Ir Anomalies Research Articles

Coexisting altered glass and Fe–Ni oxides at the Cretaceous–Tertiary boundary, Stevns Klint (Denmark): direct evidence of meteorite impact

The Cretaceous–Tertiary (K–T) boundary at Stevns Klint, Denmark, is noteworthy for its large Ir anomaly that is taken as evidence of extraterrestrial components, but the origin of the smectite in this marl has been variously interpreted to have a detrital, meteorite impact, or volcanic origin. We have carried out scanning electron microscopy and transmission electron microscopy (TEM)/analytical electron microscopy observations of the impact and contiguous layers within the K–T marl at Stevns Klint. TEM images show abundant smectite, much of which occurs with layers curving around and grading into cores of nanometer-scale glass shards. The smectite composition is unusual in having both significant octahedral Al and Mg. The glass and smectite major element compositions are similar and unique relative to glasses of terrestrial and extraterrestrial origin with the exception for one kind of glass at the K–T boundary in Haiti. Abundant 10–20-nm diameter iron oxides having as much as 10% Ni and minor Zn are intergrown with smectite. We interpret these domains to be altered meteorite fragments, which formed when impact glass was transformed to smectite. The direct association of unique glass and meteorite fragments is unambiguous evidence for meteorite impact. These data may imply fall-out of globally distributed impact-derived particles over an extended time period. The relations imply that TEM observations may be a powerful tool in detecting other impact events in the geological record.

The Paleocene-Eocene boundary in a flysch sequence from Goriška brda (Western Slovenia): Oxygen and carbon stable isotope variations

Stable (δ13Ccarb., δ13Corg., δ18O) isotopic records were studied to elucidate the problem of the Paleocene/Eocene (P/E) boundary in a flysch sequence at Nožno in Goriška Brda, W. Slovenia. A remarkable disturbance in organic carbon as well as in carbonate C and O isotopic records and associated Ir anomalies provide strong evidence that something unusual happened at the time of the Paleocene-Eocene transition. The diverse character of the negative excursion of δ13C of carbonate and sedimentary organic carbon indicates a significant perturbation in the global carbon cycle and climatic changes, most probably controlled by a combination of volcanic activity and fluctuations in bioproductivity. The corresponding δ18O variability may be related to global warming due to a higher greenhouse effect and/or diagenetic alteration.

Cretaceous age for Ir‐rich Deccan intertrappean deposits: palaeontological evidence from Anjar, western India

Micropalaeontological investigations of an iridium‐bearing, lacustrine intertrappean sedimentary sequence at the western margin of the Deccan volcanic province near Anjar, have revealed a profuse occurrence of theropod eggshell fragments (ornithoid type) in beds overlying the iridium‐enriched levels. Associated late Cretaceous ostracods, lack of evidence of reworking, and the absence of any exclusively Palaeocene taxa above the iridium levels, taken together, indicate that the extinction of dinosaurs in the Indian subcontinent occurred after the deposition of Ir layers at Anjar, and that these Ir anomalies may significantly predate the K–T boundary.

Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, Sweden

A distinct Ir anomaly with an Ir content of up to 4.5 ppb is found in resurge deposits tied to the Middle Ordovician Lockne impact structure, Jämtland, Sweden. The impact took place in a marine environment and the sea water played an important role in the impact pro-cess and in the aftermath. The samples yielding the Ir anomaly also contain abundant shocked quartz, which proves that the structure was formed by the impact of an extraterrestrial body. The mid-Ordovician volcanism close to Scandinavia did not generate any distinct Ir content. About 20% of the meteorite Ir remained in the resurge deposit around the crater. The Ir from the impact-related deposits in Lockne correlates well with Cr. There is a weaker correlation between Ir and chalcophile and siderophile (e.g., Fe, Sb, Zn, and Ni, Co) elements. The low-temperature hydrothermal system, which acted after the impact event, mobilised some of the chalcophile and siderophile elements and, thus, resulted in their weaker correlation with Ir in some samples.

Multidisciplinary high-resolution correlation of the Permian–Triassic boundary

Biostratigraphic correlation of the ten most important Permian–Triassic boundary sections throughout Tethys enables establishment of four conodont and ammonoid subdivisions within a stratigraphic interval, one to a few metres thick, representing less than 1 Ma. In ascending order, they are conodonts ( Clarkina changxingensis–C. deflecta Zone; Hindeodus typicalis Interval; Isarcicella parva Zone and I. isarcica Zone) and ammonoids ( Pseudotirolites–Pleuronodoceras Zone, lower Otoceras Zone, upper Otoceras Zone and Ophiceras Zone). Most of them can be traced to North America and the Arctic region. Carbon isotope investigations of 24 sections along Tethys and in Greenland and Spitzbergen confirm the consistency of δ 13C negative excursions at the Permian–Triassic boundary, together with an inconsistent Ir anomaly. Eight sections show that in most cases there is a succession of: Ir anomaly (denoting the end-Permian catastrophic environment); δ 13C excursion (biomass loss, extinction); P/T boundary (origination of newcomers; potentially capable of intercontinental correlation within that short time interval). There is an intercontinental sequence boundary at the top of the Permian, and a transgressive surface at the P/T boundary followed shortly by a maximum flooding surface. An intercontinental anoxia event accompanied the transgression. Three delineations of the mass extinction phases and three population explosions have been recognized and can be more or less correlated in South China and the Southern Alps. Radiometric dating of the volcanogenic boundary clays of Meishan, Shangsi and the main-stage Siberian Tunguss Traps give an almost identical age of 250 Ma, thus implying a synchronous interregional volcanic event. There are a few palaeomagnetic transforms within the PTB strata. Thus, multidisciplinary research of the P/T boundary strata allows subdivision of this short interval into more than ten intercontinentally correlatable parts. Each subdivision averages less than 100 ka in duration. This may be the highest resolution obtained so far in pre-Cretaceous rocks. High-frequency, Milankovitch-type cyclic deposits within the P/T boundary beds enable correlation of even higher resolution in the Lower Yangtze area. This high resolution is made possible because the P/T transition is a time of saltation and catastrophy in geological history. Causality of the aggregation of events merits further investigation.

Geochemical Data on Marine Sediments of the Permian-Triassic Boundary Interval: A Review

Described as the turning point in the Pangean environmental history, the Permian-Triassic boundary interval (PTBI) records major global events as biotic crisis, variations of sea level and a large volcanic activity. The increasing number of geochemical studies across the PTBI have outlined variations of unusual amplitudes. A survey of 13 tethyan sections across the PTBI (Baud et al., 1989) confirmed that the variations of ~13C values of carbonates described previously had a global character. The most detailed geochemical study across a Permian-Triassic boundary section has been carried out on the Gartnerkofel core, Southern Alps (papers in Holser and Sch6nlaub, 1991) with three main objectives: the calibration of the 813Ccarb and ~13Corg curves, the research of rare earth and Ir anomalies and the analysis of anoxia. Among the reviews on the Permian-Triassic boundary that consider geochemical data, the most recent one has been published by Hallam and Wignall (1997). The goal of the present note is to review the most recent literature and to discuss the use of geochemistry in problems related to the PTBI.

Stable Isotopes, Ir Anomaly and Other Elemental Markers Near the Palaeocene - Eocene Transition: Evidence from the Western Tethys

The geological history of the Earth is characterized by several global events recording dramatic climatic, oceanographic and biotic changes. One of them is a short-term Palaeocene-Eocene (P/E) deep sea benthic foraminiferal extinction event (BEE), also marked by distinct negative 813C and 81So anomalies, recorded in several marine and terrestrial sections all over the world (see Corfield, 1994; Koch et al., 1995; Stott at al., 1996 and references therein), as well as geochemical and sediment compositional changes (Lu et al., 1996; Schmitz et al., 1997). In this study we present preliminary results of complex stable isotope and geochemical investigations of the P/E boundary interval in the flysch sequence from the western Tethys (Gorika Brda section, Slovenia), and discuss their implications in a view of better understanding of the nature and the causes of the of the global changes across the P/E transition in this part of the Tethys.

Extraterrestrial impact events: the record in the rocks and the stratigraphic column

Abstract The known terrestrial impact record is a biased sample of a much larger population of impact events. The biases are due to the modifying effects of terrestrial geologic processes, coupled with incomplete searches for impact structures and impact-related materials. Terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but, because of post-impact modification by terrestrial geologic process, are recognised by the occurrence of shock metamorphic effects. In some cases, siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body. Similar shock metamorphic effects and a siderophile anomaly in K–T boundary materials are indicative of a major impact event, which has been correlated with the formation of the Chicxulub structure, Mexico. Evidence of a small number of other impacts occur in the stratigraphic record, most commonly as tektite or microtektite horizons. In some cases they are known to be accompanied by geochemical anomalies. In other cases a number of Ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact. The majority of known impact events in the stratigraphic record are from relatively recent geologic time. Logic dictates, however, that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts ( D ≥20 km) have the potential to cause atmospheric blow-out and, thus, global dispersion of some of the impact products. Geochemical detection, however, of such events may not be easy; in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material. In addition, non-chondritic bodies may result in no appreciable geochemical anomaly. In view of this, any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials; although, in the case of impacts into oceanic crust, this too will be difficult. Given the K–T experience, however, and the fact that large-scale impact on Earth is a natural consequence of the character of the solar system, the potential of impacts to provide local and global marker horizons can not be ignored. Similarly, the fact that impacts may have the potential to result in short-term biologic or climatic excursions can not be dismissed arbitrarily, when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record.

Satellite thermal survey—a new tool for the study of seismoactive regions

Abstract The National Oceanic and Atmospheric Administration (NOAA)series satellite thermal images (STI) study showed the presence of positive anomalies of the outgoing Earth radiation flux recorded at night time and associated with largest linear structures and fault systems of the crust. The analysis of a continuous series (100-250 days) of nightly STI data for a period of 10 years allowed identification of a set of IR radiation anomalies in the Central Asian seismoactive region, Iran, Egypt, etc. About 10 000 NOAA images were analysed. It was actually discovered that there was a statistically significant correlation between the activity of IR anomalies (mean value of area per year or month) and the seismic activation of the Central Asian seismoactive region. At present the nature of stable and non-stable IR anomalies is not clear. In all probability hydrogeological factors or the greenhouse effect play a main role in forming the anomalies. Thus, satellite thermal survey is a tool for investigations of seismoactive regions and for earthquake predictions.

Iridium geochemistry of volcanic ash layers from the early Eocene rifting of the northeastern North Atlantic and some other Phanerozoic events

This study shows that some types of explosive volcanism can result in significant Ir anomalies (up to 750 ppt); however, these are readily discerned from impact-related, extraterrestrial Ir anomalies by other chemical and mineralogical criteria. Iridium and major and trace elements were analyzed in 34 of the >180 volcanic ash layers in the Fur Formation of early Eocene age in Denmark. These ashes originate from explosive volcanism during rifting of the Greenland-Eurasia continent and subsequent sea-floor spreading in the young northeastern North Atlantic. In addition, 15 older Phanerozoic ashes, associated with other dramatic global events, have been analyzed for Ir. The Fur Formation ashes are unusually well preserved and have a wide compositional range–for example, rhyolitic, dacitic, tholeiitic and alkaline basaltic, mafic alkaline, and different salic alkaline ashes, and ashes with intermediate or unknown magma origin. The original magma compositions have been determined by immobile-element plots (Nb/Y-Zr/Ti) and other criteria. Iridium shows a bimodal distribution, where all types of salic (alkaline as well as subalkaline) ashes have very low Ir concentrations ( Analyses of 15 of the

When the sky fell on our heads: Identification and interpretation of impact products in the sedimentary record

Research on impact products, mechanisms and consequences has benefited greatly in the last ten years from the highly publicized Cretaceous‐Tertiary (KT) boundary debate. This debate has led to an increasing awareness among Earth scientists of the importance and potential effects of impact events in Earth history. With more and more researchers realizing the role played by impacts and becoming familiar with the characteristics of impact materials, it is likely that in the coming years, an increasing number of impact events will be identified, in agreement with the Earth cratering rate estimations (Grieve, 1984; Shoemaker et al., 1990).The increasing recognition of impact event is demonstrated by the discovery or identification in recent years of several impact craters, some of large diameter: Chicxulub, >180 km in Yucatan; Mjølnir, 40 km in the Barents Sea; Ames, 16 km in Oklahoma; Avak 12 km in Alaska; Pretoria Saltpan, 1.1 km and Kalkkop, 640 m both in South Africa. Sediments record impact events in the form of impact glass, shocked minerals, Ir anomalies, and perhaps also polymic‐chaotic breccias and/or tsunami deposits. Identification of impact products, particularly in ancient rocks, is hampered by poor preservation, diagenetic alteration, metamorphism, misinterpretation, or even lack of interest or awareness of their significance. Impact products represent very short term events, most likely geographically localized and stratigraphically concentrated over a thin interval of sediments and often diluted by normal background deposition. Finding these impact signatures requires very detailed petrographic and geochemical investigations with closely spaced sampling (a few cm). Such detail is not commonly used when studying ancient Mesozoic and Paleozoic sediments.

Global iridium anomaly, mass extinction, and redox change at the Devonian-Carboniferous boundary

Iridium abundance anomalies have been found on a global scale in the Devonian-Carboniferous (D-C) boundary interval, which records one of the largest Phanerozoic mass-extinction events, an event that devastated many groups of living organisms, such as plants, ammonoids, trilobites, conodonts, fish, foraminiferans, brachiopods, and ostracodes. At or very close to the D-C boundary, there exists a geographically widespread black-shale interval, and Ir abundances reach anomalous maxima of 0.148 ppb (Montagne Noire, France), 0.138 ppb (Alberta, Canada) 0.140 ppb (Carnic Alps, Austria), 0.156 ppb (Guangxi, China), 0.258 ppb (Guizhou, China), and 0.250 ppb (Oklahoma). The discovery of global D-C Ir anomalies argues for an impact-extinction model. However, nonchondritic ratios of Ir to other important elements and a lack of physical evidence (shocked quartz, microtektites) do not support such a scenario. The fact that all Ir abundance maxima are at sharp redox boundaries in these sections leads us to conclude that the Ir anomalies likely resulted from a sudden change in paleo-redox conditions during deposition and/or early diagenesis. 36 refs., 2 figs., 1 tab.

Cretaceous/Tertiary and Permian/Triassic boundary events compared

The Cretaceous/Tertiary ( K T ) and Permian/Triassic ( P Tr ) events differ in many more aspects than they are similar. This alone makes it unlikely that they are parts of a common web of periodic mass extinction events (MEE). Unlike the K T , most MEE have no Ir anomalies and where (in the comparatively complete section of the P Tr at Gartnerkofel, Austria) weak Ir concentrations are found, their ratios to siderophile elements (Co, Ni, Cr) are far less than chondritic. In contrast, ratios to chalcophile elements (As, Sb, Zn) never approach chondritic, even in K T peaks and locality differences suggest local depositional processes that may also affect the siderophiles. Profiles of isotopes ( δ 13 C carb , 87Sr 86Sr , and possibly δ 18 O carb ) across these boundaries also differ radically. Whereas the K T profile of surface ocean δ 18 O carb has been shown to be mainly a drop in primary productivity, the P Tr profile is interpreted at least in part to be a consequence of a large and accelerating oxidation of previously stored C org, with greatly differing consequences for climate. These and other differences of proximate forcings for these two MEE lead us to a hypothesis that such external exigencies were ever (intermittently and severally) with us, but were effective only when the biotic web had already internally unravelled in some crucially strained patch. Gaia became Antigaia!

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

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.

Hiatus distributions and mass extinctions at the Cretaceous/Tertiary boundary

Research Article| May 01, 1991 Hiatus distributions and mass extinctions at the Cretaceous/Tertiary boundary Norman MacLeod; Norman MacLeod 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar Gerta Keller Gerta Keller 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar Author and Article Information Norman MacLeod 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Gerta Keller 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 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 (5): 497–501. https://doi.org/10.1130/0091-7613(1991)019<0497:HDAMEA>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 Norman MacLeod, Gerta Keller; Hiatus distributions and mass extinctions at the Cretaceous/Tertiary boundary. Geology 1991;; 19 (5): 497–501. doi: https://doi.org/10.1130/0091-7613(1991)019<0497:HDAMEA>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 Much disagreement over the interpretation of data bearing on various Cretaceous/Tertiary (K/T) extinction scenarios results from a failure to view these data within their appropriate stratigraphic context. Combined biostratigraphic and chronostratigraphic analyses of K/T boundary sequences have revealed systematic differences in patterns of sediment accumulation within continental-shelf and deep-sea depositional settings. Although virtually all deep-sea boundary sequences are marked by intervals of nondeposition or hiatus formation during the latest Cretaceous and earliest Tertiary, many continental shelf-slope sequences appear to be temporally complete over this same interval. This differential pattern of sediment accumulation can be related to the latest Maastrichtian-earliest Danian sea-level rise, during which deep-sea sediment- accumulation rates would be expected to drop as the locus of sediment deposition migrated across the continental shelf. Our data suggest that the abrupt shifts in carbon-isotope abundances, single-peak Ir anomalies, and apparently instantaneous mass extinctions of marine plankton—which are routinely reported from deep-sea K/T boundary sequences and used to support a causal relation between Late Cretaceous bolide impacts and K/T mass extinctions—may be artifacts of a temporally incomplete (or extremely condensed) deep-sea stratigraphic record. 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.

Fires at the K/T boundary: Carbon at the Sumbar, Turkmenia, site

We have measured carbon at the marine K/T boundary site SM-4 at the Sumbar river in Turkmenia, USSR, which has an undisturbed Ir profile and the largest known Ir anomaly (580 ng/cm 2). Twenty samples, ranging from −100 to +100 cm, were analyzed to determine the concentration and δ 13C of elemental carbon and kerogen, using a Cr 2O 7 oxidation method (Wolbach and Anders, 1989) to resolve these components. The samples were unusually complex, containing 3 kerogen components of distinctive IR spectra and half-lives in Cr 2O 7 (~200, ~100, and ⪡60 h), in addition to soot and charcoal from the K/T fire. The elemental C has δ 13C = −25.96 ± 0.6 l%o, close to the mean for 11 K/T sites (-25.8 ± 0.6%; 11 mg/cm 2). The first of the 2 kerogen components (δ 13C = −22.8%), which dominates in the Cretaceous, appears to come from detrital carbonaceous shale; the second (δ 13C = −27.7%o) occurs only in the boundary clay and may come from land plant material swept to sea, or phytoplankton grown in the presence of excess, light CO 2 from fires on land. The great thickness of the boundary clay (10.5 cm) permits determination of the compositional profiles with unprecedented spatial resolution. Ir and shocked quartz—both representing impact ejecta—rise sharply at the boundary, peak in the basal layer, and then decline. Soot and total elemental C show a similar spike in the basal layer but then rise rather than fall, peaking at 7 cm. Apparently fires started before the basal layer had settled, implying that ignition and spreading of major fires became possible if not immediately then very soon after the impact. There is no basis for claims by Hansen et al. (1987) that K/T carbon black first occurs 3.5 m below the Danish K/T boundary and has δ 13C of −27.4 to −27.8%o. Reexamination of a sample from Nye Kløv shows that these authors confused kerogen with carbon black.

Intrasample REE Variability in Pacific Ocean Fe-Mn Crusts: A Potential Paleoceanographic Indicator: ABSTRACT

Recent studies of the microscale chemistry and morphology of marine Fe-Mn crusts suggest that prevailing oceanographic conditions at the time of formation are recorded in individual layers of the deposits. Using current radiometric crust growth-rate estimates of 1 to 5 mm per million years, thick Fe-Mn oxide accumulations may represent a growth record ranging from 10 to 100 million years. Detailed chemical and mineralogical analyses were performed on 5 mm layers of selected thick (5-10 cm) crusts from our collection. The analyses reveal significant variations in the distribution and abundance of the REE as a function of depth within the crusts. The trivalent REE generally exhibit sympathetic concentration changes in pure Fe-Mn oxides but display a greater degree of fractionation between the light and heavy REE in layers enriched in phosphatic matter. The compositional variability and the extent of REE fractionation can be explained partially by mineralogical control but are also believed to reflect differing depositional conditions and the composition of the seawater from which the deposits formed. Changes in the Ce anomaly throughout the crust correlates with Pt and Ir anomalies observed by other workers and appears to be a very promising indicator of major events. The potential usemore » of Ce as a paleoceanographic tracer stems from its electronic structure, which leads to a preference for the highly insoluble tetravalent CeO{sub 2}, whereas other REE exist principally in the trivalent oxidation state. Because Fe-Mn crusts accrete primarily under oxidizing conditions, variations in the Ce anomaly of crusts may be useful as a paleoredox indicator as previously suggested by other investigators. The REE patterns observed in this study will be discussed in terms of their relation to the major element composition and mineralogy of the crusts.« less

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.

Geochemical Anomaly at the Devonian / Carboniferous Boundary, Huangmao, Guangxi, China

Abstract The abundances of nearly 40 elements, Ir included, have been measured using radio‐chemical and instrumental neutron activation analysis (RNAA and INAA) across a Devonian / Carboniferous (D / C) boundary section at Huangmao, Guangxi, China. The Ir anomaly has been found in the D / C boundary bed. Its peak value is 156 ppt, richer by a factor of 12 than that in the underlying strata. Besides, as with Ir, other siderophile and chalcophile elements such as Au, Ni, Co, As and Sb are also enriched. The cause for the abundance anomalies of Ir and other elemets is discussed. Neither volcanic eruption nor extraterrestrial impact can explain it satisfactorily. The real mechanism for the anomalies awaits further study.

Iridium anomaly from the Acraman impact ejecta horizon: impacts can produce sedimentary iridium peaks

THE observation of anomalously high iridium concentrations at the Cretaceous/Tertiary (K/T) boundary1 triggered the still unresolved debate concerning the origin of sedimentary Ir anomalies. Several hypotheses have been presented, including meteorite impacts1,2 and intense volcanism3,4. Here we present data from the Acraman impact ejecta horizon, South Australia5, which conclusively links high sedimentary iridium concentrations to a major meteoroid-impact structure and its widely dispersed ejecta. An Ir anomaly (up to 2 parts per 109 (p.p.b.) Ir as well as Au, Pt, Pd, Ru and Cr anomalies) is present within the ejecta from the Acraman impact event preserved in late Precambrian (∼600 Myr BP) shales within the Adelaide Geosyncline. The highest Ir (and Cr) concentrations within the ejecta are associated with the coarsergrained clasts of acid volcanics (boulder to medium-sand size) suggesting that most of the Ir is carried by these fragments. The target rocks at the impact site are comparable acid volcanics that have very low Ir, Au, Pt, Pd, Ru and Cr concentrations, indicating a meteoritic origin for the anomalous Ir levels within the ejecta. To our knowledge, this is the first example in which an accepted, widely dispersed impact-ejecta blanket, itself uniquely linked to a major terrestrial impact structure5,6 has been shown to be anomalously high in cosmogenic siderophile elements. This finding strongly supports the hypothesis that Ir anomalies in sediments can be produced by terrestrial meteorite impact events.