KT boundary

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 ...

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.

Large impacts are credited with the most devastating mass extinctions in Earth’s history and the Cretaceous – Tertiary (K/T) boundary impact is the strongest and sole direct support for this view. A review of the five largest Phanerozoic mass extinctions provides no support that impacts with craters up
to 180 km in diameter caused significant species extinctions. This includes the 170 km-diameter Chicxulub impact crater regarded as 0.3 million years older than the K/T mass extinction. A second, larger impact event may have been the ultimate cause of this mass extinction, as suggested by a
global iridium anomaly at the K/T boundary, but no crater has been found to date. The current crater database suggests that multiple impacts, for example comet showers, were the norm, rather than the exception, during the Late Eocene, K/T transition, latest Triassic and the Devonian – Carboniferous
transition, but did not cause significant species extinctions. Whether multiple impacts substantially contributed to greenhouse warming and associated environmental stresses is yet to be demonstrated. From the current database, it must be concluded that no known Phanerozoic impacts, including the
Chicxulub impact (but excluding the K/T impact) caused mass extinctions or even significant species extinctions. The K/T mass extinction may have been caused by the coincidence of a very large impact (4250 km) upon a highly stressed biotic environment as a result of volcanism. The consistent
association of large magmatic provinces (large igneous provinces and continental flood-basalt provinces) with all but one (end-Ordovician) of the five major Phanerozoic mass extinctions suggests that volcanism played a major role. Faunal and geochemical evidence from the end-Permian, end-
Devonian, end-Cretaceous and Triassic/Jurassic transition suggests that the biotic stress was due to a lethal combination of tectonically induced hydrothermal and volcanic processes, leading to eutrophication in the oceans, global warming, sea-level transgression and ocean anoxia. It must be concluded that major magmatic events and their long-term environmental consequences are major contributors, though not the sole causes of mass extinctions. Sudden mass extinctions, such as at the K/T boundary, may require the coincidence of major volcanism and a very large Impact.

Impact stratigraphy is an extremely useful correlation tool that makes use of unique events in Earth’s history and places them within spatial and temporal contexts. The K-T boundary is a particularly apt example to test the limits of this method to resolve ongoing controversies over the age of the Chicxulub impact and whether this impact is indeed responsible for the K-T boundary mass extinction. Two impact markers, the Ir anomaly and the Chicxulub impact spherule deposits, are ideal
because of their widespread presence. Evaluation of their stratigraphic occurrences reveals the potential and the complexities inherent in using these impact signals. For
example, in the most expanded sedimentary sequences: (1) The K-T Ir anomaly never contains Chicxulub impact spherules, whereas the Chicxulub impact spherule layer never contains an Ir anomaly. (2) The separation of up to 9 m between the Ir anomaly and spherule layer cannot be explained by differential settling, tsunamis, or slumps. (3) The presence of multiple spherule layers with the same glass geochemistry as melt
rock in the impact breccia of the Chicxulub crater indicates erosion and redeposition of the original spherule ejecta layer. (4) The stratigraphically oldest spherule layer is in undisturbed upper Maastrichtian sediments (zone CF1) in NE Mexico
and Texas. (5) From central Mexico to Guatemala, Belize, Haiti, and Cuba, a major K-T hiatus is present and spherule deposits are reworked and redeposited in early Danian (zone P1a) sediments. (6) A second Ir anomaly of cosmic origin is present in
the early Danian. This shows that although impact markers represent an instant in time, they are subject to the same geological forces as any other marker horizons— erosion, reworking, and redeposition—and must be used with caution and applied on a regional scale to avoid artifacts of redeposition. For the K-T transition, impact stratigraphy
unequivocally indicates that the Chicxulub impact predates the K-T boundary, that the Ir anomaly at the K-T boundary is not related to the Chicxulub impact, and that environmental upheaval continued during the early Danian with possibly
another smaller impact and volcanism.

The Cretaceous–Tertiary boundary (KTB) mass extinction is primarily known for the demise of the dinosaurs, the Chicxulub impact, and the frequently rancorous thirty-years-old controversy over the cause of this mass extinction. Since 1980 the impact hypothesis has steadily gained support, which culminated in 1990 with the discovery of the Chicxulub crater on Yucata´n claimed as the KTB impact site and ‘‘smoking gun’’ that virtually proved this hypothesis. In a perverse twist of fate, this discovery also began the decline of the impact hypothesis, because for the first time it could be tested directly based on the impact crater and impact ejecta in sediments throughout the Caribbean, Central America, and North America. Two decades of multidisciplinary studies amassed a database with a sum total that overwhelmingly reveals the Chicxulub impact as predating the KTB mass extinction in the impact-crater cores, in sections throughout northeastern Mexico and in Brazos River sections of Texas, U.S.A. This paper recounts the highlights of
the KTB controversy, the discovery of facts inconsistent with the impact hypothesis, and the resurgence of the Deccan volcanism hypothesis as the most likely cause for the mass extinction.

Arambourg was the first to conduct methodical vertebrate palaeontological studies in the Oulad Abdoun and Ganntour phosphatic basins of Morocco between the 1930s and 1950s. As early as 1935, he identified the main stratigraphical levels of the phosphatic series, characterizing them by a specific association of vertebrates (mainly selachians), and proposed stratigraphical correlations between the phosphatic levels of these two basins. During the last decade, due to a French-Moroccan program of collaboration, vertebrate fossils have been collected in great abundance. Here we present an updated overview of the latest Cretaceous to Ypresian reptilian faunas from the Oulad Abdoun and Ganntour basins, on the basis of published data and new field records. In addition to advances in the study of the already known taxa (i.e., squamates, crocodyliforms, plesiosaurs), recent field works reveal new major reptilian taxa that were unknown (or undescribed) at Arambourg’s time: very abundant and diversified marine chelonians (Maastrichtian to Ypresian), scarce dinosaurs and pterosaurs remains (Maastrichtian), and a well diversified marine avifauna (Thanetian and Ypresian). A significant increase in the number of described taxa (52 versus 13) and an improvement of the quality of the specimens found (articulated skeletons versus isolated remains) is worthy of consideration. The Maastrichtian reptilian assemblages are dominated by mosasaurid squamates whereas those of the Palaeogene are by the mirroring crocodyliforms (dyrosaurids and eusuchians).

–The suevite breccia of the Chicxulub impact crater, Yucatàn, Mexico, is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters. Detailed studies (microscope, electron microprobe, SEM, XRF) have been carried out on a noncontinuous set of samples from the drill hole Yucatàn 6 (Y6) located 50 km SW from the center of the impact structure. Three subunits can be distinguished in the suevite: the upper unit is a fine-grained carbonate-rich suevite breccia with few shocked basement clasts, mostly altered melt fragments, and formerly melted carbonate material; the middle suevite is a coarse-grained suevite with shocked basement clasts and altered silicate melt fragments; the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts. The matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene. The composition of the upper members of the suevite is dominated by the sedimentary cover of the Yucatàn target rock. With depth in well Y6, the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly. Even at the thin section scale, melt phases of different chemistry can be identified, showing that no widespread homogenization of the melt took place. The melt compositions also reflect the heterogeneity of the deep Yucatàn basement. Calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt. The proportion of carbonate to evaporite clasts is less than 5:1, except in the lower suevite where large evaporite clasts are present. This proportion constrains the amount of CO 2 and SO X released by the impact event.

A 40 m stratigraphic section at Gorgonilla Island, Colombia, provides a unique deep-marine, low-latitude, Southern Hemisphere record of events related to the end-Cretaceous Chicxulub impact and the global Cretaceous/Paleogene boundary (KPB). The KPB is marked by a 20-mm-thick, densely packed spherule bed as defined by planktic foraminifera, in contrast to complex relationships found in high-energy, impact-proximal sites in the Gulf of Mexico and Caribbean basins. The absence of basal Danian foraminiferal Zone P0 may indicate a possible hiatus of <10 ka immediately above the spherule bed, but is most probably an artifact of deposition below the calcite compensation depth as suggested by the nearly complete absence of calcareous fossils for 20 m below the Zone Pα. A weighted mean  40 Ar/ 39 Ar age of 66.051 ± 0.031 Ma for 25 fresh glassy spherules unequivocally establishes both their derivation from Chicxulub, and the association between the impact and the KPB. The spherule bed, and Maastrichtian strata below it, display soft-sediment deformation features consistent with strong seismic motion, suggesting that seismic activity in the immediate aftermath of the Chicxulub impact continued for weeks. We discovered a fern-spike immediately above the spherule bed, representing the first record of this pioneer vegetation from the South American continent, and from a low-latitude (tropical) environment.

Twelve fossil species of Proteaceous pollen, predominantly attributable to Proteacidites and Beaupreaidites, were recovered from the MaastrichtianePaleocene sedimentary succession of the Garden Cove Formation on Campbell Island, the southernmost landmass of the Zealandia continent. Among
these are two new species, Proteacidites campbellensis and Proteacidites hortisinus. The high diversity of Proteaceae pollen in the sediments encompassing the CretaceousePaleogene boundary on Campbell Island is consistent with the fossil record from neighbouring landmasses but strongly contrasts with the impoverished record of the family in the extant New Zealand flora. Examples of Beauprea- and Knightialike pollen in the Campbell Island assemblages confirm the presence of these lineages on Zealandia by the end of the Cretaceous and suggest that their present endemism in New Caledonia and New Zealand
can be explained in terms of relictual vicariant distributions, perhaps modified by northward tracking of warmer climates on Zealandia through the Cenozoic.

Abstract Terrestrial pollen and spores in late Maastrichtian to early Paleocene marine strata at mid-Waipara, New Zealand, permit reconstruction of contemporary vegetation and paleoclimates. During the latest Cretaceous, spore-pollen assemblages reflect a temperate rainforest with a prominent
podocarp and tree ferns component, angiosperm pollen being mainly represented by Nothofagus and Proteaceae. Disruption of the vegetation at the Cretaceous/Tertiary (K/T) boundary is recorded by an increase in fern spores, reduction in gymnosperm pollen, and temporary loss of angiosperm pollen both in mid Waipara and in the terrestrial sections of Moody Creek Mine and Compressor Creek. Following an interval of fern dominance, gymnosperms and later angiosperms return to the palynological record. The floral turnover at the K/T boundary is comparable to palynological records from North America and Japan, indicating that disruption of vegetation was global. Fern dominance is estimated to have lasted several thousands of years, based on foraminiferal biostratigraphy of immediate post-K/T boundary strata. This is orders of magnitude greater than seen in normal seral successions following deforestation. We suggest that the observed vegetation succession is due to a prolonged period of low ambient light levels, sufficient for photosynthesis but favouring plants already adapted to these levels (such as forest ground stratum), accompanied by a moderate temperature and moisture regime.

Bolide impact and flood volcanism compete as leading candidates for the cause of terminal-Cretaceous mass extinctions. High-precision (40)Ar/(39)Ar data indicate that these two mechanisms may be genetically related, and neither can be considered in isolation. The existing Deccan Traps magmatic system underwent a state shift approximately coincident with the Chicxulub impact and the terminal-Cretaceous mass extinctions, after which ~70% of the Traps&#39; total volume was extruded in more massive and more episodic eruptions. Initiation of this new regime occurred within ~50,000 years of the impact, which is consistent with transient effects of impact-induced seismic energy. Postextinction recovery of marine ecosystems was probably suppressed until after the accelerated volcanism waned.

The Cretaceous–Tertiary boundary (KTB) is one of the easiest epoch boundaries to identify, whether based on lithological changes in the field, geochemical analysis in the laboratory, or fossil content. A set of five KTB-identifying criteria, originally proposed by the ICS working group during the late 1980s–1990s, have proven globally applicable and independently verifiable: (1) mass extinction of Cretaceous planktic foraminifera, (2) evolution of the first Danian species, (3) KTB clay and red layer, (4) Ir anomaly, and (5) d13C shift. Despite this successful track record, it was recently proposed to reduce the five KTB-identifying criteria to just two, the mass extinction and impact signals, based on the assumption that the Chicxulub
impact caused the mass extinction and therefore defines the KTB. Because this assumption is contradicted by stratigraphic data in many places, this has led to contentious arguments, whereas defining the Chicxulub impact as KTB in age has led to circular reasoning. This study demonstrates the contradictions, pitfalls, and erroneous assumptions that accompany the use of these reduced impact-event-based KTB criteria. Returning the definition of the KTB to its GSSP based on all five criteria, and where this is not possible based on the mass extinction, the first appearance of Danian species, and the d13C shift provide the most reliable KT boundary markers.

The devastating effect on terrestrial plant communities of a bolide impact at the Cretaceous-Tertiary boundary is shown in fossil pollen and spore assem-blages by a diverse flora being abruptly replaced by one dominated by a few species of fern. Well documented in North America, this fern spike signals
widespread deforestation due to an impact winter or massive wildfires. A Southern Hemisphere record of a fern spike, together with a large iridium anomaly, indicates that the devastation was truly global. Recovery of New Zealand plant communities followed a pattern consistent with major climatic perturbations occurring after an impact winter that was possibly preceded by
global wildfires.

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A new heterosporous fern species, Azolla boliviensis sp. nov., is described from latest Maastrichtian (latest Cretaceous) to Paleocene (earliest Palaeogene) terrestrial sediments
of the Eslabón and Flora Formations, Subandean belt, Bolivia. The species is represented by dissociated but abundantly co-preserved megasporocarps, megaspores, microsporangia, massulae and microspores. The genus consistently characterizes warm-climate lacustrine settings. Fossil Azolla is first identified around the Early to mid-Cretaceous but the genus
apparently underwent dramatic radiation during the Late Cretaceous. Abundant Azolla remains in Bolivia add to this portrait of rapid geographic dispersal and diversification
near the close of the Cretaceous. The ranges of many Azolla species span the Cretaceous- Palaeogene boundary and the potential of Azolla to withstand altered environmental conditions, such as periodic frost damage, drought, and salinity change, and its ability to undergo rapid vegetative regeneration in association with nitrogen-fixing cyanobacterial symbionts, suggest that the survival of this group was favoured during the adverse conditions of the end-Cretaceous event.

A gavialoid crocodylian from the Maastrichtian of the Oulad Abdoun phosphatic Basin (Morocco) is described, representing the oldest known crocodylian from Africa. The specimen consists of a skull that exhibits several features not found in other gavialoids, and a new genus and species is erected, Ocepesuchus eoafricanus. A phylogenetic analysis has been conducted including 201 characters and 71 taxa, where Ocepesuchus eoafricanus appears as the most basal African gavialoid, and the South American gavialoids are paraphyletic. This paraphyly has strong biogeographic implications, and the previous hypothesis of South American and Asian assemblages derived from African gavialoids should be reviewed. The historical biogeography of Gavialoidea is probably more complex than previously supposed. The phosphatic deposits of Morocco provide a unique opportunity to study the vertebrate faunal turnover across the Cretaceous-Tertiary (KT) boundary. The crocodyliforms are very scarce in the Maastr...

Thanks to an intensive field work undertaken in spring 2000 with the collaboration between various French and Moroccan organisations, numerous Crocodyliformes remains has been recovered from the phosphatic marine layers from the Cretaceous and Paleogene of the Oulad Abdoun Basin, Morocco.
Two taxa have been found in these deposits, eusuchians and dyrosaurids, represented by four and eight species respectively.
Among eusuchians, the occurence of three gavialoids can be noted. One is a thoracosaur, which are reported for the first time in Africa. A phylogenetic analysis reveals that Thoracosaurus is a monophyletic taxa, and Maroccosuchus zennaroi, a previously described eusuchian from the phosphates of Morocco but completed with new material, is the most primitive tomistomine.
Until know, the dyrosaurids forms a poorly known taxa. A taxonomic revision shows that “Hyposaurus” bequaerti should be transferred to Congosaurus, that Hyposaurus wilsoni, H. nopcsai and Rhabdognathus rarus are nomina dubia, and that two skulls from Rhabdognathus are two new species. Three new dyrosaurids from Morocco are described, and a phylogenetic analysis of Dyrosauridae within the Crocodyliformes setting shows that convergences due to the longirostrine morphology remain problematic in phylogenetic analyses.
The diversity observed in the Oulad Abdoun Basin indicates a strong diversification during the Paleocene, whereas the Crocodyliformes were extremely rare during the Maastrichtian. Then, they have apparently not been affected by the K-T crisis, but probably have taken advantage of the other marine reptile disappearance, to colonise the main part of their ecological niches as soon as the Paleocene.