Iridium Anomaly Research Articles

Tracing impact events in clay samples with iridium anomaly at and above the Cretaceous/Paleogene boundary at Byala, Eastern Bulgaria

Elemental composition of two groups of 12 clay samples, at and above the Cretaceous/Paleogene (K/Pg) boundary at Byala (Black Sea shore, Eastern Bulgaria), is studied by Neutron Activation Analysis (NAA) in order to trace impact events. Iridium anomaly and a certain set of trace elements, together with biostratigraphic data, point to two possible impact events recorded: the first one (at the K/Pg boundary), linked to the giant Chicxulub impact in Mexico; and a second one (above the K/Pg boundary), possibly linked to the later in age, smaller and closer in distance, Boltysh impact in Ukraine.

A compound scenario for the end-Cretaceous mass extinctions

Two major controversies have arisen in research on the end-Cretaceous mass extinctions, concerning the extent to which they were sudden or gradual, and terrestrially or extraterrestrially induced. A review of recent work supports more or less gradual extinction for a number of terrestrial and marine groups such as dinosaurs and-ammonites, but the spectacular crash of the calcareous plankton and correlative ecological disaster in land plants in part of the northern hemisphere suggest a short-term catastrophic event. Chemical and physical signatures of the Cretaceous-Tertiary boundary, including iridium anomalies, shocked quartz, microspherules and carbon, oxygen and strontium isotopes, are discussed and the evidence for and against a bolide impact-induced or volcanic catastrophe is reviewed, with the conclusion that it is not yet decisive either way. Evidence for longer-term changes is also cited, with strontium-isotope data supporting that from stratigraphy in suggesting a significant fall in sea level shortly before the end of the Cretaceous. A large body of evidence also supports a latest Cretaceous fall in seawater and air temperature, but this has recently been disputed for western North America on the basis of leaf studies. It is concluded that both longer term causes, intrinsic to this planet, and a final catastrophe either involving bolide impact or volcanism on a spectacular scale, or perhaps a combination of the two, are required to account for the pattern of end-Cretaceous extinctions.

Timing and causes of forest fire at the K–Pg boundary

We report K–Pg-age deposits in Baja California, Mexico, consisting of terrestrial and shallow-marine materials re-sedimented onto the continental slope, including corals, gastropods, bivalves, shocked quartz grains, an andesitic tuff with a SHRIMP U–Pb age (66.12 ± 0.65 Ma) indistinguishable from that of the K–Pg boundary, and charred tree trunks. The overlying mudstones show an iridium anomaly and fungal and fern spores spikes. We interpret these heterogeneous deposits as a direct result of the Chicxulub impact and a mega-tsunami in response to seismically-induced landsliding. The tsunami backwash carried the megaflora offshore in high-density flows, remobilizing shallow-marine fauna and sediment en route. Charring of the trees at temperatures up to > 1000 °C took place in the interval between impact and arrival of the tsunami, which on the basis of seismic velocities and historic analogues amounted to only tens of minutes at most. This constrains the timing and causes of fires and the minimum distance from the impact site over which fires may be ignited.

Unraveling the record of a tropical continental Cretaceous-Paleogene boundary in northern Colombia, South America

The Cretaceous-Paleogene (K-Pg) boundary event, triggered by the Chicxulub asteroid impact in Mexico ∼66 Myr ago, is recognized as one of the major environmental crises in Earth's history. Its effects on the evolution of tropical vegetation were drastic, giving rise to the closed-canopy tropical rainforest. Despite of its importance, the actual K-Pg boundary event layer has not yet been identified in terrestrial sections within tropical latitudes. Here, we study a sediment core (Diablito-1) from northern Colombia and described its lithofacies, palynostratigraphic content, magnetic susceptibility and geochemical composition, including the concentration of key elements linked to a meteoritic contribution such as iridium. We found an 8.2 cm-thick (0.27 inches) interval with gravel-size calcareous intraclasts, broken skeletal fragments and siliceous spherules that were transported during a short-duration high-energy event into the coastal plain. Very fine-to medium-grained spherules, concentrated in several layers of the flow structure, could be interpreted as derived from the K-Pg impact event. This thin intraclastic packstone interval lies within the extinction level of the palynomorph Echimonocolpites franciscoi, an important biostratigraphic event associated with the K-Pg boundary in Colombia. In addition, this layer is associated with both a small iridium anomaly and high concentration of several other major and trace elements that are occurring above a low Magnetic Susceptibility (MS) interval and within the initial phase of a high MS interval. These lines of evidence support the position of the K-Pg event layer within the thin interval that includes the spherules in the Diablito-1 core.

Widespread elevated iridium in Upper Triassic\u2013Lower Jurassic strata of the Newark Supergroup: implications for use as an extinction marker

Anomalous levels of iridium in sedimentary strata are associated with mass extinction events caused by impact events. In the case of the end-Triassic extinction event, the anomalies as well as the extinctions are linked to the eruption of the Central Atlantic Magmatic Province (CAMP) flood basalts. We report new data on concentrations of iridium in continental strata of the Fundy, Deerfield, Hartford and Newark basins, both above and below the oldest CAMP flows in these basins, that demonstrate that these anomalies are more common than previously known. We conclude that the enrichments were at least in some instances likely derived locally by concentration due to leaching directly from the lavas into sediments proximal to the CAMP flows due to post-eruptive hydrothermal activity. In other instances, the enrichments likely record the global fallout of aerosols and/or ash particles during the eruptions of the CAMP basalts. The common association of the highest levels of enrichment with organic matter suggests either redox control or stabilization by formation of organometallic complexes following post-eruptive redistribution. These findings demonstrate the importance of considering the distribution and magnitude of iridium anomalies in considering the source of the iridium and possible extinction mechanisms.

Iridium profiles and delivery across the Cretaceous/Paleogene boundary

We examined iridium (Ir) anomalies at the Cretaceous/Paleogene (K/Pg) boundary in siliciclastic shallow marine cores of the New Jersey Coastal Plain, USA, that were deposited at an intermediate distance (∼2500 km) from the Chicxulub, Mexico crater. Although closely spaced and generally biostratigraphically complete, the cores show heterogeneity in terms of preservation of the ejecta layers, maximum concentration of Ir measured (∼0.1–2.4 ppb), and total thickness of the Ir-enriched interval (11–119 cm). We analyzed the shape of the Ir profiles with a Lagrangian particle-tracking model of sediment mixing. Fits between the mixing model and measured Ir profiles, as well as visible burrows in the cores, show that the shape of the Ir profiles was determined primarily by sediment mixing via bioturbation. In contrast, Tighe Park 1 and Bass River cores show post-depositional remobilization of Ir by geochemical processes. There is a strong inverse relationship between the maximum concentration of Ir measured and the thickness of the sediments over which Ir is spread. We show that the depth-integrated Ir inventory is similar in the majority of the cores, indicating that the total Ir delivery at time of the K/Pg event was spatially homogeneous over this region. Though delivered through a near-instantaneous source, stratospheric dispersal, and settling, our study shows that non-uniform Ir profiles develop due to changes in the regional delivery and post-depositional modification by bioturbation and geochemical processes.

ICHNOLOGICAL EVIDENCE FOR ENDOBENTHIC RESPONSE TO THE K–PG EVENT, NEW JERSEY, U.S.A.

Degree of bioturbation, Thalassinoides isp. morphology, and diameters were compared across the Cretaceous–Paleogene (K–Pg) boundary interval at three localities along the New Jersey coastal plain. Within this regionally extensive ichnoassemblage, mean burrow diameters decrease abruptly by 26–29% (n = 1767) at the base of the Main Fossiliferous Layer (MFL) or laterally equivalent horizons. The base of the MFL has been previously interpreted as the K–Pg boundary based on the last occurrence of Cretaceous marine reptiles, birds, and ammonites, as well as iridium anomalies and associated shocked quartz. Along with the mean, the maximum and minimum burrow diameters exhibit a negative shift, which indicates that the changes are the result of a directional reduction in diameter, rather than an artifact of decreased variance. As a proxy for the size of the tracemaker, a change in burrow diameter indicates a decrease in thalassinid crustacean body size. We interpret this shift as dwarfing within the endobenthic community as detrital food sources became scarce following the mass extinction. Despite the difference in size, there is no change in framework geometry. Ichnofabric indices generally increase up-section at each site across the K–Pg chronostratigraphic boundary, indicating a regional reduction in sedimentation rate, which is supported by a gradual increase in glauconite maturity. Overall, the ichnological evidence at these localities suggests that a prolonged period of negative feedback followed a short-term positive endobenthic response across the K–Pg boundary.

Iridium contents in the Late Cretaceous-Early Tertiary clays in relation to the K/T boundary, North Jordan

The mineralogy, lithology, and geochemistry of five discrete laminations across the K/T boundary of clayey shale at the Yarmouk River area, Jordan, were examined. There were no marked changes in the mineralogy of the clayey shale within the K/T boundary. This outcrop consists of more than 100 m of Maastrichtian oil shale overlying about 20 m limestone. Marly limestone included many clay laminations from organic and volcanic origins, which are considered an evidence of the K/T boundary through detected iridium anomalies. Any of these particular lamellae range from 2 mm to 5 mm in thickness. Smectite was the predominant clay mineral in smectitic shale laminations. It was located at eight meters above the K/T boundary and includes some anomalous concentrations of iridium and traces of other elements. The analysis of geochemical platinum group at the K/T boundary clays showed anomalous enrichments of iridium, compared with other carbonate rocks as a result of weathering processes of oil shale, or through concentration from weathering of basalt flows, but not pointing to an impact process. The clays in late Maastrichtian have Ir–Sc prevailed anomalies and synchronize with increasing of terrigenous and volcanogenic traced elements. Kaolin, smectite, and volkonskoite were the dominant clay minerals at the K/T boundary with high concentrations of iridium. The concentration levels of iridium in some laminations of the Yarmouk sediments ranged between 1.6 and 7.8 ppb.

The influence of extraterrestrial material on the late Eocene marine Os isotope record

A reconstruction of seawater 187Os/188Os ratios during the late Eocene (∼36–34Ma), based upon bulk sediment analyses from the sub-Antarctic Southern Atlantic Ocean (Ocean Drilling Program (ODP) Site 1090), Eastern Equatorial Pacific Ocean (ODP Sites 1218 and 1219) and the uplifted (land-based) Tethyan section (Massignano, Italy), confirms that the previously reported abrupt shift to lower 187Os/188Os is a unique global feature of the marine Os isotope record that occurs in magnetochron C16n.1n. This feature is interpreted to represent the change in seawater 187Os/188Os caused by the Popigai impact event. Higher in the Massignano section, two other iridium anomalies previously proposed to represent additional impact events do not show a comparable excursion to low 187Os/188Os, suggesting that these horizons do not record multiple large impacts. Comparison of records from three different ocean basins indicates that seawater 187Os/188Os begins to decline in advance of the Popigai impact event. At Massignano this decline coincides with a previously reported episode of elevated 3He flux, suggesting that increased influx of interplanetary dust particles (IDPs) contributed to the pre-impact shift in 187Os/188Os and not to the longer-term latest Eocene 187Os/188Os decline that occurred ∼1million year after the Popigai impact event.

Iridium anomaly in the cretaceous-paleogene boundary at Højerup (Stevns Klint, Denmark) and Woodside Creek (New Zealand): The question of an enormous proportion of extraterrestrial component

The Cretaceous-Paleogene boundary clays at H?jerup and Woodside Creek show anomalous enrichments of iridium compared with the marine sedimentary rocks. For the average iridium content of 465 ppb in CI chondrite the estimate of the carbonaceous chondritic proportions in the decarbonated iridium-rich boundary layers, based on the integrated iridium fluencies, is about 26% at H?jerup and 65% at Woodside Creek. These proportions are most likely too high due to a significant Ir influx from the nearby marine or continental site to these sections.

Prominent occurrence of iron oxides at KTB mass extinction: a review

Out of the five major mass extinction, which have taken place in the history of life, Cretaceous–Tertiary boundary (KTB) mass extinction was the 2nd most disastrous, the most severe being that at Permian–Triassic boundary (PTB). On the basis of iridium anomaly at a number of KTB sites it has been established that the cause of extinction was impact with an extraterrestrial bollide. The iron oxide/oxyhydroxide and iron minerals such as illite, phyllosilicate, jarosite etc. present in the rock samples from KTB form a major part. One can study different phases of iron oxide/oxyhydroxide present in the KTB samples with the help of Mossbauer spectroscopy. The Mossbauer studies showed that at KTB sites nanophase goethite and/or hematite are present without coexistence of their counterpart bulk size iron oxide/oxyhydroxide minerals. The amount of nanophase iron oxides well correlate with iridium concentration showing that the nanophase iron minerals are genetically related to the events at KTB. Thus these nanophase iron compounds were formed in the geochemical conditions created by the impact, and not by slow weathering. The iron mineralogy at some other extinction sites is found to be very similar to that at KTB although there is no iridium anomaly. This raises an interesting suggestion that iron mineralogy i.e. presence of nano-sized oxide and oxhydroxide particles alone without the existence of bulk iron-oxide in sedimentary layers can be used as impact markers. Studies carried out on some off-KTB rock samples also showed iron mineral composition similar to that at KTB without having iridium anomaly. This opens up the new questions whether these layers are also result of some large meteoritic impact? Keywords : Mossbauer spectroscopy, KT boundary, nanophase iron, iron mineralogy International Journal of Engineering, Science and Technology , Vol. 2, No. 8, 2010, pp. 110-117

Mechanisms and models of iridium anomaly shape across the Cretaceous–Paleogene boundary

The interpretation of the Cretaceous–Paleogene (K–Pg) iridium anomaly – and other impact ejecta – as the result of a single, large asteroid impact has been the subject of much debate, in part due to the distribution of impact markers beyond the narrow confines of the K–Pg boundary sedimentary layer. Here, we revisit the hypothesized processes leading to the shape of K–Pg iridium profiles including geochemical remobilization and/or diffusion, prolonged deposition, volcanism, multiple impacts, and sediment mixing. Using evidence from the literature and modeling of one North Pacific site, we find that sediment mixing of a single impact event provides the most parsimonious mechanism for iridium profile shape in open ocean oxic sediments, while the increase in background iridium bracketing the boundary likely has a volcanic origin. In some past studies, a sediment mixing mechanism for iridium profile shape was ruled out based on an overly simplified set of expectations for the effect of sediment mixing on markers of geologically instantaneous events. Thus, we introduce and use a Lagrangian sediment mixing model to illustrate the theoretical effects of mixing on records of rapid events. The sediment mixing origin for iridium anomaly shape, the correspondence in mixing extent between iridium and microfossils, and the fit of sediment mixing models to an empirical iridium profile indicate that iridium may provide a better tracer of mixing than previously proposed K–Pg mixing tracers such as Ni-spinels.

Relationship between mass extinction and iridium across the Cretaceous-Paleogene boundary in New Jersey

We directly link iridium (Ir) anomalies in New Jersey to the mass extinction of marine plankton marking the Cretaceous-Paleogene (K-Pg) boundary. We confirm previous reports of an Ir anomaly 20 cm below the extinction of Cretaceous macrofauna (the “ Pinna ” bed) with new results from a muddy sand section from Tighe Park, Freehold, New Jersey (United States), but we also show that Ir anomalies correlate with marine mass extinctions at three other clay-rich New Jersey sections. Thus, we attribute the anomaly at Freehold to the downward movement of Ir and reaffirm the link between impact and mass extinction.

Neutron activation analysis in geochemical characterization of Jurassic–Cretaceous sedimentary rocks from the Nordvik Peninsula

As a part of geophysical, stratigraphical and paleontological study aimed at precise correlation of the Jurassic-Cretaceous (J/K) boundary interval in the Tethyan and Boreal Realms, detailed magnetostratigraphic and biostratigraphic profiles with well-calibrated J/K boundary have been selected from several localities for geochemical characterization. Instrumental and radiochemical neutron activation analyses (INAA and RNAA, respectively) were employed in the characterization of a vertical profile around the supposed J/K boundary in the Boreal Realm situated on the Nordvik Peninsula, Northern coast of Middle Siberia (Laptev Sea), Russia, for which an iridium anomaly has been reported in literature. INAA enabled determination of about forty major and trace elements. An RNAA procedure for assaying platinum group elements (PGE) has been tested consisting in sample decomposition by alkaline-oxidative fusion, reduction of PGE, and pre- cipitation of their sulphides. Despite several difficulties, the procedure enabled to found Pt and Ir at ppb and sub-ppb levels, respectively, in several samples, namely in pyrite aggregates originated probably in diagenetic reductive processes following decomposition of burried organic matter. However, the existence of a pronounced PGE spike reported previously for the J/K boundary on the Nordvik Peninsula has not been confirmed.

Plants and the K–T Boundary

Plants and the K–T Boundary

Explosive bolide impact designates the Cambrian Explosion, terminating the Cambrian event in New York

In 1978 as chairman of the Geology and Geography Section of the American Association for the Advancement of Science (AAAS) I planned a program for the 1980 annual convention in San Franscisco, California. The 1980 program included Louis W. Alvarez (1911–1988) who reported that an asteroid 10 kilometers in diameter struck the earth at the end of the Cretaceous. This conclusion resulted from Alvarez and colleagues discovery in Gubbio, Europe, of a centimeter-thick clay layer among carbonate rock containing iridium, a siderophile element, at the Cretaceous-Tertiary boundary. Involvement in this discussion stimulated my interest in the study of Taconic carbonate deposits of my home and university setting in the Troy, New York and contiguous areas dating to the base of the Cambrian. Analyses of the New York carbonate samples gave comparable iridium results to those from the end of the Cretaceous in Europe. From these iridium anomalies I concluded that an extraterrestrial source, namely an asteroid, produced the iridium anomalies in the Cambrian of New York.

Peritidal sedimentary depositional facies and carbon isotope variation across K/T boundary carbonates from NW Adriatic platform

The K/T boundary in the Karst region (Italy and Slovenia) is well documented in correspondence to shallow water limestones. Its evidence is usually related to palaeontological data (disappearance of Cretaceous taxa and subsequent appearance of Tertiary ones), palaeomagnetism (Ch 29r) iridium anomaly and negative shift of δ 13C. In Italy, the Padriciano section highlights the K/T boundary included in the basal breccia of a peritidal cycle. Recent investigations brought to light a short well exposed new stratigraphic sequence containing the K/T boundary, 100 m east of Padriciano, as demonstrated by lithological, palaeontological and new isotopic data. The recording of a new peritidal setting denotes stressed environmental conditions. The findings of the biota mostly correspond to the lagoonal limestones of the peritidal cycle. It consists of small opportunistic thin shelled foraminifers, ostracods and gastropods. At Padriciano, the environmental stability occurred after episodes of strong fresh water influence. The intraclasts, oncolites and stromatolites have been observed in the Padriciano section. The broken shell fragments, algae and foraminifera have been found associated with fenestral fabric. These features indicate deposition in a shallow marine (intertidal to supratidal) environment. Algal boundstones and microbial (stromatolitic) laminae, crenulated domal laminae and crenulated conical laminae have been studied in petrographic sections for microstructures. The K/T boundary section has been analysed for the carbon and oxygen isotopic ratios. The δ 13C values range from − 3.62 to − 10.01‰ (V-PDB) and the δ 18O values range from − 3.85 to − 5.47‰ (V-PDB). This extreme depletion in the δ 13C record has already been reported from other K/T boundary sections both in the Padriciano area (Italy) and in Slovenia (Dolenja Vas sections) and may be related to global climate change but also to local environmental changes occurring in the NW Adriatic Platform.

THE HISTORY OF THE LAWRENCE BERKELEY NATIONAL LABORATORY INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS PROGRAMME FOR ARCHAEOLOGICAL AND GEOLOGICAL MATERIALS

The Lawrence Berkeley National Laboratory pottery provenance group developed standards and instrumental neutron activation analysis (INAA) methods that are used at many archaeometry laboratories around the world. The background and development of ‘Standard Pottery’ and of methods for INAA are described. Early pottery provenance studies are described, and other research programmes, involving obsidian and magmatic mixing, the origin of the stone used for the Colossi of Memnon, and the ‘Plate of Brass’, are mentioned. Research work by the Laboratory included the discovery of the world‐wide iridium anomaly and extensive subsequent research on what has come to be known as the ‘Asteroid Impact Theory’. Characteristics of the analytical programme for pottery provenance work, including overall aims, precision and accuracy, intercalibration, and irradiation and measurement protocols, are discussed. New research areas developed in the past 15 years, to broaden the usefulness of chemical compositional data for archaeological investigation, and examples of recent work, are described. This research, which makes use of high‐precision X‐ray fluorescence analysis in addition to INAA measurements on sample splits, includes distinguishing the products of different workshops located at the same production site, studies on the significance of the distribution of silver in archaeological pottery and the use of high‐precision chemical compositional data as an aid for making chronological distinctions.

Asteroid mega-impacts and Precambrian banded iron formations: 2.63 Ga and 2.56 Ga impact ejecta/fallout at the base of BIF/argillite units, Hamersley Basin, Pilbara Craton, Western Australia

Abstract The temporal association between late Archaean to earliest Proterozoic asteroid impact ejecta/fallout units and overlying banded iron formations suggests that, in some instances, these impacts were closely followed by significant transformation in the nature of source terrains of the sediments. The Jeerinah Impact Layer (JIL) [B.M. Simonson, D. Davies, S.W. Hassler, Discovery of a layer of probable impact melt spherules in the late Archean Jeerinah Formation, Fortescue Group, Western Australia. Aust. J. Earth Sci. 47 (2000) 315–325; B.M. Simonson, S.W. Hassler, Revised correlations in the early Precambrian Hamersley Basin based on a horizon of resedimented impact spherules. Aust. J. Earth Sci. 44 (1997) 37–48; B.M. Simonson, B.P. Glass, Spherule layers — records of ancient impacts. Ann. Rev. Earth Planet. Sci. 32 (2004) 329–361; A.Y. Glikson, Early Precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47–2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. Astrobiology 4 (2001) 19–50; S.W. Hassler, B.M. Simonson, D.Y. Sumner, D. Murphy, Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups, Western Australia: stratigraphic and depositional implications of re-correlation. Aust. J. Earth Sci. 52 (2005) 759–772; B. Rasmussen, C. Koeberl, Iridium anomalies and shocked quartz in a late Archean spherule layer from the Pilbara Craton: new evidence for a major asteroid impact at 2.63 Ga. Geology 32 (2004) 1029–1032; B. Rasmussen, T.S. Blake, I.R. Fletcher, U–Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah–Carawine impact ejecta layer. Geology, 33 (2005) 725–728.] overlies an argillite-dominated unit (Jeerinah Formation, 2684 ± 6 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and lies directly below a thin volcanic tuff (2629 ± 5 Ma, [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and banded iron formation (BIF) (upper part of Marra Mamba Iron Formation, 2597 ± 5 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]). The Spherule Marker Bed (SMB) [B.M. Simonson, Geological evidence for an early Precambrian microtektite strewn field in the Hamersley Basin of Western Australia. Geol. Soc. Am. Bull. 104 (1992) 829–839; B.M. Simonson, S.W. Hassler, K.A. Schubel, Lithology and proposed revisions in stratigraphic nomenclature of the Wittenoom Formation (Dolomite) and overlying formations, Hamersley Group, Western Australia. Geol. Surv. W. Aust. Rep. 345 (1993) 65–79; S.W. Hassler, B.M. Simonson, D.Y. Sumner, D. Murphy, Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups, Western Australia: stratigraphic and depositional implications of re-correlation. Aust. J. Earth Sci. 52 (2005) 759–772. [5] ], which includes two impact cycles [A.Y. Glikson, Early Precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47–2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. Astrobiology 4 (2001) 19–50.], is located at the top of a carbonate/calcareous siltstone-dominated sequence (Bee Gorge Member, Wittenoom Formation, 2565 ± 9 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and below a carbonate-poor siltstone–chert–BIF sequence (Mount Sylvia Formation, Bruno's Band [BIF], Mount McRae Shale, 2504 ± 5 Ma [B. Rasmussen, T.S. Blake, I.R. Fletcher, U–Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah–Carawine impact ejecta layer. Geology, 33 (2005) 725–728.]). No ferruginous sediments overlie impact layers hosted by stromatolitic carbonates (

Asteroid/comet impact clusters, flood basalts and mass extinctions: Significance of isotopic age overlaps

Morgan et al. [J. Phipps Morgan, T.J. Reston, C.R. Ranero. Earth Planet. Sci. Lett. 217 (2004) 263–284.], referring to an overlap between the isotopic ages of volcanic events and four epoch/stage extinction boundaries, suggest a dominant role of Continental Flood Basalts (CFB) and of explosive CO 2-rich volcanic pipes (“Verneshots”) as mass extinction triggers. Here I point out that Morgan et al. overlook 3 overlaps between the ages of extraterrestrial impacts, volcanic and mass extinction events, and 3 overlaps between the ages of extraterrestrial impact and volcanic events. These overlaps suggest that both extraterrestrial impacts and volcanism served as extinction triggers separately or in combination. A protracted impact cluster overlaps extinctions at the end-Devonian (∼374–359 Ma) and impact-extinction age overlaps occur in the end-Jurassic (∼145–142 Ma), Aptian (∼125–112 Ma); Cenomanian–Turonian (∼95–94 Ma); K–T boundary (∼65.5 Ma) and mid-Miocene (∼16 Ma) ( Table 1). Morgan et al. appear to question the uniqueness of shock metamorphic and geochemical criteria used to identify asteroid/comet impacts. However, shock pressures at 8–35 GPa, indicated by intra-crystalline planar deformation features (PDF), exceed lithospheric and volcanic explosion pressures by an order of magnitude and are not known to be associated with explosive volcanic diatremes, kimberlites or lamproites. These authors make reference to apparent iridium anomalies of volcanic origin. However, platinum group element (PGE) abundance levels, volatile/refractory PGE ratios, and Cr and Os isotopes of meteoritic materials are clearly distinct from those of terrestrial volcanics. Given a Phanerozoic time-integrated oceanic/continent crustal ratio > 2.5 and the difficulty in identifying oceanic impacts, I suggest the effects of large impacts on thin thermally active oceanic crust–capable of triggering regional to global mafic volcanic events and ensuing environmental effects–provide an essential clue for understanding the relationships between impacts and volcanic events which, separately or in combination, result in deleterious environmental effects, in some instances leading to mass extinctions.