected enormous quantities of particles and aerosols, shading the solar illumination as attested to by the elimination of the marine photosynthesizing plankton. Such a dark period must have affected life on land. The apparent cold-blooded non-avian dinosaurs, which were used to living in open terrains to absorb the solar illumination, became inactive during the dark period and were incapable of withstanding predators. This was in contrast to cold-blooded crocodilians, turtles and lizards that could hide in refuge sites on land and in the water. Dinosaur relics discovered in Cretaceous Polar Regions were attributed to perennial residents, surviving the nearly half-year-long dark winter despite their ability to leave. The polar concentrations of disarticulated dinosaur bones were suggested as having resulted from a catastrophic burial of a population by floods. However, this should have fossilized complete skeletons. Alternatively, herds of dinosaurs living in high latitudes might have been sexually driven to spend the half year of continuously illuminated polar summer for mating rather than for nourishment, in which the lower latitudes provided as well. The aggressive mating competitions would have left victims among the rivals and of young ones incidentally trampled over, all being consumed and their skeletons disarticulated. Accordingly, the alleged ‘polar dinosaurs’ do not challenge the logical conclusion that the non-avian dinosaurs were cold-blooded, as a result of which they became inactive and subjected to predation during the end-Cretaceous dark period.
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We interpret the near K/T boundary clastic deposits of northeastern Mexico as deposited over an extended time period, during the last 170 to 200 k.y. of the Maas-trichtian and by normal sedimentary processes that include gravity flows and turbidity currents, rather than impact-generated tsunami waves. This deposition scenario is indicated by multiple horizons of bioturbation within the top unit 3 and near the base of the middle unit 2; the presence of thin layers enriched in fine clay-minerals and planktic foraminifera, suggesting hemipelagic sedimentation within unit 3; the presence of a marl layer of Maastrichtian age above the clastic deposit; the occurrence of distinct layers enriched in zeolites in unit 3; and the presence of lithologically, sedimentologically, and mineralogically distinct units and subunits that are correlatable over more than 300 km. Such correlations do not support a chaotic deposition as predicted for an impact-generated tsunami event. We interpret the clastic beds of northeastern Mexico as having accumulated during the major eustatic sealevel lowstand near the end of the Maastrichtian. In this scenario, the unconformity at the base of the clastic deposit represents a type 1 sequence boundary, where deltaic sediments were eroded and transported into deeper waters, depositing the spherulerich layer of unit 1. Continued sea-level lowering resulted in erosion and bypass of shelf sediments and the deposition of the sandstone of unit 2. Subsequently, stabilization of the sea-level lowstand resulted in episodes of decreased erosion and sediment transport alternating with normal hemipelagic sedimentation, thus depositing the sand and the silt and shale layers of unit 3. The sea-level rise during the last 50 to 100 k.y. of the Maastrichtian resulted in the normal hemipelagic sedimentation observed in the pre-K/T boundary marl layer above the clastic deposit.
Article
Biostratigraphic analysis of four Cretaceous/Tertiary (K/T) boundary sections in northeastern Mexico (Mimbral, Lajilla, Mulato, Parida) reveals that all, except Mimbral, contain a thin (5-to 10-cm) layer of Maastrichtian marl above a siliciclastic deposit that was considered to be an impact generated tsunami-deposit. This marl layer contains typical late Maastrichtian planktic foraminiferal assemblages with no apparent size sorting, grain-size gradation, or reworked shallow-water benthic foraminifera, suggesting an interval of normal hemipelagic sedimentation between deposition of the siliciclastic sediments and the K/T boundary. Siliciclastic sediments were deposited prior to the K/T boundary and within the last 170 to 200 k.y. of the Maastrichtian, as indicated by the presence of the latest Maastrichtian index species Plummerita hantkeninoides and Micula prinsii either below, within, or above this deposit but always below the K/T boundary. Deposition of the siliciclastic sediments occurred not as a single event but as a series of events, as indicated by the presence of bioturbation in several layers within unit 3 and at the base of unit 2 of this deposit. Sedimentologic, biostratigraphic, and trace fossil data all point to multievent deposition prior to the K/T boundary, with depositional events separated by scouring and erosion as well as alternating episodes of terrigenous influx and hemipelagic sedimentation. The spherule-rich sediments of unit 1, with its rare glass fragments of composition similar to that of the glass spherules from Beloc, Haiti, represent an unusual feature of the siliciclastic deposit. Whatever the origin of these glasses, the event that produced them, whether impact or volcanism, preceded the K/T boundary.
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Impact stratigraphy is an extremely useful correlation tool that makes use of unique events in Earth's history and places them within spatial and temporal contexts. The K-T boundary is a particularly apt example to test the limits of this method to resolve ongoing controversies over the age of the Chicxulub impact and whether this impact is indeed responsible for the K-T boundary mass extinction. Two impact markers, the Ir anomaly and the Chicxulub impact spherule deposits, are ideal because of their widespread presence. Evaluation of their stratigraphic occurrences reveals the potential and the complexities inherent in using these impact signals. For example, in the most expanded sedimentary sequences: (1) The K-T Ir anomaly never contains Chicxulub impact spherules, whereas the Chicxulub impact spherule layer never contains an Ir anomaly. (2) The separation of up to 9 m between the Ir anomaly and spherule layer cannot be explained by differential settling, tsunamis, or slumps. (3) The presence of multiple spherule layers with the same glass geochemistry as melt rock in the impact breccia of the Chicxulub crater indicates erosion and redeposi- tion of the original spherule ejecta layer. (4) The stratigraphically oldest spherule layer is in undisturbed upper Maastrichtian sediments (zone CF1) in NE Mexico and Texas. (5) From central Mexico to Guatemala, Belize, Haiti, and Cuba, a major K-T hiatus is present and spherule deposits are reworked and redeposited in early Danian (zone P1a) sediments. (6) A second Ir anomaly of cosmic origin is present in the early Danian. This shows that although impact markers represent an instant in time, they are subject to the same geological forces as any other marker horizons- erosion, reworking, and redeposition-and must be used with caution and applied on a regional scale to avoid artifacts of redeposition. For the K-T transition, impact stratigraphy unequivocally indicates that the Chicxulub impact predates the K-T boundary, that the Ir anomaly at the K-T boundary is not related to the Chicxulub impact, and that environmental upheaval continued during the early Danian with possibly another smaller impact and volcanism.
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In the Gulf of Mexico area, Cretaceous-Tertiary sections are characterized by a rather unusual depositional sequence. The lithological units, which are not all present in all sections, are a bed of spherules containing rare glass particles, succeeded by a sandstone, with a rather thick series of marly ripple beds at the top. Ir and Ni-rich spinels, markers derived from extraterrestrial material, are found only in the uppermost part of the sequence. The clear stratigraphical separation of the units and the late deposition of the cosmic markers are not easily explained by a single collisional event. Tentatively, we propose a multiple collision process resulting from the fragmentation of a single-incident bolide, colliding with the Earth at grazing incidence, with ricochet and rebound of fragments.