The breakup of the L-chondrite parent body 466 Ma and its terrestrial effects – a search for a mid-Ordovician biodiversity event

About a third of all meteorites that fall on Earth today, the stony L-chondrites, originate from a major breakup event in the asteroid belt 466 Ma, in the early Darriwilian. This is the largest asteroid breakup in the past three billion years documented by K-Ar gas-retention ages of recently fallen meteorites. There has been a debate whether the breakup had any effects on Earthâs biota. Based mainly on brachiopod data from western Russia, some authors have argued for the existence of a major biodiversity âeventâ at approximately the time of the L-chondrite breakup. An analysis of the distribution of three fossil groups (conodonts, ostracods and trilobites) across the late Dapingian and early Darriwilian in three sections in southern Sweden shows no evidence of any biodiversity event. The only biotic changes outside a normal trend are those related to a sea-level fall following the arrival of large amounts of dust from the asteroid breakup. We conclude that the Great Ordovician Biodiversification Event represents a sequence of changes over about 20 Myr, coinciding with an asteroid shower from the breakup of the L-chondrite parent body

Unusual sources of fossil micrometeorites deduced from relict chromite in the small size fraction in ~467 Ma old limestone

Extraterrestrial chrome spinel and chromite extracted from the sedimentary rock record are relicts from coarse micrometeorites and rarely meteorites. They are studied to reconstruct the paleoflux of meteorites to the Earth and the collisional history of the asteroid belt. Minor element concentrations of Ti and V, and oxygen isotopic compositions of these relict minerals were used to classify the meteorite type they stem from, and thus to determine the relative meteorite group abundances through time. While coarse sediment-dispersed extraterrestrial chrome-spinel (SEC) grains from ordinary chondrites dominate through the studied time windows in the Phanerozoic, there are exceptions: We have shown that ~467 Ma ago, 1 Ma before the breakup of the L chondrite parent body (LCPB), more than half of the largest (>63 μm diameter) grains were achondritic and originated from differentiated asteroids in contrast to ordinary chondrites which dominated the meteorite flux throughout most of the past 500 Ma. Here, we present a new data set of oxygen isotopic compositions and elemental compositions of 136 grains of a smaller size fraction (32–63 μm) in ~467 Ma old pre-LCPB limestone from the Lynna River section in western Russia, that was previously studied by elemental analysis. Our study constitutes the most comprehensive oxygen isotopic data set of sediment-dispersed extraterrestrial chrome spinel to date. We also introduce a Raman spectroscopy-based method to identify SEC grains and distinguish them from terrestrial chrome spinel with ~97% reliability. We calibrated the Raman method with the established approach using titanium and vanadium concentrations and oxygen isotopic compositions. We find that ordinary chondrites are approximately three times more abundant in the 32–63 μm fraction than achondrites. While abundances of achondrites compared to ordinary chondrites are lower in the 32–63 μm size fraction than in the >63 μm one, achondrites are approximately three times more abundant in the 32–62 μm fraction than they are in the present flux. We find that the sources of SEC grains vary for different grain sizes, mainly as a result of parent body thermal metamorphism. We conclude that the meteorite flux composition ~467 Ma ago ~1 Ma before the breakup of the LCPB was fundamentally different from today and from other time windows studied in the Phanerozoic, but that in contrast to the large size fraction ordinary chondrites dominated the flux in the small size fraction. The high abundance of ordinary chondrites in the studied samples is consistent with the findings based on coarse extraterrestrial chrome-spinel from other time windows

Late Eocene 3He and Ir anomalies associated with ordinary chondritic spinels

Abstract During the late Eocene there was an enigmatic enhancement in the flux of extraterrestrial material to Earth. Evidence comes from sedimentary 3He records indicating an increased flux of interplanetary dust during ca. 2 Myr, as well as two very large impact structures, Popigai (100 km diameter) and Chesapeake Bay (40–85 km), that formed within 10–20 kyr at the peak of the 3He delivery. The Massignano section in Italy has one of the best sedimentary records of these events, including a well-defined 3He record, an Ir-rich ejecta bed related to the Popigai impact event, and two smaller Ir anomalies. Recently we showed that the Popigai ejecta is associated with a significant enrichment of chromite grains (>63 μm) with an H-chondritic elemental composition (17 grains in 100 kg of rock). Most likely these grains are unmelted fragments from the impactor. Slightly higher up (ca. 20 cm) in the section, where a small Ir anomaly possibly related to the Chesapeake Bay impact has been measured, we found a weak enrichment in L-chondritic grains (8 grains in 208 kg of rock). Here we report an extended data set increasing the total amount of sediment dissolved in acid and searched for extraterrestrial chromite grains from 658 to 1168 kg. In altogether 760 kg of background sediment from 17 levels over 14 m of strata outside the interval corresponding to the Popigai and Chesapeake Bay impacts, we only found 2 extraterrestrial chromite grains. Both grains have L-chondritic compositions and were found in a 100 kg sample from the ca. 10.25 m level in the section where the second of the smaller Ir anomalies has been reported. A correlation appears to exist between Ir, 3He and chromite from ordinary chondrites. We also report oxygen three-isotope measurements of the extraterrestrial chromite grains associated with the Popigai ejecta and confirm an H-chondritic composition. The new results strengthen our scenario that the upper Eocene 3He and Ir enrichments originate from the asteroid belt rather than the Oort cloud as originally proposed when the 3He anomaly was discovered. The generally low background concentrations of extraterrestrial chromite through the section speak against any major single asteroid breakup event such as in the mid-Ordovician after the break-up of the L-chondrite parent body. Instead the data reconcile with a small, possibly a factor of 2–3, increase in the flux of extraterrestrial material to Earth, but of both H- and L-chondritic composition. We also report the composition of all the 2310 terrestrial chrome spinel grains recovered, and show that their chemical composition indicates a dominantly regional ophiolitic source. Four anomalous chrome spinel grains with high Ti and V concentrations were found in the Popigai ejecta. These grains originate from Siberian Traps basalts in the Popigai crater at the time of impact

Exceptionally Preserved Cambrian Trilobite Digestive System Revealed in 3D by Synchrotron-Radiation X-Ray Tomographic Microscopy

The Cambrian ‘Orsten’ fauna comprises exceptionally preserved and phosphatised microscopic arthropods. The external morphology of these fossils is well known, but their internal soft-tissue anatomy has remained virtually unknown. Here, we report the first non-biomineralised tissues from a juvenile polymerid trilobite, represented by digestive structures, glands, and connective strands harboured in a hypostome from the Swedish ‘Orsten’ fauna. Synchrotron-radiation X-ray tomographic microscopy enabled three-dimensional internal recordings at sub-micrometre resolution. The specimen provides the first unambiguous evidence for a J-shaped anterior gut and the presence of a crop with a constricted alimentary tract in the Trilobita. Moreover, the gut is Y-shaped in cross section, probably due to a collapsed lumen of that shape, another feature which has not previously been observed in trilobites. The combination of anatomical features suggests that the trilobite hypostome is functionally analogous to the labrum of euarthropods and that it was a sophisticated element closely integrated with the digestive system. This study also briefly addresses the preservational bias of the ‘Orsten’ fauna, particularly the near-absence of polymerid trilobites, and the taphonomy of the soft-tissue-harbouring hypostome

Review of uppermost Furongian trilobites from Scania, Southern Sweden, based on type material

The Scandinavian trilobites described in the classical papers by Moberg and Moller (1898) and Moberg (1898) are reviewed, based on the original type collections. The ten species and subspecies originally described are here considered to represent 12 species and subspecies, ranging in age from the latest Furongian (late Cambrian) through the Tremadocian (Early Ordovician). Lectotypes are selected for Parabolina (Parabolina) acanthura (Angelin), Acerocare ecorne Angelin and Westergaardia lata (Matthew). Leptoplastus claudicans (Moberg and Moller) is regarded as a junior subjective synonym of L. stenotus Angelin. These, and lectotypes formerly only figured by hand-drawings, are illustrated by high-resolution photographs

Upper middle Cambrian through Furongian of Scandinavia with focus on trilobites, paleoenvironments and correlations

This thesis is based on studies of upper middle Cambrian through Furongian strata in Scandinavia with focus on trilobite biostratigraphy, taxonomy, paleoecology, and intercontinental correlation. The material studied derives mainly from Västergötland and Scania, southern Sweden. Three new trilobite species are described: Macropyge (Promacropyge) scandinavica, Elkanaspis kinnekullensis, and Parabolinella sandtorpensis. The former two proved important for correlations with China and North America, respectively, whereas the latter is important for our understanding of the systematic position of the genus Parabolinella. All these species were recovered from Mount Kinnekulle, Västergötland, Sweden. The strata studied at Kinnekulle show features here interpreted as indicative of an intrabasinal paleohigh, including current-oriented fossils, the presence of thin sand wedges, the large proportion of limestone, different limestone lithologies, and gaps in the succession. This is in contrast to the biostratigraphically complete, upper middle Cambrian through Furongian successions in Scania, where the strata are unaffected by water movements and reflect a deeper water setting on the outer shelf. The succession in Scania is, however, punctuated by five major fossil-barren intervals. The upper part of such an interval, in the uppermost middle Cambrian Agnostus pisiformis Zone, yielded phosphatocopines, protoconodont elements, and enigmatic trace fossils. Thus, these intervals, or at least parts of them, are, not necessarily barren of fossils, but of the normal, trilobite-dominated faunas. Moreover, the phosphatocopine-dominated and trilobite-dominated intervals represent two interdigitating biofacies of which the former, anomalous facies, correlates with a trilobite extinction event in North America and the onset of the globally recognizable Steptoean Positive Carbon Isotope Excursion (SPICE). The lowermost Furongian biozone in Scandinavia, the Olenus & A. (Homagnostus) obesus Zone, has here been replaced by two new zones; the globally recognized Glyptagnostus reticulatus Zone and the Olenus Zone. A drill core comprising middle Cambrian (Ptychagnostus atavus Zone) to Furongian (Parabolina spinulosa Zone) strata from Andrarum, Scania, southernmost Sweden, was sampled for ?13Corg-analysis. The resulting curve has a distinct positive ?13Corg excursion corresponding to the SPICE. This is the first time that the SPICE has been recorded in Baltica and from organic matter in an alum shale setting. A study from another drill core comprising upper Furongian strata recovered from c. 2500 m depth at Håslöv, in the southwesternmost tip of Scania, revealed a succession comparable to other sections and cores in Scania regarding biostratigraphy, lithology and the presence of major barren intervals. Moreover, from this drilling the conodont Cordylodus proavus Müller? was recovered from non-reworked sediments for the first time in Sweden. A review of 10 species and subspecies of upper Furongian trilobites originally described by Moberg and Möller (1898) and Moberg (1898), revealed that they, in fact, represent 12 species and subspecies. One species name was regarded as a junior subjective synonym, and lectotypes for three species were selected

Upper Cambrian trilobite biostratigraphy and taphonomy at Kakeled on Kinnekulle, Vastergotland, Sweden

A section through the Upper Cambrian black shales and limestones at Kakeled on Kinnekulle, Vastergotland, Sweden, extends from the lower-middle part of the Agnostus pisiformis Zone into the Peltura scarabaeoides Zone. Fossils are usually preserved only in the stinkstones, but in the A. pisiformis Zone trilobites can be found also in the shales. Lithologically, the stinkstones can be subdivided into primary coquinoid limestone, which include the majority of the fossils, and early diagenetically formed limestone. The orientation of cephala and pygidia of A. pisiformis were measured on four shale surfaces and one stinkstone surface. The majority of the shields were deposited with the convex side up and showed a preferred orientation, suggesting that their positions were affected by currents. Above the A. pisiformis Zone the section comprises the Olenus/Homagnostus obesus Zone (0.30 m), the upper part of the Parabolina spinulosa Zone (0.05 m), the Peltura minor Zone (1.15 m), and the Peltura scarabaeoides Zone (2.50 m). The Leptoplastus and Protopeltura praecursor zones are missing. The Olenus/H. obesus Zone is represented only by the O. gibbosus and O. wahlenbergi subzones, whereas the O. truncatus, O. attenuatus, O. dentatus, and O. scanicus subzones are missing

Upper Cambrian trilobite faunas and biostratigraphy at Kakeled on Kinnekulle, Västergötland, Sweden

Upper Cambrian strata are well exposed in an old quarry at Kakeled on south-western Kinnekulle, south-central Sweden. The exposures consist of finely laminated alum shale with lenses and beds of dark grey limestone (stinkstone or orsten), and a few thin layers of sandstone. A 6.20 m thick section in the central part of the quarry has been measured and sampled in order to establish the succession of trilobite species. This section extends from the lower-middle part of the Agnostus pisiformis Zone into the Peltura scarabaeoides Zone. A 150 cm thick stinkstone bed ("The Great Stinkstone Bed") occurs in the Olenus/Homagnostus obesus Zone and in the upper part of the A. pisiformis Zone. It comprises a lower part measuring 30 cm and an upper part measuring 120 cm. These are separated by a thin (10 cm) bed of alum shale. Above "The Great Stinkstone Bed" nine additional stinkstone beds, with thicknesses varying from 10 to 55 cm, are present. Fossils are usually preserved only in the stinkstones, but in the A. pisiformis Zone trilobites can be found in both the shales and the stinkstones. The A. pisiformis Zone is dominated almost entirely by the zonal index, which occurs in abundance. A. pisiformis occurs throughout the A. pisiformis Zone and ranges up into the Olenus gibbosus Subzone. Nearly all specimens of A. pisiformis are disarticulated. The orientation of cephala and pygidia of A. pisiformis were measured on four shale surfaces and one stinkstone bedding plane. The majority of the shields were deposited with the convex side up and showed a preferred orientation, suggesting that their positions were affected by currents. The orientation of the shields generally indicates a current direction roughly towards the present north. Above the A. pisiformis Zone the section comprises the Olenus/Homagnostus obesus Zone (30 cm), the upper part of the Parabolina spinulosa Zone (5 cm), the Peltura minor Zone (105 cm), and the Peltura scarabaeoides Zone (260 cm). The Leptoplastus and Protopeltura praecursor Zones are missing. The Olenus/H. obesus Zone is represented only by the O. gibbosus and O. wahlenbergi Subzones, whereas the O. truncatus, O. attenuatus, O. dentatus, and O. scanicus Subzones are missing.Populärvetenskaplig sammanfattning: I Kakeled på Kinnekulles sydvästsluttning, Västergötland, finns en blottad lagerföljd från överkambrium. Överkambrium är en tidsperiod som sträckte sig från ca 525 till 500 miljoner år sedan. Avsättningen i det överkambriska havet var extremt långsam (1-10 mm/1000 år). Bottenförhållandena var mer eller mindre syrefria, varför nedbrytningen av det organiska materialet var ofullständigt. Dessa förutsättningar gav upphov till den så kallade alunskiffern som är en svart skiffer med upp till 25 % organiskt material som lätt klyver sig i tunna skivor. Lagerföljden med alunskiffer innehåller också linser och lager med oren kalksten, s.k. orsten. Alunskifferlagerföljderna är ofta mycket rika på fossil, men faunorna är inte särskilt mångformiga pga. av den extrema avsättningsmiljön. Bland fossilen märks i synnerhet trilobiterna. Denna djurgrupp tillhör leddjuren och är det dominerande inslaget i faunor från kambrium. De hade ett hårt yttre skelett på ryggsidan som lätt bevarades som fossil. Trilobiterna kan användas för att fastställa lagernas inbördes åldersförhållande, s.k. biostratigrafi. En biostratigrafisk undersökning har gjorts på lagerföljden i Kakeled. Denna undersökning har resulterat i att fem zoner och sex subzoner har kunnat urskiljas här. Den traditionella uppfattningen om alunskifferns avsättningsmiljö har varit att den avsatts på relativt djupt vatten där strömmar och andra vattenrörelser inte förekommit. För att se om denna uppfattning stämmer gjordes en undersökning gällande orienteringen hos en trilobits (Agnostus pisiformis) skal. Denna trilobit förekommer i stor mängd i A. pisiformis-zonen och täcker ofta hela alunskifferytor. Vid en bottenmiljö fri från strömmar där endast gravitationen påverkar det nedfallande materialet, kan det förväntas att skalen avsätts med den konvexa sidan ner och med en slumpmässig orientering. På de fem ytor (fyra alunskifferytor och en orstensyta) som undersöktes visade det sig att majoriteten skal var avsatta med den konvexa sidan upp och att de dessutom hade en generell orientering som indikerade en strömriktning mot norr. Detta pekar starkt på att avsättningsmiljön var påverkat av vattenrörelser

Asteroid break-ups and meteorite delivery to Earth the past 500 million years

The meteoritic material falling on Earth is believed to derive from large break-up or cratering events in the asteroid belt. The flux of extraterrestrial material would then vary in accordance with the timing of such asteroid family-forming events. In order to validate this, we investigated marine sediments representing 15 time-windows in the Phanerozoic for content of micrometeoritic relict chrome-spinel grains (>32 μm). We compare these data with the timing of the 15 largest break-up events involving chrome-spinel–bearing asteroids (S- and V-types). Unexpectedly, our Phanerozoic time windows show a stable flux dominated by ordinary chondrites similar to today’s flux. Only in the mid-Ordovician, in connection with the breakup of the L-chondrite parent body, do we observe an anomalous micrometeorite regime with a two to three orders-of-magnitude increase in the flux of L-chondritic chrome-spinel grains to Earth. This corresponds to a one order-of-magnitude excess in the number of impact craters in the mid-Ordovician following the L-chondrite break-up, the only resolvable peak in Phanerozoic cratering rates indicative of an asteroid shower. We argue that meteorites and small (<1-km-sized) asteroids impacting Earth mainly sample a very small region of orbital space in the asteroid belt. This selectiveness has been remarkably stable over the past 500 Ma