Multiple impacts across the Cretaceous–Tertiary boundary

Abstract

The stratigraphy and age of altered impact glass (microtektites, microkrystites) ejecta layers from the Chicxulub crater are documented in Late Maastrichtian and Early Danian sediments in Mexico, Guatemala, Belize and Haiti. In northeastern Mexico, two to four ejecta layers are present in zone CF1, which spans the last 300 ky of the Maastrichtian. The oldest ejecta layer is dated at 65.27±0.03 Ma based on sediment accumulation rates and extrapolated magnetostratigraphy. All younger ejecta layers from the Maastrichtian and Early Danian Parvularugoglobigerina eugubina zone Pla(l) may represent repeated episodes of reworking of the oldest layer at times of sea level changes and tectonic activity. The K/T boundary impact event (65.0 Ma) is not well represented in this area due to widespread erosion. An Early Danian Pla(l) Ir anomaly is present in five localities (Bochil, Actela, Coxquihui, Trinitaria and Haiti) and is tentatively identified as a third impact event at about 64.9 Ma. A multiimpact scenario is most consistent with the impact ejecta evidence. The first impact is associated with major Deccan volcanism and likely contributed to the rapid global warming of 3–4 °C in intermediate waters between 65.4 and 65.2 Ma, decrease in primary productivity and onset of terminal decline in planktic foraminiferal populations. The K/T boundary impact marks a major drop in primary productivity and the extinction of all tropical and subtropical species. The Early Danian impact may have contributed to the delayed recovery in productivity and evolutionary diversity.

Introduction

The Chicxulub structure in Yucatan, Mexico, is generally considered the K/T boundary impact that caused one of the major mass extinctions in the Earth's history. Impact ejecta layers have now been widely recognized in numerous localities around the Gulf of Mexico (see review by Smit, 1999) and linked to the Chicxulub impact based on their geographic distribution, ³⁹Ar/⁴⁰Ar ages close to the K/T boundary Sigurdsson et al., 1991, Swisher et al., 1992, Dalrymple et al., 1993 and chemical similarity to Chicxulub melt rock Izett et al., 1991, Blum et al., 1993, Koeberl et al., 1994, Chaussidon et al., 1996. Controversies persist with respect to the stratigraphic position of the ejecta layer at or near the K/T boundary, and the nature and tempo of emplacement, whether by tsunami (Smit et al., 1996) or gravity flows and sea level changes Adatte et al., 1996, Bohor, 1996, Stinnesbeck et al., 1996, Keller et al., 1997, and the presence of multiple altered impact glass spherule layers in the Late Maastrichtian and Early Danian Keller et al., 2001, Keller et al., 2002a, Keller et al., 2002b, Soria et al., 2001. A better understanding of these events may reconcile the two divergent K/T mass extinction hypotheses. The impact–extinction hypothesis calls for sudden mass extinctions due to a single large impact (Alvarez et al., 1980) now considered to be the Chicxulub structure. Paleontologists have long argued that the fossil record does not support a single cause for the mass extinction and, therefore, proposed multievent scenarios that include major volcanism, rapid climate and sea level changes Archibald, 1996, Keller, 1996, MacLeod et al., 1997 and one or more impacts Keller et al., 1997, Keller et al., 2002a, Keller, 2001.

These controversies have remained unsolved, in part, because most impact-related investigations have been geographically limited to a narrow region surrounding Chicxulub, and temporally limited to an interval spanning the K/T boundary clay, the ejecta layer and a few samples above and below. No significant effort has been made to examine older or younger sediments for additional impact ejecta or other environmental signals. The wider context of the mass extinction event, including the half million years before and after the K/T boundary, is well studied based on fossil assemblages, climate and sea level changes, all of which show major changes preceding the K/T boundary (see review in Keller, 2001). Only recently have investigations of impact ejecta in Haiti and Mexico included the upper part of the Late Maastrichtian and Early Danian and revealed the presence of multiple impact ejecta layers (microtektites and microkrystites) in both Late Maastrichtian and Early Danian sediments, as well as Ir and Platinum group element (PGE) anomalies in the Early Danian Keller et al., 2001, Keller et al., 2002a, Stinnesbeck et al., 2001, Stinnesbeck et al., 2002, Odin et al., 2001, Stüben et al., 2002.

In this paper, we review the stratigraphy and biochronology of the K/T boundary ejecta deposits and provide new evidence of multiple ejecta deposits from sections in central and southern Mexico, southern Belize and eastern Guatemala (Fig. 1). The first part introduces the biostratigraphic scheme of the K/T boundary, the globally recognized boundary markers and the age and biozonation control of the Late Maastrichtian and Early Danian based on which the age and continuity of the sedimentary record is evaluated. In the second part, we review and document the stratigraphy and age of thick microtektite and microkrystite ejecta deposits from Mexico, Guatemala, Belize, Haiti and deep-sea sites, and provide new mineralogical (Cheto smectite) analyses that suggest a common glass origin. Finally, we provide a regional synthesis of the age of spherule deposition in Central America and the Caribbean and propose a multievent scenario for the K/T transition that is consistent with current impact, climate and fossil data.

Field sections were examined, measured and sampled based on standard methodologies. Biostratigraphic analysis was based on planktic foraminifera processed following the standard method of Keller et al. (1995). The smaller (36–63 μm) size fraction was examined for the first occurrence of tiny Early Danian species. Individual clasts from breccias, conglomerates and spherule layers were processed separately and analyzed for planktic foraminifera in order to determine the biostratigraphic ages of these sediments prior to erosion and redeposition.

Clay mineral analyses were conducted at the University of Neuchatel, Switzerland, based on XRD (SCINTAG XRD 2000 Diffractometer, Geological Institute) and ESEM (Phillips environmental microprobe equipped with EDEX analyzer, Institute of Microtechnique) following the procedures outlined by Kübler (1987) and Adatte et al. (1996). Platinum group elements (PGE) were analyzed at the Institute for Mineralogy and Geochemistry, University of Karlsruhe, by isotope dilution HR-ICP-MS after preconcentration and matrix reduction by Ni-fire assay Kramar et al., 2001, Stüben et al., 2002.