deccan volcanic contribution of ir and other trace elements near the k t boundary in india abstract the concentrations of ir and some other elements in the takli intertrappean sediments representing a 2 m thick shallow continental deposit in the deccan province around the k t boundary ktb event were measured to determine their volcanic contribution each unit of the intertrappeans consisting of lava ash clay and marl sandwiched between two basalt flows was analysed the concentrations of fe and co are found to be generally lower than in the adjacent basalts whereas cr is similar but occasionally higher in the volcanic ash concentrations of eight rare earth elements ree other than ce are also similar to their levels in basalts except at a few horizons in the ash where higher concentrations by a factor of 2 5 are found ir concentration is determined to be

sanidine spherules at the cretaceous tertiary boundary indicate a large impact event the hypothesis that a catastrophic impact of an extraterrestrial body caused the terminal cretaceous mass extinctions of dinosaurs planktonic foraminfera and other species is now accepted as respectable following the discovery of a worldwide iridium enrichment in the cretaceous tertiary k t boundary clay1 5 in the basal lamina of the k t boundary clay of caravaca spain 7 numerous spherules were discovered composed of finely crystallized almost pure k feldspar in the structural state of high sanidine it is concluded here that these spherules solidified from a melt and were probably derived from the impacting body this poses problems as high k values are not reported from bulk analyses of meteorites6 the k feldspar phenocrysts reported in some iron meteorites23 suggest the body may have been a metal sulphide silicate planetesimal a cometary body is suggested as an alternative

shock induced microdeformations in quartz and other mineralogical indications of an impact event at the cretaceous tertiary boundary abstract the event terminating the cretaceous period and the mesozoic era caused massive extinctions of flora and fauna worldwide theories of the nature of this event can be classed as endogenic volcanic climatic etc or exogenic extraterrestrial causes mineralogical evidence from the boundary clays and claystones strongly favor the impact of an extraterrestrial body as the cause of this event nonmarine k t boundary claystones are comprised of two separate layers an upper layer composed of high angle ejecta material shocked quartz altered glass and spinel and a basal kaolinitic layer containing spherules clasts and altered glass together with some shocked grains recognition of this dual layered nature of the boundary clay is important for the determination of the timing and processes involved in the impact event and in the assignment and interpretation of geochemical signatures multiple sets of shock induced microdeformations planar features in quartz grains separated from k t boundary clays provide compelling evidence of an impact event this mineralogical manifestation of shock metamorphism is associated worldwide with a large positive anomaly of iridium in these boundary clays which has also been considered indicative of the impact of a large extraterrestrial body global distributions of maximum sizes of shocked quartz grains from the boundary clays and the mineralogy of the ejecta components favor an impact on or near the north american continent spinel crystals magnesioferrite occur in the boundary clays as micrometer sized octahedra or skeletal forms their composition differs from that of spinels found in terrestrial oceanic basalts magnesioferrite crystals are restricted to the high angle ejecta layer of the boundary clays and their small size and skeletal morphology suggest that they are condensation products of a vaporized bolide hollow spherules ranging up to 1 mm in size are ubiquitously associated with the boundary clays in nonmarine sections where a high angle ejecta layer and an underlying kaolinitic layer can be distinguished the spherules are found only in the kaolinitic layer the morphologies and surface features of these spherules suggest that they are original forms and not secondary growths or algal bodies these impact spherules closely resemble microtektites in size and shape all of these features of the boundary clay are uniquely associated with impact and cannot have been formed by volcanic or other terrestrial processes

mean magnetic contrasts between oceans and continents abstract the magnetic contrast between continents and oceans is an important constraint for understanding the regional distribution of magnetic sources in the crust and upper mantle this magnetic difference was investigated using satellite 2 c scalar magnetic anomalies collected by nasa s magsat mission over the atlantic and eastern pacific oceans north and south america europe and africa reducing the magnetic anomalies differentially to vertical polarization yields mean anomaly values which correlate with the tectonic ages of the various continental and oceanic regions the range of magnetic anomaly values of tectonically older lithosphere is greater than that of younger lithosphere statistically at the 99 9 confidence level the mean radially polarized magnetic anomaly of the continents is greater than that of the oceans a careful inversion was performed for regional effective susceptibility contrasts which accurately fit the magsat data assuming a model in which the moho represents a boundary between crustal magnetic and predominantly non magnetic mantle rocks the root mean square of the magnetic properties of oceans compares favorably with magnetic properties predicted from models constrained by magnetic studies of the oceanic rocks and which incorporate viscoremanent components from rocks near the base of the crust the mean magnetic anomaly and property contrasts of the oceans are negative and show large variability indicating widespread deviations from the standard oceanic crust the effective susceptibility contrasts obtained facilitate quantitative testing of inferences concerning large scale petrologic variations crustal thickness and thermal perturbations and other geologic processes which control the regional distribution of magnetic properties within the lithosphere

environmental perturbations caused by the impacts of asteroids and comets we review the major impact associated mechanisms proposed to cause extinctions at the cretaceous tertiary geological boundary we then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions but we also stress that there are many uncertainties we conclude that impacts with energies less than about 10 mt are a negligible hazard for impacts with energies above 10 mt and below about 104 mt i e impact frequencies less than one in 6 104 years corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m respectively blast damage earthquakes and fires should be important on a scale of 104 or 105 km which corresponds to the area damaged in many natural disasters of recent history however tsunami excited by marine impacts could be more damaging flooding a kilometer of coastal plain over entire ocean basins in the energy range of 104 105 mt intervals up to 3 105 years corresponding to comets and asteroids with diameters up to 850 m and 1 4 km respectively water vapor injections and ozone loss become significant on the global scale in our nominal model such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects but if a higher fraction of pulverized rock than we think likely reaches the stratosphere stratospheric dust causing global cooling would also be important in this energy range thus 105 mt is a lower limit where damage might occur beyond the experience of human history the energy range from 105 to 106 mt intervals up to 2 106 years corresponding to comets and asteroids up to 1 8 and 3 km diameter is transitional between regional and global effects stratospheric dust sulfates released from within impacting asteroids and soot from extensive wild fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10 moreover the ejecta plumes of these impacts may produce enough no from shock heated air to destroy the ozone shield between 106 and 107 mt intervals up to 1 5 107 years corresponding to comets and asteroids up to 4 and 6 5 km diameter dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km and the resulting smoke would reduce light levels even further at energies above 107 mt blast and earthquake damage reach the regional scale 106 km tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world s ocean basins fires would be set globally light levels may drop so low from the smoke dust and sulfate as to make vision impossible at energies approaching 109 mt 108 years the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids the cretaceous tertiary impact in particular struck evaporate substrates that very likely generated a dense widespread sulfate aerosol layer with consequent climatic effects the combination of all of these physical effects would surely represent a devastating stress on the global biosphere

dinoflagellate cysts sea level changes and planktonic foraminifers across the cretaceous tertiary boundary at el haria northwest tunisia abstract dinoflagellate cysts and planktonic foraminifers have been studied from the cretaceous tertiary k t boundary interval at el haria northwest tunisia a high resolution integrated biostratigraphy is presented the k t boundary is drawn at the level of extinction of cretaceous planktonic foraminifers and is coincident with the first occurrence of the dinoflagellate cyst species danea californica the final extinction of planktonic foraminifers is foreshadowed by a reduction in their total abundance some 5 kyr earlier at the base of the boundary clay this reduction is coeval with reported anomalies in siderophyle elements and 13 c values in the same area dinoflagellate cysts do not show accelerated rates of extinction at k t time associations of dinoflagellate cysts however change drastically and parallel changes in relative numbers of sporomorphs spores and pollen and in the quantity of land derived organic matter jointly these changes reflect a rapidly falling sea level during the final 17 kyr of the mesozoic which culminates at the level of the k t boundary this steep sea level fall at k t time represents a peak regressive pulse at the end of the well documented latest cretaceous regressive trend this short term sea level fall might show to be a wide spread phenomenon which could have caused an excess shrinking of the already reduced areal extent of marginal seas since deep waters in cretaceous oceans were primarily produced in shallow marginal seas the rate of formation of deep water might have been minimized at k t time minimum rates of formation of deep water might have curtailed the slow upward mixing of relatively cool and nutrient rich deeper water through which the thermocline weakened and surficial waters became depleted in nutrients consequently phytoplankton productivity rapidly diminished which in combination with a weakened thermal gradient pushed the highly depth stratified cretaceous planktonic foraminiferal fauna to extinction over a period of time of some 5 kyr guembelitria cretacea was the sole planktonic foraminifer which could accommodate to the low productivity conditions the oscillating rise in sea level at the beginning of the cenozoic reinforced the upward mixing of relatively cool and nutrient rich deeper water steepened the thermocline and replenished the photic layer with nutrients concomitant niche differentiation in the photic layer progressively stimulated morphological innovation amongst early cenozoic planktonic foraminifers the final return of normally sized planktonic foraminifers and of stable and well balanced dinoflagellate cyst associations at about 125 kyr after the k t boundary seems to indicate that primary productivity and niche differentiation in the photic layer begin to revert to optimum levels the earliest cenozoic planktonic foraminiferal species globoconusa minutula and parvularugoglobigerina fringa are thought to have developed from a benthic foraminiferal species rather than having a planktonic ancestry

siderophile interelement variations in the cretaceous tertiary boundary sediments from caravaca spain a detailed profile of the siderophiles fe co ni pd re os ir pt and au from the cretaceous tertiary boundary at caravaca spain indicates that the major fractionation mechanisms are local redistribution reworking and diagenetic mobilization large fractions of the fe co and re and perhaps a small amount of pd and au appear to be terrestrially derived but nearly all of the ni os ir and pt must be derived from an extraterrestrial source only the basal layers at caravaca and stevns klint denmark are representative of the initial fallout and integrated ir fluences across entire sections can be misleading these suggest a concentration oft 120mg g projectile in the fallout and a global fluence of chondritic matter oft 140mg cm 2

a preliminary comparison of the magsat data and aeromagnetic data in the continental u s a preliminary regional scale magnetic anomaly map derived from the magsat data over the continental u s shows reasonably good correlations when compared with the corresponding aeromagnetic map this conclusion is based upon the analysis of the fine attitude scalar magsat data of about a two month duration starting november 2 1979 and the analysis of the project magnet u s aeromagnetic data in the same region

magnesioferrite from the cretaceous tertiary boundary caravaca spain abstract magnesioferrite grading toward magnetite has been identified as a very small but meaningful constituent of the basal iron rich portion of the cretaceous tertiary k t boundary clay at the barranco del gredero section caravaca spain this spinel type phase and others of the spinel group found in k t boundary clays at many widely separated sites have been proposed as representing unaltered remnants of ejecta deposited from an earth girdling dust cloud formed from the impact of an asteroid or other large bolide at the end of the cretaceous period the magnesioferrite occurs as euhedral frequently skeletal micron sized octahedral crystals the magnesioferrite contains29 11 ppb ir which accounts for only part of the ir anomaly at this k t boundary layer 52 1 ppb ir major element analyses of the magnesioferrite show variable compositions some minor solid solution exists toward hercynite spinel and chromite magnesiochromite a trevorite nichromite nife 2 o 4 nicr 2 o 4 component is also present the analyses are very similar to those reported for sites at furlo and petriccio umbria italy on the basis of the morphology and general composition of the magnesioferrite grains rapid crystallization at high temperature is indicated most likely directly from a vapor phase and in an environment of moderate oxygen fugacity elemental similarity with metallic alloy injected into rocks beneath two known impact craters suggests that part of the magnesioferrite may be derived from the vaporized chondritic bolide itself or from the mantle there is no supporting evidence for its derivation from crustal target rocks

geochemical variability of the yucatan basement constraints from crystalline clasts in chicxulub impactites the 65 ma old chicxulub impact structure with a diameter of about 180 km is again in the focus of the geosciences because of the recently commenced drilling of the scientific well yaxcopoil 1 chicxulub is buried beneath thick post impact sediments yet samples of basement lithologies in the drill cores provide a unique insight into age and composition of the crust beneath yucatan this study presents major element sr and nd isotope data for chicxulub impact melt lithologies and clasts of basement lithologies in impact breccias from the pemex drill cores c 1 and y 6 as well as data for ejecta material from the k t boundaries at la lajilla mexico and furlo italy the impact melt lithologies have an andesitic composition with significantly varying contents of al ca and alkali elements their present day 87sr 86sr ratios cluster at about 0 7085 and 143nd 144nd ratios range from 0 5123 to 0 5125 compared to the melt lithologies that stayed inside the crater data for ejecta material show larger variations the 87sr 86sr ratios range from 0 7081 for chloritized spherules from la lajilla to 0 7151 for sanidine spherules from furlo the 143nd 144nd ratio is 0 5126 for la lajilla and 0 5120 for the furlo spherules in an tchur nd tur sr diagram the melt lithologies plot in a field delimited by cretaceous platform sediments various felsic lithic clasts and a newly found mafic fragment from a suevite granite gneiss and amphibolite have been identified among the fragments from crystalline basement gneiss their 87sr 86sr ratios range from 0 7084 to 0 7141 and their 143nd 144nd ratios range from 0 5121 to 0 5126 the tnddm model ages vary from 0 7 to 1 4 ga pointing to different source terranes for these rocks this leads us to believe that the geological evolution and the lithological composition of the yucatn basement is probably more complex than generally assumed and gondwanan as well as laurentian crust may be present in the yucatn basement

iridium sulfur isotopes and rare earth elements in the cretaceous tertiary boundary clay at stevns klint denmark microbial activity and redox controlled precipitation have been of major importance in the process of metal accumulation in the strongly ir enriched cretaceous tertiary k t boundary clay the fish clay at stevns klint in denmark two important findings support this view 1 1 kerogen recovered by leaching the fish clay in hcl and hf shows an ir concentration of 1100 ppb this represents about 50 of the ir present in the bulk sample fish clay strong organometallic complexes is the most probable carrier phase for this fraction of ir kerogen separated from the k t boundary clay at caravaca spain similarly exhibits enhanced ir concentrations 2 2 sulfur isotope analyses of metal rich pyrite spherules which occur in extreme abundance about 10 by weight in the basal fish clay give a 34s value of 32 this very low value shows that sulfide formation by anaerobic bacteria was intensive in the fish clay during early diagenesis since the pyrite spherules are major carriers of elements such as ni co as sb and zn microbial activity may have played an important role for concentrating these elements in the fish clay large amounts of rare earth elements have precipitated from sea water on fish scales analyses reveal that compared with sea water the fish clay is only about four times less enriched in sea water derived lanthanides than in ir this shows that a sea water origin is plausible for elements that are strongly enriched in the clay but whose origin cannot be accounted for by a lithogenic precursor

end cretaceous brachiopod extinctions in the chalk of denmark the results of a detailed study of the brachiopods of the most complete cretaceous tertiary boundary in denmark nye kl v show an extinction pattern for this marine invertebrate group compatible with that reported for pelagic foraminifera and coccoliths and with the impact scenario the extinction is abrupt coinciding with the maastrichtian danian boundary there is no warning in the form of decreasing density decreasing diversity or early extinction of specialized groups the basal few meters of the danian are almost devoid of brachiopods and a danian brachiopod fauna starts almost as abruptly as the maastrichtian fauna disappeared the new fauna is similar to the maastrichtian as regards density and diversity and at maximum six species are common to both stages the northwest european masstrichtian chalk is composed mainly of the remains of coccoliths and pelagic foraminifera the mass extinction of these groups led to a total cessation of chalk production the chalk is overlain by a thin clay bed deposited partly under anoxic conditions this combination of anoxia and clay deposition coupled with a cessation of productivity led to the extinction of specialized groups such as the chalk brachiopods the surviving species included forms that could survive in well aerated shallow marine waters on substrates other than chalk

compatibility of high altitude aeromagnetic and satellite altitude magnetic anomalies over canada results from equivalent source distributions derived jointly from high altitude average 4 km aeromagnetic and magsat derived average 400 km magnetic anomalies over canada indicate that long wavelength components 500 2500 km in these fields are extremely compatible with one another with a correlation coefficient of 0 95 the jointly estimated anomaly field at the earth s surface can be used as a long wavelength adjustment surface for regional near surface magnetic anomaly compilations and in assessing the performance of other downward continuation techniques because near surface anomalies are not available over all regions of the world we compare the jointly estimated anomaly field to the results of two different downward continuation techniques the evaluation of anomalies at the earth s surface from spherical harmonic coefficients derived from satellite altitude data and the use of downward continuation methods based on harmonic splines numerical and visual comparisons of these downward conti

comparison between the recent u s composite magnetic anomaly map and magsat anomaly data in order to test the validity of the magsat anomaly data the recently published u s geological survey society of exploration geophysicists composite magnetic anomaly map cmam of the conterminous united states was used as a standard and a comparison was made with these satellite data the cmam data set digitized by the phoenix corporation was resampled and continued upward to 320 km altitude this upward continued data set was compared with an independently derived magsat field at the same altitude our initial results were unsatisfactory the cmam data are dominated by a long wavelength surface increasing in amplitude by some 400 nt in a northwesterly direction across the unites states this broad trend surface was suppressed by removing a general two dimensional polynominal function having nine orders in longitude and five in latitude obtained through least squares fitting the resulting residual upward continued field was compared with the magsat data by means of correlation techniques correspondence was found between these data sets for many large amplitude anomalies for example off the east coast in kentucky oklahoma mississippi embayment georgia arizona and nebraska dissimilarities occur over the rio grande rift minnesota and wyoming the negative anomaly over the rio grande rift and the positive anomaly over the midcontinent geophysical anomaly are more pronounced in the cmam data than in the magsat results these latter discrepancies are attributed to the method used to remove external field variations from the satellite data which suppresses north south trending structures i e those lying in the orbit direction cmam base level problems localized over minnesota and wyoming are believed to cause observed discordance between satellite and aeromagnetic data in these regions this study confirms with ground truth the validity of these satellite measurements and demonstrates that satellite derived anomaly maps can be used to identify base level problems when compiling local or regional aeromagnetic surveys

have distal impact ejecta changed through geologic time sand size spherules of former silicate melt have been identified in 10 thin early archean to paleoproterozoic layers that appear to be reworked ejecta from large impacts melt spherules are common in phanerozoic layers of distal impact ejecta but the aggregate thickness of spherules in early precambrian layers is generally 10 to 100 times greater and they display textures rarely seen in phanerozoic spherules these contrasts suggest secular variations in the nature of both impactors and target materials such as a progressive decrease in the size of impactors and a progressive increase in the fraction of earth s surface covered by continental crust

the cretaceous tertiary boundary event in the north pacific planktonic foraminiferal results from deep sea drilling project site 577 shatsky rise a detailed micropaleontologic analysis of sediments from deep sea drilling project site 577 from the shatsky rise north pacific was undertaken to describe extinction and radiation patterns of plank tonic foraminifera in a continuous carbonate sequence spanning the cretaceous tertiary k t boundary a 15 m section containing the boundary was closely sampled and contained planktonic foraminiferal assemblages characteristic of the late maastrichtian abathomphalus mayaroensis zone through the danian globorotalia uncinata zone the cretaceous tertiary boundary was placed at the abrupt disappearance of almost all maastrichtian planktonic foraminifera coincident with these extinctions was the presence of a large number of microtektitelike spherules the most important faunal change in the late maastrichtian was an abrupt increase in the relative abundance of the high latitude species hedbergella monmouthensis together with the appearance of diminutive populations of guembelitria cretacea and globigerina eugubina approximately 20 000 years before the boundary these changes are interpreted as indicating a cooling of surface water conditions globigerina eugubina and g cretacea were the only planktonic foraminiferal species to survive the cretaceous tertiary boundary event at this locality the o 18 and c 13 values of g eugubina from below the k t boundary are nearly the same as those from above the boundary but in both cases the o 18 values indicate relatively cooler paleotemperatures than those from other coexisting planktonic foraminiferal species in addition the c 13 values of g eugubina are lighter than those of the other planktonic foraminifera such a relationship might suggest that g eugubina lived in cooler shallow intermediate water masses advected from higher latitudes during a latest maastrichtian cooling episode better preservation of planktonic foraminifera a progressive change in the c 13 of benthic foraminifera and a brief positive excursion in c 13 of the carbonate fine fraction and of planktonic foraminifera within 20 to 30 cm below the k t boundary all suggest important paleoceanographic changes that preceded the extinction event a large population of aberrant g eugubina and eoglobigerina was observed in the earliest danian these forms are characterized by the development of secondary apertures bullae and abnormal final chambers these abnormal morphotypes are considered to be ecophenotypic variants reflecting ecologic stress or instability in the earliest cenozoic marine environment

comparative mineralogical and geochemical clay sedimentation in the betic cordilleras and basque cantabrian basin areas at the cretaceous tertiary boundary abstract the mineralogy and geochemistry of the bulk rock and clay fraction of sediments deposited at the cretaceous tertiary transition and in the ktb layer have been investigated by various methods in two sections from the betic cordilleras agost caravaca and five sections from the basque cantabrian basin monte urko sopelana zumaya hendaye biarritz active terrigenous supply prevailed in both regions uring the latest maastrichtian and earliest paleocene very different clay associations occur in the regions smectite being dominant to the south and illite to the north the betic cordilleras series mainly reflects climate and weathering conditions as well as volcanic activity with the ktb layer displaying a specific source and diagenesis control expressed as increased amounts of smectite the sequences cropping out in the basque cantabrian basin mainly reflect tectonic activity and burial diagenesis the effects of which differ according to the palaeogeographical location the clay sedimentation patterns in the north and south iberian sections across the cretaceous tertiary passage are controlled overwhelmingly by regional or local geodynamic and diagenetic phenomena thus any global namely extraterrestrial influence cannot be identified

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

age and provenance of the target materials for tektites and possible impactites as inferred from sm nd and rb sr systematics sm nd and rb sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects tektites have large negative e nd 0 values and are uniform within each tektite group while the e sr 0 are large positive values and show considerable variation within each group chemical trace element and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust each tektite group is characterized by a uniform nd model age t chur nd interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites 1 1 15 ae for australasian tektites 2 1 91 ae for ivory coast tektites 3 0 9 ae for moldavites 4 0 65 ae for north american tektites and 5 0 9 ae for high si irghizites sr model ages t ur sr are variable within each group reflecting rb sr fractionation and in the favorable limit of very high rb sr ratios approach the time of sedimentation of the parent material which melted to form the tektites australasian tektites are derived from 0 25 ae sediments moldavites from 0 0 ae sediments ivory coast tektites from 0 95 ae sediments possible parent sediments of other tektite groups have poorly constrained ages our data on moldavites and ivory coast tektites are consistent with derivation from the ries and bosumtwi craters respectively irghizites are isotopically distinct from australasian tektites and are probably not related sanidine spherules from a cretaceous tertiary boundary clay have initial e nd 2 e sr 5 and are not derived from old continental crust or meteoritic feldspar they may represent a mixture of basaltic oceanic crust and sediments implying an oceanic impact these isotopic results are also consistent with a volcanic origin for the spherules

nondestructive spectroscopic and petrochemical investigations of paleoarchean spherule layers from the icdp drill core barb5 barberton mountain land south africa a paleoarchean impact spherule bearing interval of the 763 m long international continental scientific drilling program icdp drill core barb5 from the lower mapepe formation of the fig tree group barberton mountain land south africa was investigated using nondestructive analytical techniques the results of visual observation infrared ir spectroscopic imaging and micro x ray fluorescence xrf of drill cores are presented petrographic and sedimentary features as well as major and trace element compositions of lithologies from the micrometer to kilometer scale assisted in the localization and characterization of eight spherule bearing intervals between 512 6 and 510 5 m depth the spherule layers occur in a strongly deformed section between 517 and 503 m and the rocks in the core above and below are clearly less disturbed the xrf element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in 1 sorting of two types of spherules and 2 occurrence of primary minerals ni cr spinel and zircon we favor a single impact scenario followed by postimpact reworking and subsequent alteration the spherule layers are al2o3 rich and can be distinguished from the al2o3 poor marine sediments by distinct al oh absorption features in the short wave infrared swir region of the electromagnetic spectrum infrared images can cover tens to hundreds of square meters of lithologies and thus may be used to search for al oh rich spherule layers in al2o3 poor sediments such as eoarchean metasediments where the textural characteristics of the spherule layers are obscured by metamorphism

magnetic characterization of cretaceous tertiary boundary sediments rock magnetic properties across several k t boundary sections have been investigated to reveal any possible magnetic signature associated with the remains of the impact event at the end of the cretaceous studied sections locations vary in distance to the chicxulub structure from distal agost and caravaca spain through closer odp hole 1049a blake nose north atlantic to proximal el mimbral and la lajilla mexico a clear magnetic signature is associated with the fireball layer in the most distal sections consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization sirm and a decrease in remanence coercivity magnetic properties in these sections point to a distinctive ferrimagnetic phase probably corresponding to the reported mg and ni rich highly oxidized spinels of meteoritic origin at closer and proximal sections magnetic properties are different although there is an increase in susceptibility and sirm associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections and with a similar limonitic layer on top of the spherule bed that defines the boundary at blake nose the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine grained goethite based on previous geochemical studies at blake nose and new geochemical and pge abundance measurements performed in this work at el mimbral this goethite rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of fe there is not enough evidence to assert that this fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material magnetic geochemical and iridium data reject it as a primary meteoritic phase

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

downward continuation of magsat lithospheric anomalies to the earth s surface summary the assumed lithospheric component of magsat data is downward continued to the earth s surface over continental sized areas using the depleted basis method with a regularizing constraint parker shure 1982 to make the problem numerically tractable and stable respectively the depleted basis is arranged to have a high density of points over the area of interest lower elsewhere all three components at satellite altitudes are used in the inversion for three orthogonal components at the earth s surface edge effects are seen if the depleted basis changes rapidly from a high to low spatial density which are eliminated by inserting a region of intermediate density basis points the downward continued field fits the data to within their probable uncertainties regularizing the solution produces a much smoother solution for very little increase in misfit some of the anomalies in the downward continued field over africa correlate with known tectonic features although the relationship between magnetic highs or lows and basins or cratons is not clear cut this magsat derived downward continued field can be used to control the longer wavelength component of the crustal magnetic field that is not defined by aeromagnetic surveys the method is illustrated on an aeromagnetic compilation of about 30 surveys from south africa however there is a mismatch in amplitudes between the aeromagnetic data and downward continued magsat data

altered spherules of impact melt and associated relic glass from the k t boundary sediments in haiti partially to wholly altered glass spherules produced by impact induced shock melting have been found in the k t boundary sediments of haiti which also contain grains of shocked quartz the relic glass has an approximately dacitic composition and although grossly similar in composition to most previously described tektite glasses it is slightly enriched in ca and slightly depleted in si suggesting the haitian glass was produced either from a target with a greater fraction of carbonate and anhydrite lithologies and fewer silicate units than the targets from which most other tektites were produced and or from one with a significant mafic component the composition of the glass can best be reconciled with a continental margin terrane consistent with studies of shocked mineral phases reported elsewhere the thickness of the deposit in which the impact spherules occur indicates the source of the ejecta was in the proto caribbean region

chemical profiles in k t boundary section of meghalaya india cometary asteroidal or volcanic abstract the cretaceous tertiary boundary ktb exposed as a 1 5 cm thick limonitic layer in the um sohryngkew river basin in meghalaya is marked by enhanced concentrations of ir co ni os fe zn sb by factors of 4 to 1200 and ree by factors of 1 7 to 5 compared to the cretaceous shales ir concentration is generally 10 in 2 m thick broad band around the ktb abruptly reaching a peak concentration of 12 1 ng g 1 within the limonitic layer the os ir ratio at the ktb is 0 37 but in the adjacent shales it is 1 8 suggesting that the sources of these elements in the peak and the broad band may be different cretaceous planktonic foraminifera and dynocysts start disappearing before the ir rich limonitic layer was deposited but several diminutive cretaceous foraminifera survive after this layer the observations indicate that the extinctions are confined to a short span of time but are probably not as abrupt as anticipated by the asteroidal impact hypothesis the chemical data are discussed in terms of cometary asteroidal and volcanic hypotheses the profiles of ir and os ir can be understood in the frame work of a model of cometary impact in which cometary debris falls onto the earth preceding and succeeding the fall of the cometary nucleus

an assessment of long wavelength magnetic anomalies over canada the reliability of the long wavelength portion 300 km of the magnetic field over canada as represented by the national aeromagnetic anomaly database compiled by the geological survey of canada gsc is assessed by comparison with two independent data sets a high altitude country wide survey carried out by the former earth physics branch epb and data from the magsat and pogo satellite missions the different altitudes at which each data set was measured 300 m 4 km and 400 km and their different resolution and time span of observations allow a determination of the integrity of selected wavelength bands in each data set the upward continued epb and magsat pogo fields compare well for wavelengths of 300 2500 km the gsc data show significant differences to the former indicating that the levelling and merging of several hundred individual surveys has degraded the longer wavelength components of the magnetic field replacing the gsc wavelength components 300 km with those from the epb field

long wavelength aeromagnetic anomaly map of the conterminous united states total intensity magnetic anomaly unflltered and low pass filtered profile and contour maps of the conterminous united states have been prepared from data provided by the u s naval oceanographic office9s project magnet survey the maps are useful for regional geological investigations because the anomalies particularly on the filtered maps that emphasize long wavelength anomalies can be correlated with known major geologic features the most intense positive long wavelength anomaly occurs over precambrian mafic basement rocks in eastern tennessee and kentucky other examples of prominent positive anomalies occur over the great valley of california and the colorado plateau and along the appalachian fold belt in contrast prominent negative long wavelength magnetic anomalies occur over the cascade mountains mississippi embayment and southern rocky mountains and marginally to the positive anomaly related to the mid continent rift

experimental evidence that an asteroid impact led to the extinction of many species 65 million years ago an overview is presented of the theory that an asteroid hit the earth 65 million years ago wiping out large numbers of land and marine species this theory predicts that the clay layer found at the boundary of the cretaceous and tertiary periods would be substantially different chemically from the clays in the limestone above and below the boundary layer it also predicts the iridium enhancement seen in this boundary layer would be found worldwide both predictions have been found to be true a third prediction was made but not fulfilled that the five major extinctions which occurred in the last 570 million years were caused by an asteroid impact the transport mechanism of the dust created by the impact and four mechanisms for the extinction of species are discussed criticisms of the theory are also discussed

initial scalar magnetic anomaly map from magsat a scalar magnetic anomaly map between 50 geographic latitude was derived from magsat data acquired during the november 1979 to june 1980 mission lifetime separation of anomaly fields from core and external fields was accomplished utilizing techniques developed for pogo data mayhew 1979 langel et al 1980 comparison with a similar map from pogo data shows that the basic anomaly patterns are essentially reproducible except in a few regions such as the atlantic and pacific at latitudes south of 15 increased resolution due to a lower measurement altitude is evident in the magsat map

chemostratigraphy across the cretaceous tertiary boundary and a critical assessment of the iridium anomaly the elevated concentration of iridium one of the platinum group elements pge at the cretaceous tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end cretaceous mass extinctions new chemostratigraphic data for cross boundary sections from both hemispheres are not easily explained in terms of such an impact event for example the observation that the pge patterns show marked differences between the hemispheres the new constraints indicate that models of mantle derived pge should be seriously considered and that pge anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earth s history

geophysical characterization of the chicxulub impact crater 1 geophysical data indicate that the 65 5 million years ago chicxulub impact structure is a multi ring basin with three sets of semicontinuous arcuate ring faults and a topographic peak ring pr slump blocks define a terrace zone which steps down from the inner rim into the annular trough fault blocks underlie the pr which exhibits variable relief due to target asymmetries the central structural uplift is 10 km and the moho is displaced by 1 2 km the working hypothesis for the formation of chicxulub is a 50 km radius transient cavity lined with melt and impact breccia formed within 10 s of the impact and within minutes weakened rebounding crust rose kilometers above the surface the transient crater rim underwent localized deformation and collapsed into large slump blocks resulting in a inner rim at 70 85 km radius and outer ring faults at 70 130 km radius the overheightened structural uplift collapsed outward buried the inner slump blocks and formed the pr most of the impact melt was ultimately emplaced as a coherent 3 km thick melt sheet within the central basin that shallows within the inner regions of the pr smaller pockets of melt flowed into the annular trough subsequently slope collapse ejecta ground surge and tsunami waves infilled the annular trough and annular basin with sediments up to 3 km and 900 m thick respectively testing this working hypothesis requires direct observation of the impactites within and adjacent to the pr and central basin

emplacement of cretaceous tertiary boundary shocked quartz from chicxulub crater observations on shocked quartz in cretaceous tertiary k t boundary sediments compellingly tied to chicxulub crater raise three problems first in north america shocked quartz occurs above the main k t ejecta layer second shocked quartz is more abundant west than east of chicxulub third shocked quartz reached distances requiring initial velocities up to 8 kilometers per second corresponding to shock pressures that would produce melt not the moderate pressure shock lamellae observed shock devolatilization and the expansion of carbon dioxide and water from impacted wet carbonate producing a warm accelerating fireball after the initial hot fireball of silicate vapor may explain all three problems

nickel rich magnesiowiistite in cretaceous tertiary boundary spherules crystallized from ultramafic refractory silicate liquids we have found trace inclusions of ni rich magnesiowastite within grains of magnesioferrite spinel recovered from cretaceous tertiary boundary sediments from dsdp site 596 south pacific 23 51 20 s 169 39 27 w and dsdp site 577 north pacific 3 6 51 n 157 43 40 e measured compositions of these inclusions range from mg 85ni 74fe 17 o to mg 74ni 09fe 1 7 o coexisting magnesioferrite and magnesiowastite can only crystallize from ultramafic refractory mg rich liquids with mgsi 2 atom ratio such liquid compositions cannot form as a result of fractional crystallization and are unknown to occur as a result of terrestrial igneous processes or meteoroid ablation we infer that these minerals crystallized from liquid droplets that equilibrated with silicate vapor at high temperatures probably 2300 c resulting in fractionation of volatile sio2 from more refractory mgo the most plausible source of this high temperature vapor is in the fireball of the major impact event that terminated the cretaceous

verification of the crustal component in satellite magnetic data to investigate the utility of satellites for mapping crustal magnetic anomalies pogo polar orbiting geophysical observatory and preliminary magsat magnetometer data are compared with scalar aeromagnetic data obtained by the u s naval oceanographic office noo vector magnetic survey of the conterminous u s a pogo and available magsat data demonstrate remarkable consistency over the study region the noo aeromagnetic data are low pass filtered for wavelengths larger than about 4 and spherically upward continued to 450 km elevation by equivalent point source inversion for direct comparison with pogo satellite magnetometer observations the upward continued noo data show that most of the energy is in the long wavelength 1000 3000 km anomalies removal of these wavelengths by suitable filtering reveals a residual anomaly field that corresponds well with the satellite anomalies thus demonstrating that the satellite data are useful for mapping crustal magnetic anomalies a number of correlations between the noo pogo and preliminary magsat data are evident at satellite elevations including a prominent transcontinental magnetic high which extends from the anadarko basin of the eastern texas panhandle to the cincinnati arch the transcontinental magnetic high is breached by negative anomalies located over the rio grande rift and mississippi river aulacogen

the end of the cretaceous sharp boundary or gradual transition evidence indicates that the cretaceous tertiary boundary is very sharp and within the limits of resolution it is apparently synchronous at the various boundary localities arguments to the contrary particularly those of officer and darke are shown to invalid

fe rich and k rich mafic spherules from slumped and channelized chicxulub ejecta deposits in the northern la sierrita area ne mexico spherule deposits commonly interpreted as ejecta from the chicxulub impact at yucatan mexico are present in many k t cretaceous tertiary sections geological mapping of the northern la sierrita area ne mexico revealed the presence of 1 multiple spherule deposits embedded in late maastrichtian marls which are folded or disaggregated breccia like they are up to 6 m thick locally present in two outcrop areas and show limited lateral continuity these deposits consist of mm cm sized spherical to drop shaped vesiculated spherules angular to filamentous ejecta fragments and abundant carbonate they are interpreted as primary ejecta fallout deposits that have been affected by subsequent local slumps slides liquefaction and debris flows welded components suggest an initial ground surge like ejecta dispersion mode 2 a spherule deposit 10 60 cm thick that constitutes the base of a channelized sand siltstone deposit at or close to the k t boundary and is characterized by wide lateral continuity it is of similar petrologic composition to deposit 1 though slightly enriched in terrigeneous detritus thus reflecting influx from proximal shelf areas it is interpreted to result from debris flows and turbidite currents though no size sorting and abrasion of ejecta has been observed petrological mineralogical and geochemical criteria suggest that ejecta components from both types of spherule deposits are similar and originated from the chicxulub impact with multiple deposits produced by subsequent remolding reworking and redeposition spherules and fragments have an fe 25 30 wt al mg rich and si poor 25 wt sio2 composition and are altered to chlorite and iron oxides though rare k rich mafic glass 50 wt sio2 5 8 wt k is also present they contain ti fe k rich schlieren fe mg rich globules and rare m sized metallic and sulfidic ni co rich inclusions carbonate as clasts and within spherules and fragments shows textures indicative of quenching and or liquid immiscibility although potential ejecta fractionation and alteration make accurate evaluation difficult this composition suggests an ejecta origin mainly from mafic lithologies and carbonaceous sediments in addition to a contribution from intermediate felsic rocks and the possibility of meteoritic contamination

siderophile rich magnetic spheroids from the cretaceous tertiary boundary in umbria italy the magnetic fraction of some cretaceous tertiary k t boundary sediments sampled at two locations in italy contains a spheroidal component which has retained some relict mineral textures and possibly mineral chemistry from the k t event the spheroids contain small crystals 1 50 m of magnetite in a highly altered groundmass these crystals are commonly skeletal indicating formation by rapid growth from a high temperature liquid variable concentrations of ni mg and al differing by at least a factor of 5 in the magnetite may also reflect a relict chemistry these spheroids contain higher concentrations of ir and probably other siderophiles than the bulk sediment and may have been an important carrier for these elements at 65 myr this discovery supports evidence for a major accretionary event at the end of the cretaceous

mineralogy and diagenesis of 3 24ga meteorite impact spherules abstract spherules in the s3 bed of the barberton greenstone belt south africa are distal fallout formed within an impact plume after a large impact event at 3 24 ga since that time diagenesis and lower greenschist grade metamorphism of the spherules has changed the mineralogy though shape and texture are largely preserved alteration of the s3 bed has resulted in spherules composed of quartz phyllosilicates ti and fe oxides and some ni rich chromites initially glassy spherules were palagonitized and silica cementation of the spherules began during low temperature interaction with seawater further alteration by si and k rich fluids resulted in a mineralogical assemblage of quartz feldspar and clays crystalline minerals were replaced by dissolution precipitation processes preserving relict textures further silica cementation resulted in complete lithification of the bed most of this alteration occurred at the seafloor and during shallow burial with continued burial amorphous silica recrystallized to microcrystalline quartz later recrystallization of clays to micas occurred during regional metamorphism at peak temperatures of 300 320 c late stage shearing and mineralization preferentially affected the northern region of the belt samples from different sections record highly variable local conditions water depth the amount of pre depositional transport location within the belt and proximity to igneous dikes all affect the diagenesis of the s3 spherules silica and barite concentrations are lower and carbonate concentrations are higher in the deep water depositional environments element mobility during diagenetic and lower greenschist grade metamorphism can be inferred based on studies of multiple sections throughout the bgb the most immobile elements are al zr ti sc and the high field strength elements and present element ratios can be used to infer original composition of the spherules the large ion lithophile elements are highly mobile as are the light rare earth elements rees which are particularly susceptible to mobilization during carbonate diagenesis and phosphate authigenesis of the rees ce and eu show the largest variability suggesting significant mobilization during diagenesis and low grade metamorphism consistent cr ir ratios particularly in the high concentrations suggest limited mobility and segregation of platinum group elements into ni rich chromite bearing spherules sulfide mineralization has not affected the ir concentration in the s3 layer

the geochemistry and mineralogy of the cretaceous tertiary boundary at agost southeast spain abstract this paper gives the mineralogical and geochemical results of research made on the k t boundary in the agost stratigraphic sequence belonging to the intermediate units of the betic cordilleras spain in this sequence the abathomphalus mayaroensis globigerina cretacea and parvularugoglobigerina eugubina zones are present and the k t level 2 mm is very well defined the k t boundary is characterized by a decrease in carbonate content an increase in the proportion of smectites and the presence of spherules of fe oxides and k feldspar we have detected a considerable enrichment in platinum group elements ir 24 4 ppb as sb zn fe ti cr ni v co sr among other elements and an depletion in ca and rare earth elements the fe oxide spherules are enriched in most of these trace elements we suggest that these spherules were formed as a product of the alteration of pyrite originating in an oxygen depleted marine environment in which abundant sulphides and organic complexes would have formed and scavenged the metals 87 sr 86 sr ratio for the k t boundary is similar to the values reported for oceanic waters of the same age the pge values normalized to c1 chondrites are very different to those corresponding to meteoritic composition post depositional processes could have brought about a differential enrichment of these elements although meteorite impact cannot be discounted we suggest that volcanic phenomena and early diagenetic processes were related to the chemical anomalies of the k t boundary

geochemical markers of the cretaceous tertiary boundary event at brazos river texas usa the cretaceous tertiary boundary sites around the gulf of mexico are close to the chixculub impact site and are relatively well studied yet much remains to be learned about them therefore the first integrated study of carbon soot and fullerenes in a cretaceous tertiary boundary section was undertaken at the brazos 1 site on the brazos river in texas at the most complete section of end cretaceous and basal paleocene deposits on the texas segment of the gulf coast area up to 409 ppm of native sulfur so were serendipitously discovered in a spherule bearing unit of the br 1 section and lesser amounts were found in spherule bearing units of nearby brazos riverbed sections in a section on darting minnow creek the isotopic composition 33s 12 97 34s 24 89 and 36s 46 4 implies that this so cannot have come to earth by the impactor that formed the chicxulub crater but most likely was produced by sulfate reducing bacteria during a local transient bacterial bloom for which the sulfate was provided by caso4 bearing spherules carbon and soot were determined in twelve samples representing all units of br 1 from the cretaceous corsicana kemp formation to the tertiary kincaid formation a significant increase of c and soot contents up to 2 2 104 ppm and 1 4 104 ppm respectively occurs in a sandy bed at the top of the kt complex fullerenes were determined in fifty four samples from all units of the same br 1 section less than 1 ppb was reliably detected at the same sandy bed where the strongest ir anomaly of the section is known to occur it is suggested here that the chicxulub impact 65 ma ago ignited local wildfires that produced c soot and fullerene which settled onshore or near shore whence they were transported to the brazos site by coastal flooding and associated sediment laden water plumes moving offshore

mass extinctions the view of a sceptic the concept of periodic mass extinctions by extraterrestrial impacts has contributed very strongly to the current revival of interest in the problem of mass extinctions in this context several radical claims have been made asserting that the newly emerging picture of mass extinctions as a separate class of biological phenomena caused by a separate class of processes demands a far reaching revision of the the neodarwinian paradigm of evolution and implies a vindication of the cuvierian perspective on earth9s history the validity of these claims however is questionable at all levels from the interpretation of particular events to the concept of mass extinctions as a special class of phenomena to their proposed causation by impacts and to their alleged historical pedigree and implications for current evolutionary theory mass extinctions may actually be clusters of more or less accidentally aggregated in time but causally distinct extinction episodes

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

magnetic properties of k t and e o microspherules origin by combustion abstract magnetic spherules and spheroids are found in association with ir anomalies at the cretaceous tertiary k t boundary and near to the eocene oligocene e o boundary detailed magnetic analyses have been performed on these particles and on bulk k t boundary clay from marine and continental sections coercivity spectra and blocking and curie temperatures of the spheroids and k t boundary clays have been determined and natural remanence and susceptibility measured for the bulk clay and associated strata marine k t boundary clays are strongly magnetic with remanent coercivities similar to those of the spheroids in contrast the continental clays are diamagnetic except for the thin overlying impact layer which contains abundant paramagnetic jarosite along with shocked quartz and feldspar the morphologies mineralogies and magnetic properties of the k t magnetic spheroids are closely analogous to magnetic fly ash spherules this similarity may be related to the presence of carbonaceous soot particles in some marine k t boundary clays and the recent identification of plerospheres and cenospheres in continental boundary clay in contrast tektites and microtektites differ from the k t magnetic spherules in a number of fundamental ways the late eocene particles are magnetically distinct from the k t spheroids both in their coercivity and thermal spectra inaa results on upper cretaceous combustible shales indicate that the anomalous abundances of certain chalcophile and lithophile elements at the k t boundary may be explained in terms of the natural combustion of bituminous sediments

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

microspherules in devonian sediments origins geological significance and contamination problems we have studied various microspherules 0 5 mm across that were found in devonian sediments from canada and china during our micropaleontological studies e g conodont picking these spherules are similar in appearance but they are quite different in origin a combined scanning electron microscope energy dispersive x ray x ray microdiffraction and electron microprobe analysis enables us to determine their probable origins phosphatic conodont pearls of biological origin silicate glass microtektites of terrestrial meteorite impact origin and black magnetic iron spherules of probable extraterrestrial origin we also observed some contaminant spherules glue spatters and steel slag of artificial origins studies of natural spherules in sediments can provide important geological information that may be otherwise undetectable

minimal crustal magnetizations from satellite data abstract the minimum amplitude crustal magnetization needed to explain magsat vector anomaly data is found using harmonic splines as basis functions shure et al 1982 phys earth planet inter 28 215 229 and choosing a subset of the original basis parker and shure 1982 geophys res lett 9 812 815 to represent magnetization we suggest the optimum model of this type is obtained by associating representers only with north and vertically downwards data points given the larger uncertainties associated with east component magsat data following parker et al 1987 rev geophys 25 17 40 we find a model minimizing the r m s volume integral of magnetization over the magnetized lithosphere the numerical effort involved in producing a regional or continental scale model with given horizontal spatial detail with our method is less than that for an equivalent source dipole model when the magnetization is not assumed induced approximately equal to that for an induced magnetization equivalent dipole model and considerably less than that for a spherical harmonic apparent susceptibility model the method is demonstrated by producing a solution for two regions the contiguous usa for comparison with previously published models and africa and its continental margins magnetization models fit the data slightly worse than those of the downward continued anomaly field at the earth s surface comparison of our component of magnetization for the usa in the direction of the core field with the induced magnetization equivalent dipole model of mayhew 1984 earth planet sci lett 71 290 296 is very good as expected for magnetization either induced by or frozen remanently in an approximately dipole field either parallel or antiparallel to the current rotation axis the east component of magnetization is much smaller than the north and vertically downwards components we make no assumptions about the relative contributions of remanent and induced magnetization and the resulting models have a substantial remanent component however our models suffer from the severe non uniqueness common to all magnetization models whereby magnetization in a uniform susceptibility and thickness shell concentric with the earth s centre owing to any internal magnetic field that is then switched off generates no external magnetic field

limitations on digital filtering of the dnag magnetic data set for the conterminous u s the magnetic data set compiled for the decade of north american geology dnag project presents an important digital data base that can be used to examine the north american crust the data represent a patchwork from many individual airborne and marine magnetic surveys however the portion of data for the conterminous u s has problems that limit the resolution and use of the data now that the data are available in digital form it is important to describe the data limitations more specifically than before the primary problem is caused by datum shifts between individual survey boundaries in the western u s the dnag data are generally shifted less than 100 nt in the eastern u s the dnag data may be shifted by as much as 300 nt and contain regionally shifted areas with wavelengths on the order of 800 to 1400 km the worst case is the artificial low centered over kentucky and tennessee produced by a series of datum shifts a second significant problem is lack of anomaly resolution that arises primar

end cretaceous mass extinction event argument for terrestrial causation the end cretaceous mass extinctions were not a geologically instantaneous event and were selective in character these features are incompatible with the original alvarez hypothesis of their being caused by a single asteroid impact that produced a world embracing dust cloud with devastating environmental consequences by analysis of physical and chemical evidence from the stratigraphic record it is shown that a modified extraterrestrial model in which stepwise extinctions resulted from encounter with a comet shower is less plausible than one intrinsic to the earth involving significant disturbance in the mantle

new types of spherules from antarctica meteoritic impact origin spherules collected form antarctic ice have been studied by using instrumental neutron activation analysis energy dispersive x ray spectrometry and x ray diffration photography peculiar spherules ca ti rich perovskite type cts and fe cr ni rich type fcn were found in the mizuho ice core at depths of 32 to 33 5 m both types have rare earth element ree abundances i e significant relative enrichment of light ree over heavy ree which is commonly seen for terrestrial perovskite however with anomalous enrichment of nd and sm in the allan hills bare ice only a chondritic type without depletion of au and s cas was recognized size distributions and influx rates of spherules for these ices and the mizuho surface snow indicate that antarctic spherules are composed of steady falling and occasional populations all the results combine to suggest that cts and fcn may be droplets strewn by the impact of a huge meteorite and cas must be debris from one of the chondrites that fell on the source region of the allan hills bare ice and survive terrestrial alterations

rb sr sm nd k ca o and h isotopic study of cretaceous tertiary boundary sediments caravaca spain evidence for an oceanic impact site isotopic ratios and trace element abundances were measured on samples of ir enriched clay at the cretaceous tertiary boundary and in carbonate and marl from 5 cm below and 3 cm above the boundary samples were leached with acetic acid to remove carbonate and with hydrochloric acid leachates and residues were measured the sr nd o and h isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials the data indicate that most of the clay material was derived from a terrestrial source with relatively low87sr 86sr and high143nd 144nd ratios the 18o data suggest that the detritus has been modified by submarine weathering k ca and rb sr systematics as well as o isotope ratios of k feldspar spherules within the boundary clay suggest that they are predominantly authigenic and may have formed after the time of deposition however sm nd and rb sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus the isotopic evidence for foreign terrestrial detritus in the boundary clay the low rare earth element concentrations and high ni concentration support the hypothesis of a terminal cretaceous asteroidal impact that produced a global layer of fallout the data are most easily explained if the impact site was on oceanic crust rather than continental crust and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere 3 km this would probably require a single large impactor

a multi isotopic and trace element investigation of the cretaceous tertiary boundary layer at stevns klint denmark inferences for the origin and nature of siderophile and lithophile element geochemical anomalies abstract os sr nd and pb isotope data were collected from a profile across the cretaceous tertiary k t boundary layer at stevns klint denmark nd t 65 ma values from within the boundary layer fish clay are lower by 1 unit than those of the underlying maastrichtian limestone and the overlying danian chalk sequences systematic profile upward changes of pb sr and os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the fish clay while os nd and pb isotopes indicate the admixing of less radiogenic components to the fish clay sr isotopes show elevated radiogenic values in the boundary layer relative to the carbonate sequences beneath and above it the sudden change in lithophile e g sr pb and nd isotope compositions at the base of the fish clay and profile upward trends of 87sr 86sr and 206pb 204pb ratios towards those of the overlying danian chalk are interpreted to reflect recovery from enhanced acid rain induced continental local weathering input to the seawater however a continental crustal source is invalid for the siderophile element os in the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements pges and certain moderately siderophile elements cr ni co v in k t boundary sediments including stevns klint shukolyukov and lugmair science 282 1998 927 929 and supported by the finding of projectile debris bauluz et al earth planet sci lett 182 2000 127 136 and the occurrence of abundant ni rich spinel at many k t sites robin et al nature 363 1993 615 617 kyte nature 396 1998 237 239 we favor to explain the sudden drop of 187os 188os ratios from 0 210 to 0 160 at the k t boundary to derive from global fall out of extraterrestrial matter the present 186os 188os ratio of 0 119836 0 000004 measured in the basal layer of the fish clay is within the uncertainty a chondritic value we therefore exclude the possibility of a major contribution of pges to the sediment from iron meteorites chondrite normalized ru ir n ratios of 0 95 0 14 and os ir n ratios of 0 93 0 14 in the fish clay cannot distinguish between abundance ratios of different types of chondrites and strongly sub chondritic pt ir n ratios of 0 62 0 09 2 suggest differential pge remobilization through the sedimentary column and consequently the alteration of inter element ratios pges and the moderately siderophile elements cr ni v and co form an elemental association with systematically upward decreasing concentrations in the fish clay low co ni ratios of 0 12 in the fish clay relative to values of 0 35 in the over and underlying carbonate sequences support mixing of meteorite derived co ni 0 05 and terrestrial upper mantle crustal co ni 0 3 sources while lithophile element isotope data indicate an increased continental crustal input to the fish clay at the k t transition the uncertainty with respect to possible post depositional alteration of abundance patterns of siderophile and moderately siderophile elements though not affecting the chondritic isotopic composition of os does not allow confirmation of indications from chromium isotopes for a carbonaceous cv type meteorite as the preferred k t impactor type by shukolyukov and lugmair science 282 1998 927 929

an equivalent layer magnetization model for the united states derived from magsat data long wavelength anomalies in the total magnetic field measured by magsat over the united states and adjacent areas are inverted to an equivalent layer crustal magnetization distribution the model is based on an equal area dipole grid at the earth s surface model resolution defined as the closest dipole spacing giving a solution having physical significance is about 220 km for magsat data in the elevation range 300 500 km the magnetization contours correlate well with large scale tectonic provinces

limitations of the long wavelength components of the north american magnetic anomaly map the magnetic anomaly map of north america and its related data set provide the opportunity not only to view the obvious short wavelength anomalies 2600 km are strongly affected by core field components that have not been completely removed from the north american data set furthermore the piecewise matching of the magnetic anomalies of adjacent survey areas in the map compilation has contaminated the intermediate wavelength anomalies 300 2600 km

coexisting altered glass and fe ni oxides at the cretaceous tertiary boundary stevns klint denmark direct evidence of meteorite impact abstract the cretaceous tertiary k t boundary at stevns klint denmark is noteworthy for its large ir anomaly that is taken as evidence of extraterrestrial components but the origin of the smectite in this marl has been variously interpreted to have a detrital meteorite impact or volcanic origin we have carried out scanning electron microscopy and transmission electron microscopy tem analytical electron microscopy observations of the impact and contiguous layers within the k t marl at stevns klint tem images show abundant smectite much of which occurs with layers curving around and grading into cores of nanometer scale glass shards the smectite composition is unusual in having both significant octahedral al and mg the glass and smectite major element compositions are similar and unique relative to glasses of terrestrial and extraterrestrial origin with the exception for one kind of glass at the k t boundary in haiti abundant 10 20 nm diameter iron oxides having as much as 10 ni and minor zn are intergrown with smectite we interpret these domains to be altered meteorite fragments which formed when impact glass was transformed to smectite the direct association of unique glass and meteorite fragments is unambiguous evidence for meteorite impact these data may imply fall out of globally distributed impact derived particles over an extended time period the relations imply that tem observations may be a powerful tool in detecting other impact events in the geological record

international geomagnetic reference field its evolution and the difference in total field intensity between new and old models for 1965 1980 the total magnetic field near the surface of the earth is a sum of several constituent fields part of the total field consists of fields that are transient or rapidly varying these fields are caused either directly or indirectly by electric currents in the upper atmosphere and beyond the part of the total field that is more permanent arises from sources that are located inside the earth evidence suggests that this part has two principal constituents the main field and the crustal field

quench textures in altered spherules from the cretaceous tertiary boundary layer at agost and caravaca se spain abstract abundant spherules are observed in a well preserved cretaceous tertiary boundary ktb layer at the agost and caravaca sections betic cordilleras se spain these spherules have been diagenetically altered to goethite or k feldspar different morphologies are observed ranging from perfect spheres to droplet shapes this is the first time that dendritic and fibroradial textures are reported in goethite spherules with clearly splash form morphology this morphology suggests that they originated from melt in the hot vapour plume and not as condensate as unaltered ktb spherules are composed of clinopyroxene this could be the probable precursor of part of the altered spherules at agost and caravaca which may have been derived from an expanding vapour plume with a significant contribution of extraterrestrial material in addition c rich cores have been detected in k feldspar which are enriched in ir pt pd and ni although postdepositional processes and microbial activity could have led to severe modification of the original element concentrations the high content of noble metals and ni suggests extraterrestrial contamination

geochemistry of impact glasses from the k t boundary in haiti relation to smectites and a new type of glass we have individually analyzed twelve black and seven yellow glasses from the k t boundary section at beloc haiti plus three smectite mantles around black glasses the main chemical differences between black and yellow glasses are higher s ca mg zn as br sb and au but lower k na and si abundances in the yellow glasses we have found high cao yellow glasses with low s contents 0 08 0 17 wt which may be explained by fusion of carbonate rich sediments microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10 100 m level while yellow glasses show much more variability and different intra sample elemental correlations one of the black glasses has higher sio2 and k2o abundances but lower concentrations of all other major elements this suggests the existence of a third glass type which we have named the high si k variety hsi k glass the glass shows areas consisting of pure sio2 maybe lechatelierite and some schlieren with lower sio2 content the chemical composition indicates that the hsi k glass may have originated from melting of a sedimentary source rock such as shale or argillite severe changes in major and trace element composition occur during alteration of the glass as documented by analyses of adhering smectites the ree patterns in the glass are similar to average upper crust but the smectites exhibit much lower abundances and flat patterns not unlike the patterns observed in claystones at numerous k t boundary sites the suggestion that the chicxulub structure in yucatan may be the source crater for the k t impact glasses is not fully supported by the currently available data impact glasses and tektites are produced from the surface layers of their targets at chicxulub the target stratigraphy comprised mainly carbonates and evaporites although we cannot exclude chicxulub as a source the present data do not provide any firm chemical evidence for a connection we conclude however that the haiti glasses have formed by impact and have later undergone alteration by low temperature hydrothermal processes to produce clays the boundary claystones at k t boundary sites may very well be alteration products of these impact glasses

clay mineralogy of the cretaceous tertiary boundary clay analyses of the clay mineralogy of samples from the cretaceous tertiary boundary layer at four localities show that the boundary clay is neither mineralogically exotic nor distinct from locally derived clays above and below the boundary the significant ejecta component in the clay that is predicted by the asteroid impact scenario was not detected

tem study of meteorite impact glass at new zealand cretaceous tertiary sites evidence for multiple impacts or differentiation during global circulation abstract study by transmission electron microscopy of samples from the cretaceous tertiary k t boundary clay at flaxbourne river and woodside creek new zealand has revealed the occurrence of nanometer sized meteorite impact derived glass the average glass composition is exceptionally ca rich and is distinct from other glass found on earth apart from glass inferred to be of impact origin at mexican and haitian k t sites the glass shards are partially altered to montmorillonite like smectite with the dominant interlayer cation ca reflecting the composition of the parent glass the data imply a heterogeneous global distribution in composition of k t boundary impact glass si rich and ca rich in mexico and haiti si rich in denmark and ca rich in new zealand this heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the chicxulub impact site however recent discovery of an impact crater of k t boundary age in ukraine raises the possibility of impact clusters which produce material of heterogeneous composition

the cretaceous tertiary boundary chronostratigraphic position and sequence of events the cretaceous tertiary boundary has always been considered to be exceptional and linked to special events and mechanisms where catastrophes have often been advocated the boundary stratotype stevns klint in denmark has been paleomagnetically analysed the c t boundary is now given an absolute age of 66 7 m y the boundary is shown to have occurred under stable geomagnetic conditions about 165 000 years before the polarity reversal of the base of anomaly 29 and about 0 7 1 0 m y before the extinction of dinosaurs in new mexico significant disturbances in the earth s rate of rotation and direction of spin axis at the passing from the cretaceous into the tertiary are strongly contradicted the cat boundary events cannot be explained by a single large catastrophe a sequence of different events and some gradualism need also to be included in present and future scenarios of the boundary events

the historical record in the scaglia limestone at gubbio magnetic reversals and the cretaceous tertiary mass extinction the scaglia limestone in the umbria marche apennines well exposed in the gubbio area offered an unusual opportunity to stratigraphers it is a deepwater limestone carrying an unparalleled historical record of the late cretaceous and palaeogene undisturbed by erosional gaps the scaglia is a pelagic sediment largely composed of calcareous plankton calcareous nannofossils and planktonic foraminifera the best available tool for dating and long distance correlation in the 1970s it was recognized that these pelagic limestones carry a record of the reversals of the magnetic field abundant planktonic foraminifera made it possible to date the reversals from 80 to 50 ma and subsequent studies of related pelagic limestones allowed the micropalaeontological calibration of more than 100 myr of geomagnetic polarity stratigraphy from ca 137 to ca 23 ma some parts of the section also contain datable volcanic ash layers allowing numerical age calibration of the reversal and micropalaeontological time scales the reversal sequence determined from the italian pelagic limestones was used to date the marine magnetic anomaly sequence thus putting ages on the reconstructed maps of continental positions since the breakup of pangaea the gubbio scaglia also contains an apparently continuous record across the cretaceous tertiary boundary which was thought in the 1970s to be marked everywhere in the world by a hiatus a geochemical study of the gubbio cretaceous tertiary boundary accidentally uncovered an unexpectedly large concentration of iridium in the boundary clay providing the first evidence for a large comet or asteroid impact at the time of the cretaceous tertiary mass extinction the sudden foraminiferal extinction in the essentially complete gubbio scaglia showed that the cretaceous tertiary extinction was a catastrophic event after publication of the gubbio cretaceous tertiary iridium anomaly in 1980 a decade followed in which more and more evidence for a catastrophic impact challenged the traditional uniformitarian viewpoint of geologists in 1991 the impact site was discovered at chicxulub in the yucata n peninsula of mexico the gubbio area has thus emerged as a classical geological locality it has been critical in the development of geomagnetic reversal stratigraphy a third system for dating earth history complementing biostratigraphy and radiometric age dating gubbio was also central to the recognition that occasional catastrophic events like great impacts require a rejection of strict uniformitarianism

foraminiferal biostratigraphy and paleoenvironmental reconstruction at the yaxcopoil 1 drill hole chicxulub crater yucatan peninsula the yaxcopoil 1 yax 1 drill hole comprises cretaceous limestones and calcarenites the k p boundary cocktail unit including impact breccia and a danian marly clay layer overlain by calcareous marls the biostratigraphy paleobathymetry and environmental turnover across the k p interval were inferred after analyzing the planktic and benthic foraminiferal assemblages the cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle campanian to maastrichtian age while low diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic occasionally stressed environment the impact breccia and the redeposited suevite are overlain by a 46 cm thick dolomitic calcareous sandstone unit that contains scarce reworked planktic foraminiferal specimens this unit probably represents the uppermost part of the initial infill of the crater the uppermost centimeters of this unit are bioturbated and its top represents a hiatus that spans at least the g cretacea pv eugubina and part of the p pseudobulloides biozones this unit is overlain by a 3 4 cm thick marly clay layer that represents a condensed layer benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer the increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the g compressa biozone toward the a uncinata p2 biozone the danian planktic and benthic foraminiferal assemblages indicate a deeper probably bathyal environment

heat flow pattern at the chicxulub impact crater northern yucatan mexico abstract along an east west profile crossing the chicxulub impact structure in northern yucatan mexico curie depths were obtained from statistical spectral analysis of a grid of aeromagnetic data 256 km wide and 600 km long these depths were corrected for flight height and depth to the sea floor to determine the geothermal gradient assuming a temperature of 580 c for the curie temperature heat flow was then calculated from the geothermal gradients using a value of 2 67 w m k for the mean crustal thermal conductivity the results show a conspicuous heat flow high above on the impact basin in this location the heat flow is 80 mw m 2 approximately available offshore estimates of the depth to the crustal magnetic source bases on the northern yucatan platform and onshore heat flow determination on 8 shallow bore holes and in a 1511 m deep one support the existence of this major high heat flow anomaly associated with the impact crater this high heat flow might be related to the impact through 1 an uplift of the crystalline basement rocks in the center of the crater and 2 impact induced radioactive element concentration into the crust below the impact structure higher thermal conductivities at the lower crust might also play a key role available seismological and thermal property data are compatible with these mechanisms