Analyses of shocked quartz at the global K-P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub

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Abstract

The precise cause and timing of the Cretaceous–Paleocene (K–P) mass extinction 65 Ma ago remains a matter of debate. Many advocate that the extinction was caused by a meteorite impact at Chicxulub, Mexico, and a number of potential kill-mechanisms have been proposed for this. Although we now have good constraints on the size of this impact and chemistry of the target rocks, estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact. An oblique impact is likely to have been far more catastrophic than a sub-vertical one, because greater volumes of volatiles would have been released into the atmosphere. The principal purpose of this study was to characterize shocked quartz within distal K–P ejecta, to investigate whether the quartz distribution carried a signature of the direction and angle of impact. Our analyses show that the total number, maximum and average size of shocked quartz grains all decrease gradually with paleodistance from Chicxulub. We do not find particularly high abundances in Pacific sites relative to Atlantic and European sites, as has been previously reported, and the size-distribution around Chicxulub is relatively symmetric. Ejecta samples at any one site display features that are indicative of a wide range of shock pressures, but the mean degree of shock increases with paleodistance. These shock- and size-distributions are both consistent with the K–P layer having been formed by a single impact at Chicxulub. One site in the South Atlantic contains quartz indicating an anomalously high average shock degree, that may be indicative of an oblique impact with an uprange direction to the southeast ± 45°. The apparent continuous coverage of proximal ejecta in this quadrant of the crater, however, suggests a relatively high impact angle of > 45°. We conclude that some of the more extreme predictions of the environmental consequences of a low-angle impact at Chicxulub are probably not applicable.

Introduction

Alvarez et al. [1] identified an extraterrestrial signature within the K–P boundary clay layer and, following a lengthy debate, this layer is now universally agreed to have been formed by a meteorite impact. The K–P layer also contains highly shocked terrestrial minerals (quartz, feldspar and zircon) that originated from rocks at the impact site, as well as impact-derived Ni-rich spinel [2], [3], [4], [5], [6], [7], [8]. After the Chicxulub impact crater was discovered [9] numerous studies of this crater and the K–P boundary led to widespread agreement that Chicxulub was the site of a K–P boundary impact, with a few notable exceptions [10], [11]. The cause of the K–P mass extinction is still contentious, and the precise role of the Chicxulub impact and Deccan volcanism remains a matter of some debate [e.g. [12], [13]]. The impact would have released dust and climatically active gasses into the atmosphere, and caused a period of extreme temperature change and darkness, but calculations of the nature, extent and duration of these changes differ [e.g. [14], [15], [16], [17]]. Estimates of climatic effects are dependent, in part, upon knowledge of the energy of impact, the chemistry of the target rocks, and the obliquity of impact. An oblique impact would have been much more damaging than a sub-vertical one because the shock wave would be more focused within the near-surface, volatile-rich, sedimentary rocks, and this would cause the release of higher volumes of climatically active carbon- and sulfur-rich gasses [17], [18], [19]. The impact size and target chemistry are now reasonably well constrained [e.g. [9], [20]], but the obliquity of impact is unknown.

The best indicator of direction of impact for craters on other planetary bodies is found in the asymmetric distribution of their proximal ejecta (Fig. 1a). Highly oblique impacts (< 10°) produce elliptical craters and a butterfly ejecta pattern with ejecta-free zones in the up and downrange directions. For impact angles of greater than ∼ 15° near-circular craters are produced and, for oblique impact angles up to ∼ 45°, ejecta is missing in the uprange direction [21]. Chicxulub is roughly circular [9], [20], suggesting an impact angle of > 15°. Although asymmetries in the apparent crater structure have been used to estimate impact angle and direction [18], [22], the feature applied (offset of the central uplift) has been shown not to be diagnostic of impact direction for craters on Venus [23]. At Chicxulub much of the proximal ejecta is buried and, as the impact site was submerged continental shelf, is likely to have been re-distributed by tsunamis or erosion immediately or soon after impact.

During the excavation stage of cratering the so-called ejecta curtain material is ejected at particle velocities of up to ∼ 2 kms-1 to form the proximal ejecta deposits [24], [25], and for Chicxulub these deposits should extend up to ∼ 400 km from the impact site. This ejection mechanism cannot explain the presence of shocked quartz in the global K–P layer as velocities of > 9 kms-1 are required to eject quartz to the other side of the planet. The only plausible explanation is that these ejecta were accelerated to high speeds within the expanding vapor plume [25]. The expanding plume would consist of a mixture of vapor, melt and clastic minerals metamorphosed at different shock pressures. However, the precise kinematics of ejection are unknown and different assumptions about the velocity/mass distribution lead to quite different final distributions of ejecta [26], [27].

Observational, experimental and numerical modeling of oblique impacts all indicate that vapor plumes initially expand in the downrange direction [19], [28], [29]. Hence it appears plausible that the shocked quartz could be ejected in an asymmetric pattern from the impact site. Previous studies of the K–P ejecta have suggested some intriguing asymmetries in the distal ejecta pattern [18], [30], [31], with shocked quartz being more abundant in sites to the west of Chicxulub [30], [31]. However, these observations are based upon data acquired by a number of groups, who have adopted different sampling and/or analytical procedures. Detailed mineralogical studies that report the absolute abundance and grain size of shocked quartz are rare [31], [32], [33], and some sites are better documented than others. Hence, it is difficult to make a quantitative assessment of asymmetries in the distribution of the distal ejecta using the existing data alone.

This paper reports the results of a new systematic study of shocked quartz from the global K–P boundary that covered a range of paleodistances and azimuths (Fig. 1b; Table 1). Observations of the grain size and degree of shock in quartz have been made using optical and scanning electron microscopy (SEM). The objective of this study was to evaluate whether the shocked quartz is asymmetrically distributed around the crater and, if so, whether that distribution offers a clue to the direction and angle of impact at Chicxulub. In contrast to previous studies, site-to-site comparisons of these new data can be made with confidence because the same protocol was used to analyze each sample.

Section snippets

Sampling and analytical procedures

To cover a range of paleodistances and azimuths, a number of sites in Europe, Brazil, New Zealand, North America, and the Pacific and Atlantic ocean regions were chosen for this study (Fig. 1b; Table 1). Samples were collected from a number of outcrops at each location, to investigate whether there was significant local variation in the K–P layer. In each case we attempted to sample the entire shocked quartz-bearing layer. For North American sites, the Fireball Layer and Boundary Layer (see

Optical analyses

In Table 2 we show the number of shocked quartz grains observed as well as an estimate of the total number of quartz grains, and the maximum and average size for each sample. During the course of the optical analysis we noted that the shallow marine sites from Europe and New Zealand contained large amounts of unshocked quartz with grain sizes > 60 μm. The high number of unshocked quartz grains in these samples is indicative of their depositional environment in shallow marine seas (Table 1) (e.g.

Apparent paleodistance and azimuth

As discussed in the introduction, the only plausible mechanism for distributing shocked quartz around the globe is by their acceleration to high velocity within the expanding vapor plume. If the impact at Chicxulub had been sub-vertical, we would expect a symmetrical ejection pattern in all directions. One factor that affects the final ejecta distribution is the Earth's rotation, as sites to the west of Chicxulub move closer as the ejecta is traveling towards them. To partially remove this

Conclusions

A large volume of material was examined from the proposed K–P boundary at Poty Quarry in Brazil. We did not find shocked quartz within these samples, and conclude that this section is unlikely to be K–P in age. This is the first systematic study of shocked quartz at many K–P sites. The size and number of shocked quartz grains decrease with apparent distance from the Chicxulub crater, and this decrease is fairly symmetric. This contradicts previous studies that proposed that shocked quartz was

Acknowledgments

This research was funded by a grant from the Leverhulme Trust. The authors would like to thank the Natural History Museum for their technical support, and in particular John Sprat, Terry Williams and Tony Willington. We also thank Finn Surlyk, University of Copenhagen, for support during sampling of the Danish site. Jan Smit and Frank Kyte are acknowledged for sharing their experience on the K–P boundary. Uwe Reimold and an anonymous reviewer are thanked for their reviews. This research used

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