noble gases in individual l chondritic micrometeorites preserved in an ordovician limestone we have measured the he and ne concentrations and isotopic ratios of individual sediment dispersed extraterrestrial chromite grains 63 180 mu m in diameter from an ordovician limestone in southern sweden in the same sediment many fossil meteorites were found and have been attributed to the l chondrite parent body breakup event similar to 470 ma ago in this analysis of 37 individual extraterrestrial chromite grains of l chondritic major element composition at least 35 similar to 95 contain surface implanted helium and neon of fractionated solar wind composition implying that these grains are relict parts of fossil micrometeorites of asteroidal origin similar to what has been observed in recent micrometeorites collected in the polar regions significant amounts of cosmogenic ne 21 were found in several grains resulting in cosmic ray exposure cre ages of up to similar to 77 myr these ages exceed both dynamical lifetimes for asteroidal micrometeorites of this size as well as cre ages observed in chromites from fossil meteorites from the same sediment beds significant contributions from terrestrial sources like nucleogenic and cosmic ray muon induced ne 21 can be excluded in the extraterrestrial chromites since 11 terrestrial chrome spinel grains from the same sediment beds did not contain any measurable ne 21 excesses having found micrometeorites of undisputed asteroidal origin with cosmic ray exposure ages of several 10 7 years implies that high cosmic ray exposure ages alone are not a good indicator of cometary origin in opposition to what has been suggested for recent micrometeorites and idps we propose instead that these grains collected their cosmogenic ne 21 while residing in the regolith layer of their parent body c 2009 elsevier b v all rights reserved less

graptolite derived organic matter in the wufeng longmaxi formations upper ordovician lower silurian of southeastern chongqing china implications for gas shale evaluation abstract the organic rich shales of the wufeng longmaxi formations ordovician silurian are one of the most important hydrocarbon source rocks and shale gas exploration targets in china graptolites are abundant in these shales but their contribution to the dispersed organic matter om has been overlooked this lack of attention could seriously affect the evaluation of om type om maturity hydrocarbon generation potential reservoir characteristics and gas content of these shales optical microscope and focused ion beam scanning electron microscope fib sem techniques were employed to describe the optical characteristics and micro nanopores of the graptolites to determine the contribution of the graptolites and to evaluate the thermal maturity of the wufeng longmaxi formations non granular graptolites which show a stronger anisotropy and higher reflectance under polarized light than the granular graptolites were primarily observed in these sediments graptolites account for 20 93 of the dispersed om in the wufeng longmaxi formations and the level of their contribution has been attributed to the radiation of graptolites at the late ordovician kaptian and the recovery of graptolites at the early silurian rhuddanian the bireflectance values of the non granular graptolites fall between 2 22 and 4 32 indicating a behavior similar to biaxial anthracitic vitrinite the equivalent vitrinite reflectance eqvro values of the sediments in the wufeng longmaxi formations calculated from the mean maximum reflectance of non granular graptolite fall in the range of 3 08 4 29 and these values have a precision that is greater than in previous studies on the wufeng longmaxi formations abundant organic pores were observed in the non granular graptolites and they are aligned parallel to the fusellar layers of the graptolites this indicates that the fusellar layers of non granular graptolites controlled the development and distribution of organic pores such abundant organic pores played a significant role in the accumulation of shale gas in the wufeng longmaxi formations of the upper yangtze platform

shock barometry of the siljan impact structure sweden the siljan impact structure in sweden is the largest confirmed impact structure in western europe despite this the structure has been poorly studied in the past and detailed studies of shock metamorphic features in the target lithologies are missing here we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at siljan at 21 localities from an area approximately 20 km in diameter located centrally in the structure the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter the most strongly shocked samples recording pressures of up to 20 gpa occur at the very central part of the structure and locally in these samples higher pressures causing melting conditions in the affected rocks were reached pressures recorded in the studied samples decrease outwards from the center of the structure forming roughly circular envelopes around the proposed shock center based on the distribution pattern of shocked quartz at siljan the original transient cavity can be estimated at approximately 3238 km in diameter after correcting for erosion we conclude that the original rim to rim diameter of the siljan crater was somewhere in the size range 5090 km less

extraterrestrial chromite in latest maastrichtian and paleocene pelagic limestone at gubbio italy the flux of unmelted ordinary chondrites the distribution of sediment dispersed extraterrestrial ordinary chondritic chromite ec grains 63 m has been studied across the latest maastrichtian and paleocene in the bottaccione gorge section at gubbio italy this section is ideal for determining the accumulation rate of ec because of its condensed nature and well constrained sedimentation rates in a total of 210 kg of limestone representing eight samples of 14 28 kg distributed across 24 m of the bottaccione section only 6 ec grains were found an average of 0 03 ec grains kg 1 in addition one probable pallasitic chromite grain was found no ec grains could be found in two samples at the cretaceous tertiary k t boundary which is consistent with the k t boundary impactor being a carbonaceous chondrite or comet low in chromite the average influx of ec to earth is calculated to 0 26 grain m 2 kyr 1 this corresponds to a total flux of 200 tons of extraterrestrial matter per year compared to 30 000 tons per year as estimated from os isotopes in deep sea sediments the difference is explained by the ec grains representing only unmelted ordinary chondritic matter predominantly in the size range from 0 1 mm to a few centimeters in diameter sedimentary ec grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere the average of 0 03 ec grain kg 1 in the gubbio limestone contrasts with the up to 3 ec grains kg 1 in mid ordovician limestone that formed after the disruption of the l chondrite parent body in the asteroid belt at 470 ma the two types of limestone were deposited at about the same rate and the difference in ec abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup

the asaphid trilobite fauna its rise and fall in baltica abstract the baltica terrane is well known as the asaphus province of the ordovician world being situated in intermediate latitudes and surrounded by fairly wide oceans baltica had a benthic fauna that developed in a relatively endemic direction the most common trilobites were the asaphids the present study discusses how they appeared evolved in various environments around baltoscandia and finally disappeared the total range of the asaphids based on data from 381 species is estimated as extending from the furongian latest cambrian to the end ordovician the remnant cambrian olenid fauna which was adapted to black bituminous shale facies survived into the tremadocian this cambrian fauna includes the first asaphids promegalaspides niobella eoasaphus with a shift to lighter coloured and carbonatic sediments in the middle tremadocian this fauna was replaced by the immigrating ceratopyge fauna the latter contains the first five widely distributed asaphid genera promegalaspides niobe niobella niobina and asaphellus as carbonate sedimentation became more widespread in many areas from the floian onwards the asaphids increased in number and formed a stabilised asaphid fauna during the dapingian in baltoscandia a gradual development of endemism related to facies belts is observed during the darriwilian records of the mud related niobines megistaspis and ptychopyge faunas together with some other benthic macrofaunas are not found beyond the boundary of the kunda aseri regional stages middle darriwilian their disappearance may be due to a catastrophic event in contrast asaphus s l survived this interval drift of baltica towards lower latitudes and fluctuations in sea level influenced the asaphid fauna leading to its gradual collapse by late sandbian time influx of the islotelines during the early katian gave way to a set of illaenimorph species found together with corals that are related to a reefal environment this morph disappears prior to the hirnantian glaciation

the mid ordovician osmussaar breccia in estonia linked to the disruption of the l chondrite parent body in the asteroid belt the middle ordovician 466 ma osmussaar breccia situated along the northwestern coast of estonia is rich in angular chromite grains of extraterrestrial origin 13 grains kg 1 and shocked quartz the angularity of the chromite grains implies that they have not been transported or reworked to any large extent connoting that the brecciation is the result of a contemporary impactor either as a direct consequence of the impact or as a result of an earthquake triggered by the impact and thus is not as previously suggested redeposited material from the nearby similar to 70 m y older neugrund impact structure the chemical composition of the chromite indicates that the impactor was an ordinary chondrite of l type which concurs well with the hypothesis that the influx of large bodies to earth increased during this period due to the breakup of the l chondrite parent body this in turn gives support to the recent suggestion that abundant coeval mega breccias worldwide are impact triggered the presence of extraterrestrial chromite also strengthens the theory that physical pieces of a large celestial body can survive upon impact with earth less

palaeontology meteoritic spur to life from about 470 million years ago the middle ordovician period witnessed a rapid increase in biodiversity this explosion in numbers of species is almost perfectly contemporaneous with an increased frequency of meteorite impacts

the middle darriwilian ordovician δ13c excursion mdice discovered in the yangtze platform succession in china implications of its first recorded occurrences outside baltoscandia abstract the middle darriwilian 13 c excursion mdice one of the least known of the ordovician 13 c excursions has previously been recorded only from the middle ordovician of baltoscandia analysis of many recently collected limestone samples from the guniutan formation at two yangtze platform localities show elevated 13 c values in the same biostratigraphic interval microzarkodina ozarkodella conodont subzone as the mdice in baltoscandia which justifies identification of the chinese 13 c excursion as the mdice these occurrences which are in strata that show striking lithological and conodont faunal similarity to the swedish holen limestone and some coeval units in estonia indicate that the mdice the stratigraphically oldest of the named ordovician 13 c excursions is likely to have a world wide distribution and to have great potential for local and long range chemostratigraphic correlations

extensive soft sediment deformation structures in the early darriwilian middle ordovician shallow marine siliciclastic sediments formed on the baltoscandian carbonate ramp northwestern estonia abstract the nearshore middle ordovician early darriwilian deposits of the baltoscandian basin formed on the flank of tectonically stable fennoscandian shield comprise numerous soft sediment deformation structures the areal extent of the deformed interval extends up to 9000 km 2 from nw estonia to se sweden the deformation structures occur within a meter scale sandstone horizon that includes load casts flame structures ball and pillow morphologies up to 2 m wide sedimentary dykes autoclastic breccias sand volcanoes and other related soft sediment deformation structures evidently these deformation structures formed during a single seismic event that caused liquefaction and fluidization of the water saturated unconsolidated shallow marine sediments within almost the entire sandstone unit pakri formation probably a unique catastrophic earthquake of magnitude 7 or higher occurred in this region penecontemporaneously with the deposition of these sediments the ultimate cause of seismic event of such magnitude in the tectonically stable baltoscandian basin is unclear however deposition and deformation of the pakri formation fall within the middle ordovician meteoritic bombardment period which resulted from the disruption of a large parent body in the asteroid belt approximately 470 ma ago this in turn suggests a potential impact origin for the single catastrophic earthquake in the tectonically stable study region although impact crater of suitable age is currently not known from the region then previous studies have documented some impact metamorphic shock lamellae i e pdfs and abundant extraterrestrial chromite grains from the deformed sediments

asteroid breakup linked to the great ordovician biodiversification event the great ordovician biodiversification event coincides 470 million years ago with the break up of a large asteroid and the resultant frequent bombardment of earth with asteroid fragments

pulsed cephalopod diversification during the ordovician abstract during the ordovician radiation cephalopods expanded their habitat from the shallow marine environments of the low latitudes to global occurrences in almost all marine environments the expansion of habitat was accompanied by a drastic morphological and taxonomical diversification the diversification was not a constant linear process but involved intervals of diversity crises and fundamental faunal change at the global scale a new database comprising more than 5100 cephalopod occurrences with high stratigraphical resolution allowed for a subsampled and normalised calculation of mean standard diversities at the scale of the igcp 503 time slices the resulting curves reveal a pulsed diversification of ordovician cephalopods with three main diversification phases which were in each case preceded by a significant faunal change and a diversity crisis each diversification pulse led to higher diversity levels reaching a katian diversity climax the cephalopod turnover rates decreased during the ordovician but only a weak diversity dependence of turnover rates and diversity can be detected the diversification events are characterised by 1 a poor correlation with major physical events in the ordovician world but good agreement with important events of the diversity evolution of trilobites brachiopods and reefbuilders and by 2 a statistically significant general trend of decreasing origination rates a major cephalopod faunal change at the lower middle ordovician boundary can be directly compared with the contemporaneous faunal changes in brachiopods and trilobites we interpret the canonical pulsed diversification of ordovician cephalopods as a result of ecosystem dependent processes potentially an increasing ecosystem complexity an increasing stability of the trophic structure and an increasing ecosystem resilience led to the expansion of cephalopod habitats and the multiplication of niches

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

a global rain of micrometeorites following breakup of the l chondrite parent body evidence from solar wind implanted ne in fossil extraterrestrial chromite grains from china previous studies of limestone beds of mid ordovician age from both sweden and china show that the earth saw an at least two orders of magnitude increase in the influx of extraterrestrial material approximately 470 ma following the disruption of an l chondrite parent body in the asteroid belt recovered extraterrestrial material consists of fossil meteorites and sediment dispersed extraterrestrial chromite sec grains both with l chondritic origin ne isotope analysis of sec grains from one of the swedish limestone sections revealed that the vast majority of the grains were delivered to earth as micrometeorites in this study we extend the previous work both in time and geographically by measuring concentrations and isotopic ratios of ne in individual sec grains 60120 mu m in diameter from three different beds from a contemporary middle ordovician limestone section in china all of the chinese sec grains 44 in total contain surface implanted ne of fractionated solar wind composition implying that these grains were as in the case of the swedish sec grains delivered to earth as micrometeorites this gives further compelling evidence that the two to three orders of magnitude increase in the influx of micrometeoritic material following the breakup of the l chondrite parent body was indeed a global event the rain of micrometeorites prevailed for at least 2 myr the estimated time of the deposition of the topmost chinese bed after the breakup event less

a single asteroidal source for extraterrestrial ordovician chromite grains from sweden and china high precision oxygen three isotope sims analysis we examined oxygen three isotope ratios of 48 extraterrestrial chromite ec grains extracted from mid ordovician sediments from two different locations in sweden and one location in south central china the ages of the sediments similar to 470 ma coincide with the breakup event of the l chondrite parent asteroid elemental compositions of the chromite grains are generally consistent with their origin from l or ll chondrite parent bodies the average delta o 17 parts per thousand deviation from the terrestrial mass fractionation line measured in situ from 15 mu m spots by secondary ion mass spectrometry sims of ec grains extracted from fossil meteorites from thorsberg and brunflo are 1 17 0 09 parts per thousand 2 sigma and 1 25 0 16 parts per thousand respectively and those of fossil micrometeorites from thorsberg and puxi river are 1 10 0 09 parts per thousand and 1 11 0 12 parts per thousand respectively within uncertainty these values are all the same and consistent with the l chondrite group average delta o 17 1 07 0 18 parts per thousand but also with the ll chondrite group average delta o 17 1 26 0 24 parts per thousand clayton et al 1991 we conclude that the studied ec grains from correlated sediments from sweden and china are related and most likely originated in the same event the l chondrite parent body breakup we also analyzed chromites of modern h l and ll chondrites and show that their delta o 17 values coincide with averages of delta o 17 of bulk analyses of h l and ll chondrites this study demonstrates that in situ oxygen isotope data measured by sims are accurate and precise if carefully standardized and can be used to classify individual extraterrestrial chromite grains found in sediments c 2009 elsevier ltd all rights reserved less

40ar 39ar step heating ages of north american tektites and of impact melt rock samples from the chesapeake bay impact structure abstract this study presents 40ar 39ar step heating ages of four north american tektites three bediasites and one georgiaite and of two groundmass samples extracted at different depths from clast rich impact melt rocks cb w61 and cb w84 recovered by the usgs icdp eyreville b drill core about 9 km from the centre of the chesapeake bay impact structure radiometric age determination on both north american tektites and impact melt rocks from within chesapeake bay crater offers the first possibility to confirm the origin of these tektites for this aim argon isotopic data from 13 samples aliquots of tektite rims cores and bulk and 4 samples aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions age spectra of all tektite samples show plateaux comprising 62 98 of the 39ar release over consecutive intermediate and high temperature heating steps few low temperature extractions indicate excess 40ar inverse isochron 40ar 36ar intercepts of tektite samples are indistinguishable from air 295 5 however impact melt rock spectra presented complex ar release affecting primarily the low temperature heating steps inverse isochrones indicate excess argon from which the 40ar 36ar intercept was used to correct the age calculation cb w61 and cb w61 2 40ar 36ar intercepts are 354 5 2 5 and 327 2 6 3 respectively and those for cb w84 and cb w84 2 are 332 0 7 3 and 329 6 5 6 respectively the inverse isochron weighted mean age according to currently suggested k decay constants revisions by schwarz et al 2011 and renne et al 2011 for all four tektites is 34 86 0 25 ma mswd 0 96 p 0 41 n 4 and for the two impact melt rocks is 37 16 3 65 ma mswd 0 83 p 0 36 the combined tektite and impact melt rocks isochron mean age of 34 86 0 23 0 32 ma mswd 0 87 p 0 43 is slightly though not significantly higher than the plateau mean age of 34 55 0 27 0 36 ma mswd 0 66 p 0 62 placing this age in the global stratotype section and point gssp marine section exposed at massignano italy it falls below the eocene oligocene e o boundary overlapping with the 10 28 m ir anomaly these results agree within errors with previously reported ages of 35 20 0 54 ma especially those derived from k ar and ar ar total fusion analysis an age of 34 86 0 32 ma sets the chesapeake bay impact event close to the youngest of the three ir anomalies at 35 0 ma in the case the impactor was ir rich e g a chondrite primitive achondrite stony iron or iron meteorite the concordance with the e o boundary at 33 9 ma seems only marginally possible and only if the ir contribution from the ejecta were potentially due either to its small amount becoming diluted in the geologic record or the impactor being ir poor e g of differentiated achondritic composition this study also brings to front the need to re establish the stratigraphic and palaeo magnetic correlations across the globe for the ir anomalies and the magneto stratigraphy during the mid to late eocene and early oligocene and the need to re evaluate the markers for the eocene oligocene boundary

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

extraterrestrial chromite in the resurge deposits of the early late ordovician lockne crater central sweden the distribution of extraterrestrial chromite grains 63 mu m has been studied in the resurge deposit of the early late ordovician 458 ma lockne impact structure in central sweden a 1 kg sample of resurge sediment from the rim of the crater is extremely rich in extraterrestrial chromite in total the sample contains 125 chromium spinel grains kg 1 most grains being chromite due to post depositional alterations the origin of several of the grains is dubious although it is clear that the major part of the chromites is of extraterrestrial origin 75 grains kg 1 most likely derived from the impactor the alterations are primarily due to the hydrothermal system induced by the impactor with zinc enrichment in the chromites as the most common characteristic the element chemistry of the least altered chromite grains indicates that the impactor was an ordinary l chondrite this concurs with a suggested increase in the flux of l chondritic asteroids to earth following the disruption of the l chondrite parent body at ca 470 ma more than 170 impact craters are known from around the earth but only few of the impactors have been identified mainly due to the low survival rate of projectile material further studies of impact craters will reveal if survival of extraterrestrial chromite is unique for the lockne structure c 2006 elsevier b v all rights reserved less

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

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

bentonite geochronology marine geochemistry and the great ordovician biodiversification event gobe abstract attribution of ordovician climate forcing to explosive volcanism and the potential global importance of volcanism in ordovician biodiversification suggest the necessity of evaluating the relationships between k bentonite deposition and increasingly high resolution records of marine biogeochemical change globally ordovician strata preserve an extensive record of explosive volcanism including the widely recognized lower to middle ordovician famatina k bentonite suite in argentina and the upper ordovician millbrig deicke kinnekulle suite of north america and europe here we present high resolution id tims u pb zircon ages of k bentonites from measured sections of the san juan formation talacasto and cerro la chilca section of the argentine precordillera k bentonites from the argentine precordillera provide stratigraphically consistent i e younging upward ages that range from 473 45 0 70 ma to 469 53 0 62 ma and constrain the age of a low magnitude 2 globally recorded negative carbon isotope excursion evaluation of the timing of k bentonite deposition in the argentina precordillera relative to marine biostratigraphic and biogeochemical records provides insight into relationships between explosive volcanism and regional to global environmental change from a regional standpoint these ages provide critical direct evidence for a dapingian to earliest darriwilian age of the upper san juan formation at sampled localities these ages are consistent with carbon isotope data suggesting that the san juan formation in the region of its type section is coeval with only the base of the often correlated table head group of western newfoundland this data thus highlights the difficulties in using regional biostratigraphic data particularly within erosionally truncated or otherwise diachronous units to define the timeframe of carbon isotope chemostratigraphy new geochronological data also indicate that a discrete negative carbon isotope excursion within the san juan and table head formations is correlative to a globally recognized pre mdice negative excursion and indicates that this aspect of the marine carbon isotope record can be used as a discrete chronologic marker san juan formation bentonites however cannot be discretely correlated with observed environmentally significant changes in the middle ordovician marine geochemical records of carbon sulfur strontium or sea surface temperature these results suggest that the extent of volcanism represented by the famatina bentonite suite was insufficient to affect global surface environments and that the relationship between explosive volcanism and environmental change may not be straightforward as previously suggested

asteroidal source of l chondrite meteorites abstract establishing connections between meteorites and their parent asteroids is an important goal of planetary science several links have been proposed in the past including a spectroscopic match between basaltic meteorites and 4 vesta that are helping scientists understand the formation and evolution of the solar system bodies here we show that the shocked l chondrite meteorites which represent about two thirds of all l chondrite falls may be fragments of a disrupted asteroid with orbital semimajor axis a 2 8 au this breakup left behind thousands of identified 1 15 km asteroid fragments known as the gefion family fossil l chondrite meteorites and iridium enrichment found in an 467 ma old marine limestone quarry in southern sweden and perhaps also 5 large terrestrial craters with corresponding radiometric ages may be tracing the immediate aftermath of the family forming collision when numerous gefion fragments evolved into the earth crossing orbits by the 5 2 resonance with jupiter this work has major implications for our understanding of the source regions of ordinary chondrite meteorites because it implies that they can sample more distant asteroid material than was previously thought possible

the origin of the brunflo fossil meteorite and extraterrestrial chromite in mid ordovician limestone from the garde quarry jamtland central sweden the brunflo fossil meteorite was found in the 1950s in mid ordovician marine limestone in the garde quarry in jamtland it originates from strata that are about 5 million years younger than similar limestone that more recently has yielded 50 fossil meteorites in the thorsberg quarry at kinnekulle 600 km to the south based primarily on the low tio2 content about 1 8 wt of its relict chromite the brunflo meteorite had been tentatively classified as an h chondrite the meteorite hence appears to be an anomaly in relation to the kinnekulle meteorites in which chromite composition chondrule mean diameter and oxygen isotopic composition all indicate an l chondritic origin reflecting an enhanced flux of meteorites to earth following the disruption of the l chondrite parent body 470 ma new chondrule size measurements for the brunflo meteorite indicate that it too is an l chondrite related to the same parent body breakup chromite maximum diameters and well defined chondrule structures further show that brunflo belongs to the l4 or l5 type chromites in recently fallen l4 chondrites commonly have low tio2 contents similar to the brunflo chromites adding support for brunflo being an l4 chondrite the limestone in the circle quarry is relatively rich about 0 45 grain kg 1 in sediment dispersed extraterrestrial chromite grains 63 mu m with chemical composition similar to those in l chondrites and the limestone 1 3 grains kg 1 at kinnekulle suggesting that the enhanced flux of l chondrites prevailed although somewhat diminished at the time when the brunflo meteorite fell less

the marine osmium isotope record over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research several analytical developments particularly in the past few years have significantly increased our ability to study the behaviour of osmium in the surficial environment the 187os 188os and osmium concentration of seawater river water rain and hydrothermal vent fluids have been measured directly recently the behaviour of osmium in estuaries critical for estimating the marine residence time of osmium has been studied our knowledge of the surficial osmium cycle has thus significantly improved in addition reconstructions of past variations in the marine 187os 188os recently have been extended back into the mesozoic this review attempts to summarize our current understanding of the marine osmium system present and past the 187os 188os of seawater during the cenozoic to first order mimics the marine 87sr 86sr record it is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic tectonic processes however the marine osmium isotope system differs fundamentally from the marine strontium isotope system this review emphasizes three important differences first large impacts are capable of resetting the 187os 188os to unradiogenic values without significantly affecting the marine strontium system second organic rich sediments are characterized by high 187re 188os resulting 187os 188os ingrowth trajectories are similar to the average slope of the cenozoic 187os 188os seawater record trends towards more radiogenic 187os 188os seawater therefore can be caused by weathering of organic rich sediments at a constant rate third the marine residence time of osmium is sufficiently short to capture short periodic glacial interglacial fluctuations that are inaccessible to the buffered marine strontium isotope system this offers the opportunity to discriminate between high frequency climatic and low frequency tectonic forcing

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

from the mid ordovician into the late silurian changes in the micrometeorite flux after the l chondrite parent breakup we present the first reconstruction of the micrometeorite flux to earth in the silurian period we searched 321 kg of condensed marine limestone from the late silurian cellon section southern austria for refractory chrome spinel grains from micrometeorites that fell on the ancient sea floor a total of 155 extraterrestrial spinel grains 10 grains 63 m and 145 in the 32 63 m fraction were recovered for comparison we searched 102 kg of similar limestone from the mid ordovician komstad formation in southern sweden this limestone formed within 1 ma after the breakup of the l chondrite parent body lcpb in the asteroid belt in the sample we found 444 extraterrestrial spinel grains in the 63 m fraction and estimate a content of at least 7000 such grains in the 32 63 m fraction our results show that in the late silurian 40 ma after the lcpb the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude almost down to background levels among the ordinary chondrites the dominance of l chondritic micrometeorites has waned off significantly from 99 in the post lcpb mid ordovician to 60 in the late silurian with 30 h and 10 ll chondritic grains in the late silurian primitive achondrite abundances are similar to today s value contrasting to the much higher abundances observed in pre lcpb mid ordovician sediments

high resolution carbon isotopic records from the ordovician of south china links to climatic cooling and the great ordovician biodiversification event gobe abstract the great ordovician biodiversification event gobe represented the largest increase in diversity in the marine biosphere during the phanerozoic eon yet its causes and consequences remain poorly understood patterns of isotopic variation in high resolution 13ccarb and 13corg records from a well exposed section at honghuayuan in south china may provide important insights regarding the gobe the honghuayuan isotopic profiles which can be correlated with c isotopic records from contemporaneous sections globally reveal large perturbations to the global carbon cycle during the ordovician a 8 increase in 13corg values in the floian implies a large albeit transient increase in the burial rate of organic matter during the mid early ordovician that may have contributed to climatic cooling and played an important role in triggering the gobe a 4 increase in 13ccarb and high frequency variation in 13corg in the darriwilian to sandbian suggest a second episode of elevated organic carbon burial rates accompanied by substantial instability in the global carbon cycle during the late middle and early late ordovician this pattern may mark the onset of climate changes culminating in the end ordovician hirnantian glaciation and mass extinction event that terminated the gobe

back transformation of high pressure minerals in shocked chondrites low pressure mineral evidence for strong shock post shock annealing of meteorites can destroy their shock induced features particularly high pressure minerals and complicate the estimation of impact pressure temperature conditions however distinguishing post shock annealing features from thermal metamorphism effects can be practically difficult here we report results from mbale a highly shocked l chondrite to investigate the mechanisms kinetics and identification criteria for post shock annealing of high pressure signatures olivine fragments within shock melt veins in mbale occur as chemically heterogeneous nanocrystalline aggregates that contain trace wadsleyite and ringwoodite their strong variation in fayalite content provides evidence of iron partitioning during transformation of olivine to wadsleyite followed by back transformation to olivine after decompression experimental studies of transformation kinetics show that wadsleyite transforms to olivine in seconds at temperatures above 1200 k and in hours at temperatures between 900 and 1200 k thermal models of shock melt cooling show that shock veins in mbale cooled to 1200 k in 1 s the shock pulse must have been shorter than 1 s to provide the high temperature conditions for post shock back transformation of wadsleyite many highly shocked l chondrites which have abundant high pressure minerals must have experienced relatively long shock durations combined with rapid cooling of shock melt regions to preserve high pressure phases the most highly shocked samples such as impact melt breccias lack high pressure phases because of post shock back transformations

middle upper ordovician darriwilian sandbian paired carbon and sulfur isotope stratigraphy from the appalachian basin usa implications for dynamic redox conditions spanning the peak of the great ordovician biodiversification event abstract the evans ferry section from the appalachian basin of eastern north america has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the great ordovician biodiversification event gobe paired stable isotope 13c and 34s analyses were used in this carbonate dominated locality from the appalachian basin to reconstruct the marine redox states during this key period of rapid biodiversification this succession is one of the most expanded sandbian stage upper ordovician deposits known from north america and it allows new high resolution reconstructions of long term global carbon and sulfur cycle fluctuations the integrated geochemical and sequence stratigraphic investigation presented here from the laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the middle late ordovician that may have been linked to global marine paleoredox dynamics utilizing new conodont based 87sr 86sr isotope stratigraphy along with previous conodont biostratigraphy we can confidently correlate our stable isotope profiles to other carbonate successions globally carbonate facies indicate that this area experienced partial restriction from open marine conditions during certain intervals in the sandbian these semi restricted environments record carbon 13ccarb isotope trends that are secular in nature as they can be correlated to other successions across laurentia and baltica we identify several intervals when 13ccarb and 34scas carbonate associated sulfate trends are decoupled these inverse stratigraphic trends are at times followed by parallel positive shifts in 13ccarb and 34scas causal mechanisms for the observed decoupled 13c and 34s trends may include a wide variety of factors such as more closed system local biogeochemical processes associated in part with diagenesis or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions the positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments

quantifying the community turnover of the uppermost wenlock and ludlow silurian conodonts in the baltic basin abstract the homerian to ludfordian interval of the mid to late silurian period was a time of significant changes in conodont communities global climate oceanographic patterns and biogeochemical cycles the mulde and the lau events are preeminent examples of globally recognized conodont extinction episodes from this interval in earth s history the silurian baltic basin is the most suitable locality globally for studying these perturbations to the ocean atmosphere system since there is an extensive record of conodont taxa occurrences their communities and their environments spanning across shore face to open ocean settings in this study we present new conodont and 13c data from the upper homerian to ludlow interval from two core sections g luva 99 and g luva 118 representing shelfal environments and the numerical conodont data from the vidukl 61 section from deep water settings and compare them with patterns of conodont diversity change as revealed in the data rich milaiciai 103 core section for this purpose we explored the stratigraphically tied time series of conodont diversity changes employing recurrence and cross recurrence plots the binary similarity matrices that are used for deciphering complex spatial and temporal dynamic patterns the cross recurrence plots were used as a means of synchronizing the geological sections by applying dynamic time warping and the newly described moving window median recurrence point search algorithms the results revealed that the conodont community compositional data are sufficiently temporally and spatially coherent to be reasonably used for synchronizing geological records moreover the sudden state transitions detected in the cross recurrence plots suggest that the lau event was of great importance for conodont community evolution in the studied time slice

a search for h chondritic chromite grains in sediments that formed immediately after the breakup of the l chondrite parent body 470 ma ago abstract a large abundance of l chondritic material mainly in the form of fossil meteorites and chromite grains from micrometeorites has been found in mid ordovician 470 ma old sediments globally the material has been determined to be ejecta from the l chondrite parent body breakup event a major collision in the asteroid belt 470 ma ago in this study we search the same sediments for h chondritic chromite grains in order to improve our understanding of the extraterrestrial flux to earth after the asteroid breakup event we have used sims in conjunction with quantitative sem eds to determine the three oxygen isotopic and elemental compositions respectively of a total of 120 randomly selected sediment dispersed extraterrestrial chromite grains mainly representing micrometeorites from 470 ma old post breakup limestone from the thorsberg quarry in sweden and the lynna river site in russia we show that 99 or more of the grains are l chondritic whereas the h chondritic fraction is 1 or less the l h chondrite ratio after the breakup thus was 99 compared to 1 1 in today s meteoritic flux this represents independent evidence in agreement with previous estimates based on sediment dispersed extraterrestrial chromite grain abundances and sedimentation rates of a two orders of magnitude higher post breakup flux of l chondritic material in the micrometeorite fraction finally we confirm the usefulness of three oxygen isotopic sims analyses of individual extraterrestrial chromite grains for classification of equilibrated ordinary chondrites the h and l chondritic chromites differ both in their three oxygen isotopic and elemental compositions but there is some overlap between the groups in chromite tio 2 is the oxide most resistant to diagenesis and the combined application of tio 2 and oxygen three isotope analysis can resolve uncertainties arising from the compositional overlaps

sediment dispersed extraterrestrial chromite traces a major asteroid disruption event abundant extraterrestrial chromite grains from decomposed meteorites occur in middle ordovician 480 million years ago marine limestone over an area of 250 000 square kilometers in southern sweden the chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to earth during the mid ordovician as previously indicated by fossil meteorites extraterrestrial chromite grains in mid ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar system history

early middle ordovician brachiopod diversification in the middle yangtze region of south china the evolution of the early middle ordovician brachiopod diversity in the middle yangtze region of south china has been analyzed the brachiopods in this region originated in the early tremadocian a

global mass wasting during the middle ordovician meteoritic trigger or plate tectonic environment abstract mass wasting at continental margins on a global scale during the middle ordovician has recently been related to high meteorite influx although a high meteorite influx during the ordovician should not be neglected we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 ma having strong arguments against the impact related hypothesis we propose an alternative explanation which is based on a re evaluation of the mass wasting sites considering their plate tectonic distribution and the global sea level curve a striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism terrane accretion and continental or back arc rifting respectively related to subduction of oceanic lithosphere such processes are commonly connected with seismic activity causing earthquakes which can cause downslope movement of sediment and rock considering all that it seems more likely that most of this mass wasting was triggered by earthquakes related to plate tectonic processes which caused destabilisation of continental margins resulting in megabreccias and debris flows moreover the period of mass wasting coincides with sea level drops during global sea level lowstand in some cases sea level drops can release pore water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins reduced pore water overpressure can also destabilise gas hydrate bearing sediment causing slope failure and thus resulting in submarine mass wasting overall the global mass wasting during the middle ordovician does not need meteoritic trigger

remnants of altered meteorite in the cretaceous paleogene clay boundary in poland fossil iron meteorites are extremely rare in the geological sedimentary record the paleometeorite described here is the first such finding at the cretaceous paleogene k pg boundary in the boundary clay from the outcrop at the lechowka quarry poland fragments of the paleometeorite were found in the bottom part of the host layer the fragments of meteorite 2 6 mm in size and meteoritic dust are metallic gray in color and have a total weight of 1 8181 g geochemical and petrographic analyses of the meteorite from lechowka reveal the presence of ni rich minerals with a total ni amount of 2 3 wt the identified minerals are taenite kamacite schreibersite ni rich magnetite and ni rich goethite no relicts of silicates or chromites were found the investigated paleometeorite apparently represents an independent fall and does not seem to be derived from the k pg impactor the high degree of weathering did not permit the chemical classification of the meteorite fragments however the recognized mineral inventory lack of silicates and their pseudomorphs and texture may indicate that the meteorite remains were an iron meteorite

a rain of ordinary chondritic meteorites in the early ordovician forty fossil meteorites with a total original mass of v7 7 kg have been recovered in the first systematic search for fossil meteorites pursued in an active quarry in lower ordovician 480 ma marine limestone in southern sweden the meteorites represent at least 12 different falls over a seafloor area of v6000 m 2 during 91 75 myr making the quarry one of the most meteorite dense areas known in the world geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably l chondrites mechanisms for meteorite delivery from the asteroid belt to earth were the same 480 ma as today however the flux was one to two orders of magnitude higher most likely reflecting the disruption of the l chondrite parent body at about that time this is a major event in late solar system history which may also have led to an enhanced flux of asteroids to earth during v30 myr fl 2001 elsevier science b v all rights reserved

integrated conodont biostratigraphy and carbon isotope chemostratigraphy in the lower middle ordovician of southern sweden reveals a complete record of the mdice one of the few and most complete records of the mdice middle darriwilian isotope carbon excursion is herein documented from baltoscandia based on a core section penetrating the condensed lower middle ordovician succession 46 m on the island of oland southeastern sweden we provide an integrated scheme for carbon isotope chemostratigraphy 313 samples and conodont biostratigraphy 29 samples for this period the carbonate succession in the tingskullen core records 12 conodont zones and 6 subzones including the oepikodus evae trapezognathus diprion baltoniodus triangularis b navis b norrlandicus lenodus antivariabilis l variabilis yangtzeplacognathus crassus eoplacognathus pseudoplanus microzarkodina hagetiana and microzarkodina ozarkodella subzones e suecicus pygodus serra e foliaceus e reclinatus e robustus and e lindstroemi subzones and pygodus anserinus zones in ascending order the 13ccarb record reveals an apparently complete record of the mdice including a rising limb a well defined peak and a falling limb the anomaly covers a thickness of c 27 m in the core and spans the eoplacognathus pseudoplanus e suecicus pygodus serra and p anserinus conodont zones combined with the new detailed conodont biostratigraphy the mdice in the tingskullen core can be used for detailed correlation with successions from baltica north america the argentine precordillera south china and north china

polychaete palaeoecology in an early late ordovician marine astrobleme of sweden the post impact dalby limestone kulcruse upper ordovician of the tvaren crater southeastern sweden has been analysed with regards to polychaetes as represented by scolecodonts a palaeoecological succession is observed in the tvaren 2 drill core sequence as the vacant ecospace was successively filled by a range of benthonic nektonic and planktonic organisms scolecodonts belong to the first non planktonic groups to appear and constitute one of the most abundant fossil elements the polychacte assemblage recorded has an overall composition characteristic of that of the upper ordovician of baltoscandia oenonites vistulella mochtyella and the enigmatic xanioprion represent the most common genera whereas pteropelta protarabellites atraktoprion and xanioprion are considerably more rare the assemblage differs from coeval ones particularly in its poorly represented ramphoprionid fauna and the relatively high frequency of xanioprion a taxonomic succession and changes in abundance and relative frequency of different taxa is observed from the deepest part of the crater and upwards towards more shallow water environments the initial post impact assemblage does not however necessarily represent a benthonic colonization of the crater floor instead it seems to be a taphocoenosis as indicated by its taxonomic correspondence to the rim facies fauna recovered from dalby limestone erratics of the ringson island the tvaren succession has yielded considerably richer scolecodont assemblages than hitherto recorded from the approximately coeval lockne crater possibly as a consequence of shallower water settings in the former area

petrographic classification of middle ordovician fossil meteorites from sweden the maximum diameter of chromite fecr2o4 grains within l chondrites reflects the petrographic type of the sample on the basis of our measurements of nine recent l chondrites l3 chromite d mac 34 50 mu m l4 87 150 mu m l5 76 158 mu m and l6 253 638 mu m this variation reflects the crystallization of the chromite grains during parent body thermal metamorphism we use this calibration to classify six fossil meteorites from the middle ordovician in sweden as type 3 or 4 to 6 the high flux of l chondrites at 470 ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent l chondrite falls the fossil meteorites have in places preserved recognizable chondrule textures including porphyritic olivine barred olivine and radiating pyroxene a large relict clast and fusion crust have also been tentatively identified in one fossil meteorite apart from chromite all of the original meteorite minerals have been replaced by carbonate and sheet silicate and sulfate during diagenesis within the limestone host the preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules the range 0 4 0 6 mm is consistent with measurements made in the same way on recent l chondrites less

in situ multiphase u pb geochronology and shock analysis of apatite titanite and zircon from the lac la moinerie impact structure canada in situ u pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite titanite and zircon from the 8 km diameter lac la moinerie impact structure canada the three analyzed phases are inherited from target rocks that were transformed to impact melt bearing breccias and clast laden impact melts apatite yields an array of u pb ratios populating a region between a younger regression with a refined age of 453 5 ma and an older regression with a lower intercept age of 1708 10 ma titanite defines a younger regression with an age of 444 15 ma and an older array with a lower intercept at 1844 24 ma zircon yields an upper intercept age of 1810 13 ma and a lower intercept age of 433 21 ma within error of apatite and titanite the oldest ages from all three phases define paleoproterozoic igneous metamorphic events in granitoid target rocks of the de pas suite isotopic resetting in apatite and titanite with relatively high pb diffusion rates was controlled by a combination of proximity to impact generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization zircon resetting was controlled by shock effects and metamictization which resulted in significant pb loss we outline an approach for discriminating true impact ages from metamict zircons with high common pb that have experienced post impact pb loss lower intercept ages of all phases constrain the age of formation for the lac la moinerie impact structure with apatite yielding the most precise age of 453 5 ma with an age of 453 ma lac la moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the middle to late ordovician with potential connection to the l chondrite parent body breakup event and a period of enhanced bombardment on earth

global mass wasting at continental margins during ordovician high meteorite influx the break up of an asteroid led to the rapid delivery of meteorites to earth around 470 million years ago the widespread occurrence of sedimentary megabreccias deposited at the same time suggests this high influx of meteorites could have triggered mass wasting downslope movement of rock and sediment at continental margins by inducing the destabilization of sedimentary slopes

new 40ar 39ar dating of the clearwater lake impact structures quebec canada not the binary asteroid impact it seems abstract the two clearwater lake impact structures quebec canada are generally interpreted as a crater doublet formed by the impact of a binary asteroid here arguments are presented that raise important questions about the proposed double impact scenario new 40 ar 39 ar dating of two virtually fresh impact melt rock samples from the 36 km west clearwater lake impact structure yielded two statistically robust early permian plateau ages with a weighted mean of 286 2 2 2 2 6 ma 2 mswd 0 33 p 0 57 in contrast 40 ar 39 ar results for two chloritized melt rocks from the 26 km east clearwater lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component although individually weakly robust age spectra corrected for the trapped argon component and inverse isochron plots for the east clearwater melt rocks consistently yielded apparent ages around 460 470 ma no permian signal was found in either of these melt aliquots our new 40 ar 39 ar results reproduce earlier 40 ar 39 ar plateau ages 283 ma and 465 ma respectively for the two impact structures by bottomley et al 1990 and are in conflict with a previous statistically non robust rb sr age of 287 293 26 ma for east clearwater the combined cluster of apparent ages of 460 470 ma derived from four different samples across the impact melt sheet is very unlikely to represent a false age effect due to the incorporation of extraneous argon into the melt instead it strongly favors a middle ordovician age for the east clearwater impact and impact induced hydrothermal chloritization moreover the clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories an effect unexpected in the case of a permian double impact whereas the west clearwater impact affected ordovician carbonates incorporated into the impact breccia drill core reports from the 1960s concluded that clasts of ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the east clearwater lake impact structure which is overlain by 100 m of post impact sandstones shales and carbonates no resolvable impactor contamination has so far been detected in the west clearwater impact melt rocks whereas east clearwater carries a distinct ordinary possibly l chondritic impactor signature in its melt rocks east clearwater lake might thus represent one among a long list of ordovician impact structures in north america and northern europe that were presumably formed in response to the l chondrite asteroid breakup event 470 ma ago paleogeographic reconstructions show that the ordovician east clearwater impact probably occurred in a near coastal to shallow marine setting while the permian west clearwater impact hit continental pangaea along with the new 40 ar 39 ar data the paleomagnetic sedimentologic and geochemical findings suggest that the close spatial arrangement of the two clearwater lakes is most likely pure coincidence the two impact structures seem to represent a false doublet struck by impacts separated by 180 million years in time the new results for the clearwater lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on earth and on other planetary bodies in the inner solar system

attenuation models qp and qs in three dimensions of the southern california crust inferred fluid saturation at seismogenic depths we analyze high dynamic range waveform spectra to determine t values for both p and s waves from earthquakes in southern california we invert the t values for three dimensional 3 d frequency independent q p and q s regional models of the crust the models have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth and extend from the u s mexico border to the coast ranges in the south and sierra nevada in the north in general qp and qs increase rapidly with depth consistent with crustal densities and velocities the 3 d q p and q s models image the major tectonic structures and to a much lesser extent the thermal structure of the southern california crust the near surface low q p and q s zones coincide with major sedimentary basins such as the san bernardino chino san gabriel valley los angeles ventura and santa maria basins and the salton trough in contrast at shallow depths beneath the peninsular ranges southern mojave desert and southern sierras we image high q p and q s zones which correspond to the dense and high velocity rocks of the mountain ranges several clear transition zones of rapidly varying q p and q s coincide with major late quaternary faults and connect regions of high and low q p and q s at midcrustal depths the q p and q s exhibit modest variation in slightly higher and lower qp or qs zones which is consistent with reported crustal reflectivity in general for the southern california crust q s q p is greater than 1 0 suggesting partially fluid saturated crust a few limited regions of q s q p less than 1 0 correspond to areas mostly outside the major sedimentary basins including areas around the san jacinto fault suggesting a larger reduction in the shear modulus compared to the bulk modulus or almost complete fluid saturation

the nature of the l chondrite parent body s disruption as deduced from high pressure phases in the sixiangkou l6 chondrite the disruption of the l chondrite parent body lcpb at similar to 470ma is currently the best documented catastrophic celestial impact event based on the large number of l chondritic materials associated with this event uranium lead u pb dating of apatite and its high pressure decomposition product tuite in the sixiangkou l6 chondrite provides a temporal link to this event the u pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the u pb system correlates closely with the extent of apatite decomposition this suggests that the u pb system of apatite could be substantially disturbed by high temperature pulse during shock compression from natural impacts at least on the scale of mineral grains although many l chondrites can be temporally related to the catastrophic lcpb impact event the shock conditions experienced by each individual meteorite vary this could be due to the different geologic settings of these meteorites on their parent body the shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions although they could provide lower limits to the impact conditions the sixiangkou shock duration similar to 4s estimated from high pressure transformation kinetics provides a lower limit to the high pressure pulse of the lcpb disruption impact combined with available literature data of l chondrites associated with this impact event our results suggest that the lcpb suffered a catastrophic collision with a large projectile with a diameter of at least 18 22km at a low impact velocity 5 6kms 1 this is consistent with astronomical estimates based on the dynamical evolution of l chondritic asteroids

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

u pb geochronology of apatite and zircon from the brent impact structure canada a late ordovician sandbian katian boundary event associated with l chondrite parent body disruption in situ la icp ms u pb geochronology has been performed on apatite and zircon within thermally recrystallized clast laden and clast poor impact melt rocks from the brent impact structure a total of 377 laser analyses on 120 impact melt grown n 9 and impact recrystallized zircon grains n 111 were obtained from which a concordia age of 452 8 2 7 ma mswd 0 57 n 11 and a weighted average mean 206pb 238u age of 453 2 2 9 ma mswd 0 60 n 11 are calculated a total of 300 laser analyses from 100 relict apatite grains were obtained with an unanchored regression through all data yielding a lower intercept age of 453 2 6 0 ma mswd 5 8 n 300 that overlaps within error of zircon 207pb 206pb ratios obtained from feldspar clasts within clast laden impact melt retain the same initial pb composition as the target rocks from which they are derived while feldspars that crystallized from impact melt have 207pb 206pb ratios indicative of isotopic re equilibration between basement lithologies of two different ages a similar variability in 207pb 206pb is recorded by apatite this provides evidence for the involvement of neoproterozoic lake nipissing alkaline suite as well as mesoproterozoic grenville gneisses in the production of impact melt at brent recrystallized apatite grains exhibit enrichments in light rare earth elements lrees along neoblast grain boundaries indicative of trace element substitution and phase precipitation during impact induced recrystallization an age of 452 8 2 7 ma from zircon and 453 2 6 0 ma from apatite places the impact event in the late ordovician at or near the sandbian katian boundary confirming brent s involvement in the middle to late ordovician crater cluster event a period of enhanced impactor flux to earth related to the l chondrite parent body disruption

late ordovician brachiopod distribution and ecospace partitioning in the tvaren crater system sweden abstract patterns of distribution and ecospace utilization of late ordovician brachiopods in a recently formed contemporary meteorite crater are described and analyzed rhynchonelliformean brachiopod communities dominated by a wide range of orthides and strophomenides colonized the newly formed crater at the crater rim communities were established early on although the crater depression was not inhabited until deposition of the upper third of the remaining crater fill the crater formed a protected but restricted microenvironment where sediments four times the thickness of the nearby basinal succession accumulated within this narrow space environments varied from shallow water to deeper waters about 200 m in depth and from well oxygenated to hypoxic such varied environments generated a rough ecological landscape facilitating niche partitioning across a relatively small geographic area analysis of the guild structure of the fauna permits explanation of a local biodiversity hotspot in otherwise low diversity strata elsewhere in the scandinavian region the tvaren impact event had an important palaeobiologic effect upon the fossil record as it served as a local pump and reservoir for biodiversity moreover the development of new community types and narrowly defined niches helped further drive both and biodiversity during a critical phase of the great ordovician biodiversification event

modelling curved surface wave paths membrane surface wave synthetics summary earth s heterogeneity near the surface is so severe that surface waves with period 20 s exhibit complicated propagation effects such as curved paths and multiple rays in order to take account of these effects a new method to analyse surface waves is developed the method which we call the membrane surface wave method is an approximation to the green s function method in a 3 d medium and is valid for a laterally smoothly varying media we also present the formulae to invert the seismograms as an example we apply the method to a preliminary california model use of the method for the forward calculation shows that heterogeneity in the model is so severe that the wavefront due to the whittier narrows earthquake is split in two by the time it reaches the san francisco bay area the kernels in the inverse formula to the six berkeley stations show curved paths multipathings and finite wavelength effects and visually demonstrate how waves sample the medium the method is attractive in that these complications cause no problems in its application

biotic dynamics and carbonate microfacies of the conspicuous darriwilian middle ordovician taljsten interval south central sweden enclosed in the darriwilian middle ordovician part of the reddish orthoceratite limestone of baltoscandia is a conspicuous c 1 5 m thick unit colloquially known as the taljsten interval it has a wide geographical distribution in the baltoscandian paleocontinent but is particularly well exposed in south central sweden the unit is characterized by its deviant gray color and beds with a mass occurrence of cystoid echinoderms moreover the recent discovery of an anomalous abundance of extraterrestrial chromite grains and micrometeorites in the taljsten and immediately adjacent strata bear witness of a time of deposition correlating to an increased influx of extraterrestrial matter to earth in this study we analyze the faunal dynamics based primarily on microfossils and carbonate microfacies across this intriguing interval generally known as a macrolithologically monotonous succession the taljsten proved more heterogeneous than previously believed in terms of fossil content and preservation as well as mineral content and composition the general texture is wackestone packstone but local heterogeneities are frequent a microfaunal reorganization occurred with a transition from a trilobite and brachiopod dominated community to one dominated by echinoderms which is mirrored also macroscopically by the cystoids the latter pelmatozoans seem to have flourished and might even have aided in the formation of the numerous hardgrounds the increase in echinoderms is linked to a volkhovian increase in echinoderms in eastern baltica in the immediate post taljsten strata there is no distinct change back to the pre taljsten biota conodont and ostracod faunal diversity data reveal peak values in the taljsten compared to the enclosing strata suggesting that the paleoenvironment was more suitable for higher diversity faunas of these metazoan groups all data collectively support a regression in the beginning of the taljsten likely followed by transgression in its upper part c 2012 elsevier b v all rights reserved less

calibration of a conodont apatite based ordovician 87sr 86sr curve to biostratigraphy and geochronology implications for stratigraphic resolution the ordovician 87 sr 86 sr isotope seawater curve is well established and shows a decreasing trend until the mid katian however uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 sr 86 sr decrease may have varied which has implications for both the stratigraphic resolution possible using sr isotope stratigraphy and efforts to model the effects of ordovician geologic events we measured 87 sr 86 sr in conodont apatite in north american ordovician sections that are well studied for conodont biostratigraphy primarily in nevada oklahoma the appalachian region and ohio valley our results indicate that conodont apatite may provide an accurate medium for sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 sr 86 sr during the middle ordovician darriwilian stage our 87 sr 86 sr results suggest that sr isotope stratigraphy will be most useful as a high resolution tool for global correlation in the mid darriwilian to mid sandbian when the maximum rate of fall in sr

the millikan shaking experiments and high frequency seismic wave propagation in southern california in order to study high frequency seismic wave propagation seismic wavefields generated by resonant shaking experiments of the millikan library on the campus of california institute technology pasadena california usa were analysed because the resonant shaking frequencies are 1 12 hz the east west direction and 1 64 hz the north south direction this active source experiment can provide opportunities for studying high frequency seismic wave propagation in southern california because they are very narrow frequency band data the analyses must be quite different from ordinary time domain analyses we show theoretically that the signals must be dominated by surface waves adopting this surface wave assumption we proceed to make two separate analyses one on spectral amplitude and the other on phase we present a new method to derive group velocity from phase based on the cross correlations between the station in the millikan library mik and stations in the regional network our results support that an active source experiment by resonant shaking of a building is a feasible approach for high frequency seismic wave studies the authors 2014 published by oxford university press on behalf of the royal astronomical society

records of the moon forming impact and the 470 ma disruption of the l chondrite parent body in the asteroid belt from u pb apatite ages of novato l6 novato a newly observed fall in the san francisco bay area is a shocked and brecciated l6 ordinary chondrite containing dark and light lithologies we have investigated the u pb isotope systematics of coarse cl apatite grains of metamorphic origin in novato with a large geometry ion microprobe the u pb systematics of novato apatite reveals an upper intercept age of 4472 31 ma and lower intercept age of 473 38 ma the upper intercept age is within error identical to the u pb apatite age of 4452 21 ma measured in the chelyabinsk ll5 chondrite this age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the moon forming giant impact on earth the lower intercept age is consistent with the most precisely dated ar ar ages of 470 6 ma of shocked l chondrites and the fossil meteorites and extraterrestrial chromite relicts found in ordovician limestones with an age of 467 3 1 6 ma in sweden and china the lower intercept age reflects a major disturbance related to the catastrophic disruption of the l chondrite parent body most likely associated with the gefion asteroid family which produced an initially intense meteorite bombardment of the earth in ordovician period and reset and degassed at least approximately 35 of the l chondrite falls today we predict that the 470 ma impact event is likely to be found on the moon and mars if not mercury

determining the impactor of the ordovician lockne crater oxygen and neon isotopes in chromite versus sedimentary pge signatures abundant chromite grains with l chondritic composition in the resurge deposits of the lockne impact crater 458 myr old dia similar to 10 km in sweden have been inferred to represent relict fragments of an impactor from the break up of the l chondrite parent body at 470 ma this view has been challenged based on it cr and platinum group element pge patterns of the same resurge deposits and a reinterpretation of the origin of the chromite grains an impactor of the non magmatic iron meteorite type was proposed instead here we show that single grain oxygen and noble gas isotope analyses of the chromite grains from the resurge deposits further support an origin from an l chondritic asteroid we also present pge analyses and ir cr ratios for fossil l chondritic meteorites found in mid ordovician marine limestone in sweden the l chondritic origin has been confirmed by several independent methods including major element and oxygen isotopic analyses of chromite although the meteorites show the same order of magnitude pge and cr concentrations as recent l chondrites the elements have been redistributed to the extent that it is problematic to establish the original meteorite type from these proxies different pge data processing approaches can lead to highly variable results as also shown here for the lockne resurge deposits we conclude that the lockne crater was formed by an l chondritic impactor and that considerable care must be taken when inferring projectile type from pges in sedimentary ejecta deposits c 2011 elsevier b v all rights reserved less

diachroneity of the clearwater west and clearwater east impact structures indicated by the u th he dating method the u th he method has been applied to constrain the formation ages of the clearwater west and east impact structures of quebec canada zircons were separated from impact melt samples derived from a surface exposure at clearwater west 32 km diameter and from a drill core at clearwater east 26 km diameter the u th he results indicate ages of 280 27 ma 2 n 7 for clearwater west and 450 56 ma 2 n 8 for clearwater east our u th he date for clearwater west supports the findings of previous rb sr 266 15 ma 2 and 40ar 39ar 280 4 ma and 283 8 2 2 ma 2 impact melt studies our u th he date for clearwater east also overlaps with previously published 40ar 39ar dating results which yielded u shaped spectra with maximum and best estimate dates of 460 470 ma our results support the contention previously based solely on 40ar 39ar data that the clearwater west and east impact structures do not comprise an impact doublet that formed coevally from a binary asteroid pair

chromium isotopes in late eocene impact spherules indicate a likely asteroid belt provenance abstract spherules and magnetic fractions extracted from two late eocene impact deposits massignano italy and ocean drilling program hole 709c western indian ocean have anomalous concentrations of ir and high concentrations of cr ni co and fe an excess of 53 cr relative to 53 cr 52 cr ratios in terrestrial samples shows that much of the cr in these samples is extraterrestrial the absence of relatively large 54 cr excesses as found in carbonaceous chondrites precludes a carbonaceous chondrite source and the data best fit ordinary chondrites the two localities produced samples with somewhat different elemental and isotopic compositions but these are likely different components of the same impact rather than from two distinct impacts the multiple impacts and dust deposition in the late eocene were more likely caused by disruption of a large asteroid in the inner asteroid belt than by a comet shower the brangane asteroid family may be the result of this disruption the source of the dust and impactors in the late eocene and its members may be composed of h chondrite lithologies based upon the coeval 35 ma peak in h chondrite cosmic ray exposure ages

late holocene environmental history of southern choco region pacific colombia sediment diatom and pollen analysis of core el caimito abstract we present a multi proxy study of pollen diatoms sediment characteristics and major elements of a 610 cm sediment core from lake el caimito located in the humid rain forest of southern choco pacific colombia we propose an integrated reconstruction of the local basin development and of the regional vegetation which is possibly related to the tectonic activity of an unstable coastal area time control is based on 7 ams 14 c dates that show that the record represents the last 3850 cal yr bp from 3850 to 2700 cal yr bp sandy deposits low carbon content absence of diatoms and low diversity of the pollen spectra indicate that the site was under the influence of the fluvial system erosive event s removed part of the sediment record and we observed a hiatus representing 700 years after that the basin became more isolated from the river drainage system from 2010 to 1430 cal yr bp mainly clay was deposited and repeatedly interrupted by river pulses that left sandy and silty horizons in the record benthic and littoral benthic diatom species indicate a shallow water body and a stable water chemistry mangrove forest was close to the lake apparently growing along the close by inlets regionally the main vegetation elements were palms and taxa in the families of moraceae urticaceae melastomataceae fabaceae and from a number of other families and genera characteristic of tropical lowland rain forest from 1430 to 810 cal yr bp the river impact gradually diminished each fluvial event that affected the local forest is shown in the pollen record by an expansion of cecropia dominated pioneer forest decreasing intensity of forest disturbance coincides with an increase in the diversity of fossil pollen taxa possibly reflecting an increasing plant diversity of the forest mangrove pollen declined indicating that the coastline moved seaward and suggesting tectonic uplift of the coastal area between 810 and 580 cal yr bp mangrove forest was closer to the lake again reflecting an inland migration of the coastline suggesting tectonic subsidence from 580 to 300 cal yr bp the last fluvial events were recorded diatom associations indicate oligotrophic and acidic water the mangrove belt moved seaward again suggesting tectonic uplift palms and cecropia became more abundant suggesting increased human impact in the near shore lowland forest during the last 300 years stable lacustrine conditions and lowland rain forest with the highest floral diversity is registered

a russian record of a middle ordovician meteorite shower extraterrestrial chromite at lynna river st petersburg region numerous fossil meteorites and high concentrations of sediment dispersed extraterrestrial chromite ec grains with ordinary chondritic composition have previously been documented from 467 1 6 ma middle ordovician darriwilian strata these finds probably reflect a temporarily enhanced influx of l chondritic matter following the disruption of the l chondrite parent body in the asteroid belt 470 6 ma in this study a volkhovian kundan limestone marl succession at lynna river northwestern russia has been searched for ec grains 63 mu m eight samples forming two separate sample sets were collected five samples from strata around the asaphus expansusa raniceps trilobite zone boundary in the lower middle kundan yielded a total of 496 ec grains in 65 5 kg of rock average 7 6 ec grains kg 1 but up to 10 2 grains kg 1 these are extremely high concentrations three orders of magnitude higher than background levels in similar condensed sediment from other periods ec grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite three stratigraphically lower lying samples close to the a lepidurusa expansus trilobite zone boundary at the volkhov kunda boundary yielded only two ec grains in 38 2 kg of rock 0 05 grains kg 1 the lack of commonly occurring ec grains in the lower interval probably reflects that these strata formed before the disruption of the l chondrite parent body the great similarity in ec chemical composition between this and other comparable studies indicates that all or most ec grains in these russian mid ordovician strata share a common sourcethe l chondrite parent body less

the conodont paroistodus horridus barnes and poplawski as a new biostratigraphical tool for the middle darriwilian ordovician abstract this contribution develops an analysis on the conodont paroistodus horridus barnes and poplawski from several argentine precordilleran sections from these collections we have established that p horridus occurs from the lenodus crassus zone to the amorphognathus tvaerensis zone darriwilian to early sandbian this study recognizes several primary morphological features of p horridus which vary through the darriwilian and three evolutionary morphological types can be identified early form middle form with two stages and late form the co occurrence of index conodonts with the successive stages of this evolutionary development allowed a significant biostratigraphical value to be assigned for each different morphological type of p horridus based on this information we propose a detailed biostratigraphical control and correlation with equivalent strata from canada china australia united states norway and western central asia

the winneshiek biota exceptionally well preserved fossils in a middle ordovician impact crater the winneshiek shale middle ordovician darriwilian was deposited in a meteorite crater the decorah impact structure in ne iowa this crater is 5 6 km in diameter and penetrates cambrian and ordovician cratonic strata it was probably situated close to land in an embayment connected to the epicontinental sea typical shelly marine taxa are absent the konservat lagerstatte within the winneshiek shale is important because it represents an interval when exceptional preservation is rare the biota includes the earliest eurypterid a giant form as well as a new basal chelicerate and the earliest ceratiocarid phyllocarid conodonts some of giant size occur as bedding plane assemblages bromalites and rarer elements including a linguloid brachiopod and a probable jawless fish are also present similar fossils occur in the coeval ames impact structure in oklahoma demonstrating that meteorite craters represent a novel and under recognized setting for konservat lagerstatten

middle late ordovician darriwilian sandbian decoupling of global sulfur and carbon cycles isotopic evidence from eastern and southern laurentia abstract middle late ordovician sequences from the appalachian basin and arbuckle mountain regions of north america were analyzed for carbonate associated sulfate 34 s cas and pyrite 34 s pyr paired with carbonate 13 c carb and organic matter 13 c org chemostratigraphy two major negative drops in 34 s cas 12 excursions are recognized the older decline in 34 s cas occurs within the histiodella holodentata phragmodus polonicus conodont zones and the younger drop is within the cahabagnathus sweeti amorphognathus tvaerensis baltoniodus gerdae subzone zones these overall these negative shifts in 34 s cas have an antithetical relationship with positive shifts in 34 s pyr 10 and 13 c carb 2 recorded in the same successions the older negative 34 s cas shift is coincident with the widely documented mid darriwilian 13 c excursion mdice and the younger negative 34 s cas shift is coincident with another positive 13 c carb shift in the early sandbian geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative 34 s cas trends additionally a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends while increased crustal weathering is broadly consistent with a sea level lowstand and the seawater 87 sr 86 sr isotope record of change in continental weathering in the late darriwilian stage of the ordovician geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in 34 s cas these antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates pulses of oceanic ventilation would have expanded habitable environments for marine organisms and thus is broadly consistent with major increases in biodiversification during this period of the ordovician

major oceanic redox condition change correlated with the rebound of marine animal diversity during the late ordovician to understand probable influences of environmental changes on the decrease and rebound of biodiversity during the late ordovician a detailed analysis of total organic carbon toc pyrite v v ni ratios and s isotopic compositions of pyrite in the late ordovician sediments from poland was performed the significant enrichments of toc and pyrite as well as high v v ni ratios 0 6 for the jeleniow formation of the latest darriwilian to middle katian stages provide evidence for predominant anoxic bottom water conditions in contrast low toc and pyrite as well as low v v ni ratios for the wolka formation of late katian age indicate oxic depositional conditions the s isotopic data are consistent with the interpretations of oceanic redox conditions based on multiple proxies for paleoredox the results from the baltica area reveal a major oceanic redox condition change during the late katian and suggest that oceanic oxygenation may have contributed to the rebound of marine animal diversity during the late ordovician

extraterrestrial chromite in middle ordovician marine limestone at kinnekulle southern sweden traces of a major asteroid breakup event the distribution of sediment dispersed extraterrestrial chromite grains and other cr rich spinels 63 mu m has been studied in middle ordovician orthoceratite limestone from two quarries at kinnekulle southern sweden in the thorsberg quarry all similar to 3 2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment dispersed extraterrestrial chromite grains typically 1 3 grains are found per kilogram of limestone in the nearby hallekis quarry the same beds show similarly high concentrations of extraterrestrial chromite grains but in samples representing the 9 m downward continuation of the section exposed at this site only 5 such grains were found in a total of 379 kg of limestone the extraterrestrial equilibrated ordinary chondritic chromite grains call be readily distinguished by a homogeneous and characteristic major element chemistry including 2 0 3 5 wt tio2 and stable v2o3 concentrations close to 0 7 wt terrestrial cr rich spinels have a wide compositional range and co exist with extraterrestrial chromite in some beds these grains may be derived for example from mafic dykes exposed and weathered at the sea floor considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied instead the difference may be primarily related to all increase in the ancient flux or extraterrestrial matter to earth in connection with the disruption of the l chondrite parent body in the asteroid belt at about this time the coexistence in some beds of high concentrations of chondritic chromite and terrestrial cr rich spinels however indicates that redistribution of heavy minerals on the sea floor related to changes in sea level and sea floor erosion and currents must also be considered less

extraterrestrial chromite distribution across the mid ordovician puxi river section central china evidence for a global major spike in flux of l chondritic matter previous studies of mid ordovician limestone in sweden have shown that over a stratigraphic interval representing a few million years there is a two orders of magnitude enrichment in fossil l chondritic meteorites empty set 1 21 cm and sediment dispersed extraterrestrial chromite ec grains 63 mu m this has been interpreted as a dramatic increase in the flux of l chondritic matter to earth following the breakup of the l chondrite parent body which based on ar ar gas retention ages 470 6 ma of recently fallen meteorites occurred at about this time here we show that the general trend in the distribution of sediment dispersed ec grains can be reproduced in the puxi river section in central china a total of 288 kg of limestone was searched for chrome spinels in samples spanning the lower 8 m of the section representing the paroistodus originalis and lenodus antivariabilis conodont zones a total of 110 kg of limestone yielded only one ec grain similarly to the swedish sections ec grains begin to be common in the overlying l variabilis zone and remain common throughout the upper 9 m of the section representing the l variabilis yangtzeplacognathus crassus and l pseudoplanus zones in this part of the section 178 kg of limestone yielded 290 ec grains with an average chemical composition very similar to chromite from recent l chondrites in most of the beds over this interval one finds 1 4 ec grains per kilogram rock a clear two orders of magnitude enrichment relative to the lower part of the section small bed by bed variations in the ec content over the upper interval most likely reflect small variations in sedimentation rates the puxi river section contains only very rare terrestrial chrome spinel grains which can be distinguished already by their rounded abraded appearance compared to the angular pristine extraterrestrial spinels in the mid ordovician based on paleoplate reconstructions the puxi river site was positioned at mid latitudes on the southern hemisphere a couple of thousand kilometers east of the swedish sites the prominent enrichment of ec grains over the same stratigraphic interval in china and sweden is supporting evidence for a dramatic increase in the flux of l chondritic matter to earth shortly after the disruption of the l chondrite parent body in the asteroid belt c 2010 elsevier inc all rights reserved less