No evidence of multiple impact scenario across the Cretaceous/Paleogene boundary based on planktic foraminiferal biochronology
Published:June 21, 2022
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José A. Arz*, I. Arenillas*, J.M. Grajales-Nishimura*, C.L. Liesa*, A.R. Soria*, R. Rojas*, T. Calmus*, V. Gilabert*, 2022. "No evidence of multiple impact scenario across the Cretaceous/Paleogene boundary based on planktic foraminiferal biochronology", From the Guajira Desert to the Apennines, and from Mediterranean Microplates to the Mexican Killer Asteroid: Honoring the Career of Walter Alvarez, Christian Koeberl, Philippe Claeys, Alessandro Montanari
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Large bolide impacts seem to have strongly affected biological evolution, causing detrimental effects on the biosphere. The best-known case is the Chicxulub impact (Yucatan Peninsula, Mexico), the most probable trigger of the Cretaceous/Paleogene boundary (KPB) mass extinction. Nevertheless, after four decades of intensive research, a consensus on the causal relationship between the impact and the mass extinction has not yet been reached. Most of the scientific community has established multiple, strong arguments for the Chicxulub impact as the most likely and principal cause of the KPB mass extinction. However, a variety of hypotheses link the mass extinction mainly to the volcanism of the Deccan Traps and one or more bolide impact events within a relatively short time through the KPB: one impact in the late Maastrichtian (66.3 Ma), corresponding to the Chicxulub impact, a larger one at the KPB (66 Ma), and a final one in the early Danian (65.9 Ma).
Here, we report on the controversies relating to the correlation of the Chicxulub impact event with the mass extinction, with a focus on the stratigraphy and biostratigraphy of sections in Mexico, Cuba, and Haiti, which include ejecta-rich clastic deposits linked to the Chicxulub impact. High-resolution biostratigraphy and quantitative data with planktic foraminifera reveal that these deposits are synchronous with the ejecta-rich airfall layer and the KPB mass extinction horizon of the El Kef, Tunisia, stratotype. Our results provide no support for a multiple impact scenario but confirm that the Chicxulub impact event is indeed the KPB impact event. Furthermore, we have not found any biostratigraphic evidence to support an additional Danian impact event near the Gulf of Mexico region.
The hypothesis that the impact of a large asteroid caused the massive extinction of the Cretaceous/Paleogene boundary (KPB), as proposed by Alvarez et al. (1980) and Smit and Hertogen (1980), is by now a well-established theory among most KPB specialists (see Schulte et al., 2010). In the 1980s, various findings made it possible to delimit the area where this asteroid could have impacted. For example, Bourgeois et al. (1988) found evidence of a tsunami deposit coinciding with the KPB at Brazos River, Texas, USA, directly below an iridium anomaly. They hypothesized that the most likely source of the tsunami deposit was a bolide-water impact at the KPB. Izett et al. (1990) and Hildebrand and Boynton (1990) reported further evidence of an impact within the Gulf of Mexico–Caribbean region after analyzing in detail impact glass spherules and shocked minerals from a coarse mafic deposit at Beloc, Haiti, located below an iridium anomaly. Based on planktic foraminiferal biostratigraphy, Maurrasse et al. (1979) and Maurrasse (1981, 1982) had previously interpreted this layer as a basaltic turbidite deposited at the KPB.
The discovery of the Chicxulub impact structure on the Yucatan Peninsula (Hildebrand et al., 1991) verified the Alvarez hypothesis and seemed to settle the debate about the best explanation for the KPB extinction event. Shortly afterwards, the finding of the impact structure was supported by further publications on the Beloc impact glass (Maurrasse and Sen, 1991; Izett et al., 1991) and by the discovery of extensive new oceanic clastic deposits containing glass spherules, shocked minerals, and high concentrations of iridium, such as those identified in Deep Sea Drilling Program (DSDP) Leg 77 Sites 536 and 540, in the southeastern Gulf of Mexico (Alvarez et al., 1992a), and El Mimbral, northeastern Mexico (Alvarez et al., 1992b; Smit et al., 1992). According to these authors, the lithological and structural complexity of these clastic units close to the impact site is a consequence of the high-energy processes triggered by the Chicxulub impact at the KPB, such as giant tsunamis caused by the main impact and perhaps other closely spaced impacts (Maurrasse and Sen, 1991), subsequent seiche waves (Smit et al., 1992) and submarine landslides, and finally the settling of the silt-size fraction suspended by the impact-generated waves (Alvarez et al., 1992b). It is of great merit that the main processes controlling the deposition of the ejecta-rich units in the Gulf of Mexico–Caribbean region were identified by these authors at such an early date. However, the early proposal by Maurrasse and Sen (1991) of closely spaced impacts around the KPB is now discredited.
More early lines of evidence confirmed the identification of the Chicxulub structure as a buried impact crater and the source of the KPB ejecta layer found worldwide. For example, geochemical evidence provided by Blum et al. (1993) genetically linked the Haitian impact glasses and the Chicxulub melt rock, and Krogh et al. (1993) reported that the impact breccia from Chicxulub contains shocked zircons that are identical in age, texture, and lead loss pattern to the zircons founded in ejecta layers of the Raton Basin, Colorado, USA, and the Beloc section.
Since the discovery of the Chicxulub impact structure, more than 50 offshore ejecta-rich clastic deposits have been described in detail in outcrops around this region, such as those in Haiti (Maurrasse and Sen, 1991; Izett et al., 1991; Leroux et al., 1995; Keller et al., 2001; Maurrasse et al., 2005), Mexico (Smit et al., 1992, 1996; Stinnesbeck et al., 1993; Bohor, 1996; Keller et al., 1997; Grajales-Nishimura et al., 2000, 2003; Soria et al., 2001; Arz et al., 2001a, 2001b; Lawton et al., 2005; Arenillas et al., 2006; Schulte et al., 2012), Guatemala and Belize (Ocampo et al., 1996; Stinnesbeck et al., 1997), Cuba (Takayama et al., 2000; Tada et al., 2002, 2003; Matsui et al., 2002; Alegret et al., 2005; Goto et al., 2008), Texas (Yancey, 1996; Schulte et al., 2006; Keller et al., 2007), and Missouri, USA (Campbell et al., 2008).
In addition, more than 100 boreholes drilled by the DSDP, Ocean Drilling Program (ODP), International Ocean Discovery Program (IODP), and International Continental Scientific Drilling Program (ICDP) in the Gulf of Mexico and the Caribbean (Bralower et al., 1998; Dressler et al., 2004; Gulick et al., 2019; Whalen et al., 2020), as well as industry-drilled, deep-sea wells in the northern Gulf of Mexico (Denne et al., 2013; Sanford et al., 2016; Poag, 2017) and in the Campeche Escarpment (Paull et al., 2014), have shed considerable light on the complex stratigraphy of these Chicxulub ejecta-rich units and the mechanisms involved in the remobilization and transport of sediments and their subsequent re-deposition. These mechanisms mainly include submarine landslides, tsunami run-up and backwash, sediment fluidization and liquefaction, slumping, gravity flows, turbidite currents, and the final settling of suspended fine material rich in iridium and fine ejecta (see Smit, 1999; Soria et al., 2001; Lawton et al., 2005; Goto et al., 2008; Schulte et al., 2010; Sanford et al., 2016).
The radiometric dating (40Ar/39Ar) of pristine glassy spherules confirmed the synchrony between the KPB, the KPB mass extinction, and the Chicxulub impact, with a margin of error of only 32 k.y. (Renne et al., 2013). However, some KPB specialists hold that the Chicxulub impact predates the KPB by ~200–300 k.y. (Keller et al., 1997, 2002a, 2004, 2009; Stinnesbeck et al., 2001). The stratigraphic distribution of impact spherules and iridium anomalies in the Gulf of Mexico and the Caribbean led Keller et al. (2003) to propose multiple bolide impacts through the KPB: an older Maastrichtian impact (66.3 Ma), which for them is the Chicxulub impact; another larger impact at 66 Ma that distributed the iridium at the KPB; and a third impact in the early Danian (65.9 Ma). According to Keller (2005), only the ~200-km-diameter Chicxulub impact structure has been discovered, whereas the estimated 250- or 350-km-diameter KPB impact structure and the potentially smaller Danian impact structure are still hidden. Furthermore, Keller (2005) has stated that it was not these multiple impacts that were responsible for the KPB mass extinction, but rather the massive volcanism of the Deccan Traps in India. The ages of the impact events proposed by these authors are based mainly on planktic foraminiferal biostratigraphy, which is a useful tool for minimizing the margin of error inherent in radiometric dating techniques.
In this study, we apply high-resolution biostratigraphy based on planktic foraminifera across the KPB in sections of the Gulf of Mexico and the Caribbean (Fig. 1) to test the precise age of the Chicxulub impact, as well as the evidence for the multiple asteroid impact hypothesis. Furthermore, we carry out a historical review of some of the main micropaleontological and sedimentological controversies relating to the age of the Chicxulub impact, which have sometimes been biased by a neglect of the well-established physical processes that occur during a large impact event. This report is the result of over two decades of collaborative work on KPB outcrops in the Gulf of Mexico–Caribbean region as well as in the Tethyan realm.
BIOSTRATIGRAPHIC AND CHRONOSTRATIGRAPHIC FRAMEWORK
Brief Historical Account of the Definition of the KPB
In 1987, the Cretaceous/Palaeogene Working Group selected four key horizons as candidates to be the Global Boundary Stratotype Section and Point (GSSP) for the base of the Danian, or the KPB: the base of the tsunamite, the base of the boundary clay, the first occurrence of the dinoflagellate Danea californica, and an iridium maximum in the boundary clay. Four candidate sections to host the GSSP (the Brazos River, El Kef, Stevns Klint, and Zumaia) were also proposed. After a vote via postal ballot in 1988, the chosen GSSP was the base of the “boundary clay” as the stratigraphic level, and El Kef as the geographical location (Cowie et al., 1989). At the 28th International Geological Congress in Washington, D.C., in 1989, the GSSP for the KPB was officially designated and ratified by the International Union of Geological Sciences (IUGS).
The final step in its definition, i.e., the publication of the GSSP in a prestigious stratigraphy journal, was not taken at that time. Only a very short note was published by Cowie et al. (1989) in the journal Episodes, in a report on the activities of the International Commission on Stratigraphy (ICS) from 1984 to 1989. In 1999, despite concerns raised about the accessibility and exposure of the GSSP outcrop at El Kef, the ICS (chaired by Jürgen Remane) finally ratified El Kef as the stratotype section in an issue of Episodes and proposed Elles and Aïn Settara, Tunisia, as auxiliary sections (Remane et al., 1999). However, Remane et al. (1999) confused the GSSP for the KPB (that is, the point at the base of the “boundary clay” where the “golden spike” should be placed) with the stratotype itself (that is, the El Kef section). Due to this imprecise location of the GSSP, an intense debate was launched on what criteria to use to place the KPB in a particular section (e.g., Keller et al., 1996, versus Smit, 1999). Finally, in 2006, the chairman (Eustoquio Molina) of the International Subcommission on Paleogene Stratigraphy (ISPS), at the request of the ICS (see Gradstein and Ogg, 2004, 2005), formally published in Episodes the unambiguous definition of the GSSP for the KPB (Molina et al., 2006).
Key Horizons to Identify the KPB
The KPB clay sequence at El Kef consists of a millimeter-thick, iridium-rich, rust-colored ferruginous clay layer (airfall layer), overlain by a dark clay bed with low values of %CaCO3 and δ13C. This sequence is similar in all continuous Tethyan sections, and in general in all of the continuous pelagic sections farthest from the Chicxulub impact structure (Smit, 1999; Arenillas et al., 2006; Molina et al., 2009). As explained above, the KPB was defined at the base of the “boundary clay” at El Kef, which is at the same stratigraphic level as the base of the airfall layer (fig. 2 in Molina et al., 2006). This level marks the GSSP and contains a high percentage of impact-derived ejecta (impact glasses, Ni-spinels, shocked quartz, etc.) that would have fallen from the air and the seawater column (Molina et al., 2006). In addition, the base of the iridium-rich airfall layer coincides with the mass extinction horizon of planktic foraminifera, which evidences, pursuant to the majority, the cause-effect relationship between the Chicxulub impact and the KPB mass extinction (see Schulte et al., 2010). According to this definition (summarized in Fig. 2), the Chicxulub impact-derived ejecta material overlies the KPB, and consequently any lithological unit containing this material must be considered Danian in age (Smit et al., 1996; Arz et al., 2001a, 2001b, 2004; Arenillas et al., 2011).
Even though the GSSP and thus the precise chronostratigraphic position of the KPB was already well established, the debate over the criteria that should be used to place it in a local stratigraphic section persisted. Ignoring the official definition of the GSSP by Molina et al. (2006), Keller (2008) claimed that the valid KPB-identifying criteria had been summarized in Keller et al. (1996) and granted herself the authority to go against the ICS. According to Keller (2008), the KPB was defined in terms of: (1) the base of the red airfall layer of the “boundary clay”; (2) the iridium anomaly concentrated in the red airfall layer; (3) the marine plankton or planktic foraminiferal mass extinction, which is nearly coincident with the base of the red airfall layer; (4) the first appearance of Danian species immediately above the extinction horizon; and (5) a negative shift in δ13C values.
Keller (2008) considered that all five have remained remarkably consistent KPB markers in marine sequences worldwide. From the chronostratigraphic point of view, Keller ignores the fact that in the vote to define the GSSP for the KPB, in which she herself participated, only one criterion was chosen by a majority of 75% of the voting members (Cowie et al., 1989): the base of the “boundary clay” or, what amounts to the same thing, the base of the airfall layer. All of the other criteria or key horizons are closely associated with these and can be used for correlation but are not part of the chronostratigraphic definition of the KPB, as Smit (1999) highlights.
Calibration of Biozones
For biochronological interpretations, we have used the planktic foraminiferal zonations of Arz and Molina (2002) for the uppermost Maastrichtian, and Arenillas et al. (2004) for the lower Danian. For the purposes of this study, we have considered the last two biozones of the Maastrichtian (the Pseudoguembelina hariaensis and Plummerita hantkeninoides Zones) and the first three of the Danian (the Guembelitria cretacea, Parvularugoglobigerina eugubina, and Parasubbotina pseudobulloides Zones). The latter are subdivided into several subbiozones: the Muricohedbergella holmdelensis and Parvularugoglobigerina longiapertura Subzones (for the Gb. cretacea Zone), the Parvularugoglobigerina sabina and Eoglobigerina simplicissima Subzones (for the Pv. eugubina Zone), and the Eoglobigerina trivialis, Subbotina triloculinoides, and Globanomalina compressa Subzones (for the P. pseudobulloides Zone) (Fig. 3). In light of the recent astrochronological age calibration performed at Zumaia by Gilabert et al. (2022), the base of the Pt. hantkeninoides Zone can be dated at ~99 k.y. before the KPB, and the bases of the Pv. longiapertura, Pv. sabina, E. simplicissima, E. trivialis, S. triloculinoides, and G. compressa Subzones at ~7 k.y., 18 k.y., 26 k.y., 68 k.y., 210 k.y., and 473 k.y. after the KPB, respectively.
For a better reading of the biostratigraphic framework, these biozonations have been compared with that of Li and Keller (1998) for the uppermost Maastrichtian and those of Keller et al. (1996) and Wade et al. (2011) for the lower Danian. Although the taxonomy used by these authors differs, the approximate correspondence of these biozonations is illustrated in Figure 3.
The first Danian species to appear were Chiloguembelitria danica and Pseudocaucasina antecessor ~2 k.y. after the KPB, and they are consequently recorded in the middle part of Biozone P0 or the Mh. holmdelensis Subzone. Ps. antecessor is the ancestor of the so-called parvularugoglobigerinids, which include the genera Parvularugoglobigerina and Palaeoglobigerina in addition to Pseudocaucasina (Arenillas and Arz, 2017). In biostratigraphic studies of the lower Danian, the species of Pseudocaucasina and Palaeoglobigerina have been routinely classified as belonging to primitive forms of the genera Eoglobigerina and/or Globoconusa. For this reason, many authors consider that the latter appeared immediately after the KPB and are recorded even within Zone P0 (e.g., Keller, 1988, 2008, 2014).
From a biostratigraphic point of view, the KPB is placed at the boundary between the Maastrichtian Pt. hantkeninoides Zone and the Danian Gb. cretacea Zone, i.e., at the catastrophic mass extinction horizon of Maastrichtian planktic foraminifera (Arenillas et al., 2000a, 2000b). The KPB should never be placed at the lowest occurrence of Danian species, since their first appearance occurred ~2 k.y. later (Fig. 3).
To minimize taxonomic difficulties concerning the identification of Biozone P0, Arenillas et al. (2006) proposed the use of so-called planktic foraminiferal acme stages (PFAS) after the previous identification of several distinctive acme stages in the lower Danian by Arenillas et al. (2000a, 2000b). The use of quantitative data and the identification of apogee intervals (acmes), mainly when referring to relative abundances of a set of species (e.g., a genus or a set of genera), minimizes the subjectivity inherent in taxonomic determinations of particular species. Three PFAS were proposed: the dominance of triserial species of the genus Guembelitria (PFAS-1), the dominance of tiny trochospiral species of the genera Parvularugoglobigerina and Palaeoglobigerina (PFAS-2), and the dominance of biserial species of the genera Woodringina and Chiloguembelina (PFAS-3). These acme stages have been identified worldwide, mainly in the Tethys, North Atlantic, and Gulf of Mexico–Caribbean regions (e.g., Arenillas et al., 2000a, 2000b, 2002, 2018; Alegret et al., 2004; Gallala et al., 2009; Lowery et al., 2018), which suggests that they are useful for global biochronostratigraphic correlation. Stratigraphically, PFAS-1 spans a little more than the dark clay bed, i.e., the Mh. holmdelensis Subzone and the lower part of the Pv. longiapertura Subzone (Fig. 3). The base of the lowest acme stage (PFAS-1 or the Guembelitria acme) can be used as an alternative biostratigraphic marker for the KPB, free from taxonomic subjectivity, as proposed by Arenillas et al. (2006, 2011). PFAS-1 is completely different from other Danian guembelitriid blooms, not only because of the biochronostratigraphic position of these other acmes, but also because of easily recognizable differences in the planktic foraminiferal species/assemblages with which they are associated.
STRATIGRAPHY OF THE KPB SEQUENCE IN REGIONS NEAR THE CHICXULUB STRUCTURE
The impact processes, the paleophysiography of the Gulf of Mexico–Caribbean region at the time of the Chicxulub impact, as well as the nature and distribution of pre-KPB sediments and rock units and sediment relocation played a decisive role in the nature of the KPB sedimentary sequence in the region. In Figure 4, we show the correlation among the main stratigraphic units of the KPB sedimentary sequence in the sections studied by us in Mexico, Cuba, and Haiti. Field views of the outcrops showing sedimentological features are illustrated in Figures 5–8.
The KPB Sedimentary Sequence inside the Chicxulub Structure
Near the center of the impact area (e.g., Site M0077), the KPB sequence consists of a suite of ~130 m of impact deposits (impactites) followed by a 75-cm-thick, fine-grained, carbonate-rich “Transitional Unit” that overlies the impact-deformed granitic basement (Lowery et al., 2018; Whalen et al., 2020). This impactite sequence mainly consists of a distinctive suite of deposits of melt rock and impact breccia (suevite), which was redeposited after the resurge of seawater into the impact crater. The impact breccia especially governs the upper sorted suevite (Gulick et al., 2019). The base of the thin, hemipelagic, gray-green marlstone layer that overlies the Transitional Unit at Site M0077 contains a distinct Ir anomaly in excess of ~1.0 ppb, which is interpreted as the local iridium-rich airfall layer (Goderis et al., 2021).
In the outer parts of the impact structure (e.g., the Yaxcopoil-1 borehole), the impactite sequences are thinner (up to ~87 m) than in the inner parts, but the upper reworked and sorted suevite deposit is thicker and reaches a thickness of up to 13.39 m (Dressler et al., 2004).
The KPB Sedimentary Sequence outside the Chicxulub Structure
In the Gulf of Mexico–Caribbean region, this sedimentary sequence was termed clastic complex by Smit et al. (1996), K-T boundary cocktail by Bralower et al. (1998), and—finally—a complex clastic unit (CCU) by Arenillas et al. (2006), which is the name used here.
The CCU at Guayal and Bochil, located ~600 km from the center of the Chicxulub impact structure, is a fining-upward sedimentary succession (51 m and 86 m thick, respectively), which can be subdivided, from base to top, into four distinct subunits (Grajales-Nishimura et al., 2003, 2009; Arenillas et al., 2006). Subunit 1 mainly consists of a very coarse-grained carbonate breccia with abundant rudists and carbonate megablocks up to 2 m in diameter, which grades to fine-grained carbonate breccia. Subunit 2 is a fine-grained calcareous breccia and coarse-grained calcareous sandstone mixed with scarce impact material. Subunit 3 comprises very fine-grained yellow and red rippled sandstone and siltstone that is rich in ejecta and includes shocked mineral phases and distinctive accretionary lapilli horizons at Guayal. Finally, Subunit 4 is a thin, yellow–red shaly layer that includes the finest ejecta and has a distinctive horizon that is anomalously enriched in iridium.
In the northeastern Mexican sections (>1000 km westwards from the impact zone), the CCU only consists of a distinctive, 1–5-m-thick (locally up to 12-m-thick) clastic sandstone complex intercalated between two thick pelagic marl successions, the upper Maastrichtian Méndez and the Danian Velasco Formations. Based on the general fining-upward trend and other sedimentological features, this clastic sequence has been subdivided into four lithological units (Smit et al., 1992, 1996). Unit I is made up of poorly sorted, coarse-grained sediments, usually pebbly sandstones that are locally rich in spherules, filling irregular scours and channels. Unit II consists of fining-upward medium- to fine-grained sandstones that display a wide variety of sedimentary structures such as parallel lamination, ripples, climbing ripples, and rare antidune-like ripple and sand bar structures. Unit III consists of thinning- and fining-upward, small-scale cross-bedded, fine sandstone layers alternating with thin layers of silt/mud draping over the sand layers. The silt layers contain anomalous iridium concentrations and Ni-rich spinels (Smit et al., 1992). Unit IV is a 5–10-cm-thick calcareous siltstone or mudstone layer and is also enriched in iridium and Ni-rich spinels (Smit et al., 1996).
In western Cuba, ~500–600 km from the Chicxulub impact structure, the CCU is 30 m (e.g., Santa Isabel section) to >180 m thick (e.g., Peñalver section; Goto et al., 2008). It is a calcareous CCU that fines upward from calcirudite with limestone intraclasts up to 2 m in diameter common in the lower part, to calcilutite that lacks bioturbation but contains foraminifera and rudist fragments as well as altered vesicular glass and shocked quartz grains (see detailed descriptions in Takayama et al., 2000; Tada et al., 2003; Goto et al., 2008).
In other KPB sections in Cuba (e.g., the Loma Capiro and Moncada sections), the CCU is <10 m thick. According to Alegret et al. (2005), the Loma Capiro section consists of three subunits: first, a lower subunit comprising breccia of subangular to subrounded blocks of various sizes; then, a gradational, pebble-granule microconglomerate to coarse-grained sandstone subunit; and, finally, an upper subunit of fine-grained sandstone grading to silt with abundant impact glass spherules, shocked quartz, and other impact markers. The CCU at Moncada is composed of ~1.7-m-thick sandstone overlain by a 2–3-cm-thick unit of alternating dark-colored, calcareous claystones and very fine calcareous sandstones, and it ends with a 1–2-cm-thick, olive-gray, fine sandstone layer with a yellowish rim. This section is relevant because it contains a dark clay bed enriched in iridium (Tada et al., 2002; Arenillas et al., 2016).
The CCU recorded in Haiti, ~500–700 km from the Chicxulub impact structure, is much thinner than in Mexico and Cuba, with a maximum thickness of 75 cm in the stratotype of the Beloc Formation. It mainly consists of a spherule-rich bed (Maurrasse and Sen, 1991), embedded in the Beloc Formation, which is a succession of gray and pale yellowish brown fossiliferous pelagic limestones and marls. Here, we present the preliminary results from a new Haitian section, Nan Pak, located on the Route de l’Amitié (Friendship Road), which lies halfway between Leogane and Jacmel. At Nan Pak, the thickness of the spherule-rich bed is 15 cm; it is constituted mainly by 0.5–4.0 mm altered spherules, clay, and carbonate. Spherules are more abundant in the basal part and represent 30–50% of the volume of the bed. Larger spherules contain pale brown, corroded glass, as shown by optical microscope. Below the CCU, the sequence is very homogeneous and composed of a fine alternation of marls with more resistant fine limestones just below the ejecta level. The CCU is covered by a limestone bed ~10 cm in thickness, above which a reworked but coherent ejecta lens is interbedded with marls and marly limestones, which suggests a rapid amalgamation of the ejecta materials before reworking.
THE CHICXULUB IMPACT PREDATES THE KPB?
Planktic Foraminiferal Record inside the Chicxulub Impact Structure: The PEMEX Drilling Campaigns
The geophysical data and the subsurface stratigraphy from the Chicxulub-1, Yucatan-6, and Sacapuc-1 wells, drilled by Petróleos Mexicanos (PEMEX) in the mid-twentieth century, were crucial to the proof of the existence of the Chicxulub impact structure by Hildebrand et al. (1991). Previous PEMEX reports had mentioned the presence inside the impact structure of an igneous-textured unit of andesitic composition. Today this is interpreted as part of the Chicxulub impactite sequence, which includes diverse polymict, impact melt-rich breccia and suevite units and is covered by post-impact Cenozoic carbonates (Dressler et al., 2004). Shortly after these studies, radiometric, paleomagnetic, and geochemical studies of core samples provided more evidence of the meteoritic origin of the Chicxulub impact structure and its KPB age (e.g., Sharpton et al., 1992; Swisher et al., 1992; Koeberl et al., 1994).
The first discrepancies in the KPB age of the impact-induced clastic deposits came from the planktic foraminiferal biostratigraphic analysis carried out by Ward et al. (1995) at the Yucatán-1, -2, -4, -5A, and -6 wells. In the matrix of the breccia units, these authors found specimens belonging to the species Abathomphalus mayaroensis, Globotruncanita conica, Rosita patelliformis, Pseudoguembelina palpebra, Racemiguembelina fructicosa, and Muricohedbergella monmouthensis, leading them to infer a late Maastrichtian age (instead of a KPB age) for the deposition of this unit. However, given the hybrid and detrital nature of the breccia, all of these specimens must be considered reworked (ex situ) as a result of the erosion of older sediments, and they do not provide a direct and unequivocal age for the Chicxulub impact. Ward et al. (1995) also reported an 18-m-thick marly unit overlying the breccia unit at the Chicxulub-1 well and located near the center of the peak ring. They point out that López-Ramos (1973) identified “Maastrichtian” planktic foraminiferal species in this interval, such as Globotruncana rosetta, Globotruncana ventricosa, Globotruncana lapparenti, Globotruncana fornicata, Pseudoguembelina excolata, Heterohelix globocarinata, Pseudotextularia elegans, Planoglobulina carseyae, and Globigerinelloides volutus, which suggested that this was further evidence that the Chicxulub impact occurred before the KPB. Nevertheless, if these planktic foraminiferal specimens were in situ (not reworked), this would mean that the Chicxulub impact had occurred at least 2 m.y. before the KPB according to the biozonation of Arz and Molina (2002) and not by ~300 k.y., as suggested by Keller et al. (2004), among others. Furthermore, Ward et al. (1995) also found A. mayaroensis in the underlying impact breccia unit.
According to the most recent time scale (GTS2012; Gradstein et al., 2012), the last appearances of G. lapparenti and G. ventricosa were calibrated at 70.90 Ma and 70.14 Ma, respectively. Both bioevents are therefore older than the first appearance of A. mayaroensis (69.18 Ma), which indicates that the biostratigraphic interpretations of Ward et al. (1995) were incoherent. These biostratigraphic inconsistencies can only be explained as resulting from the mixing and reworking of specimens of different age, which is compatible with the infilling of the impact structure that followed the Chicxulub impact (Claeys et al., 1998). In addition, Marín et al. (1994) reported carbonate sediments with well-preserved Danian planktic foraminiferal assemblages directly above the suevite at the well Y6-N12 (1000–1003 m). These assemblages include species such as Parasubbotina pseudobulloides and Subbotina triloculinoides (Sharpton et al., 1996), which indicates that the carbonates overlying the suevite belong to the S. triloculinoides Subzone.
Normal Maastrichtian Hemipelagic Sedimentation Just above the CCU in NE Mexico?
El Mimbral and La Lajilla are two of the few localities worldwide that contain, albeit scarcely, preserved glassy spherules linked to the Chicxulub impact (Smit et al., 1996; Renne et al., 2018). In NE Mexico, the spherules are concentrated in Unit I of Smit et al. (1996), i.e., in the basal unit of the CCU in this region (Fig. 4A). High-resolution planktic foraminiferal biostratigraphic studies in the La Lajilla section led López-Oliva and Keller (1996) to disconnect the spherule-rich Unit I from the KPB. Their main evidence was the identification of diversified Maastrichtian planktic foraminiferal assemblages in a 4-cm-thick marly bed that locally overlies the CCU. If these were in situ specimens, they would indicate that the Chicxulub impact preceded the KPB by a few thousand years (Keller and Stinnesbeck, 1996). At La Lajilla, López-Oliva and Keller (1996) placed the KPB at the top of this thin, marly bed, coinciding with the lowest occurrences of Danian species such as Parvularugoglobigerina eugubina, Eoglobigerina fringa, and Eoglobigerina edita. According to these authors, a short hiatus is present that affects the basal Danian Biozones P0 and P1a (= Pα).
To ascertain whether the planktic foraminiferal specimens used to date the thin, marly bed are in situ or ex situ, we performed a similar high-resolution study from the ≥63-µm-sized fraction (Fig. 9). The Maastrichtian planktic foraminiferal assemblages of the Méndez Formation at La Lajilla belong to the Pt. hantkeninoides Zone and the uppermost part of the P. hariaensis Zone and are characterized by poor preservation (Fig. 10) but high diversity and a wide range of test sizes and morphological variability. A total of 61 species belonging to 17 genera were identified, and all Maastrichtian ecogroups are well represented. Reworked specimens of most of these species have been sporadically recognized in the spherule-rich Unit I of the CCU as well as in the overlying Units II and III.
In the thin, marly bed overlying the CCU at La Lajilla, there is evidence that the planktic foraminiferal specimens are ex situ. If the thin, marly bed had an ordinary hemipelagic origin as López-Oliva and Keller (1996) proposed, it should contain diversified planktic foraminiferal assemblages of the late Maastrichtian, similar to those recorded in the Méndez Formation (Keller et al., 1997). However, this is not the case. Despite an intensive search in the washed residues, we were only able to identify 12 small-sized species (Fig. 9). Their relative abundances are still similar to those found in the Méndez Formation, which suggests a settlement of fine sediment and foraminiferal tests previously suspended in the water column. The thin, marly bed was called Unit IV by Smit et al. (1996) and represented, according to these authors, the final phase of the local deposition of CCU, which occurred after the influence of tsunami waves had decreased and the suspended silt and clay was able to settle on the seafloor. At La Lajilla, the iridium concentration in Unit IV is the highest in the entire CCU, which indicates that fine particles rich in iridium settled at the same time as the rest of the silty and clayey particles that compose this unit (Smit, 1999).
Because the Cretaceous planktic foraminifera are reworked and there are no earliest Danian species, we consider the entire CCU at La Lajilla, including the upper Unit IV, to be a lowermost Danian barren interzone sandwiched between the Méndez and the Velasco Formations. The first centimeters of the Velasco Formation already contain Danian species of the Pv. longiapertura Subzone, which confirms the existence of a short hiatus (Fig. 9). According to the definition of the GSSP for the base of the Danian, the KPB must be placed here at the base of the spherule-rich Unit 1.
The Basal Glass Spherule Unit in NE Mexico and the Sandy Limestone Layer (SLL)
In northeastern Mexico, a 10–90-cm-thick, spherule-rich unit (SRU), such as Unit I of Smit et al. (1996) at La Lajilla, characterizes the basal part of the CCU (Figs. 4A and 6E). The spherules and droplets have been interpreted as remains of Chicxulub molten ejecta (impact glasses), because some spherules from El Mimbral contain a relict core of impact glass like those from Beloc (Maurrasse and Sen, 1991; Smit et al., 1992). The impact glasses were subsequently infilled and replaced by calcite or chlorite-smectite minerals, and the preserved impact glasses are, in fact, very rare (Figs. 11A–11B).
The SRU is quite heterogeneous in northeastern and central-eastern Mexico. Tabular or channeled beds contain not only spherules but also limestone and marly, flat rip-up clasts and muddy pebbles from the underlying Méndez Formation (Smit et al., 1996; Alegret et al., 2002; Schulte et al., 2003; Figs. 6G and 11D). These components are usually embedded in terrigenous grains and foraminifers. At El Mimbral, some unstratified marls and fused rip-up clasts derived from the Méndez Formation were plastically deformed seemingly from laterally continuous marly layers, alternating with spherule-rich sand layers (Fig. 6D), as Smit et al. (1992, 1996) had already observed.
The El Peñón section has attracted considerable attention since Stinnesbeck et al. (1993) reported an SRU composed of two spherule beds separated by a 10–20-cm-thick bed of well-cemented packstone, which was termed the sandy limestone layer (SLL). These authors claimed that the deposition of the SLL possibly lasted tens of thousands of years (Keller et al., 1997), which proves to be incompatible with the geologically instantaneous deposition previously suggested for the CCU (Smit et al., 1992). This bed has since been referred to by several other geologists not as a hemipelagic limestone but as a sandstone. For example, Schulte et al. (2010, Supporting Online Material, p. 14) described it as “a 10 cm-thick calcareous sandstone layer” and showed large, marly rip-up clasts at the base of this calcareous sandstone bed, similar to those found in the spherule-rich deposits in other localities. Similarly, Rehrmann et al. (2012, p. 325) described it as “a ca. 20 cm thick well indurated layer of fine- to medium-grained sandstone with a calcareous matrix.”
No thin sections of this layer have ever been illustrated to date, which has made it difficult to ascertain the true lithological nature of the SLL. Our petrographic analysis reveals that the so-called SLL is actually a fine- to medium-grained, well-sorted sandstone made up of mineral grains, mostly angular quartz and feldspar and very rounded carbonate, and occasionally (< 1%) undetermined bioclasts, glauconite, and plagioclase (Fig. 11C). The quartz is predominantly monocrystalline, although polycrystalline quartz is also identified. The micritic matrix is scarce, and sparry calcite cement is abundant. This sandstone bed does not differ much from the other sandstones in the lower part of the massive sandstone Unit II, as Rehrmann et al. (2012) pointed out. Similar SLLs have been reported at La Lajilla, El Mulato, and El Mimbral (Stinnesbeck et al., 1996; Keller et al., 1997), but we have not been able to confirm them in the field (Figs. 6C–6E and 6G).
According to Keller’s group, the SLL of El Peñón presents occasional J-shaped, spherule-filled burrows that are truncated by erosion, which reflects invertebrate colonization on the seafloor and long-term deposition (Keller et al., 2002a, 2003, 2009; Keller, 2014). Previously, these structures had been interpreted as water-escape structures resulting from the soft-sediment deformation of unconsolidated sediments due to the rapid deposition of the massive sandstones of Unit II (Smit et al., 1996). To date, only two structures resembling J-shaped burrows have been illustrated, and their stratigraphic position seems debatable. For example, Keller et al. (2002a) reported that the two ichnofossils shown in their figures 4B–4C come from the base of Unit II at El Peñón and Rancho Canales, respectively. Later, these same two ichnofossils were illustrated by Keller et al. (2009; their fig. 3), this time asserting that they both come from El Peñón (in the SLL and near the base of Unit II, respectively). Finally, Keller (2014) placed the same two J-shaped burrows in the SLL of El Peñón (their figures 16A–16B). To judge by our field observations, the presence of burrows is an extremely rare phenomenon in the CCU of northeastern Mexico, and they are concentrated exclusively at the top of the CCU, as also reported by Smit (1999) and Schulte et al. (2010). If these infrequent, burrow-like sedimentary structures are real ichnofossils, they may also be interpreted as escape structures (fugichnia) generated by animals buried and/or carried away by the sediment, as previously observed in turbidite sequences (e.g., Einsele, 1988).
The Discovery of the “Original” Glass Spherule Bed near the Base of the Pt. hantkeninoides Zone
The debate about the age of the Chicxulub impact took a new turn when Stinnesbeck et al. (2001) reported three or four SRUs embedded in the late Maastrichtian Méndez Formation in the La Sierrita area, NE Mexico. They interpreted them as resulting from an impact prior to the KPB that was followed by several episodes that reworked the “original” impact spherules, which initially accumulated in nearshore areas and were later re-deposited in younger, deeper sediments. According to these authors, the “original” SRU with the Chicxulub impact-derived ejecta is placed below the CCU (9 m and 2 m in the Loma Cerca and Mesa Juan Pérez sections, respectively). They also asserted that the SRUs in the La Sierrita area were separated by pelagic marls belonging to the Pt. hantkeninoides Zone, which suggests that the “original” SRU, and consequently the Chicxulub impact, predate the KPB by ~270 k.y. according to the timescales of that time.
Almost at the same time, Soria et al. (2001) studied these outcrops, albeit referred to as the El Tecolote area, and concluded that the multiplicity of spherule beds in the area was due to folding and other processes that disrupted the bed related to slump development, which implies that they are all the same and of KPB age. Slumps affect not only the SRU but also marls of the uppermost meters of the Méndez Formation (Fig. 6A) and, locally, the basal part of the sandy Unit II (Fig. 2B in Soria et al., 2001). The paper by Soria et al. (2001) sparked an exchange (Keller et al., 2002b; Soria et al., 2002) in which more arguments for “multiple spherule layers in a context of normal pelagic sedimentation” versus “multiplicity by slumping due to gravity flows triggered by the Chicxulub impact” were discussed.
Later, Keller (2005) reported a similar “original” pre-KPB SRU at El Peñón, 5 m below the CCU. It consists of a 40–200-cm-thick deposit with spherules cemented by a calcite matrix. According to Keller (2008), these spherules generally contain multiple air cavities and some concave-convex contacts, which is characteristic of Chicxulub impact glass, in contrast to the spherules of the SRU in the basal part of the CCU, which are embedded in a matrix rich in clasts and clastic minerals. Unfortunately, most of the micrographs of thin sections of the “original” Chicxulub spherules provided by Keller (2008, their figs. 10D–10F) and Keller et al. (2009, their fig. 7) fail to show the nature of the matrix, as they focus on showing one or two spherules in detail. Nonetheless, the spherules from the SRU of El Peñón and El Mimbral illustrated in Figures 11A–11B seem quite similar to those reported by Keller (2008) and Keller et al. (2009). These spherules, some of which are drop-shaped, show concave-convex contacts and commonly exhibit a bubbly internal texture that is infilled with calcitic cement and iron oxides; the occurrence of clastic grains in the matrix is highly variable but sometimes low. The spherules from the putatively pre-KPB SRU closely resemble those found in the slumped SRU of El Tecolote (Soria et al., 2001). Chemical and isotopic signatures of several glass types (yellow, green, and black) have been reported from the spherule-rich beds of the El Mimbral and Beloc sections, which indicates melt mixing of the target rocks (Blum et al., 1993; Belza et al., 2015, and references therein). From a stratigraphic, textural, and morphological point of view, we have not found significant differences between those types of spherules, which thus suggests that a single spherule deposit of KPB age was slumped and folded together with marls of the Méndez Formation.
The Maastrichtian deposits located above the allegedly pre-KPB SRU in the El Peñón section display some differences with respect to those in the La Sierrita/El Tecolote area. At El Peñón, Keller et al. (2009) described these deposits as a ~4-m-thick marl interval, horizontally bedded and bioturbated, which includes two marly limestone and two thin, rust-colored layers. According to these authors, the latter represent intervals of condensed sedimentation. The pelagic nature of these deposits is invoked as evidence of the resumption of normal sedimentation after the deposit of the “original” SRU that is linked to the Chicxulub impact. We were unable to confirm the presence of this stratigraphic succession at El Peñón, and we find it exceedingly strange that the marly limestones, bioturbation, and thin, rust-colored layers were not previously mentioned by Keller (2005, 2008).
For his Ph.D. thesis, Schulte (2003) carried out the most detailed fieldwork in NE Mexico. After detailed geological mapping, sampling, and analysis, he concluded that the SRUs embedded in the Méndez Formation in both La Sierrita and El Peñón areas show sedimentological evidence of having been modified after their initial deposition by further mass movements, constituting isolated, lenticular units of recumbent folded layers or disaggregated (breccia-like) spherule deposits of chaotic appearance. Schulte (2003) interpreted these sedimentary features as being indicative of downslope sediment movements associated with soft-sediment deformation, in accordance with Soria et al. (2001, 2002).
The overwhelming conclusion from the above considerations is that the hypothesis that the “original” SRU, and accordingly the Chicxulub impact, predates the KPB by ~270 k.y. finds little support in the sedimentary record. Synsedimentary deformational structures and slope instability induced by Chicxulub-derived seismic disturbances are the most reasonable explanation for the multiplication of spherule beds, contrary to what Keller (2008, 2014) claimed. Extensive sediment fluidization, liquefaction, large-scale slumping, dyke-like injections, and slope failure affecting unlithified uppermost Maastrichtian sediments below the CCU have been reported throughout the Gulf of Mexico–Caribbean region in outcrop sections, DSDP and ODP cores, oil wells, and seismic data (e.g., Keller et al., 2001; Soria et al., 2001; Day and Maslin, 2005; Schulte et al., 2012; Denne et al., 2013; Paull et al., 2014). They have also been reported in South Dakota (Jannett and Terry, 2008), the Atlantic continental margins (Klaus et al., 2000; Norris et al., 2000; Norris and Firth, 2002; MacLeod et al., 2007), and the Colombian Island of Gorgonilla in the Pacific Ocean (Renne et al., 2018; Bermúdez et al., 2019), among other locations.
Back to the Impact Structure: The ICDP Yaxcopoil-1 Borehole
In 2001 and 2002, the ICDP Chicxulub Scientific Drilling Program drilled the Yaxcopoil-1 borehole, located ~60–70 km from the center of the Chicxulub impact structure, on its southern flank. One of the main objectives of this program was to determine the role of the Chicxulub impact event in the KPB mass extinction, yet a consensus was not reached this time either. The Yaxcopoil-1 core includes a 100-m-thick impactite sequence recorded from 894.94 m to 794.14 m (Fig. 4B). The impactite sequence consists of polymict and monomict impact breccia and suevite, with a 13-m-thick redeposited suevite unit toward its top. Overlying the redeposited suevite, there is a 46-cm-thick laminated dolomitic calcareous sandstone unit with common amalgamated dolomite crystals that is referred to here as the critical unit (CU). A 2–3-cm-thick, dark, laminated, marly clay bed overlies the dolomitic calcareous sandstone unit. The interval between 794.10 m and 793.99 m consists of calcareous marls of the S. triloculinoides and G. compressa Zones, which suggests that a significant part of the lower Danian is missing (Arz et al., 2004). This hiatus was also recognized by Smit et al. (2004) and Keller et al. (2004).
Diverging interpretations of the age of the CU at Yaxcopoil-1 fueled a renewed controversy over the age of the Chicxulub impact. The CU was first described by Stinnesbeck et al. (2003) as a 50-cm-thick unit of dolomitic limestones barren of fossils. Just a year later, Keller et al. (2004) changed this description substantially, considering the unit to be finely laminated dolomitic and micritic limestones with abundant in situ planktic foraminiferal specimens, supposedly belonging to diverse late Maastrichtian assemblages and deposited in a low-energy, hemipelagic environment. According to Keller et al. (2004), the analysis of thin sections revealed the presence of a total of 15 late Maastrichtian species, including Pt. hantkeninoides. They therefore concluded that “normal” Maastrichtian hemipelagic limestones were deposited at Yaxcopoil-1 after the Chicxulub impact and before the KPB mass extinction, and that consequently the Chicxulub impact predated the KPB by ~300 k.y.
In contrast, Smit et al. (2004) described the CU as a cross-bedded unit with dolomitic sandstones (not micritic limestones) alternating with thin conglomerates formed by bio- and lithoclasts, shocked quartz grains, and altered to smectite impact glass. They did not find any planktic foraminiferal specimens in thin sections and interpreted the micrographs provided by Keller et al. (2004) as images of dolomite crystals that mimic foraminifers, because the idiomorphic overgrowth has the same thickness as a foraminiferal test wall. Smit et al. (2004) interpreted this unit as being the result of tsunami backwash and crater infill after the Chicxulub impact.
Our group (Arz et al., 2004) reached conclusions similar to those of Smit et al. (2004). In 2002, we received a set of samples from the CU placed in levels stratigraphically equivalent to those studied by Keller et al. (2004). Instead of studying them by means of thin sections, we disaggregated the samples with dilute acids and studied them from washed residues to achieve more accurate taxonomic identifications. Our results are summarized in Figure 12. Despite an intensive search, we were only able to find 11 planktic foraminiferal specimens in all of the samples studied from the CU, which contradicts the assertion by Keller et al. (2004) that this interval was rich in foraminifera. In addition, the mix of species of different ages is relevant, ranging from Albian–Turonian (e.g., Praeglobotruncana delrioensis, Muricohedbergella planispira, and Planomalina buxtorfi) to Campanian–Maastrichtian (e.g., Heterohelix navarroensis and Rugoglobigerina scotti). None of these specimens can be considered in situ as, by definition, all grains in a sandstone are reworked, including the foraminiferal tests. Keller (2011, p. 15) misrepresented these results by assuming without evidence that the specimens of H. navarroensis and R. scotti were in situ in support of her hypothesis that the CU was of Maastrichtian age.
Our observations of the washed samples confirmed that most of the micrographs of “late Maastrichtian” foraminifera illustrated by Keller from thin sections seem to be amalgamated dolomite crystals, which are very abundant in the residue of the studied samples (Arz et al., 2004). For example, according to Keller et al. (2004), the sample Yax1-794.55 m contains a diversified planktic foraminiferal assemblage. In Figure 12B, we illustrate a micrograph of the washed residue of the same sample. The residue is entirely composed of dolomite crystals, and there is no evidence of any planktic foraminiferal tests, as in most of the CU samples studied. Stinnesbeck et al. (2004) also illustrated thin-section micrographs from Upper Cretaceous and lower Paleogene samples in Yaxcopoil-1 to provide evidence of the presence of planktic foraminifera. The authors illustrated Danian specimens in their figures 4.1–4.12 and late Cenomanian specimens in figures 4.20–4.26. All of them are indubitably planktic foraminiferal tests, since they clearly display the chamber arrangement, chamber shape, test wall, chamber infill, as well as other characteristics of such tests. Conversely, the illustrated specimens from the CU, which were assigned to the latest Maastrichtian species Pt. hantkeninoides, Rugoglobigerina macrocephala, R. rugosa, Globotruncana insignis (?), and Rosita contusa (figs. 4.13–4.19), do not exhibit test walls or any other attributes of foraminiferal tests and therefore must be considered pseudo-fossils. The micropaleontological evidence at Yaxcopoil-1 suggesting a pre-KPB age for the Chicxulub impact seems to be unsubstantiated.
Was Chicxulub a “Dirty-Snowball”–type Comet that Blasted near the Gulf of Mexico Without Significant Biotic Effects?
Since they found no iridium anomaly in the pre-KPB “original” impact ejecta layer in the La Sierrita and El Peñón area, Keller (2008) suggested that Chicxulub was a “dirty-snowball”-type comet whose impact did not eject appreciable amounts of iridium. The assertion that Chicxulub was a comet whose collision occurred before the KPB should have been supported by stronger evidence, such as the presence of shocked minerals or accretionary lapilli in the allegedly “original” SRU and/or relevant changes in the post-impact microplankton assemblages recorded in the basal part of the Pt. hantkeninoides Zone. However, such evidence does not appear to be on hand.
High-resolution quantitative analyses of planktic foraminifera are very powerful tools for evaluating paleoenvironmental, paleoclimatic, and paleoceanographic changes, for they are highly sensitive to changes in physicochemical pelagic conditions and oceanic productivity. Keller et al. (2002a) and Keller et al. (2009) used this methodology to evaluate the biotic effects of the Chicxulub impact on the planktic foraminiferal assemblages in the aforementioned sections of Loma Cerca, Mesa Juan Pérez (El Tecolote), and El Peñón. Significantly, they found no biotic changes during and/or after the deposition of the “original” SRU. Keller et al. (2009) concluded that none of the 52 planktic foraminiferal species went extinct until the top of the Maastrichtian, nor did they undergo any major changes in their relative abundance before and after the Chicxulub impact. This is indeed surprising because, shortly afterwards, Keller (2014) argued that the Chicxulub impact and the latest Maastrichtian warming event (LMWE), a global warming episode that has commonly been linked to the onset of massive volcanic eruptions in the Deccan Traps (e.g., Hull et al., 2020), were contemporaneous and, consequently, their negative effects on microplankton communities should have been cumulative.
The hypothesis that the Chicxulub impact did not cause any extinction does not seem a plausible one. Models based on the size, velocity, and angle of the impactor and/or on the impactite sequence itself in cores within the Chicxulub impact structure, among other data, predict that, regardless of its age, the Chicxulub impact would have been bound to trigger severe disturbances in the biosphere (e.g., Kring, 2007; Schulte et al., 2010, and references therein). Moreover, such a large impact (produced even by a comet) would have led to a supra-regional to global ejecta distribution within late Maastrichtian sediments, which has never been observed anywhere.
PLANKTIC FORAMINIFERS, IRIDIUM ANOMALIES, AND A LOWER DANIAN ASTEROID IMPACT
According to Keller et al. (2003), the multiple asteroid impact scenario through the KPB is supported by multiple SRUs and iridium-rich levels recorded in early Danian outcrops in east-central and southern Mexico, Guatemala, Belize, Haiti, and Cuba. However, we have not been able to verify this scenario in any of the sections we have studied. One of the best sections to shed light on this matter is the Bochil section in southern Mexico, because it contains microfossil-rich pelagic deposits, a thick CCU, and a clearly marked iridium anomaly.
The 86-m-thick CCU at Bochil is sandwiched between two pelagic formations rich in planktic foraminifera: the underlying Jolpabuchil Formation (Campanian–Maastrichtian) and the overlying Soyaló Formation (Paleocene). An iridium anomaly of 1.6 ppb was reported just above the CCU in a 6–10-cm-thick, intensively bioturbated, silty chalk bed below an irregular ~10-cm-thick micritic limestone bed (Montanari et al., 1994). These stratigraphic levels are easily recognizable in the field, as can be seen in Figures 5B–5C.
Keller and Stinnesbeck (1996), Keller et al. (2003), and Stüben et al. (2005) reported several studies of the Bochil section that supported their hypothesis of multiple impacts through the KPB. Nevertheless, the age suggested for the CCU, as well as the lithostratigraphic descriptions and sedimentological interpretations, have changed over time. First, they suggested that the breccia unit of the CCU at Bochil is pre-KPB in age, arguing that the overlying uppermost meter of the CCU contains Pt. hantkeninoides and consists of “normal” Maastrichtian hemipelagic sand and shales deposited below the iridium-anomalous horizon (Keller and Stinnesbeck, 1996). Later, they proposed that this stratigraphic interval (i.e., the uppermost meter of the CCU) consists of brown shales and shaly marls containing planktic foraminiferal assemblages of the early Danian P1a(1) Subzone; consequently, their interpretation was that the iridium-anomalous horizon was the record of an impact event in the early Danian (Keller et al., 2003). This latter proposal was based on the earliest Danian planktic foraminiferal species found in a discrete, 3-cm-thick, white marly level placed just below a 5-cm-thick, burrowed interval within the uppermost meter of the CCU. The latter was then re-described by the authors as a ~1-m-thick microconglomerate unit with rudists, ostracods, and spherule debris (Fig. 13, after Keller, 2008). Later, Stüben et al. (2005) observed a sharp iridium anomaly of up to 1.2 ng/g directly overlying the bioturbated layer, in the same stratigraphic level as Montanari et al. (1994), but again suggested that it is early Danian in age.
In Figure 13, the stratigraphic columns and lithological units at Bochil according to Keller (2008) and Arenillas et al. (2006) are shown for comparison. The latter performed a high-resolution quantitative study of planktic foraminifera at Bochil and showed that the iridium anomaly is not Danian but KPB in age. Unlike Keller et al. (2003), they did not find any earliest Danian planktic foraminifers below the iridium anomaly. However, their presence cannot be completely ruled out in this layer, because it is located just below the intensely burrowed, thin siltstone bed. This fine bed, which topped the CCU, may be contaminated with infiltrated Danian microfossils due to the bioturbational activity of Danian invertebrates, as proven in other KPB outcrops and sites (Rodríguez-Tovar et al., 2020, and references therein).
LINKING THE CHICXULUB IMPACT TO THE KPB
What really happened at the KPB, according to the multi-impact scenario? Despite geochemical evidence that clearly links the impact melt of Chicxulub with the impact-derived ejecta (e.g., Swisher et al., 1992; Blum et al., 1993), Keller (2005) assumed that the iridium-rich airfall layer that marks the GSSP at El Kef is not linked to Chicxulub but to a significantly larger impactor whose crater could have reached at least 250–300 km in diameter, which is consistent with the global distribution of the iridium anomaly and the mass extinction event. As long as this undiscovered gigantic impact structure and/or the colossal, ejecta-rich deposits associated with this hypothetical impact are not found, we think that there is plenty of evidence to support the claim that the Chicxulub impact occurred at the KPB and was the trigger for the mass extinction, as most previous research has concluded.
Biomarkers and Impact Markers Confirming that the KPB Impact Event and the Chicxulub Event Are One and the Same
Biozone P0, Planktic Foraminiferal Acme Stages, and the KPB Dark Clay Bed
Although the Chicxulub-derived lithological unit, i.e., the CCU, is extraordinarily thick in the Gulf of Mexico–Caribbean region, Biozone P0 (= the Mh. holmdelensis Subzone) and the KPB dark clay bed are absent in most sections of this region, which implies a short hiatus affecting the lowermost Danian (López-Oliva and Keller, 1996; Keller et al., 1997; Arz et al., 2001a, 2001b, 2004; Alegret et al., 2005; Molina et al., 2009). PFAS-1 (the Guembelitria acme), or at least its lower part, is also missing or partially absent in some localities in the Gulf of Mexico and the Caribbean. For example, PFAS-1 has not been identified at La Lajilla, and PFAS-2 (or the parvularugoglobigerinid acme) overlies the local CCU in this section (Fig. 9). We have found four notable exceptions: Bochil and Guayal in southern Mexico (Arenillas et al., 2006), Moncada in western Cuba (Arenillas et al., 2016), and Nan Pak in Haiti (this report). In all four sections, both the dark clay bed (Figs. 5C–5E, 7D, and 8E) and Biozone P0 (Figs. 14–17) were identified just above the local CCU, which indicates a complete sedimentary record of the lowermost Danian, since Biozone P0 spans the first 7 k.y. of the Danian. Planktic foraminiferal images of relevant species are illustrated in Figures 18 and 19. In the Nan Pak section, the KPB dark clay bed appears to be proportionally less thick than in the other sections or in the El Kef stratotype (Fig. 20), since it only spans the lower part of Biozone P0. This may be the result of a depositional process, since the upper part of the dark clay bed could be locally replaced by silica-rich limestones, perhaps due to a proliferation of calcareous algae and radiolarians. The sedimentary continuity in the lowermost Danian of Bochil, Moncada, and Nan Pak is verified by the identification of PFAS-1 (the Guembelitria acme) by Arenillas et al. (2006) in the three localities (Fig. 20).
Lowery et al. (2018) identified a high abundance of Guembelitria in the lowermost Danian limestone sample at Site M0077, which contains the most continuous lowermost Danian record inside the Chicxulub impact structure known to date. This sample belongs to Biozone Pα and consists of dark micrite, so it could correspond to the uppermost part of PFAS-1. Planktic foraminiferal images of relevant species from Site M0077 are illustrated in Figure 18.
The rapid evolutionary radiation of new Danian planktic foraminiferal species after the KPB mass extinction provides excellent biochronostratigraphic markers. Keller (2014, p. 61) accused Arenillas et al. (2006) and Molina et al. (2006), among others, of circular reasoning: Chicxulub is KPB in age by definition since Molina et al. (2006) re-defined the GSSP for the KPB based solely on impact markers. As we have discussed previously, this statement is a fallacy. The recognition of Biozone P0 and PFAS-1, and the KPB dark clay bed in the Gulf of Mexico–Caribbean region, constitute strong evidence that the Chicxulub impact is KPB in age, with a margin of error less than 7 k.y. based on biochronological criteria. This is the most accurate dating of the impact ever obtained by any method.
Distribution of Impact Markers
Keller et al. (2003) claimed that the hypothesis of multiple impacts through the KPB is the one most consistent with the stratigraphic evidence and questioned the idea that the separation of ejecta spherules and the iridium anomaly in the CCUs is due to expected, impact-induced sedimentary processes. This scenario was suggested on the basis only of glass spherules and iridium anomalies; it was argued that these are the most widespread evidence and the most easily recognized impact markers. However, when the stratigraphic distribution of other impact markers, such as Ni-rich spinels or shocked quartz, is added to the database, the most plausible scenario is a single impact event exactly at the KPB (Claeys et al., 2002; Schulte et al., 2010). Furthermore, Ni-rich spinels and shocked minerals, as well as shatter cone structures, are found in the CCU but not in what Keller and her colleagues refer to as the “original” Chicxulub layer (Schulte et al., 2010, and references therein).
The Chicxulub impact-derived material also includes accretionary lapilli, which are spherules up to 2 cm in size formed by the accretion of small mineral and lithic fragments. They are considered to be part of the distal fallout suevite and were formed in the turbulent part of the impact vapor cloud (Claeys, 2006). The accretionary lapilli are concentrated in discrete beds, and their large size and fine concentric structure allow them to be easily recognized with a field loupe. This is a relevant impact marker that Keller’s group systematically omits. Accretionary lapilli similar to those reported from the Miocene Ries impact structure, Germany (Graup, 1981) or the Paleoproterozoic Sudbury impact structure, Canada (Huber and Koeberl, 2017), are widely found in the CCU over a wide area bordering the Chicxulub impact structure.
The high fragility of accretionary lapillus grains implies that they are not reworked. In situ accretionary lapilli have been described in places as far apart as Mesa Juan Pérez, Rancho Canales, El Mimbral, and Guayal in Mexico (Grajales-Nishimura et al., 2003; Schulte, 2003; Schulte and Kontny, 2005; Claeys, 2006), Armenia in Belize (Pope et al., 2005), Loma Capiro in Cuba (Alegret et al., 2005), the Brazos River in Texas, and the Bass River borehole in New Jersey, USA (Yancey and Guillemett, 2008). Lapillistone beds and individual lapillus grains from the CCU of Guayal are illustrated in Figures 5F and 11E, respectively. According to Keller (2014), the deposition of the CCU, rich in accretionary lapilli and shocked quartz grains, would have predated the KPB by ~50–80 k.y. This would entail the impact of another, larger asteroid near the Gulf of Mexico and the Caribbean, after the Chicxulub impact and before the alleged giant asteroid impact occurring at the KPB. For this to be confirmed, another as yet undiscovered impact structure would have to be located within the region where the accretionary lapilli were found.
Recently, DePalma et al. (2019) reported an exceptional KPB seismically induced seiche deposit in North Dakota, USA. Termed the Tanis Event Deposit, this contains an iridium anomaly, shocked minerals with multiple intersecting sets of planar deformation features, microkrystites, and unaltered glass spherules geochemically linked with Chicxulub black glass. DePalma et al. (2019) showed a variety of evidence that the Tanis spherules are not reworked from older levels, such as the presence of spherules clustered in the gill region of paddlefish specimens and in amber samples, and of down-warped laminations with a single spherule at their base caused by incoming ejecta that perturbed the fine sediment laminations. The Tanis Event Deposit provides relevant new geochemical, sedimentological, and paleobiological evidence for the link between the Chicxulub ejecta and the KPB.
Microfossils in the CCU Supporting the Erosion of a High Volume of Upper Cretaceous Sediments
In the Gulf of Mexico–Caribbean region, a persistent unconformity occurs underneath the CCU as a result of the seismic pulse generated by the Chicxulub impact and the subsequent earthquakes, tsunamis, submarine landslides, and erosion of Cretaceous materials. The time span of the stratigraphic gap of the uppermost Cretaceous is highly variable: < 100 k.y. in northeastern and east-central Mexico (López-Oliva and Keller, 1996; Arz et al., 2001a, 2001b) and Haiti (Nan Pak; this report), ~150 k.y. at Loma Capiro, and ~400–550 k.y. at Bochil and Guayal (Grajales-Nishimura et al., 2009) and Santa Isabel and Peñalver (Takayama et al., 2000). At Moncada, an extensive hiatus was identified, because the deposits below the CCU are Albian–Cenomanian in age (Tada et al., 2003; Arenillas et al., 2016). The eroded Cretaceous pelagic sediments included huge quantities of foraminifera and calcareous nannofossils that were suspended along the water column and redeposited in the CCU. The reworked and mixed Cretaceous microfossils are currently a major part of the CCU clastic sediments, which Bralower et al. (1998) very appropriately called the K-T boundary cocktail.
The mixture of planktic foraminifera of different ages is very evident in the 30-m-thick calcilutite unit in the upper part of the CCU at Santa Isabel and Peñalver. For example, we have identified more than 60 species with ages from Aptian to Maastrichtian in the highest sample of the upper calcilutite unit at Peñalver; these include Planomalina buxtorfi (upper Albian), Whiteinella aprica (upper Cenomanian to Turonian), Marginotruncana coronata (upper Turonian to lower Campanian), Sigalia decoratissima (upper Santonian), Radotruncana calcarata (upper Campanian), Abathomphalus mayaroensis and Racemiguembelina powelli (upper Maastrichtian), and Pt. hantkeninoides (uppermost Maastrichtian). Some of these species are illustrated in Figure 19. Dicarinella concavata, Globotruncanita elevata, and Gansserina gansseri, which are biostratigraphic markers of Upper Cretaceous stages, are also frequent in the 35 samples studied from the CCU of Santa Isabel and Peñalver. As Bralower et al. (1998) suggested, these mixed assemblages are indicative of a catastrophic event involving the erosion, transport, and redeposition in the Caribbean Sea of a high volume of Upper Cretaceous sediments.
CCU deposits with abundant, reworked Upper Cretaceous foraminifera and calcareous nannofossils were also reported in northeastern Mexico (Smit et al., 1999; Arz et al., 2001a, 2001b), southern Mexico (Grajales-Nishimura et al., 2003, 2009; Arenillas et al., 2006; Paull et al., 2014), in an industry-drilled, deep-water well in the northern Gulf of Mexico (Denne et al., 2013; Sanford et al., 2016; Poag, 2017), in western North Atlantic sites (ODP Site 1049: Norris et al., 1999; DSDP Sites 386 and 387: Norris et al., 2000; and ODP Leg 207: MacLeod et al., 2007), and even within the Chicxulub impact structure in the Yaxcopoil-1 borehole (Arz et al., 2004) and Site M0077 (Lowery et al., 2018; Whalen et al., 2020).
The huge amount of sediment deposited at the KPB as the CCU and distributed up to 1000 km around the Chicxulub impact structure was clearly governed by multiple high-energy geological processes such as sediment liquefaction and slope failure (e.g., Bralower et al., 1998; Grajales-Nishimura et al., 2000, 2003, 2009; Day and Maslin, 2005), the resurge of oceanic water in the crater (e.g., Matsui et al., 2002; Smit et al., 2004; Gulick et al., 2019), and tsunamis (e.g., Smit, 1999; Lawton et al., 2005; Goto et al., 2008; Schulte et al., 2012) and seiche waves (e.g., Maurrasse et al., 2005; DePalma et al., 2019). This evidently contradicts the claim by the Keller group (Keller et al., 1997, 2003; Keller, 2014) that the sedimentary record of the alleged undiscovered KPB asteroid impact is largely missing in the Gulf of Mexico and Caribbean continental shelves due to extensive erosion. In fact, the CCU has been judged to be the most widespread and biggest deposit on Earth (Norris et al., 2000). By mapping thickness with seismic and borehole data, several authors have calculated the volume of Chicxulub impact-remobilized sediments in the Gulf of Mexico and the Caribbean, concluding that the global volume could range between ~105 × 103 km3 (Denne et al., 2013; Sanford et al., 2016) and ~198 × 103 km3 (Pope et al., 2004).
Global stratigraphic, sedimentological, petrographic, and micropaleontological records provide strong evidence for a single large asteroid impact at the Cretaceous/Paleogene boundary (KPB), as proposed by Alvarez et al. (1980). The seismic energy released dramatically altered the sedimentation in the Gulf of Mexico–Caribbean region and triggered the extensive deposition of an ejecta-rich, complex clastic unit widely distributed throughout the region. The Chicxulub impact structure was finally discovered because all previous evidence had originally pointed to the Gulf of Mexico–Caribbean region as the impact site. The alternative multi-impact hypothesis proposed by the Keller group is inconsistent with the data. If the Chicxulub impact predated the KPB and a second larger impact was responsible for the global iridium deposition at the KPB, it would also be expected that this alleged second impact produced a huge clastic deposit, but there is currently no supporting evidence. The evidence presented here does not rule out the possibility that other, minor causal factors might have played a role in the KPB mass extinction and the delayed ecosystem recovery in the early Danian; however, it casts doubt on claims that there were multiple impacts across the KPB and provides strong arguments that the Chicxulub impact was the main cause of the KPB mass extinction.
We deeply thank science editor Nancy Riggs, volume editor Christian Koeberl, and the two reviewers, Peter Schulte and Florentin Maurrasse, for thoughtful and constructive reviews that improved the manuscript. This work was supported by MCIU/AEI/FEDER, UE (PGC2018-093890-B-I00) and by the Aragonese government/FEDER, UE (DGA groups E33_20R and E32_20R). V. Gilabert acknowledges support from the Spanish Ministerio de Economía, Industria y Competitividad (FPI grant number BES-2016-077800). The authors thank Alfonso Meléndez, Gustavo Murillo, and María del Carmen Rosales for their logistical support and discussions during the fieldwork campaigns in Mexico. We thank Roberte Bien-Aimé Momplaisir for her help during the fieldwork in Haiti, give particular thanks for the 4×4 vehicle of the University of Port au Prince, and are very grateful to Hildegonde Cenatus Amilcar and Helliot Amilcar for the discussion in the field. This study includes data from samples provided by the International Ocean Discovery Program (IODP) and the International Continental Scientific Drilling Project (ICDP). We acknowledge the use of the Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza, Spain, for providing thin sections and scanning electron microscope micrographs and thank Rupert Glasgow for improving the English text.