Palaeogeography, Palaeoclimatology, Palaeoecology
Biotic effects of the Chicxulub impact, K–T catastrophe and sea level change in Texas
Introduction
Biotic effects of the Cretaceous–Tertiary (K–T) catastrophe are well known primarily from the extinction of dinosaurs, ammonites and other invertebrates, planktic foraminifera and nannofossils (reviews in MacLeod et al., 1997, Keller, 2001, Twitchett, 2006). Among these only planktic foraminifera suffered a dramatic and sudden mass extinction at the K–T boundary. The Chicxulub impact is commonly believed to be the single cause despite the gradual extinction patterns, associated climate changes, sea level fluctuations and volcanism preceding the mass extinction. The K–T age for this impact is based on the controversial interpretation of a sandstone complex with reworked Chicxulub impact spherules at the base as impact generated tsunami deposits in NE Mexico and Texas (e.g., Bourgeois et al., 1988, Smit et al., 1992, Smit et al., 1996, Smit et al., 2004, Schulte et al., 2006, Kring, 2007, Schulte et al., 2008). Such geologically instantaneous deposition is required to bridge the stratigraphic separation between the K–T boundary and Chicxulub spherule ejecta layer. But multiple burrowing horizons within the sandstone complex in Mexico and Texas are incompatible with tsunami deposition (Ekdale and Stinnesbeck, 1998, Keller et al., 2003a, Gale, 2006). Recent discoveries of an older and apparently the original Chicxulub spherule ejecta layer 4–9 m below the sandstone complex in two sections in NE Mexico and 45–60 cm below in Texas indicate that this impact predates the K–T boundary by about 300,000 years (Keller et al., 2003a, Keller et al., 2007a).
Brazos River sections have long been known to contain a complete K–T transition comparable to the El Kef stratotype section in Tunisia (Jiang and Gartner, 1986, Keller, 1989a, Keller, 1989b, Barrera and Keller, 1990), but with the added advantage of a sandstone complex with Chicxulub spherules up to 1.6 m below the K–T boundary (Keller et al., 2007a, Keller et al., 2008a). Nevertheless, Schulte et al., 2006, Schulte et al., 2008 placed the K–T boundary at the base of the sandstone complex arguing that the Chicxulub impact defines the K–T boundary. With the recent discovery of the original Chicxulub spherule ejecta layer in undisturbed claystones below the sandstone complex (Keller et al., 2007a), the Brazos sections have also become the most unique in their preservation of these three stratigraphically well-separated events that represent the Chicxulub impact, sea level fall and K–T mass extinction. These sections thus provide an unprecedented opportunity to unravel the history of events leading up to the K–T mass extinction in a marginal continental shelf environment.
The first and only previous attempt to use quantitative planktic foraminiferal studies and stable isotopes to understand the K–T transition at Brazos was about 20 years ago and prior to the discovery of the Chicxulub impact crater (Keller, 1989a, Keller, 1989b, Barrera and Keller, 1990). Since that time, hundreds of K–T sections have been analyzed globally and the biotic and environmental changes are much better understood, which warrants a comprehensive analysis of new Brazos sections and drill cores. In this study, the main objectives are to quantitatively document the faunal changes in planktic foraminifera in order to evaluate the biotic and environmental effects of the Chicxulub impact, the sea-level fall and the K–T boundary event and reconstruct the history leading up to the end-Cretaceous mass extinction.
Section snippets
Location, materials and methods
The Brazos K–T transitions are found in central Texas along a 3 km stretch extending from Highway 413 south along the Brazos River of Falls County and its tributaries the Cottonmouth and Darting Minnow Creeks (Fig. 1; Yancey, 1996). These sections have long been the focus of numerous studies evaluating the K–T mass extinction by focusing on a prominent sandstone complex as link to the Chixulub impact (e.g., Jiang and Gartner, 1986, Hansen et al., 1987, Bourgeois et al., 1988, Hansen et al., 1993
Lithology and depositional environment
The lithology, mineralogy, stable isotopes and depositional environment of core Mull-1 and the Cottonmouth Creek (CMA and CMB) sections were detailed in Keller et al. (2007a) and only a brief summary is provided here. The late Maastrichtian sediments below the sandstone complex consist of undisturbed bedded and burrowed dark grey claystone with invertebrate shells, including the small ammonite, Discoscaphites iris (Conrad, 1858), which is indicative of the uppermost Maastrichtian ammonite zone
K–T transition — age control
This study uses the high-resolution planktic foraminiferal zonation of the El Kef stratotype section developed by Keller et al. (1995), Pardo et al. (1996) and Li and Keller, 1998a, Li and Keller, 1998b, Fig. 4). In the Brazos sections, the Maastrichtian was recovered in core Mull-1, but outcrop exposures along Cottonmouth Creek are limited to the uppermost Maastrichtian beginning about 1 m below the sandstone complex. In Mull-1, the uppermost Maastrichtian zone CF1 index species Plummerita
Stable isotopes
Preservation of Maastrichtian specimens is excellent (Fig. 5), as also observed by Barrera and Keller (1990). Examination by scanning electron microscope (SEM) did not reveal signs of calcite overgrowth or recrystallization in Maastrichtian species. Benthic δ18O values at Brazos vary between − 0.5‰ to − 4‰ and are generally lower than typical Late Maastrichtian deep-sea values that vary between + 0.9‰ to − 0.5‰ (Li and Keller, 1998c, Barrera and Savin, 1999, Li and Keller, 1999, Frank et al., 2005
Paleodepth and planktic/benthic ratio
In a study of benthic foraminifera in Brazos core KT3 and outcrops Keller (1992, p. 82) observed very low species richness (∼ 10 species) as compared with 20–25 species at El Kef, Caravaca or the Negev where middle to outer shelf or upper slope paleodepths were estimated. She concluded that deposition at Brazos occurred in a much shallower middle to outer neritic environment. This paleodepth comparison was overly optimistic. Current data on planktic foraminiferal depth ranking, the
Chicxulub impact caused no extinctions
The oldest and original Chicxulub impact spherule ejecta layer in the Cottonmouth Creek (CMAW) section occurs in the lower part of zone CF1 between 45–60 cm below the sandstone complex and consists of a 3–4 cm thick spherule layer altered to cheto smectite (Keller et al., 2007a). Negative δ18O and δ13C excursions suggest significant short-term environmental effects (Fig. 6, Fig. 7). The biotic effects can be evaluated from the planktic foraminiferal record in both small (> 63 μm) and larger (>
Biotic response to environmental stress
Species richness and the relative abundances of individual species populations are two commonly used proxies to assess environmental changes. In recent years, isotopic depth ranking studies of planktic foraminifera have greatly advanced our understanding of the nature of environmental stress associated with faunal turnovers (Abramovich et al., 2003, Frank et al., 2005). Based on these studies diversity changes and biotic stress conditions across a spectrum of environments from open marine to
Conclusions
- 1.
K–T sections along the Brazos River, Texas, reveal three stratigraphically well separated events: the Chicxulub impact spherules layer, sea-level fall (sandstone complex) and K–T mass extinction.
- 2.
The Chicxulub impact spherule layer (altered to cheto smectite) is present in upper Maastrichtian (lower zone CF1) claystones, ∼ 300 ky prior to the K–T boundary. No species extinctions or significant populations changes were caused by this impact.
- 3.
A Sea-level fall from middle to inner neritic depths
Acknowledgement
We thank the reviewers, particularly M. Hart, for their thoughtful comments. The material for 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, BSF Grant No. 2004045 and the Swiss National Fund No. 21-67702.02/1.
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2019, Cretaceous ResearchCitation Excerpt :In fact, the total richness drops by 46% (i.e., from 52 to 28) within 3.75 m interval prior to the K-Pg boundary. Similar decline in the species richness throughout the P. hantkeninoides Zone has been recorded in many areas, such as El Kef, Elles, Negev, Agost, Bidart, Brazos River, DSDP Site 525 and Meghalaya (Keller, 1988, Molina et al., 1996; Li and Keller, 1998; Abramovich et al., 1998; Abramovich and Keller, 2002; Gallala et al., 2009; Keller et al., 1995, 2009, 2011, 2012). The opportunistic Guembelitria cretacea blooms and the decline in the total richness could signal environmental stress prevailed before the K-Pg boundary in the Haymana Basin.
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2015, GeobiosCitation Excerpt :The catastrophic vs. gradual disappearance or change of biota or recovery of the involved communities is still a matter of debate and controversy (Kring, 2007; MacLeod et al., 1997; MacLeod, 1998). Some authors (Kring, 2007; Keller et al., 2009; Schulte et al., 2010) point out the absence of a similar reaction of different biota at the boundary, suggesting influences of several factors leading to variable effects on marine vs. continental communities, vertebrates vs. invertebrates, planktonic vs. benthic, K- vs. r-strategists, or deposit vs. suspension feeders (Powell and MacGregor, 2011; Labandeira et al., in press). Marine deposit-feeders living in deep-sea substrates and feeding upon detritus or dead organisms (Arthur and Zachos, 1987; Twichett, 2006) were less affected by the extinction episode than suspension and filter feeders living in the shallow water conditions, having more direct trophic links to photosynthesis (Sheehan and Hansen, 1986).
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2014, Earth-Science ReviewsCitation Excerpt :These deposits shed some light on events that took place approximately 66 Ma (see stratigraphy.org for the new absolute age of the Cretaceous/Paleogene boundary). They provide evidence of extraterrestrial impact, but prior to the Cretaceous/Paleogene boundary and the mass extinction event (Keller et al., 2007, 2009). Another remarkable geosite that exhibits deposits of a regional-scale event is the Jaja-Petropavlovskij section in Southern Siberia (Russia) (Fig. 2C).