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KT Controversies - the "Science" letters

The following texts are all comments on the 40-signature paper by Schulte et al., published in Science.  (Schulte, P. and 40 others, 2010, The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary: Science, v. 327, p. 1214-1218.)

The journal refused to print all but one letter and comments by Gerta Keller et al. and Courtillot and Fluteau.   However the version that was published was highly expurgated.  We publish here Keller et al.'s full comment, as well as the rejected comments from other authors. 


Comment on the ‘Review’ article by Schulte and 40 co-authors: The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary.

  • Gerta Keller, Geosciences Department, Princeton University, Princeton NJ 08544, USA
  • Tel. 609 258-4117, [email protected]
  • Thierry Adatte, Geological and Paleontological Institute, University of Lausanne, CH 1015 Lausanne, Switzerland, [email protected]
  • Alfonso Pardo, Escuela Politécnica Superior de Huesca, Universidad de Zaragoza, Crta. Cuarte s/n 22071- Huesca, Spain, [email protected]
  • Sunil Bajpai, Department of Earth Sciences, Indian Institute of Technology, Roorkee 247 667, Uttarakhand, India, [email protected]
  • Ashu Khosla, Center for Advanced Study in Geology, Panjab University, Chandigarh-160014, India, [email protected]
  • Bandana Samant, Department of Geology, Nagpur University, Nagpur-440 001, Maharashtra, India, [email protected]

The SCIENCE (v. 327, p. 1214, 2010) article by Schulte et al., written as review of the 30 year old controversy over the cause of the end-Cretaceous mass extinction concluded that the original theory of 1980 was right – a large asteroid impact on Yucatan was the sole cause for this catastrophe.

To arrive at this conclusion, the authors used a rather selective review of data and interpretations by proponents of this viewpoint, but ignore the vast body of evidence accumulated by scientists across disciplines (paleontology, stratigraphy, sedimentology, geochemistry, geophysics, volcanology) that documents a complex long-term scenario involving impacts, volcanism and climate change that is inconsistent with their conclusion. Here we briefly point out some of the key evidence regarding the age and biotic and environmental effects of the Chicxulub impact and Deccan volcanism and their implications.

1. The underlying basis for Schulte et al’s claim that the Chicxulub impact is the sole cause for the Cretaceous-Paleogene (K-Pg or KT) mass extinction is the assumption that the Iridium anomaly at the KT boundary (KTB) and Chicxulub are genetically linked and therefore the same age. There is no evidence to support this assertion. No Ir anomaly has ever been identified in association with undisputed Chicxulub impact ejecta (impact glass spherules) and no impact spherules have ever been identified in the Ir-enriched KTB clay in Mexico or elsewhere (1,2). In rare deep-sea sites where the Ir anomaly is just above impact spherules it is due to condensed sedimentation and/or non-deposition. Defining Chicxulub impact ejecta as the KTB is circular reasoning and a prioi excludes any assessment of the true age of the Chicxulub impact.
2. A Chicxulub impact generated tsunami is another basic assumption of Schulte et al. to account for the impact spherules in late Maastrichtian sediments (including a sandstone complex) in Mexico and Texas. Multiple lines of evidence contradict this assumption and demonstrate long-term deposition prior to the KTB:
  • 2-3 impact spherule layers are present (1,2).
  • Spherule layers are reworked and transported from shore areas (2,3).
  • Limestone layer with spherule-filled burrows separates spherule layers (1-2).
  • Burrowed horizons present in the upper part of the sandstone complex (4-7).
  • Presence of correlatable zeolite layers indicate discrete ash falls (8).
  • Clasts with impact spherules testify to an older age for the Chicxulub impact (7)
3. Evidence of the pre-KT age of the Chicxulub impact can also be found in sediments above the sandstone complex in Texas and NE Mexico and above the impact breccia in the Chicxulub crater. This evidence (a-h) demonstrates that the KTB is not linked to the sandstone complex and impact spherules.
  • The KTB red layer, Ir anomaly and mass extinction are always above the sandstone complex in Mexico (1,2,7,9).
  • Three wells on Yucatan contain Maastrichtian limestone between the KTB and impact breccia (10,11).
  • Chicxulub crater well Yaxcopoil-1: limestone deposition in C29r, KTB markedby negative ?13C shift and mass extinction, some reworked older species present (10,12).
  • Five burrowed glauconite layers within this limestone mark long-term deposition preceding the KTB (10).
  • At the base of the limestone a 10 cm thick interval with cm-scale oblique beddingindicates minor wave action, which does not support Schulte et al.’s interpretationof impact tsunami deposition (10).
  • The limestone is dolomitized, which accounts for the grain size that Schulte et al.erroneously interpret as tsunami debris (10).
  • At Brazos, Texas, 80 cm of burrowed and laminated claystones with uppermost Maastrichtian ?13C values, planktic foraminifera, nannofossils and palynomorphs separate the KTB mass extinction and ?13C shift from the sandstone complex (7, 9, 13).
4. Evidence that supports the pre-KT age of the Chicxulub impact is also found in sediments below the sandstone complex in NE Mexico and Texas.
  • In Texas a thin yellow clay layer in laminated claystone 45 cm below the sandstone complex consists of impact spherules altered to cheto smectite (7).
  • The same cheto smectite is present in the impact spherule layers at the base of thesandstone complex (7, 9).
  • XRD analysis shows almost no sanidine in the yellow clay, contrary to claims by Schulte et al. who interpret a volcanic origin on the basis of sanidine (7).
  • In NE Mexico (Penon) 4 m of horizontally bedded marl separates the sandstone complex from a Maastrichtian age impact spherule layer traced over 100 m (14).
  • Absence of shallow debris, and presence of melt glass and concave/convex spherule contacts mark this as the primary fallout layer (14, 15).
  • Spherule deposition occurred in the latest Maastrichtian zone CF1; no species extinctions occurred across this impact layer (14).
  • There is no significant evidence of “…liquefaction and slumping consistent with the single very-high-energy Chicxulub impact” as claimed by Schulte et al. (14).
5. Deccan volcanism is dismissed by Schulte et al. as much older and of no consequence in the KT mass extinction. Recent Deccan volcanism studies show the contrary.
  • There are three phases of volcanic eruptions with the main phase (80%) in C29r below the KTB (16,17).
  • Eruptions were sporadic and rapid, including the longest lava flows known on Earth, spanning over 1500 km across India and out into the Bay of Bengal (16-19).
  • The four longest Deccan lava flows occurred very close to the KTB with the fourth coinciding with the mass extinction (18, 20).
  • Rapid latest Maastrichtian global warming by 4ºC in ocean bottom waters, followed by 3-4ºC cooling before the KTB may be related to Deccan volcanism (21-23).
  • Major faunal and floral changes accompanied these climatic changes prior to the mass extinction (24-26).
  • Ir anomaly and KTB coincide with the end of the main phase of Deccan volcanism suggesting the possibility of another large impact.
  • Os ratios mark Deccan signal, but current correlation (27) is not supported.

When all this evidence is taken into account, it is clear that the massive Chicxulub and Deccan database indicates a long-term multi-causal scenario and is inconsistent with the model proposed by Schulte et al.


  1. G. Keller, Geol. Soc. Amer. Special Paper 437, 147 (2008).
  2. G. Keller, W. Stinnesbeck, T. Adatte, D. Stueben, Earth Sci. Rev. 62, 327 (2003).
  3. L. Alegret, E. Molina, E. Thomas, Geology 29, 891 (2001).
  4. G. Keller, J.G. Lopez-Oliva, W. Stinnesbeck, T. Adatte, Geol. Soc. Amer. Bull. 109, 410 (1997).
  5. A.A. Ekdale, W. Stinnesbeck, Palaios, 13, 593 (1998).
  6. A. Gale, Proc. Geol. Assoc. 117, 1 (2006).
  7. G. Keller, et al. Earth Planet. Sci. Lett. 255, 339 (2007).
  8. T. Adatte, W. Stinnesbeck, G. Keller, Geol. Soc. Amer. Spec. Paper 307, 197 (1996).
  9. G. Keller, S. Abramovich, Z. Berner, T. Adatte, Paleogeogr. Paleoclimatol. Paleoecol. 271, 52 (2009).
  10. G. Keller et al. Proc. Natl. Acad. Sci. U.S.A. 101 (ll), 3753 (2004).
  11. W.C. Ward, G. Keller, W. Stinnesbeck, T. Adatte, Geology 23, 873 (1995).
  12. J. A. Arz, L. Alegret, I. Arenillas, Meteorit. Planet. Sci. 39, 1099 (2004).
  13. M.L. Prauss, Paleogeogr. Paleoclimatol. Paleoecol. 283,195 (2009).
  14. G. Keller, T. Adatte, A.J. Pardo, J.G. Lopez-Oliva, J. Geol. Soc. London, 166, 393 (2009).
  15. G. Keller, Australian J. Earth Sci. 52, 725 (2005).
  16. A.-L. Chenet, X. Quidelleur, F. Fluteau, V. Courtillot, Earth Planet. Sci. Lett. 263, 1 (2007).
  17. A.-L. Chenet, F. Fluteau, V. Courtillot, M. Gerard, K.V. Subbarao, J. Geophys. Res. 113, B04101 (2008).
  18. G. Keller, T. Adatte, S. Gardin, A. Bartolini, S. Bajpai, Earth Planet. Sci. Lett. 268, 293 (2008)
  19. S. Self, A.E. Jay, M. Widdowson, L.P. Keszthelyi, J. Volc. & Geoth. Res. 172, 3 (2008)
  20. G. Keller, et al. Earth Planet. Sci. Lett. 282, 10 (2009).
  21. L. Li, G. Keller, Geology 26, 995 (1998).
  22. S. Abramovich, G. Keller, Marine Micropaleontology 48, 225 (2003).
  23. P. Wilf, K. R. Johnson, B. T. Huber, Proc. Natl. Acad. Sci. U.S.A. 100, 599 (2003).
  24. B. Samant, D.M. Mohabey, Gond. Geol. Magz. Spl. 8, 151 (2005).
  25. B. Samant, D.M. Mohabey, J Biosci. 34, (5), 811 (2009).
  26. G. Keller et al. Geol. Soc. Amer. Abstract 41(7), 239 (2009).
  27. N. Robinson, et al. Earth Planet. Sci. Lett. 281, 159 (2009).

Letter to the Editor of Science, by Abramovich and Benjamini

Schulte et al., in SCIENCE v. 327, p. 1214-1218 (2010) emphasized the abruptness of the K-Pg event on world biota, and failed to do justice to the nature and tempo of environmental perturbation and concomitant biotic response in last few million years of the Cretaceous. Maastrichtian long-term cooling was accompanied by a burst of evolutionary modifications in planktonic foraminifera that was interrupted by two warming events (Li and Keller, 1998, 1999); the latter was an abrupt volcanism-generated (Chenet et al., 2007, 2008) warming of at least 3???° within chron C29r a few hundred thousand years prior to K-Pg.

Stress caused by warming resulted in reduced planktonic foraminiferal diversity, dwarfing of taxa, and decrease in photosymbiotic activity (Abramovich et al., 2003). Stable isotopic data of species inhabiting different water depths indicate decrease in available niches and significant vertical constriction of water mass strata. Repeated late Maastrichtian extinction of specialized globotruncanid species were recorded worldwide, in the Tethys [Abramovich et al., 1998; Li and Keller, 1998c; Abramovich and Keller, 2002;Keller et al., 2002; Keller, 2004], at South Atlantic DSDP Hole 525A [Li and Keller, 1998a], and in Madagascar [Abramovich, et al., 2003]. Thermal stress resulted also in widespread blooms of disaster/opportunist Guembelitria species (Abramovich et al., in press). Stress-related Guembelitria blooms and survival of small species, considered by Schulte et al. as the primary oceanic response to the K-Pg event, clearly had repeated antecedents in characteristic stressed environments of the Maastrichtian [Keller1989; Kroon and Nederbragt, 1990; Schmitz et al., 1992; Abramovich et al., 1998; Keller et al., 1998; Abramovich and Keller, 2002; Abramovich et al., 2003; Adatte et al., 2002; Keller, 2003; Keller and Pardo, 2004, Keller, 2007; Pardo and Keller, 2008].

Thus, the response of oceanic biota to the K-Pg event needs to be evaluated in the light of a developing Paleogene mode of oceanic stratification with origins in the Late Cretaceous.

Letter to the Editor of Sciene from Dr Andrew Kerr

Re: Schulte et al. 2009. The Chicxulub impact and mass extinction at the Cretaceous-Paleogene boundary. Science, 327, 1214-1218.

Dear Sir,

I write in respect of the above recently published article. My concerns in relation to this paper relate to the generally one-sided nature of the discussion and the disingenuous way in which the substantial body evidence (e.g. Macleod et al. 1997; Courtillot, 1999; Gale, 2006; Keller et al. 2007; Keller, 2008) for alternative volcanic and multi-causal models for the K-Pg are either not properly discussed, summarily dismissed or completely ignored.

Continent-scale volcanic eruptions (large igneous provinces) that erupted in excess of 1million km3 of lava, often in less than 1-2 million years, correlate (almost perfectly) with mass extinction events and indeed other smaller extinction events over the last 300 millions years (e.g. Courtillot, 1999). In contrast the only putative evidence of an asteroid impact coinciding with a mass extinction event is at the K-T boundary. The significant role of volcanism as a ‘killing-factor’ in these other mass extinctions is widely accepted, which makes the inadequate treatment of the volcanic effects of the Deccan eruptions by Schulte et al. quite bewildering. Of particular concern, is the cursory manner in which the age and duration of the Deccan eruptions is discussed: i.e. some of the most recent key papers are not referenced at all (e.g. Chenet et al. 2008; Self et al. 2008; Keller et al. 2008) and those that are referenced are only mentioned briefly, in connection with the significant age-data they contain, in the caption to Figure 1.

In publishing this paper without giving the advocates of alternative (multi-causal and volcanic) models of the K-Pg extinction an equivalent voice, ‘Science’ has unfortunately missed a great opportunity to ignite further debate on this far-from-resolved issue. I would therefore strongly encourage you to solicit a review article from those who propose alternative and perhaps more viable models for the K-Pg extinction event.


Chenet, A.L., et al. 2008. Journal of Geophysical Research: Solid Earth, v. 113, B04101.
Courtillot,V. 1999. Evolutionary Catastrophes. The science of mass extinctions. Cambridge University Press. 171pp.
Gale, A.S., 2006. Proceedings of the Geologists Association, v. 117, p. 173-185.
Keller, G., 2008. Cretaceous Research, v. 29, p. 754-771.
Keller, G., et al., 2007. Texas. Earth and Planetary Science Letters, v. 255, p. 339-356.
Keller, G., et al., 2008. Earth and Planetary Science Letters, v. 268, p. 293-311.
MacLeod, N., et al., 1997. Journal of the Geological Society, London, v. 154, p. 265–292.
Self, S., et al., 2008. Journal of Volcanology and Geothermal Research, v. 172, p. 3-19.
Yours sincerely

Dr Andrew C Kerr

Reader in Petrology
Cardiff University, UK

Letter to the Editor of Science from Prof. Emer. Ashok Sahni*

K-Pg Mass Extinctions: the Final Word?

This has reference to the Schulte et al. paper on the Chicxulub Crater and impact-generated deposits and the Cretaceous-Paleogene (K-Pg) boundary mass extinction events published recently in your journal (March 5, 2010 in volume 327). The authors make a strong case for a single ejecta event represented by deposits that can be compositionally linked to the Chicxulub impact at the K-Pg boundary extending from the Gulf of Mexico through the Caribbean Islands as far eastwards as Spain. However, two issues discussed in the paper, namely the timing of the K-Pg extinctions 1,2 and global extinction scenarios, do not adequately address the large volume of work carried out on the Deccan Volcanic geochronology and resulting catastrophic ecological consequences 3-8. The debate on the causes of the K-Pg extinctions is now nearly three decades old and needs to be discussed in a balanced perspective. Your journal has always maintained this balance not only in the review process but also in the published article. The current article gives a one-sided view of the K-Pg extinctions and does not really discuss the mismatch in ages between the impact and extinction events. One gets the impression that the final word has been said. Science knows better!
  1. Keller, G., Stinnesbeck, W., Adatte, T., Stüben, D., Earth Sci. Rev. 62, 327 (2003).
  2. Keller, G., Adatte, T., Berner, Z., Pardo, A., Lopez-Oliva, L., Paleogeogr.,Paleoclimatol., Paleoecol., 271, 52-68.
  3. Courtillot,V. Evolutionary Catastrophes. The science of mass extinctions, Cambridge University Press 1-171 (1999)
  4. Courtillot, V. Palaeogeography, Palaeoclimatology and Palaeoecology 89, 291-299 (1990)
  5. Chenet, A-L., Quidelleur, X., Fluteau, F. and Courtillot, V., EPSL 263, p. 1-15.
  6. Chenet A-L, Fluteau F, Courtillot V, Gerard M, Subbarao KV., J. Geophys. Res. 113: B04101.
  7. Keller, G., Adatte, T., Gardin, S., Bartolini, A., Bajpai, S., EPSL 268, 293-311.
  8. Keller, G., Adatte, T., Bajpai, S., Khosla, A., Sharma, R., Widdowson, M.,  Khosla, S.C., Mohabey, D.M., Gertsch, B., Sahni,. EPSL 282, 10-23.

* Ashok Sahni, Emeritus Professor, Centre of Advanced Study in Geology, Lucknow University, Lucknow 226001
10 March, 2010

Letter to the Editor of Science from Andy Saunders, Mark Reichow and Steve Self

Re: The Chicxulub Impact and Mass Extinction at the Cretaceous-Paleogene boundary by Schulte et al. Science v. 327, p.1214-1218.

12 May, 2010

Sirs,  Schulte et al review data and observations to conclude that the cause of the Cretaceous-Tertiary mass extinction was, definitively, an asteroid impact1. Several aspects of this review are not in doubt: that there was impact (as preserved at Chicxulub), that there are substantive related fall-out layers, and the effects of such an impact would have been calamitous for the Earth’s surface environment. The dismissal of other influences, in particular the Deccan Traps volcanism, is, however, unfortunate. A review of this type should be less partisan, particularly when the subject has been the topic of debate for the last 30 years.

There is strong evidence2-4 that the Deccan volcanism peaked at the time of the K-Pg mass extinction. It is pointless to consider long-term time-averaged release of volcanic gases because the eruptions were spasmodic, occurring in immense events, and consequently the emission rates of gases and the resulting atmospheric aerosols, especially sulfates, would have been much higher5 than those mentioned by Schulte et al. Furthermore these flux rates would have been maintained for a much longer period (decades for individual eruptive events; millennia for the main pulse of volcanism) than from a single impact (a few hours), leading to greater cumulative loading5.

It is striking that the three other major mass extinction events of the last 300 million years, the Guadalupian (260 Ma), end-Permian (251 Ma), end-Triassic (200 Ma), in addition to several oceanic anoxic (OA) events, all occur at the same time, within error of the dating methods, as the eruption of flood basalt provinces4,6. This is unlikely to be pure coincidence6. Furthermore, none of these mass extinction or OA events appears to be associated with a major impact crater or fallout layer. There is clear scope for both processes being causally related to the K-Pg mass extinction; the volcanism leading to progressively worsening environmental conditions, and the impact(s) offering the coup de grace. The question to address is this: would the K-Pg mass extinction have occurred without an impact? Other extinctions appear to have done so.


  1. Schulte P et al. (2010) Science 327: 1214-1218.
  2. Chenet A-L et al. (2008) Journal of Geophysical Research 113: B04101.
  3. Keller G et al. (2008) Earth and Planetary Science Letters 268: 293-311.
  4. Courtillot V and Renne P (2003) Comptes Rendus Geoscience 335: 113-140.
  5. Self S et al. (2008) Science 319: 1654-1657.
  6. White R and Saunders A (2005) Lithos 79: 299-316.

Andy Saunders and Marc Reichow
Department of Geology
University of Leicester
Leicester LE1 7RH

[email protected]

Steve Self
Department of Earth and Environmental Sciences
Open University
Walton Hall
Milton Keynes MK7 6AA