1. Chicxulub predates the KT boundary and is not the cause for the end-Cretaceous mass extinction
2. Chicxulub Crater
2.1. PEMEX Cores: Maastrichtian Breccia?
2.2. New Evidence: Yaxcopoil-1
2.3. Backwash and Crater Infill?
2.4. Cross-bedding? Grain size grading?
2.5. Polymict clasts? Breccia? melt rock?
2.6. Glauconite
2.7. Age of Sediments above Impact Breccia
2.8. Microfossils or Crystals?
2.9. Reworked microfossils?2.9.1 Stable isotopes
2.9.2. Iridium
2.9.3. Paleomagnetic Chron 29r
Summary
3.
Conclusions: Chicxulub impact predates KT by 300 kyr
References
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2. Chicxulub Crater
In l981 a circular structure on northern Yucatan was discovered based
on gravity and magnetic anomalies that suggested an impact crater (31)
(Fig. 15). This observation lay dormant for the next 10 years. In l99l
investigation of Cores drilled by PEMEX, the Mexican state petroleum
company supported the earlier suggestion (32) and this time the scientific
community took note, leading to a 10-year hunt for evidence in central
America by a multitude of scientists (46).
Figure 15. Magnetic and gravity anomaly map of the circular subsurface structure at Chicxulub (Sharpton, LPI). .
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2.1. PEMEX Cores: Maastrichtian Breccia?
PEMEX cores, and particularly the stratigraphic position of the breccia
and the age of the limestones overlying it, were controversial from the
beginning. At issue was a report by PEMEX investigators of the l970’s
(Meyerhoff and others (33) and Lopez Ramos (34, 35) who reported limestones
with diverse planktic foraminiferal assemblages of upper Maastrichtian
age overlying the breccia in wells C1 and Y6 (Fig. 15). If true, then
the impact breccia below it could not be of KT age.
Ward et al. (36) re-examined the stratigraphy, paleontology and sedimentology
of PEMEX cores Yucatan core Y1, Y2, Y4, Y5A, and Y6, no samples could
be obtained from wells Sacapuc 1 (S1), Chicxulub 1 (C1) and Ticul 1 (T1)
(Fig. 15), and they relied on e-logs and published lithologic and paleontologic
descriptions for these wells. For all wells resistivity-spontaneous potential
logs were available and these were examined and correlated (Fig. 16).
Note also that sediments underlying the breccia are normally stratified in all but wells C1 and Y6 near the center of the impact crater.
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Core recovery for the KT boundary was generally poor and no samples could be obtained from well C1. One sample obtained from A. Hildebrand for well Y6 (N12) at about 1000-1003 m yielded a Paleocene (zone P3) age. Hildebrand et al (32, writtine comm.. 2003) argued that this Paleocene age invalidates the late Maastrichtian age assigned by earlier studies. However, sample Y6 N12 is about 40 to 50 m above the breccia and no conclusion can be reached regarding the age of the breccia. For example, in the new well Yaxcopoil-1, Paleocene zone P3 faunas are present already at 70cm above the impact breccia. The amount of Maastrichtian or early Paleocene sediments present above the breccia is dependent on erosion events and the paleotopography.
Because of the lack of samples for examination by Ward and others (36) of the critical interval, no precise age could be independently determined for the sediments directly overlying the breccia. However, e-log correlations indicate that “the top of the breccia could be as high as 938m, the lowest depth at which presumably undisturbed marls and limestones can be correlated on electric log characteristics with equivalent beds in S1.” Ward et al. concluded that “There is some evidence that the breccia unit is overlain by about 18 m of uppermost Maastrichtian marl, suggesting an impact before the Cretaceous-Tertiary boundary.”
2.2. New Evidence: Yaxcopoil-1
In 2000-2001 the International Continental Drillling Program (ICDP) supported the drilling of a new core by the Chicxulub Scientific Drilling Program (CSDP). One of the major objectives of this project was to settle the controversial issue of the age of the impact breccia and hence the age of the Chicxulub impact (37). A site on the Hacienda Yaxcopoil was chosen. The site was believed to be in the center of the crater and drilling was expected to recover the impact melt sheet and thick breccia deposits (Fig. 17). This was not the case. No melt sheet was recovered and the breccia was only 100 m thick, significantly less than the 300-600 m recovered by PEMEX. This suggests that drilling occurred near the crater rim on uplifted fault blocks (Fig. 18).
Figure 17. Model of the Chicxulub impact crater, showing the central uplift, impact melt sheet and breccias. The new core Yax-1 was expected to penetrate the melt sheet and several hundred meters of breccia. Instead, no melt sheet was recovered and the breccia is only a100m thick. This suggests that drilling occurred near the crater rim on uplifted fault blocks.
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Figure 18. Thickness of breccia units recovered by drill cores on Yucatan.
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Figure 19. Core Yaxcopoil-1, Chicxulub, showing the uppermost part of the suevite breccia and the lower 20 cm of the overlying dolomitized, laminated micritic breccia. Note that the upper 15m of the suevite breccia are reworked and current bedded. Melt glass and spherules are common in this interval.
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Figure 20. Core Yaxcopoil-1, Chicxulub, showing the green clay of the KT boundary (30cm up from the core base) and early Tertiary limestones.
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2.3. Backwash and Crater Infill?
The 50 cm thick laminated, partially dolomitized, micritic limestone that unconformably overlies the suevite breccia and underlies the KT boundary holds the key to determining the age of the Chicxulub impact and the nature of the post-impact paleoenvironment (Fig. 21). In order to have a common origin for the suevite breccia and the KT boundary, this 50 cm layer must be interpreted as part of the impact event, such as backwash and crater infill, as commonly argued (32, 38a,b) Hildebrand, written comm. 2003). In support of this interpretation, it is claimed that this 50cm interval shows high-energy activity in the form of cross bedding and grain size grading, abundant impact melt rock now altered to green clay layers and quartz minerals.
Figure 21. Lithology of the critical 50cm interval between the impact breccia and the KT bondary in well Yaxcopoil-1. If the impact breccia is KT age, then this 50 cm interval of sediments must be interpreted as backwash and crater infill deposited over a geologically instantaneous time span. Sediment and microfossil analyses indicate deposition occurred during the last 300 ky of the Maastrichtian in variable but low energy environments.
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Backwash and crater infill
is expected to erode, transport and redeposit material from the surrounding
crater walls via slumps or gravity flows due to earthquakes or unstable
topography, and by currents. Hildebrand argues that Gulf of Mexico waters
rushed in through a 100km wide and 1km deep trough, sliced off by the impact
in the northern part of the crater, transporting a mix of reworked sediments
and faunas and depositing them in the crater (written comm.. 2003). These
scenarios can be evaluated based on the sediments overlying the Chicxulub
breccia. To validate the backwash and crater infill scenario the sediments
must contain:
1. Sedimentary structures indicative of high-energy currents (e.g. cross-bedding, flaser bedding, grain size grading.In contrast, normal pelagic sedimentation is indicated if the sediments show:
2. Highly variable lithoclasts from the diverse underlying strata, such as shallow water limestones, gypsum, dolomite, anhydrite, plus breccia clasts.
3. A significant amount of melt rock either as glass fragments or glass altered to Cheto smectite.
4. Reworked microfossil and macrofossil faunas from the underlying sediments, such as shallow water benthic foraminifera and invertebrates that are common in the lagoonal to subtidal Cretaceous environments of the Yucatan platform, though no planktic foraminifera.
5. Reworked fauna must show evidence of diverse ages reflecting the deep erosion and disturbance of underlying sediments.
1. No grain size grading or other high-energy current bedding,Our sediment analyses have addressed these issues based on thin section investigations, XRD, ESEM, isoluble residues, and microfossils. We discuss the results below.
2. Lack clasts of breccia, melt rock, or other lithologies,
3. Lack reworked faunas and burrowing organisms,
4. Contain evidence of in situ accumulated sediments such as galuconite
5. Contain microfossil assemblages characteristic of a narrow age interval.
2.4. Cross-bedding? Grain size grading?
The sediments of the 50 cm interval predominantly consist of laminated micritic limestone with patches or microlayers of dolomite. The micritic limestone is finely crystalline, whereas the dolomite crystals are large and angular. At first sight this may give the impression of grain size grading. However, dolomite formed by diagenetic replacement of the precursor (micritic) limestone with the laminated texture still visible. The micritic limestone indicates deposition in quiet water conditions.
Figure 22. Limestone lithologies from Yax-1. Numbrs refer to samples indicated in Figure 21. 12. Laminated microlayers in micritic limestone, 16, laminated microlayers micritic limestone enhanced by partial dolomitization of some layers, 18. laminated microlayers micritic limestone enhanced by early stage dolomitization, 21. Dolostone, advanced diagenetic alteration of the original micritic limestone and replacement by angular dolomite crystals.
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Core Yax-1 shows three <1 cm thick layers of oblique bedding within
an 8 cm thick interval between 794.45 and 794.53 m that has been referred
to as cross-bedding (Fig. 21). Cross-bedding must have grain size changes
of at least medium to fine grained sand. We found no grain size changes
and samples dissolved in acid show almost no insoluble residues and no
quartz grains. We could not confirm Smit’s claim of common sand grains
with dolomite overgrowth (38b). If such sand grains were present, they
should be in the insoluble residues. The apparent grain size change in
these sediments is diagenetic due to dolomitization (see Fig. 22).
Grain size change is also characteristic of flaser bedding. Based on visual
examination we thought there was flaser bedding, though insoluble residues
revealed only some glauconite or glauconite coated microclasts. Burrowing
structures account for the apparent mottled character of these intervals.
Based on our analyses we could not confirm the presence of cross-bedding,
flaser-bedding or grain size grading. Diagenetic growth of dolomite crystals,
burrowing organism and glauconite formation appear to account for the
features identied as evidence of high-energy current bedding.
2.5. Polymict clasts? Breccia?
melt rock?
By visual core examination, we originally identified several very thin
(<0.5 cm) microconglomerate layers as possibly supporting reworking
and redeposition from the breccia. Closer examination revealed five thin
green clayey microclast layers interbedded with the laminated micritic
limestone (794.43, 794.34-35, 794.24, 794.19, 794.11)(Fig. 21). Insoluble
residues revealed these microclasts to be of glauconitic origin or have
in situ glauconite coating (see below). We observed only very rare clasts
and very rare glass shards (Fig. 23). These insignificant amounts argue
against backwash and crater infill from reworked and transported material.
Figure
23a. Impact glass in Sample 10 below KT boundary.
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2.6. Glauconite
The five green clay and glauconite layers at the KT boundary and below it have been interpreted by Smit and others (38a,b) as evidence of abundant altered impact glass.
Impact glass alters to a characteristic, almost pure Cheto smectite (16, 39). XRD and ESEM analyses indicate that Cheto smectite is present in the impact breccia. However, the green clay at the KT boundary, or in the other four intervals, is of glauconitic origin and represents a mature glauconite (40) (Fig. 24).
Figure 24. Evidence for pre-K-T age of the Chicxulub Crater. New evidence from the Chicxulub core Yaxcopoil-1, which was drilled on the flank of the crater, reveals a pre-K-T age for this impact event. The evidence is based on several independent proxies, including sedimentology, mineralogy, magnetostratigraphy and micropaleontology. The age determination is based on sediments between the impact breccia and the K-T boundary, which contains planktic foraminifera characteristic of the last 300,000 years of the Cretaceous and paleomagnetic chron 29R which spans the last 500,000 years of the Cretaceous and first 270,000 years of the early Tertiary. These sediments were deposited in a normal marine environment where laminated limestones alternated with deposition of five glauconite layers. The top glauconite layer marks the K-T boundary and a hiatus. Glauconite forms over tens of thousands of years in slightly agitated bottom waters. The sediments also contain fossil burrows, which indicate that invertebrates colonized the ocean floor during deposition. Together this evidence rules out chaotic and rapid deposition via tsunami or backwash after the impact.
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The five layers of glauconite formation thus effectively rule out a scenario of rapid deposition, high-energy currents, backwash and crater infill.
2.7. Age of Sediments above Impact Breccia
The depositional age of the 50 cm thick laminated micritic limestones between the top of the suevite breccia and the KT boundary can be determined from the presence of planktic foraminifera. This critical interval contains diverse planktic foraminiferal assemblages characteristic of zone CF1, which spans the last 300 kyr of the Maastrichtian (Fig. 25a). These assemblages include Globotruncana stuarti, G. insignis, G. arca, G. falsocalcarata, Abathomphalus mayaroensis, Rosita contusa, R. petaloidea, R. walfishensis, Rugoglobigerina rugosa, R. macrocephala, Plummerita hantkeninoides, Globotruncanella petaloidea, Heterohelix, Hedbergella sp. and Globigerinelloides aspera (Fig. 25b). Small benthic foraminifera are also present (mostly buliminellids), but there are no reworked shallow water benthic foraminifera from below the breccia. No planktic foraminifera lived within the lagoonal to subtidal environment of the pre-impact Yucatan platform.
These foraminiferal assemblages indicate deposition occurred after the impact event and prior to the KT boundary mass extinction, sometime during the last 300 kyr of the Maastrichtian, similar to the age of spherule deposition in NE Mexico.
Figure 25b. Planktic foraminifera from the early Danian zone Pla and zone CF1, which spans the last 300 kyr of the late Maastrichtian at Yax-1, Chicxulub. Zone Pla: 1. Woodringina hornerstownensis, 2. Parvularugoglobigerina eugubina, 3. Parasubbotina pseudobulloides, 4. Morozovella inconstans (sample 4, zone Plc). 5. Plummerita hantkeninoides (sample 20); 6. Rugoglobigerina macrocephala (sample 9); 7. R. rugosa (sample 12); 8. R. hexacameratea (sample 10); 9 & 10. Globotruncana insignis (sample 20); 11. Rosita contusa (sample 9).
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2.8.
Microfossils or Crystals?
The images in Figure 25 are demonstrably those of planktic foraminiferal species. This is evident when the thin section images are compared with 3D images of the same species from well-preserved specimens at El Kef, Tunisia (Figure 26a). It has been suggested that the microfossils identified by GK are just
crystals (Smit, in (41) and written comm.. Sept. 26, 2003, CCNET). Smit
states that “neither the Zaragoza group nor I were able to find any
determinable foraminiferal remains in any of the samples. Instead, we
found in thin sections exclusively rhomb-like idiomorphic dolomite overgrowths
of sand grains. The rhombs resemble in size and thickness somewhat the
testwalls of foraminifers.” Smit illustrated this with the micrograph show in Figure 26b.
Figure 26a. Micrographs of Late Maastrichtian planktic foraminifera from the Chicxulub Yax-1 core between the impact breccia and the K/T boundary and comparison with the pristine 3D images of the same species from the El Kef section of Tunisia. (1. Globotruncana insignis, 2. Rugoglobigerina rugosa, 3. R. macrocephala, 4. Plummerita hantkeninoides). Note that the Yax-1 specimens show the same number of chambers, chamber arrangement and coil, size and morphology as the pristine species. This demonstrates that these images are foraminiferal species, rather than dolomite rhombs as claimed by Smit.
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Micritic limestones are notorious for poor foraminiferal preservation and no nannofossils are preserved. Micrite recrystallizes the limestone and the calcite of the foram shells. Frequently all that is left is the knobby structure of the shell rim or keel and the white color of the original test calcite showing the morphology and test chambers of the species (Fig. 26). At very high magnification the foraminiferal shapes dissolves into the surrounding crystalline matrix. There is only a limited range of magnification in which the shapes can be viewed and illustrated. This limits relatively clear illustrations to the largest species (globotruncanids and some rugoglobigerinids, with smaller species fuzzy at the magnification required for illustration. In sediments that are also partially dolomitized, preservation of foraminiferal species is even more difficult since dolomite rhombs are rather large (as shown in Figure 22, dolostone sample 21) and consume larger parts of the test shells. But they never resemble testwalls of foraminifers. This claim by Smit suggests that he and Arz searched for the foraminifera in dolomitic intervals, where indeed there are no recognizable foraminifera preserved.
However, the argument by Smit and Arz that no Maastrichtian foraminifera are preserved in these micritic limestones is now mute because Arz and others confirmed their presence as published in the July 2004 volume of Meteoritics and Planetary Science (see Arz et al., MAPS 39(7), 1099-1111 (2004).
Figure 26b. Smit's illustration of a dolomite rhomb that he thinks resembles a foraminifera.
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2.9. Reworked microfossils?
Could these microfossils be reworked? Smit (38b) claims that “even if the foraminiferal fossils were missed by the Zaragoza (Arz) and Amsterdam (Smit) groups, they would not permit any conclusion about the age of the crater” because “cross- and parallel beds tell any sedimentologist that such sediments are deposited by currents and waves and that all grains in those beds, including foraminiferal shells, are transported from another source.”
It is important to remember here that Smit’s “cross-beds” refer to three < 1cm thick micritic limestone layers with oblique bedding,which contain no sand grains, or size grading, except for (diagenetic) dolomite crystals (see section 2.4). Similarly, the “parallel beds” are millimeter thin micritic limestone laminations (see Fig. 21). Moreover, there are five thin glauconitic layers interbedded within this 50cm thick interval (sec. 2.6). As demonstrated above (sections 2.3 to 2.6), micritic limestone and finely laminated limestones, such as in Yax 1 between the KT and the impact breccia, form in quiet water environments and glauconite forms in slightly agitated waters with little or no sediment accumulation. Neither of these sediments lend any credence to an interpretation of significant transport and reworking, much less “all grains” transported from “the crater rim or the direct surroundings of the crater”(38b).
Apart from the sedimentological evidence for quiet pelagic sedimentation, there is also no evidence for significant reworking in the form of transported clasts from the breccia or of microfossils within them (sec. 2.5), or from the sediment below. Moreover, the shallow lagoonal to subtidal platform environment that prevailed prior to the Chicxulub impact did not support planktic foraminifera. Therefore, the late Maastrichtian assemblages would have had to be transported a long distance by strong backwash or tsunami waves from the open marine Gulf of Mexico. But there is no evidence for such high energy deposition. Moreover, such backwash and tsunami waves would have ripped up and transported sediments, clasts, and microfossils from various older Cretaceous ages. But there is no evidence for such reworking (sec. 2.5). The planktic foraminiferal assemblages are characteristic of the narrow interval of zone CF1, which spans the last 300 kyr of the late Maastrichtian.
We conclude that the zone CF1 assemblages that characterize the 50 cm interval between the KT boundary and suevite breccia were deposited after the Chicxulub impact in a normal pelagic environment that was deep enough to support planktic foraminifera.
2.9.1 Stable isotopes
Carbon Isotopes (42) of bulk rock samples in the 50 cm thick micritic limestone above the impact breccia reveal consistently high d13C values characteristic of the late Maastrichtian, followed by the signature 2‰ negative excursion at the K-T boundary (Fig. 27). Only sample 21 shows anomalously low d13C (0.76‰ and 1.7‰) and d18O (- 4.33‰ and -1.24‰) values as a result of diagenesis in this completely dolomitized limestone (see Fig. 22).
These data reveal the carbon and oxygen isotope values in the micritic limestone to be within the range of late Maastrichtian sediments, indicating the absence of any significant diagenetic overprint. The drop of about 2‰ in _13C at the top of the glauconitic layer is in a typical range observed for the K/T transition worldwide. The relative constant C- and O-isotope values argue against chaotic reworking and transport of this sequence. In addition, trace element concentrations are nearly constant through this section (42), and Ir concentrations are steadily increasing up to the K/T boundary. These features also argue against major reworking of this sedimentary sequence.
By itself, the stable isotope data does not conclusively show that these sediments are not reworked, because sediment settling from the water column after the impact event could yield similar Cretaceous isotope signals. However, together with the Ir data, other trace elements, absence of reworking,or high-energy current transport in sediments, and presence of glauconite layers all support normal pelagic sedimentation.
Figure 27. Stable isotopes, Ir concentrations and paleomagnetic stratigraphy in the micritic limestone above the Chicxulub suevite breccia. Note the late Maastrichtian carbon isotope signal and characteristic negative excursion that marks the KT transition in low and middle latitudes. Ir concentrations are low and probably within background values. The absence of an Ir anomaly at the KT boundary is due to a major hiatus.
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2.9.2. Iridium
Iridium concentrations at Yax-1 are minor (0.06-0.08 ng/g) in the first
20 cm of the micritic limestone overlying the suevitic breccia and increase
only slightly to reach maximum values of 0.3 ng/g in the glauconitic layer
at 794.08 - 794.14m that marks the KT boundary (Fig. 27). Two centimeters
above Ir concentrations return to values of 0.06 - 0.08 ng/g. The absence
of an Ir anomaly at the KT boundary is due to a hiatus that spans the lower
Danian (lower part of Pla) and probably the uppermost Maastrichtian interval.
2.9.3. Paleomagnetic Chron 29r
Paleomagnetic analysis (43) of the 56 cm micritic limestone above the impact
breccia reveal reversed polarity of C29r, which spans approximately the
last 500 kyr of the Maastrichtian and first 270 kyr of the Tertiary (Fig.
27). The fact that only about 6 cm of the early Tertiary C29r interval is
present confirms the hiatus identified by planktic foraminifera (44). Thus,
paleomagnetic data are consistent with the planktic foraminiferal data that
indicate zone CF1, which spans the last 300 kyr of the Maastrichtian, the
stable isotope data, and the evidence of normal pelagic sedimentation.
Summary
The Chicxulub impact predates the K-T boundary mass extinction by about 300,000 years based on the new evidence from sediments, geochemistry, paleomagnetism and microfossils in the Chicxulub crater core Yax-1. This supports the earlier evidence from northeastern Mexico where Chicxulub impact glass spherules (microtektites) are interbedded in late Maastrichtian sediments that predate the K-T boundary by 300,000 years. The Chicxulub impact coincided with a major climate warming as a result of Deccan Trap volcanism, but caused no species extinctions (Figure 28). The mass extinction at the K-T boundary coincided with a major iridium anomaly which indicates a second, probably much larger, impact for which no crater has been found to date. This second K-T boundary impact also coincided with a major increase in Deccan volcanism. The killing mechanism of the K-T mass extinction seems to be the result of the unfortunate coincidence of major volcanism and a large impact, rather than a single large impact alone.
Figure 28. Multiple impacts and massive volcanism are the twin causes of the K-T boundary mass extinction. The Chicxulub impact occurred 300,000 years before the K-T and coincident with a major Deccan volcanism induced greenhouse warming, but caused no species extinctions. The ultimate cause for the K-T mass extinction is the unfortunate coincidence of major volcanism and a large impact.
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Figure 28A. The results of the Chicxulub crater core were published in The Proceedings of the National Academy of Sciences in March 2004.
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1. Chicxulub predates the KT boundary
3. Conclusions: Chicxulub impact predates KT by 300 kyr
References
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