A dual-layer Chicxulub ejecta sequence with shocked carbonates from the Cretaceous–Paleogene (K–Pg) boundary, Demerara Rise, western Atlantic
Introduction
Impact ejecta layers are very useful geological tools. In sites distal to the impact, they can be knife-sharp time markers that provide a precise biostratigraphic age and a reference horizon against which impact-related environmental effects can be compared. In proximal sites, ejecta fallout and other high-energy deposits are directly linked to the impact, allowing an assessment of the magnitude of the impact event (for reviews see Smit, 1999, Simonson and Glass, 2004). In addition, composition and structure of ejecta deposits reflect pre-impact target stratigraphy, physico-chemical processes in the ejected material, and in specific cases, projectile composition (e.g., Smit, 1999, Kring and Durda, 2002, Simonson and Glass, 2004, Trinquier et al., 2006). In the case of the deeply buried ∼180 km-diameter Chicxulub impact structure, Yucatán peninsula, Mexico (e.g., Morgan et al., 1997), for which only few drill cores exist (e.g., Ward et al., 1995, Claeys et al., 2003, Stöffler et al., 2004), breccias inside the crater and ejecta provide the only access to the basement in the region. Of more importance, the study of Chicxulub ejecta deposits is crucial for evaluating the mechanisms that linked the Chicxulub impact event with the Cretaceous–Paleogene (“K–Pg”, formerly “K–T”) boundary mass extinction (papers in Ryder et al., 1996, Koeberl and MacLeod, 2002).
The Chicxulub cratering event affected materials down to the base of the crust, and a part of this crustal material was ejected (Morgan et al., 1997, Christeson et al., 2001). Previous studies have documented a range of ejecta compositions, as well as compositional trends with increasing distance from Chicxulub (marine sections) and within ejecta deposits (terrestrial sections, Western Interior of North America). These trends indicate a concentration of melted material from relatively shallow target rocks in the early ejecta, and prevalence of deeper crustal components (including shocked minerals) together with material of the projectile concentrated in the so-called fireball layer (Fig. 1 and Izett, 1990, Pollastro and Bohor, 1993, Bohor and Glass, 1995, Kring and Durda, 2002). Of crucial importance for understanding the catastrophic effects of the Chicxulub event on the biosphere is the impact-related input of carbon dioxide and sulfur oxides in the atmosphere by devolatilization of the up to 3 km thick carbonate and evaporite platform sediments that covered the target area at the time of the impact (Pope et al., 1997, Toon et al., 1997). Published results on the amount of dissociated carbonate and evaporites differ by several orders of magnitude and no consensus has been reached over the related climatic or sedimentological effects (e.g., Pierazzo et al., 2003, MacLeod et al., 2001, Ivanov and Deutsch, 2002).
In this context, observations on K–Pg ejecta sites may provide boundary conditions for improving numerical models of the vapor plume (Agrinier et al., 2001, Langenhorst et al., 2002). Experiments show that degassed carbonates (i.e., the residual CaO) display a porous texture with very high surface area, causing fast back-reaction with CO2 to reform calcite (Agrinier et al., 2001). Consequently, the amount of CO2 and SO2/SO3 that is released into the atmosphere could be strongly dependent on the kinetics of the back-reactions of CO2 and SO2 with hot CaO, but so far unambiguous evidence for degassing and back-reaction with CaO has not been observed in Chicxulub-related suevite or ejecta deposits. Yet, the presence of Ca-rich plagioclase and pyroxene in the silicate impact-melt lithologies clearly indicates assimilation of carbonate and anhydrite clasts from the sedimentary cover (Deutsch and Langenhorst, 2007). Evidence for carbonate melting has been found occasionally in Chicxulub suevite (Jones et al., 2000, Claeys et al., 2003, Deutsch and Langenhorst, 2007) and – quite abundant – in proximal K–Pg sites in the surroundings of the Gulf of Mexico (Fig. 1, Electronic annex EA-1, and Schulte and Kontny, 2005, Schulte et al., 2006).
Distal K–Pg sites generally include of a mm-thick ejecta layer containing spherules and shocked quartz topped by the iridium-rich boundary clay (e.g., Smit, 1999, Molina et al., 2006). These sites lack a record of carbonate phases either due to dilution by enclosing marine carbonates, due to dissolution within terrestrial environments (Izett, 1990), or because they were never deposited there. Evidence for carbonate dispersal by the Chicxulub impact in distal K–Pg sites has been restricted to the presence of few calcite and dolomite clasts (ODP Leg 171 and 174; Norris et al., 1999, Martínez-Ruiz et al., 2002) and nm-scale Ca-rich relic glass (Stevns Klint, Denmark; Bauluz et al., 2000).
This study provides – for the first time – evidence for the occurrence of shocked and unshocked carbonate phases at a paleo-distance of about 4500 km to the Chicxulub crater (see Fig. 1). Thirteen holes at five sites were drilled at the Demerara Rise, western equatorial Atlantic, during ODP Leg 207, and six of these included an up to 2-cm thick spherule-rich ejecta deposit (Fig. 2 and Electronic annexes EA-2 and EA-3; Erbacher et al., 2004). The ejecta deposit occurs precisely at (and only at) the biostratigraphic boundary (MacLeod et al., 2007). The deposit also correlates with a sharp negative δ13C excursion of –2.5‰, a sharp drop of the carbonate productivity from >80% to <20%, and a collapse in the abundance of calcareous microfossils; it contains shocked quartz grains and a well-defined iridium anomaly (MacLeod et al., 2007, Schulte et al., 2008). The ejecta deposit is normally graded with no evidence for subsequent reworking and bioturbation, and hence, it has been considered being undisturbed (Fig. 2). Whilst the named papers have demonstrated the correlation between the Chicxulub impact and the K–Pg mass extinction, compositional and textural details of the ODP Leg 207 ejecta deposit were not the focus of these studies. We use a multi-disciplinary approach to address this shortcoming and argue that the Demerara spherule deposit integrates key differences that have been noted for marine vs. terrestrial and proximal vs. distal K–Pg records: The most exciting discovery is the presence of calcite and dolomite clasts next to shocked tectosilicates in the topmost millimeter; these carbonate clasts show very distinct, in part exotic features that are clearly related to the Chicxulub impact event.
Section snippets
Materials and methods
Polished thin sections were generated from cuts normal and parallel to bedding in the spherule deposit as well as from the transition to the Maastrichtian and Danian sediments in ODP Leg 207 Sites 1258B Core 27R, 1259B Core 13R, and 1259C Core 8R. The fragile samples were dried at 50 °C for 24 h, then immersed with Resin-Cure (Bühler GmbH, Düsseldorf) in an exsiccator, and finally polished with 1 μm diamond spray on a pellon-cloth, using Aralux (petroleum) as wetting agent to avoid swelling of the
Stratigraphy and lithology
Six of the ODP Leg 207 drill sites recovered a complete K–Pg boundary sequence including a single, 1.5- to 2-cm thick spherule deposit that occurs precisely at the contact between the Cretaceous and the Paleogene (see Fig. 2, Electronic annexes EA-2, EA-3 and Erbacher et al., 2004, MacLeod et al., 2007). The K–Pg boundary is underlain by moderately bioturbated, light greenish gray nannofossil chalk with foraminifers that belong to the latest Maastrichtian planktic foraminifera Plummerita
Interpretation and discussion
The Chicxulub ejecta deposits in ODP Leg 207, Demerara Rise, are complex and exhibit sedimentological, petrological, and geochemical features that allow the evaluation of (1) ejecta components, target lithologies, and impact processes, (2) ejecta distribution and depositional processes, and (3) the relation to Chicxulub ejecta at the K-P boundary worldwide. (4) Finally, we discuss the implications from the Sr–Nd isotope data of the ejecta.
Conclusions
ODP Leg 207 recovered an exceptional K–Pg boundary record being remarkably undisturbed and similar in six of 13 cores. The 1.5- to 2-cm thick spherule deposit shows normal grading. Microchemical analysis reveals distinct microstratigraphy and a complex composition of the K–Pg spherule deposit:
- (i)
The lower part of the deposit consists predominantly of spherules, interpreted as impact glass altered mostly to smectite. These spherules are massive, hollow, or collapsed. Massive spherules show globular
Acknowledgments
This research used samples provided by the Ocean Drilling Program (ODP). Funding for this research was provided by the Deutsche Forschungsgemeinschaft (Grants SCHU 2248/2 and DE 401/13) as well as by Bruker AXS Microanalysis GmbH, Berlin. We acknowledge the Ocean Drilling Program (ODP), Jochen Erbacher (BGR Hannover), the ODP 207 curators, and Henk Brinkhuis (Dept. of Biology, Univ. Utrecht) who made the samples available (ODP-LEG 207, sample requests 18468A and 18539A). Matthias Göbbels and
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