DiscussionComment on the paper “Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas” by Keller et al. (2007)
Section snippets
Misplacement of the K–P boundary
The “golden spike” of the global boundary stratotype section and point (GSSP) of the basal Danian (= basal Paleocene and basal Paleogene) is located at the base of the (black) boundary clay at El Kef, Tunisia. The primary criteria for correlation of the base of the Danian (i.e. the K–P boundary) by the International Stratigraphic Commission (ICS, www.stratigraphy.org) and outlined in Molina et al. (2006) are:
- (1)
evidence for asteroid impact (Iridium anomaly, Ni-rich spinels, shocked quartz,
An untenable reworking and sea-level scenario
The identification of a cm-thick yellow clay layer, 40 cm below the well-known spherule-rich event deposit, as the original Chicxulub ejecta layer is an extraordinary conclusion. Extraordinary conclusions demand extraordinary evidence. Yet, the authors fail to provide this in any aspect. According to the interpretation presented by Keller et al., the ejecta spherules in the spherule-rich event bed are the result of repeated reworking of the original spherule deposit about 200,000 years (!)
Lack of impact evidence for the yellow clay layer
Keller et al. (2007) refer to a cm-thick yellow clay layer in the Brazos succession as “…original impact spherule layer…” or as “…original ejecta layer with relic glass spherules mostly altered to clay.” (page 349 and Fig. 9 of Keller et al., respectively). However, nowhere in the paper it is clarified whether the authors have found the remnants of true Chicxulub ejecta spherules, i.e. round- or drop-shaped spherules with internal cavities and vesicles. The micrograph in their Fig. 9 from the
Omission of evidence from K–P sections inside and outside the Gulf of Mexico
Various independent proxy data (e.g., concentric ejecta grain-size distribution, similar isotopic ages, and distinct compositional range of ejecta phases) from many continental and marine K–P boundary sections as well as from more than 24 recent Ocean Drilling Program (ODP) K–P drillcores all provide strong support for the genetic relationship between the Chicxulub impact event and the worldwide distributed K–P boundary ejecta layer (Sigurdsson et al., 1997, Olsson et al., 1997, Smit, 1999,
Conclusions
Based on the following major points of criticism, we suggest that Keller et al. (2007) have not made any case for Chicxulub as a pre-K–P boundary impact:
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The Brazos K–P boundary level suggested by Keller et al. is misplaced since they rely exclusively on secondary – at best – and poor stratigraphic markers. Therefore, it does not fit the primary criteria of the International Commission of Stratigraphy (ICS) and the Cretaceous–Paleogene Working Group for positioning the K–P boundary (Molina et
Note added in proof
Each ODP Leg (e.g., Leg 207) drills at up to four sites (e.g., ODP Leg 207 Sites 1258, 1259, 1260, and 1261) and at each site, up to three single cores are drilled (e.g., 1259a, b, and c). Therefore, an ODP Leg may provide multiple records of the K–P boundary. For a detailed list of more than 48 DSDP and ODP K–P boundary cores please visit the ODP website at http://www-odp.tamu.edu/database/.
Acknowledgments
We acknowledge Jose Antonio Arz Sola (University of Zaragoza) for helpful comments on the K–P stratigraphy. We are grateful to Kenneth G. MacLeod (University of Missouri) and Alex Deutsch (University of Münster) for detailed and constructive reviews that significantly improved the paper. We also thank Claude Jaupart for careful editorial handling and valuable suggestions.
References (60)
- et al.
Chicxulub impact event is Cretaceous/Paleogene boundary in age: new micropaleontological evidence
Earth Planet. Sci. Lett.
(2006) - et al.
Dinoflagellate cysts, sea level changes and planktonic foraminifers across the Cretaceous–Tertiary boundary at El Haria, Northwest Tunisia
Mar. Micropaleontol.
(1988) - et al.
Dinoflagellate-based sea surface temperature reconstructions across the Cretaceous–Tertiary boundary
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(1998) Sequence stratigraphy of clastic systems: concepts, merits, and pitfalls
J. South. Afr. Earth Sci.
(2002)- et al.
Distinguishing primary and resedimented vitric volcaniclastic layers in the Burdigalian carbonate shelf deposits in Monferrato (NW Italy)
Sediment. Geol.
(1999) - et al.
Iridium anomaly at the Cretaceous–Tertiary boundary in Texas
Earth Planet. Sci. Lett.
(1981) - et al.
Sedimentology and extinction patterns across the Cretaceous–Tertiary boundary interval in east Texas
Cretac. Res.
(1987) - et al.
Patterns of molluscan extinction and recovery across the Cretaceous–Tertiary boundary in east Texas: report on new outcrops
Cretac. Res.
(1993) Extinction, survivorship and evolution of planktic foraminifera across the Cretaceous/Tertiary boundary at El Kef, Tunisia
Mar. Micropaleontol.
(1988)- et al.
Paleoecology of the Cretaceous/Tertiary mass extinction in planktonic foraminifera
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2002)
Chicxulub impact predates K–T boundary: new evidence from Brazos, Texas
Earth Planet. Sci. Lett.
The Chicxulub impact event and its environmental consequences at the Cretaceous–Tertiary boundary
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Rare earth element composition as evidence of the precursor material of Cretaceous–Tertiary boundary sediments at distal sections
Chem. Geol.
Analyses of shocked quartz at the global K–P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub
Earth Planet. Sci. Lett.
Depositional processes of primary to reworked volcaniclastics on an alluvial plain: an example from the lower Pliocene Ohta tephra bed of the Tokai Group, central Japan
Sedimentary Geology
Primary or secondary distal volcaniclastic turbidites: how to make the distinction? An example from the Miocene of New Zealand (Mahia Peninsula, North Island)
Sediment. Geol.
The Cretaceous–Paleogene (K–P) boundary at Brazos, Texas: sequence stratigraphy, depositional events and the Chicxulub impact
Sediment. Geol.
Emplacement of Cretaceous–Tertiary boundary shocked quartz from Chicxulub crater
Science
An independent test of planktic foraminiferal turnover across the Cretaceous/Paleogene (K/P) boundary at El Kef, Tunisia: catastrophic mass extinction and possible survivorship
Micropaleontology
Sequence stratigraphic concepts applied to Paleogene outcrops, Gulf and Atlantic basins
A revised Cenozoic geochronology and chronostratigraphy
Origin and diagenesis of K/T impact spherules — from Haiti to Wyoming and beyond
Meteoritics
Selective calcareous nannoplankton survivorship at the Cretaceous–Tertiary boundary
Geology
Leg 198 summary
Baseline studies of the Clay Minerals Society source clays: powder X-ray diffraction analyses
Clays Clay Miner.
Distribution of Chicxulub ejecta at the Cretaceous–Tertiary boundary
The implications of the reworking on the mineralogy and chemistry of lower Carboniferous K-bentonites
Clay Miner.
The Sedimentary Record of Sea-Level Change
Organic carbon fluxes and ecological recovery from the Cretaceous–Tertiary mass extinction
Science
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2016, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :The K/Pg boundary records a number of synchronous extinction events (Keller et al., 2009) in both the marine and terrestrial realms, including some invertebrates (e.g., ammonites, but see Landman et al., 2014), calcareous nannofossils, planktic foraminifera and non-avian dinosaurs. The relative roles of the Chicxulub bolide impact and Deccan volcanism in the various extinction hypotheses are still in debate within the geoscience community (Keller et al., 2004a,b, 2007, 2008; Schulte et al., 2006, 2008, 2009; Adatte et al., 2011; Keller and Adatte, 2011; Schoene et al., 2015), often causing some degree of acrimony. In particular, Keller et al. (2004a,b, 2007, 2008) have suggested that a yellow claystone a few centimetres below the ‘Event Bed’ in Cottonmouth Creek, Brazos River area, Texas, was indicative of a pre-extinction event impact, although this suggestion has been vigorously contested (Schulte et al., 2010).
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