relating wave attenuation to pancake ice thickness using field measurements and model results wave attenuation coefficients m a1 were calculated from in situ data transmitted by custom wave buoys deployed into the advancing pancake ice region of the weddell sea data cover a 12 day period as the buoy array was first compressed and then dilated under the influence of a passing low pressure system attenuation was found to vary over more than 2 orders of magnitude and to be far higher than that observed in broken floe marginal ice zones a clear linear relation between and ice thickness was demonstrated using ice thickness from a novel dynamic thermodynamic model a simple expression for in terms of wave period and ice thickness was derived for application in research and operational models the variation of was further investigated with a two layer viscous model and a linear relation was found between eddy viscosity in the sub ice boundary layer and ice thickness

u pb isotopic study of relict zircon inclusions recovered from muong nong type tektites abstract isotopic compositions of u and pb were determined for relict zircon inclusions recovered from muong nong type tektites from the australasian and north american strewn fields these inclusions are generally opaque white with granular textures probably representing mechanical disaggregation and or local melting and recrystallization all zircons have strongly discordant u pb isotopic ratios and the isotopic ratios of most of the zircons are dominated by common pb this is interpreted as being the result of complete resetting of the isotopic system during the impact event and exchange with pb from the target sediments ages that may represent the target rocks were only retained by one sample representing a zircon that was extracted from a muong nong type georgiaite a value of 0 6 ga was obtained which is similar to the nd and sr model ages obtained for the north american tektites our results indicate that the u pb isotopic system in almost all zircons from tektites has been totally reset during the impact event this is in contrast to zircons extracted from impact melt rocks at some impact structures and or distal ejecta some of which are only partly reset

fine structure microstructure and vertical mixing processes in the upper ocean in the western weddell sea the upward flux of heat from the subsurface core of warm deep water wdw to the perennially ice covered sea surface over the continental slope in the western weddell sea is estimated using data obtained during february june 1992 from a drifting ice station through the permanent pycnocline the diapycnal heat flux is estimated to be about 3 w m 2 predominantly because of double diffusive convection there is no evidence that shear driven mixing is important in the pycnocline the estimated mean rate of heat transfer from the mixed layer to the ice is 1 7 w m 2 although peak heat fluxes of up to 15 w m 2 are found during storms it is hypothesized that isopycnal mixing along sloping intrusions also contributes to the loss of heat from the wdw in this region however we are unable to quantify the fluxes associated with this process intrusions occur intermittently throughout this experiment but are most commonly found near the boundary of the warm core current and the shelf modified water to the east these heat fluxes are significantly lower than the basin averaged value of 19 w m 2 fahrbach et al 1994 that is required to balance the heat budget of the weddell gyre other studies suggest that shelf processes to the west of the ice station drift track and more energetic double diffusive convection in the midgyre to the east could account for the difference between our flux estimates for this region and those based on the basin scale heat budget

turbulent heat flux in the upper ocean under sea ice turbulence data from three arctic drift station experiments demonstrate features of turbulent heat transfer in the oceanic boundary layer time series analysis of several w t records shows that heat and momentum flux occur at nearly the same scales typically by turbulent eddies of the order of 10 20 m in horizontal extent and a few meters in vertical extent probability distribution functions of w t have large skewness and kurtosis where the latter confirms that most of the flux occurs in intermittent events with positive and negative excursions an order of magnitude larger than the mean value an estimate of the eddy heat diffusivity in the outer ekman part of the boundary layer based on measured heat flux and temperature gradient during a diurnal tidal cycle over the yermak plateau slope north of fram strait agrees reasonably well with the eddy viscosity with values as high as 0 15 m2 s 1 an analysis of measurements made near the ice ocean interface at the three stations shows that heat flux increases with both temperature elevation above freezing and with friction velocity at the interface it also reveals a surprising uniformity in parameters describing the heat and mass transfer e g the thickness of the transition sublayer from a modified version of the yaglom kader theory is about 10 cm at all three sites despite nearly a fivefold difference in the under ice roughness z0 which ranges from approximately 2 to 9 cm a much simplified model for heat and mass transfer at the ice ocean interface suggested by the relative uniformity of the heat transfer coefficients at the three sites is outlined

u pb memory behavior in chicxulub s peak ring applying u pb depth profiling to shocked zircon abstract while the zircon u pb system is one of the most robust geochronometers individual crystals can be affected differently by the passage of a hypervelocity impact generated shock wave and heating unraveling the potentially complex thermal history recorded by zircon crystals that experienced different levels of shock metamorphism as well as their pre and post impact thermal evolution has been difficult using conventional geochronological methodologies the existing high precision 40ar 39ar age constraints for the k pg chicxulub event and the previous u pb dating of the basement rocks from the impact site make chicxulub an ideal location to study impact induced effects on the zircon u pb systematics and the pre impact u pb signature within individual zircon crystals recent iodp icdp drilling of the chicxulub impact structure recovered 580 m of uplifted shocked granitoid and 130 m of melt and suevite providing an unprecedented opportunity to study zircon crystals subjected to a range of shock pressures thermal and deformational histories zircon morphologies were classified using scanning electron microscopy sem imaging and then samples were depth profiled using laser ablation inductively coupled plasma mass spectrometry la icp ms to document the range of preserved age domains from rim to center within individual crystals the results show u pb ages range from 66 to 472 ma which are consistent with both inherited carboniferous and late paleozoic basement ages as well as pb loss ages in response to the k pg impact event while the bulk of the zircons preserve paleozoic ages high u metamict zones within fractured zircon crystals exhibited an age within uncertainty 66 6 2 ma of the impact age 66 038 0 049 ma indicating that inherited intragrain u pb kinetics and or hydrothermal fluid flow control age resetting in zircon rather than impact induced shock and heating alone moreover the calculated decay doses show that the zircon crystals experienced stage 1 or early stage 2 radiation damage accumulation therefore we suggest that the lowered crystal annealing temperature in crystals that experienced radiation damage make the zircon u pb clock either more susceptible to the relatively short heat pulse of the impact event at the and or to the moderate pressure and temperature conditions in the peak ring and or to hydrothermal fluid flow in the impact induced hydrothermal system

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

wintertime mixed layer measurements at maud rise weddell sea 1 sea ice plays a crucial role in the exchange of heat between the ocean and the atmosphere and areas of intense air sea ice interaction are important sites for water mass modification the weddell sea is one of these sites where a relatively thin first year ice cover is constantly being changed by mixing of heat from below and stress exerted from the rapidly changing and intense winds this study presents mixed layer turbulence measurements obtained during two wintertime drift stations in august 2005 in the eastern weddell sea close to the maud rise seamount turbulence in the boundary layer is found to be controlled by the drifting ice directly measured heat fluxes compare well with previous studies and are well estimated from the mixed layer temperatures and mixing heat fluxes are also found to roughly balance the conductive heat flux in the ice hence little freezing melting was observed the under ice topography is estimated to be hydraulically very smooth comparison with a steady 1 d model shows that these estimates are made too close to the ice ocean interface to be representative for the entire ice floe the main source and sink of turbulent kinetic energy are shear production and dissipation observations indicate that the dynamics of the under ice boundary layer are influenced by a horizontal variability in mixed layer density and an increasing amount of open leads in the area

isotope data from ice station weddell implications for deep water formation in the weddell sea helium isotope data 3he 4he ratios and 4he concentrations and h218o h216o ratios obtained from stations occupied during the drift of ice station weddell february to june 1992 are used together with hydrographic data to study formation of deep and bottom water in the western weddell sea the data indicate deep and bottom water formation along the entire track of the ice station 71 4 to 65 8 s 53 w ice shelf water isw seems to contribute significantly to the formation of weddell sea deep water wsdw and weddell sea bottom water wsbw in the southern part of the drift track toward the north the fraction of isw contained in wsdw wsbw decreases this trend is overlaid by high isw fractions in the deep and bottom waters found in the vicinity of the larsen ice shelf the fraction of western shelf water wsw in wsbw shows the opposite trend increasing from south to north the combined fraction of isw and wsw in waters with potential temperatures below 0 c is about 20 corresponding to a roughly 200 m thick layer overall wsw seems to contribute approximately 2 to 3 times more water than isw to the water column below the 0 c isotherm using the estimated flow of isw over the sill north of the filchner depression of 1 sv foldvik et al 1985a together with the ratio of wsw plus winter water ww to isw we calculate a value of about 5 sv for the formation rate of wsbw with a potential temperature of 0 7 c about one third of this flux represents near surface waters wsw ww which have recently been equilibrated with the atmosphere whereas pure isw contributes about 10

retention of u and pb in zircons from shocked granite in the siljan impact structure sweden the siljan ring structure central sweden was formed by a devonian impact the u pb zircon isotopic system of the shocked proterozoic granite basement appears to be undisturbed although the zircons were mechanically disrupted by the impact

sedimentation in marine impact craters insight from the ritland impact structure abstract this paper is intended to compare the sedimentary processes active in the ritland impact structure with the processes discussed from gardnos kardla lockne and chesapeake bay marine impact structures sedimentary processes e g rock avalanches slides and slumps debris and density flows turbidity and suspension deposition occur approximately in the same stratigraphical order in all these craters resulting in comparable crater infills the thickness and compositional variations can be explained by differences in crater size target lithology and water depth rock avalanches dominate in crystalline target impact sites with shallow water depth as observed in the ritland and gardnos craters impact into deeper water e g lockne does not reveal large scale gravity driven collapses of the transient sea floor cavity crystalline target sites with shallow water depth and thin sedimentary cover as in the ritland and gardnos impact sites resulted in elevated rim formations reducing resurge energy and yielded comparable reduced debris flow events impact into thick stratified sedimentary targets resulted in relatively weaker rim formation in kardla the related brim developments in the lockne and chesapeake bay impact structures can be reflected in multiple events of large scale debris flows resurge turbidites are observed in the kardla and lockne crater infilling successions turbidites in the ritland and gardnos are dominantly originated through later slope failure and reworking of the impact generated debris suspension dominated late post impact sedimentation within all the craters is largely controlled by their regional geological settings

turbulent mixing under drifting pack ice in the weddell sea by providing cold dense water that sinks and mixes to fill the abyssal world ocean high latitude air sea ice interaction is the main conduit through which the deep ocean communicates with the rest of the climate system a key element in modeling and predicting oceanic impact on climate is understanding the processes that control the near surface exchange of heat salt and momentum in 1992 the united states russian ice station weddell 1 traversed the western weddell sea during the onset of winter providing a platform for direct measurement of turbulent heat flux and reynolds stress in the upper ocean data from a storm early in the drift indicated i well formed ekman spirals in both velocity and turbulent stress ii high correlation between mixed layer heat flux and temperature gradients iii that eddy viscosity and eddy thermal diffusivity were similar about 0 02 square meters per second and iv that the significant turbulent length scale 2 to 3 meters through most of the boundary layer was proportional to the wavelength at the peak in the weighted vertical velocity spectrum the measurements were consistent with a simple model in which the bulk eddy viscosity in the neutrally buoyant mixed layer is proportional to kinematic boundary stress divided by the coriolis parameter

the age and thermal evolution of the vredefort impact structure a single grain upb zircon study abstract single grain u pb shrimp analyses of a heterogeneous zircon population from a small granite body in the archean basement core of the vredefort dome indicate an age of 2017 5 ma for the granite which is identical within errors to the previously reported age determinations for the vredefort impact event petrographic evidence from the granite indicates that it crystallized after the impact shock event but some doubt remains whether it is a true impact melt a decompression melt generated during rapid uplift of the crater basement following the impact event or a melt generated during an earlier 2 05 2 06 ga granulite facies anatectic event that was still at least partially molten at the time of the vredefort impact event rare zircon xenocrysts displaying planar microdeformation features and giving discordant archean ages are overgrown by polycrystalline zircon rims with u th compositions similar to that of the main 2 02 ga zircon population but with discordant 207pb 206pb ages indicating more recent pb loss

impact melting of lunar meteorite dhofar 458 evidence from polycrystalline texture and decomposition of zircon abstract dhofar 458 is a lunar meteorite consisting mainly of olivine plagioclase intergrowths pyroxene plagioclase intergrowths and plagioclase fragments pyroxene plagioclase globules are also common in this study we report the discovery of a polycrystalline zircon in this lunar meteorite the polycrystalline zircon contains small vesicles and rounded baddeleyite grains at its margin the polycrystalline and porous texture of the zircon indicates high pressure shock induced melting and degassing baddeleyite grains are derived from decomposition of zircon under high postshock temperature the shock features in zircon indicates that the shock pressure in dhofar 458 was greater than approximately 60 gpa and the postshock temperature greater than approximately 1700 c the polycrystalline and degassing texture and decomposition zircon also strongly indicates that dhofar 458 is a clast rich impact melt rock during this shock event most components were melted and grains of mafic minerals are interstitial to lath like plagioclase grains large fragments of olivine and chromite also formed polycrystalline texture at margins and chemically reequilibrated with surrounding melts we suggest that pyroxene plagioclase globules could be remains of melted target clasts whereas vesicles may form during shock induced degassing of the rock the u pb isotopic data plot on a well defined discordant line yielding the age of the zircon of 3434 15 ma 2 this age is interpreted as the time of the impact event that melted dhofar 458 and caused decomposition and recrystallization of this zircon in dhofar 458 which reset this zircon s u pb age

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

a middle late triassic 40ar 39ar age for the paasselka impact structure se finland abstract 40ar 39ar dating of recrystallized feldspar glass particles separated from clast rich impact melt rocks from the approximately 10 km paasselka impact structure se finland yielded a middle to late triassic ladinian karnian pseudo plateau age of 228 7 3 0 3 4 ma 2 this new age makes paasselka the first known triassic impact structure dated by isotopic methods on the baltic shield the new paasselka impact age is within uncertainty coeval with isotopic ages recently obtained for the lake saint martin impact structure in canada indicating a new middle to late triassic impact crater population on earth the comparatively small crater size however suggests no relationship between the paasselka impact and a postulated extinction event at the middle late triassic boundary

shallow brine driven free convection in polar oceans nonhydrostatic numerical process studies a three dimensional nonhydrostatic convection model which accounts for small scale ice ocean interactions is used to study convection in shallow sea coastal ice formation regions which contribute significantly to water mass formation in both the arctic and antarctic ocean for certain conditions the results presented in this paper are also transferable to shallow open ocean convection the model is applied to an initial well mixed ocean at rest with a temperature close to the freezing point the ocean initially free of ice is exposed to cold and dry polar air we consider situations in which the mean wind stress is negligible but wind fluctuations result in small sensible and latent heat fluxes corresponding to a wind speed of 2 m s 1 cellular convection patterns develop in the ocean finally occupying a mean aspect ratio of 2 convection is driven by salt release during frazil ice formation due to supercooling newly forming sea ice is collected along convergent downwelling regions at the surface thus showing also cellular structures because the area of insulating sea ice remains small new ice can be formed continuously and the surface buoyancy forcing remains large this collection of ice in small fractions of the sea surface results in a latent heat polynya type which is very effective in terms of dense water mass formation a comparison of the three dimensional model and a two dimensional slice model shows that key results can be reproduced with the slice model in summary the results of the process studies indicate that cellular features in the sea ice cover which may be detectable by remote sensing techniques are closely related to active brine driven convection

low p t caledonian resetting of u rich paleoproterozoic zircons central sweden uranium rich zircons from a paleoproterozoic high grade deformation zone in the fennoscandian shield central sweden show an almost complete resetting of the u pb system in early phanerozoic time a mylonitic gneiss in the deformation zone contains two types of highly discordant 70 u rich zircons large brown cloudy prisms and small milky white irregularly shaped grains the gneiss also contains mostly clear prismatic zircon of lower u content with mildly discordant to concordant u pb ages laser raman spectroscopy reveals that the dark cathodoluminescent areas in brown zircons have a highly metamict crystal structure whereas the structures of both the dark cathodoluminescent milky white grains and the bright cathodoluminescent clear prisms have higher degrees of crystallinity age dates obtained by u pb sims analysis of 40 zircons of the three types described above range continuously from concordant at 1871 11 ma to 98 discordant at 384 15 ma the strongly discordant zircons clearly have suffered severe disturbance at about the time of the caledonian orogeny however caledonian metamorphic temperatures and pressures in this region did not exceed 150 200 c and 1 3 kbar too low to strongly disturb the u pb systematics in non metamict zircon by thermal means alone independent evidence indicates that saline fluids were circulating in the paleoproterozoic basement rocks at this time possibly driven by hydrological gradients generated in front of the encroaching caledonian orogenic wedge these low temperature saline fluids are inferred to be responsible for causing both strong pb loss in the mostly metamict brown zircons via a diffusive process and the formation of small milky white zircon via a low temperature recrystallization or dissolution re precipitation process

two high precision u pb zircon ages for the sudbury nickel irruptive two independent u pb zircon ages were measured using norite samples from the north and south ranges of the sudbury nickel irruptive ages of and ma were obtained with a probability of fit within experimental error of 51 and 57 respectively previous determinations by the rb sr whole rock method gave ages between about 1700 and 2000 ma with errors of the order of 100 ma the difference in precision is inherent in the two methods zircons from the norite are highly radiogenic and almost concordant whereas rb sr whole rock ages are determined by very small increments in radiogenic 87sr the two zircon ages have completely different discordia so that all simple models for lead loss by continuous diffusion can be excluded

u pb systematics of zircon and titanite from the gardnos impact structure norway evidence for impact at 546 ma abstract zircon and titanite were investigated in impactites of the gardnos structure a crater formed in sveconorwegian ca 1 ga crust which was then overridden in the devonian by caledonian nappes observed deformation features in zircons are granular texture planar microstructures and likely the incorporation of organic carbon during impact causing black staining of the zircon grains the grains were studied by scanning electron microscopy sem and cathode luminescence cl and dated by u pb isotope dilution thermo ionization mass spectrometry id tims zircon grains without impact related features have u pb data showing moderate discordance 5 13 and indicating formation ages mostly in the range of 1600 1000 ma except detrital zircon ages as old as 2481 ma reflecting the diversity of target rocks in the area titanite with concordant ages of 995 999 ma dates metamorphism during final juxtaposition of the telemarkia on the idefjorden terrane to the east zircon grains with demonstrated or presumed shock features yield highly discordant 14 40 u pb data with a majority of them plotting along an array with a lower intercept of about 340 ma reflecting the influence of the caledonian orogeny and recent pb loss one zircon grain was totally reset at 379 ma during late caledonian metamorphism which also caused local growth of new titanite a specific group of zircon grains yields data with relatively high discordance for moderate u contents and five of these analyses including that of a grain with proven granular or aggregate texture fit a discordia line with an upper intercept of 546 5 ma these features are interpreted as indicating zircon break down to an amorphous state during impact with subsequent recrystallization into microcrystalline aggregates causing extensive to complete pb loss we further suggest that their crystallinity prevented pb loss during the caledonian orogeny while the small subgrain size and increasing metamictisation allowed more recent disturbances we thus interpret the 546 ma age as the approximate time of impact

ion microprobe uranium lead dating of zircons from the lappajarvi impact crater western finland the lake lappajarvi impact crater lies in paleoproterozoic svecofennian metasedimentary rocks on the western side of the central finland granitoid complex 1 9 ga two conflicting ages have been reported for the meteorite impact an age of 77 3 0 4 ma on the basis of ar ar whole rock data from impact melt samples and a paleomagnetic age of 195 ma during studies on impact crater indicator minerals at lappajarvi zircons with an atypical appearance were found in suevite boulders these zircons seemed to have been affected by impact shock metamorphism and it was considered that they would be good candidates for ion microprobe u pb dating allowing a new and independent age estimate for the impact event at lappajarvi four spot analyses on two black coated zircons plotted close to the upper intercept end of the concordia curve giving an approximate age of 1 8 ga for the source rock seventeen analyses were done on three dull zircon grains showing patchy impact related partial recrystallization most of these data fell fairly well on a single discordia line with intercept ages of 73 3 2 5 3 ma and 1854 2 5 1 ma however five of the data spots near the lower intercept end fell on the younger side of the line this was interpreted to indicate post impact loss of lead importantly the new ion microprobe u pb age of 73 3 f 5 3 ma is in a very good agreement with the previously reported ar ar age

observations of upper ocean boundary layer dynamics in the marginal ice zone 1 vertical profiles of turbulent kinetic energy dissipation and small scale hydrography were collected in the upper ocean boundary of both ice covered and ice free stations in the marginal ice zone of the barents sea in spring 2005 together with shipboard wind measurements and current profiles the mixed and mixing layer dynamics are studied during the survey shear production by the stress at the surface or under the ice dominated large upward turbulent heat fluxes 300 500 w m 2 were calculated for the mixing layers overlying the warm atlantic water which compared well with those obtained from an independent parameterization at the top of the pycnocline corresponding heat fluxes were 10 20 w m 2 significant stabilizing buoyancy fluxes were estimated for ice drift stations owing to the melting of ice in response to the large heat fluxes the mean dissipation profile in the ice free reference station agreed with the constant stress wall layer scaling within 30 on the contrary the observed dissipation profiles were enhanced in the upper half of under ice mixing layer or within 2 5 times the assessed pressure ridge keel depth deeper in the mixing layer the profiles relaxed toward the wall scaling the variability of dissipation in the mixed layer was better captured by the shear production profile when local friction speed was used together with a mixing length profile modified by buoyancy fluxes significant correlations were found between dissipation integrated over the mixing layer and work done by stress under the ice and the wind work at 10 m height the low correlation between the mixed layer depth and length scale for neutral conditions significantly increased when the reduction in the mixing length due to buoyancy fluxes was accounted for the mixing depth is observed to be strongly correlated with the outer neutral planetary length scale

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

entrainment induced by near inertial drift of sea ice and its impact on under ice biogeochemical processes in marginal ice zones abstract mooring observation of hydrography hydrodynamics and suspended particles distribution under a drifting sea ice revealed the mixing and entrainment pattern in the upper mixed layer ml of the marginal ice zone the ice floe where the mooring system was installed drifted as near inertial motion with approximately 12 h cycle the mixing pattern induced by this near inertial drift can be divided into two distinct regimes first simple entrainment upward fluxes from the seasonal pycnocline to sea ice water boundary are induced by shear across ml and seasonal pycnocline during the period when ice floes drift toward pack ice the entrainment speed was in the range of 0 25 2 m h 1 which matches well with thickening and thinning of the ml during a near inertial period turbulent wakes on the boundary between sea ice and open water occurred behind the advancing edge of ice in the second regime when ice floes drift toward open ocean the turbulent wakes at the advancing edge of ice are combined with the entrainment caused by near inertial motion which results in a complex mixing pattern of both upward and downward fluxes in the ml the echo intensity observed by the acoustic doppler current profiler and beam attenuation from transmissometer revealed the elevated concentration of suspended particulate materials in the ml which can be direct evidence visualizing the mixing pattern results suggest that the mixing and entrainment found in our study sustain particulate matters in suspension within upper ml for a few months this may provide a potential mechanism to sustain abundant organic particulates in the ml and upper pycnocline for months after under ice bloom under strong wind events like storms the entrainment induced by near inertial motion may also get enhanced which causes elevated supply of nutrients from the deeper permanent pycnocline to the ml

argon 40 argon 39 analyses of samples from the gardnos impact structure norway abstract we have conducted 40arpar age dating on a sample of impact melt from the gardnos impact structure in norway in an attempt to better constrain the formation age of the crater current estimates of the age of the gardnos crater cover a wide range and are as old as 900 ma dons and naterstad 1992 french et al 1997 the age spectra that we obtained from three samples are consistent with a thermal event at 385 5 ma la because this differs greatly from the best stratigraphic age of 600 ma and because the minerals present in the dated sample are a metamorphic assemblage we believe we have not dated the formation age of the crater instead we have probably dated the effect of the early devonian collapse of the late caledonian scandian orogeny on these materials dons and naterstad 1992 french et al 1997 although it may be possible it will be dif icult to determine the age of the impact by isotopic means alone because of this widespread metamorphism detailed stratigraphic analyses of the crater fill sediments may be the most promising method for constraining the crater age

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

geochemistry of carbonaceous impactites from the gardnos impact structure norway abstract the gardnos impact structure in southern norway is one of only two known impact structures among 175 whose impactites contain significant amounts typically 0 2 1 0 wt of carbon or 5 to 10 times the amount present in the target rocks sudbury canada is the other this study extends a previous investigation of the geochemistry and petrology of gardnos impactites french et al 1997 with additional sampling and a detailed investigation of the nature and possible origin of the carbonaceous material present two principal carbon components have been identified in gardnos impactites 1 impact produced diamonds 0 5 to 1 m in size with a cubic crystal structure predominantly hexagonal morphologies with platey layers and an estimated concentration of geochemical data suggests that there are no suitable target rocks that could provide a single source for the carbon in gardnos impactites however raman spectroscopy stable isotope analysis and transmission electron microscopy of the impact diamonds and graphitic carbon suggests that there were at least two episodes of c emplacement in gardnos impactites an impact related incorporation and shock transformation of graphitic material from target rocks followed by later mobilization of c possibly during postimpact cooling or later regional metamorphism

sea ice dynamics in the weddell sea simulated with an optimized model the advection of sea ice and associated freshwater salt fluxes in the weddell sea in 1986 and 1987 are investigated with a large scale dynamic thermodynamic sea ice model the model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the weddell sea ice the skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30 day sea ice drift a large number of sensitivity studies show how simulated sea ice transports and associated freshwater salt fluxes respond to variations in physical parameterizations the model reproduces the observed ice drift well provided ice dynamics parameters are set to appropriate values optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters the optimized model yields a mean northward sea ice volume export out of the southern weddell sea of 1693 km3 in 1986 and 2339 km3 in 1987 this shows the important role of sea ice transport for the freshwater budget of the weddell sea and gives an indication of its high interannual variability

anatomy of impactites and shocked zircon grains from dhala reveals paleoproterozoic meteorite impact in the archean basement rocks of central india abstract the dhala structure in central india has been a topic of global interest ever since the report of an ancient meteorite impact event there here we present an integrated study of the petrology geochemistry and zircon u pb zircon geochronology and rare earth element geochemistry from the structure along with and an analysis of the grain morphology and textural features our results provide new insight into the nature and timing of the impact event the zircon grains from the impactites show textures typical of shock deformation which we correlate with the impact event we also identified the presence of reidite based on raman spectroscopy and characteristics such as a persistent planar fracture bright backscattered electron images and a lack of zoning which are all diagnostic features of this mineral formed during an impact event our zircon u pb data from the various rock types in the basement show magma emplacement at ca 2 5 2 47 ga and the pb loss features suggest that the impact might have occurred between ca 2 44 ga and ca 2 24 ga another minor group of late paleoproterozoic zircons with concordant ages of 1826 and 1767 ma in the brecciated quartz reefs along the margins of the impact crater from unfractured grains represent an younger thermal event after the impact the rare earth element patterns of the neoarchean to early paleoproterozoic zircon population reflect the effects of hydrothermal alteration on a peralkaline host rock the abnormally high concentration of k2o in the impactite up to 15 91 wt is also consistent with metasomatic alteration associated with the impact event

convectively driven turbulent mixing in the upper ocean abstract two experiments were performed to study the characteristics of turbulence in convective mixed layers in the upper ocean in the first a diurnal convective mixed layer developed in the bahamas under the influence of the cycle of daytime solar heating and nighttime evaporative cooling the mixed layer reached as deep as 100 m each night and restratified each day in the second the mixed layer of a warm core gulf stream ring deepened from less than 50 m to more than 150 m in a little more than a day when subjected to rapid cooling during a cold air outbreak although individual profiles of the rate of viscous dissipation of turbulent kinetic energy had considerable spatial and temporal variability the mean dissipation profiles were similar to those in convecting atmospheric boundary layers the is established by the surface buoyancy flux jb0 jb0 0 61 and 0 72 in the bahamas and ring data respectively compared with 0 64 in convecting atmospheric mixed layers the mean profiles decre

chicxulub crater source for shocked zircon crystals from the cretaceous tertiary boundary layer saskatchewan evidence from new u pb data new u pb data for five of six individual shocked zircon grains from the cretaceous tertiary k t boundary fireball layer rock creek site south central saskatchewan yield a source age of 548 6 ma the collinear data points are variably discordant in proportion to the intensity of shock metamorphism exhibited by each zircon the age and zircon shock features establish a strong temporal and genetic link with k t distal ejecta from the fireball layer at the berwind canyon site in the raton basin colorado where texturally similar zircons from a previous study gave an indistinguishable source age of 544 5 ma these ages are in agreement with independent ages obtained from a previous study for single shocked zircons from the chicxulub crater mexico and from beloc haiti the u pb zircon data unequivocally support a meteorite impact origin for the global k t boundary layer and imply that the chicxulub crater is the sole north american target source

isotope systematics and shock wave metamorphism ii u pb and rb sr in naturally shocked rocks the haughton impact structure canada minerals and whole rocks from crystalline fragments collected in the allochthonous polymict breccia of the 23 ma old haughton impact crater on devon island arctic canada were analysed for u pb k and rb sr in order to constrain degrees of shock wave induced perturbations in these systems the results show that shock wave metamorphism at 50 gpa with onset of total melting does not significantly affect the u pb chronometers in monazite and zircon monazite preserved its age of crystallization during high grade metamorphism at 1928 2 2 1 2 0 ma 2 errors as dated by the upper intercept of 2 9 discordant monazites the same analyses yield a lower intercept age of 277 5 35 1 35 0 ma in the concordia diagram demonstrating that discordancy is not a direct effect of shock wave metamorphism but a product of time integrated pb diffusion prior to the impact event which occurred 23 ma ago this constrains maximal shock induced pb loss to less than 3 including possible alteration related pb diffusion from the shock damaged grains in 1903 1 1 4 1 1 ma old monazite from an unshocked reference gneiss recent pb loss also reaches 3 corroborating the occurrence of weak alteration related upb fractionation the data substantiate that u pb discordancy patterns are not a reliable chronometer to date impact events small rock fragments and a coherent block of shock stage i to iii vi consist of heterogeneous unequilibrated rb sr sub systems on the 10 m scale with drastically varying isr shock related rbsr fractionation is very limited and in the large block stage iii three biotite fractions yield an rb sr isochron age of 1791 34 ma 2 which is undistinguishable from the 1814 8 ma age of biotite in the unshocked basement gneiss in contrast to this m3 sized fragment rb sr systematics in a fist size sample of shock stage iii do not yield any reliable age information due to small scale 1 cm3 isotopic disequilibrium it appears that such small scale disequilibrium is essentially due to a take up of radiogenic 87sr in feldspar glass from biotite during shock wave passage this prevents dating of the impact events by the rb sr internal isochron method and it can be concluded that total rock melting is the first condition necessary to reset the rb sr chronometer in shocked crystalline rocks

nanosims study of seismically deformed zircon evidence of y yb ce and p redistribution and resetting of radiogenic pb lattice defects in zircon can cause trace elements redistribution and disturbance of isotopic systems this study investigates in detail how seismically induced deformation microstructures in zircon correlate with trace element and isotope re equilibration felsic mylonites with pseudotachylyte veins from the ivrea verbano zone in northern italy were studied with special focus on deformed zircon we have revealed the following post growth deformation microstructures planar deformation bands pdbs planar fractures pfs non planar fractures both healed and open and finite strain patterns pdbs are planar portions of crystal lattice that are strictly parallel to 100 crystallographic planes and are rotated to up to 3 with respect to the host grain they are from 0 5 to 1 m wide and have average spacing of 5 m pdbs originate in seismically active environment at elevated differential stress strain rate and temperature several grains in which pdbs are observed were analyzed with ion microprobe ion maps indicate redistribution of radiogenic pb isotopes associated with pdb formation isotopic redistribution preferably occurs in pdbs with larger crystallographic misorientation profiling demonstrated clear spatial correlation of pdbs with variations of ree abundances both gain and loss and possible correlations with increased and decreased hf ti and p abundances trace elements can be depleted or enriched compared to the abundance in surrounding matrix in deformed domains depending on the spacing of pdbs and the proximity of the analyzed volume to grain boundary or to detrital core 207 pb 206 pb ratio demonstrates systematic pb loss in the pdb bearing lattice domains with respect to pdb free domains in some cases pb gain is observed where the pdbs source radiogenic pb from older detrital cores our study has important implications for geochronology and microchemistry of zircon from seismically deformed sections of ivrea verbano zone and from other paleo seismic zones of the world zircon found in seismically deformed rocks near pseudotachylyte veins may demonstrate distorted and even reset isotopic ages and altered trace element abundances enhanced trace element exchange between deformed zircon and host mylonite can influence mass balance calculations for the bulk rock

stratified ekman layers under fair weather conditions the time averaged wind driven current forms a spiral in which the current vector decays and turns clockwise with increasing depth these spirals resemble classical ekman spirals they differ in that their rotation depth scale exceeds the e folding depth of the speed by a factor of 2 to 4 and they are compressed in the downwind direction a related property is that the time averaged stress is evidently not parallel to the time averaged vertical shear we develop the hypothesis that the flat spiral structure may be a consequence of the temporal variability of stratification within the upper 10 20 m of the water column this variability is associated primarily with the diurnal cycle and can be treated by a time dependent diffusion model or a mixed layer model the latter can be simplified to yield a closed solution that gives an explicit account of stratified spirals and reasonable hindcasts of midlatitude cases at middle and higher latitudes the wind driven transport is found to be trapped mainly within the upper part of the ekman layer the diurnal warm layer at tropical latitudes the effects of diurnal cycling are in some ways less important and ekman layer currents are likely to be significant to much greater depths in that event the lower part of the ekman layer is likely to be affected by stratification variability that may be nonlocal

deformational features and impact generated breccia from the sierra madera impact structure west texas deformational features from the sierra madera impact structure west texas were studied to estimate shock temperature and shock pressure values involved during its formation sandstone limestone and polymict impact generated breccia samples occur within the central uplift the sandstones host several shock related deformational features including toasted quartz grains and planar microstructures planar fractures and planar deformation features in quartz zircon mineral grains separated from the sandstone samples provide evidence of shock deformation in the form of planar fractures and granular surface textures both sandstone and carbonate within the central uplift contain shatter cones whereas polymict impact generated breccia samples contain shatter cone fragments and clasts of mixed lithology most high temperature features e g high temperature mineral phases and a melt sheet typically associated with less eroded structures of similar size are lacking however some small millimeter size possible devitrified glass clasts within polymict breccias occur at several locations deformational features within zircon mineral grains shatter cones and quartz grains with multiple planar microstructures record pressure and temperature conditions during the impact of 3 20 gpa and postshock temperatures between 350 and 1000 c deformational conditions during the impact event were almost certainly higher given that erosion has presumably removed most of the more severely shocked rocks

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

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

melt bearing impactites suevite and impact melt rock within the gardnos structure norway abstract melt bearing impactites dominated by suevite and with a minor content of clast rich impact melt rock are found within the central part of the gardnos structure they are preserved as the eroded remnants in the relatively small complex impact structure with a present diameter of 5 km these rocks have been mapped in the field and in the branden drill core and described according to mineralogy petrology including matrix litho clast and melt content as well as geochemistry based on our extensive field mapping a simple 3 d model of the original crater was constructed to estimate tentative volumes for the melt bearing impactites the variations in lithic and melt fragment content and chemistry of suevite matrix can mostly be explained by incorporation of mafic rocks into a dominant mixture of granitic gneissic and quartzitic target rocks reflecting mixing of material from different parts of the crater melt fragments within suevite occur with a variety of shapes and textures probably related to different original target rock composition to the various temperatures the individual fragments were subjected to during the impact event and deposition processes this study discusses the impact related deposits based on a sedimentological approach their overall composition and structures indicate dominating gravity flow processes in the final transportation and deposition of the suevite

the petrology and chronology of nwa 8009 impact melt breccia implication for early thermal and impact histories of vesta abstract studies of petrology mineralogy and geochronology of eucrites are keys to reconstruct the thermal and impact history of 4 vesta the proposed parent body for hed meteorites here we report the petrography mineralogy and geochemistry of nwa 8009 a newly found eucritic impact melt breccia and present sims u pb ages of zircon and phosphates nwa 8009 consists of coarse and fine grained lithic and mineral clasts set in fine grained recrystallized matrix it was derived from a protolith of monomict non cumulate eucrite evidence for intense shock metamorphism observed in nwa 8009 includes mosaicism deformed exsolution lamellae and partial melting of pyroxene melting and incipient flow of plagioclase planar fractures and granular textures of zircon these shock effects indicate nwa 8009 was subjected to an impact metamorphism with peak pressure of 50 60 gpa and post shock temperature of 1160 1200 c nwa 8009 is among the most intensely shocked heds reported yet after the impact the sample was buried near the surface in target rocks and experienced rapid cooling 23 c h and annealing resulting in recrystallization of the matrix and devitrification of plagioclase and silica glasses u pb isotopic system of apatite within plagioclase groundmass of lithic clasts is completely reset and constrains the timing of impact at 4143 61 ma providing a new robust impact age on vesta combined with the presence of synchronous impact resetting events especially those recorded by lu hf sm nd and pb pb isotopic systems we identified a period of high impacts flux at ca 4 1 4 2 ga on vesta this impact flux occurred coincident with the uptick at ca 4 1 4 2 ga in impact age spectra of the moon probably reflects widespread intense bombardment throughout the inner solar system at ca 4 1 4 2 ga based on evidence from zircon chemical zoning petrographic occurrences as well as the distinctive zr hf ratios we suggested that zircons in nwa 8009 have had a metamorphic instead of magmatic origin they mainly crystallized from melts produced by partial melting of mesostasis area due to reheating event during early global thermal metamorphism rather than by zr release from zr rich minerals the u pb isotopic system in zircons was not disturbed by subsequent impacts the weighted mean 207 pb 206 pb age of 4560 8 ma represents the timing of zircon growth during thermal metamorphism zircons from nwa 8009 and other eucrites may share a common origin during metamorphic growth events and constraining the global thermal metamorphism on vesta at ca 4 55 ga the main heat sources responsible for global metamorphism in basaltic crust of vesta might be heating from the hot interior especially heat flow related to magmatism rather than impact

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

planar microstructures in zircon from paleo seismic zones pseudotachylytes resulted from frictional melts associated with ultramylonites in high grade metapelitic rocks from the ivrea verbano zone in the southern alps northern italy were studied with focus on the deformation microstructures in zircon the aims were to investigate the characteristics of zircon deformation in seismic zones and to recognize specific microstructures generated in zircon during earthquakes which could be useful for mineral dating of paleo seismic events helps to understand how seismic energy is released at depth and interacts with metamorphic processes the interior of polished zircon grains ranging from 30 to 150 m in length were investigated with optical microscope and scanning electron microscope sem techniques including secondary electron se backscattered electron bse forward scattered electron fse cathodoluminescence cl imaging and crystallographic orientation mapping by electron backscatter diffraction analysis ebsd grains were studied in situ and as separated fractions embedded in epoxy disks among different cataclastic and crystal plastic deformation microstructures in zircon we identified characteristic planar deformation bands pdbs planar fractures pfs and curviplanar fractures cfs planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain which varies from 0 4 to 2 7 pdbs are usually parallel to 100 crystallographic planes have width from 0 3 to 1 m and average spacing of 5 m in 2d sections planar deformation bands appear as contrast lamellae in orientation contrast images and in ebsd maps and in rare cases can be observed with the optical microscope pdbs form in specifically oriented grains due to high differential stresses high temperatures and high strain rates generated in seismically active environment and or due to shearing in the vicinity of frictional melts discovered structures represent a result of crystal plastic deformation of zircon grains with operating dislocations having 010 glide system and misorientation axis therefore they can be classified as a new type 4 lattice distortion pattern according to the existing classification for zircon piazolo et al 2012 kovaleva et al 2014 we have demonstrated that formation of planar fractures in zircon takes place not only during impacts but also in seismically active zones we observe at least two cases of formation of pfs with 100 orientation 1 as a result of evolution of pdbs and 2 as micro cleavage this study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events but also as a result of seismic events at depth due to unusually high differential stress strain rate and temperature according to the new findings pdbs in zircon from the deep crust are supposed to represent newly recognized evidence of seismicity

ocean heat flux in the central weddell sea during winter abstract seasonal sea ice which plays a pivotal role in air sea interaction in the weddell sea a region of large deep water formation with potential impact on climate depends critically on heat flux from the deep ocean during the austral winter of 1994 an intensive process oriented field program named the antarctic zone flux experiment measured upper ocean turbulent fluxes during two short manned ice drift station experiments near the maud rise seamount region of the weddell sea unmanned data buoys left at the site of the first manned drift provided a season long time series of ice motion mixed layer temperature and salinity plus a truncated high resolution record of temperature within the ice column direct turbulence flux measurements made in the ocean boundary layer during the manned drift stations were extended to the ice ocean interface with a mixing length model and were used to evaluate parameters in bulk expressions for interfacial stress a rossby similarity drag law and ocean to

the gardnos impact structure norway petrology and geochemistry of target rocks and impactites abstract the gardnos structure norway is an approximately circular area of anomalously fractured and brecciated rock about 4 5 km in diameter emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite the original recognition of gardnos as a deeply eroded impact structure between 900 and 400 ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks impactites deep erosion of the structure has erased the original rim removed much of the crater fill deposits and exposed large areas at or near the original crater floor however a wide variety of distinctive impactites fractured and blackened quartzites in the sub crater basement rocks lithic breccias and melt bearing breccias are still preserved these impactites show petrographic and chemical characteristics that confirm an impact origin distinctive planar deformation features pdfs in quartz and feldspar incipient melting of feldspar clasts in the melt bearing breccias close matches between the chemical composition of the breccias and mixtures of the target lithologies and the detection of an extraterrestrial component a minor extraterrestrial component 0 15 was detected in the melt bearing breccias based on significantly elevated ir and os contents and lower 187os 188os ratios compared to those in the target rocks the gardnos impactites are significantly enriched in c 5 10x over the exposed target rocks this may reflect the presence of a c rich shale overlying the metamorphic basement at the time of impact this idea is supported by 13c values of 28 1 to 31 5 measured in the impactites mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60 90 wt granite gneiss 0 30 wt amphibolite 0 12 wt quartzite and 3 19 wt of a c rich shale component the deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult one possible scenario involves the impact of a 300 m diameter stony meteorite that released 1019 j of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m the original crater was filled with at least 0 3 km3 of allochthonous melt matrix breccias containing about 0 06 km3 of impact melt the structure underwent low grade greenschist metamorphism in caledonian time about 400 ma ago and was subsequently eroded to its present appearance the value of the gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone in the preservation of a possibly complete sequence of crater fill breccias beneath a cap of elastic sediments and in the unusual carbon enrichment of its impactites

lagrangian pathways of upwelling in the southern ocean the spatial and temporal variability of upwelling into the mixed layer in the southern ocean is studied using a 1 10 ocean general circulation model virtual drifters are released in a regularly spaced pattern across the southern ocean at depths of 250 500 and 1000 m during both summer and winter months the drifters are advected along isopycnals for a period of four years unless they outcrop into the mixed layer where lateral advection and a parameterization of vertical mixing is applied the focus of this study is on the discrete exchange between the model mixed layer and the interior localization of interior mixed layer exchange occurs downstream of major topographic features across the indian and pacific basins creating hotspots of outcropping minimal outcropping occurs in the atlantic basin while 59 of drifters outcrop in the pacific sector and in drake passage the region from 140 w to 40 w a disproportionately large amount even when considering the relative basin sizes due to spatial and temporal variations in mixed layer depth the lagrangian trajectories provide a statistical measure of mixed layer residence times for each exchange into the mixed layer the residence time has a rayleigh distribution with a mean of 30 days the cumulative residence time of the drifters is 261 194 days over a period of four years these results suggest that certain oceanic gas concentrations such as co 2 and 14 c will likely not reach equilibrium with the atmosphere before being re subducted

preferential dissolution of 234u and radiogenic pb from α recoil damaged lattice sites in zircon implications for thermal histories and pb isotopic fractionation in the near surface environment abstract zircons from two archean samples a 2732 ma trondhjemite and a 2728 ma dacite both from the western superior province were washed in hydrofluoric acid hf at moderate temperatures zircon residues and hf wash solutions were analyzed for u pb isotope systematics and 234u 238u ratios in an attempt to study the effect of hf washing on selective dissolution of radiogenic isotopes in areas damaged by recoil on a conventional concordia diagram the datum from one analyzed wash solution plots below concordia near the array of discordant data from magnetic unabraded zircon fractions which record a lower concordia intersection of about 230 ma data points from hf wash residues of two different zircon fractions from the trondhjemite are also discordant and collinear with near concordant data from abraded unleached fractions which record the true age of crystallization however this array has a negative lower concordia intercept of 749 ma the residue and wash from hf treatment of a single zircon fraction from the dacite are collinear with the crystallization age of the sample as defined by near concordant data from unleached fractions this line has a lower concordia intersection of 1388 ma also negative but distinct from the trondhjemite residue line the discordance of strongly washed residues is proportional to the calculated recoil dosage of the grains leach residues show negative 234u 238u anomalies while the leaches themselves show 234u excess on the order of 10 these results can be explained by the bulk dissolution of altered and metamict crystal domains during the hf wash accompanied by leaching of 234u and radiogenic pb from recoil damaged lattice sites in alteration free zircon residues studies were also carried out on zircons washed in nitric acid these show small ca 0 5 negative 234u anomalies this suggests that small amounts of radiogenic pb can be leached from damaged lattice sites by nitric acid the negative age lower concordia intercepts of pb loss lines defined by hf leach residues can be explained if early radiation damage has been annealed so that only more recently formed radiogenic pb atoms are in damaged crystal lattice sites and are susceptible to leaching since this pb fraction is relatively young the residue pb is biased toward anomalously old values a model is proposed for early burial and heating of the rocks above the annealing temperature for recoil damage followed by uplift and cooling several hundred million years after crystallization this model predicts a cooling time of 2500 20 ma for the trondhjemite and 2295 35 ma for the rhyolite comparison with metamorphic and cooling histories from ar ar studies suggest that the long term annealing temperature for low levels of recoil damage in zircon is in the range 250 300 c approximately the same as for fission tracks these results demonstrate that hf and possibly to a small extent hno3 can preferentially leach radioactive daughter elements from damaged zircon lattice sites in addition to dissolving metamict domains the threshold recoil dosage for dissolution of damaged zircon appears to be slightly in excess of 1 5 1015 decay events mg the isotopic composition of the leached pb is sensitive to the annealing history of the zircon thus in addition to supplying the magmatic crystallization age of the sample the hf leaching of zircon might be useful for constraining its later low temperature thermal history leaching from recoil damaged sites also occurs in the natural environment and this may lead to significant enhancement of 206pb 204pb in groundwater

internal waves and tides in the western weddell sea observations from ice station weddell the upper ocean current and temperature fields in the western weddell sea were measured from the drifting pack ice at ice station weddell 1 isw and nearby sites using a vertical profiler and an array of moored sensors in january june 1992 these data document the structure and variability of the internal gravity wave field and tidal currents in this remote region the variance of the internal wave continuum frequency n at isw was 0 2 0 6 of the garrett munk gm universal level for the first 60 days increasing to near gm levels during the final 10 days of the deployment in contrast the energy density at site c 50 km west of isw and farther up the continental slope was always near gm levels variations may be due to a combination of spatial and temporal gradients of the internal wave field at isw coherence between vertically separated sensors was used to estimate vertical wave number bandwidth energy and bandwidth estimates are compared with previous studies in both ice covered and temperate oceans using our measurements of the internal wave field and existing parameterizations of mixing we estimate the vertical heat flux from the warm deep water toward the surface at isw the upward heat flux due to mixing associated with the internal waves was about 1 w m 2 much less than the 20 w m 2 average flux required to balance the heat budget for the weddell gyre tidal currents contributed significantly to the total measured horizontal velocity variance the tides were primarily barotropic and increased toward the west in both the semidiurnal and diurnal frequency bands it is suggested that the stronger tidal currents to the west over the shallower water of the upper continental slope are indirectly responsible for the higher internal wave energy at site c relative to isw

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

fingerprinting the k t impact site and determining the time of impact by upb dating of single shocked zircons from distal ejecta abstract u pb isotopic dating of single 1 3 g zircons from k t distal ejecta from a site in the raton basin colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site data for the least shocked zircons are slightly displaced from the 544 5 ma primary age for a component of the target site white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65 5 3 0 ma such shocked and granular zircons have never been reported from any source including explosive volcanic rocks zircon is refractory and has one of the highest thermal blocking temperatures hence it can record both shock features and primary and secondary ages without modification by post crystallization processes unlike shocked quartz which can come from almost anywhere on the earth s crust shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers with u pb zircon dating it is now possible to correlate ejecta layers derived from the same target site test the single versus multiple impact hypothesis and identify the target source of impact ejecta the ages obtained in this study indicate that the manson impact site iowa which has basement rocks that are mid proterozoic in age cannot be the source of k t distal ejecta the k t distal ejecta probably originated from a single impact site because most grains have the same primary age

observations of mixed layer deepening during an antarctic gale 1 observations of mixed layer deepening made during a gale in february 2005 near an ice shelf fimbulisen antarctica are reported the observations were made from the rrs james clark ross in the lee of the ice shelf using repeated downcasts yo yo of a conductivity temperature depth package together with shipboard meteorological and other measurements the mixed layer deepened from less than 40 m to over 120 m over the course of 27 h with a very rapid deepening from 80 m to 120 m over a period of under 11 h the mixed layer became both colder and fresher with the change in salinity and heat content likely to be caused by melting ice oxygen isotope results suggest the source of the fresh water was melting sea ice rather than precipitation or ice shelf melt the input of melt water at the surface stabilizes the mixed layer so extra energy is required to deepen the mixed layer the observations suggest that approximately 1 8 of the available wind work energy was used to mix the upper water column while the stabilizing surface buoyancy flux inhibits the turbulence in the mixed layer limiting the mixing length to 1 6 m the eventual depth of the mixed layer is in line with estimates based on the planetary length scale u f the rate of mixed layer deepening is given by ue u 0 035 the apparent peak ice melting rate was approximately 60 mm hr 1 although this is likely to be exaggerated by convergence and downwelling