Biotic effects of the Chicxulub impact, K–T catastrophe and sea level change in Texas

Abstract

Biotic effects of the Chicxulub impact, the K–T event and sea level change upon planktic foraminifera were evaluated in a new core and outcrops along the Brazos River, Texas, about 1000 km from the Chicxulub impact crater on Yucatan, Mexico. Sediment deposition occurred in a middle neritic environment that shallowed to inner neritic depths near the end of the Maastrichtian. The sea level fall scoured submarine channels, which were infilled by a sandstone complex with reworked Chicxulub impact spherules and clasts with spherules near the base. The original Chicxulub impact ejecta layer was discovered 45–60 cm below the sandstone complex, and predates the K–T mass extinction by about 300,000 years.

Results show that the Chicxulub impact caused no species extinctions or any other significant biotic effects. The subsequent sea level fall to inner neritic depth resulted in the disappearance of all larger (> 150 μm) deeper dwelling species creating a pseudo-mass extinction and a survivor assemblage of small surface dwellers and low oxygen tolerant taxa. The K–T boundary and mass extinction was identified 40–80 cm above the sandstone complex where all but some heterohelicids, hedbergellids and the disaster opportunistic guembelitrids went extinct, coincident with the evolution of first Danian species and the global δ¹³C shift. These data reveal that sea level changes profoundly influenced marine assemblages in near shore environments, that the Chicxulub impact and K–T mass extinction are two separate and unrelated events, and that the biotic effects of this impact have been vastly overestimated.

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.