Cretaceous-Tertiary Boundary Layer Research Articles

SPHERULE LAYERS—RECORDS OF ANCIENT IMPACTS

▪ Abstract A large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand-size particles that get deposited hundreds to thousands of kilometers from the point of impact. These particles, known as impact spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years to 3.47 billion years. Unaltered impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Impact spherule layers form very rapidly and can be very extensive, even global in extent [e.g., the Cretaceous-Tertiary (K/T) boundary layer], so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context, spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A record of impacts whose craters have since been obliterated, most notably those in pre-Mesozoic oceanic crust, could survive in the form of spherule layers. Secular changes in surface environments and/or the nature of the impactors striking Earth through its history could also be reflected in differences in spherules and spherule layers as a function of geologic age. In this paper, we briefly review what spherules and spherule layers are and the processes that create them, then speculate about what might be learned through wider identification of and more extensive study of impact spherule layers.

Thermal decomposition pattern and particle size estimation of iron minerals associated with the Cretaceous‐Tertiary boundary at Gubbio

Abstract— Mössbauer studies of the samples from the Cretaceous‐Tertiary (K‐T) boundary layer at Gubbio, Italy show that iron appears mainly in two phases, magnetically ordered hematite and a paramagnetic silicate phase. The average particle size of hematite is estimated to be in the range of 16 to 27 nm from transmission electron micrographs and lack of a Morin transition. The hyperfine magnetic field at the iron nucleus is observed to be somewhat less than that of bulk hematite, which may be explained by collective magnetic excitation. Stepwise heating up to 1000°C shows a decomposition pattern of the paramagnetic phase, which suggests it to be a tri‐octahedral layer silicate. The iron‐bearing phases found in the bulk sedimentary K‐T boundary material are different from those found in the spherules separated from this material indicating that the redox conditions changed rapidly after the impact, becoming more oxidizing during the period these bulk phases were formed.

New evidence for a large Palaeoproterozoic impact: spherules in a dolomite layer in the Ketilidian orogen, South Greenland

An unconformable sedimentary succession deposited between c. 2130–1848 Ma on Archaean gneisses of the foreland of the Palaeoproterozoic Ketilidian orogen includes a layer with coarse sand-sized silicate spherules. The layer is c. 1 m thick and consists mainly of coarse diagenetic dolomite. In addition to c. 18% spherules, the layer also contains 3% well-sorted, very fine quartzose sand and 6% larger intraclasts of chert and carbonate. The spherules were previously interpreted as microfossils ( Vallenia sp.) because of their spheroidal shapes and inclusions of carbonaceous matter. The spherules are reinterpreted as replaced impact ejecta because they have shapes typical of splash-form microtektites, some contain possible examples of replaced skeletal spinel crystals, perlitic cracks and devitrification spherulites, and non-spherical particles with shapes and textures of typical glassy and scoriaceous volcanic ash are absent. The carbonaceous matter is attributed to hydrocarbons that migrated into the spherule layer from elsewhere in the sedimentary succession. The spherules were reworked after deposition, probably as a result of turbidity currents or storm- or impact-induced waves. Analysis of one spherule-bearing sample revealed only 0.02 ppb iridium, a value comparable with low iridium abundances in distal layers of other terrestrial impact ejecta. The spherules in South Greenland are the first distal impact ejecta recognized in mid-Precambrian strata. They represent a major impact because their aggregate thickness exceeds the thickest spherule accumulations reported from the Cretaceous-Tertiary boundary layer. Given their loosely constrained age and the implied scale of the impact, the Ketilidian spherules could be distal ejecta from either the Vredefort, South Africa, ( c. 2025 Ma) or Sudbury, Canada, ( c. 1850 Ma) impacts.

Quench textures in altered spherules from the Cretaceous-Tertiary boundary layer at Agost and Caravaca, SE Spain

Abundant spherules are observed in a well preserved Cretaceous-Tertiary boundary (KTB) layer at the Agost and Caravaca sections (Betic Cordilleras, SE Spain). These spherules have been diagenetically altered to goethite or K-feldspar. Different morphologies are observed ranging from perfect spheres to droplet shapes. This is the first time that dendritic and fibroradial textures are reported in goethite spherules with clearly splash-form morphology. This morphology suggests that they originated from melt in the hot vapour plume and not as condensate. As unaltered KTB spherules are composed of clinopyroxene, this could be the probable precursor of part of the altered spherules at Agost and Caravaca, which may have been derived from an expanding vapour plume with a significant contribution of extraterrestrial material. In addition C-rich cores have been detected in K-feldspar which are enriched in Ir, Pt, Pd and Ni. Although postdepositional processes and microbial activity could have led to severe modification of the original element concentrations, the high content of noble metals and Ni suggests extraterrestrial contamination.

Chemical characteristics of the Cretaceous-Tertiary boundary layer at Gubbio, Italy

Platinum metals (Pd, Ir, Pt) and Au were determined by radiochemical neutron activation analysis (RNAA) on Cretaceous-Tertiary (KT) boundary samples collected at Gubbio, Italy. In addition, about thirty elements were determined by instrumental NAA (INAA) for both the Cretaceous and the Tertiary layer samples, as well as the boundary clay samples. Iridium and other Pt metals were observed to be similarly enriched, relative to CI-chondrite, in the boundary layers, i.e., they occur in chondritic ratios. On the other hand, correlations among the metals are not very good, suggesting that Pt metals were not incorporated into clay materials in a single phase, but rather, they behaved separately according to their own solution chemistries. In the Gubbio section, the chemical composition does not change so much across the KT boundary layer, except for Ir (and possibly other Pt group elements) and is very similar to that of the “North American shale composite (NASC)”. Arsenic and Sb were observed not to be anomalously enriched in the Gubbio KT boundary layers. This implies that their abundances cannot be a clue in elucidating an event which must have happened at the KT boundary period. NASC-normalized REE abundance patterns for the boundary samples show characteristic features due to some REE precipitation out of seawater. REE abundances are not variable among the Cretaceous, the Tertiary, and the boundary samples, suggesting that conditions for precipitation and sedimentation were similar across the KT boundary at Gubbio.

Vertical Variation of Rhenium, Cadmium, Silver and Platinum-group Elements within a Transitional Bed of Danish Cretaceous-Tertiary Boundary Layer.

Contents of Re, Cd, Ag and platinum-group elements in specimens from four sub-zones of a 3cm-thick bed of a K-T boundary layer from Stevns Klint in Denmark, were determined by stable isotope dilution using ICP mass spectrometer. The chondrite-normalized Re value linearly decreases in the upper zone. Also the content of Ag linearly diminishes upward. These relations lead us to a conclusion that, at the earliest stage of the formation of this bed, the marine site studied was hi ghlyreducing, and at the final stage of the bed formation, the sea recovered to the “normal” oxidation-reduction potential. That is, it is reasoned that the anomalously reducing condition at the site persisted for a relatively short period. Some discussions are given to effect of chemical valency state on distributional features and to fractionation in the dust fallout.

Molecular activation analysis for iridium

Molecular activation analysis (MAA) of iridium in geological samples, based on a combination of a newly-developed chemical sequential dissolution method and radiochemical and instrumental NAA, was described to study the distribution pattern and chemical species of iridium in various fractions (soluble carbonate, metal, sulfide, oxide, silicate and acid-insoluble residue) of geological boundary samples, meteorites, ultrabasic rock and volcanic lava. The correlations of Ir with Au, Os, siderophile, chalcophile and lithophile elements were discussed. In addition, the role of kerogen and noble-nugget in the Ir enrichment was scrutinized. The MAA results of Ir favors a mixed effect of asteroid impact, volcanic eruption and post-depositional redistribution to interpret the extant Ir anomaly at Cretaceous-Tertiary boundary layer.

Chicxulub crater source for shocked zircon crystals from the Cretaceous-Tertiary boundary layer, Saskatchewan: Evidence from new U-Pb data

New U-Pb data for five of six individual shocked zircon grains from the Cretaceous-Tertiary (K-T) boundary fireball layer, Rock Creek site, south-central Saskatchewan, yield a source age of 548 ± 6 Ma. The collinear data points are variably discordant in proportion to the intensity of shock metamorphism exhibited by each zircon. The age and zircon shock features establish a strong temporal and genetic link with K-T distal ejecta from the fireball layer at the Berwind canyon site in the Raton basin, Colorado, where texturally similar zircons from a previous study gave an indistinguishable source age of 544 ± 5 Ma. These ages are in agreement with independent ages obtained from a previous study for single shocked zircons from the Chicxulub crater, Mexico, and from Beloc, Haiti. The U-Pb zircon data unequivocally support a meteorite impact origin for the global K-T boundary layer and imply that the Chicxulub crater is the sole North American target source.

Fullerenes in the Cretaceous-Tertiary Boundary Layer

High-pressure liquid chromatography with ultraviolet-visible spectral analysis of toluene extracts of samples from two Cretaceous-Tertiary (K-T) boundary sites in New Zealand has revealed the presence of C(60) at concentrations of 0.1 to 0.2 parts per million of the associated soot. This technique verified also that fullerenes are produced in similar amounts in the soots of common flames under ambient atmospheric conditions. Therefore, the C(60) in the K-T boundary layer may have originated in the extensive wildfires that were associated with the cataclysmic impact event that terminated the Mezozoic era about 65 million years ago.

Platinum-group elements in the cores of potassium feldspar spherules from the Cretaceous-Tertiary boundary at Caravaca (Spain)

The abundant spherules present in the Cretaceous-Tertiary boundary layer at Caravaca are diagenetically transformed to potassium feldspar. Before our study no possible relicts of the precursor material had been reported. but in this paper we describe the presence of cores in these spherules that could represent a relict of the «unknown precursor». These cores are made up of C mixed with Si. Mg. AL Cr. Ca among other elements. Laser Ablation System analysis also reveals an enrichment in pe;E could suggest an extraterrestrial origin for this material. PI. Pd and Ir do not show a chondritic ratio: however. asevere modification of their concentration could be expected during the early diagenetic processes.

Formation mechanisms of planar deformation features in naturally shocked quartz

Shock waves induce peculiar defects in quartz: shock mosaics, high-pressure polymorphs (coesite and stishovite), diaplectic glass and ‘planar deformation features’ (PDFs). Together, these features are the indices of ‘shock metamorphism’, PDFs appear under the optical microscope as straight and narrow defects parallel to the { 10 1n } rhombohedral planes with n = 3 and 2 as most frequent values, although n = 1, 4 and ∞ (i.e. basal plane) are also found. We report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of PDFs in shocked quartz grains from a variety of sites (Slate Islands, La Malbaie and Manson in North America; the Vredefort complex in South Africa; the Ries Crater in Germany and the Cretaceous-Tertiary boundary layer at Raton Basin, Colorado). We distinguish four PDF fine structures: (1) bands of dislocations; (2) lamellae of mixtures in various proportions of amorphous silica and small crystallites; (3) thin Brazil twin lamellae; (4) short, parallel lamellae forming serrated ladder structures. The possible formation mechanisms of these PDFs are discussed in the second part of the paper. It is suggested that the dislocation bands are probably not original PDF structures, but stem from a later recrystallization stage. The Brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave. The other PDF structures, which consist of partial amorphization in the { 10 1n } planes, must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes. Theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 GPa. The growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave.

Impacts, Tsunamis, and the Haitian Cretaceous-Tertiary Boundary Layer

The marker bed at the Cretaceous-Tertiary boundary of the Beloc Formation (southern Haiti) contains abundant coarse-grained microtektites and minor amounts of shocked quartz grains in the basal part. The upper part is composed of medium-grained marl with amalgamated microtektite lenses and finer-grained marl lenses disseminated throughout. Field and petrographic observations, and the distribution of planktonic foraminifera suggest that the bed formed from a complex sequence of events. A bolide impact nearby produced microtektites that sett1led to form a nearly pure layer at the base. Vaporized materials with anomalously high extraterrestrial components settled last, along with carbonate sediments. The entire bed became sparsely consolidated. Subsequently, another major disruptive event, perhaps a giant tsunami, partly reworked the initial deposit. Cohesive fragments of the original marker bed mixed with exotic materials were redeposited as lenticular bodies. This process also may have caused further mixing of Cretaceous and Tertiary microfossils, as observed at Beloc and elsewhere.

Tektites and microtektites: key facts and inferences

Tektites are generally small, black, rounded, silicate glass bodies found scattered over several widely separated areas of the Earth's surface called strewn fields. Four major tektite strewn fields are known: (1) Australasian, (2) Ivory Coast, (3) Czechoslovakian and (4) North American. Tektites from these four strewn fields have been dated at 0.7, 1, 15, and 34 Ma respectively. Most tektites are splash forms (i.e., spheres, teardrops, dumbbells, etc.), but some have undergone a second period of melting to produce ablated forms. Tektites with a blocky, layered appearance are called Muong Nong-types. Microtektites (microscopic tektites) belonging to the Australasian, Ivory Coast and North American strewn fields have been found in deep-sea sediments. The geographic occurrence and abundance of microtektites at each site have been used to determine the size and mass of each strewn field. The Australasian, Ivory Coast and North American strewn fields contain 100 million, 20 million and 1 billion tonnes of glass respectively. Although tektites resemble obsidian, they can be distinguished from terrestrial volcanic glasses by their composition and petrography. Tektites, for example, are drier than volcanic glasses and have higher FeO/Fe 2O 3 ratios. In addition, tektites, unlike volcanic glasses, contain lechatelierite particles (SiO 2 glass) and are devoid of primary crystallites. Although some authors favor a lunar volcanic origin, most of the compositional and petrographic data indicate a terrestrial impact origin. The major, trace, and isotopic compositions of tektites indicate that the parent material was a terrestrial sedimentary deposit, and the presence of lechatelierite, coesite and shocked relict inclusions favors an impact origin. Furthermore, two tektite strewn fields have been associated with known impact craters, and tektites from one strewn field have been found associated with impact ejecta. Unlike the Cretaceous-Tertiary boundary layer, microtektite layers are not associated with mass extinctions or iridium anomalies.

Stishovite at the Cretaceous-Tertiary Boundary, Raton, New Mexico

Stishovite, a dense phase of silica, has become widely accepted as an indicator of terrestrial impact events. Stishovite occurs at several impact structures but has not been found at volcanic sites. Solid-state silicon-29 magic-angle spinning nuclear magnetic resonance (silicon-29 MAS NMR) and X-ray diffraction of samples from the Cretaceous-Tertiary boundary layer at Raton, New Mexico, indicate that stishovite occurs in crystalline mineral grains. Stishovite was indicated by a single, sharp resonance with a chemical shift value of -191.3 ppm, characteristic of silicon in octahedral coordination, that disappeared after heating the sample at 850 degrees Celsius for 30 minutes. An X-ray diffraction pattern of HF residuals from the unheated sample displayed more than 120 peaks, most of which correspond to quartz, zircon, rutile, and anatase. Eight unambiguous weak to moderate reflections could be ascribed to d-spacings characteristic of stishovite.

Impact on the earth, ocean and atmosphere

On the basis of finite-difference techniques, cratering flow calculations are used to obtain the spatial attenuation of shock pressure with radius along the impact axis for the impact of silicate rock and iron impactors on a silicate half-space at speeds of 5 to 45 km/sec. Upon impact of a 10 to 30 km diameter silicate or water object onto a 5 km deep ocean overlying a silicate half-space planet at 30 km/sec, it is found that from 12 to 15 percent of the incident energy is coupled into the water. The mass of atmosphere lost due to impacts of 1 to 5 km radius projectiles is calculated.

Iridium in sea-water

Iridium in sea-water has been measured (after isolation from the saline matrix by reduction with magnesium) by neutron bombardment, radiochemical purification and high-resolution γ-ray spectroscopy. The concentration obtained in a Pacific coastal water was 1.02 ± 0.26 × 10 −14 g per g of sea-water. At such extremely low concentrations, seawater is an extremely unlikely source for anomalously high iridium concentrations measured in the Cretaceous-Tertiary boundary layer of deep-sea sediments.

Clay Mineralogy of the Cretaceous-Tertiary Boundary Clay

Analyses of the clay mineralogy of samples from the Cretaceous-Tertiary boundary layer at four localities show that the boundary clay is neither mineralogically exotic nor distinct from locally derived clays above and below the boundary. The significant ejecta component in the clay that is predicted by the asteroid-impact scenario was not detected.