preservation of extraterrestrial 3he in 480 ma old marine limestones we have measured the helium abundance and isotopic composition of a suite of lower ordovician marine limestones and associated fossil meteorites from kinnekulle sweden limestone 3he 4he ratios as high as 11 5 times the atmospheric value in fused samples and up to 23 times atmospheric in a single step heat fraction indicate the presence of extraterrestrial helium and demonstrate that at least a fraction of the extraterrestrial 3he carried by interplanetary dust particles must be retained against diffusive and diagenetic losses for up to 480 ma the carrier phase has not been identified but is not magnetic extrapolation of high temperature 3he diffusivities in these sediments is consistent with strong retention of extraterrestrial 3he under ambient earth surface conditions combination of the observed helium concentrations with sedimentation rates estimated from conodont biostratigraphy suggest that the flux of extraterrestrial 3he in the early ordovician was about 0 5 10 12 cm 3 stp cm 2 ka 1 ignoring potential post deposition helium loss this value is indistinguishable from the average 3he flux estimated for the cenozoic era in contrast previous studies of fossil meteorites ir abundances and os isotopic ratios in the limestone suggest that the total accretion rate of extraterrestrial material during the studied interval was at least an order of magnitude higher than the cenozoic average this disparity may reflect significant post depositional loss of 3he from idps within these old limestones if so the match between the ordovician flux and the cenozoic average would be fortuitous alternatively the size distribution of infalling objects during the early ordovician may have been enriched only in extraterrestrial material too large to retain 3he during atmospheric entry heating 30 m the fossil meteorites themselves also preserve extraterrestrial helium meteorite 3he concentrations of 2 to 9 10 12 cm 3 stp g 1 are several orders of magnitude lower than found in most modern meteorites suggesting very substantial helium loss probably 99 9 from these chemically altered objects the meteorites carry 3he concentrations only a factor of a few higher than the host limestones the meteorites themselves cannot be the source of the extraterrestrial 3he observed in the limestones

precessional climate cyclicity in late cretaceous early tertiary marine sediments a high resolution chronometer of cretaceous tertiary boundary events we report well dated late cretaceous and early tertiary precessional climatic cycles recorded by rhythmic carbonate maxima and minima in south atlantic deep sea sites spectral analyses of digitized sediment color a suitable carbonate proxy show prominent regularities in the spacing marl carbonate beds magnetostratigraphic dating over a number of magnetic chrons constrains the duration of the cycles which can be detected over at least 20 myr of sedimentation at 7 coring locations their mean absolute period of23 5 4 4kyr agrees closely with the predicted late cretaceous precessional period of 20 8 kyr because they can be matched to a physical forcing mechanism with a known repeat time the cycles offer a new high resolution tool to measure rates of climate change before and after the cretaceous tertiary k t boundary from counts of carbonate cycles we derive the position of the k t boundary within c29r at 350 kyr after the base of the reversal the constancy of cycle thickness linearly related to sedimentation rate and amplitude up to the boundary clay does not give evidence for climate instability preceding the boundary orbital chronometry records a step function decrease in sediment accumulation rate at the cretaceous tertiary boundary that is consistent with a geologically instantaneous event

empirical and theoretical comparisons of the chicxulub and sudbury impact structures chicxulub and sudbury are 2 of the largest impact structures on earth research at the buried but well preserved chicxulub crater in mexico has identified 6 concentric structural rings in an analysis of the preserved structural elements in the eroded and tectonically deformed sudbury structure in canada we identified ring like structures corresponding in both radius and nature to 5 out of the 6 rings at chicxulub at sudbury the inner topographic peak ring is missing which if it existed has been eroded reconstructions of the transient cavities for each crater produce the same range of possible diameters 80110 km the close correspondence of structural elements between chicxulub and sudbury suggests that these 2 impact structures are approximately the same size both having a main structural basin diameter of 150 km and outer ring diameters of 200 km and 260 km this similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt melt produced directly upon decompression from high pressure impact and impact melt shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall in large impacts our empirical comparisons suggest that sudbury has 70 more impact melt than does chicxulub 31 000 versus 18 000 km3 and 85 more shock melt 27 000 km3 versus 14 500 km3 to examine possible causes for this difference we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts we use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models the results demonstrate that the differences in melt volumes can be readily explained if chicxulub was an asteroid impact and sudbury was a comet impact the estimated 70 difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked preheating of the target rocks at sudbury by the penokean orogeny cannot explain the excess melt at sudbury the majority of which resides in the suevite the greater amount of suevite at sudbury compared to chicxulub may be due to the dispersal of shock melt by cometary volatiles at sudbury

a 100 kyr periodicity in the flux of extraterrestrial 3he to the sea floor most of the helium 3 in oceanic sediments comes from interplanetary dust particles idps and can therefore be used to infer the accretion rate of dust to the earth through time 3he records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years probably owing to cometary and asteroidal break up events muller and macdonald have proposed that periodic changes in this accretion rate due to a previously unrecognized 100 kyr periodicity in the earth s orbital inclination might account for the prominence of this frequency in climate records of the past million years here we report variations in the 3 he flux to the sea floor that support this idea we find that the flux recorded in rapidly accumulating quaternary sediments from the mid atlantic ridge oscillates with a period of about 100 kyr we cannot yet say however whether the 100 kyr climate cycle is a consequence of a cause of or an effect independent of these periodic changes

comparing dust flux records from the subarctic north pacific and greenland implications for atmospheric transport to greenland and for the application of dust as a chronostratigraphic tool we present a new record of eolian dust flux to the western subarctic north pacific snp covering the past 27 000 years based on a core from the detroit seamount comparing the snp dust record to the north greenland ice core project ngrip ice core record shows significant differences in the amplitude of dust changes to the two regions during the last deglaciation while the timing of abrupt changes is synchronous if dust deposition in the snp faithfully records its mobilization in east asian source regions then the difference in the relative amplitude must reflect climate related changes in atmospheric dust transport to greenland based on the synchronicity in the timing of dust changes in the snp and greenland we tie abrupt deglacial transitions in the 230th normalized 4he flux record to corresponding transitions in the well dated ngrip dust flux record to provide a new chronostratigraphic technique for marine sediments from the snp results from this technique are complemented by radiocarbon dating which allows us to independently constrain radiocarbon paleoreservoir ages we find paleoreservoir ages of 745 140 years at 11 653 year b p 680 228 years at 14 630 year b p and 790 498 years at 23 290 year b p our reconstructed paleoreservoir ages are consistent with modern surface water reservoir ages in the western snp good temporal synchronicity between eolian dust records from the subantarctic atlantic and equatorial pacific and the ice core record from antarctica supports the reliability of the proposed dust tuning method to be used more widely in other global ocean regions

new insights into the carrier phase s of extraterrestrial 3he in geologically old sediments to better understand the composition characteristics of helium diffusion and size distribution of interplanetary dust particles idps responsible for the long term retention of extraterrestrial 3he we carried out leaching stepped heating and sieving experiments on pelagic clays that varied in age from 0 5 ma to 90 myr the leaching experiments suggest that the host phase s of 3he in geologically old sediments are neither organic matter nor refractory phases such as diamond graphite al2o3 and sic but are consistent with extraterrestrial silicates fe ni sulfides and possibly magnetite stepped heating experiments demonstrate that the 3he release profiles from the magnetic and non magnetic components of the pelagic clays are remarkably similar because helium diffusion is likely to be controlled by mineral chemistry and structure the stepped heating results suggest a single carrier that may be magnetite or more probably a phase associated with magnetite furthermore the stepped outgassing experiments indicate that about 20 of the 3he will be lost through diffusion at seafloor temperatures after 50 myrs while sedimentary rocks exposed on the earth s surface for the same amount of time would lose up to 60 the absolute magnitude of the 3he loss is however likely to depend upon the 3he concentration profile within the idps which is not well known contrary to previous suggestions that micrometeorites in the size range of 50 100 m in diameter are responsible for the extraterrestrial 3he in geologically old sediments stuart f m harrop p j knott s turner g 1999 laser extraction of helium isotopes from antarctic micrometeorites source of he and implications for the flux of extraterrestrial 3he flux to earth geochim cosmochim acta63 2653 2665 our sieving experiment demonstrates that at most 20 of the 3he is carried by particles greater than 50 m in diameter the size distribution of the 3he bearing particles implies that extraterrestrial 3he in sediments record the idp flux rather than the micrometeorite flux

size fractionation reproducibility and provenance of helium isotopes in north equatorial pacific pelagic clays helium isotope compositions were measured in late quaternary pelagic clays from the north equatorial pacific where terrestrial and extra terrestrial end member components are identified and quantified in different grain size populations the results provide important constraints on the nature of interplanetary dust particles idps in sedimentary environments and on their application as constant flux proxies in paleoceanographic studies approximately 92 95 of the extra terrestrial 3 he 3 heet is concentrated in particles smaller than 20 mm within this grain size group 44 of 3 heet is concentrated in the 20 10 and 10 2 mm fractions while more than 50 of the total 3 heet is found in fine particles smaller than 2 mm the peak concentration of 3 heet in the very fine particle size population explains a consistently better reproducibility of 3 heet in sediments compared to model predictions that is related to the relatively homogenous distribution of clay size particles in sediments compared to coarser particles especially in deep sea sediments and ice cores the 3 he 4 he ratios are highest in the coarser than 20 mm fractions peaking between 63 and 38 mm 1 7871 13 10 4 compared to the bulk and the o20 mm fractions 3 2072 62 10 5 and 2 1170 88 10 5 respectively the isotopic composition of helium in each of the size fractions reflects a mixture between a pure extraterrestrial end member and a terrigenous component which is itself comprised of varying mixtures of volcanic material and eolian material associated with the fine grain size fraction of loess deposits in south east asia poor 3 heet reproducibility observed here and elsewhere inhibiting its application as a constant flux proxy stems from the presence of a small number of aliquots displaying significantly higher 3 heet contents compared to the average of their reference group these helium rich particles hrps are probably characterized by a o20 mm diameter and carry significant amounts of extraterrestrial 3 he and 4 he careful rejection of such aliquots improves the signal reproducibility by more than two fold

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

rapid change in strontium isotopic composition of sea water before the cretaceous tertiary boundary a critical aspect of the debate over the origin of the chemical biological and climatological perturbations that characterize the cretaceous tertiary boundary k t boundary is the timescale and detailed pattern of the extinctions and geochemical changes whether the development of the anomalies was catastrophic over tens to hundreds of years or more gradual 103 to 106 years whether the anomalies occur exactly at the boundary or precede it and whether the chemical and biological signatures represent one event or can be resolved into several discrete events are issues fundamental to identifying the cause of the marked changes to determine the timing and rate of change of strontium isotope evolution of sea water during the five million years preceding the k t boundary we analysed foraminifera from an exceptionally thick palaeontologically well characterized marine k t section exposed at bidart southwest france we found a rapid increase in 87sr 86sr of ocean water 1 5 to 2 3 myr before the boundary the increase may be explained by a 10 increase in continental strontium flux to the oceans over a period of about 1 myr

chemostratigraphic evidence of deccan volcanism from the marine osmium isotope record continental flood basalt cfb volcanism is hypothesized to have played a causative role in global climate change and mass extinctions uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of cfb volcanism our results document a 25 decline in the marine 187 os 188 os record that predates the cretaceous tertiary boundary ktb and coincides with late maastrichtian warming we argue that this decline provides a chemostratigraphic marker of deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of deccan volcanism was a transient global warming event of 3 to 5 c that is fully resolved from the ktb mass extinction

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

orbital cyclicities above and below the cretaceous paleogene boundary at zumaya n spain agost and relleu se spain abstract the rhythmicity of hemipelagic limestone marl successions covering the cretaceous paleogene k pg boundary is investigated and evidence of orbital control is presented three sections were surveyed and sampled in detail two are located near agost and relleu in the province of alicante and one near zumaya in the basque country the distance between agost and relleu is about 30 km between agost and zumaya about 600 km the carbonate content of 2779 samples was determined and used as a proxy to determine cyclicities lithologically all three successions are very similar although in the maastrichtian of the zumaya succession 63 7 m 339 light green siliciclastic turbidite layers varying in thickness between 0 1 and 12 cm intercalate with the calcisiltites conventional paleomagnetic and micropaleontologic datum planes are few and show the usual associated uncertainties power spectra based on the few reliable datum planes available are unstable more datum planes were created in an unconventional way i e by visual correlation of the local maxima and minima of the carbonate curves to those of the relevant part of the theoretical eccentricity curve the validity of the method is based on the assumption that the eccentricity periods are considered practically stable throughout geologic time the results support the hypothesis that the carbonate rhythmicity at agost relleu and zumaya are orbitally controlled most likely orbital forcing and random processes play a role in the frequency of occurrence and the thickness of turbidites in the maastrichtian of zumaya the age of the k pg boundary is estimated at 64 56 0 4 ma

extraterrestrial 3he in seafloor sediments evidence for correlated 100 kyr periodicity in the accretion rate of interplanetary dust orbital parameters and quaternary climate we have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the ontong java plateau western equatorial pacific ocean odp site 806 these results provide a two million year record of the burial flux of extraterrestrial 3he which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles the 3he burial flux prior to 700 ka was relatively low 0 5 pcc cm 2 kyr 1 but from 700 ka to the present the burial flux gradually increased to a value of 1 0 pcc cm 2 kyr 1 100 kyr periodicity in the 3he burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate with high 3he burial fluxes associated with interglacial periods this periodicity and phase are consistent with previous 3he measurements in north atlantic sediments although 100 kyr periodicity in 3he burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris the measurements and predictions differ by one half cycle in phase nevertheless our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and or inclination relative to the solar system invariable plane such control is a necessary but not sufficient condition for the hypothesis of muller and macdonald 1995 that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period we also identify several brief 25 kyr intervals of strongly enhanced 3he burial possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles

helium isotopes in ferromanganese crusts from the central pacific ocean abstract helium isotopes have been measured in samples of two ferromanganese crusts va13 2 and cd29 2 from the central pacific ocean with the exception of the deepest part of crust cd29 2 the data can be explained by a mixture of implanted solar and galactic cosmic ray produced gcr he in extraterrestrial grains and radiogenic he in wind borne continental dust grains 4he concentrations are invariant and require retention of less than 12 of the in situ he produced since crust formation loss has occurred by recoil and diffusion high 4he in cd29 2 samples older than 42 ma are correlated with phosphatization and can be explained by retention of up to 12 of the in situ produced 4he 3he 4he of va13 2 samples varies from 18 5 to 1852 ra due almost entirely to variation in the extraterrestrial he contribution the highest 3he 4he is comparable to the highest values measured in interplanetary dust particles idps and micrometeorites mms helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in falling terrestrial and extraterrestrial grains of fe mn crusts the extraterrestrial 3he concentration of the crusts rules out whole undegassed 4 40 m diameter idps as the host instead it requires that the extraterrestrial he inventory is carried by numerous particles with significantly lower he concentrations and occasional high concentration gcr he bearing particles

age measurement of the submarine montagnais impact crater seven laser generated 40ar 39ar age spectra of melt rocks from the newly described submarine montagnais crater are presented while the age spectra are not completely straight forward to interpret the evidence is very strong that the impact occurred approximately 51 m y ago thus the montagnais event is associated with neither the k t nor the north american tektite events such an age is also totally out of phase with the 26 32 m y cycles proposed recently for the meteorite impact and extinction records as it falls roughly midway between two of the most precisely dated points in such proposed cycles the k t boundary and the north american tektite event

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

a meteorite from the cretaceous tertiary boundary cretaceous tertiary boundary sediments are now widely recognized to contain the record of a large asteroid or comet impact event1 probably at the site of the chicxulub crater on the yucatan peninsula2 after nearly two decades of intensive research however much remains unknown about the specific nature of the projectile and of the impact event itself here we describe a 2 5 mm fossil meteorite found in sediments retrieved from the cretaceous tertiary boundary in the north pacific ocean that we infer may be a piece of the projectile responsible for the chicxulub crater geochemical and petrographic analyses of this meteorite indicate that it probably came from a typical metal and sulphide rich carbonaceous chondrite rather than the porous aggregate type of interplanetary dust considered typical of cometary materials3 the fact that meteorite survival should be enhanced by impacts at low asteroidal velocities4 also implies that this meteorite had an asteroidal rather than a cometary origin

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

stratigraphic record of the asteroidal veritas breakup in the tortonian monte dei corvi section ancona italy the discovery of elevated concentrations of the cosmogenic radionuclide 3he in deep sea sediments from ocean drilling program odp site 926 atlantic ocean and odp site 757 indian ocean points toward accretion of extraterrestrial matter probably as a result of the catastrophic disruption of a large asteroid that produced the veritas family of asteroids at ca 8 3 0 5 ma and which may have had important effects on the global climatic and ecologic systems here we investigated the signatures possibly related to the veritas event by performing a high resolution multiproxy stratigraphic analysis through the late tortonian early messinian monte dei corvi section near ancona italy closely spaced bulk rock samples through a 36 m thick section approximately spanning from ca 9 9 ma to ca 6 4 ma show an 5 fold 3he anomaly starting at ca 8 5 ma and returning to background values at ca 6 9 ma confirming the global nature of the event we then analyzed at 5 cm intervals bulk rock samples for sedimentary and environmental proxies such as magnetic susceptibility calcium carbonate content total organic carbon and bulk carbonate 18 o and 13 c through a 21 m thick section encompassing the 3he anomaly available high resolution sea surface temperature data via alkenone analyses for this site show a temperature decrease starting exactly at the inception of the 3he anomaly cyclostratigraphic fast fourier transform spectral analyses of the proxies indicate an age of 8 47 0 05 ma for the inception of the 3he anomaly a search for impact ejecta analogous to what is present in the late eocene where both a 3he anomaly and large scale impact events are recorded was not successful detailed cyclostratigraphic analyses of our data suggest that the changes in the stable isotope series and environmental proxy series through this late tortonian time interval had a common forcing agent and that perturbations of orbitally forced climate cycles are present exactly through the interval with the enhanced influx of extraterrestrial 3he thus the chemostratigraphic evidence for a collisional event that created the veritas family of asteroids coinciding with climate perturbations on earth suggests yet another form of interaction between earth and the solar system

reconstructing the marine 187os 188os record and the particulate flux of meteoritic osmium during the late cretaceous abstract paired analyses of os isotope composition and concentration of bulk sediment and leachable os in a metalliferous pelagic clay sequence from the north pacific odp site 886c are used to reconstruct the marine os isotope record and the particulate meteoritic os flux between 65 5 and 78 ma measured 187os 188os of bulk sediments ranges from approximately 0 64 to 0 32 and those of leach analyses are very similar to bulk analyses hydrogenous os dominates the sedimentary os inventory throughout most of the studied interval as a result the measured 187os 188os of leachable os approximates that of contemporaneous seawater the odp 886c record shows rising 187os 188os in the deepest portion of the core with a local maximum of 0 66 close to 74 ma the 67 72 ma portion of the record is characterized by nearly constant 187os 188os ratios close to 0 6 the structure of the marine os isotope record from odp 886c differs markedly from the seawater 87sr 86sr curve which rises monotonically throughout the time interval studied here calculated particulate meteoritic os fluxes are between 0 5 and 2 pg cm 2 kyr 1 throughout most of the studied interval two discrete intervals of the core one of which is within cretaceous tertiary boundary ktb interval are characterized by higher fluxes of meteoritic os excluding these two intervals the average background flux of particulate meteoritic os is roughly half of that estimated from analyses of cenozoic marine sediments these are the first os isotope data to provide evidence of resolvable temporal variations in the background flux of particulate meteoritic material to the earth

strontium isotope profiles across k t boundary sequences in denmark and antarctica strontium isotope profiles derived from minimally altered samples across marine k t boundary sequences exposed at kjolby gaard and nye klov denmark and on seymour island antarctica show neither the boundary excursions nor the boundary spikes in 87sr 86sr that have been reported for k t boundary sequences from elsewhere nor do our data conform to modelled predictions of the sr isotopic response of the oceans to a positive spike in 87sr 86sr at the terminal cretaceous boundary values for 87sr 86sr with 95 confidence intervals are 0 707828 3 in denmark and 0 707832 7 in antarctica our data suggest that 87sr 86sr stopped increasing and started decreasing at least 90 ka before the k t boundary

extraterrestrial 3 he as a tracer of marine sediment transport and accumulation the deposition rate of deep sea sediments and their focused redeposition by deep sea currents can be evaluated from analyses of sedimentary 230th with a temporal resolution limited only by bioturbation6 7 10 11 230th is produced uniformly throughout the ocean by radioactive decay of dissolved 234u and is removed sufficiently fast by sorption onto sinking particles to act as a constant flux tracer of sedimentation rates but the half life of 230th 75 kyr limits its use for this purpose to the past 200 250 kyr here we explore the use of extraterrestrial 3he from interplanetary dust particles1 4 idps as a constant flux proxy that is free from this limitation a comparison of 3he with 230th in two cores from the equatorial pacific ocean indicates that the variability in the mean flux of idps over the past 200 kyr is less than 75 but in contrast to this relatively constant rate of supply of 3he to the deep sea the local burial rates of 3he and 230th have varied by a factor of five over the past 450 and 200 kyr respect ively we interpret this variability as reflecting sediment focusing with a temporal pattern that suggests regular cycles of climate driven reorganization of near bottom currents in the deep pacific ocean

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

a short duration of the cretaceous tertiary boundary event evidence from extraterrestrial helium 3 analyses of marine carbonates through the interval 63 9 to 65 4 million years ago indicate a near constant flux of extraterrestrial helium 3 a tracer of the accretion rate of interplanetary dust to earth this observation indicates that the bolide associated with the cretaceous tertiary k t extinction event was not accompanied by enhanced solar system dustiness and so could not have been a member of a comet shower the use of helium 3 as a constant flux proxy of sedimentation rate implies deposition of the k t boundary clay in 10 2 10 3 years precluding the possibility of a long hiatus at the boundary and requiring extremely rapid faunal turnover

high 3 he 4 he ratio in ocean sediments two decades ago merrihue1 reported 3he 4he ratios of 10 4 in ferromagnetic separates from a pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to 1 of the sediment a decade later krylov et al 2 compiled 3he 4he isotopic data on ocean sediments measured in the soviet union and observed that the 3he 4he ratio is generally higher in pelagic sediments where the sedimentation rate is lower they suggested that the high 3he 4he ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor however these important discoveries were almost completely neglected until we re examined the problem we have measured 39 sediments from 12 different sites 10 sites from the western to central pacific and two sites from the atlantic ocean we find 3he 4he ratios 5 10 5 for six sites well above the values generally observed in common terrestrial materials the very high 3he 4he ratio in the sediments is probably due to input of extraterrestrial materials input of stratospheric dust of 1 p p m which corresponds to a fallout rate of 2 000 tons per year can explain the observation

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

40ar 39ar age of the manson impact structure iowa and correlative impact ejecta in the crow creek member of the pierre shale upper cretaceous south dakota and nebraska a set of 34 laser total fusion 40 ar 39 ar analyses of sanidine from a melt layer in crater fill deposits of the manson impact structure in iowa has a weighted mean age of 74 1 0 1 ma this age is about 9 0 m y older than 40 ar 39 ar ages of shocked microcline from the manson impact structure reported previously by others the 74 1 ma age of the sanidine which is a melt product of precambrian microcline clasts indicates that the manson impact structure played no part in the cretaceous tertiary k t mass extinction at 64 5 ma moreover incremental heating 40 ar 39 ar ages of the sanidine show that it is essentially free of excess 40 ar and has not been influenced by postcrystallization heating or alteration an age spectrum of the matrix of the melt layer shows effects of 39 ar recoil including older ages in the low temperature increments and younger ages in the high temperature increments at 17 places in eastern south dakota and nebraska shocked quartz and feldspar grains are concentrated in the lower part of the crow creek member of the pierre shale upper cretaceous the grains are largest 3 2 mm in southeastern south dakota and decrease in size 0 45 mm to the northwest consistent with the idea that the manson impact structure was their source the ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the crow creek member suggests it is an event bed recording an instant of geologic time ammonites below and above the crow creek member limit its age to the zone of didymoceras nebrascense of earliest late campanian age plagioclase from a bentonite bed in this zone in colorado has a 40 ar 39 ar age of 74 1 0 1 ma commensurate with our sanidine age of 74 1 ma for the manson impact structure 40 ar 39 ar ages of bentonite beds below and above the crow creek are consistent with our 74 1 0 1 ma age for the manson impact structure and limit its age to the interval 74 5 0 1 to 73 8 0 1 ma recently two origins for the crow creek have been proposed eastward transgression of the late cretaceous sea and a manson impact triggered tsunami we conclude that most data are in accord with an impact origin for the crow creek member and are at odds with the marine transgression hypothesis

evaluating the accretion of meteoritic debris and interplanetary dust particles in the gpc 3 sediment core using noble gas and mineralogical tracers abstract extraterrestrial et noble gases helium and neon in 35 sediment samples from central pacific core ll 44 gpc 3 demonstrate the variable flux of interplanetary dust particles idps and major meteorite impacts over the past 70 ma 21 72 ma spinel mineralogical and chemical compositions clearly distinguish major impact events from the continuous flux of idps including the well established cretaceous tertiary k t and late eocene e o impact boundaries no spinel grains with chemical or mineralogical evidence of a distinctly et origin were found in an extensive survey of background samples i e non e o or k t boundary suggesting that either the carrier grains for et noble gas occur within the fe ti oxide mineral fraction observed in this study found to include ilmenite and ulvospinel or are too small for identification by sem the presence of ilmenite and ulvospinel suggest lunar regolith is a potential source for et noble gas rich particles noble gas analysis on both the emf extractable magnetic fraction and the bulk minus emf bulk emf show that the he and ne compositions are consistent with partially degassed noble gas signatures of zero age magnetic grains z mag and stratospheric interplanetary dust particles idps conclusive evidence for a planetary ne a noble gas signature is found only in the bulk sediments at the k t boundary although all gpc 3 k t fractions bulk emf and hf digestion plot along a mixing line between planetary ne a and solar wind sw spinels from major impact boundaries k t e o exhibit dendritic texture and elevated ni consistent with previous reports in contrast to the otherwise consistent 3he signal from idps the 3he at the known impact boundaries k t and e o actually decreases these anomalously low 3he are accompanied by significantly elevated ne and significantly lower 3he 20ne solar ratios 10 lower produced by both preferentially degassing of he relative to ne at times of increased flux of larger et material degassed degassed he enriched ne profile noble gas characteristics occur in two sample intervals that do not correspond to any known impact events 47 and 71 ma explained by an influx of larger particles sem analysis of the 47 ma sample shows spinels with dendritic textures but without distinctive markers of large meteorite impacts e g elevated ni particle size increases and degassed signatures may be caused by major bolides micrometeorites comet showers or simply a flux of larger idps potentially with a different source

rapid recovery of life at ground zero of the end cretaceous mass extinction the cretaceous palaeogene mass extinction eradicated 76 of species on earth1 2 it was caused by the impact of an asteroid3 4 on the yucatan carbonate platform in the southern gulf of mexico 66 million years ago 5 forming the chicxulub impact crater6 7 after the mass extinction the recovery of the global marine ecosystem measured as primary productivity was geographically heterogeneous 8 export production in the gulf of mexico and north atlantic western tethys was slower than in most other regions8 11 taking 300 thousand years kyr to return to levels similar to those of the late cretaceous period delayed recovery of marine productivity closer to the crater implies an impact related environmental control such as toxic metal poisoning 12 on recovery times if no such geographic pattern exists the best explanation for the observed heterogeneity is a combination of ecological factors trophic interactions 13 species incumbency and competitive exclusion by opportunists 14 and chance 8 15 16 the question of whether the post impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems if there is a relationship between the distance from the impact and the recovery of marine productivity we would expect recovery rates to be slowest in the crater itself here we present a record of foraminifera calcareous nannoplankton trace fossils and elemental abundance data from within the chicxulub crater dated to approximately the first 200 kyr of the palaeocene we show that life reappeared in the basin just years after the impact and a high productivity ecosystem was established within 30 kyr which indicates that proximity to the impact did not delay recovery and that there was therefore no impact related environmental control on recovery ecological processes probably controlled the recovery of productivity after the cretaceous palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events

terrigenous helium in deep sea sediments abstract we have measured the isotope ratios of helium contained in various terrigenous materials that contribute to deep sea sediments these materials include ice rafted debris from the north atlantic chinese loess and sediment collected at or near the mouths of three large rivers the amazon the ganges and the yangtze we observe terrigenous 3he 4he ratios that vary from 1 8 10 9 to 4 6 10 7 i e values that are higher than the theoretically derived range of 10 9 to 10 8 atlantic and pacific deep sea sediment 3he 4he ratios can be explained by mixing of helium from similar extraterrestrial but different terrigenous sources terrigenous sources for north atlantic and north pacific sediments are characterized by 3he and 4he contents that are higher and 3he 4he ratios that are lower than those for central and eastern equatorial pacific sediments this is consistent with the supply to the north atlantic and north pacific oceans of older cratonic continental material that contains high amounts of in situ produced nucleogenic and radiogenic helium terrigenous material transported to central and eastern equatorial pacific sediments contains lower amounts of 3he and 4he and higher 3he 4he ratios indicative of supply from a more juvenile andean source in the equatorial atlantic core v31 135 we have used previously published 230th data to determine an extraterrestrial 3he flux of 1 16 0 15 10 12 cm3stp cm 2 ka 1 within error of that previously determined in sediments from the equatorial pacific ocean 0 78 0 29 10 13 cm3stp cm 2 ka 1 marcantonio et al 1996

the geochronology of large igneous provinces terrestrial impact craters and their relationship to mass extinctions on earth the relationship between sudden catastrophic events and sudden mass extinctions on earth has attracted a great deal of research effort including measurements of the absolute and relative ages of these events the result has been a rapid improvement in the precision and accuracy of radiometric dates for large igneous provinces lips for which there is now a consensus on the peak eruption times over the last 250 ma in parallel there has been a more gradual improvement in the radiometric dates for craters formed by hypervelocity impacts hvis of asteroids and comets with earth the absolute ages of the largest craters are known to within a few million years but the ages of many smaller craters are poorly constrained in addition whereas relatively few lips have formed in the last 250 ma at least one hvi crater larger than 20 km diameter has formed on earth every million years only 25 such craters have been discovered so coincidence between the ages of hvi crater formation and ages of mass extinctions is related as much to crater preservation as to coincidence of the events the difference in quality of geochronology between hvis and lips is the result of differing availability of samples lips cover large areas and present an array of basalt samples suitable for dating whereas hvi craters are often poorly preserved and present few datable samples the radiometric data for preserved hvis and lips are now sufficiently precise and accurate for us to be confident that there is no one to one correlation between either lips or hvis and global mass extinctions of life on earth however there is a close association between a significant number of lips and sudden environmental change and a very close association between at least one massive hvi crater formation and a mass extinction further there is no apparent correlation between hvis and lips and thus no reason to suggest a causative relationship finally the radiometric dating of hvi craters indicates several clusters in the geological record but the correlation with sudden environmental change is no better than for large craters

accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record the flux of particulate extraterrestrial et matter to the deep sea has been calculated using a four component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the pacific ocean nineteen published bulk leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation the calculated annual particulate flux averaged over several 100 ka by slow accumulation and bioturbation ranges from 18 000 to 67 000 tons with a mean value of 37 000 13 000 tons the data indicate no significant variability in the flux with time except at the k t boundary and are thus compatible with results based on ir accumulation in deep sea sediments the inferred constancy of the et matter flux is in contrast to recent results based on the et 3he flux that integrates only a small size fraction of the entire et matter flux 3he data indicate variability by a factor of 6 over the past 70 ma and a fivefold increase in the et flux between 3 ma and 1 ma that is not seen in the bulk et matter flux based on osmium isotope data the apparent constancy of the cosmic matter flux over the past 80 ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record except at the k t boundary a comparison between osmium isotope and ir data from two pelagic sediment cores from the pacific dsdp site 596 and ll44 gpc3 indicates that the seawater soluble fraction of et os cannot exceed 36 kg a and most likely is significantly smaller the maximum value of 36 kg a can account for approximately one half of the seawater soluble et matter flux necessary to balance the radiogenic continental runoff of os without any additional unradiogenic source simple mass balance calculations indicate that an additional unradiogenic source of os to the oceans most likely alteration of oceanic crust is required to balance the present day seawater osmium isotopic composition this source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

extraterrestrial ni rich spinel in upper eocene sediments from massignano italy evidence of a major cosmic event at the end of the eocene is given by the finding of at least one possibly two or more impact horizons containing microtektites microkrystites shocked quartz and unusually high iridium concentrations we report here the discovery in the global stratotype of the eocene oligocene boundary at massignano in italy of ni rich spinel a mineral that forms by the interaction of meteoritic bodies with the earth s atmosphere a maximum concentration of 800 crystals mg 1 corresponding to a flux of 2 107 crystals cm 2 is observed in a well dated layer 35 7 0 4 ma that also contains shocked quartz and iridium the crystals are found clustered in flattened spheroids the probable relic of their host bodies the chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects as proposed for the spinel bearing spheroids found at the cretaceous tertiary boundary the most likely explanation is that the dust particles were generated by a large cometary impact whether this impact is the one that produced microtektites and or microkrystites or another one that may have occurred shortly after or before is still questionable additional searching for ni rich spinel at other upper eocene sections may help to answer this question

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

kara and ust kara impact structures ussr and their relevance to the k t boundary event ample evidence exists for at least one major meteorite impact event at the time of the cretaceous tertiary k t boundary and it is therefore important to establish if any recognized terrestrial impact craters are k t in age the kara ussr impact structure consists of two adjacent large impact craters a rare and interesting geologic phenomenon and it has been suggested that this twin impact structure might be related to the k t boundary event however newly determined 40 ar 39 ar and k ar ages presented here suggest that these structures are slightly older than 70 ma and may thus be too old for a 66 ma k t boundary event still these two craters represent a substantial impact event that could have initiated regional if not global degradation of the biosphere their age suggests a possible relation with the campanian maastrichtian boundary

geochemical evidence for a comet shower in the late eocene analyses of pelagic limestones indicate that the flux of extraterrestrial helium 3 to earth was increased for a 2 5 million year my period in the late eocene the enhancement began 1 my before and ended 1 5 my after the major impact events that produced the large popigai and chesapeake bay craters 36 million years ago the correlation between increased concentrations of helium 3 a tracer of fine grained interplanetary dust and large impacts indicates that the abundance of earth crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced these observations provide evidence for a comet shower triggered by an impulsive perturbation of the oort cloud

temporal variation of 3he 4he ratio recorded in deep sea sediment cores he isotopic composition was measured for 72 deep sea sediments from 16 sites in the pacific and atlantic oceans and the japan sea 3he content ranges from less than 10 12 cm3 stp g to about 6 10 11 cm3 stp g and 3he 4he from about 10 8 to 1 67 10 4 the he isotopic data lie fairly well on a mixing curve in a 3he 4he 3he diagram where the end members of the mixing curve are extraterrestrial he in interplanetary dust particles idp s and terrestrial he in ocean sediments this confirms the view that there are significant amounts of extraterrestrial he in ocean sediments from global fallout of idp s into deep sea sediments 3he flux estimated for two long cores gives a value of about 1 5 1 0 10 15 cm3 stp cm2 yr over a time span of 40 m y which corresponds to an annual fallout rate of idp s size 10 m of about 400 t in one of the cores we observe a 3he spike which is associated with anomalously high ir content 1 86 0 56 ppb this may be due to meteorite debris

an alternative age model for the paleocene eocene thermal maximum using extraterrestrial 3he a continuous age model for the brief climate excursion at the paleocene eocene boundary has been constructed by assuming a constant flux of extraterrestrial 3he 3he et to the seafloor 3he et measurements from odp site 690 provide quantitative evidence for the rapid onset few kyr and short duration 120 kyr of global warming and of the associated disturbance to the earth s surficial carbon budget at this time these observations support astronomically calibrated age models indicating extremely rapid release of isotopically light carbon possibly from seafloor methane hydrate as the proximal cause of the event however the 3he et technique indicates a previously unrecognized and extreme increase in sedimentation rate coincident with the return of climate proxies to pre event values the 3he et based age model thus suggests a far more rapid recovery from the climatic perturbation than previously proposed or predicted on the basis of the modern carbon cycle and so may indicate additional or accelerated mechanisms of carbon removal from the ocean atmosphere system during this period 3he et was also measured at odp site 1051 to test the validity of the site 690 chronology comparison of these data sets seems to require removal of several tens of kyr of sediment within the climatic excursion at site 1051 an observation consistent with sediment structures and previous age modeling efforts the site 1051 age model shows a 30 kyr period in which climate proxies return toward pre event values after which they remain invariant for 80 kyr if this rise represents the recovery interval identified at site 690 then the 3he et based age models of the two sites are in good agreement however alternative interpretations are possible and work on less disrupted sites is required to evaluate the reliability of the proposed new chronology of the climate excursion regardless of these details this work shows that the 3he et technique can provide useful independent evidence for the development and testing of astronomically calibrated age models

laser extraction of helium isotopes from antarctic micrometeorites source of he and implications for the flux of extraterrestrial 3he to earth helium isotopes have been measured in 45 putative micrometeorites in the size range 50 to 400 m that were extracted from antarctic ice forty of the antarctic micrometeorites amms are irregular in shape and display varying degrees of partial melting whereas five are spherical and thought to represent completely melted particles of the 30 50 to 100 m suspected micrometeorites only 8 released neither 3he or 4he sixteen released 3he and 4he 0 004 3 5 10 3 cm3 stp g having 3he 4he in the range 1 5 10 4 to 4 1 10 4 six particles released only 3he yielding minimum 3he 4he of 1 6 to 10 6 10 4 5 of which fall in the range of values for amms that had measurable 3he and 4he only 2 of the 15 100 to 400 m amms released measurable 4he 4 44 10 7 cc g 1 these have 3he 4he of 6 10 4 and 20 10 4 respectively significantly higher than the majority of the ratios of 50 to 100 m amms implanted solar energetic particles are the dominant source of he in the 50 to 100 m amms with small contributions of solar wind and or spallogenic he present in 10 particles the absence of sw he is likely due to the loss of the outermost layers of the micrometeorites by ablation during atmospheric entry and abrasion during residence in ice and sampling spallation he is most important in the 100 to 400 m amms consistent with their smaller surface area to volume ratio maximum spallation 3he exposure ages of six particles are close to that predicted from poynting robertson spiral in times of asteroidal micrometeorites the two terrestrial particles identified by chemical composition and mineralogy released only 4he yielding radiogenic he isotope compositions an inverse relationship between particle sizes and solar he abundance is explained by he loss during entry helium loss is likely controlled by phyllosilicate breakdown where present and diffusion controlled bubble growth and rupture rather than melting the 50 to 100 m micrometeorites deliver 8 10 8 cm3 stp 3he g to earth this is more than 100 times less than transported by 20 m particles 2 10 5 cm3 stp 3he g but because 50 to 100 m micrometeorites deliver approximately 20 times more extraterrestrial matter to earth than 20 m particles the larger particles may contribute 5 of the total flux of extraterrestrial 3he to terrestrial sediments the mean he concentrations of the 50 to 100 m amms antarctic micrometeorites and the five spherules 2 1 10 9 cm3 stp 3he g 1 overlap those of the magnetic fraction of deep sea sediments 0 8 68 10 9 cm3 stp g a combination of this material could provide the extraterrestrial 3he measured in ancient deep sea sediments without calling upon the survival of friable 20 m chondritic interplanetary dust particles for tens of millions of years

late cretaceous eocene nannofossil and magnetostratigraphic correlations near gubbio italy abstract using a modified sample preparation technique we have been able to establish a detailed lower campanian to upper eocene nannofossil stratigraphy in the bottaccione and contessa highway sections near gubbio appearance and extinction levels of virtually all the commonly used calcareous nannofossil zonal markers have been recognized and can now be closely correlated with the planktonic foraminifera zonation and the magnetic reversal stratigraphy previously established in these sections comparisons with the nannofossil calibrations of the oceanic magnetic anomaly sequence in deep sea drilling project dsdp sites suggest that magmetic subchrons c17n and c25n are missing in the bottaccione section the observed variability of the relative stratigraphic position of most plankton events is confirmed to less than one magnetic subchron absolute abundance paleobiogeographic restriction and differential preservation render some of the traditionally used biostratigraphic events less reliable than others

reevaluation of siderophile element abundances and ratios across the cretaceous paleogene k pg boundary implications for the nature of the projectile abstract the discovery over 30 years ago at gubbio italy and caravaca spain of an enrichment in the concentrations of iridium ir and the other platinum group elements pge by up to four orders of magnitude irmax 0 10 87 ng g compared to average continental crustal background levels remains one of the most important discoveries in the earth sciences since then similar anomalies have been detected in more than 120 cretaceous paleogene k pg boundary sites worldwide highly elevated ir and other siderophile element abundances in roughly chondritic ratios are considered strong indicators for the presence of a meteoritic contribution in impact related lithologies melt rocks impact ejecta material etc delivered when an extraterrestrial object strikes earth the presented work adds 113 unpublished pge analyses of 38 k pg sections worldwide to the existing literature the analytical protocol relied on for this purpose consisted of a combination of a nickel sulfide fire assay pre concentration technique and subsequent trace metal determination via inductively coupled plasma mass spectrometry icp ms through repeated determination of key siderophile elements i e cr co ni and pge the importance of sampling nugget effects and analytical methodologies applied becomes more apparent even more critically these analytical effects are superimposed by the local syn and post depositional conditions that have affected the pristine meteoritic signature of the k pg impactor including potential fractionation during vaporization and condensation dissimilar pge carrier phases terrestrial pge input sedimentation rate reworking diagenesis bioturbation and chemical diffusion while chondrite normalized pge patterns of individual sites appear relatively flat i e chondritic strong variations in siderophile element content and inter element ratios exist between k pg locations inter laboratory measurements and replicate analyses hampering a precise projectile identification using highly siderophile elements only when considering improved databases of siderophile element concentrations in meteorites in combination with linear regression analysis to calculate inter element ratios from a large suite of ejecta deposit sites the nature of the k pg projectile can be resolved application of this methodology to an extensive data set of continental and marine sites very proximal to distal to the chicxulub impact structure supports a carbonaceous chondritic impactor type cm or co

inoceramid extinction in the gubbio basin northeastern apennines of italy and relations with mid maastrichtian environmental changes abstract a detailed study of five representative maastrichtian sections of deep water pelagic limestones in the umbria marche apennines of italy shows that inoceramids became extinct synchronously throughout this region some 3 5 ma before the cretaceous tertiary boundary the decline and disappearance of these bivalves are immediately preceded by two closely spaced peak abundance zones and are linked with an important change in environmental conditions during the mid maastrichtian period as is indicated by modifications in the 13c composition of sea water and variations in planktonic foraminiferal assemblages the restoration of stable environmental conditions after this biotic crisis took about half a million years in this region excursions of the 13c signal contemporaneous to the extinction of inoceramids in other maastrichtian basins throughout the world i e basque country and southern oceans indicate that these bivalves were extremely sensitive to environmental changes however accurate magneto and biostratigraphic correlations between the italian basque and southern hemisphere sections unambiguously indicate that the extinction of inoceramids was widely diachronous throughout the world of as much as 4 ma

a 35 myr record of helium in pelagic limestones from italy implications for interplanetary dust accretion from the early maastrichtian to the middle eocene we have determined the helium concentration and isotopic composition of a suite of early maastrichtian through middle eocene pelagic limestones in the italian apennines the results provide a 35 myr record of the implied flux of extraterrestrial 3he which is a proxy for the accretion rate of interplanetary dust particles idps our measurements show that the 3he flux was fairly constant in the maastrichtian except for possible minor increases factor of two or less from 70 5 ma to 68 ma and at 66 ma which probably reflect transient increases in the accretion rate of asteroidal and or cometary idps we find no evidence for an increase in idp accretion at or immediately before the k t boundary implying that the k t impact was not associated with enhanced solar system dustiness this observation precludes the possibility that the k t impactor was a member of a major comet shower and is more consistent with impact of a lone comet or asteroid our data suggest a 2 to 4 fold increase in idp accretion between 57 and 54 ma followed by a factor of three decrease over an 4 myr period in the early to middle eocene the duration and magnitude of this variability is inconsistent with previous observations attributed to a shower of long period comets and is more likely the result of collisions in the asteroid belt and or kuiper belt in the entire 35 myr record we find no evidence for major enhancements of the idp accretion rate of the type expected from comet showers our results in combination with earlier 3he measurements do not support models that predict recurrent comet showers with periods of 38 myrs if there is a periodicity in the cratering record that is caused by periodic modulation of the oort cloud it is not evident in the apennine sediment data along with the 3he measurements we also obtained 4he concentrations which record temporal changes in the flux or composition of terrigenous matter the most significant change in 4he occurs in the last 4 myrs of the cretaceous over which the concentration of 4he in the detrital component rises by 300 this rise tracks a strong increase in the seawater 87 sr 86 sr ratio suggesting a globally significant change in the composition of continental detritus delivered to the oceans possibly arising from increased continental weathering

shocked quartz from the late eocene impact evidence from massignano italy this paper reports the discovery of shocked quartz grains within a well dated upper eocene marlstone at the massignano outcrop ancona italy the shocked quartz reaches a maximum concentration of 1 6 0 4 grains per cubic centimetre of bulk rock a modest ir anomaly is also coincident with the shocked quartz and together they strongly support an impact origin moreover this impact layer has a precise interpolated radiometric age of 35 7 0 4 ma although impact evidence found throughout the world indicates that at least two large impacts did occur during this time the actual number timing and environmental effects of these impact events remain unclear the late eocene was a critical time in earth history because of significant climatic biotic and environmental crises

comet showers as a cause of mass extinctions if at least some mass extinctions are caused by impacts why do they extend over intervals of one to three million years and have a partly stepwise character the solution may be provided by multiple cometary impacts astronomical geological and palaeontological evidence is consistent with a causal connection between comet showers clusters of impact events and stepwise mass extinctions but it is too early to tell how pervasive this relationship may be

4he as a tracer of continental dust a 1 9 million year record of aeolian flux to the west equatorial pacific ocean the mass accumulation rates mars of aeolian dust in the ocean basins provide an important record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns responsible for dust transport in the geologic past the incorporation of other terrigenous components such as volcanic ashes in seafloor sediments however often obscures the aeolian dust record we describe a new approach which uses the delivery rate of crustal 4he to seafloor sediments as a proxy for the mass accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components we have determined the flux of crustal 4he delivered to the seafloor of the ontong java plateau ojp in the western equatorial pacific over the last 1 9 myrs crustal 4he fluxes vary between 7 7 and 30 ncc cm 2 kyr 1 and show excellent correlation with global climate as recorded by oxygen isotopes with high crustal 4he fluxes associated with glacial periods over the entire interval studied furthermore the onset of strong 100 kyr glacial interglacial climate cycling is clearly seen in the 4he flux record about 700 kyrs ago these data record variations in the supply of asian dust in response to climate driven changes in the aridity of the asian dust sources consistent with earlier work on asian dust flux to the northern pacific ocean however in contrast to previous studies of sites in the central and eastern equatorial pacific ocean there is no evidence that the inter tropical convergence zone an effective rainfall barrier to the southward transport of northern hemisphere dust across the equator in the central and eastern pacific has influenced the delivery of asian dust to the ojp the most likely carrier phase for crustal helium in these sediments is zircon which can reasonably account for all the 4he observed in the samples as a first order estimate these results suggest that the mass accumulation rate of asian dust on the ojp over the last 1 9 myrs varied from about 4 to 15 mg cm 2 kyr 1 in contrast previous studies show that over the same interval the total mar of terrigenous dust i e asian dust plus local volcanics on ojp varied between about 34 and 90 mg cm 2 kyr 1

high resolution iridium δ13c δ18o foraminifera and nannofossil profiles across the latest paleocene benthic extinction event at zumaya spain in the expanded upper paleocene lower eocene section 30 m of zone p5 sediments at zumaya northern spain the highest occurrence of many late paleocene deep sea benthic foraminifera species 40 extinction coincides with a transition from marl to calcite free clay our high resolution studies chemical elements 613c 6180 calcareous nannofossils planktic and benthic foraminifera show that below the marl clay transition there is a 40 50 cm thick interval corresponding to 10 20 kyr containing a detailed record of a gradual succession of faunal and geochemical events culminating in the benthic extinctions planktic foraminiferal and nannofossil changes e g the onset of demise in fasciculithus genus occur a few meters below the marl clay transition in the limestone 50 cm below the base of the clay a prominent glauconite maximum indicates that sea floor oxygenation suddenly decreased glauconite continues to be common until the onset of clay deposition a whole rock negative 613c shift 1 6 0 most likely reflecting an original sea water trend is gradually developed over the 40 cm of greenish brown marls immediately below the clay at the base of these marls there is a small significant iridium anomaly of 133 ppt ir compared with an average background of 38 ppt in the marls the demise of the fasciculithus species accelerates gavelinella beccariiformis becomes extinct and the abundance of acarinina species begins to increase the superjacent 4 m of clay is devoid of original calcite in its lower part and has a low calcareous content higher up at calcareous levels in the clay an unusual planktic foraminifera fauna occurs dominated by acarinina species when marl deposition returns 613c gradually increases and then stabilizes at values about 0 5 o lower than before the isotopic excursion the 613c excursion spans in total 5 m probably corresponding to 200 400 kyr the fasciculiths disappear shortly after the stabilization of 613c here we also present a whole rock 6 3c profile through the entire paleocene section at zumaya the profile is very similar to previous profiles registered in well preserved deep sea material suggesting that whole rock 613c at zumaya can be used for correlation

eocene magnetic stratigraphy at gubbio italy and its implications for paleogene geochronology the sequence of polarity zones established by alvarez and others for the cretaceous and paleocene in the scaglia formation in the bottaccione gorge at gubbio has now been extended through the uppermost paleocene into the upper eocene globigerinatheka semiinvoluta zone anomaly 18 the relationships between the standard paleogene foraminiferal zones and the sea floor anomalies are defined with the exception of the mid eocene zones of morozovella lehneri orbulinoides beckmanni and the base of the truncorota loides rohri zone which are obscured by inadequate preservation and a fault gap the base of the eocene lies above anomaly 25 the lower to middle eocene contact base lutetian lies in the upper part of anomaly 22 the base of the bartonian lies below the base of anomaly 18 and the base of the priabonian upper eocene lies within anomaly 18 radiometric ages appear to diverge systematically from a geochronology based on steady sea floor spreading rates suggesting that spreading may have been as much as a factor of 3 slower in the paleocene and the late eocene than in the early and middle eocene

determination of regional soil geochemical baselines for trace metals with principal component regression a case study in the jianghan plain china abstract the soil geochemical baseline is an important index in environmental assessment detailed baseline studies are necessary in large areas with complex geological settings landforms and soil types the jianghan plain a major industrial and agricultural region located in central china has a soil geochemical baseline that has yet to be fully defined the objective of this paper is to study the baseline of cd pb and zn in the topsoil of the jianghan plain in a subarea using principal component regression pcr a total of 9030 samples were collected from the surface layer and 2 soil profiles and 2 sedimentary columns were sampled near the yangtze and han rivers fifty two elements and two parameters were analyzed data processing and the creation of spatial distribution maps of the elements were performed using mapgis r and spss software the results show that the distributions of cd pb and zn are mainly controlled by parent material drainage system and soil type the study area is divided into 3 subareas with factors reflecting the physico chemical characteristics of the soil using factor analysis fa the geochemical baseline model is established in every subarea to predict the cd pb and zn values using principal component regression analysis pcr the exceptional values as a result of anthropogenic input or mineralization are distinguished by residuals and the natural background values and anthropogenic contributions are clearly distinguished therefore the pcr method in these subareas is objective and reasonable and the conclusion provides effective evidence of exceptional high values for further environmental assessment

a late miocene dust shower from the break up of an asteroid in the main belt throughout the history of the solar system earth has been bombarded by interplanetary dust particles idps which are asteroid and comet fragments of diameter approximately 1 1 000 microm the idp flux is believed to be in quasi steady state particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution dynamical ejection and planetary or solar impact because idps are rich in 3he seafloor sediment 3he concentrations provide a unique means of probing the major events that have affected the idp flux and its source bodies over geological timescales here we report that collisional disruption of the 150 km diameter asteroid that created the veritas family 8 3 0 5 myr ago also produced a transient increase in the flux of interplanetary dust derived 3he the increase began at 8 2 0 1 myr ago reached a maximum of approximately 4 times pre event levels and dissipated over approximately 1 5 myr the terrestrial idp accretion rate was overwhelmingly dominated by veritas family fragments during the late miocene no other event of this magnitude over the past approximately 10 8 yr has been deduced from main belt asteroid orbits one remarkably similar event is present in the 3he record 35 myr ago but its origin by comet shower or asteroid collision remains uncertain

the age of the kara impact structure russia for the 65 km sized kara impact structure close to the polar ural we report an age of 70 3 2 2 ma 2s defined by the mean of 40ar 39ar plateau ages for three glassy or crystalline impact melt rocks cleaned from mineral and rock clasts the fine structure of the age spectra of these samples can quantitatively be simulated by modeling taking into account 39ar recoil effects without assuming the presence of excess ar the calculations corroborate our age results by showing that 39ar recoil does not affect the plateau fractions previously kara has been proposed as a probable k t impact site or was related to the campanian maastrichtian boundary at 73 ma at the 2s level both suggestions are ruled out by the well constrained age for the kara impact structure

helium and argon isotopes in rocks minerals and related ground waters a case study in northern switzerland abstract to address the problems related to the transfer of helium and argon isotopes from rocks into related aquifers the concentrations of u th li and k and helium and argon isotopes were measured in a sedimentary sequence from tertiary to permo carboniferous and in crystalline rocks in northern switzerland in addition to whole rock samples mineral separates have also been investigated the observed concentrations of 3 he 4 he and 40 ar in rocks minerals and groundwaters are compared to calculated values which would result from in situ radiogenic production in a closed system this comparison shows that the rocks and minerals have almost completely lost radiogenic in situ produced helium loss of 3 he and 4 he has been controlled by different retention capacities of the rock forming and accessory minerals and fillings therefore measured 3 he 4 he ratios in these rocks can differ from the calculated production ratios the deviation is generally less than a factor of 3 however in certain chemical sediments e g anhydrites elevated ratios of 3 he 4 he up to 15 10 8 which is 20 times the production ratio were observed it is postulated that 3 h the precursor of 3 he might be chemically bound in such sediments during the 12 3 years half life of 3 h and that the beta decay energy of 3 h is too small to liberate 3 he the calculated closed system concentrations of helium in groundwater exceed the measured ones by up to three orders of magnitude implying that the water rock system has not been a closed system since the time of sedimentation and that movable waters have removed helium to a discharge area and to the atmosphere stagnant old waters however could contain high concentrations of helium and supply it to younger movable waters aquitards with low permeability permian shales appear to be a more important source of helium isotopes than water bearing rocks permian sandstones high abundances of all parent elements in the shales almost complete loss of radiogenic helium and the enhanced production ratio of 3 he 4 he 7 2 10 8 in these rocks similar to the ratio observed in the neighbour permian aquifer 9 4 10 8 suggest the shale as a major source of helium isotopes for this aquifer and diffusion as a principal process for helium transfer from the stagnant porewaters in shales into the movable waters in sandstones some sedimentary rocks e g permian sandstones and shales contain more helium than could have been produced since the time of sedimentation while permian sandstones mainly have retained this excess rare gas component they have released radiogenic helium the difference in the retention of these two components most probably demonstrates that migration of radiogenic helium from damage tracks along crystal imperfections to grain boundaries is an important process controlling its loss the abundances of excess 3 he and 4 he in rocks cannot be predicted by in situ production calculations studies of rocks and minerals are required the average ratio of 40 ar 36 ar 385 in groundwaters collected from permian sediments from the weiach borehole is much higher than that in the aquifers from the crystalline basement below the sediments radiogenic 40 ar in permian sediments is mainly liberated by diagenetic water rock interactions the low contribution of radiogenic 40 ar in deep aquifers of the crystalline basement and the fact that chlorine concentrations in groundwaters from the basement are also much lower than in the overlaying permian aquifer eliminates a hypothetical upwards fluid flux from this study we envisage intrabasin sources for helium and argon isotopes in groundwaters with mainly lateral migration of waters through sedimentary layers at different rates and diffusion of helium from slower velocity older waters in more stagnant zones shales into flow passes with highervelocity younger waters in sections with higher permeability sandstones an external source for noble gases is not required in such a model

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

an extraterrestrial impact at the permian triassic boundary becker et al 1 presented geochemical evidence that suggests that the largest mass extinction in earth history at the permian triassic boundary ptb 250 million years ago ma coincided with an extraterrestrial impact comparable in size to the one that likely caused the end cretaceous extinctions 65 ma 2 although becker et al analyzed material from sections in hungary japan and china the hungarian section yielded no extraterrestrial signature and their identification of the ptb in the japanese section is questioned in the accompanying comment by isozaki below thus only their analyses of the chinese section provide hitherto uncontested evidence for an impact at the boundary in the form of data on the abundance and composition of fullerenes in the boundary clay a volcanic ash layer called bed 25 at meishan china 3 although fullerenes may be purely terrestrial see e g 4 becker et al report that the fullerenes from the meishan ash carry extraterrestrial noble gases in the cage structure rich in 3 he and with distinctive 3 he 36 ar and 40 ar 36 ar ratios and that this signature therefore derived from a bolide impact here we report that we are able to detect fullerene hosted extraterrestrial 3 he neither in aliquots of the same meishan material analyzed by beckeret al nor any in samples of a second chinese ptb section and that we thus find no evidence for an impact

a record of the extraterrestrial 3he flux through the late cretaceous abstract late cretaceous 100 73 ma pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3he as a tracer in the bottaccione section near gubbio italy three intervals of elevated 3he concentration were detected k1 in the campanian stage at 79 ma k2 in the santonian stage at 85 ma and k3 in the turonian stage at 91 ma all three of these episodes are associated with high 3he 4he and 3he non carbonate ratios consistent with their derivation from an enhanced extraterrestrial 3he flux rather than decreased carbonate sedimentation or dissolution while k2 is modest in magnitude and duration and thus is of limited significance k1 and k3 are each identified by a few myr interval with an 4 fold enhancement in mean 3he flux compared with pre event levels samples from odp hole 762c in the indian ocean spanning both k2 and k3 93 83 ma confirm the presence of a peak in the turonian stage suggesting that k3 is a global event the k1 and k3 3he events are similar in most respects to the two peaks previously detected in the cenozoic suggesting a similar origin these have been attributed to a major asteroid collision in the late miocene and to a shower of either comets or asteroids in the late eocene based on the age and temporal evolution of k1 we suggest that it most likely records the collision which produced the baptistina asteroid family independently dated at 80 ma the k3 event is less easily explained it is characterized by an unusually spiky and erratic temporal progression suggesting an unusual abundance of very 3he rich particles not previously seen in the sedimentary 3he record we suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust producing lunar impacts

the chicxulub impact event and its environmental consequences at the cretaceous tertiary boundary abstract an impact mass extinction hypothesis for the cretaceous tertiary k t boundary transition has been confirmed with multiple lines of evidence beginning with the discovery of impact derived ir in k t boundary sediments and culminating in the discovery of the chicxulub impact crater likewise a link between the chicxulub impact crater and k t boundary sediments has been confirmed with multiple lines of evidence including stratigraphic petrological geochemical and isotopic data the environmental effects of the chicxulub impact event were global in their extent largely because of the interaction of ejected impact debris with the atmosphere the environmental consequences of the chicxulub impact event and their association with the k t boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet