melt in the impact breccias from the eyreville drill cores chesapeake bay impact structure usa the center of the 35 3 ma chesapeake bay impact structure 85 km diameter was drilled during 2005 2006 in an icdp usgs drilling project the eyreville drill cores include polymict impact breccias and associated rocks 1397 1551 m depth tens of melt particles from these impactites were studied by optical and electron microscopy electron microprobe and microraman spectroscopy and classified into six groups m1 clear or brownish melt m2 brownish melt altered to phyllosilicates m3 colorless silica melt m4 melt with pyroxene and plagioclase crystallites m5 dark brown melt and m6 melt with globular texture these melt types have partly overlapping major element abundances and large compositional variations due to the presence of schlieren poorly mixed melt phases partly digested clasts and variable crystallization and alteration the different melt types also vary in their abundance with depth in the drill core based on the chemical data mixing calculations were performed to determine possible precursors of these melt particles the calculations suggest that most melt types formed mainly from the thick sedimentary section of the target sequence mainly the potomac formation but an additional crystalline basement schist gneiss precursor is likely for the most abundant melt types m2 and m5 sedimentary rocks with compositions similar to those of the melt particles are present among the eyreville core samples therefore sedimentary target rocks were the main precursor of the eyreville melt particles however the composition of the melt particles is not only the result of the precursor composition but also the result of changes during melting and solidification as well as postimpact alteration which must also be considered the variability of the melt particle compositions reflects the variety of target rocks and indicates that there was no uniform melt source original heterogeneities resulting from melting of different target rocks may be preserved in impactites of some large impact structures that formed in volatile rich targets because no large melt body exists in which homogenization would have taken place

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

the canyon diablo impact event projectile motion through the atmosphere meteor crater is one of the first impact structures systematically studied on earth its location in arid northern arizona has been ideal for the preservation of the structure and the surviving meteoric material the recovery of a large amount of meteoritic material in and around the crater has allowed a rough reconstruction of the impact event an iron object 50 m in diameter impacted the earths surface after breaking up in the atmosphere the details of the disruption however are still debated the final crater morphology deep bowl shaped crater rules out the formation of the crater by an open or dispersed swarm of fragments in which the ratio of swarm radius to initial projectile radius cd is larger than 3 the final crater results from the sum of the craters formed by individual fragments on the other hand the lack of significant impact melt in the crater has been used to suggest that the impactor was slowed down to 12 km s by the atmosphere implying significant fragmentation and fragments separation up to 4 initial radii this paper focuses on the problem of entry and motion through the atmosphere for a possible canyon diablo impactor as a first but necessary step for constraining the initial conditions of the impact event which created meteor crater after evaluating typical models used to investigate meteoroid disruption such as the pancake and separated fragment models we have carried out a series of hydrodynamic simulations using the 3d code sova to model the impactor flight through the atmosphere both as a continuum object and a disrupted swarm our results indicate that the most probable pre atmospheric mass of the meteor crater projectile was in the range of 4x10 8 to 1 2x10 9 kg equivalent to a sphere 4666 m in diameter during the entry process the projectile lost probably 30 to 70 of its mass mainly because of mechanical ablation and gross fragmentation even in the case of a tight swarm of particles cd 3 small fragments can separate from the crater forming swarm and land on the plains tens of km away from the crater as individual meteorites starting from an impactor pre atmospheric velocity of 18 km s which represents an average value for earth crossing asteroids we find that after disruption the most probable impact velocity at the earths surface for a tight swarm is around 15 km s or higher a highly dispersed swarm would result in a much stronger deceleration of the fragments but would produce a final crater much shallower than observed at meteor crater

lithium in tektites and impact glasses implications for sources histories and large impacts abstract lithium li abundances and isotope compositions were determined in a representative suite of tektites moldavites muong nong type tektites and an australite ivory coast tektites and bediasites impact related glasses libyan desert glass zhamanshinites and irghizites a glass fragment embedded in the suevite from the ries impact crater and sedimentary materials in order to test a possible susceptibility of li to fractionation during hypervelocity impact events and to de convolve links to their potential parental sources the overall data show a large spread in li abundance 4 7 58 ppm li and 7 li values 3 2 to 26 0 but individual groups of tektites and impact glasses have distinctive li compositions most importantly any significant high temperature li isotope fractionation can be excluded by comparing sedimentary lithologies from central europe with moldavites instead we suggest that li isotope compositions in tektites and impact related glasses are probably diagnostic of the precursor materials and their pre impact geological histories the muong nong type tektites and australite specimen are identical in terms of li concentrations and 7 li and we tentatively endorse their common origin in a single impact event evidence for low temperature rayleigh fractionation which must have operated prior to impact induced melting and solidification is provided for a subset of muong nong type tektites although li isotope variations in most tektites are broadly similar to those of the upper continental crust libyan desert glass carries high 7 li 24 7 which appears to mirror the previous fluvial history of parental material that was perhaps deposited in lacustrine environment or coastal seawater lithium isotopes in impact related glasses from the zhamanshin crater define a group distinct from all other samples and point to melting of chemically less evolved mafic lithologies which is also consistent with their major and trace element patterns extreme shock pressures and the related extreme post shock temperatures alone appear not to have any effect on the li isotope systematics therefore useful information on parental lithologies and magmatic processes may be retrieved from analyses of martian and lunar meteorites moreover lack of significant li depletion in tektites provides further constraints on the loss of moderately volatile elements during the moon forming impact

co2 record between 40 and 8 kyr b p from the greenland ice core project ice core co2 ice core records show an increase in the atmospheric concentration of 80 100 parts per million by volume ppmv from the last glacial maximum lgm to the early holocene we present co2 measurements performed on an ice core from central greenland drilled during the greenland ice core project grip this co2 profile from grip confirms the most prominent co2 increase from the lgm with a mean concentration of 200 ppmv to the early holocene with concentrations between 290 and 310 ppmv some structures of the new co2 record are similar to those previously obtained from the dye 3 ice core greenland which indicated a dilemma between greenland and antarctic co2 records oeschger et al 1988 both greenland cores show high co2 values for rather mild climatic periods during the last glaciation whereas co2 records from antarctica do not show such high co2 variations during the glaciation and furthermore the co2 values in the early holocene are about 20 30 ppmv higher in the grip record than in antarctic records there is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the co2 concentration under certain conditions if in situ processes can change the co2 concentration in the air bubbles the question arises about how reliably do co2 records from ice cores reflect the atmospheric composition at the time of ice formation the discrepancies between the co2 profiles from greenland and antarctica can be explained by in situ production of excess co2 due to interactions between carbonate and acidic species since the carbonate concentration in antarctic ice is much lower than in greenland ice co2 records from antarctica are much less affected by such in situ produced co2

ab initio calculations on the free energy and high p t elasticity of face centred cubic iron ab initio finite temperature molecular dynamics simulations have been used to calculate the free energy and elasticity of face centred cubic fcc iron at a state point representative of the earth s inner core whilst the free energy of this phase is found to be higher than that of hexagonal close packed hcp iron the difference is only 14 mev atom it is possible that this difference might be overcome by the presence of light elements as previous calculations at zero kelvin have shown that the addition of elements such as silicon stabilise fec fe with respect to hcp fe by at least 40 mev atom the calculated elastic constants at core pressures and temperatures of pure fec fe and of alloys of fe with sulphur and nickel fe3s and fe3ni derived from the fee structure lead to average shear wave velocities that are considerably higher than those inferred from seismology however these mineralogical and seismological results could be reconciled by the presence of partial melt in the inner core the calculated p wave anisotropy of fec fe is comparable with the seismological values but only if there is a high degree of crystal alignment although the necessity for alignment can be reduced if a layered model for the inner core is invoked the results presented in this paper therefore suggest that fcc fe cannot be ruled out as a candidate for the dominant phase of the earth s inner core c 2008 elsevier b v all rights reserved

drill core lb 08a bosumtwi impact structure ghana petrographic and shock metamorphic studies of material from the central uplift during a recent drilling project sponsored by the international continental scientific drilling progam icdp two boreholes lb 07a and lb 08a were drilled into the crater fill of the bosumtwi impact structure and the underlying basement into the deep crater moat and the outer flank of the central uplift respectively the bosumtwi impact structure in ghana west africa which is 10 5 km in diameter and 1 07 myr old is largely filled by lake bosumtwi here we present the lithostratigraphy of drill core lb 08a recovered between 235 6 and 451 33 m depth below lake level as well as the first mineralogical and petrographic observations of samples from this core this drill core consists of approximately 25 m of polymict clast supported lithic breccia intercalated with suevite which overlies fractured brecciated metasediment that displays a large variation in lithology and grain size the lithologies present in the central uplift are metasediments composed dominantly of fine grained to gritty meta graywacke phyllite and slate as well as suevite and polymict lithic impact breccia the suevites principally present between 235 6 and 240 5 m and between 257 6 and 262 2 m display a fine grained fragmental matrix about 39 to 45 vol and a variety of lithic and mineral clasts that include meta graywacke phyllite slate quartzite carbon rich organic shale and calcite as well as melt particles fractured quartz unshocked quartz unshocked feldspar quartz with planar deformation features pdfs diaplectic quartz glass mica epidote sphene and opaque minerals the crater fill suevite contains calcite clasts but no granite clasts in contrast to suevite from outside the northern crater rim the presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 gpa quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to rarely 4 sets of pdfs per grain the shock pressures recorded by the pdfs in quartz grains in the polymict impact breccia range from 10 to 30 gpa we also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth meta graywacke samples from the basement are heterogeneously shocked with shock pressures locally ranging up to 2530 gpa suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim fallback impact breccia as well as a lack of ballen quartz which is present in the external breccias similar variations of melt particle abundance and shockmetamorphic grade between impact breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the ries impact structure germany

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

the canyon diablo impact event 2 projectile fate and target melting upon impact abstract despite its centennial exploration history there are still unresolved questions about meteor crater the first recognized impact crater on earth this theoretical study addresses some of these questions by comparing model results with field and laboratory studies of meteor crater our results indicate that meteor crater was formed by a high velocity impact of a fragmented projectile ruling out a highly dispersed swarm as well as a very low impact velocity projectile fragmentation caused many fragments to fall separately from the main body of the impactor making up the bulk of the canyon diablo meteorites most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression and were redistributed randomly around the crater thus the distribution of canyon diablo meteorites is not representative of projectile trajectory as is usual for impactor fragments in smaller strewn fields at least 50 of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles spheroids and highly shocked solid fragments shrapnel when compared with the known distribution model results suggest an impactor from the sw overall every model case produced much higher amounts of pure projectile material than observed the projectile target mixing was not considered in the models however this process could be the main sink of projectile melt as all analyzed melt particles have high concentrations of projectile material the fate of the solid projectile fragments is still not completely resolved model results suggest that the depth of melting in the target can reach the coconino sandstone formation however most of the ejected melt originates from 30 40 m depth and thus is limited to moenkopi and upper kaibab material some melt remains in the target based on the estimated volume of the breccia lens at meteor crater our models suggest at most a 2 content of melt in the breccia finally a high water table at the time of impact could have aided strong dispersion of target and projectile melt

martian rampart crater ejecta experiments and analysis of melt water interaction abstract viking images of martian craters with rampart bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar sized craters on the moon and mercury topographic control of distribution lobate and terraced margins cross cutting relationships and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage it is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement the dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation ejecta particle size and the energy and mode of melt ejecta dispersal study of terrestrial lobate volcanic ejecta produced by steam blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse this deposition results from flow yield strength increasing above shear stress due to interparticle friction

quaternary stratigraphy and ice cores abstract ice cores contain highly detailed continuous stratigraphic records extending from the present to 800 ka bp a large variety of climate proxies related to atmospheric and climate changes can be analysed and dated with an accuracy ranging from a few years for recent centuries to a few millennia for older records over the past two decades the contribution of ice core studies to the reconstruction and interpretation of past climate changes became fundamental they allowed the direct measurement of greenhouse gas changes in the past and the detection of rapid climatic changes this paper presents a short summary of ice core results restricted only to those relevant for the stratigraphy of the quaternary at the base of the epica dome c dc ice core the matuyama bruhnes m b magnetic reversal has been indirectly identified at the end of marine isotopic stage mis 19 and dated 764 776 6 ka bp allowing the correlation with the m b boundary observed in the deep sea sediments the details of the m b transition observed in the epica dc ice core are of interest for an improved characterization of this time marker which is crucial for the identification of the lower middle pleistocene boundary a clear increase in the amplitude of the glacial interglacial cycles is observed at 430 ka bp at the mis 12 11 transition in the epica dc climatic curves this mid bruhnes event marks a change in the pattern of climatic changes the middle upper pleistocene boundary at the base of the eemian stage is clearly detectable in the antarctic ice cores as a sharp increase in the methane signal coinciding with the initial peak of temperature derived from stable isotope profile around 128 ka bp in good agreement with vegetation changes on land in the northern hemisphere in greenland ice cores the last climatic cycle is marked by high amplitude millennial scale climatic variations bounded by abrupt limits that appear to correlate with stadial interstadial stages these climatic events are also present in the antarctic ice cores but smoothed smaller in amplitude and out of phase with respect to their northern counterparts therefore the stratigraphy of the deglaciation and of the transition from the upper pleistocene to the holocene differs in the two opposite hemispheres the holocene in ice cores is different from previous interglacials and it deserves a specific identification in the geological history despite its short duration compared to other epochs the formal definition of the pleistocene holocene boundary has been recently proposed and ratified on the ngrip ice core and fixed at 11 7 ka before ad 2000 about 200 years before present the sharp increase of co 2 and methane above the natural interglacial level as well as the sudden increase of atmospheric pollutants are a clear stratigraphic evidence for a recent global event caused by the human impact

petrography geochemistry and alteration of country rocks from the bosumtwi impact structure ghana samples of the country rocks that likely constituted the target rocks at the 1 07 myr old bosumtwi impact structure in ghana west africa collected outside of the crater rim in the northern and southern parts of the structure were studied for their petrographic characteristics and analyzed for their major and trace element compositions the country rocks mainly meta graywacke shale and phyllite of the early proterozoic birimian supergroup and some granites of similar age are characterized by two generations of alteration a pre impact hydrothermal alteration often along shear zones is characterized by new growth of secondary minerals such as chlorite sericite sulfides and quartz or replacement of some primary minerals such as plagioclase and biotite by secondary sericite and chlorite a late argillic alteration mostly associated with the suevites is characterized by alteration of the melt glass clasts in the groundmass of suevites to phyllosilicates suevite which occurs in restricted locations to the north and to the south southwest of the crater rim contains melt fragments diaplectic quartz glass ballen quartz and clasts derived from the full variety of target rocks no planar deformation features pdfs in quartz were found in the country rock samples and only a few quartz grains in the suevite samples show pdfs and in rare cases two sets of pdfs based on a total alkali element silica tas plot the bosumtwi granites have tonalitic to quartz dioritic compositions the nb versus y and ta versus yb discrimination plots show that these granites are of volcanic arc tectonic provenance provenance studies of the metasedimentary rocks at the bosumtwi crater have also indicated that the metasediments are volcanic arc related compared to the average siderophile element contents of the upper continental crust both country rocks and impact breccias of the bosumtwi structure show elevated siderophile element contents this however does not indicate the presence of an extraterrestrial component in bosumtwi suevite because the birimian country rocks also have elevated siderophile element contents which is thought to result from regional hydrothermal alteration that is also related to widespread sulfide and gold mineralization

gas records from the west greenland ice margin covering the last glacial termination a horizontal ice core abstract certain sites along ice sheet margins provide an easily accessible and almost unlimited supply of ancient ice at the surface measurements of gases in trapped air from ice outcropping at pakitsoq west greenland demonstrate that ancient air is mostly well preserved no alterations in 18 o atm and 15 n of n 2 are apparent and alterations in methane are found in only a few ice sections using measurements of these gases we have unambiguously identified a stratigraphic section containing ice from the end of last glacial period as well as bolling allerod younger dryas and preboreal intervals extensive sections of ice from the holocene and most ages within the last glacial period are probably also present very accurate dating has been possible in the ice section containing the younger dryas preboreal abrupt climate transition signal the ice at pakitsoq is folded and non uniformly thinned with many cross cutting bands of bubble free ice and dust the cross cutting features are associated with anomalies in both the gas and the ice records with careful sampling to avoid these the ice at pakitsoq is suitable for recovery of large volume samples of the ancient atmosphere for analysis of trace constituents such as 14 ch 4

modeling the formation of the k pg boundary layer abstract in this paper we investigate the formation of the cretaceous paleogene k pg boundary layer through numerical modeling the k pg layer is widely agreed to be composed of meteoritic material and target rock from the chicxulub impact site that has been ejected around the globe and mixed with local material during final deposition the observed composition and thickness of the k pg boundary layer changes with azimuth and distance from the impact site we have run a suite of numerical simulations to investigate whether we can replicate the observational data with a focus on the distal k pg layer and the impact glasses at proximal sites such as beloc haiti previous models of the k pg ejecta have assumed an initial velocity distribution and tracked the ejecta to its final destination here we attempt to model the entire process from impact to the arrival of the ejecta around the globe our models replicate the observed ejecta thickness at proximal sites and the modeled ejecta is composed of sediments and silicate basement rocks in agreement with observational data models that use a 45 impact angle are able to replicate the total ejecta and iridium volume at distal sites and the majority of the ejecta is composed of meteorite and target sediments sub vertical impacts generate too little iridium and oblique impacts of 30 degrees generate too much however in contrast to observations models that involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance and are unable to transport shocked minerals quartz and zircon from the chicxulub basement rocks around the globe we suggest that much of the k pg ejecta is transported non ballistically and that the most plausible mechanism is through re distribution from a hot expanding atmosphere the results are important for future investigations of the environmental effects of the chicxulub impact

ivory coast microtektite strewn field description and relation to the jaramillo geomagnetic event abstract during the present study the ivory coast microtektite layer was found in cores from five equatorial atlantic sites bringing the total number of ivory coast microtektite bearing cores to eleven the strewn field appears to be restricted to between 9 n and 12 s latitude there is a general increase in the concentration of microtektites towards the bosumtwi crater which is generally thought to be the source of the ivory coast tektites the relationship between the onset of the jaramillo subchron and the ivory coast microtektite layer has been investigated in six cores a plot of the difference in depth between the base of the jaramillo subchron and the microtektite layer versus sediment accumulation rate was used to determine the average post depositional remanent magnetization pdrm acquisition depth and the age difference between the onset of the jaramillo subchron and the deposition of the microtektites assuming that the pdrm acquisition depth does not vary with sediment accumulation rate we find that the average pdrm acquisition depth is 7 cm and that the microtektites were deposited approximately 8 ky after the onset of the jaramillo subchron this indicates that the impact responsible for the ivory coast tektites and microtektites could not be causally related to the geomagnetic reversal at the base of the jaramillo subchron

shock metamorphic petrography and microraman spectroscopy of quartz in upper impactite interval icdp drill core lb 07a bosumtwi impact crater ghana standard and universal stage optical microscope and microraman spectroscopic examination of quartz from the upper impactite interval of the international continental scientific drilling program icdp lake bosumtwi crater drill core lb 07a demonstrates widespread but heterogeneous evidence of shock metamorphism in the upper impactite which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333 4 415 7 m quartz occurs as a major component within metasedimentary lithic clasts and as abundant isolated single crystal grains within matrix the noted quartz shock metamorphic features include phenomena related to a deformation such as abundant planar microstructures grain mosaicism and reduced birefringence b phase transformations such as rare diaplectic quartz glass and very rare coesite c melting such as isolated colorless to dark glassy and devitrified vesicular melt grains and d secondary post shock features such as abundant variable decoration of planar microstructures and patchy grain toasting common to abundant planar deformation features pdfs in quartz are dominated by 1013 equivalent crystallographic planes although significant percentages of 1012 and other higher index orientations also occur notably c 0001 planes are rare significantly the quartz pdf orientations match most closely those reported elsewhere from strongly shocked crystalline target impactites barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 gpa were widely reached pressures exceeding 40 45 gpa were more rare the relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz together with the nature and distribution of melt within suevite suggest a water or volatile rich target for the bosumtwi impact event

a tentative reconstruction of the last interglacial and glacial inception in greenland based on new gas measurements in the greenland ice core project grip ice core parameters the grip d 18 oice chronological sequence is obtained by comparing a new set of d 18 o of atmospheric o2 and ch4 measurements from the bottom section of the grip core with their counterpart in the vostok antarctic profiles this comparison clearly identifies ice from the penultimate glacial maximum mis 6 190 130 kyr b p in the grip core further it allows rough reconstruction of the last interglacial period and of the last glacial inception in greenland which appears to lay its antarctic counterpart our data suggest that while antarctica is already entering into a glaciation greenland is still experiencing a warm maximum during mis 5e index terms 1040 geochemistry isotopic composition chemistry 1827 hydrology glaciology 1863 3344 meteorology and atmospheric dynamics paleoclimatology keywords interglacial ice cap firn

possible reasons of shock melt deficiency in the bosumtwi drill cores pre drilling numerical modeling of the bosumtwi impact event predicted a 200 m thick coherent melt layer as well as abundant highly shocked target material within the central part of the crater structure however these predictions are in disagreement with data from drill core obtained in 2004 2005 here i provide a brief overview of previous results and discuss possible reasons behind melt deficiency such as specific impact scenarios low impact velocity and or low impact angle and specific target properties different composition high porosity high content of volatiles i conclude that the most likely explanation is the dispersion of impactites due to the vaporization of pore water which was not included in the original numerical model

chronology reconstruction for the disturbed bottom section of the gisp2 and the grip ice cores implications for termination ii in greenland and d 18 oatm in the trapped gases our reconstructed ages for basal ice samples are based on comparison of published measurements of ch4 and d 18 oatm from the disturbed section of the grip and gisp2 cores with the same properties in the vostok ice core ngrip d 18 oice values are also used to constrain the chronology during the end of marine isotope stage 5e for each sample we assign an age that represents the unique or most probable time of gas trapping given its gas composition of 157 samples with ch4 and d 18 oatm data 10 give unique ages twenty five newly measured values of the triple isotope composition of o2 from the disturbed section of the gisp2 core add a third time dependent gas property that agrees with our reconstruction our reconstruction supports earlier conclusions of landais et al 2003 that the disturbed section primarily includes ice from the last interglacial mis 5e and the penultimate glacial period mis 6 the oldest ice in the basal layer of gisp2 and grip has an age 237 ka the climate history we derive suggests that the last interglacial at summit greenland around 127 ka was slightly warmer than the current interglacial period reduction of various ion concentrations in ice and thickening of the ice sheet during termination ii was similar to that in termination i

petrography of the impact breccias of the enkingen subo 18 drill core southern ries crater germany new estimate of impact melt volume a new drill core subo 18 that intersects a continuous sequence of impact breccias has been obtained at the town of enkingen at the southern edge of the ries inner crater detailed petrographic analysis of this core provides important new information regarding the nature and origin of within crater suevite which is not only relevant for the understanding of the formation of the ries impact structure but has strong implications for impact breccia deposition in general in the enkingen core below 21 m of crater sediment 90 m of impact breccia have been sampled most of this material is what in the past has been referred to as suevite defined as groundmass of clastic material entraining cogenetic melt fragments and target rock clasts only the bottommost 10 m of core are melt dominated impact breccia any relatively thick melt rock intersection along this core is revealed to be composed of individual centimeter to decimeter sized melt bodies best described as agglomerate due to its close resemblance to volcaniclastics of that type detailed macro meso and microscopic petrographic analysis has established that the subo 18 impact breccias cannot be divided into distinct units the groundmass is extensively altered to phyllosilicate smectite or chlorite or carbonate minerals results of a detailed scanning electron microscopic investigation of the groundmasses of samples from all parts of this sequence emphatically support the traditional definition of suevite irrespective of scale modal analysis at macroscopic to submicroscopic scales has shown that the overall average melt content of enkingen suevite is on the order of 48 vol this exceeds by far previous estimates and brings the ries crater melt volume in line with values traditionally suggested from crater and melt volume scaling for similar sized impact structures in crystalline and mixed targets

the icdp lake bosumtwi impact crater scientific drilling project ghana core lb 08a litho log related ejecta and shock recovery experiments the 1 07 myr old lake bosumtwi impact crater in ghana was drilled within the framework of the international continental scientific drilling project icdp hole lb 08a drilled into the outer flank of the central uplift and with a total depth of 451 m yielded 215 71 m of impact related rocks this paper summarizes observations of the lithological logging on core lb 08a between a depth of 235 6 and 260 m the section consists of a melt bearing allochthonous polymict and mostly clast supported impact breccia down to 418 m the section comprises a rather uniform unit of metagraywacke alternating with phyllite to slate lower greenschist facies few par autochthonous impact breccia bodies and rare impact dike breccias are present the lowermost part of the section contains several centimeter to decimeter thick melt bearing breccia dikes in country rocks identical to those occurring above omnipresent fracturing was mapped in a qualitative manner most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals the maximum shock pressure as evidenced by quartz is below 26 gpa the allochthonous breccias occasionally contain a few vol of melt particles suevites occur outside the crater rim carrying diaplectic crystals coesite and ballen quartz as well as true melt glasses and a variety of lithic clasts among those spectacular staurolite rich mica schists the recorded shock level in the uplifted target rocks is lower than expected and modeled shock recovery experiments with analogue carbonaceous graywackes at 34 and 39 5 gpa yielded nearly complete transformation of quartz into diaplectic glass we therefore exclude a specific shock behavior of the soft fluid rich target material carbonaceous graywackes shales slates in core lb 08a as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the lake bosumtwi impact crater

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

stable carbon cycle climate relationship during the late pleistocene a record of atmospheric carbon dioxide co2 concentrations measured on the epica european project for ice coring in antarctica dome concordia ice core extends the vostok co2 record back to 650 000 years before the present yr b p before 430 000 yr b p partial pressure of atmospheric co2 lies within the range of 260 and 180 parts per million by volume this range is almost 30 smaller than that of the last four glacial cycles however the apparent sensitivity between deuterium and co2 remains stable throughout the six glacial cycles suggesting that the relationship between co2 and antarctic climate remained rather constant over this interval

origin and emplacement of the impact formations at chicxulub mexico as revealed by the icdp deep drilling at yaxcopoil 1 and by numerical modeling we present and interpret results of petrographic mineralogical and chemical analyses of the 1511 m deep icdp yaxcopoil 1 yax 1 drill core with special emphasis on the impactite units using numerical model calculations of the formation excavation and dynamic modification of the chicxulub crater constrained by laboratory data a model of the origin and emplacement of the impact formations of yax 1 and of the impact structure as a whole is derived the lower part of yax 1 is formed by displaced cretaceous target rocks 610 m thick while the upper part comprises six suevite type allochthonous breccia units 100 m thick from the texture and composition of these lithological units and from numerical model calculations we were able to link the seven distinct impact induced units of yax 1 to the corresponding successive phases of the crater formation and modification which are as follows 1 transient cavity formation including displacement and deposition of cretaceous megablocks 2 ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity 3 deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity brecciated impact melt rock 4 collapse of the ejecta plume and deposition of fall back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification 5 continued collapse of the ejecta plume and deposition of the upper suevite 6 late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere 7 final phase of fall back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite and 8 return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions our results are compatible with a 180 km and 100 km for the diameters of the final crater and the transient cavity of chicxulub respectively as previously proposed by several authors and b the interpretation of chicxulub as a peak ring impact basin that is at the transition to a multi ring basin

ch4 and δ18o of o2 records from antarctic and greenland ice a clue for stratigraphic disturbance in the bottom part of the greenland ice core project and the greenland ice sheet project 2 ice cores the suggestion of climatic instability during the last interglacial period eem based on the bottom 10 of the greenland ice core project grip isotopic profile has been questioned because the bottom record from the neighboring greenland ice sheet project 2 gisp2 core 28 km away is strikingly different over the same interval and because records of the 18o of atmospheric o2 from both cores showed unexpected rapid fluctuations here we present detailed methane records from the vostok antarctica grip and gisp2 cores over the relevant intervals the grip and gisp2 data show rapid and large changes in methane concentration which are correlative with variations of the 18o of the ice while the vostok record shows no such variations this discrepancy reinforces the suggestion that the bottom sections of the greenland records are disturbed by combining the methane data with measurements of 18o of o2 in the same samples we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the grip core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation

the melt rocks of the boltysh impact crater ukraine ussr the 100 12 m y 25 km diameter boltysh impact crater was formed in precambrian granites and granite gneisses of the ukrainian shield the crater deposits have undergone minimal post impact erosion and it is possible to study a complete vertical section of the underlying 200 m thick melt sheet the melt rocks as sampled in two drill holes can be subdivided into two major textural classes microcrystalline and glassy the microcrystalline melt rocks form an uppermost and two lowermost units with the glassy variety occupying the middle of the melt sheet the microcrystalline units contain 25 zoned plagioclase phenocrysts set in a microcrystalline matrix of intergrown alkali feldspar and quartz pyroxene has been replaced by sheet silicates mineral and lithic clasts make up 5 15 and show varying degrees of shock and resorption the glassy melt rocks are characterized by 10 30 zoned plagioclase and 5 10 orthopyroxene set in a fresh to partially devitrified glassy matrix clast content is 5 chemically the melt rocks are relatively homogeneous and correspond to a mixture of kirovograd granites and gneisses in the ratio of 5 to 1 with ni ir and cr showing slight enrichments over the target rocks there are minor differences in the fe2o3 feo ratio and the alkalis between the microcrystalline and glassy varieties the increase in matrix crystallinity at the upper and lower contacts is contrary to observations at other impact melt sheets where greater matrix crystallinity occurs in the interiors of the melt sheets one possible explanation is that the melt matrix was originally glassy throughout due to its high sio2 content and the microcrystalline matrix is the result of extensive devitrification involving minor alkali exchange with circulating ground waters

popigai crater origin and distribution of diamond bearing impactites about three decades ago a formerly unknown genetic type of natural diamonds was discovered in impact craters impact diamonds are currently known from a number of impact structures in europe asia and north america and it s likely that the number of finds will increase with time the popigai crater northern siberia where impact diamonds were first found was specifically investigated in terms of geology geophysics petrography and mineralogy large resources of industrial impact diamonds were discovered and these minerals were studied in detail authigenic impact diamonds occur in situ in shocked graphite bearing gneisses that are found as inclusions in impact melt rocks tagamites and suevites according to the observed transformation of coexisting minerals the lower estimated pressure of the coherent martensite transition of graphite to diamonds is 35 gpa impact diamonds inherit the original shape of graphite crystals and are composed of a polycrystalline structured aggregate of cubic and probably hexagonal carbon microcrystals 1 5 m across numerous properties of diamonds depend on the high density of defects in the crystal lattice allothigenic impact diamonds occur in rocks produced by the homogenization and solidification of impact melt which originated from the complete fusion of graphite bearing precursor gneisses these diamonds usually reflect the influence of the hot melt and are strongly corroded diamond bearing tagamites and suevites in the popigai crater interior occur as extended lens and sheets bodies and also as irregular small bodies diamond distribution depends on the original abundance of precursor graphite in the target rocks on the superimposed shock metamorphic zonation and on the character of the ejection of shocked and melted material along different trajectories and azimuths this has resulted in radial and concentric inhomogeneities in diamond distribution in the crater interior on a second order the distribution depends on the scale of melt contamination by clasts and fragments and by the duration of cooling of certain melt bodies and their constituents enrichment in diamonds at the margins of thick tagamite sheets is the result of rapid cooling which prevents combustion of diamonds a positive correlation between diamond content and the amount of phosphorus pentoxide in impactites indicates links to c and p probably in organic matter of the primary sedimentary rocks which were subjected to granulitic metamorphism 2 4 ga ago and melted at the time of impact 35 7 ma ago

termination v in the vostok antarctica ice core the age depth relationship of the vostok antarctica ice core has been reconstructed in the depth interval 3300 3347 m by comparing three gas properties in ice co 2 ch 4 and 18 o atm with those in the epica dome c antarctica core fourteen vostok depths were examined in this interval and it was found that nine samples are uniquely dated if candidate ages are restricted to the interval between 400 and 650 kyr one of these samples is uniquely dated without restriction the analysis supports previous reports that this section contains ice from termination v but that the stratigraphic order of ice is reversed the top of the overturned layer lies between 3316 and 3319 m at least one other stratigraphic disturbance was found between 3340 and 3343 m as indicated by another reversal of the age depth relationship finally the oldest ice in this section is dated at 440 kyr confirming the existence of ice from the cold marine isotope stage mis 12 interval

petrology and geochemistry of target rocks from the bosumtwi impact structure ghana and comparison with ivory coast tektites the 10 5 km diameter bosumtwi crater in ghana west africa is the most likely source crater for the ivory coast tektites as the tektites and the crater have the same age 1 07 ma and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks the crater is excavated in 2 1 2 2 ga old metasediments and metavolcanics of the birimian supergroup here we present the first integrated petrographic and geochemical study of rocks from the bosumtwi impact crater a variety of target rocks from the bosumtwi impact structure were selected to represent the major rock types that have been described before resulting in four groups shale phyllite graywacke and two different types of granites from dispersed dikes and from the so called pepiakese intrusion at the northeastern side of the crater these rocks were analyzed for their major and trace element composition and their petrographic characteristics in addition representative samples were also analyzed for their o sr and nd isotopic compositions the target rocks do not show any unambiguous evidence of shock metamorphism i e planar deformation features pdfs distinct impact characteristic shock effects pdfs were identified only in clasts within suevite derived melt fragments the compositional range of the target rocks is significantly wider than that of the ivory coast tektites but overlap the tektite compositions a best fit line for the bosumtwi crater rocks in a rb sr isotope evolution diagram yields an age of 1 98 ga and an initial 87sr 86sr ratio of 0 701 which is close to results previously obtained for granitoid intrusions in the birimian of ghana our nd isotopic data yield depleted mantle model ages ranging from 2 16 to 2 64 ga and nd values of 17 2 to 25 9 harmonic least squares hmx mixing calculations were able to reproduce the composition of ivory coast tektites from a mixture of bosumtwi country rocks that include about 70 phyllite graywacke 16 granite dike and 14 pepiakese granite the oxygen isotopic composition of the metasedimentary rocks and granite dikes 18o 11 3 13 6 and the tektites 18o 11 7 12 9 also this work agree fairly well while the pepiakese granites have lower values 18o 8 6 9 0 indicating that only minor amounts of these rocks were incorporated in the formation of the ivory coast tektites the large variation in sr and nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions but in both a 87sr 86sr vs 1 sr plot and an sr vs nd diagram the tektites plot within the area occupied by the metasedimentary and granitic bosumtwi crater rocks thus our data support the interpretation that the composition of the ivory coast tektites is similar to that of rocks exposed at the bosumtwi impact structure indicating formation during the same impact event

impact and volcanism a momentum scaling law for erosion the controversy that raged through the 1950s 1970s over similarities and differences between meteorite impact and volcanic processes is revisited in this paper we propose that there are quantitative similarities in erosion caused by high speed ejecta produced by either impacts or volcanic processes field and petrographic data from the manicouagan impact crater canada are used to demonstrate that during the emplacement of the impact melt sheet erosion occurred at a rate of 2562 kg m 2 s 1 field data for the mount st helens lateral blast of may 18 1980 suggest an erosion rate of 2 kg m 2 s 1 and field data for a small pyroclastic flow on august 7 1980 suggest an erosion rate of 14 kg m s 1 it is proposed that these three rates were determined dominantly by the momentum of the ejecta and a quantitative formulation based on lofting theory is given a new application of the monte carlo approach to analysis provides minimum most likely and maximum estimates for both the field and the theoretical analyses the substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

lithostratigraphic and petrographic analysis of icdp drill core lb 07a bosumtwi impact structure ghana lithostratigraphic and petrographic studies of drill core samples from the 545 08 m deep international continental scientific drilling program icdp borehole lb 07a in the bosumtwi impact structure revealed two sequences of impactites below the post impact crater sediments and above coherent basement rock the upper impactites 333 38 415 67 m depth comprise an alternating sequence of suevite and lithic impact breccias the lower impactite sequence 415 67 470 55 m depth consists essentially of monomict impact breccia formed from meta graywacke with minor shale as well as two narrow injections of suevite which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts the basement rock 470 55 545 08 m depth is composed of lower greenschist facies metapelites shale schist and minor phyllite meta graywacke and minor meta sandstone as well as interlaminated quartzite and calcite layers the basement also contains a number of suevite dikelets that are interpreted as injection veins as well as a single occurrence of granophyric textured rock tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre impact granitoid complexes impact melt fragments are not as prevalent in lb 07a suevite as in the fallout suevite facies around the northern crater rim on average 3 6 vol of melt fragments is seen in the upper suevites and up to 18 vol in the lower suevite occurrences shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz 1 to 3 sets rare diaplectic quartz glass and very rare diaplectic feldspar glass notably no ballen quartz which is abundant in the fallout suevites has been found in the within crater impact breccias an overall slight increase in the degree of shock metamorphism occurs with depth in the impactites but considerably lower shock degrees are seen in the suevites of the basement rocks which show similar features to each other the bulk of the suevite in lb 07a appears to have been derived from the 35 gpa shock zone of the transient crater

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events

the co2 concentration of air trapped in gisp2 ice from the last glacial maximum holocene transition measurements of the co2 concentration of air trapped in greenland ice sheet project two ice show an increase from approximately 190 ppm to 270 ppm during the 8000 years between the end of the last glacial maximum and the beginning of the holocene this increase is in agreement with other parallel records from both greenland and antarctica although the gisp2 data exhibit considerable variability over periods as small as 50 100 years and show that co2 can be affected by processes occurring within the ice after bubble formation the in situ reactions which contribute to the modification of the concentrations of paleoatmospheric co2 may include both co2 uptake as well as co2 production the existence of two major episodes of co2 enrichment in addition to the variability of the data prevents a precise determination of the progression of the increase in atmospheric co2 although the 80 ppm rise between 18 0 kybp and 10 4 kybp is evident these results strongly suggest that some intervals of the gisp2 co2 record must be interpreted in the context of the chemical composition of the ice and that records of atmospheric co2 in polar ice can be considered accurate only if the ice is either essentially carbonate free or contains abundant carbonate