arenigian early ordovician sea level history and the response of conodont communities western newfoundland four cluster analyses were performed which recognized 17 conodont communities in the arenigian lower ordovician of western newfoundland the analyses include 69 598 identifiable conodont specimens recovered from 153 conodont bearing samples from four stratigraphical sections representing the environmental settings of the platform upper proximal slope lower proximal slope and distal slope the distribution of conodont communities along the platform to slope environmental gradient shows that sea level changes simultaneously affected the development and replacement of the conodont communities in the different facies the pattern of change in conodont communities allows an interpretation of sea level change that is correlated precisely into the detailed graptolite biozonation a gradual transgression lasted most of tetragraptus approximatus zone time which was followed by a brief regression a transgression regression cycle occurred in the t akzharensis zone time a major transgression caused a highst

the classification of micrometeorites due to their small size the mineralogical and chemical properties of micrometeorites mms are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites identifying which groups of mm are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical textural and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies albeit with minimal bias towards preconceived genetic models specifically the interpretation of mms requires a rigorous and meaningful classification scheme at present the classification of mms is however at best ambiguous a unified petrological chemical classification scheme is proposed in the current study and is based on observations of several thousand mms collected from antarctic ice

asteroid impact ejecta units overlain by iron rich sediments in 3 5 2 4 ga terrains pilbara and kaapvaal cratons accidental or cause effect relationships abstract the significance of temporal and spatial associations between asteroid comet impact ejecta units and overlying iron rich sediments including banded iron formation bif jaspilite and ferruginous shale observed in the pilbara craton western australia and the barberton greenstone belt bgb kaapvaal craton south africa is considered such associations include 1 3470 1 1 9 ma impact spherule units and tsunami type breccia overlain by jaspilite in the antarctic chert member central pilbara craton 2 3258 3 ma impact spherule unit s2 in the bgb overlain by bif jaspilite and ferruginous shale 3 3243 4 ma impact unit s3 in the bgb overlain by iron rich sediments ulundi formation 4 two formations of bif and ferruginous shale at the base nimmingarra iron formation and lower part paddy market formation of the gorge creek group central pilbara craton which overlie 3235 3 ma felsic volcanics along a boundary correlated with the bgb s3 unit 5 2629 5 ma impact spherule unit and tsunami type deposit overlain by bif and ferruginous shale marra mamba iron formation central pilbara craton 5 2481 4 ma spherule unit intercalated at the lower part of the dales gorge iron member of the brockman iron formation hamersley basin and possible equivalents in the kuruman formation western transvaal no significant thicknesses of iron rich sediments are known to overlie carbonate hosted impact ejecta units including the bee gorge member bgm of the wittenoom formation 2561 8 ma carawine dolomite eastern hamersley basin pilbara craton monteville formation and reivilo formation griqualand west basin transvaal group or the graensco vallen spherule occurrence southwest greenland the juxtaposition between impact ejecta units and overlying bif and jaspilite may be accidental or purely related to preservation of these units in below wave base environments alternatively this association may hint at enrichment of seawater in soluble iron under low oxygen atmosphere hydrosphere conditions due to enhanced denudation of mafic volcanic terrains uplifted exposed through impact induced tectonic movements and or impact triggered mafic volcanic and hydrothermal activity the general absence of iron rich sediments above carbonate hosted impact ejecta units may be attributed to lack of fe enrichment or to ph and eh conditions unsuitable for the precipitation of iron oxides the near absence of bif in high energy shallow water environments overlying impact ejecta units may reflect retardation of colloidal chemical sedimentation in such environments a systematic association between impact ejecta and iron rich sediments if confirmed may yield useful stratigraphic tracers in the search for impact signatures in early precambrian terrains

the leonard award address presented 1996 july 25 berlin germany the elemental composition of stony cosmic spherules five hundred stony cosmic spherules collected from deep sea sediments polar ice and the strato sphere have been analyzed for major and some minor element composition typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids the samples are small and modified by atmospheric entry but they are an important source of information on the composition of asteroids the spherules in this study were all analyzed in an identical manner and they provide a sampling of the solar system s asteroids that is both different and less biased than provided by studies of conventional meteorites volatile elements such as na and s are depleted due to atmospheric heating while siderophiles are depleted by less understood causes the refractory nonsiderophile elements appear not to have been significantly dis turbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the earth typical spherules have cm like composition that is distinctively different than ordinary chondrites and most other meteorite types we assume that c type asteroids are the primary origin of spherules with this composition type s asteroids should also be an important source of the spherules and the analysis data provide constraints on their composition a minor fraction of the spher ules are melt products of precursor particles that did not have chondritic elemental compositions the most common of these are particles that are dominated by olivine the observed compositions of spherules are in consistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry

micrometeorite flux on earth during the last 50 000 years the flux of micrometeorites is estimated by using cosmic spherule counts from a seafloor area of 2 50 m 2 from the indian ocean the spherules are recovered from sediment samples in close spaced locations from the indian ocean after sieving 293 kg of sediment the terrestrial age of the spherules has a range of 0 50 000 years the spherules have a size range of 57 750 m average size 265 92 m the diameter of the spherules increases from scoriaceous barred cryptocrystalline glassy types the time averaged flux of the spherules is 160 tonnes yr a sizeable mass 60 resides in the 300 m fraction the slope of distribution is similar to that of deepsea spherules dss but significantly different from other collections which have lower average diameters it is observed that a significant population of cosmic dust resides in the higher sizes which can be recovered by sampling large areas in time and space the proportions of textural types observed are similar to that of unbiased collections this collection comprises particles that are larger than other unbiased collections and the proportion of textural types does not vary with size further this collection samples the flux over a longer time span indicating that the textural types of cosmic dust that have been raining on the earth during the last 50 ky have been constant regardless of size major element chemistry of a majority of the spherules shows elemental ratios that are close to a cm or ci chondritic parent body a single spherule 0 2 of the population suggests an achondritic parent body large unbiased collections spanning large areas temporally and spatially expand the inventory of the earth crossing meteoroid complex and provide valuable inputs for models on cosmic dust accretion

spinel from archean impact spherules abstract large meteorite impacts are recorded in distal fall deposits preserved in two early archean greenstone belt sequences 3 2 3 5 ga mapping stratigraphie and geochemistry studies have been used to identify at least four impact layers in the barberton greenstone belt of southern africa and one in the pilbara greenstone belt of western australia spherules that represent impact melt droplets distinguish these impact layers from associated tuffs spinel with unusual compositional traits occurs within spherules in two of these impact layers spinel is the only preserved primary phase and is found within up to 10 of the spherules in the best preserved layers some spherules contain up to 40 spinel which is often the only primary phase that crystallized within the spherules spinel morphologies include dendrites and skeletal octahedra typical of those found in high temperature melts spinel compositions are similar to those of cenozoic impacts including very high ni and ferric iron contents but the archean spinel is distinctive in being composed dominantly of the chalcophile elements fe ni cr and v this spinel is not metamorphic or metasomatic in origin closely associated komatiites in the underlying onverwacht group both fresh and highly altered have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel characteristics of spinel preserved in these archean impact spherules suggest a rather complex series of steps following a large bolide impact initial high temperature reduction produced melt droplets some of which were immiscible silicate metal sulfide density differences allowed separation of some of these immiscible melts which subsequently evolved independently and formed spherules with highly variable chalcophile element contents oxidation during atmospheric reentry increased ni fe and fe 3 fe 2 in the chalcophile siderophile melts archean impact spinel has higher ni fe than cenozoic impact spinel though distinctly lower fe 3 fe 2 which probably reflects a substantially less oxidizing atmosphere during the archean the archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric h 2 o h 2 bolide size is estimated to be 24 km in diameter based on the average diameter of 0 85 mm for the spinel bearing spherules this estimate is consistent with the observed ir fluence in spinel bearing impact layers of approximately 5 3 mg cm2 which would be produced by a 30 km diameter chondritic bolide

micrometer and nanometer sized platinum group nuggets in micrometeorites from deep sea sediments of the indian ocean abstract we examined 378 micrometeorites collected from deep sea sediments of the indian ocean of which 175 180 and 23 are i type s type and g type respectively of the 175 i type spherules 13 contained platinum group element nuggets pgns the nuggets occur in two distinct sizes and have distinctly different elemental compositions micrometer m sized nuggets that are 3 m contain dominantly ir os and ru iridium platinum group element or ipge and sub m or nanometer sized 1 m nuggets which contain dominantly pt rh and pd palladium pge or ppge the m sized nuggets are found only one per spherule in the cross section observed and are usually found at the edge of the spherule by contrast there are hundreds of nanometer sized nuggets distributed dominantly in the magnetite phases of the spherules and rarely in the wustite phases both the nugget types are found as separate entities in the same spherule and apparently nugget formation is a common phenomenon among i type micrometeorites however the m sized nuggets are seen in fewer specimens 2 5 of the observed i type spherules in all we analyzed four nuggets of m size and 213 nanometer sized nuggets from 13 i type spherules for platinum group elements chemically the m sized pgns contain chondritic ratios of os ir but are depleted in the more volatile pge pt rh and pd relative to chondritic ratios on the other hand the nanometer sized nuggets contain dominantly pt and rh importantly the refractory pges are conspicuous by their absence in these nanometer nuggets palladium the most volatile pge is highly depleted 1 1 with respect to chondritic ratios in the m sized pgns and is observed in only 17 of 213 nanometer nuggets with concentrations that are just above the detection limit 0 2 distinct fractionation of the pge into ipge ir os ru and ppge seems to take place during the short span of atmospheric entry these observations suggest several implications 1 the observation of fractionated pge in an fe ni system gives rise to the possibility that earth s core could contain fractionated pge 2 the present data support the processes suggested for the fractionated pge patterns observed in the ejecta of ancient meteorite impacts 3 meteoric metals released in the troposphere could contain fractionated pgns in large numbers

fractionation and fragmentation of glass cosmic spherules during atmospheric entry glass cosmic spherules are a sub type of s type cosmic spherules that are almost at the end phase of fractionation and volatilization during atmospheric entry a focused investigation of 36 glass spherules out of several hundred spherules reveals fractionation and fragmentation phenomena during atmospheric entry some of the glass spherules largely preserve relict pyroxene normative crystals within the spherules appear to have experienced temperatures of 1700 o c that resulted in vaporization of fe and other volatile elements such as s na and k from the silicate spherule resulting in the enrichment of sio2 mgo and cao in weight percent calculations with a chemical ablation model suggest an entry velocity of 11 16 km s to achieve the temperature required for glass spherule formation under these conditions the micrometeoroid undergoes a mass of loss of 20 any increase in entry velocity will increase the mass loss drastically and change its chemical composition most glass spherules beyond a certain threshold diameter of 300 m appear to undergo fragmentation resulting in two different spherules which could lead to an apparent enhancement of the percentage of glass spherules in unbiased collections siderophile and volatile element loss and subsequent development of a platinum group element pge nugget are observed in one of the spherules this appears to be the first reported observation of a platinum group nugget pgn in a glass spherule the nugget has a diameter of 3 m and contains all the six pge although some of the more volatile pge are depleted relative to an average chondritic composition interestingly the pge data of the nugget seem to have a trend which is similar to that of a pge nugget from a calcium aluminium rich inclusion cai the trace element data of these glass spherules suggest that their parent bodies were carbonaceous chondrites

an increased abundance of micrometeorites on earth owing to vesicular parachutes micrometeorites mms are extraterrestrial dust particles that survive atmospheric entry and can be recovered from sedimentary rocks fossil mms allow events beyond the earth such as the collisional breakup of asteroids to be identified here the effects of vesicle formation during melting of dust are investigated through numerical modeling and observations of antarctic mms vesicle formation is shown to cause a parachute effect that causes rapid deceleration decreasing peak temperature vesicular parachuting enhances the abundance of melted mms formed from phyllosilicate bearing c type asteroid dust on the earth surface by a factor of 2 micrometeorites recovered from the geological record therefore are biased toward breakup events involving hydrated c type asteroids whilst those involving phyllosilicate poor particles are diluted by the enhanced background flux of hydrous dust the parachute effect is also likely to increase the delivery of 3he to ocean sediments by c type asteroid dust

middle ordovician darriwilian conodonts from southern tibet the indian passive margin implications for the age and correlation of the roof of the world

numbers types and compositions of an unbiased collection of cosmic spherules abstract abstract micrometeorites collected from the bottom of the south pole water well spww may represent a complete well preserved sample of the cosmic dust that accreted on earth from 1100 1500 a d we classified 1588 cosmic spherules in the size range 50 800 pm the collection has 41 barred olivine spherules 17 glass spheres 12 cryptocrystalline spherules 11 porphyritic olivine spherules 12 relic grain bearing spherules 3 scoriaceous spherules 2 i type spherules 1 ca al ti rich cat spherules and 1 g type spherules we also found bubbly glass spherules spherules with glass caps and ones with sulfide coatings particles that are absent from other collections a classification sequence of the stony spherules scoriaceous relic grain bearing porphyritic barred olivine cryptocrystalline glass and cat is consistent with progressive heating and evaporation of fe from chondritic materials the modem day accretion rate and size distribution measured at the spww can account for the stony spherules present in deep sea collection through preferential dissolution of glass and small stony spherules however weathering alone cannot account for the high accretion rate of i type spherules determined for two deep sea collections the spww collection provides data to constrain models of atmospheric entry heating and to assess the effects of terrestrial weathering

meteoric smoke fallout over the holocene epoch revealed by iridium and platinum in greenland ice an iridium anomaly at the cretaceous tertiary boundary layer has been attributed to an extraterrestrial body that struck the earth some 65 million years ago1 it has been suggested that during this event the carrier of iridium was probably a micrometre sized silicate enclosed aggregate2 or the nanophase material of the vaporized impactor3 but the fate of platinum group elements such as iridium that regularly enter the atmosphere via ablating meteoroids remains largely unknown here we report a record of iridium and platinum fluxes on a climatic cycle timescale back to 128 000 years ago from a greenland ice core4 we find that unexpectedly constant fallout of extraterrestrial matter to greenland occurred during the holocene whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust laden atmosphere of the last glacial age we suggest that nanometre sized meteoric smoke particles5 6 formed from the recondensation of ablated meteoroids in the atmosphere at altitudes 70 kilometres are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps this implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the holocene

middle ordovician late dapingian darriwilian conodonts from the cow head group and lower head formation western newfoundland canada1this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology middle ordovician late dapingian darriwilian conodonts from the shallow bay and green point formations cow head group and the lower head formation are recorded from three sections in gros morne national park the collection was investigated to clarify local age relationships between the uppermost part of the cow head group and the interbedded to overlying sediments of the lower head formation conodonts from st pauls inlet north section indicate a middle dapingian age for the upper lower bed 13 latest dapingian to early darriwilian age for the upper bed 13 an early darriwilian dw 1 age for the top beds or bed 15 of the shallow bay formation at lower head and the lower head formation is referred to the darriwilian the uppermost part of the lower bed 13 contains periodon hankensis n sp gothodus sp a and diaphorodus delicatus followed by periodon macrodentatus ansella longicuspica erraticodon n sp a and spinodus wardi n sp in the lowermost part of upper bed 13 the fauna with p macrod

sedimentology of middle ordovician carbonates in the argentine precordillera evidence of regional relative sea level changes the las aguaditas formation in the argentine precordillera of san juan is the only ordovician carbonate sequence deposited on a slope spiculites mudstones and calcisiltites represent the autochthonous sediments characterized by a fine lamination rare fossils and their dark colour the pelagic fauna consists of rare radiolarians calcispheres trinucleid trilobites graptolites and conodonts the latter are typical of an open marine environment and proved a llanvirn llandeilo age for the las aguaditas formation

noble metal abundances in an early archean impact deposit we report detailed analyses on the concentrations of the noble metals pd os ir pt and au in an early archean spherule bed s4 of probable impact origin from the lower fig tree group barberton greenstone belt south africa compared to other sedimentary deposits of known or suspected impact origin some noble metals are present in exceptionally high concentrations noble metal abundances are fractionated relative to abundances in chondrites with ratios of osir ptir pdir and auir at only 80 80 41 and 2 of these values in ci chondrites although an extraterrestrial source is favored for the noble metal enrichment the most plausible cause of the fractionation is by regional hydrothermal metasomatic alteration

ordinary chondritic micrometeorites from the indian ocean extraterrestrial particulate materials on the earth can originate in the form of collisional debris from the asteroid belt cometary material or as meteoroid ablation spherules signatures that link them to their parent bodies become obliterated if the frictional heating is severe during atmospheric entry we investigated 481 micrometeorites isolated from 300 kg of deepsea sediment out of which 15 spherules appear to have retained signatures of their provenance based on their textures bulk chemical compositions and relict grain compositions seven out of these 15 spherules contain chromite grains whose compositions help in distinguishing sub groups within the ordinary chondrite sources there are seven other spherules which comprise either entirely of dusty olivines or contain dusty olivines as relict grains two of these spherules appear to be chondrules from an unequilibrated ordinary chondrite in addition a pop porphyritic olivine pyroxene chondrule like spherule is also recovered the bulk chemical composition of all the spherules in combination with trace elements the chromite composition and presence of dusty olivines suggest an ordinary chondritic source these micrometeorites have undergone minimal frictional heating during their passage through the atmosphere and have retained these features these micrometeorites therefore also imply there is a significant contribution from ordinary chondritic sources to the micrometeorite flux on the earth

in situ oxygen isotope compositions in olivines of different types of cosmic spherules an assessment of relationships to chondritic particles cosmic spherules collected from deep sea sediments of the indian ocean having different textures such as scoriaceous 4 relict bearing 16 porphyritic 35 and barred olivine 2 were investigated for petrography as well as high precision oxygen isotopic studies on olivine grains using secondary ion mass spectrometry sims the oxide feo mgo ratios of large olivines 20 m in cosmic spherules have low values similar to those seen in the olivines of carbonaceous chondrite chondrules rather than matching the compositions of matrix the oxygen isotope compositions of olivines in cosmic spherules have a wide range of 18o 17o and 17o values as follows 9 to 40 13 to 22 and 11 to 6 our results suggest that the oxygen isotope compositions of the scoriaceous relict bearing porphyritic and barred spherules show provenance related to the carbonaceous cm cv co and cr chondrites the different types of spherules that has experienced varied atmospheric heating during entry has not significantly altered the 17o values however one of the relict bearing spherules with a large relict grain has 17o 5 7 suggesting that it is derived from 16o poor material that is not recognized in the meteorite record a majority of the spherules have 17o ranging from 4 to 2 similar to values in chondrules from carbonaceous chondrites signifying that chondrules of carbonaceous chondrites are the major contributors to the flux of micrometeorites with an insignificant fraction derived from ordinary chondrites furthermore barred spherule data shows that during atmospheric entry an increase in 10 of 18o value surges 17o value by 1

paleoecology of llandovery conodonts anticosti island quebec abstract anticosti island quebec has perhaps the most fossiliferous and complete llandovery early silurian sequence in the world the undeformed limestones are well exposed and yield extremely well preserved conodonts whose taxonomy and biostratigraphy has been studied in detail in addition to documenting the general temporal and spatial distribution of llandovery conodonts a set of 107 conodont samples was taken from four cliff sections in the lower and upper becscie lower gun river and upper jupiter formations these samples were used to examine the relationship of conodont elements and communities with microfacies and explore their change through time over 9300 conodonts elements were identified and subjected to cluster analysis the sections comprise mainly the background sedimentation of lime mudstones deposited in 30 100 m of water on a gentle ramp these mudstones are variably affected by interbeds or channel fill deposits interpreted as tempestites a pattern of evolving conodont communities is documented for the lime mudstone facies through time different communities are found in the tempestite deposits that do not suggest a reworking of the mudstones but rather the importation of deeper offshore faunas during periods of storm surge onto the ramp the tempestites contain the highest species diversity and show no evidence of reduced diversity through post mortem selective sorting by the storms

ordovician floian lower darriwilian conodont biofacies of the san juan formation in the cerro viejo of huaco argentine precordillera abstract the conodont biofacies from the san juan formation at the cerro viejo of huaco central precordillera of argentina are analyzed for the identification of faunal dynamics and sea level changes the prioniodus elegans and oepikodus evae zones floian in age lower ordovician are identified in the lower san juan formation at the inner part of the huaco anticline in the western flank of the anticline the succession of the carbonate stratigraphic unit extends from the floian dapingian boundary lower middle ordovician up to the lower darriwilian middle ordovician lenodus variabilis zone for the interpretation of conodont biofacies through the los gatos creek section 5044 conodonts from rock samples of the floian to the lower darriwilian are studied conodont abundance and generic diversity graphs and cluster analysis reveal the following biofacies the juanognathus bergstroemognathus protopanderodus reutterodus drepanodus protopanderodus oepikodus juanognathus semiacontiodus and rossodus periodon protopanderodus biofacies are determined which indicate various deposits stacking from inner to middle and outer carbonate ramp environments of the san juan formation the associated analysis of biofacies and lithology allow for the recognition of two transgressive events in the san juan formation at the los gatos creek section which could be related to transgressive systems tracts tst that occurred during the lower and middle ordovician the recovered conodonts have a cai that varies from 2 to 2 5 indicating a burial paleotemperature between 60 and 155 c furthermore the variation of cai in conodonts of the same sample the presence of conodonts with corroded surface lamella recrystallization and crystal overgrowths and the presence of stylolites in limestone beds of the san juan formation indicate the circulation of hydrothermal fluids

oxygen isotopic composition of relict olivine grains in cosmic spherules links to chondrules from carbonaceous chondrites most olivine relict grains in cosmic spherules selected for the present study are pristine and have not been disturbed during their atmospheric entry thereby preserving their chemical mineralogical and isotopic compositions in order to understand the origin of the particles oxygen isotope compositions of relict olivine grains in twelve cosmic spherules collected from deep sea sediments of the indian ocean were studied using secondary ion mass spectrometry most of the data lie close to the ccam carbonaceous chondrite anhydrous mineral line with 17 o ranging from 5 to 0 the data overlap oxygen isotopic compositions of chondrules from carbonaceous chondrites such as cv ck cr and cm which suggests that chondrules from carbonaceous chondrites are the source of relict grains in cosmic spherules chemical compositions of olivine in cosmic spherules are also very similar to chondrule olivine from carbonaceous chondrites several olivine relict grains in three cosmic spherules are 16 o rich 17 o 21 9 to 18 7 similar to oxygen isotopic compositions observed in calcium aluminium rich inclusions cais amoeboid olivine aggregates aoas and some porphyritic chondrules from carbonaceous chondrites these grains appear to have recorded the initial oxygen isotopic composition of the inner solar nebula three olivine grains from two cosmic spherules have 18 o values 20 which could be interpreted as mixing with stratospheric oxygen during atmospheric entry

biostratigraphy and palaeoecology of middle late ordovician conodont and graptolite faunas of the las chacritas river section precordillera of san juan argentina a conodont graptolite biostratigraphic study was carried out on the top strata of the san juan las chacritas and las aguaditas formations in the la trampa range precordillera of san juan in western argentina significant conodont records in the san juan and las chacritas formations allow for the recognition of the yangtzeplacognathus crassus eoplacognathus pseudoplanus microzarkodina hagetiana and m ozarkodella subzones and eoplacognathus suecicus zones of darriwilian age index species and co occurrences of graptolites and conodonts were recorded in the las aguaditas formation allowing the identification of the nemagraptus gracilis and the pygodus anserinus zones which represent the sandbian stage these data indicate a hiatus between the las chacritas and the las aguaditas formations corresponding to the pygodus serra zone and the pterograptus elegans and hustedograptus teretiusculus zones upper darriwilian a total of 7287 identifiable conodont elements were recorded from the study section the species frequency registered for each zone shows that periodon and paroistodus are the most abundant taxa which are indicative of open marine environments the records of particular conodont taxa such as histiodella periodon microzarkodina eoplacognathus and baltoniodus allow a precise global correlation with other regions such as south central china baltoscandia north america great britain southern australia and new zealand the graptolite fauna identified here are recognized worldwide in equivalent strata in the baltic region great britain north america china southern australia and new zealand the presence of graptolites in the ribbon limestones of the las chacritas formation is documented for the first time

darriwilian middle ordovician conodont biofacies from the central precordillera of argentina abstract conodont biofacies structure was recognized throughout detailed stratigraphic sampling at three localities from the central precordillera of argentina from north to south las aguaditas creek las chacritas river and cerro la chilca sections a total of 50 carbonate samples yielded over 9900 identified specimens including a new conodont species fahraeusodus jachalensis which is described due to its paleoecological connotations the yangtzeplacognathus crassus eoplacognathus pseudoplanus and periodon macrodentatus zones are dominated by the ubiquitous species periodon macrodentatus and paroistodus horridus at the three study areas this enables the recognition of the periodon paroistodus biofacies in the central precordillera numerical cluster methodology allowed the identification of conodont sub biofacies the protopanderodus semiacontiodus sub biofacies in the las aguaditas creek the protopanderodus parapaltodus sub biofacies in the las chacritas river the parapaltodus sub biofacies at the cerro la chilca and the fahraeusodus ansella sub biofacies in all of the study sections these sub biofacies present distinctive attributes which are correlative with changes in the depositional environment the diverse protopanderodus parapaltodus sub biofacies developed with a shallowing of the basin while the fahraeusodus ansella sub biofacies is characteristic of deep environments shallower water settings resulted in higher species richness diversity and evenness of conodont communities the eustatic change that led to the drowning of the carbonate platform produced a shift to deeper anoxic benthic conditions that in turn induced a change in the taxonomic composition of the fauna and affected the diversity of the conodont communities

major and trace element geochemistry of s type cosmic spherules micrometeorites that pass through the earth s atmosphere undergo changes in their chemical compositions thereby making it difficult to understand if they are sourced from the matrix chondrules or calcium aluminum rich inclusions cais these components have the potential to provide evidence toward the understanding of the early solar nebular evolution the variations in the major element and trace element compositions of 155 different type scoriaceous relict bearing porphyritic barred cryptocrystalline and glass of s type cosmic spherules are investigated with the intent to decipher the parent sources using electron microprobe and laser ablation inductively coupled plasma mass spectrometry the s type cosmic spherules appear to show a systematic depletion in volatile element contents but have preserved their refractory trace elements the trends in their chemical compositions suggest that the s type spherules comprise of components from similar parent bodies that is carbonaceous chondrites large fosteritic relict grains observed in this investigation appear to be related to the fragments of chondrules from carbonaceous chondrites furthermore four spherules two of these spherules enclose spinels and one comprised entirely of a ca al rich plagioclase show enhanced trace element enrichment patterns that are drastically different from all the other 151 cosmic spherules the information on the chemical composition and rare earth elements rees on cosmic spherules suggest that the partially to fully melted ones can preserve evidences related to their parent bodies the ce eu and tm anomalies found in the cosmic spherules have similar behavior as that of chondrites distinct correlations observed between different rees and types of cosmic spherules reflect the inherited properties of the precursors

the origins of i type spherules and the atmospheric entry of iron micrometeoroids the earth s extraterrestrial dust flux includes a wide variety of dust particles that include feni metallic grains during their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed i type spherules these particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 au this article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials the results indicate that iron micrometeoroids experience peak temperatures 300 800 k higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of 50 m the lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates which enhances their abundance among micrometeorites by a factor of 2 the abundance of i types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite derived mms arriving at earth furthermore carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the i type spherule flux events involving such objects therefore cannot be recognized from i type spherule abundances in the geological record

the conodont fauna in the middle ordovician eoplacognathus pseudoplanus zone of baltoscandia conodont faunas from the eoplacognathus pseudoplanus zone in five main areas of middle ordovician rocks in sweden are described and discussed and compared with faunas of comparable age from neighbouring areas the investigation is based on 118 samples which together yielded 175 114 conodont elements the swedish conodont faunas are intermediate between shallower water faunas from the eastern part of baltica and deeper water faunas towards the west correlation with the yangtze platform of south central china is fairly easy and the occurrence of key taxa such as histiodella makes it also possible to correlate some levels with laurentia although faunal lists for the swedish localities are almost identical the conodont taxa have their relative abundance maxima in separate areas species of protopanderodus and periodon are most common in deeper water settings eoplacognathus baltoniodus and microzarkodina characterize intermediate areas and semiacontiodus is typical of shallow subtidal environments less

an urban collection of modern day large micrometeorites evidence for variations in the extraterrestrial dust flux through the quaternary we report the discovery of significant numbers 500 of large micrometeorites 100 m from rooftops in urban areas the identification of particles as micrometeorites is achieved on the basis of their compositions mineralogies and textures all particles are silicate dominated s type cosmic spherules with subspherical shapes that form by melting during atmospheric entry and consist of quench crystals of magnesian olivine relict crystals of forsterite and iron bearing olivine within glass four particles also contain ni rich metal sulfide beads bulk compositions are chondritic apart from depletions in the volatile moderately volatile and siderophile elements as observed in micrometeorites from other sources the reported particles are likely to have fallen on earth in the past 6 yr and thus represent the youngest large micrometeorites collected to date the relative abundance ratio of barred olivine to cryptocrystalline spherule types in the urban particles of 1 45 is shown to be higher than a quaternary average of 0 9 suggesting variations in the extraterrestrial dust flux over the past 800 k y changes in the entry velocities of dust caused by quasi periodic gravitational perturbation during transport to earth are suggested to be responsible variations in cosmic spherule abundance within the geologic column are thus unavoidable and can be a consequence of dust transport as well as major dust production events

melted micrometeorites from antarctic ice with evidence for the separation of immiscible fe ni s liquids during entry heating we report the discovery of four large 50 m cosmic spherules css and a single scoriaceous micrometeorite smm that contain evidence for the separation of immiscible fe ni s liquids during atmospheric entry heating the particles contain segregated fe rich regions dominated by either ni s bearing fe oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids the oxides which may be hydrous are thought to result from alteration of metal and sulphide the compositions of the silicate portions of the css are equivalent to spherules without fe rich regions implying that metallic liquids are exsolved during the heating of most spherules but completely separate the single smm has a very different composition from other scoriaceous particles and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating the pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually c rich this smm might be therefore an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle idp the exsolution of immiscible fe ni s liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt

an ordinary chondrite impactor for the popigai crater siberia with a diameter of 100 km popigai in northern siberia is the largest crater known in the cenozoic the concentrations in platinum group elements pge were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile the method selected was preconcentration by nis fire assay followed by inductively coupled plasma mass spectrometry icp ms this technique measures all the pge except os and by using aliquots 10g the results are highly reproducible the major and trace element composition of the impact melt resembles that of gneissic lithologies of the anabar shield which are representative of the target rock the pge are enriched in the melt by factors of 3 to 14 compared to the main target lithology but the meteoritic contamination is only around 0 2 wt using plots of elemental ratios such as ru rh vs pt pd or ru rh vs pd ir the popigai impactor is clearly identified as an ordinary chondrite and most likely l chondrite this study indicates that pge elemental ratios allow discrimination of the type of impactor even in the case of low meteoritic contamination this study confirms that a significant fraction of the crater forming projectiles presently documented could have an ordinary chondrite composition their probable source the s type asteroids appears to form the majority of the bodies in the main asteroid belt and among near earth objects neos the ordinary chondrite origin of the popigai projectile supports an asteroidal origin for the late eocene impacts as a plausible alternative to the comet shower scenario proposed by farley et al 1998 copyright 2005 elsevier ltd

biostratigraphy and biofacies of the middle darriwilian ordovician conodonts from the laoshidan section in the western margin of the north china craton abstract middle ordovician conodonts from the sandaokan and zhuozishan formations of the laoshidan section at the western margin of the north china craton consist of 39 identified and six indeterminate species belonging to 21 genera and three taxa left in open nomenclature at both genus and species levels of particular biostratigraphic importance is the presence of morphologically advanced species of histiodella associated with several age diagnostic endemic taxa making it possible to recognize international conodont biozones in the main part of the sequence three histiodella based zones are recognized in the studied section including the histiodella cf holodentata histiodella kristinae and histiodella bellburnensis zones in ascending order they can be directly correlated with the conodont zonations established in western newfoundland canada and tarim western china and compared well with the conodont biozones of baltoscandia and yangtze central china however the same stratigraphic interval on the north china platform is barren of conodonts multivariate statistical studies on these conodonts allow recognition of four conodont biofacies scalpellodus biofacies panderodus biofacies drepanoistodus biofacies and phragmodus biofacies turnovers of the conodont biofacies are related to either sea level changes or mixing of shallow water conodonts due to down slope transportation excluding the effect of the allochthonous conodonts the transgressive regressive patterns demonstrated by the conodont biofacies compare closely to the published sea level curves for the western margin of the north china craton

early middle ordovician conodont biofacies on the yangtze platform margin south china applications to palaeoenvironment and sea level changes 9172 conodonts have been recovered from the uppermost hunghuayuan formation and the zitai formation at two sections in shitai county southern anhui province south china which was situated close to the margin of the lower yangtze platform during the early to middle ordovician systematic and multivariate statistical studies on these conodonts permit recognition of seven conodont biofacies tropodus biofacies diaphorodus biofacies oepikodus biofacies baltoniodus biofacies paroistodus biofacies periodon biofacies and protopanderodus biofacies each biofacies is restricted to a particular lithofacies and stratal position and shows a consistent order and or position within the succession turnover of these conodont biofacies is related to sea level changes the transgressive regressive patterns demonstrated by the conodont biofacies compare closely to published sea level curves for south china and highlight the utility of conodont biofacies as a means of confirming sedimentological evidence of relative sea level change c 2014 elsevier ltd all rights reserved less

placers of cosmic dust in the blue ice lakes of greenland a concentration process occurring in the melt zone of the greenland ice cap has produced the richest known deposit of cosmic dust on the surface of the earth extraterrestrial particles collected from this region are well preserved and are collectable in large quantities the collected particles are generally identical to cosmic spheres found on the ocean floor but a pure glass type was discovered that has not been seen in deep sea samples iron rich spheres are conspicuously rare in the collected material

the mineralogy and petrology of i type cosmic spherules implications for their sources origins and identification in sedimentary rocks abstract i type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and feni metal i types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time we report the results of a study of the mineralogy and petrology of 88 i type cosmic spherules recovered from antarctica in order to evaluate how they formed and evolved during atmospheric entry to constrain the nature of their precursors and to establish rigorous criteria by which they may be conclusively identified within sediments and sedimentary rocks two textural types of i type cosmic spherule are recognised 1 metal bead bearing met spherules dominated by ni poor 100 and suggest that metal from h group ordinary cm cr and iron meteorites may form the majority of particles oxidation during entry heating increases in the series met 80 wt ni comprising a particle mass fraction of

uranium in foraminiferal calcite as a recorder of seawater uranium concentrations abstract we present results of an investigation of uranium calcium ratios in cleaned foraminiferal calcite as a recorder of seawater uranium concentrations for accurate reconstruction of past seawater uranium content shell calcite must incorporate uranium in proportion to seawater concentration and must preserve its original uranium composition over time laboratory culture experiments with live benthic amphistegina lobifera and live planktonic globigerinella calida foraminifera show that the u ca ratio of cleaned calcite tests is proportional to the concentration of uranium in solution after correcting results for the presence of initial calcite the apparent distribution coefficient d u ca calcite u ca solution 10 6 0 3 10 3 for a lobifera and d 7 9 0 1 10 3 for g calida u ca ratios in planktonic foraminifera from core tops collected above 3900 m in the equatorial atlantic and above 2100 m in the pacific ocean show no significant difference among the species analyzed d estimated from core top samples ranges from 7 6 0 4 10 3 for o universa to 8 4 0 5 10 3 for g ruber in benthic species c wuellerstorfi d 7 0 0 8 10 3 u ca and mg ca in g tumida and g sacculifer from core tops taken near and below the regional lysocline decrease with water depth smaller decreases in u ca and mg ca with depth were observed in c wuellerstorfi in the planktonic species we believe that u ca and mg ca are lower in the more dissolution resistant fraction of calcite leading to lower u ca in more highly dissolved samples

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 co2 rich volcanic pipes bverneshotsq 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 timeintegrated oceanic continent crustal ratio n2 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 d 2005 elsevier b v all rights reserved

the occurrence of platinum group element and gold minerals in the bon accord ni oxide body south africa two samples from the enigmatic ni oxide body of bon accord barberton greenstone belt south africa have been investigated with the hydroseparation technique to obtain heavy mineral concentrates the concentrates contain abundant pt pd and gold minerals never reported before from the bon accord ni oxide body the grains occur as 1 minute inclusions 3 m in trevorite ideally nife3 o4 and 2 larger 5 70 m free aggregates liberated from the host phase the first group comprises several pgm compounds of pd sb pd sb as pd cu sb pt sb pt as s ru as s ru s along with free grains of ni fe as the second consists of sperrylite ptas2 members of the sobolevskite kotulskite series and electrum these results are in good agreement with previous analyses of pge au in bulk rock paragenetic relationships indicate that the pgm and electrum are of secondary origin probably generated during low temperature metamorphism of the ni rich mineralization they have a terrestrial origin and are related with a low sulfidation regime that usually accompanies hydrothermally driven serpentinization of mafic ultramafic bodies the ligands in the newly formed pgm as sb bi te and o probably proceed from the same source of the hydrothermal solutions in this model the metals ni pge au were original components of the primary mineral assemblage of the bon accord precursor whereas as sb bi te and o might have been contributed by the metasomatizing fluids during near surface evolution of the ore body the data on the high grade heavy mineral concentrates obtained by hydroseparation have provided new knowledge about the mineral deportment of pd pt and au

darriwilian middle ordovician delta c 13 carb chemostratigraphy in the precordillera of argentina documentation of the middle darriwilian isotope carbon excursion mdice and its use for intercontinental correlation although documented from estonia latvia sweden eastern north america and china the middle ordovician darriwilian positive delta c 13 excursion known as the mdice has previously not been recognized with certainty in south america the most promising region in south america for detailed middle ordovician carbon isotope research is the precordillera of western argentina where there are many excellent exposures of biostratigraphically well dated carbonate successions spaning the tremadocian through sandbian stratigraphic interval for this project we collected numerous isotope and conodont samples from the middle darriwilian las chacritas and aguaditas formations at their type localities which yielded important biostratigraphic data as well as informative delta c 13 carb values in the e pseudoplanus zone in the upper half of the las chacritas formation there is a relatively modest but distinct delta c 13 carb excursion because its stratigraphic position and magnitude closely agree with the mdice in other parts of the world we recognize it as the first firm record of this excursion in south america the fact that the delta c 13 carb curve from the las aguaditas formation shows no such excursion is due to the existence of a stratigraphic gap between the lower and middle members of this formation that cuts out the excursion interval the precordilleran mdice is used for detailed long range correlations with successions in baltoscandia newfoundland and china illustrating the usefulness of also this delta c 13 carb excursion as a global stratigraphic tool a recent proposal of a greatly extended chronostratigraphic range of the table head group on newfoundland is rejected based on well established biostratigraphic evidence c 2013 elsevier b v all rights reserved less

use of platinum group elements for impactor identification terrestrial impact craters and cretaceous tertiary boundary abstract this study presents the abundances of ru ir pt pd rh and au platinum group elements pge in the ejecta layer at the cretaceous tertiary boundary and melt rocks from east clearwater lake brent new quebec and lake wanapitei impact craters all believed to have been formed by the impact of chondritic projectiles the objective was to evaluate the pure application of pge interelement ratios e g ru ir for impactor identification to compare the utility of both melt rocks and ejecta deposits for this purpose and to expand the pge database for these craters the similarity of pge interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition introduction of indigenous pge from country rock may contaminate the original meteoritic signature in addition the extent of pge fractionation during impact and melt sheet cooling and of post sedimentary pge remobilization must be evaluated in each case for east clearwater the pge chondrite normalized plot is flat indicating a chondritic impactor based on ru ir and cr ir ratios a cl or l chondrite is most likely however variation of ratios with depth in the core makes any fine classification of the projectile tenuous for the brent and new quebec craters the flat chondrite normalized pge plot indicates a chondritic impactor but finer classification is precluded by possible post syn depositional pge fractionation at lake wanapitei ni cr and cr ir ratios indicate a ll chondrite impactor wolf et al 1980 but sparse pge data for this chondrite type does not allow further classification of the projectile a flat chondrite normalized pge plot indicates a chondritic rather than an iron impactor at lake wanapitei the global ejecta layer at the cretaceous tertiary boundary has a chondritic pge signature but only when the integrated flux of pge on a global scale is considered individual sites may not show chondritic ratios due to differential pge remobilization resulting from local differences in depositional environment continental vs marine and to vaporization and or condensation pge fractionation occurring during impact the results for melt rocks and the k t ejecta deposit indicate that pge can be used for impactor identification but primarily in combination with other siderophile chalcophile meteoritic elements and their ratios