Eric Robin

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

Abstract Magnetic spinels produced by oxidation of extraterrestrial objects in the atmosphere have a composition distinct from terrestrial spinels. They are characterized by a high iron oxidation state, arising from crystallization under high oxygen fugacities, and a high nickel concentration due to the relatively high abundance of this element in extraterrestrial material. The iron oxidation state increases from micrometeorites, to meteoroid ablation material and to impact-generated products. This reflects a progressive increase of the oxygen fugacity, corresponding to decreasing altitudes of crystallization. Spinels found at the Cretaceous-Tertiary boundary are similar to those that crystallized from meteoroid ablation material and impact-generated products, supporting the view that a collisional event did occur at the end of the Cretaceous.

Cosmic markers, 40Ar/39Ar dating and paleomagnetism of the KT sections in the Anjar Area of the Deccan large igneous province

Abstract Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424] reported the discovery of iridium-bearing sediments sandwiched between basalt flows in the Anjar area (Kutch province, India). They concluded that the signature of the K/T impact had been recorded and that onset of volcanism in the Deccan traps preceded the K/T boundary, excluding the possibility of a causal connection. This paper reports complementary analyses of Anjar outcrops by a joint Indo–French team, where we focused on cosmic markers (iridium and spinels) in the intertrappean sediments and 40Ar/39Ar dating and paleomagnetism of the lava flows. Anomalous Ir concentrations (up to 0.4 ng/g) are confirmed, with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections. Despite careful search, no Ni-rich spinels were found. Eight basalt samples provided 40Ar/39Ar results, four on plagioclase bulk samples, four on whole rocks. Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. Sci. Lett. 180 (2000) 13–28]. The three flows underlying the Ir-bearing sediments are dated at ∼66.5 Ma, and two overlying flows at ∼65 Ma. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. There are reports [Bajpai, Geol. Soc. India Mem. 37 (1996) 313–319; Bajpai, J. Geol. Soc. London 157 (2000) 257–260] that the intertrappean sediments contain uppermost Maastrichtian dinosaur and ostracod remains above the uppermost Ir-bearing level, and may not be mechanically disturbed. We propose the following scenario to interpret these multiple field and analytical observations. Deccan trap volcanism started within uppermost Maastrichtian normal chron C30N at ∼66.5–67 Ma in the Anjar area. Volcanism then stopped at least locally, and lacustrine sediments were deposited over a period that could be in the order of 1–2 Ma. The K/T bolide impact was recorded as a deposit of Ir, and possibly (though not necessarily) spinels. Volcanism resumed shortly after the K/T boundary, within reversed chron C29R, as witnessed by the three reversely magnetised overlying basalt flows dated ∼65 Ma. This was responsible for erosion and destruction of part of the uppermost sediments (including spinels if there were any) and heterogeneous and non-uniform redeposition of Ir at a number of underlying sedimentary levels. This was also responsible for the partial remagnetisation of the underlying flows. These findings generally confirm and complement those of Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424], and are compatible with the occurrence of the K/T impact at the paleontological K/T boundary, and of Deccan trap volcanism straddling the boundary and starting before the impact. Anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors, though still within normal chron C30N. There is no indication contradicting the view that the bulk of Deccan trap volcanism occurred over only three chrons (C30N, C29R, C29N) [Courtillot, Evolutionary Catastrophes: the Science of Mass Extinctions, Cambridge University Press, 1999; Courtillot et al., Earth Planet. Sci. Lett. 80 (1986) 361–374; Vandamme et al., Rev. Geophys. 29 (1991) 159–190].

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

Abstract Ni-rich spinels have been found throughhout the world at the Cretaceous-Tertiary boundary. These minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris, suggesting that the Cretaceous-Tertiary boundary spinels result from an extraterrestrial event. Their stratigraphic distribution, which has been observed at the Cretaceous-Tertiary boundary sites of El Kef (Tunisia), Caravaca (Spain) and Hole 761C (ODP Leg 122, Indian Ocean), are markedly different from site to site and are all much less extended than the Ir anomalies. Regional differences in Ni-rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions. The wider stratigraphic dispersion of Ir is attributed to postdepositional chemical processes. At El Kef, the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate, the 1–2 mm thick Ni-rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years. This upper limit is consistent only with the collision of an asteroid or a comet at the close of the Cretaceous.

Extraterrestrial Ni-rich spinel in upper Eocene sediments from Massignano, Italy

Evidence of a major cosmic event at the end of the Eocene is given by the finding of at least one, possibly two or more, impact horizons containing microtektites, microkrystites, shocked quartz, and unusually high iridium concentrations. We report here the discovery, in the global stratotype of the Eocene-Oligocene boundary at Massignano in Italy, of Ni-rich spinel, a mineral that forms by the interaction of meteoritic bodies with the Earths atmosphere. A maximum concentration of ≈800 crystals mg−1, corresponding to a flux of 2 × 107 crystals cm−2, is observed in a well-dated layer (35.7 ± 0.4 Ma) that also contains shocked quartz and iridium. The crystals are found clustered in flattened spheroids, the probable relic of their host bodies. The chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects, as proposed for the spinel-bearing spheroids found at the Cretaceous-Tertiary boundary. The most likely explanation is that the dust particles were generated by a large cometary impact. Whether this impact is the one that produced microtektites and/or microkrystites, or another one that may have occurred shortly after or before, is still questionable. Additional searching for Ni-rich spinel at other upper Eocene sections may help to answer this question.

The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area

Abstract The K-T boundary at Beloc in Haiti is characterized by a bed containing more or less spheroidal globules composed of either smectite or calcite. A small fraction of these objects still contains well-preserved glass particles which have been interpreted as the remains of tektites resulting from a bolide impact in the Caribbean area. However, Beloc glasses are markedly different from tektites and microtektites: their structure and composition suggest that they are not impact-generated products, but are likely of volcanic origin. On the other hand, in sections which have probably preserved their original stratigraphy, the globule bed, showing well-developed grading, appears first in the sedimentary column followed by the ubiquitous Cretaceous-Tertiary boundary (KTB) clay layer containing Ir- and Ni-rich spinels: granulometric considerations indicate that the globule bed and the Ir- and spinel-rich clay layer correspond to two distinct events. We conclude that the globules at the KTB depositional sequence of Beloc represent a local volcanic event slightly preceding the great KTB cosmic catastrophe. Our observations do not provide any unequivocal evidence that a cratering event occurred nearby.

Crystalline micrometeorites from Greenland blue lakes: Their chemical composition, mineralogy and possible origin

Among the different populations of cosmic particles found in the Greenland cryoconites, the irregular-shaped population is probably the most interesting one because it provides direct information about the nature of small meteoroids that impact the Earth. Petrographic and trace element analyses of this material reveal two main families labelled as coarse-grained (CG) and fine-grained (FG) which have, respectively, compact and porous structures. We have restricted our study to the coarse-grained particles which contain large relic minerals and thus are well adapted to the SEM and microprobe analysis techniques. We compare the mineralogical associations of 49 coarse-grained particles to that of meteorites and find that a majority of them belong to the same population of mineral grains previously found in CM chondrites and a minority of ordinary chondrites.

The K/T boundary at Beloc (Haiti): Compared stratigraphic distributions of the boundary markers

At Beloc, Haiti the Cretaceous-Tertiary boundary (KTB) is characterized by a spherule bed containing glass particles. These particles are considered by some authors as remains of tektites resulting from a nearby impact. However, because of the stratigraphic complexity of the Beloc sections the genetic link between the KTB cosmic event and the spherule bed is not obvious. In this paper, we report new data on shocked quartz and Ni-rich spinels at Haitian KTB sites. The detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence. The first and largest shocked quartz is found in the upper part of the spherule layer. They are abundant and size graded over the 25–30 cm of carbonate-rich sediments overlying this layer. The first Ni-rich spinels, which are also rich in Cr, appear in the carbonate sediments. The size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event. Although different from typical impact glasses (tektites), Beloc glass particles must be considered as impact-derived products. The enormous fluence of shocked quartz (≈104 grains/cm2) is consistent with a proximal event. The upper part of the sequence is more complex. A second distribution of shocked quartz associated with Ni-rich spinels of different compositions appears in the layer containing the maximum Ir concentration. We propose that these features, which are not easily explained by a sedimentary artifact, result from a second collisional event.

Enregistrement des événements remarquables de la limite Crétacé-Tertiaire dans la coupe d'Ellès (Tunisie)

Abstract The review of the Cretaceous–Paleogene interval deposits of the Elles section based on a detailed sampling gives a good characterization of the K/T boundary and reconstitution of the geological events underlining this boundary. Thus, the discovery of a thin Ir-rich layer, with Ni and Cr-rich spinel and shocked quartz is a well preserved record of the known cosmic event that occurred at that time. This sudden event is also corroborated by the quantitative and qualitative organic components distribution along the K/T interval. Added to some other long-term events (e.g. climatic, eustatic), it generated a mass extinction, at the K/T boundary, of specialists among the Globotruncanids and Heterohelicids planktic foraminifera species. Following this biological crisis, the biotic turnover into the Danian is slow. The underlining of all the Cretaceous–Tertiary interval biozones attests that the Elles section is complete. If compared to the K/T stratotype and El Kef II sections, the Elles section is found to display a better exposure of the K/T interval and shows more expanded zones and subzones. Such characteristics promote the Elles section to be considered as a parastratotype.

Sedimentological and diagenetical conditions of the impact level of the Cretaceous/Tertiary boundary in Tunisia; no anoxia required

The Ain Settara section (Kalaat-Senan area, Tunisia) shows a well-exposed and expanded Cretaceous/Tertiary (K/T) boundary that can be divided into several units of contrasted lithologies. The bolide impact level is identified thanks to the occurrence of a peak of iridium and Ni-rich spinel abundance. The impact level is a jarosite nodule-rich horizon upon which the emphasis is placed: the jarosite nodules, initially present as pyrite nodules, may be interpreted as reflecting reducing depositional conditions at the K/T boundary. Trace metal geochemistry indicates that depositional conditions were not reducing and that the nodules must not have formed during earliest diagenesis. Consequently, water column-scaled anoxia cannot be put forward to account for the mass extinctions observed at the K/T boundary at Ain Settara.