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

nanoscale deformation twinning in xenotime a new shocked mineral from the santa fe impact structure new mexico usa shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact related deformation in a wide variety of planetary materials here we describe the first occurrence of shock deformation in xenotime ypo 4 from a shocked quartz bearing shatter cone in granite at the santa fe impact structure new mexico usa backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced planar fractures high resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in 112 contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations other features attributed to impact such as planar low angle boundaries and planar deformation bands record crystal plastic deformation shatter cone formation and co existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5 10 gpa an upper limit of 20 gpa is tentatively assigned based on the absence of ypo 4 polymorphs and shock twins in co existing zircon we propose that 112 deformation twins in xenotime constitute a diagnostic record of shock metamorphism similar to 112 twins in zircon they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation documentation of deformation twins in xenotime a widely applied u pb geochronometer can be used to identify hypervelocity deformation in shocked rocks detrital grains and other materials and may be particularly ideal for recording low pressure 20 gpa impact conditions that do not produce diagnostic shock microstructures in zircon

isotope systematics and shock wave metamorphism i u pb in zircon titanite and monazite shocked experimentally up to 59 gpa this study reports the first u pb isotope analyses on experimentally shocked zircon titanite and monazite extracted from proterozoic granitoid rocks in all three types of minerals shock waves produce drastic changes in the crystal lattices causing strong lowering of birefringence turbidization and decolorization of the individual grains moreover x ray patterns indicate transition of the crystals into polycrystalline aggregates of 10 5 mm block size precisely dated grains with concordant or nearly concordant ages were embedded in kbr and shocked at 35 47 5 and 59 gpa u pb isotope analyses on these grains show that shock metamorphism does not fractionate pb isotopes within the analytical precision of 0 1 as far as chemical fractionation is concerned there is no difference in degree of concordancy between shocked and unshocked monazite and small degrees 2 of relative upb fractionation in shocked zircon and titanite are due to time integrated pb loss and not to the shock experiment in consequence the data document that shock wave metamorphism alone does not measurably effect the u pb chronometer questioning the view that lower intercept ages of discordant u pb data reflect shock induced re equilibration of the chronometer in moderately to highly shocked rapidly cooling rocks

changing fluvial environments across the permian triassic boundary in the karoo basin south africa and possible causes of tetrapod extinctions abstract the karoo supergroup is a 12 km thick succession of sedimentary rocks that accumulated in a large intracratonic retroarc foreland basin in southwestern gondwana the strata record 100 million years of almost continuous sediment accumulation from the permo carboniferous 300 ma through to the early jurassic 190 ma under a range of climatic regimes and within several tectonically controlled sub basins alluvial sediments dominate the succession from the late permian onwards fossils of synapsid reptiles and early dinosaurs are sufficiently common to be used in a ten fold biostratigraphic subdivision of these strata the permian triassic boundary in the karoo succession is marked by a major extinction of the herbivorous dicynodonts which co incides with a rapid change in fluvial facies this study uses field observations of the sedimentary facies palaeosols and in situ fossils of well exposed p tr boundary sequences in the southern karoo basin to provide evidence of environmental changes that may have caused the tetrapod extinctions in the main karoo basin below the p tr boundary sequence the sediments comprise drab greenish grey mudrocks with a few thick 3 12 m multistoried laterally accreted fine grained sandstone bodies the mudrocks host hydromorphic palaeosols with palustrine carbonate horizons and mostly disarticulated skeletal remains of dicynodon assemblage zone fauna above a transition zone of approximately 20 m thickness these strata change into reddish brown mudrocks with numerous thin 0 5 5 m ribbon sandstone bodies and more continuous sheet sandstones calcic palaeosols with rhizocretions nodular horizons and pervasive calcareous encrustation of articulated skeletons of lystrosaurus and associated fauna of the lystrosaurus assemblage zone within a 20 m thick interval of the transitional strata fossils of dicynodon and lystrosaurus occur together the facies transition that co incides with the latest permian dicynodont extinction is interpreted as a change in fluvial style from meandering to low sinuosity channels with general drying of the floodplain habitats and redistribution of vegetation belts the horsetails in particular became significantly reduced in areal distribution the changes in fluvial style in the karoo basin at the end of the permian were triggered by a pulse of thrusting in the southerly source area dated at 247 ma 2 ma which brought about rapid progradation of a large sandy braided fan system the katberg sandstone fm into the central parts of the basin a concomitant basinward migration of depositional environments allowed the lystrosaurus communities that were adapted to dry floodplains to colonise the entire basin to the detriment of most of the wet floodplain dicynodon fauna that lived there

the last glaciation in central magellan strait southernmost chile abstract glacial landforms and drift stratigraphy in central magellan strait southernmost chile document repeated fluctuations during the last glacial cycle of outlet lobes from an ice cap centered over the southern andes the lobes developed comparatively low gradient profiles because of low basal shear stresses over soft deformable beds and this made them sensitive to even small scale changes in the mass balance such low profiles and rapid calving in deep proglacial lakes during deglaciation may have made the magellan ice lobe particularly responsive to climatic fluctuations during the last glacial cycle and to advance and retreat over considerable distances study of the glacial landforms and drift stratigraphy has led to the identification of at least five glacier advances to limits at and south of the segunda angostura fragments of mollusc shells contained in basal till indicate marine incursions between some advances thus documenting extensive deglaciation a partial chronology based on amino acid studies and radiocarbon dating suggests that five of these advances occurred during the last glacial cycle the most extensive advances may have culminated during substages of marine isotope stage 5 substage 5b or 5d and or during stage 4 slightly less extensive advances occurred between ca 28 000 and 14 000 yr b p

the sea level fingerprints of ice sheet collapse during interglacial periods abstract studies of sea level during previous interglacials provide insight into the stability of polar ice sheets in the face of global climate change commonly these studies correct ancient sea level highstands for the contaminating effect of isostatic adjustment associated with past ice age cycles and interpret the residuals as being equivalent to the peak eustatic sea level associated with excess melting relative to present day of ancient polar ice sheets however the collapse of polar ice sheets produces a distinct geometry or fingerprint of sea level change which must be accounted for to accurately infer peak eustatic sea level from site specific residual highstands to explore this issue we compute fingerprints associated with the collapse of the greenland ice sheet west antarctic ice sheet and marine sectors of the east antarctic ice sheet in order to isolate regions that would have been subject to greater than eustatic sea level change for all three cases these fingerprints are more robust than those associated with modern melting events when applied to infer eustatic sea level because 1 a significant collapse of polar ice sheets reduces the sensitivity of the computed fingerprints to uncertainties in the geometry of the melt regions and 2 the sea level signal associated with the collapse will dominate the signal from steric effects we evaluate these fingerprints at a suite of sites where sea level records from interglacial marine isotopes stages mis 5e and 11 have been obtained using these results we demonstrate that previously discrepant estimates of peak eustatic sea level during mis5e based on sea level markers in australia and the seychelles are brought into closer accord

amino acid racemization geochronology of reworked quaternary mollusks on u s atlantic coast beaches implications for chronostratigraphy taphonomy and coastal sediment transport approximately 200 shells primarily mercenaria from 21 beach sites between new jersey and florida have been analyzed for the extent of racemization epimerization of their fossilized amino acids the greatest concentration of sites is in north carolina in regions least affected by human modification of natural beach sediment transport processes these results can be used to estimate the frequency of age mixing of pleistocene shell material in holocene depositional environments selected shells have also been dated by 14c conventional and or ams to provide direct calibration of the amino acid epimerization ratios shell taphonomic characteristics particularly color can be qualitatively related to apparent ages data for shell fragmentation combined with amino acid age estimates provide insights into probable transport distances of reworked shells shells with apparent pleistocene ages have epimerization values equivalent to those seen in onshore exposures of pleistocene coastal units amino acid epimerization measurements are a comparatively rapid and inexpensive chronologic tool for use in taphonomic and sediment shell transport studies paired amino acid 14c analyses on single shells provide insights into geochemical alteration of shells and permit modelling of the residence time of shells on beaches greater numbers of analyses at each site are needed before statistically valid estimates of age mixing can be obtained but the apparent distribution of pleistocene shells on atlantic coast beaches appears to be related to the distribution of pleistocene units in the shoreface and inner shelf and the thickness of holocene sedimentary cover in specific areas these observations provide useful criteria for the evaluation of possible age mixing of shells collected at outcrops of pleistocene units in the coastal plain

comment on bedout a possible end permian impact crater offshore of northwestern australia nearly a decade ago gorter 1 1 suggested that the bedout basement high offshore western australia might represent an impact structure in view of the site s well defined circular bouguer anomaly and seismic reflection data indicating a possible ring syncline recently becker et al 2

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

the lappajarvi impact structure finland age duration of crater cooling and implications for early life abstract an in depth approach of 40 ar 39 ar dating of the 23 km lappajarvi impact structure finland was performed using carefully selected single grain aliquots of optically fresh clast poor impact melt rock and recrystallized k feldspar melt particles separated from impact metamorphosed granite pegmatite step heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven k feldspar samples the melt rocks yielded fully concordant ages with a weighted mean of 76 37 0 46 ma dating of the k feldspar melt particles resulted in a series of younger plateau ages ranging from 75 11 0 36 to 76 11 0 35 ma the melt rock results combined with the oldest syn melt rock k feldspar age and including all sources of uncertainties yielded a weighted mean age of 76 20 0 29 ma 2 mswd 1 02 p 0 41 which is interpreted to represent the best estimate age of the lappajarvi impact the age spread of at least 1 1 0 5 ma between the crystallization of the impact melt rocks and the k feldspar melt particles warns against the use of single 40 ar 39 ar analysis to derive a meaningful impact age in turn the age offset monitors both rapid cooling of the lappajarvi melt sheet versus slow cooling and impact induced hydrothermal circulation within the crater basement most likely driven by long lasting heat flow from the hot central uplift of the impact structure this interpretation is in line with the grain size dependent argon diffusion parameters and apparent closure temperatures of 230 410 c determined for various domain sizes of k feldspars observed in this study our study shows that even in comparatively small impact craters post impact hydrothermal activity can be estimated to last between 600 ka and 1 6 ma and is technically resolved by means of the 40 ar 39 ar dating technique therefore the longevity of hydrothermal systems in medium sized impact craters might be an order of magnitude longer than previously estimated prolonged post impact heat flow in medium sized craters in analogy to lappajarvi might have played an important role in the emergence of life on early earth and possibly mars

mixed carbonate siliciclastic microfacies from permian deposits of western gondwana evidence of gradual marine to continental transition or episodes of marine transgression abstract at the end of the paleozoic several intracratonic basins developed in gondwana including the parana basin in western gondwana permian sedimentary units include the passa dois group which encompasses four formations the irati serra alta teresina and rio do rasto and represents a transition from marine to continental depositional environments its strata correlate extensively from argentina to south africa and the irati and teresina formations in particular present mixed carbonate siliciclastic successions this interval has been speculated to have formed under several different paleoenvironmental scenarios however permian gondwana basin microfacies with no open sea influence have rarely been studied in addition in these basins there is a scarcity of micropaleontological studies and geochronological calibration besides an absence of marine index fossils in view of this a comprehensive microfacies study for the unit is developed with a large and representative surface and subsurface sample analysis of the passa dois group divided into three large geographic domains north central and south of the parana basin brazil thin sections distributed among the four units were analyzed with an emphasis on carbonate layers where the microfossils have a better chance to be recognized the data were interpreted within a revised biochronostratigraphic framework and integrated with bivalves macroflora palynology conchostracans and vertebrates biozones microfacies analysis resulted in the recognition of 13 microfacies four siliciclastic eight carbonate and one siliceous chert we observed a predominance of fine siliciclastic microfacies rich in organic matter in the southern domain rudaceous carbonate microfacies in the central domain with diverse allochemicals and the greater diversity of bioclasts and an increase on siliciclastic sediments influx in the northern domain of the basin where only the irati and teresina formations occur a shortage of microfossils was observed in the south domain where the carbonate microfacies are less varied than those to the north in the central domain coarse grained carbonates are present bivalves ostracods and sponge spicules are the main bioclasts on rudstones and grainstones from the irati and teresina formations and have been interpreted as allochthonous to parautochthonous the association of detrital and authigenic glauconite to bioaccumulations of sponge spicules is interpreted as episodes of marine transgression

impact shocked zircons discovery of shock induced textures reflecting increasing degrees of shock metamorphism abstract textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously however planar deformation features pdf due to shock metamorphism are well documented in quartz and other mineral grains from these same environments an etching technique was developed that allows sem visualization of pdf and other probable shock induced textural features such as granular polycrystalline texture in zircons from a variety of impact shock environments these textural features in shocked zircons from k t boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the u pb isotopic system

geochronological constraints on the age of a permo triassic impact event u pb and 40ar 39ar results for the 40 km araguainha structure of central brazil abstract impact cratering has been a fundamental geological process in earth history with major ramifications for the biosphere the complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination hampering the assessment of the effects of an individual event in the geological record we demonstrate the utility of a multi chronometer approach in our study of samples from the 40 km diameter araguainha impact structure of central brazil samples of uplifted basement granite display abundant evidence of shock deformation but u pb ages of shocked zircons and the 40ar 39ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history mixed results are obtained from in situ 40ar 39ar spot analyses of shocked igneous biotites in the granite with deformation along kink bands resulting in highly localized partial resetting in these grains likewise spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material the age of the glass itself and post impact devitrification the timing of crater formation is better assessed using samples of impact generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization granular aggregates of neocrystallized zircon form a cluster of ten u pb ages that yield a concordia age of 247 8 3 8 ma the possibility of pb loss from this population suggests that this is a minimum age for the impact event the best evidence for the age of the impact comes from the u th pb dating of neocrystallized monazite and 40ar 39ar step heating of three separate populations of post impact inclusion rich quartz grains that are derived from the infill of miarolitic cavities the 206pb 238u age of 254 5 3 2 ma 2 error and 208pb 232th age of 255 2 4 8 ma 2 error of monazite together with the inverse 18 point isochron age of 254 10 ma mswd 0 52 for the inclusion rich quartz grains yield a weighted mean age of 254 7 2 5 ma 0 99 2 error for the impact event the age of the araguainha crater overlaps with the timing of the permo triassic boundary within error but the calculated energy released by the araguainha impact is insufficient to be a direct cause of the global mass extinction however the regional effects of the araguainha impact event in the parana karoo basin may have been substantial

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

last interglacial isotope stage 5 glacial and sea level history of coastal chukotka peninsula and st lawrence island western beringia abstract study of glacial and marine sequences along the outer coast of chukotka peninsula bering strait is the basis for major revisions of the regional stratigraphy the val katlen suite at its type section at the mouth of the enmelen river records the youngest high sea stand of the last interglaciation on chukotka peninsula marine deposits and glacial diamicton stratigraphically below the type section record respectfully the peak of the last interglacial marine oxygen isotope substage 5e and rapid glacierization of the coastal mountains probably during substage 5d or 5b correlative deposits recording a similar sequence of intra stage 5 events but once thought to be of early and middle pleistocene age include the upper and lower pinakul suite at cape pinakul and marine deposits near the nunyamo river on the gulf of anadyr based upon amino acid analyses biostratigraphy and supporting geochronology these deposits enclose warmer than present faunas and floras comparible to last interglacial pelukian marine deposits found along the coast of alaska the last major advance of glacial ice from chukotka peninsula across anadyr strait and onto st lawrence island was likely initiated during the later part of stage 5 based on our reinterpretaion of the stratigraphy at cape pinakul near the nunyamo river and on northwestern st lawrence island chukotkan ice probably reached st lawrence island sometime during oxygen isotope stage 4 ice extend across chukotka during the lgm was very imited

aminostratigraphic correlations and paleotemperature implications pliocene pleistocene high sea level deposits northwestern alaska abstract multiple periods of late pliocene and pleistocene high sea level are recorded by surficial deposits along the coastal plains of northwestern alaska analyses of the extent of amino acid epimerization in fossil molluscan shells from the nome coastal plain of the northern bering sea coast and from the alaskan arctic coastal plain of the chukchi and beaufort sea coasts allow recognition of at least five intervals of higher than present relative sea level three late pliocene transgressions are represented at nome by the complex and protracted beringian transgression and on the arctic coastal plain by the colvillian bigbendian and fishcreekian transgressions these were followed by a lengthy period of non marine deposition during the early pleistocene when sea level did not reach above its present position a middle pleistocene high sea level event is represented at nome by the anvilian transgression and on the arctic coastal plain by the wainwrightian transgression anvilian deposits at the type locality are considerably younger than previously thought perhaps as young as oxygen isotope stage 11 410 000 bp finally the last interglacial pelukian transgression is represented discontinuously along the shores of northwestern alaska amino acid epimerization data together with previous paleomagnetic measurements radiometric age determinations and paleontologic evidence provide geochronological constraints on the sequence of marine deposits they form the basis of regional correlations and offer a means of evaluating the post depositional thermal history of the high sea level deposits provisional correlations between marine units at nome and the artic coastal plain indicate that the temperature difference that separates the two sites today had existed by about 3 0 ma since that time the effective diagenetic temperature was lowered by about 3 4 c at both sites and the mean annual temperature was lowered considerably more this temperature decrease was largely accomplished by the close of the fishcreekian beringian iii transgression ca 2 5 2 1 ma since then intervals of warm temperature must have been extremely brief these data suggest that the steep latitudinal temperature gradient and the frigid temperatures that characterize the high latitudes of alaska today are ancient features of arctic climate

zircons from the acraman impact melt rock south australia shock metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events abstract this study presents the first optical and scanning electron microscopic characterization and u pb shrimp dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large 40 90 km eroded ediacaran acraman impact structure in south australia microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer sized spots of zro 2 probably baddeleyite that pseudomorph pre impact zircon the granular zircons correspond to the highest observed level of shock metamorphism and impact induced recrystallization zro 2 bearing granular zircons indicate shock pressures in excess of 65 70 gpa which are considerably higher than previous shock pressure estimates for the acraman impactites u pb systematics of untreated and chemically abraded melt rock zircons indicate that u pb ratios of the acraman zircons were variably reset during impact weakly shocked crystalline grains yield ages on concordia at 1 59 1 60 ga reflecting the magmatic age of the gawler range volcanics only the entirely granular zircon population was apparently impact reset but based on an ediacaran age from stratigraphic constraints on the ejecta layer experienced significant post impact pb loss the microcrystalline nature of granular zircons could have promoted pb diffusion and recoil in post impact time as suggested by grain size dependent diffusion and recoil modeling a positive correlation of u concentration and shock level suggests that granularization might have preferentially occurred in initially u rich probably metamict zircons 40 ar 39 ar dating of a melted yardea dacite clast from the acraman melt rock as well as k feldspar separated from shocked yardea dacite resulted in post impact alteration plateau ages suggestive of hydrothermal events at 500 ma and 450 ma that selectively affected the impactites exposed in the central domain of the acraman impact structure our study demonstrates that the acraman impact is particularly difficult to date in the absence of accurate and precise isotopic ages for acraman the ediacaran ejecta stratigraphic age of 635 541 ma is considered the most reliable age constraint currently available for the timing of the large acraman impact

aminostratigraphic relations and age of quaternary deposits northern espanola basin new mexico abstract amino acid ratios of gastropods provide a useful basis for correlation and approximate dating of middle to late quaternary fluvial deposits from the northern espanola basin new mexico sparsely fossiliferous slackwater deposits in the rio chama rio grande floodplain were buried episodically by piedmont alluvium during periods of climatic change as the axial river system cut down 120 m during the past 620 000 yr alloisoleucine isoleucine alle ile ratios in the total hydrolysate and free fraction of amino acids in succinea and vallonia are strongly correlated with elevation ratios in succinea range from 0 01 to 0 79 in the total hydrolysate and from 0 00 to 1 15 in the free fraction for deposits that range in age from modern to 620 000 yr old amino acid ratios are tightly clustered for deposits that contain the 620 000 yr old lava creek b tephra layer demonstrating their utility for correlation and providing a calibration point for a local dating curve two 14c ages from younger deposits and limiting ages for erosion surfaces that lie above gastropod bearing units tend to support the validity of much of the curve combining the dating curve with the geologic constraints suggests that aggradation events separated by episodes of net incision occurred at about 620 000 310 000 70 000 170 000 40 000 95 000 15 000 yr ago after 19 000 yr ago and during two minor periods between 80 000 and 25 000 yr ago

the south american ancestry of the north patagonian massif geochronological evidence for an autochthonous origin new u pb and 40 ar 39 ar age data from deformed and undeformed granitoids of the north patagonian massif establish the presence of early cambrian and widespread ordovician magmatism in northern patagonia these data suggest that the pampean cambrian and famatinian ordovician magmatic belts of the sierras pampeanas are continuous into patagonia shrimp u pb age spectra from detrital zircons of cambro ordovician metasedimentary rocks show patterns very similar to those from equivalent units of the pampia block over 500 km farther north these results suggest that the north patagonian massif was likely part of the south american margin of gondwana in the early palaeozoic and strongly argue in favour of an authochtonous or para autochthonous origin for this block

rapid racemization of aspartic acid in mollusc shells and potential for dating over recent centuries dating of deposits and materials less than 350 years old is hindered by the very poor time resolution of the radiocarbon method over this period1 2 fluctuations in atmospheric 14c levels result in the existence of several possible calendric ages for any given radiocarbon age for terrestrial samples in the marine record these fluctuations are dampened however only a small change in marine radiocarbon ages occurs over this period from ad 1700 to 1950 radiocarbon ages become only 100 yr younger2 consequently age resolution is poor furthermore there is an uncertainty in the amount of the correction for the reservoir effect the apparent radiocarbon age of modern marine carbon typically 400yr which reflects the average residence time of carbon in the oceans amino acid racemization epimerization analysis has been used primarily for dating samples older than the limit of radiocarbon dating 40 000 50 000 yr bp 3 4 here i show that aspartic acid asp in mollusc shells has racemized particularly rapidly over the past few centuries analyses done on land snails from deposits in the negev desert eolianites in madeira cave deposits in jamaica and from museum collections indicate racemization rates of 2 5 per century asp racemization thus provides a means of dating recent faunal assemblages of molluscs and other biogenic carbonates as well as recent terrestrial fluvial and marine sedimentary sequences that contain molluscs

a multi proxy palaeoecological and palaeoclimatic record within full glacial lacustrine deposits western tennessee usa the fulton section along the mississippi river in western tennessee usa is a 1 km continuous exposure 20 m vertically of quaternary fluvial and lacustrine deposits inset within eocene sediments and buried by thick loess fossiliferous slackwater lake sediments record maximum aggradation during the last two major glaciations with deposition between ca 190 140 ka and q1 24 18 14 c ka bp based on amino acid and radiocarbon chronology respectively during the onset of full glacial conditions ca 24 22 14 c ka bp a relatively permanent shallow lake environment is indicated by ostracodes aquatic molluscs and both pollen and macrofossils of aquatic plants by 21 8 14 c ka bp increasing emergent plants amphibious gastropods pomatiopsis and heavier d 18 o compositions suggest marsh like conditions in a periodically drying lake the surrounding uplands consisted of picea pinus woodlands mixed with cool temperate hardwoods e g quercus populus carya grasses and herbs more open conditions ensued ca 20 14 c ka bp with loess and slopewash gradually infilling the former lake by 18 14 c ka bp modern analogue analyses of ostracodes and palaeontological evidence imply a full glacial climate similar to today s mixed boreal zone in central minnesota usa about 98c cooler in mean annual temperature than present day western tennessee copyright 2009 john wiley sons ltd

petrographic and geochemical characterization of the granitic rocks of the araguainha impact crater brazil petrographic and geochemical data obtained on the araguainha impact crater goias mato grosso states brazil indicate the existence of several molten products that originated during impact induced congruent melting of an alkali granite exposed in the inner part of the central uplift of the structure although previous studies have described these melts to some extent there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives and therefore little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the araguainha crater this paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium rich granite similar to postcollisional a type granites also found in terrains outside the araguainha crater in the brasilia orogenic belt the molten products are texturally distinct and different from the original rock but have very similar geochemical composition making it difficult to separate these lithotypes based on concentrations of major and minor elements this also applies for trace and rare earth elements ree thus indicating a high degree of homogenization during impact induced congruent melting under high pressure and postshock temperature conditions petrographic observations along with geochemical data indicate that melting occurs selectively where some of the elements are transported with the melt simultaneously there is an effective dissolution of the rock granite which leads to entrainment of the most resistant solid phases intact or partially molten minerals into the melt minerals more resistant to melting such as quartz and oxides contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact

anatomy of a mass extinction sedimentological and taphonomic evidence for drought induced die offs at the permo triassic boundary in the main karoo basin south africa abstract the southern part of the karoo basin of south africa contains a near complete stratigraphic record of the permo triassic boundary ptb isotope and magneto stratigraphy confirm that these predominantly fluvial strata are approximately the same age as zircon dated marine ptb sections 252 mya by august 2013 our team had found 579 in situ vertebrate fossils mostly of the clade therapsida in ptb exposures at three widely separated locations in the southern karoo basin biostratigraphic ranges of the various taxa found in each of the sections reveal three separate phases of die off within the same roughly 75 metre thick stratigraphic interval displaying the same sequence of sedimentary facies interpreted as indicative of climatic drying increased seasonality and the onset of an unpredictable monsoon type rainfall regime the three phases of an inferred ecologically stepped mass extinction are phase 1 45 m 30 m below ptb datum brought on by lowered watertables which led to loss of shallow rooting groundcover in the more elevated proximal floodplain areas and the disappearance of the smaller groundcover grazing herbivorous dicynodonts and their attendant small carnivores phase 2 20 0 m below ptb datum is the main extinction that occurs in massive maroon grey mudrock culminating in an event bed of laminated reddish brown siltstone mudstone couplets this facies reflects progressively unreliable rainfall leading to vegetation loss in proximal and distal floodplain areas the larger tree browsing herbivores and their attendant carnivores are confined to watercourses before finally disappearing phase 3 25 30 m above ptb datum occurs in massive maroon siltstone facies with evidence of climatic aridity including the accumulation of mummified carcasses buried by windblown dust all of the surviving permian taxa disappear within 30 m of the ptb temporal resolution based on accretion rates and pedogenic maturity of each stratigraphic section reveals that phase 1 and phase 2 die offs lasted 21 000 and 33 000 years separated by a short period of 7000 years where no disappearances are recorded and this was followed by 50 000 years of stasis for the final extinction phase lasting only 8000 years that removed all the permian survivor taxa we propose that the recorded disappearances are real rather than preferential preservation failure and that they represent drought induced die offs moving progressively up the food chain as the terrestrial ecosystem collapsed the latter mostly likely caused by volcanogenic greenhouse gas emissions and rapid global warming

spectral properties and aster based alteration mapping of masahim volcano facies se iran abstract this study applies advanced spaceborne thermal emission and reflection radiometer aster data and the mixture tuned matched filtering mtmf algorithm to map the sub pixel distribution of alteration minerals associated with the masahim volcano se iran for understanding the spatial relationship between alteration minerals and volcano facies investigations of the alteration mineralogy were conducted using field spectroscopy x ray diffraction xrd analysis and aster short wave infrared swir spectral data in order to spectrally characterize the stratovolcano deposits lithological units and alteration minerals the volcano was divided into three facies the central proximal and medial distal facies the reflectance spectra of rock samples show absorption features of a number of minerals including white mica kaolinite montmorillonite illite goethite hematite jarosite opal and chlorite the end members of key alteration minerals including sericite phyllic zone kaolinite argillic zone and chlorite propylitic zone were extracted from imagery using the pixel purity index ppi method and were used to map alteration minerals accuracy assessment through field observations was used to verify the fraction maps the results showed that most prominent altered rocks situated at the central facies of volcano the alteration minerals were discriminated with the coefficient of determination r2 of 0 74 0 81 and 0 68 for kaolinite sericite and chlorite respectively the results of this study have the potential to refine the map of alteration zones in the masahim volcano

age and timing of the permian mass extinctions u pb dating of closed system zircons the age and timing of the permian triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts zircons from nine ash beds within the shangsi and meishan sections china pretreated by annealing followed by partial attack with hydrofluoric acid result in suites of consistent and concordant uranium lead u pb ages eliminating the effects of lead loss the u pb age of the main pulse of the extinction is 252 6 0 2 million years synchronous with the siberian flood volcanism and it occurred within the quoted uncertainty

structural evolution of the 40 km wide araguainha impact structure central brazil the 40 km wide araguainha structure in central brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation the dominant structural features preserved at araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity slumping of the transient cavity walls resulted in inward verging inclined folds and a km scale anticline in the outer ring of the structure the folding stage was followed by radial and concentric faulting with downward displacement of kilometer scale blocks around the crater rim the central uplift records evidence for km scale upward movement of crystalline basement rocks from the transient cavity floor and lateral moment of sedimentary target rocks detached from the cavity walls much of the structural grain in the central uplift relates to structural stacking of km scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks outward plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets each individual sheet records an early stage of folding and thickening due to non coaxial strains shortly before sheet imbrication we attribute this folding and thickening phase to the kilometer scale inward movement of the target strata from the transient cavity walls to the central uplift the outer parts of the central uplift record additional outward movement of the target rocks possibly related to the collapse of the central uplift an inner ring structure at 10 12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks

abundance variation of iridium and trace elements at the permian triassic boundary at shangsi in china despite recent searches for anomalous iridium ir concentrations at the permian triassic boundary p tr of marine sediments1 3 no ir above a level of 0 5 p p b parts per 109 has been found detailed event stratigraphical research has revealed a comparatively high platinum group enrichment and geochemical anomalies of trace elements in the p tr boundary clay at meishan changxing county zhejiang province china4 we report here an investigation of ir and trace element abundances in the p tr boundary layer of shangsi section guangyuan county sichuan province fig 1 an ir value of 2 p p b detected in the boundary layer supports the occurrence of an extraterrestrial event at the end of the permian

chondritic meteorite fragments associated with the permian triassic boundary in antarctica multiple chondritic meteorite fragments have been found in two sedimentary rock samples from an end permian bed at graphite peak in antarctica the ni fe co ni and p fe ratios in metal grains the fe mg and mn fe ratios in olivine and pyroxene and the chemistry of fe ni p and s bearing oxide in the meteorite fragments are typical of cm type chondritic meteorites in one sample the meteoritic fragments are accompanied by more abundant discrete metal grains which are also found in an end permian bed at meishan southern china we discuss the implications of this finding for a suggested global impact event at the permian triassic boundary

sea cave temperature measurements and amino acid geochronology of british late pleistocene sea stands calibrating amino acid ratios with uranium series dates requires an accurate knowledge of current mean annual temperatures cmats over the region studied to measure these test tube sized diffusion sensors were emplaced for 1 year in 1984 1985 and 1986 both outside and inside minchin hole sea cave in south wales and belle hougue sea cave in jersey both of which have yielded oxygen isotope substage 5e uranium series ages on speleothems our outside temperatures agreed with meteorological ones our inside temperatures were over 1 c lower to allow for this a mean of empirical linear and parabolic epimerisation calculations suggests that ratios from molluscs inside the caves should be multiplied by over 1 1 for comparison with outside ratios this raises bowen et al s pennard stage ratios from inside minchin and bacon hole up towards the unnamed stage ratios outside and suggests that the unnamed sites are also from oxygen isotope substage 5e as proposed by proctor and smart the same conclusion is reached more strongly by comparisons with the ratios and temperatures inside belle hougue to the south and at eemian assumed 5e sites in the netherlands germany and denmark to the east the pennard ratios from outside sites may provide further evidence for global sea stands close to the present level later in oxygen isotope stage 5

catastrophic environmental transition at the permian triassic neo tethyan margin of gondwanaland geochemical isotopic and sedimentological evidence in the spiti valley india abstract interpreting global consequences of the permian triassic p tr extinction requires examination across paleogeographic realms of pangaea the spiti valley in india remnant of the peri gondwanan shelf preserved trails of this environmental catastrophe in the neo tethys ocean we document new sedimentological observations and high resolution trace element concentrations and carbon oxygen lead isotope data across the p tr boundary in spiti framboidal pyrites fossils and laminated lithology of the late permian shales indicate deeper anoxic depositional environment while 13 c org excursions of 2 4 and 3 1 in atargu and guling outcrops respectively identify the p tr transition across a clayey partly gypsiferous ferruginous layer sedimentological similarities of this layer to other neo tethyan sections from transcaucasia and iran indicate subaqueous oxidation of shallow marine sediments on a regional scale light rare earth element enriched late permian shales with conspicuous ce eu anomalies reflect their source from the adjacent panjal trap basalts ca 289 ma of kashmir continental crustal nb ta and zr hf anomalies appear at the p tr boundary sediments and prevail through the overlying early triassic carbonates original pb isotope ratios along with an increasing pb abundance closer to the p tr boundary distinguish the volcanic source of the late permian shales from the continental crustal siliciclastic signature of the early triassic carbonates our 13 c org trace element and pb isotope record from spiti indicate catastrophic changes in sediment sources and facies with effects on carbon cycle and are consistent with an abrupt episode of marine regression and erosional forcing also observed elsewhere along northern gondwanaland simultaneous eruption of siberian volcanics and bolide impacts in parana basin of brazil and elsewhere implicating impact triggered volcanism left catastrophic regional global imprints on sea level climate marine anoxia and tectonic stability that connected the p tr crisis across terrestrial and marine realms worlwide

insights into the morphology geometry and post impact erosion of the araguainha peak ring structure central brazil the 40 km wide araguainha structure is the largest and best exposed complex impact crater in south america it was excavated in flat lying sediments of the intracratonic parana basin where target rocks are well exposed and have remained undeformed since the impact event ca 245 ma despite the excellent state of preservation and exposure information available on the target rock stratigraphy post impact erosion and morphology of the structure is limited our combined field observations and remote sensing analysis demonstrate that araguainha preserves all features of a shallowly eroded peak ring crater the interior of the structure exposes a central peak surrounded by a 5 km wide annular basin and two main ring features 10 12 and 14 18 km from the center analysis of the pre impact stratigraphy present morphology and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin but minimal to nonexistent beyond the inner ring feature the formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls regional stratigraphic data combined with field observations indicate two periods of post impact erosion associated with exhumation of the parana basin we estimate that 250 350 m of fallback deposits and target rocks have been removed by erosion erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure in contrast to previous suggestions that the impact took place in a shallow marine environment our observations are more consistent with an extremely shallow brackish to freshwater lacustrine environment with little effect on the developing impact crater

emergent marine record and paleoclimate of the last interglaciation along the northwest alaskan coast the last interglacial high sea level stand the pelukian transgression of isotope substage 5e is recorded along the western and northern coasts of alaska by discontinuous but clearly traceable marine terraces and coastal landforms up to about 10 m altitude the stratigraphy indicates that sea level reached this altitude only once during the last interglacial cycle from the type area at nome to st lawrence island in the bering sea to the eastern limit of the beaufort sea pelukian deposits contain extralimital faunas indicating that coastal waters were warmer than present amino acid ratios in molluscs from these deposits decrease to the north toward barrow consistent with the modern regional temperature gradient fossil assemblages at nome and st lawrence island suggest that the winter sea ice limit was north of bering strait at least 800 km north of its present position and the bering sea was perennially ice free microfauna in pelukian sediments recovered from boreholes indicate that atlantic water may have been present on the shallow beaufort shelf suggesting that the arctic ocean was not stratified and the arctic sea ice cover was not perennial for some period in coastal regions of western alaska spruce woodlands extended westward beyond their modern range and in northern alaska on the arctic coastal plain spruce groves may have entered the upper colville river basin the flaxman member of the gubik formation on the alaskan arctic coastal plain was deposited during marine isotope substage 5a and records the breakup of an intra stage 5 ice sheet over northwestern keewatin

quaternary highstands in bear lake valley utah and idaho surficial deposits in bear lake valley utah and idaho demonstrate that bear lake and its surrounding wetlands expanded and contracted several times during the quaternary period deposits in 40 outcrops and the geomorphology of the valley were studied to reconstruct the water level history temporal control was provided by amino acid geochronology on 29 samples of aquatic mollusks calibrated by 14 c dating and tephrochronology samples of three molluscan genera were heated and analyzed to determine intergeneric differences in aspartic asp and glutamic glu acid racemization the covariance of asp and glu racemization in these genera was used to identify aberrant amino acid analyses the rate of glu racemization was calibrated by using a parabolic model that was used to estimate sample ages although the paucity of exposures uncertainty of the depositional setting and postdepositional uplift of deposits hamper an accurate reconstruction of water level in bear lake valley several conclusions can be drawn during the middle pleistocene lacustrine or wetland environments in northern bear lake valley may have extended up to 1830 masl meters above sea level 6000 ft twice between ca 1000 100 ka bear hollow phase during the late pleistocene the shoreline of bear lake transgressed at least three times 1 to 1817 masl 5960 ft at 47 39 cal calendar year corrected ka jensen spring phase 2 to 1814 masl 5950 ft at ca 16 15 cal ka raspberry square phase and 3 to 1814 masl at ca 9 cal ka willis ranch phase bear river currently bypasses bear lake but flowed into the lake during highstands and allowed the water level to rise to the valley threshold at nounan narrows water levels have lowered to 1806 masl 5923 ft since the middle pleistocene likely because of downcutting by bear river at nounan narrows because downcutting at nounan narrows controls the maximum attainable water level in bear lake valley effects of climate change on lake level are limited and bear lake may actually have been smaller than today when lake bonneville transgressed to its maximum shoreline during the last glacial maximum

stable isotope profiles of last interglacial pelukian transgression mollusks and paleoclimate implications in the bering strait region abstract the marine paleoclimate during the last interglaciation in the bering strait region is inferred from the stable oxygen and carbon isotope profiles of fossil bivalve mollusks specimens were collected from the marine highstand deposits of the pelukian transgression from the arctic alaskan coastal plain the nome coastal plain st lawrence island and the val katlen suite on the chukotka peninsula as a calibration modern mollusks were obtained from the major water masses in the bering chukchi seas system and from coastal environments near the fossil localities the 18 o composition of the modern mollusks accurately reflects the seawater 18 o differences among the water masses and the ambient temperature range at each site seasonal paleotemperature ranges are estimated from the 18 o profiles of the fossils by comparison with those of the modern specimens a simple model is developed to correct the paleotemperature ranges for the influence of differences in seawater 18 o between water masses and seasonal variations caused by freshwater discharge paleotemperatures calculated for the skull cliff site alaskan arctic coastal plain are close to but slightly cooler than those of modern specimens from peard bay and icy cape the isotopic composition of a pelukian specimen collected near wainwright shows the obvious influence of locally reduced salinities with a correction for lower seawater 18 o this profile yields a summer temperature of 13 c which is 3 c warmer than that from the modern icy cape specimen the isotope records of two specimens from the nome coastal plain suggest salinities that were 3 5 psu lower than the modern eastern bering shelf the paleotemperature range of 0 15 c is consistent with modern conditions in norton sound the character of the isotope profiles of the pelukian specimens from st lawrence island is very different than that of modern specimens near the island but matches the 18 o values of shells from deeper sites in anadyr water this suggests that the fossil mollusks were living in anadyr strait and were transported by a glacial advance from the chukotka peninsula the last interglacial specimens from the val katlen suite have isotope profiles with large seasonal 18 o ranges and the clear influence of a reduced salinity coastal water mass calculated paleotemperatures from the val katlen specimens range from 0 to 13 c differences in the 13 c of shell carbonate between sites appears to reflect the regional distribution of the 13 c of sedimentary organic matter som which is a mixture between terrestrial organic matter mean 13 c of 24 and marine phytoplankton 21 partitioning of 13 c between shallow and deep waters by phytoplankton productivity and remineralization may also affect the regional distribution differences in shell 13 c between taxa may relate to feeding mode and the type and isotopic composition of ingested particles the filter feeders mya and astarte typically have 13 c values 1 2 more positive than the deposit feeders macoma and serripes this effect is attributed to a 13 c difference between marine phytoplankton and som with a component of terrestrial organic carbon

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

in situ u pb dating and element mapping of three generations of monazite unravelling cryptic tectonothermal events in low grade terranes abstract in situ u pb dating of monazite and xenotime in sedimentary rocks from the mid archean soanesville group in the pilbara craton yields ages for provenance diagenesis and multiple low grade metamorphic events detrital monazite and xenotime grains give dates 3250 ma whereas diagenetic xenotime provides a new minimum age of 3190 10 ma for deposition of the basal soanesville group previously constrained between 3235 ma and 2955 ma metamorphic monazite provides evidence for three episodes of growth at 2 88 2 16 and 1 65 ga element mapping of monazite for la sm y and th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ u pb dating by sensitive high resolution ion microprobe shrimp specifically la and sm distributions closely correlate with different generations of monazite the presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology it also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire potentially multi aged crystals the ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton the 2 88 ga age coincides with a phase of regional deformation metamorphism and gold mineralization along a major crustal lineament whereas the most common monazite age population at 2 16 ga corresponds with the migration of a foreland fold and thrust belt across the craton the youngest age 1 65 ga coincides with an episode of tectonic reworking in the capricorn orogen along the southern pilbara margin the prolonged history of monazite growth may in part relate to channelized fluid flow during reactivation of long lived n to ne trending crustal structures that transect the craton despite repeated episodes of metamorphism the isotopic system in each generation of monazite remained unperturbed yielding precise dates the ability of monazite to record three separate events and in some instances two events in a single crystal distinguishes it from most other low temperature mineral chronometers which are readily reset during metamorphic overprinting low temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far field fluid flow related to tectonic processes the previously unrecognized history of crustal fluid flow in the pilbara craton has implications for chemical mineralogical and isotopic studies seeking to understand conditions on the early earth

pleistocene raised marine deposits on wrangel island northeast siberia and implications for the presence of an east siberian ice sheet two previously undocumented pleistocene marine transgressions on wrangel island northeastern siberia question the presence of an east siberian or beringian ice sheet during the last glacial maximum lgm the tundrovayan transgression 459 000 780 000 yr b p is represented by raised marine deposits and landforms 15 41 m asl located up to 18 km inland the presence of high sea level 64 000 73 000 yr ago the krasny flagian transgression is preserved in deposits and landforms 4 7 m asl in the krasny flag valley these deposits and landforms were mapped dated and described using amino acid geochronology radiocarbon optically stimulated luminescence electron spin resonance oxygen isotopes micropaleontology paleomagnetism and grain sizes the marine deposits are eustatic and not isostatic in origin all marine deposits on wrangel island predate the lgm indicating that neither wrangel island nor the east siberian or chukchi seas experienced extensive glaciation over the last 64 000 yr

linking shock textures revealed by bse cl and ebsd with u pb data la icp ms and sims from zircon from the araguainha impact structure brazil

matrix effects and calibration limitations in ion probe u pb and th pb dating of monazite abstract monazite can be used for u pb and th pb dating of a wide variety of rocks and geological processes particularly metamorphism in many instances ion microprobe analysis is the preferred method for dating monazite combining good analytical precision with m scale spatial resolution however ion microprobe dates can be compromised by matrix effects on the ionisation efficiencies of different secondary ionic species which can bias monazite pb tho n and pb uo n data by as much as 40 and 30 respectively for known monazite compositions these effects appear to be entirely attributable to the sputtering process in which case similar effects exist in data from all ion probes the effects on data ratios are modified sometimes exacerbated or reversed when data are subjected to the correlation based pb th and pb u calibrations that are now commonly used one dimensional calibrations of 206 pb 270 uo 2 and 208 pb 264 tho 2 are recommended corrections derived from data for a suite of reference materials with diverse compositions can reduce biases in 208 pb 232 th ages to 206 pb 238 u ages to 204 pb can be addressed without iterative data reduction

using nd isotopes to understand the provenance of sedimentary rocks from a continental margin to a foreland basin in the neoproterozoic paraguay belt central brazil abstract the neoproterozoic paraguay belt pb in central south america includes a thick sequence of sedimentary rocks that were deposited and deformed along the southeastern margin of the amazonian craton the stratigraphy is divided into four major successions 1 a lower succession of fine grained sedimentary rocks in the cuiaba group 2 a glacial diamictite turbidite succession in the cuiaba group and puga formation 3 a carbonate succession in the araras group and 4 an upper succession composed of sandstones siltstones and arkoses in the alto paraguay group this sequence of sedimentary rocks represents a passive margin that was deformed during late neoproterozoic cambrian epoch we investigated the nd isotope signature of the sedimentary rocks while considering the different lithostratigraphic sequences and their geographic extent within the pb nd isotopic ratios and tdm model ages suggest a dominant continental source where are related to paleoproterozoic to mesoproterozoic rocks of the amazonian craton the nd isotopic compositions do not display a major contribution from the younger neoproterozoic juvenile magmatic arc terrane of the brasilia belt tdm model ages plotted against the stratigraphy show old tdm model ages of ca 2 3 1 5 ga and strongly negative nd t values of 11 to 6 revealing sedimentary changes in the source the changing nd t values are interpreted to be function of the erosion deposition ratio that occurred due to uplift during orogenic processes causing variations in the source area compared to the lower glacial and carbonate successions the upper siliciclastic succession shows lower nd isotopic ratios that is consistent with a source from the paraguay belt or another neoproterozoic continental source this can be accommodated by the change from an extensional continental margin to a compressional foreland basin setting this provenance study of the paraguay belt sedimentary rocks has important implications for the west gondwana collision history prior to its final amalgamation at the end of the neoproterozoic and beginning of the cambrian epoch

abrupt and gradual extinction among late permian land vertebrates in the karoo basin south africa the karoo basin of south africa exposes a succession of upper permian to lower triassic terrestrial strata containing abundant terrestrial vertebrate fossils paleomagnetic magnetostratigraphic and carbon isotope data allow sections to be correlated across the basin with this stratigraphy the vertebrate fossil data show a gradual extinction in the upper permian punctuated by an enhanced extinction pulse at the permian triassic boundary interval particularly among the dicynodont therapsids coinciding with negative carbon isotope anomalies

conodont survival and low iridium abundances across the permian triassic boundary in south china the permian triassic sedimentary sequence of china includes one of the most complete and fossiliferous paleozoic mesozoic boundaries known closely spaced sampling across the boundary which is an important extinction event for most organisms has produced good conodont faunas that show little diversity change a drop in conodont abundance is the only apparent response to the extinction event a low concentration of iridium in the boundary clay 0 002 part per billion 20 percent as well as in samples immediately below and above that range from 0 004 to 0 034 part per billion do not support the proposal of an extraterrestrial impact event at this boundary in china

spatial u pb age distribution in shock recrystallized zircon a case study from the rochechouart impact structure france abstract age determination of impact structures via the zircon u pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on u pb geochronology it is therefore crucial to link the observed zircon microtextures including their temperature and pressure conditions associated with their formation directly to the u pb ages preserved here we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium sized rochechouart impact structure in the northwestern massif central of france for the rochechouart impact structure the impact age 206 92 0 32 ma 40ar 39ar as well as the tectono themal history is well established making this study site ideal to test concepts about u pb systematics in shocked zircon and to differentiate between shock driven age resetting and pre impact crystallization and metamorphic overprinting zircon microstructures were studied using scanning electron imaging cathodoluminescence imaging and electron backscatter diffraction ebsd mapping further we conducted u pb laser ablation inductively coupled plasma mass spectrometry la icp ms depth profiling analysis allowing us to interpret the resultant age data in discrete steps with increasing ablation time depth the u pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre impact pre variscan and variscan regional tectonic ages our results also reveal that the granular crystals encountered contain microstructural evidence for former reidite in granular neoblastic frign zircon reflecting both high pressure 30 gpa and high temperature 1200 c conditions this signifies that frign zircon is now known from an additional medium sized impact structure further supporting the hypothesis that this impact induced microstructure is commonly preserved in addition frign zircon has a high potential to preserve completely impact reset crystal domains 204 to 207 ma that can be identified by our combined analytical approach of u pb depth profiling and ebsd mapping and thus are suitable for determining reliable impact ages

earth impact effects program a web based computer program for calculating the regional environmental consequences of a meteoroid impact on earth we have developed a web based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on earth www lpl arizona edu impacteffects this paper details the observations assumptions and equations upon which the program is based it describes our approach to quantifying the principal impact processes that might affect the people buildings and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions the program requires six inputs impactor diameter impactor density impact velocity before atmospheric entry impact angle the distance from the impact at which the environmental effects are to be calculated and the target type sedimentary rock crystalline rock or a water layer above rock the program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse the thermal radiation emitted by the impact generated vapor plume fireball and the intensity of seismic shaking the program also approximates various dimensions of the impact crater and ejecta deposit as well as estimating the severity of the air blast in both crater forming and airburst impacts we illustrate the utility of our program by examining the predicted environmental consequences across the united states of hypothetical impact scenarios occurring in los angeles we find that the most wide reaching environmental consequence is seismic shaking both ejecta deposit thickness and air blast pressure decay much more rapidly with distance than with seismic ground motion close to the impact site the most devastating effect is from thermal radiation however the curvature of the earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon

new perspectives on beringian quaternary paleogeography stratigraphy and glacial history abstract aspects of the paleogeography stratigraphy and glacial history of beringia have been greatly revised over the past 15 20 yr access to north east russia in particular has provided the opportunity to evaluate the beringia landscape as a contiguous subcontinent during the quaternary for the first time new research has made clearer the connection between tectonic forces and the submergence of the bering strait during the middle pliocene revisions in the regional stratigraphy of glacial and interglacial deposits in northwest alaska and northeast russia provide a new foundation for assessing the causes for differences in glacial ice extent through time the consensus of all field workers verifies that glacial ice throughout most of beringia was of very limited extent during the last glacial maximum the onset of regional glaciation during the waning stages of the last interglaciation is clearly out of phase with glaciation at lower latitudes despite the lack of much glacial activity during the early holocene alaska contains a rich record of late holocene glacial response to neoglacial cooling changes in the holocene environment of beringia likely had a profound affect on early inhabitants the curiosity driven vision and spirit of both david hopkins and the late troy pewe have had a profound influence on arctic paleoenvironmental research

on the significance of crater ages new ages for dellen sweden and araguainha brazil samples of a few grams in weight from the impact melt at the dellen crater sweden and the araguainha dome brazil were analyzed by rb sr and sm nd techniques for dellen pure labradorite orthopyroxene and glass separates from a clast free impact melt rock with holohyaline ground mass yield an internal rb sr isochron with 89 0 2 7 ma isr 0 83571 0 00007 2 compared to ar ar plateau ages of 102 3 2 ma and 109 6 2 0 ma published for total melt rocks various whole rock values not plotting on this rb sr isochron indicate primary variations in 87sr86sr over the melt sheet the age of the araguainha dome brazil is bracketed by the rb sr model age of 243 19 ma isr 0 7226 0 0005 for altered cordierite in the impact melt and by the internal rb sr isochron age of 449 9 ma isr 0 7140 0 0007 for an uplifted granite shock stage ib in melt rocks from both impact structures 143nd144nd was not homogenized during total rock melting sm nd data either scatter around the primary isochron araguainha or point to a partial resetting dellen this data array may be caused by tiny refractory ree rich inclusions in the main mineral phases which dominate the sm nd system of the impact melt rocks the significant difference between the new isochron age for dellen and ar ar ages show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy

age of pre late wisconsin glacial estuarine sedimentation bristol bay alaska pleistocene glacial estuarine sediment deposited in an intertidal environment of northeastern bristol bay southwestern alaska was dated using a variety of approaches including infrared stimulated and thermoluminescence irsl and tl techniques analysis of modern and14c dated holocene tide flat mud demonstrates that the bulk of sediment in this environment is reset by solar radiation thereby lending confidence to ages obtained from similar pleistocene deposits by luminescence techniques irsl seems to be especially well suited for dating with resolution on time scales of 10 000 yr the ages of tide flat mud of the nushagak formation derived from the ahklun mountains to the northwest of bristol bay and of halfmoon bay drift derived from the alaska peninsula to the southeast suggest contemporaneous glacial estuarine deposition related to independent glacial source areas about 75 000 80 000 yr ago this age is consistent with other geochronological data that indicate a pre late wisconsin and post substage 5e age including nonfinite14c ages a lack of interglacial indicators and old crow tephra 140 000 yr atop the drift normal paleomagnetic inclinations and amino acid isoleucine epimerization ratios aile ile aile ile ratios inportlandia arctica 0 052 0 003 from a marine lag horizon at south naknek beach which separates halfmoon bay drift above from older glacial estuarine drift below are only slightly higher than inmya truncata 0 041 0 007 from last interglacial pelukian deposits at nome as laboratory heating experiments show that the two genera epimerize at similar rates these data imply correlation of the marine lag at south naknek beach with pelukian deposits hence glaciers on the alaska peninsula experienced major pre late wisconsin advances both before and after the last interglaciation shells reworked into halfmoon bay drift yield aile ile ratios of 0 028 0 005 forportlandiaat second point and 0 027 0 001 forhiatella arcticaat etolin point together with assumptions about the postdepositional temperature history these ratios indicate that the shells are at least 55 000 yr and probably closer to 90 000 yr although the uncertainty in this age estimate is broad the amino acid and luminescence data converge on an age between about 75 000 and 90 000 yr late during oxygen isotope stage 5 for a major ice advance far beyond late wisconsin limits

strontium isotopic composition of pliocene and pleistocene molluscs from emerged marine deposits north american arctic high precision strontium sr isotopic measurements were obtained for 53 pliocene and pleistocene molluscan shells from emerged marine deposits around the coasts of arctic north america to test whether such data can be used for chronostratigraphic purposes 87sr 86sr ratios from sr isotopic measurements on many marine fossils from arctic ocean borderland sites are broadly consistent with their expected values based on independent age control and on a comparison with the sr isotopic evolution of seawater recorded in deep sea cores all 87sr 86sr ratios measured for shells from middle and late pleistocene deposits are consistent with expected values but only 9 of 22 ratios in shells from older deposits are consistent with independent age estimates aberrant 87sr 86sr ratios are consistently higher than expected at nome alaska and baffin island canada all 87sr 86sr ratios are higher than expected because these shells were formed along the pacific ocean and atlantic ocean margins respectively their h

pre late wisconsin glacial history coastal ahklun mountains southwestern alaska new amino acid thermoluminescence and 40ar 39ar results abstract new stratigraphic and geochronologic data from the togiak bay area of southwestern alaska indicate that glaciers advanced from the southern ahklun mountains at least three and as many as six times prior to the late wisconsin the oldest glaciations are represented by glacial marine sediment in coastal exposures on hagemeister island the extent of amino acid isoleucine epimerization in fossil molluscs indicates that at least one and possibly four older middle pleistocene glacial intervals are represented with age estimates spanning 500 280 ka and averaging 400 100 ka the youngest glacial marine drift on hagemeister island may correlate with the eruption of the togiak tuya a new 40 ar 39 ar age on basalt that overlies pillow lava indicates that the volcano erupted through glacial ice at least 300 m thick 263 22 ka the youngest drift in the region overlies the old crow tephra 140 10 ka and a 70 10 ka basaltic lava flow dated by thermoluminescence analysis of underlying baked sediment the drift delimits flat piedmont lobes that spread out onto the continental shelf and terminated 100 km from their source areas during the early wisconsin sensu lato the glacial geologic evidence suggests that major expansions of glaciers were out of phase with global ice volume

aminostratigraphy of pliocene pleistocene high sea level deposits nome coastal plain and adjacent nearshore area alaska amino acid epimerization ratios aile ile measured in 295 fossil molluscan shells of 3 genera provide a basis to evaluate the number and relative timing of high sea level events represented by deposits blanketing the nome coastal plain and adjacent nearshore area northwestern alaska frequency distribution analysis along with a new approach that relies on the ranked data distribution was used to identify clusters aminozones in the aile ile data each of which presumably represents a discrete period of marine inundation of the study area six periods of high relative sea level were distinguished some are recognized only by redeposited shells rather than in situ bio or lithostratigraphic units the oldest period is represented by shells recovered from the deepest sections of two offshore boreholes aile ile ratios in these shells 1 0 are much higher than in any found onshore and probably date to the late miocene early pliocene the oldest aminozone lies below nonfossiliferous sediments which in turn are overlain by glacigenic and marine deposits containing largely fragmented shells with aile ile ratios ranging from 0 35 to 0 65 this range coincides with aile ile ratios from various onshore deposits previously ascribed to the middle pliocene 3 m y beringian transgression ratios in this range can be subdivided into three evenly spaced aminozones each of which may represent multiple short duration 10 4 yr high sea stands separated by a longer period 10 5 yr sea level oscillation on the basis of a new empirical model of parabolic epimerization kinetics and an assumed age of 3 2 m y for the oldest beringian aminozone the approximate age of the initial submergence of bering strait the estimated length of time represented by the three beringian aminozones is between 0 5 and 1 7 m y the youngest beringian highstand was followed in the late pliocene or early pleistocene by a lengthy period of nonmarine deposition during which local valley glaciers advanced onto the coastal plain at least once relative sea level apparently did not again surpass that of today until the middle pleistocene anvilian transgression the oldest higher than present stand of the pleistocene yet considerably younger than previously thought glaciers overran the nome coastal plain at least once following the anvilian transgression incorporating abundant shells into their drift the drift is notched by a shoreline formed during the pelukian transgression of the last interglaciation although well represented onshore no pelukian deposits have been found offshore

grenville age pseudotachylite in the lewisian laserprobe 40ar 39ar ages from the gairloch region of scotland uk infrared laserprobe 40 ar 39 ar dating has been used to date pseudotachylite and host rock minerals from a crush belt in the lewisian basement of scotland it has revealed complexity in the pseudotachylite data that is attributable to the presence of refractory host rock clasts and mineral fragments in the pseudotachylite in conjunction with the host rock mineral laserprobe 40 ar 39 ar data it has been possible to simplify the pseudotachylite data for the samples and the preferred ages for these are 980 39 ma 999 31 ma and 1024 30 ma 2 these ages are the first record of grenville aged brittle deformation in the lewisian further this study serves to illustrate the complexity of dating pseudotachylites and the advantages and limitations of the ir laserprobe applied to such materials

closing the clymene ocean and bending a brasiliano belt evidence for the cambrian formation of gondwana southeast amazon craton we report new paleomagnetic and geochronological data from ediacaran rift drift carbonates in the paraguai belt at the southern end of the suture zone between the amazon craton and the sao francisco and rio de plata cratons south america early thrusting resulted in remagnetization ca 528 36 ma or later the mean age is established by 40 ar 39 ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold thrust belt this remagnetization overlaps with a 525 ma gondwana reference pole metamorphic illite from the slate belt yields 40 ar 39 ar ages of 496 484 ma the timing of peak regional metamorphism oroclinal bending of the paraguai belt was caused by a 90 clockwise rotation of the east west limb after ca 528 ma probably refl ecting the irregular margin of the southeast amazon craton the age of the paraguai belt overlaps with that of the pampean orogeny farther south along the western margin of the rio de plata craton suggesting a coeval closure for the clymene ocean separating the amazon craton from the sao francisco and rio de plata cratons

amino acid chronology of the lower pleistocene deposits of venta micena orce granada andalusia spain abstract this paper deals with a model adjustment for the estimation of age by means of amino acid racemization analysis two model families were obtained on the basis of the different genera of molluscs analysed and were applied to a palaeontological site located in the cullar baza basin venta micena orce granada the analytical results obtained from the study of fossil gastropods have provided a very coherent average dating of 983 58 ky this coinciding to a large extent with the most widespread palaeontological geological and stratigraphical datings for the site there is however no agreement with certain recent theories that situate venta micena chronologically at the pliocene pleistocene boundary ca 1 6 my or before the validity of the dating fit was tested on the basis of another classical palaeontological site cullar baza where a coherent dating of 441 27 ky was obtained

enhanced age resolution of the marine quaternary record in the arctic using aspartic acid racemization dating of bivalve shells abstract aspartic acid asp racemization occurs at a significantly higher rate than isoleucine epimerization and consequently provides better temporal resolution of arctic marine deposits from alaska spitsbergen and baffin island heating experiments at 100 c on the bivalvesmyaandhiatellashow the asp racemization rate decreases with increasing d l values as is typical for biogenic carbonates based on these experimental racemization rates and rates determined from racemization of samples radiocarbon dated to ca 10 000 12 000 yr b p activation energies formyaandhiatellaare estimated to be 30 6 and 30 0 kcal mol respectively for asp racemization and 29 0 and 29 5 for isoleucine epimerization analysis of a time series of plio pleistocenehiatellafrom the north coast of alaska shows that last interglacial mollusks can be readily distinguished from modern samples by asp but not by isoleucine d l asp values indicate a younger age for the fishcreekian transgression than does isoleucine epimerization for spitsbergen d l asp shows a slight age difference ca 12 000 yr between two units of the episode b interstadial and suggests that the age of these units may be closer to 65 000 than to 80 000 yr b p two possible ages suggested by other evidence the age of the loks land interstadial on baffin island is likely to be greater than that indicated by radiocarbon ages within deposits from each region d l asp values are less variable among individual shells than isoleucine epimerization values this may indicate better reliability of asp for geochronology

amino acid composition as a taxonomic tool for molluscan fossils an example from pliocene pleistocene arctic marine deposits abstract amino acid compositions of more than 550 individuals of three common subarctic arctic molluscan genera hiatella mya and astarte were measured to determine the feasibility of using such data to classify unidentifiable shell fragments belonging to one of these genera to account for diagenetically induced variation in amino acid composition of the fossils equations were developed to reconstruct the original composition of each shell based on its extent of amino acid epimerization the classification procedure involved calculating the multivariate similarity coefficient of similarity cs between the adjusted amino acid composition of an unknown shell and that of a set of references css were calculated for an additional 141 identifiable shells of the three common genera to evaluate the performance of the classification method the procedure correctly classified 93 of the shells in addition css were calculated for 162 identifiable shells of five other common subarctic arctic genera portlandia spisula macoma cardita and arctica to test the ability of the method to discriminate shells belonging to genera not included in the reference set and are therefore necessarily misclassified overall css for correctly classified shells x 0 891 are higher than for misclassified shells x 0 809 and the ratio of correctly to incorrectly classified individuals increases with increasing cs based on these empirical tests only those classifications supported by css greater than 0 88 were considered reliable and of these approximately 6 may be misclassified the classification procedure was applied to sixty four unidentifiable molluscan shell fragments recovered from boreholes drilled onshore of nome alaska of which twenty six were classified as hiatella mya or astarte with a cs greater than 0 88

geochemistry of the end permian extinction event in austria and italy no evidence for an extraterrestrial component the end permian mass extinction 251 ma was the largest in earth s history and the great extent of biospheric perturbation is recorded as dramatic shifts in carbon isotope ratios of sedimentary materials both terrestrial and extraterrestrial events are commonly invoked as causative mechanisms for the crisis and the primary reason for the event remains the subject of controversy geochemical indicators sensitive to the influence of extraterrestrial material involve platinum group elements and osmium and helium isotope ratios analyses of extinction levels in two sections from austria and italy reveal no evidence of an extraterrestrial impact the end permian crisis it appears was a homegrown catastrophe

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

search for evidence of impact at the permian triassic boundary in antarctica and australia life on earth was almost destroyed some 250 m y ago in the most profound of all known mass extinction events we investigated the possible role of impact by an extraterrestrial bolide through chemical and mineralogical characterization of boundary breccias search for shocked quartz and analysis for iridium in permian triassic boundary sections at graphite peak and mount crean antarctica and wybung head australia thin claystone breccias at the isotopically and paleobotanically defined boundary at all three locations are interpreted as redeposited soil rather than impact ejecta the breccias at all three locations also yielded shocked quartz but it is an order of magnitude less abundant 0 2 vol and smaller only as much as 176 micrometers m diameter than shocked quartz at some cretaceous tertiary boundary sites faint iridium anomalies were detected up to 134 pg g 1 these values are an order of magnitude less than iridium anomalies at some cretaceous tertiary boundary sites furthermore peak iridium values are as much as 1 m below the isotopically and paleobotanically defined boundary the idea that impact caused the extinctions thus remains to be demonstrated convincingly

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic