Iridium Anomaly Research Articles

Stratigraphie séquentielle comparée et ≪ gradedatation ≫ par les foraminifères planctoniques du Campano-Maastrichtien et du Paléocène de quelques sites d'Europe sud-occidentale et d'Afrique du Nord

From the analysis of the open marine beds enclosing the K/T boundary along several cross-sections of SW Europe and North Africa, a subdivision is proposed into nine Campanian-Maastrichtian depositional sequences followed by five Palaeocene sequences. These sequences have been correlated from one section to another by means of planktonic Foraminifera classic biozones and supported in the Cretaceous by numeric ages coming from the grade-dating method applied to Globotruncanids. In the latter case, sequences belong to two different types separated by the presence or the absence of lowstand chaotic deposits such as breccias and olistostromes. The dark clays of the K/T boundary containing the Iridium anomaly can be regarded as a 3 d order sequence boundary. They also correspond to the base of a 2 d order cycle which is locally (Béam) underlined by a gap of the Early Palaeocene, the underlying Maastrichtian marls, everywhere grade-dated around 65.2–65.3 Ma, never being significantly eroded.

層状チャート相Ｐ／Ｔ境界付近でのイリジウム異常濃集の未検出について

層状チャート相Ｐ／Ｔ境界付近でのイリジウム異常濃集の未検出について

New Evidence of Extraterrestrial Cause for the Cretaceous-Tertiary Extinction——The Abundance of Ruthenium and Its CorrelationWith Iridium

1 Introduction In 1980, L. W. Alvarez suggested a hypothesis that the falling of a largeextraterrestrial body upon the earth caused the extinction event at the end of theCretaceous, according to the extraordinarily high concentration of Ir in K/T boundarydeposits. Since then, studies have indicated that the iridium anomaly distributeswidely in the sections of this age over the world. However, these studies focused onthe abundance of Ir. Little attention was paid to other platinum group metals (PGM).Analyzing the abundance of Ru in boundary samples is very difficult because of its

Iridium and dinocysts at the Cretaceous-Tertiary boundary on Seymour Island, Antarctica: Implications for the K-T event

A single iridium anomaly (40 times background concentration), correlated with the Cretaceous-Tertiary (K-T) boundary, is present within an interval of unconsolidated glauconitic silty sands in a marine clastic sequence on Seymour Island, Antarctic Peninsula. Dinocysts provide the basis for biostratigraphic identification of a 20-30 cm K-T transitional interval; calcareous microfossils have not been preserved in the glauconitic beds. Dinocysts also record an earliest Danian transgressive event 10 cm above the transition interval. The Ir spike is near the base of the dinocyst transition interval and 30 cm below the flooding horizon. The Seymour Island site, located at a high latitude (63°S) at the end of the Cretaceous, provides no compelling evidence for mass extinction at the K-T boundary.

Asteroids and early precambrian crustal evolution

Major enigmas arising from the Archaean crustal record are capable of resolution in terms of the combined role of episodic mega-impacts and of long-term mantle processes. The terrestrial impact record suggests that, for very large impact structures, the degree of preservation of proximal deformation effects decreases with the increase in the magnitude of the associated thermal effects—a consequence of extensive recrystallization and melting. This focuses attention on thermal igneous and metamorphic peaks as possible clues of impact events. Isotopic age studies define principal thermal events about 3.5, 3.0 and 2.7 (±0.1) Gyr and less well defined episodes, succeeded by protracted thermal fluctuations in infracrustal high-grade metamorphic zones. The bulk of the igneous activity associated with these episodes took place over relatively short time spans of c.20–30 × 10 6 years. Interpretations of these episodes in terms of internal dynamics of the Earth are inconsistent with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by the mantle rebound response to intermittent extra-terrestrial asteroid impacts is considered on the basis of (1) identification of major impacts from distal ejecta horizons marked by iridium anomalies and impact/earthquake-triggered diamictite breccia and olistostromes; (2) the relationships between microtektite horizons and volcanic activity; (3) geochemical and experimental evidence for mantle upwelling from the transition zone and catastrophic adiabatic melting required to generate peridotitic komatiites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem which arises from comparisons between modern continental crust and sial produced by continuous two-stage mantle melting processes, taking the temporal decline in heat flow into account. Impact shock effects on the Archaean crust have been largely obscured by (1) outpouring of voluminous basic/ultrabasic lavas, inundating shock-deformed crust and extending beyond the perimeters of impact excavated basins: (2) gravity subsidence and downfaulting of terrestrial mare, accounting for the anatexis of sub-greenstones basement; (3) extensive shearing and recrystallization of impact structures, breccias and mineral deformation features beneath impact-excavated basins, relics of which may be retained in structural windows in high grade metamorphic terrains. Isostatic subsidence and anatexis of thick mare-type piles and underlying impacted crust resulted in intracrustal comagmatic plutonic and volcanic suites within periods in the order of15–30 × 10 6 years, as controlled by post-impact mantle convection cooling. Subsequent isostatic updoming of remobilized basement gneiss terrains and the rise of their anatectic products along major normal shears, bearing an analogy to metamorphic core complexes, resulted in the multiple gneiss-granite complexes amidst greenstone-dominated terrains. Recurrent post-tectonic thermal fluctuations and magmatic activity reflect a long-term existence of anomalous mantle regions beneath original mega-impact sites and thermal perturbations related to distal impacts. A broad age zonation in some Archaean shields suggests lateral migration and terrane accretion in a convection driven plate tectonic regime.

Iridium Anomalies of Late Eocene Age at Massignano (Italy), and ODP Site 689B (Maud Rise, Antarctic)

Iridium anomalies of Late Eocene age are here reported in sequences from northern Italy and Weddell Sea, Antarctic. We have analyzed 270 samples of marly limestones from the Eocene/Oligocene boundary stratotype section at Massignano, Italy, ranging from 35.6 to 34.5 Ma (NP 18 Zone to NP 19/20 Zone). We discovered a distinct Ir peak of 156 ± 19 parts per trillion (ppt) in the lower calcareous nannofossil NP 19/20 Zone, corresponding to the mid-lower part of foraminiferal P16 Zone, and the top of magnetic Chron 16n2. The age of this Ir anomaly derived from several dated volcanic ashes found in the same section is 35.7 ± 0.4 Ma

Iridium anomaly associated with the Australasian tektite-producing impact: Masses of the impactor and of the Australasian tektites

Recently published studies of Ocean Drilling Project (ODP) cores from near southeast Asia revealed microtektite contents much higher than those in previously studied cores, suggesting that Ir contents might be enhanced in the tektite-bearing horizons. We determined a positive Ir anomaly in ODP core 758B from the Ninetyeast Ridge, eastern Indian Ocean; the peak Ir concentration of 190 pg/ g was ~2× the continuum level. The net Ir fluence is 1.8 ± 0.5 ng cm −2 over the depth interval from 10.87 to 11.32 m; a small additional peak also associated with microtektites contributes another 0.5 ng Ir cm −2. Concentrations of Ir in core 769A show more scatter, but a small Ir enhancement is associated with the peak microtektite abundance; our best estimate of the poorly constrained fluence is 1.3 ± 0.7 ng cm −2. Data on deep-sea cores show that the microtektite fluence falls exponentially away from southeast Asia, the fluence dropping a factor of 2 in ~400 km. In southeast Asia the trend merges with a roughly estimated mass fluence of ~ 1.1 g cm −2 inferred from evidence of a melt sheet in northeast Thailand. Integration of the inferred distribution yields a total mass of Australasian tektites of 3.2 · 10 16 g, much higher than previous estimates. Assuming a similar fallout distribution for the impactor and a chondritic composition allows us to calculate its mass to be 1.5 · 10 15 g, about 3 orders of magnitude smaller than the minimum mass of the impactor responsible for the extinctions at the end of the Cretaceous.

Upper Devonian iridium anomalies, conodont zonation and the Frasnian-Famennian boundary in the Canning Basin, Western Australia

Two iridium anomalies have been identified near the Frasnian-Famennian boundary (Upper Devonian) in the Canning Basin of Western Australia. Both anomalies are associated with the cyanobacterium Frutexites and are located in marginal slope facies. The first was identified in the Virgin Hills Formation on the west flank of McWhae Ridge near the southeastern end of the Devonian outcrop belt. This anomaly, initially identified as being from the Famennian Upper Palmatolepis triangularis Zone, is now known to be from the Early Palmatolepis crepida Zone, on the basis of the presence of Palmatolepsis crepida in the bed. The second anomaly, found in drill core from the Napier Formation just south of the Napier Range, is older than the first and is from the Frasnian Montagne Noire Conodont Zone 12 or 13 of Klapper (1989) = the Palmatolepis rhenana Zone of Ziegler and Sandberg (1990). These two iridium anomalies are thus significantly below and above the Frasnian-Famennian boundary and are not associated with the extinction event in the Palmatolepis linguiformis Zone. In the Canning Basin the association of two iridium anomalies with beds containing abundant Frutexites microstromatolites indicates that the concentrations of iridium are probably associated with a process of organic concentration. There is no evidence that either example is directly associated with an impact event.

Iridium and other platinum-group elements as geochemical markers in sedimentary environments

Iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions. However, platinum-group element (PGE) anomalies may result from several different processes, both syn- and postdepositional, acting separately or jointly. The sources and processes of enrichment involved are: extraterrestrial (asteroid, comet, cosmic dust, impact ejecta); volcanogenic (PGE-rich condensates); precipitation from seawater (low sedimentation rate, anoxic conditions); microbial (concentration, dissolution, reprecipitation); exhalative-hydrothermal; leaching, transport and precipitation at a redox boundary. Geochemical characteristics of PGE and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes: (1) extraterrestrial: chondritic Ru/Ir and Os/Ir ratios, low 187Os/ 186OS ratio, high Ir compared to Pt, Pd and Au; (2) volcanogenic: high iridium and Se, As, Sb and In contents; (3) exhalative-hydrothermal: high Au, Pd and Pt (relatively low Ir) contents; (4) precipitation from seawater: high Pt and low Pd contents, also high Cd and Ag contents under anoxic conditions; and (5) reduction from intermediate and low-temperature solutions: high PGE and Cu (or Mo, U, Ni) contents.

The cretaceous/paleogene boundary and planktonic foraminifera in the flyschgosau (Eastern Alps, Austria)

The studied interval extends from 2.5 m below to 1 m above the Cretaceous/Paleogene (K/P) boundary and comprises the uppermost Maastrichtian marly limestone overlain by a boundary (rusty) layer—a dark yellow orange clay 5–10 mm thick, followed by a turbidite sequence of very fine sandstones and grey-brown marls. The following planktonic foraminiferal zones are distinguished: Abathompalus mayaroensis, PO (subzones: POa— Guembelitria cretacea and POb — Globoconusa conusa), Pa or Parvularugoglobigerina eugubina and P1 (subzone: P1a or Subbotina pseudobulloides). The distribution of iridium shows a relatively sharp rise to a maximum value of 7.176 ppb about 1.2 cm above the rusty layer, followed by a rapid drop to normal background levels. The Abathomphalus mayaroensis Zone exhibits a moderately diverse planktonic foraminiferal assemblage. The main extinction episode occurs within the rusty layer; only a few species survived. Survivors are small primitive forms. The first new Paleocene species evolved immediately after the major Cretaceous extinctions. Rapid extinction of planktonic foraminifers coincides with the iridium anomaly which suggests that, at this site, the source(s) of the iridium anomaly was (were) probably responsible for K/P extinctions.

Origin and Clay-Mineral Genesis of the Cretaceous/Tertiary Boundary Unit, Western Interior of North America

AbstractA 3-cm-thick, two-layered clay unit that records mineralogic and textural evidence of a catastrophic event that occurred at a time now marked as the end of the Cretaceous Period was preserved in ancient peat-forming environments of the Western Interior Basin of North America. The two layers of this unit consist of altered distal ejecta and are easily distinguished by their distinctive texture and impact components from other clay beds, mainly tonsteins and detrital shales, occurring within the sequence of rocks enclosing the Cretaceous/Tertiary (K/T) boundary interval.The lower claystone layer of the K/T boundary unit represents melted silicic target rock that has altered mainly to kaolin minerals. Impact components and signatures of this lower layer include a relict imbricate fabric of glass fragments, shards, bubbles, hollow spherules (altered microtektites), small amounts of shocked mineral grains, and a subdued iridium anomaly. These components and textures, combined with the layer's restricted areal distribution, indicate that this layer, called the “melt ejecta layer,” is the distal part of an ejecta blanket deposit. We interpret the melt ejecta layer to be an altered deposit of mostly impact-derived, shock-melted, silicic target material that traveled through the atmosphere within a detached ejecta curtain and on other ballistic trajectories.The upper laminated layer of the K/T boundary unit consists mostly of altered vitric dust and abundant shocked minerals whose size and amounts decrease away from the putative crater site in the Caribbean area. High-nickel magnesioferrite crystals, high iridium content, geochemical signature, and worldwide distribution all suggest this upper layer originated from a cloud of vaporized bolide and entrained target-rock materials ejected above the atmosphere. The components of this layer, called the “fireball layer,” settled slowly by gravitational processes from an Earth-girdling vapor cloud and were deposited immediately on top of the already-emplaced melt ejecta layer.The clay minerals that formed in the two layers are largely a function of composition and the highly unstable, shock-modified state of the fallout materials altered in acidic, organic-rich waters of ancient peat swamps. The fireball layer is mostly altered to smectitic clay from a mafic glass condensed from the vaporized chondritic bolide, along with some kaolinite formed from blebs of melted silicic target material entrained in the vapor plume cloud during ejection. In contrast, the melt ejecta layer is mainly kaolinitic, derived from silicic glass formed from melted target rocks. In this layer, the glass rapidly altered to mostly disordered, micrometer-sized “cabbage-like” or submicrometer-sized embryonic forms of spherical halloysite, probably from an allophane precursor. These crystallization characteristics of the melt ejecta layer are much different than those which formed coarse vermicular aggregates and platy kaolinite crystals in tonsteins from outside the K/T boundary interval throughout the Western Interior. The contrast in the incipient formation of dominantly kaolinitic clay minerals in the basal melt ejecta layer and of smectitic clay minerals in the overlying fireball layer reflect silicic versus mafic starting materials, respectively, and also supports the proposed two-phased meteorite impact ejection and dispersal model.During subsequent burial and diagenesis of the K/T boundary unit, the metastable halloysite and smectite aggraded to kaolinite and mixed-layer illite/smectite, respectively. Both the ordering of kaolinite and illitization of smectite varies locally as a function of the degree of diagenetic grade or maturity, probably in response to local variations in temperature due to maximum burial depth (burial diagenesis).

Leaf-fossil evidence for extensive floral extinction at the Cretaceous-Tertiary boundary, North Dakota, USA

Forty-seven collections of plant macrofossils (megaflora) from the upper 17 m of the Hell Creek Formation (late Maastrichtian) and the lower 80 m of the Ludlow Member of the Fort Union Formation (Paleocene; Puercan) in south-western North Dakota document floral change across the Cretaceous-Tertiary (K-T) boundary. These collections were made within a detailed stratigraphic framework based on 37 measured sections in eight areas along a 60 km transect and were supplemented with a microstratigraphic palynological analysis of the K-T boundary. The boundary is defined by the extinction of 30% of the late Maastrichtian Wodehouseia spinata Assemblage Zone palynomorphs and occurs within a few meters above or below the Hell Creek-Fort Union formational contact. The boundary is documented palynologically in seven sections and, in one section, coincides with an iridium anomaly and shocked mineral grains. The megaflora of the uppermost 17 m of the Hell Creek Formation (megafloral zone HC III) is a diverse (65 taxa) angiosperm-dominated assemblage that is taxonomically distinct from the megaflora of the lower Hell Creek. The HC III megaflora suffers 80% extinction across the K-T boundary and is replaced by the lower Fort Union megaflora (FU I) which, with 73 taxa, appears more diverse than HC III but has fewer taxa per locality. The Paleocene flora, with abundant aquatic angiosperms and taxodiaceous conifers, reflects the increased abundance of pond and swamp environments. This indicates that some of the floral change near the boundary was due to sedimentological facies change. The number, abruptness and regional extent of extinctions combined with the presence of iridium and shocked minerals at the extinction horizon support an extinction scenario compatible with an extraterrestrial impact. The mechanisms of that extinction, however, remain unclear.

Tektite-bearing, deep-water clastic unit at the Cretaceous-Tertiary boundary in northeastern Mexico.

The hypothesis of Cretaceous-Tertiary (K-T) boundary impact on Yucatán, Mexico, predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves. An outcrop along the Arroyo el Mimbral in northeastern Mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic-marl sequence deposited at more than 400 m water depth. The marls were found to be unsuitable for determining magnetostratigraphy, but foraminiferal biostratigraphy places the clastic unit precisely at the K-T boundary. We interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact. The clastic unit comprises three main subunits. (1) The basal "spherule bed" contains glass in the form of tektites and microtektites, glass spherules replaced by chlorite-smectite and calcite, and quartz grains showing probable shock features. This bed is interpreted as a channelized deposit of proximal ejecta. (2) A set of lenticular, massive, graded "laminated beds" contains intraclasts and abundant plant debris, and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water. (3) At the top, several thin "ripple beds" composed of fine sand are separated by clay drapes; they are interpreted as deposits of oscillating currents, perhaps a seiche. An iridium anomaly (921 +/- 23 pg/g) is observed at the top of the ripple beds. Our observations at the Mimbral locality support the hypothesis of a K-T impact on nearby Yucatán.

Stepwise mass extinctions: the case for the Late Cenomanian event

AbstractThe various extinctions of marine taxa in the Late Cenomanian can be shown as conforming to the ‘step‐wise extinction’ model. The order of such extinctions is, however, compatible with the presence of an expanded oxygen minimum zone in the world ocean. The occurrence of iridium anomalies at this level in the succession may, however, indicate a volcanogenic control on the global environment. Detailed time‐analysis of the Late Cenomanian event has been used to produce a correlation of some key successions.

Stromatolitic iron oxides: Evidence that sea-level changes can cause sedimentary iridium anomalies

Research Article| June 01, 1991 Stromatolitic iron oxides: Evidence that sea-level changes can cause sedimentary iridium anomalies Malcolm W. Wallace; Malcolm W. Wallace 1Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, South Australia 5001, Australia Search for other works by this author on: GSW Google Scholar Reid R. Keays; Reid R. Keays 2Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia Search for other works by this author on: GSW Google Scholar Victor A. Gostin Victor A. Gostin 1Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, South Australia 5001, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Malcolm W. Wallace 1Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, South Australia 5001, Australia Reid R. Keays 2Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia Victor A. Gostin 1Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, South Australia 5001, Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (6): 551–554. https://doi.org/10.1130/0091-7613(1991)019<0551:SIOETS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Malcolm W. Wallace, Reid R. Keays, Victor A. Gostin; Stromatolitic iron oxides: Evidence that sea-level changes can cause sedimentary iridium anomalies. Geology 1991;; 19 (6): 551–554. doi: https://doi.org/10.1130/0091-7613(1991)019<0551:SIOETS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In an attempt to understand the origin of an Ir-rich unit near the Late Devonian Frasnian-Famennian (F/F) boundary in the Canning basin of Western Australia, we have examined two lithologically similar Early Cambrian and late Oligocene age horizons from southeastern Australia. Both consist of stromatolitic iron oxide and carbonate petrographically similar to the Ir-rich Frutexites microstromatolites near the F/F boundary. Significant siderophile and chalcophile element anomalies (Ir, Pt, and Ru up to 1.1, 14, and 1.2 ppb, respectively) at both horizons have a geochemistry similar to that of the F/F Frutexites anomaly. As with the F/F bed, the Cambrian and Oligocene stromatolitic beds are closely associated with synsedimentary hardgrounds that contain evidence of subaerial exposure. We suggest that all of these Ir-rich stromatolitic beds developed in response to relative sea-level change and represent periods of condensed marine sedimentation. It is probable that condensation was produced by rapid drowning following subaerial exposure. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geologic and biostratigraphic framework of the non-marine Cretaceous-Tertiary boundary interval in western North America

Palynologically defined Cretaceous-Tertiary boundary sites in nonmarine rocks in western North America exhibit similar characteristics. All are marked by abrupt disappearance of the regional uppermost Cretaceous palynoflora at the level of an iridium anomaly; most also yeild shock-metamorphosed minerals. All are in coal-bearing, fluvial or paludal depositional settings, although the boundary horizon may be below, within, above, or at some stratigraphic distance from coal seams. At many sites the lowermost Tertiary beds contain assemblages overwhelmed by fern spores that, together with extinctions of some groups of angiosperms, are taken as evidence of regional devastation of terrestrial plant communities and subsequent recolonization by pioneer species.

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.

Problemas relativos às extinções de organismos

Extinctions of taxonomic categories are so important in the geological record that they have to be viewed as an intrinsic constituent of life. Terminal extinctions, meaning extinctions which leave no descendants, may occur at slow or fast rates, geologically speaking. Often in the latter case they are inappropriately referred to as The terminal extinctions at the Cretaceous/Tertiary boundary are the most studied of the so-called mass extinctions. A paper published by ALVAREZ et al. (1980), revealing the existence of an anomaly in iridium concentration in clays deposited at this boundary in many parts of the world, started a boom of publications that attempt to explain this phenomenon and its possible relation to The worldwide distribution of this anomaly means that something unusual happened at the K/T limit that might have affected life, but detailed study of the biostratigraphical distribution of taxonomic categories in the beds laid down before, during and after the iridium anomaly clays demonstrates that the deterioration processes of these taxa started before the iridium anomaly. Thus, the phenomenon responsible for this anomaly affected already impoverished faunas. The study of extinctions during the Quaternary Period is particularly elucidative with respect to this problem. The Pleistocene extinctions were probably due to climatic fluctuations eventually enhanced by the action of predatory animais, mainly man. The encroachment of civilization has contributed to Holocene extinctions in spite of the efforts of conservationists. Viewed from a geological perspective, such extinctions would probably be packed together as a single mass extinetion and a possible catastrophe sought as its unique cause despite the fact that at least two events with distinctly different causes were involbed.

The Cretaceous-Tertiary boundary and the last of the dinosaurs

Disaster theories of the K-T extinctions, more specifically dinosaur extinctions, are presently engendering much controversy. They require (inter alia) that those extinctions were sudden and simultaneous worldwide and that they coincided with an allegedly causal disaster at the K-T boundary. This paper reviews the evidence for and against those temporal requirements. The other major requirement is of a biological nature, namely an indication of the manner in which the specified disaster might have extinguished the organisms concerned; yet this causal mechanism, whatever it might have been, apparently had no effect whatever upon other, very similar organisms. In any particular geographical region, the precise stratigraphic level at which dinosaurs became extinct can be determined only if there is a virtually unbroken succession of potentially dinosaur-bearing continental beds that pass up from the level of the highest dinosaur known to a level well above the K-T boundary. Unfortunately there are surprisingly few regions where such conditions prevail. The problem is further complicated by the difficulties of worldwide stratigraphic correlation and by the fact that specialists in different fields define the position of the K-T boundary on different criteria. Although some alleged discoveries of Palaeocene dinosaurs have long been discredited (the beds were not Palaeocene, or the bones were not dinosaurian), there does seem to be some evidence that dinosaurs died out at different times in different places, sometimes surviving whatever it was that produced the iridium anomaly and sometimes co-existing with Palaeocene palynomorphs and Tertiary-type mammals, or both. In such cases it does not seem unreasonable to postulate a Danian age for the animals in question.

Mineralogy and phase-chemistry of an Ir-enriched pre-K/T layer from the Lattengebirge, Bavarian Alps, and significance for the KTB problem

Three distinct iridium anomalies are exposed in the Cretaceous-Tertiary boundary (KTB) section of the Lattengebirge, Bavarian Alps, comprising a pelagic calcareous sequence from below a continental slope. Multiple Ir anomalies are in contrast to most reported K/T sections which are characterized by a single Ir peak. Two of these Ir spikes are contained in the 20 cm thick actual KTB termed “clayey interval” with the main peak of 4.4 ppb precisely at the boundary, and the second peak of 2.8 ppb 10 cm above. A third peak of 3.6 ppb was detected at 16 cm below the KTB in a thin, turbiditic and coal-bearing sandstone layer of Upper Maastrichtian age. The distribution of Ir anomalies provides evidence of episodic Ir-delivering events over an extended period of time. Whereas the worldwide distributed single-spiked KTB layers are composed entirely of diagenetically altered materials, various phases are discernible in the well preserved Ir-bearing pre-K/T layer: (1) sandstone clastic minerals in a carbonate matrix, (2) coal which is partly burned, (3) sulfides (chalcopyrite) included in coal, and (4) silicate melt glasses. The glasses represent the first ones reported from a K/T section, and were formed by combustion metamorphism from sedimentary parent rocks of the coal as indicated by their major element chemistry and REE patterns. Phase-chemistry of the layer components reveals Ir concentrations of up to 110 ppb indicating that Ir and chalcophile trace elements (TRE) are primarily contained in the sulfides. This carrier-phase points to Ir enrichment by processes unrelated to impact, and its stratigraphic position demonstrates availability of Ir for incorporation into sediments before the KTB. The uptake of Ir and chalcophile TRE into the coal and subsequent combustion-metamorphism occurred on the continent during a time interval of 14,000–9000 yr prior to the KTB as constrained by radiometric age determinations, stratigraphy, and sedimentation rates. In the Lattengebirge K/T sediments no indications of shock are found. The results suggest a volcanic origin for the multiple Ir-bearing horizons. Like the worldwide KTB layers, they are concomitantly enriched in Ir and chalcophile TRE and, hence, have a common source. The Lattengebirge K/T profile is largely continuous, and was recently reexamined by micropaleontological methods (contribution by D. Herm and K.F. Weidich). The preservation of multiple Ir anomalies is attributed to high sedimentation rates of 1.75 cm/kyr and the absence of carbonate dissolution and condensation of sediments usually encountered in the worldwide KTB layers.