Iridium Anomaly Research Articles

Chemostratigraphy across the Cretaceous-Tertiary Boundary and a Critical Assessment of the Iridium Anomaly

The elevated concentration of iridium-one of the platinum-group elements (PGE)-at the Cretaceous-Tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end-Cretaceous mass extinctions. New chemostratigraphic data for cross-boundary sections from both hemispheres are not easily explained in terms of such an impact event, for example the observation that the PGE patterns show marked differences between the hemispheres. The new constraints indicate that models of mantle-derived PGE should be seriously considered, and that PGE anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earth's history.

Role for microorganisms in the formation of iridium anomalies

We propose a microbial role in the enhancement, erasure, dispersal, and creation of certain sedimentary iridium anomalies such as those of the Cretaceous/Tertiary boundary. The proposal is based on laboratory experiments that have demonstrated the ability of some bacteria and fungi to concentrate iridium from solution and to dissolve iridium from source igneous and meteoritic rocks.

A Tsunami Deposit at the Cretaceous-Tertiary Boundary in Texas

At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolidewater impact.

Climatic Cooling and Mass Extinction of Paleozoic Reef Communities

A variety ofpatterns suggest that climatic cooling was the primary cause of the extinction of organic reef communities late in the Ordovician, Devonian, and Permian periods. Other potential agents exhibit serious deficiencies. Anoxic conditions, for example, cannot spread to surface waters. Eustatic sea-level lowering occurred during only two of the extinction intervals, and at other times during the Phanerozoic, lowering of sea level failed to produce mass extinction. Furthermore, sea-level drops precipitate an increase in the areas around volcanic islands available for reef growth. No iridium anomaly that might have resulted from a bolide impact has been found for any of the three Paleozoic crisis intervals, and each of these intervals spanned several million years. Patterns consistent with the idea that climatic cooling caused heavy extinction of Paleozoic reef communities are the following: 1) Non-tropical marine taxa suffered less severe extinction than tropical groups, presumably because many of their species were able to migrate equatorward. 2) During the protracted Ordovician and Permian crises, biogeographic provinces contracted toward the equator. 3) For each crisis, a disproportionately large percentage of surviving taxa were cosmopolitan in distribution. 4) In the aftermath of each event, carbonate production was reduced. 5) Reef-building was suppressed for several million years after each crisis, even though potential reef-builders survived. 6) Each mass extinction approximately coincided with the initiation of a glacial interval in a polar region.

The relationship between the iridium anomaly and palynological floral events at three Cretaceous-Tertiary boundary localities in western Canada

Profiles illustrating the abundance of iridium and the changes in the relative abundances of major groups within the palynoflora are given for three localities from western Canada. The peak abundance of iridium ranges from 1.35 to 5.60 ppb and corresponds to the base of a coal and to a floral-extinction event at the three localities. Immediately above the iridium-peak anomaly, angiosperm pollen is conspicuous in the assemblage, in contrast to the fern-spore spike described for sections in the mid-continental United States and at the Morgan Creek locality in south-central Saskatchewan.

Neutron activation determination of iridium, gold, platinum, and silver in geologic samples

Low-level methods for the determination of iridium and other noble metals have become increasingly important in recent years due to interest in locating abundance anomalies associated with the Cretaceous and Tertiary (K-T) boundary. Typical iridium anomalies are in the range of 1 to 100 μg/kg (ppb). Thus methods with detection limits near 0.1 μg/kg should be adequate to detect K-T boundary anomalies. Radiochemical neutron activation analysis methods continue to be required although instrumental neutron activation analysis techniques employing elaborate gamma-counters are under development. In the procedure developed in this study samples irradiated in the epithermal neutron facility of the U. S. Geological Survey TRIGA Reactor (Denver, Colorado) are treated with a mini-fire assay technique. The iridium, gold, and silver are collected in a 1-gram metallic lead button. Primary contaminants at this stage are arsenic and antimony. These can be removed by heating the button with a mixture of sodium perioxide and sodium hydroxide. The resulting 0.2-gram lead bead is counted in a Compton suppression spectrometer. Carrier yields are determined by reirradiation of the lead beads. This procedure has been applied to the U.S.G.S. Standard Rock PCC-1 and samples from K-T boundary sites in the Western Interior of North America.

Dinosaurs, spherules, and the “magic” layer: A new K-T boundary clay site in Wyoming

A new Cretaceous-Tertiary (K-T) boundary clay site has been found along Dogie Creek in Wyoming in the drainage of Lance Creek—the type area of the Lance Formation of latest Cretaceous age. The boundary clay was discovered in the uppermost part of the Lance Formation, 4–7 cm beneath the lowest lignite in the Paleocene Fort Union Formation and approximately 1 m above a fragmented dinosaur bone. The boundary clay consists of a basal kaolinitic claystone layer as much as 3 cm thick containing hollow goyazite spherules, overlain by a 2–3 mm smectitic layer (the “magic” layer) containing both shock-metamorphosed minerals and an iridium anomaly of 21 ppb. A palynological break coincides with the base of the claystone layer; numerous Late Cretaceous palynomorph species terminate at this boundary. The paleontological significance of this new boundary site lies in its close association with the well-studied assemblage of dinosaurs and other vertebrates and flora within the type area of the Lance Formation. The spherules at the Dogie Creek site are extremely well preserved by virtue of their replacement by the mineral goyazite. This preservation should facilitate the resolution of the origin of the spherules and of their host layer.

Stratigraphic occurrences of iridium anomalies at four Cretaceous/Tertiary boundary sites in New Zealand

Research Article| September 01, 1986 Stratigraphic occurrences of iridium anomalies at four Cretaceous/Tertiary boundary sites in New Zealand Robert R. Brooks; Robert R. Brooks 1Department of Chemistry and Biochemistry, Massey University, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar C. Percy Strong; C. Percy Strong 2New Zealand Geological Survey, P.O. Box 30368, Lower Hutt, New Zealand Search for other works by this author on: GSW Google Scholar Julian Lee; Julian Lee 3Applied Biochemistry Division, Department of Scientific and Industrial Research, Private Bag, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Charles J. Orth; Charles J. Orth 4Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Search for other works by this author on: GSW Google Scholar James S. Gilmore; James S. Gilmore 4Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Search for other works by this author on: GSW Google Scholar Douglas E. Ryan; Douglas E. Ryan 5Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada Search for other works by this author on: GSW Google Scholar Jiri Holzbecher Jiri Holzbecher 5Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Robert R. Brooks 1Department of Chemistry and Biochemistry, Massey University, Palmerston North, New Zealand C. Percy Strong 2New Zealand Geological Survey, P.O. Box 30368, Lower Hutt, New Zealand Julian Lee 3Applied Biochemistry Division, Department of Scientific and Industrial Research, Private Bag, Palmerston North, New Zealand Charles J. Orth 4Los Alamos National Laboratory, Los Alamos, New Mexico 87545 James S. Gilmore 4Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Douglas E. Ryan 5Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada Jiri Holzbecher 5Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (9): 727–729. https://doi.org/10.1130/0091-7613(1986)14<727:SOOIAA>2.0.CO;2 Article history First Online: 01 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 Robert R. Brooks, C. Percy Strong, Julian Lee, Charles J. Orth, James S. Gilmore, Douglas E. Ryan, Jiri Holzbecher; Stratigraphic occurrences of iridium anomalies at four Cretaceous/Tertiary boundary sites in New Zealand. Geology 1986;; 14 (9): 727–729. doi: https://doi.org/10.1130/0091-7613(1986)14<727:SOOIAA>2.0.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 Three new iridium anomaly sites have been discovered in Cretaceous/Tertiary boundary sequences in New Zealand. These are at Needles Point, Chancet Rocks, and Waipara, where integrated iridium deposition values were 165, 211, and 7 ng/cm2, respectively. In contrast to the previously reported Woodside Creek stratigraphic sequence that had an iridium anomaly of 187 ng/cm2, a ferruginous boundary clay is absent in the three new sites, though the base of the Tertiary is marked by limonite staining. The relatively weak anomaly at the Waipara section is probably due to extensive bioturbation coupled with a high sedimentation rate at the time of deposition. The discovery of these additional boundary rock sequences in New Zealand negates suggestions that the Woodside Creek iridium anomaly was a fortuitous occurrence caused by unusual weathering conditions. The integrated iridium deposition values at these three new sites of somewhat different geology support the previously reported high iridium level for Woodside Creek, which until now was the only iridium anomaly on land in the Southern Hemisphere. 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.

The terminal Cretaceous iridium anomaly in the Red Deer Valley, Alberta, Canada

Neutron activation analysis following fire-assay preconcentration in a nickel sulphide button allows processing of relatively large samples (25 g) for precision analysis of platinum group elements. Using this method, an iridium anomaly with maximum concentration of 3.36 ± 0.04 ppb has been located at the Cretaceous–Tertiary boundary at the base of the Nevis coal seam in the Red Deer Valley of Alberta. The iridium anomaly coincides with the disappearance of diagnostic late Cretaceous palynomorphs, including all but one species of Aquilapollenites.

Palynological and Iridium Anomalies at Cretaceous-Tertiary Boundary, South-Central Saskatchewan

The Cretaceous-Tertiary boundary in south-central Saskatchewan is marked by coincident anomalies in abundance of iridium and fern spores at the extinction level of a suite of Cretaceous pollen taxa. Evidence of disruption of the terrestrial flora includes the fern-spore abundance anomaly and local extinction of as much as 30 percent of angiosperm species. The reorganized earliest Tertiary flora is made up largely of surviving species that assumed new roles of dominance. Persistence of climatically sensitive taxa across the boundary indicates that if paleoclimate was altered by the terminal Cretaceous event, it returned quickly to the pre-event condition.

Late Eocene North American microtektites and clinopyroxene‐bearing spherules

Approximately 35 m.y. ago two events, separated by only 10,000–20,000 years, each spread roughly 1 billion metric tons of silicate spherules over large areas of the Earth's surface. The older event produced mostly clinopyroxene‐bearing spherules (cpx spherules). The cpx spherules have been found in the Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern equatorial Indian Ocean. The cpx spherule layer is associated with an iridium anomaly and appears to coincide with the extinction of several species of Radiolaria. The composition and widespread distribution of the cpx spherules indicate that they were formed by an impact event. The younger layer of spherules consists of microtektites belonging to the North American tektite strewn field based on age and elemental and isotopic composition. The North American microtektites have been found in the western equatorial Atlantic, on Barbados, in the Caribbean Sea, and in the Gulf of Mexico. The North American microtektite layer is not synchronous with an iridium anomaly, nor does it appear to be associated with any major changes in the microfossils. Neither the cpx spherule event nor the North American tektite event appears to be associated with a climatic change, but no detailed studies have been made across either layer. Excluding the cpx layer, which contains mostly crystal‐bearing spherules, the evidence at the present time indicates that there is only one late Eocene microtektite layer and it is part of the North American strewn field.

Late Eocene microtektites and radiolarian extinctions on Barbados

A late Eocene microtektite layer has been found in three exposed sections on Barbados, West Indies. These microtektites and associated tektite fragments are compositionally similar to the North American tektites and thus, like the Caribbean and Gulf of Mexico microtektites, may belong to the strewn field of ‘North American’ microtektites which have been found in deep-sea deposits from the Caribbean Sea, across the Pacific and into the northeastern Indian Ocean1,2. The ‘North American’ layer has been reported to be approximately synchronous with several radiolarian extinctions3,4 and with an anomaly in iridium concentrations5–7, and it has been hypothesized that the microtektites, the extinctions and the iridium anomaly are all related to an asteroid impact. We report here that the radiolarian extinctions predate the microtektite layer and that the highest Ir concentrations apparently coincide with the extinctions. Thus, although the extinctions may be causally related to the Ir anomaly, the microtektites appear not to be.

Mass extinction and iridium anomaly in the Upper Devonian of Western Australia: A commentary

An iridium anomaly has been detected in the Upper Devonian of the Canning Basin. The anomaly, which occurs in a 12-cm stromatolite bed with a high iron oxide content, is associated with a global extinction that involved a major decrease of biomass. The formation and preservation of geochemical anomalies in the Paleozoic may be unlikely in many sedimentary environments, while only biotic changes examined at the finest time scale will prove of value in identifying abrupt extinctions. Geochemical anomalies associated with extinctions may provide reliable tests for impact hypotheses. The anomaly may be due, however, to local chemical, physical, or biological processes, or to fallout from a meteorite impact or volcanic event.

Elemental Anomalies at the Cretaceous-Tertiary Boundary, Woodside Creek, New Zealand

Iridium and 26 other elements were determined in shale from the Cretaceous-Tertiary boundary at the locus classicus (for iridium anomalies) at Woodside Creek, New Zealand. Iridium, gold, copper, cobalt, chromium, nickel, arsenic, molybdenum, and iron were enriched in the basal 2 millimeters of the 8-millimeter shale parting as compared with the rest of the stratigraphic column. No other shale partings in the column had anomalous concentrations of any element when the data were expressed on a carbonate-free basis. The boundary material showed striking compositional similarities with the Stevns Klint Danish boundary shale. Elemental concentrations were in general much higher in the New Zealand material than in nonboundary shales from elsewhere in the world. The high concentration of iridium (153 nanograms per gram) in the basal layer of the boundary, together with the enrichment of other siderophile elements supports the idea of an extraterrestrial source for much of the material. The iridium/gold ratio of 2.1 is also in accordance with such a source. The iridium content of the basal layer is higher than for any other marine boundary shale obtained on land. The integrated iridium value is 187 nanograms per square centimeter of boundary surface.

Iridium Anomaly in the Upper Devonian of the Canning Basin, Western Australia

A moderate iridium anomaly, about 20 times the local background, has been found in Upper Devonian rocks in the Canning Basin. It occurs at or near the Frasnian-Famennian boundary, which is known to be associated with a major massextinction event of global extent. The anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions. Its occurrence suggests a causal link with some form of meteoroid impact. Moreover, carbon isotope data indicate that a large reduction in biomass could have occurred at this level. However, the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium Frutexites; iridium, platinum, iron, manganese, cobalt, arsenic, antimony, and cerium are preferentially concentrated in filaments of this organism, with concentrations ranging from two to five times that of the matrix. It is possible that Frutexites extracted these elements directly from seawater, without the need for their derivation from an extraterrestrial source.

Impact Theory of Mass Extinctions and the Invertebrate Fossil Record

There is much evidence that the Cretaceous-Tertiary boundary was marked by a massive meteorite impact. Theoretical consideration of the consquences of such an impact predicts sharp extinctions in many groups of animals precisely at the boundary. Paleontological data clearly show gradual declines in diversity over the last 1 to 10 million years in various invertebrate groups.Reexamination of data from careful studies of the best sections shows that, in addition to undergoing the decline, four groups (ammonites, cheilostomate beyozoans, brachiopods, and bivalves) were affected by sudden truncations precisely at the iridium anomaly that marks the boundary. The paleontological record thus bears witness to terminal-Cretaceous extinctions on two time scales: a slow decline unrelated to the impact and a sharp truncation synchronous with and probably caused by the impact.

Cretaceous boundary

Four years after it was first proposed, the theory that a colossal meteorite struck the earth 65 million years ago continues to build toward a consensus—this despite recent findings that volcanic eruptions might also have caused the “iridium anomaly” that is the impact theory's best evidence (Eos, February 7, 1984, p. 41).High concentrations of iridium at the Cretaceous‐tertiary boundary were first noticed in sediments from Italy, Denmark, Spain, and other locations around the world by Walter Alvarez, his father Luis, and their Berkeley colleagues. They concluded that the source of the extra iridium must have been extra‐terrestrial because the element shows up in crustal rocks only in very small amounts and no one could think of a mechanism that would distribute iridium from the mantle so widely around the surface of the globe. The Alvarez group postulated that a meteorite about 10‐km wide had collided with the earth, throwing up a planet‐encircling cloud that blocked out sunlight, ended photosynthesis, and snuffed out many of the land and sea creatures of the Mesozoic, including the dinosaurs.

Upper Eocene North American Microtektite Layer: Associated Radiolarian Extinctions, Climatic Change, and Iridium Anomaly: ABSTRACT

Tektites are glass objects believed by many authors to have been formed by meteorite (or cometary) impact. The areas on the earth's surface where tektites are found are called strewn fields. Thus, tektites found in Texas and Georgia belong to the North American strewn field. Microtektites (< 1-mm diameter tektites) have been found in upper Eocene sediments from one piston core and in cores from nine Deep Sea Drilling Project sites in the Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern Indian Ocean. Based on their fission-track age (34.6 ± 4.2 m.y.), geologic age, geographic location, and chemistry, these microtekties are thought to be part of the North American strewn field. The North American tektites have fission-track, K-Ar, and 40Ar-39Ar ages of ~ 34 m.y. The North American microtektite layer End_Page 470------------------------------ occurs in the Thyrsocyrtis bromia radiolarian Zone, the P15 foraminiferal Zone, and the Discoaster barbadiensis calcareous nannoplankton Zone. At each site, the layer is associated with the last appearance of several species of Radiolaria (e.g., Thyrsocyrtis bromia, t. tetracantha, t. rhizodon, Calocyclas turris). Previously published oxygen isotope data indicate a drop in temperature at about this time, which may be related to the tektite event and may have been responsible for the radiolarian extinctions. An iridium anomaly has recently been found associated with the microtektite layer that supports an impact origin for the tektites. The extent of the strewn field and the calculated mass of microtektite material (~ 1012 kg) indicate that the North American tektite event w s a relatively large event. End_of_Article - Last_Page 471------------

Eocene-Oligocene: A Time of Transition: ABSTRACT

The Eocene-Oligocene is a time of transition from a warm early Tertiary world with low vertical and latitudinal thermal gradients to the Neogene world with steep vertical oceanic gradients and high latitudinal gradients between equator and poles. The transition between these two regimes occurred primarily between the middle Eocene and middle Oligocene and can be observed in faunal and floral assemblage changes, associated paleotemperature changes, periodic current intensification as implied by increased carbonate dissolution and hiatuses, eustatic sea level changes, and the curious association of microtektites and iridium anomalies with several of these intervals. Population studies of planktonic foraminifers in 14 DSDP sites in the Atlantic, Pacific, and Indian Oceans indicate a general cooling trend between the middle Eocene and Oligocene. Major faunal changes indicating cooling episodes occur, however, at discrete intervals: middle Eocene 44 to 43 Ma (P13); middle/late Eocene boundary 41 to 40 Ma (P14/P15); late Eocene 39 to 38 Ma (P15/P16); Eocene/Oligocene boundary 37 to 36 Ma (P18); and late Oligocene 31 to 29 Ma (P20/P21). Each cooling episode resulted in the extinction of warmer water species and evolution and dominance of cooler water species. This trend is associated with the development of a steep vertical thermal gradient and resulting stratification in the upper water masses (0 to 300 m, 1,000 ft) in the latest Eocene. 18The presence of microtektites and iridium anomaly in latest Eocene sediments has resulted in the scenario of catastrophic extinctions due to a bolide impact. The present study reveals multiple microtektite occurrences at 43 Ma, 40 Ma, 38 Ma, 34 Ma, and 30 Ma. Moreover, these microtektite occurrences coincide with intervals of increased carbonate dissolution and/or hiatuses. This suggests that microtektites are concentrated as a result of carbonate dissolution and selective winnowing of sediments at these intervals. Consequently, a concentration of microtektites in deep-sea sediments may not always imply a bolide impact, nor is there any evidence of catastrophic extinctions during Eocene-Oligocene time. End_of_Article - Last_Page 494------------

Relationship between an iridium anomaly and the North American microtektite layer in core RC9‐58 from the Caribbean Sea

In a previous publication, an iridium anomaly was reported in core RC9‐58 from the Caribbean Sea, about 30 cm below the peak abundance of North American microtektites. In order to determine more precisely the relationship between the iridium anomaly and the North American microtektite layer, we searched for microtektites in the samples that were used for the iridium studies. We found that the North American microtektite layer is actually two layers, with the peak abundances separated by 25 cm. The upper layer consists of ‘normal’ North American microtektites and the lower layer consists of previously described clinopyroxene‐bearing spherules. The iridium anomaly was found to correlate with the lower layer. Although the two layers appear to be the result of two separate events, several lines of evidence suggest that they were produced by a single event. The separation into two layers may have been produced by differential settling in the sediment due to density variations. The correlation between the iridium anomaly and the North American microtektite layer supports the terrestrial impact origin for tektites.