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

very high temperature impact melt products as evidence for cosmic airbursts and impacts 12 900 years ago it has been proposed that fragments of an asteroid or comet impacted earth deposited silica and iron rich microspherules and other proxies across several continents and triggered the younger dryas cooling episode 12 900 years ago although many independent groups have confirmed the impact evidence the hypothesis remains controversial because some groups have failed to do so we examined sediment sequences from 18 dated younger dryas boundary ydb sites across three continents north america europe and asia spanning 12 000 km around nearly one third of the planet all sites display abundant microspherules in the ydb with none or few above and below in addition three sites abu hureyra syria melrose pennsylvania and blackville south carolina display vesicular high temperature siliceous scoria like objects or slos that match the spherules geochemically we compared ydb objects with melt products from a known cosmic impact meteor crater arizona and from the 1945 trinity nuclear airburst in socorro new mexico and found that all of these high energy events produced material that is geochemically and morphologically comparable including i high temperature rapidly quenched microspherules and slos ii corundum mullite and suessite fe3si a rare meteoritic mineral that forms under high temperatures iii melted sio2 glass or lechatelierite with flow textures or schlieren that form at 2 200 c and iv particles with features indicative of high energy interparticle collisions these results are inconsistent with anthropogenic volcanic authigenic and cosmic materials yet consistent with cosmic ejecta supporting the hypothesis of extraterrestrial airbursts impacts 12 900 years ago the wide geographic distribution of slos is consistent with multiple impactors

marine derived 14c calibration and activity record for the past 50 000 years updated from the cariaco basin abstract an expanded cariaco basin 14c chronology is tied to 230th dated hulu cave speleothem records in order to provide detailed marine based 14c calibration for the past 50 000 years the revised high resolution cariaco 14c calibration record agrees well with data from 230th dated fossil corals back to 33 ka with continued agreement despite increased scatter back to 50 ka suggesting that the record provides accurate calibration back to the limits of radiocarbon dating the calibration data document highly elevated 14c during the glacial period carbon cycle box model simulations show that the majority of observed 14c change can be explained by increased 14c production however from 45 to 15 ka 14c remains anomalously high indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today additional observations of the magnitude spatial extent and timing of deep ocean 14c shifts are critical for a complete understanding of observed glacial 14c variability

cosmogenic nuclide enhancement via deposition from long period comets as a test of the younger dryas impact hypothesis abstract we explore the idea that detectable excursions in 26 al may arise from direct deposition by any bolide and excursions in 14 c and 10 be abundances in the atmosphere may result from long period comet impacts this is very different from the usual processes of production by cosmic rays within earth s atmosphere long period comets experience greatly increased cosmic ray flux beyond the protection of the sun s magnetic field we report the computed amount of 14 c 10 be and 26 al present on long period comets as a function of comet mass we find that the amount of nuclide mass on large long period comets entering the earth s atmosphere may be sufficient for creating anomalies in the records of 14 c and 10 be from past impacts in particular the estimated mass of the proposed younger dryas comet is consistent with its having deposited sufficient isotopes to account for recorded 14 c and 10 be increases at that time the 26 al 10 be ratio is much larger in extraterrestrial objects than in the atmosphere and so we note that measuring this ratio in ice cores is a suitable definitive test for the younger dryas impact hypothesis even if the hypothetical bolide is not a long period comet and or did not contribute to the 14 c and 10 be increases

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

cometary airbursts and atmospheric chemistry tunguska and a candidate younger dryas event we find agreement between models of atmospheric chemistry changes from ionization for the a d 1908 tunguska siberia region russia airburst event and nitrate enhancement in greenland ice sheet project 2 gisp2h and gisp2 ice cores plus an unexplained ammonium spike we then consider a candidate cometary impact at the younger dryas onset yd the large estimated no x production and o 3 depletion are beyond accurate extrapolation but the ice core peak is much lower possibly because of insufficient sampling resolution ammonium and nitrate spikes in both greenland ice core project grip and gisp2 ice cores have been attributed to biomass burning at the onset of the yd a similar result is well resolved in tunguska ice core data but that forest fire was far too small to account for this direct input of ammonia from a comet into the atmosphere is adequate for yd ice core data but not for the tunguska data an analog of the haber process with hydrogen contributed by cometary or surface water atmospheric nitrogen high pressures and possibly catalytic iron from a comet could in principle produce ammonia accounting for the peaks in both data sets

cosmic ray event in 994 c e recorded in radiocarbon from danish oak we present measurements of radiocarbon in annual tree rings from the time period 980 1006 ce hereby covering the cosmic ray event in 994 ce the new radiocarbon record from danish oak is based on both earlywood and latewood fractions of the tree rings which makes it possible to study seasonal variations in 14c production the measurements show a rapid increase of c 10 0 from 993 to 994 ce in latewood followed by a modest decline and relatively high values over the ensuing 10 years this rapid increase occurs from 994 to 995 ce in earlywood suggesting that the cosmic ray event most likely occurred the the period between april june 994 ce our new record from danish oak shows strong agreement with existing 14c records from japan thus supporting the hypothesis that the 994 ce cosmic ray event was uniform throughout the northern hemisphere and therefore can be used as an astro chronological tie point to anchor floating chronologies of ancient history

beryllium 10 transport to antarctica results from seasonally resolved observations and modeling 1 cosmogenic 10be measured in polar ice cores has important application in the reconstruction of past solar activity however the processes controlling its atmospheric transport and deposition to the ice sheets are not fully understood here we use the seasonal changes in 10be concentrations in a 10 year monthly resolved ice core record from the law dome site east antarctica in conjunction with echam5 ham general circulation model gcm simulations of 10be and 7be deposition as tools to examine this problem maximum 10be concentrations are observed in the ice core during the austral late summer to early autumn summer autumn while minimum concentrations are observed during the austral winter the gcm simulations corroborated by earlier observations of 10be 7be ratios in antarctica from the georg von neumayer air sampling station suggest that the 10be concentration maximum is linked to direct input of stratospheric 10be from the antarctic stratosphere to the lower levels of the antarctic troposphere during the austral summer autumn this result contrasts with the modeled transport of 10be to greenland where the seasonal maximum in stratospheric input is seen in the late winter to spring synchronous with the timing of the seasonal maximum in midlatitude stratosphere to troposphere exchange our results suggest that a different combination of processes is responsible for the transport of 10be to the antarctic and greenland ice sheets

modes of covariability between sea surface temperature and wind stress intraseasonal anomalies along the coast of peru from satellite observations 2000 2008 1 the tropical rainfall measuring mission microwave imager sea surface temperature sst and quikscat wind stress satellite data are used to investigate the intraseasonal upwelling variability along the coat of peru over the period 2000 2008 two regions of peak variance correspond to the central peru region pisco region 15 s and the northern peru region piura region 5 s a covariance analysis reveals a significant coherency between winds and sst anomalies off pisco consistent with ekman pumping and transport dynamics the upwelling cell consists in a meridionally extended fringe of colder warmer water extending as far as 250 km from the coast at 15 s in the piura region the intraseasonal covariability pattern is represented by two modes one relevant to the direct ekman dynamics and the other one associated with the remote forcing of intraseasonal oceanic kelvin wave two regimes of variability are evidenced a low period regime 10 25 days is the signature of ekman transport pumping dynamics and is remotely forced by the migratory atmospheric disturbances across the southeastern pacific anticyclone a high period regime 35 60 day band is associated with the combined forcing of oceanic equatorial kelvin waves and migratory atmospheric disturbances in the midlatitudes in particular the modes of covariability exhibit a prominent 50 day period energy peak it is shown that this period arises from the impact of the first two baroclinic modes kelvin wave with the second baroclinic mode kelvin wave being more influential on the piura region

low p high t metamorphism and the role of heat transport by melt migration in the higo metamorphic complex kyushu japan this paper characterizes the metamorphic thermal structure of the higo metamorphic complex hmc and presents the results of a numerical simulation of a geotherm with melt migration and solidification reconstruction of the geological and metamorphic structure shows that the hmc initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels subsequently this initial thermal structure has been collapsed by e w and nne ssw trending high angle faults pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field p t array that may be divided into two segments the array at apparent higher structural levels has a low dp dt slope whereas that at apparent lower structural levels has a high dp dt slope this composite array cannot be explained by heat conduction in subsolidus rocks alone migmatite is exposed pervasively at apparent lower structural levels but large syn metamorphic plutons are absent at the levels exposed in the hmc transport and solidification of melt within migmatite is a potential mechanism to generate the composite array thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth the model also predicts that strata beneath the crossing point will consist of high grade solid metamorphic rocks and solidified melt products such as migmatite this prediction is consistent with the observation that migmatite was associated with the very high dp dt slope the melt migration model is able to generate the very high dp dt segment due to the high rate of heat transfer by advection

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

cosmogenic 26al in the atmosphere and the prospect of a 26al 10be chronometer to date old ice abstract cosmogenic radionuclides in the one million year half life range offer unique possibilities for age determinations in geophysics in measurements where the radioactive decay is being utilized as a clock uncertainties in age determinations may be reduced if the ratio of two radioisotopes with different half lives can be used as a chronometer in this work we investigate the atomic ratio of atmospheric 26al t1 2 0 717 ma to 10be t1 2 1 386 ma measured with accelerator mass spectrometry ams and its potential as a chronometer for dating old ice the 26al 10be ratio decreases with an effective half life of t1 2 26al 10be 1 49 ma for its application as a chronometer the atmospheric 26al 10be ratio has to be well characterized however the properties of atmospheric 26al have been understood only poorly so far at the vera ams facility of the university of vienna a first systematic study of the global variations of the 26al 10be ratio in the atmosphere and in surface firn has been carried out and pilot measurements of the 26al 10be ratio in deep antarctic ice have been performed our results indicate that this ratio is globally constant to within 5 in the atmosphere and in surface firn with a mean value of 1 89 10 3 the data also suggest that non atmospheric sources of 26al such as extraterrestrial in situ produced or re suspended 26al do not contribute significantly to the observed 26al 10be ratio in addition atmospheric mixing seems to exert only a minor influence in a first application of the method 26al 10be ratios were measured in chips collected in connection with the drilling of the lowest part of an ice core 2250 to 2760 m in dronning maud land antarctica surprisingly variable 26al 10be ratios ranging between 0 5 and up to 2 times the atmospheric ratio were found at different locations in this deep ice core while the cause for the ratios higher than atmospheric remains unexplained so far the ratios lower than atmospheric may be caused by radioactive decay allowing a first dating attempt using the 26al 10be ratio thus at an ice depth of 2760 m an approximate date of 6 7 2 6 105 years was established

atmospheric 14c variations derived from tree rings during the early younger dryas abstract atmospheric radiocarbon variations over the younger dryas interval from 13 000 to 11 600 cal yr bp are of immense scientific interest because they reveal crucial information about the linkages between climate ocean circulation and the carbon cycle however no direct and reliable atmospheric 14c records based on tree rings for the entire younger dryas have been available in this paper we present 1 high precision 14c measurements on the extension of absolute tree ring chronology from 12 400 to 12 560 cal yr bp and 2 high precision high resolution atmospheric 14c record derived from a 617 yr long tree ring chronology of huon pine from tasmania australia spanning the early younger dryas the new tree ring 14c records bridge the current gap in european tree ring radiocarbon chronologies during the early younger dryas linking the floating lateglacial pine record to the absolute tree ring timescale a continuous and reliable atmospheric 14c record for the past 14 000 cal yr bp including the younger dryas is now available the new records indicate that the abrupt rise in atmospheric 14c associated with the younger dryas onset occurs at 12 760 cal yr bp 240 yrs later than that recorded in cariaco varves with a smaller magnitude of 40 followed by several centennial 14c variations of 20 25 comparing the tree ring 14c to marine derived 14c and modelled 14c based on ice core 10be fluxes we conclude that changes in ocean circulation were mainly responsible for the younger dryas onset while a combination of changes in ocean circulation and 14c production rate were responsible for atmospheric 14c variations for the remainder of the younger dryas

development of migmatites and the role of viscous segregation in high t metamorphic complexes example from the ryoke metamorphic complex mikawa plateau central japan abstract the ryoke metamorphic complex of the mikawa plateau central japan contains well preserved geological and thermal structures formed during cretaceous high temperature high t metamorphism these structures can be explained by a pervasive melt migration model however the estimated flux of melt is much higher than that expected for compaction flow of felsic melt an additional tectonic force and or melt segregation to network of veins or layers is therefore required this paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple shear deformation and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust the results of numerical modeling of simple shear deformation of immiscible viscous fluids reveal that low viscosity fluids segregate into lenses and layers this segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system the rate of energy dissipation is proportional to viscosity therefore this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts and is an important process in focusing melt in partially melted rocks the structures produced by the numerical simulation are similar to those of migmatites the simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple shear deformation during deformation enhanced segregation a melt network will form dynamically if this process occurs at small to large scales in the hot crust melts will be effectively transported upward due to buoyancy forces it is possible that pervasive migmatites at low structural levels within high t metamorphic complexes may represent fossil melt pathways formed by crustal scale viscous segregation

large pt anomaly in the greenland ice core points to a cataclysm at the onset of younger dryas one explanation of the abrupt cooling episode known as the younger dryas yd is a cosmic impact or airburst at the yd boundary ydb that triggered cooling and resulted in other calamities including the disappearance of the clovis culture and the extinction of many large mammal species we tested the ydb impact hypothesis by analyzing ice samples from the greenland ice sheet project 2 gisp2 ice core across the bolling allerod yd boundary for major and trace elements we found a large pt anomaly at the ydb not accompanied by a prominent ir anomaly with the pt ir ratios at the pt peak exceeding those in known terrestrial and extraterrestrial materials whereas the highly fractionated pt ir ratio rules out mantle or chondritic sources of the pt anomaly it does not allow positive identification of the source circumstantial evidence such as very high superchondritic pt al ratios associated with the pt anomaly and its timing different from other major events recorded on the gisp2 ice core such as well understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction hints for an extraterrestrial source of pt such a source could have been a highly differentiated object like an ir poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the ydb impact hypothesis

new evidence from a black mat site in the northern andes supporting a cosmic impact 12 800 years ago abstractprevious work has ascribed a cosmic impact origin to black high temperature carbon encrusted beds 2 3 cm thick associated with the younger dryas readvance of ice at 12 8 ka during the late glacial in the northern andes of venezuela the evidence for this includes carbon spherules aluminosilicate melt rocks melted coatings of glass like amorphous carbon and fe mn on sands and clasts derived from local felsic gneiss and granite these sediments have been subjected to renewed investigation using high resolution scanning electron microscopy and energy dispersive spectrometry and new data show that spherules at site mum7b exhibit unique morphologies and compositions molar oxide weight percentages prove the spherules are not volcanic and show little overlap with cosmic materials spherule microstructures display quench melting and thus could not have formed from slow geological authigenic diagenetic or metamorphic processes instead geochemical values for the venezuelan samples plot withi

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

evidence from central mexico supporting the younger dryas extraterrestrial impact hypothesis we report the discovery in lake cuitzeo in central mexico of a black carbon rich lacustrine layer containing nanodiamonds microspherules and other unusual materials that date to the early younger dryas and are interpreted to result from an extraterrestrial impact these proxies were found in a 27 m long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial our attention focused early on an anomalous 10 cm thick carbon rich layer at a depth of 2 8 m that dates to 12 9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences collectively these changes have produced the most distinctive boundary layer in the late quaternary record this layer contains a diverse abundant assemblage of impact related markers including nanodiamonds carbon spherules and magnetic spherules with rapid melting quenching textures all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond n diamond i carbon and hexagonal nanodiamond lonsdaleite in order of estimated relative abundance this nanodiamond rich layer is consistent with the younger dryas boundary layer found at numerous sites across north america greenland and western europe we have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism it is however consistent with the younger dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst s and and or ground impact s at 12 9 ka

excursions in the 14c record at a d 774 775 in tree rings from russia and america the calibration of radiocarbon dates by means of a master calibration curve has been invaluable to earth environmental and archeological sciences but the fundamental reason for calibration is that atmospheric radiocarbon content varies because of changes in upper atmosphere production and global carbon cycling improved instrumentation has contributed to high resolution interannual radiocarbon activity measurements which have revealed sudden and anomalous activity shifts previously not observed at the common resolution of 5 10 years of most of the calibration scale one such spike has been recently reported from tree rings from japan and then again in europe at a d 774 775 for which we report here our efforts to both replicate its existence and determine its spatial extent using tree rings from larch at high latitude northern siberia and bristlecone pine from lower latitude the white mountains of california our results confirm an abrupt 15 14c activity increase from a d 774 to 776 the size and now the hemispheric extent of which suggest that an extraterrestrial influence on radiocarbon production is most likely responsible

a complete terrestrial radiocarbon record for 11 2 to 52 8 kyr b p radiocarbon 14c provides a way to date material that contains carbon with an age up to 50 000 years and is also an important tracer of the global carbon cycle however the lack of a comprehensive record reflecting atmospheric 14c prior to 12 5 thousand years before the present kyr b p has limited the application of radiocarbon dating of samples from the last glacial period here we report 14c results from lake suigetsu japan 35 35 n 135 53 e which provide a comprehensive record of terrestrial radiocarbon to the present limit of the 14c method the time scale we present in this work allows direct comparison of lake suigetsu paleoclimatic data with other terrestrial climatic records and gives information on the connection between global atmospheric and regional marine radiocarbon levels

discovery of a nanodiamond rich layer in the greenland ice sheet we report the discovery in the greenland ice sheet of a discrete layer of free nanodiamonds nds in very high abundances implying most likely either an unprecedented influx of extraterrestrial et material or a cosmic impact event that occurred after the last glacial episode from that layer we extracted n diamonds and hexagonal diamonds lonsdaleite an accepted et impact indicator at abundances of up to about 5 10 6 times background levels in adjacent younger and older ice the nds in the concentrated layer are rounded suggesting they most likely formed during a cosmic impact through some process similar to carbon vapor deposition or high explosive detonation this morphology has not been reported previously in cosmic material but has been observed in terrestrial impact material this is the first highly enriched discrete layer of nds observed in glacial ice anywhere and its presence indicates that ice caps are important archives of et events of varying magnitudes using a preliminary ice chronology based on oxygen isotopes and dust stratigraphy the nd rich layer appears to be coeval with nd abundance peaks reported at numerous north american sites in a sedimentary layer the younger dryas boundary layer ydb dating to 12 9 0 1 ka however more investigation is needed to confirm this association