The Chicxulub Debate

3. Conclusions: Chicxulub impact predates KT by 300 kyr
The new core, Yaxcopoil-1 (Yax-1) was expected to provide unequivocal evidence that Chicxulub is the KT impact crater that caused the mass extinction. But on the contrary, more evidence in support of a pre-KT impact event was discovered, consistent with the evidence in NE Mexico. The evidence is from different scientific investigations and includes stratigraphy, sedimentology, geochemistry, paleomagnetism and paleontology.

1. Evidence from NE Mexico:

a) KT boundary and Ir anomaly are abovethe impact ejecta. The “tsunami” deposit that was supposed to explain this discrepancy was deposited over an extended time period during which the ocean floor was repeatedly colonized by invertebrates.

b) The spherule layer beneath the siliciclastic deposit and KT boundary contains a 10-20 cm thick burrowed limestone layer that indicates deposition of the spherule ejecta occurred during two events separated by an extended time interval during which limestone accumulated burrowed by invertebrates.

c) Interbedded within the 12 m of Maastrichtian marls beneath these two spherule layers are up to three more spherule ejecta layers. The lowermost spherule layer consists of almost pure spherules, whereas subsequent layers contain variably high amounts of reworked clasts and foraminifera. This indicates that the stratigraphically oldest spherule layer represents the original deposit with all subsequent layers reworked.

d) These multiple spherule layers have been correlated to date over more than 100km. No major slumps or gravity flows have been observed, though there are occasionally minor slumps spanning a few meters.

e) The age of the oldest spherule layer is about 65.3 Ma, as indicated by its position near the base of planktic foraminiferal zone CF1,which spans the last 300 kyr of the Maastrichtian.

2. Evidence from Yaxcopoil-1 Chicxulub:

a) Yaxcopoil-1 contains a 50 cm thick laminated micritic limestone between the impact breccia and the KT boundary. This interval contains five thin green layers of glauconite formation with each interval bioturbated. This indicates deposition occurred in variable, but generally low energy environments interrupted by long pauses of little or no deposition and glauconite formation.

b) There are no grain-size grading (except for dolomite rhombs that may give that impression), no cross-bedding, no flaser bedding, no sand grains in insoluble residues, that would indicate high energy deposition due to backwash and crater infill. Though such evidence is present in the top 15 m of the impact breccia.

c) There is no evidence of significant reworking, no clasts from the underlying breccia or other Cretaceous lithologies, no microfossils from older Cretaceous strata, no mixed ages of planktic foraminifera. Impact glass is very rare.

d) Planktic foraminiferal assemblages are diverse and characteristic of the narrow interval of zone CF1 which spans the last 300 kyr of the Maastrichtian, similar to impact ejecta in NE Mexico sections.

e) Paleomagnetic data reveal Chron 29r , which spans the last 500 kyr of the Maastrichtian and first 270 kyr of the Tertiary, consistent with the planktic foraminiferal data.

f) Stable isotopes reveal typical late Maastrichtian carbon signals followed by the negative excursion that characterizes the KT boundary.

g) The KT Ir anomaly is absent due to the hiatus. Only background values are present in the interval above the breccia.We conclude that the stratigraphic, paleontologic and sedimentologic evidence from NE Mexico (l) and the stratigraphic, sedimentologic, paleontologic, geochemical, stable isotope and paleomagnetic evidence from the Chicxulub crater core Yaxcopoil-1 provide extremely strong, mutually supportive, multi-disciplinary evidence of the pre-KT age of the Chicxulub impact. The best age estimate of this impact currently suggests that Chicxulub predated the KT boundary by about 300 kyr.

Multiple Impact Scenario
A multi-impact scenario is most consistent with the current evidence of impact ejecta and Ir anomalies (Fig. 28). Chicxulub, the first impact, occurred at about 65.3 Ma and coincided with major Deccan volcanism, which led to greenhouse warming between 65.4-65.2 Ma and a decrease in primary productivity and continued decrease in planktic foraminiferal diversity that culminated in the KT boundary mass extinction. No significant species extinctions are associated with the Chicxulub impact, which can be partly explained by the relatively small size (~120 km in diameter) of the impact crater. Craters of about 100 km, such as Popigai, did not cause significant species extinctions.

The KT boundary impact also coincided with a major pulse in Deccan volcanism, but also with a dramatic drop in primary productivity and the mass extinction of all tropical and subtropical planktic foraminifera. This indicates that the KT boundary impact was significantly larger than the Chicxulub impact, or that Deccan volcanism was more intense. Considering the global aspect of the mass extinction, and the global Ir distribution, it is likely that a very large impact at the KT boundary exacerbated the already stressed environmental conditions due to massive volcanism.

The prolonged delay in biotic recovery after the KT mass extinction has puzzled investigators for a long time. The emerging evidence of an early Danian (zone Pla) iridium anomaly and volcanism suggests that there may have been repeated environmental perturbations. A recent study has shown that major environmental perturbations due to volcanism or impacts will elicit the same biotic responses (45). This suggests that the delay in the recovery after the KT mass extinction may have been due to repeated environmental disturbances.


1. Chicxulub predates the KT boundary and is not the cause for the end-Cretaceous mass extinction 2. Chicxulub Crater 3. Conclusions: Chicxulub impact predates KT by 300 kyr 1. Evidence from NE Mexico 2. Evidence from Yaxcopoil-1 Chicxulub Multiple Impact Scenario References Back to Chicxulub Debate Home Page 3. Conclusions: Chicxulub impact predates KT by 300 kyr The new core, Yaxcopoil-1 (Yax-1) was expected to provide unequivocal evidence that Chicxulub is the KT impact crater that caused the mass extinction. But on the contrary, more evidence in support of a pre-KT impact event was discovered, consistent with the evidence in NE Mexico. The evidence is from different scientific investigations and includes stratigraphy, sedimentology, geochemistry, paleomagnetism and paleontology. 1. Evidence from NE Mexico: a) KT boundary and Ir anomaly are abovethe impact ejecta. The “tsunami” deposit that was supposed to explain this discrepancy was deposited over an extended time period during which the ocean floor was repeatedly colonized by invertebrates. b) The spherule layer beneath the siliciclastic deposit and KT boundary contains a 10-20 cm thick burrowed limestone layer that indicates deposition of the spherule ejecta occurred during two events separated by an extended time interval during which limestone accumulated burrowed by invertebrates. c) Interbedded within the 12 m of Maastrichtian marls beneath these two spherule layers are up to three more spherule ejecta layers. The lowermost spherule layer consists of almost pure spherules, whereas subsequent layers contain variably high amounts of reworked clasts and foraminifera. This indicates that the stratigraphically oldest spherule layer represents the original deposit with all subsequent layers reworked. d) These multiple spherule layers have been correlated to date over more than 100km. No major slumps or gravity flows have been observed, though there are occasionally minor slumps spanning a few meters. e) The age of the oldest spherule layer is about 65.3 Ma, as indicated by its position near the base of planktic foraminiferal zone CF1,which spans the last 300 kyr of the Maastrichtian. 2. Evidence from Yaxcopoil-1 Chicxulub: a) Yaxcopoil-1 contains a 50 cm thick laminated micritic limestone between the impact breccia and the KT boundary. This interval contains five thin green layers of glauconite formation with each interval bioturbated. This indicates deposition occurred in variable, but generally low energy environments interrupted by long pauses of little or no deposition and glauconite formation. b) There are no grain-size grading (except for dolomite rhombs that may give that impression), no cross-bedding, no flaser bedding, no sand grains in insoluble residues, that would indicate high energy deposition due to backwash and crater infill. Though such evidence is present in the top 15 m of the impact breccia. c) There is no evidence of significant reworking, no clasts from the underlying breccia or other Cretaceous lithologies, no microfossils from older Cretaceous strata, no mixed ages of planktic foraminifera. Impact glass is very rare. d) Planktic foraminiferal assemblages are diverse and characteristic of the narrow interval of zone CF1 which spans the last 300 kyr of the Maastrichtian, similar to impact ejecta in NE Mexico sections. e) Paleomagnetic data reveal Chron 29r , which spans the last 500 kyr of the Maastrichtian and first 270 kyr of the Tertiary, consistent with the planktic foraminiferal data. f) Stable isotopes reveal typical late Maastrichtian carbon signals followed by the negative excursion that characterizes the KT boundary. g) The KT Ir anomaly is absent due to the hiatus. Only background values are present in the interval above the breccia.We conclude that the stratigraphic, paleontologic and sedimentologic evidence from NE Mexico (l) and the stratigraphic, sedimentologic, paleontologic, geochemical, stable isotope and paleomagnetic evidence from the Chicxulub crater core Yaxcopoil-1 provide extremely strong, mutually supportive, multi-disciplinary evidence of the pre-KT age of the Chicxulub impact. The best age estimate of this impact currently suggests that Chicxulub predated the KT boundary by about 300 kyr. Multiple Impact Scenario A multi-impact scenario is most consistent with the current evidence of impact ejecta and Ir anomalies (Fig. 28). Chicxulub, the first impact, occurred at about 65.3 Ma and coincided with major Deccan volcanism, which led to greenhouse warming between 65.4-65.2 Ma and a decrease in primary productivity and continued decrease in planktic foraminiferal diversity that culminated in the KT boundary mass extinction. No significant species extinctions are associated with the Chicxulub impact, which can be partly explained by the relatively small size (~120 km in diameter) of the impact crater. Craters of about 100 km, such as Popigai, did not cause significant species extinctions. The KT boundary impact also coincided with a major pulse in Deccan volcanism, but also with a dramatic drop in primary productivity and the mass extinction of all tropical and subtropical planktic foraminifera. This indicates that the KT boundary impact was significantly larger than the Chicxulub impact, or that Deccan volcanism was more intense. Considering the global aspect of the mass extinction, and the global Ir distribution, it is likely that a very large impact at the KT boundary exacerbated the already stressed environmental conditions due to massive volcanism. The prolonged delay in biotic recovery after the KT mass extinction has puzzled investigators for a long time. The emerging evidence of an early Danian (zone Pla) iridium anomaly and volcanism suggests that there may have been repeated environmental perturbations. A recent study has shown that major environmental perturbations due to volcanism or impacts will elicit the same biotic responses (45). This suggests that the delay in the recovery after the KT mass extinction may have been due to repeated environmental disturbances.
	Figure 28. The K-T mass extinction, multiple impacts, volcanism and climate. Since the early l990's the Chicxulub crater on Yucatan has been hailed as the smoking gun that proves the theory that an asteroid caused the end-Cretaceous mass extinction. New evidence suggests that multiple impacts, volcanism and climate change combined are the likely causes for the K-T mass extinction. Keller and her collaborators have found evidence that the Chicxulub impact predates the boundary by 300,000 years and coincided with major volcanism and global warming. This caused tremendous biotic stress and predisposed most animal groups to extinction. By the time of the K-T boundary impact at 65.0 m.y. ago, which also coincided with major volcanism, the stressed biota passed threshold conditions leading to the mass extinction. An early Danian impact, recognized by an iridium anomaly about 150,000 years after the K-T boundary, and continued volcanism delayed biotic recovery. . Click on the figure for a larger view.
Go to: Top of Page 1. Chicxulub predates the KT boundary 2. Chicxulub Crater References Back to Chicxulub Debate Home Page