extraction of potential energy from geostrophic fronts by inertial symmetric instabilities abstractsubmesoscale oceanic density fronts are structures in geostrophic and hydrostatic balance which are prone to inertial and or symmetric instabilities we argue in this article that drainage of potential energy from the geostrophic flow is a significant source of their growth we illustrate our point with two dimensional boussinesq numerical simulations of oceanic density fronts on the f plane a set of two dimensional initial conditions covers the submesoscale portion of a three dimensional parameter space consisting of the richardson and rossby numbers and a measure of stratification or latitude because we let the lateral density gradient decay with depth the parameter space map is nontrivial excluding low rossby low richardson combinations dissipation and the presence of boundaries select a growing mode of inertial symmetric instability consisting of flow cells that disturb isopycnal contours systematically these isopycnal displacements correspond to a drainage of potential energy from th

enhanced turbulence and energy dissipation at ocean fronts the ocean surface boundary layer mediates air sea exchange in the classical paradigm and in current climate models its turbulence is driven by atmospheric forcing observations at a 1 kilometer wide front within the kuroshio current indicate that the rate of energy dissipation within the boundary layer is enhanced by one to two orders of magnitude suggesting that the front rather than the atmospheric forcing supplied the energy for the turbulence the data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence the resulting boundary layer is stratified in contrast to the classically well mixed layer these effects will be strongest at the intense fronts found in the kuroshio current the gulf stream and the antarctic circumpolar current all of which are key players in the climate system

direct observations of microscale turbulence and thermohaline structure in the kuroshio front 1 direct observations of microstructure near the kuroshio front were conducted in august 2008 and october 2009 these show negative potential vorticity pv in the mixed layer south of the front where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind scaling these elevated dissipation rates scale better with an empirical scaling which considers local wind and ekman buoyancy flux driven by downfront wind near zero pv in the thermocline under the kuroshio mainstream is observed at 200 300 m depth with dissipation exceeding open ocean thermocline values by factors of 10 100 overall the large turbulent dissipation rates measured in the kuroshio can be categorized into two groups one characterized by low richardson number along the kuroshio front thermocline and the other characterized by high stratification away from the kuroshio mainstream the former is attributed to mixing by unbalanced frontal ageostrophic flows and the latter is attributed to internal wave breaking on average both groups appear in regions of large horizontal density gradients observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues extending upward from 500 to 100 m depth in a narrow 20 km zone suggesting down and upwelling driven by geostrophic straining which is confirmed by quasigeostrophic omega equation solutions this implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the kuroshio front likely contribute to npiw formation

evidence of enhanced turbulent dissipation in the frontogenetic kuroshio front thermocline 1 enhanced turbulent dissipation o 10 8 10 7 wkg 1 in the thermocline on the cyclonic side of the kuroshio front was observed during a period of frontogenesis using a microstructure profiler xbt and adcp along 143 e across the kuroshio front in august 2008 the eddy diffusivity corresponding to the mixing below the central jet is estimated to be o 10 4 10 3 m2 s 1 the strong turbulent mixing we observed in the kuroshio is in sharp contrast to previous field measurements which found that small scale diapycnal mixing in western boundary currents remains at levels typically measured in the open ocean thermocline the turbulence in the kuroshio is attributed to frontogenesis arising out of an estimated confluence rate of o 10 5 s 1 based on satellite altimeters suggesting the ubiquity of a forward energy cascade from mesoscale to microscale turbulence near ocean fronts as indicated by recent theoretical studies

intensification of ocean fronts by down front winds abstract many ocean fronts experience strong local atmospheric forcing by down front winds that is winds blowing in the direction of the frontal jet an analytic theory and nonhydrostatic numerical simulations are used to demonstrate the mechanism by which down front winds lead to frontogenesis when a wind blows down a front cross front advection of density by ekman flow results in a destabilizing wind driven buoyancy flux wdbf equal to the product of the ekman transport with the surface lateral buoyancy gradient destabilization of the water column results in convection that is localized to the front and that has a buoyancy flux that is scaled by the wdbf mixing of buoyancy by convection and ekman pumping suction resulting from the cross front contrast in vertical vorticity of the frontal jet drive frontogenetic ageostrophic secondary circulations ascs for mixed layers with negative potential vorticity the most frontogenetic ascs select a preferred cross front width and do not translate with

petrology and raman spectroscopy of high pressure phases in the gujba cb chondrite and the shock history of the cb parent body abstract high pressure phases majorite possibly majorite pyropess wadsleyite and coesite are present in the matrix and in barred olivine fragments in the gujba cb chondrite grossular pyrope was also observed in some small inclusions the cb chondrites are metal rich meteorites with characteristics that sharply distinguish them from other chondrite groups all of the cb chondrites contain impact melt regions interstitial to their chondrules fragments and metal and a major impact event or events on the cb chondrite parent body is clearly a significant stage in its history we studied three areas interstitial to chondrules and metal in the gujba cba chondrite from raman spectra the barred olivine fragments and matrix in these regions have various combinations of olivine and low ca pyroxene as well as majorite garnet mg4si4o12 a phase that forms by high pressure transformation of low ca pyroxene and wadsleyite a high pressure product of olivine compositions of the majorite suggest both majorite and majorite pyrope solid solution may be present the mineral assemblage of majorite and wadsleyite suggest minimum shock pressures and temperatures of 19 gpa and 2000 c respectively the occurrences of high pressure phases are variable from one area to another on the scale of millimeters or less suggesting heterogeneous distribution of shock and or back transformation to low pressure polymorphs throughout the meteorite the high pressure phases record a high temperature pressure impact event that is superimposed onto and thus postdates formation of the chondrules and other components in the cb chondrites the barred chondrules and metal in the cb chondrites are primary materials formed prior to the impact event either generated in an earlier planetesimal scale impact event or in the nebula

an early i xe age for cb chondrite chondrule formation and a re evaluation of the closure age of shallowater enstatite the iodine xenon system has been analyzed in samples of 7 chondrules from the cb chondrites gujba and hammadah al hamra hah 237 one sample from gujba defined a high temperature iodine xenon isochron corresponding to closure 1 87 0 4 ma before closure of shallowater enstatite motivated by this result we employ outlier rejection to re evaluate the shallowater age leading to a modified value of 4562 3 0 4 ma 1 in this process the datum obtained by combining our i xe age for gujba with the literature pb pb age is rejected as an outlier indicating that in this sample the i xe system closed earlier than the accepted pb pb age of chondrules from cb chondrites the need for a formation environment distinct from that of chondrules from other meteorites is thus reduced

verification and interpretation of the i xe chronometer abstract comparisons of i xe and pb pb relative ages are made for phosphate and feldspar separates from 12 different meteorites in all cases where i xe ages can be measured in phosphate i xe and pb pb chronometers agree within experimental uncertainty no discordant phosphate samples are observed with the exception of allegan i xe ages for feldspar separates generally agree with pb pb ages for the corresponding phosphate the general concordancy observed between i xe and pb pb chronometers suggests that the i xe system is a reliable and interpretable chronometer when applied to single minerals systems allegan feldspar differs from the other feldspar separates predating its phosphate by 17 ma with an i xe age indistinguishable from those of orgueil and murchison magnetites among the oldest observed

parameterization of mixed layer eddies part i theory and diagnosis ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean in this paper a parameterization for the restratification driven by finite amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal the streamfunction is proportional to the product of the horizontal density gradient the mixed layer depth squared and the inertial period hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence in this paper the parameterization is theoretically motivated confirmed to perform well for a wide range of mixed layer depths rotation rates and vertical and horizontal stratifications it is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification two companion papers discuss the numerical implementation and the climate impacts of this parameterization

destruction of potential vorticity by winds the destruction of potential vorticity pv at ocean fronts by wind stress driven frictional forces is examined using pv flux formalism and numerical simulations when a front is forced by downfront winds that is winds blowing in the direction of the frontal jet a nonadvective frictional pv flux that is upward at the sea surface is induced the flux extracts pv out of the ocean leading to the formation of a boundary layer thicker than the ekman layer with nearly zero pv and nonzero stratification the pv reduction is not only active in the ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low pv from the ekman layer with high pv from the pycnocline extraction of pv from the pycnocline by the secondary circulations results in an upward advective pv flux at the base of the boundary layer that scales with the surface nonadvective frictional pv flux and that leads to the deepening of the layer at fronts forced by both downfront winds and a destabilizing atmospheric buoyancy flux f b the critical parameter that determines whether the wind or the buoyancy flux is the dominant cause for pv

variability in satellite winds over the benguela upwelling system during 1999 2000 1 wind stress variability over the benguela upwelling system is considered using 16 months 01 august 1999 to 29 november 2000 of satellite derived quikscat wind data variability is investigated using a type of artificial neural network the self organizing map som and a wavelet analysis the som and wavelet analysis are applied to an extracted data set to find that the system may be divided into six discrete wind regimes the wavelet power spectra for these wind regions span a range of frequencies from 4 to 64 days with each region appearing to contain distinct periodicities to the north 10 23 5 s the majority of the power occurs during austral winter with a 4 16 day periodicity further investigation of national centers for environmental prediction reanalysis outgoing longwave radiation data indicates that the winter intensification of wind stress off the angolan coast is linked with convective activity over equatorial west africa the summer activity appears to be linked with the intensification of the angolan heat low convective activity over the congo basin appears to impact upon wind stress variability off the angolan coast throughout the year farther south 24 35 s the majority of the power occurs in the summer here a bimodal distribution occurs with peaks of 4 12 and 25 50 days the southernmost regions appear to be forced at higher frequencies by both midlatitude cyclones austral winter and mesoscale coastal lows austral summer at lower frequencies eastward propagating periodic wind events that originate over eastern south america appear to be important to the forcing of wind stress over the southern benguela

submesoscale cold filaments in the gulf stream abstracta set of realistic very high resolution simulations is made for the gulf stream region using the oceanic model regional oceanic modeling system roms to study the life cycle of the intense submesoscale cold filaments that form on the subtropical gyre interior wall of the gulf stream the surface buoyancy gradients and ageostrophic secondary circulations intensify in response to the mesoscale strain field as predicted by the theory of filamentogenesis it can be understood in terms of a dual frontogenetic process along the lines understood for a single front there is however a stronger secondary circulation due to the amplification at the center of a cold filament filament dynamics in the presence of a mixed layer are not adequately described by the classical thermal wind balance the effect of vertical mixing of momentum due to turbulence in the surface layer is of the same order of magnitude as the pressure gradient and coriolis force and contributes equally to a so called turbulent therm

iodine xenon chemical and petrographie studies of semarkona chondrules evidence for the timing of aqueous alteration we have performed inaa petrographie and noble gas analyses on seventeen chondrules from the semarkona meteorite ll3 0 primarily to study the relationship of the i xe system to other measured properties we observe a range of 10 ma in apparent i xe ages the three latest apparent ages fall in a cluster suggesting the possibility of a common event the initial 129i 127i ratio r0 is apparently related to chondrule type and or mineralogy with nonporphyritic and pyroxene rich chondrules showing evidence for lower r0 s later apparent i xe ages than porphyritic and olivine rich chondrules in addition chondrules with sulfides on or near the surface have lower r0s than other chondrules the 129xe 132xe ratio in the trapped xe component anticorrelates with r0 consistent with evolution of a chronometer in a closed system or in multiple similar systems on the basis of these correlations we conclude that the variations in r0 represent variations in ages and that later event s possibly aqueous alteration preferentially affected chondrules with nonporphyritic textures and or sulfide rich exteriors about 10 ma after the formation of the chondrules

a new metal rich chondrite grouplet abstract a new grouplet of primitive metal rich chondrites here called the cb c carbonaceous b bencubbinite chondrites has been recognized it includes bencubbin weatherford hammadah a1 hamra hh 237 and queen alexandra range que 94411 paired with que 94627 their mineral compositions as well as their oxygen and nitrogen isotopic compositions indicate that they are closely related to the cr and ch chondrites all of which are members of the more inclusive cr clan cb chondrites have much greater metal silicate ratios than any other chondrite group widely increasing the range of metal silicate fractionation recorded in solar nebular processes they also have the greatest moderately volatile lithophile element depletions of any chondritic materials metal has compositional trends and zoning patterns that suggest a primitive condensation origin in contrast with metal from other chondrite groups cb chondrites as well as other cr clan chondrites have much heavier nitrogen higher 15n 14n than that in other chondrite groups the primitive characteristics of the cb chondrites suggest that they contain one of the best records of early nebular processes another chondrite grosvenor mountains 9555 1 is petrographically similar to the cb chondrites but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir

mesoscale to submesoscale transition in the california current system part ii frontal processes this is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized equilibrium subtropical eastern boundary upwelling current system similar to the california current the emergent upper ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives using a combination of composite averaging and separation of distinctive subregions of the flow the initiating dynamical process for the transition is near surface frontogenesis the frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics i e uniform interior potential vorticity q and diagnostic force balance in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field however there are significant behavioral differences compared to the surface dynamics model wind stress acts on fronts through nonlinear ekman transport and creation and destruction of potential vorticity the strain induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events submesoscale vortices and excitation of even smaller scale flows intermittent submesoscale breakdown of geostrophic and gradient wind force balance occurs during the intense frontogenesis and frontal instability events

pb pb dating of individual chondrules from the cb a chondrite gujba assessment of the impact plume formation model the cb chondrites are metal rich meteorites with characteristics that sharply distinguish them from other chondrite groups their unusual chemical and petrologic features and a young formation age of bulk chondrules dated from the cba chondrite gujba are interpreted to reflect a single stage impact origin here we report high precision internal isochrons for four individual chondrules of the gujba chondrite to probe the formation history of cb chondrites and evaluate the concordancy of relevant short lived radionuclide chronometers all four chondrules define a brief formation interval with a weighted mean age of 4562 49 0 21 myr consistent with its origin from the vapor melt impact plume generated by colliding planetesimals formation in a debris disk mostly devoid of nebular gas and dust sets an upper limit for the solar protoplanetary disk lifetime at 4 8 0 3 myr finally given the well behaved pb pb systematics of all four chondrules a precise formation age and the concordancy of the mn cr hf w and i xe short lived radionuclide relative chronometers we propose that gujba may serve as a suitable time anchor for these systems

pb isotopic age of the allende chondrules we have studied pb isotope systematics of chondrules from the oxidized cv3 carbonaceous chondrite allende the chondrules contain variably radiogenic pb with a 206pb 204pb ratio between 19 5 268 pb pb isochron regression for eight most radiogenic analyses yielded the date of 4566 2 2 5 ma internal residue leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566 6 1 0 ma our best estimate for an average age of allende chondrule formation this pb pb age is consistent with the range of model 26al 26mg ages of bulk allende chondrules reported by bizzarro et al 2004 and is indistinguishable from pb pb ages of ca al rich inclusions cais from cv chondrites 4567 2 0 6 ma amelin et al 2002 and the oldest basaltic meteorites we infer that chondrule formation started contemporaneously with or shortly after formation of cv cais and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids the entire period of chondrule formation lasted from 4566 6 1 0 ma allende to 4564 7 0 6 ma cr chondrite acfer 059 to 4562 7 0 5 ma cb chondrite gujba and was either continuous or consisted of at least three discrete episodes since chondrules in cb chondrites appear to have formed from a vapor melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated krot et al 2005 there were possibly a variety of processes in the early solar system occurring over at least 4 5 myr that we now combine under the umbrella name of chondrule formation

oxygen isotopic compositions of chondrules from the metal rich chondrites isheyevo ch cbb mac 02675 cbb and que 94627 cbb abstract it has been recently suggested that 1 ch chondrites and the cbb ch like chondrite isheyevo contain two populations of chondrules formed by different processes i magnesian non porphyritic cryptocrystalline and barred chondrules which are similar to those in the cb chondrites and formed in an impact generated plume of melt and gas resulted from large scale asteroidal collision and ii porphyritic chondrules formed by melting of solid precursors in the solar nebula 2 porphyritic chondrules in isheyevo and ch chondrites are different from porphyritic chondrules in other carbonaceous chondrites krot et al 2005 krot et al 2008a krot et al 2008b in order to test these hypotheses we measured in situ oxygen isotopic compositions of porphyritic magnesian type i and ferroan type ii and non porphyritic magnesian and ferroan cryptocrystalline chondrules from isheyevo and cbb chondrites mac 02675 and que 94627 paired with que 94611 using a cameca ims 1280 ion microprobe on a three isotope oxygen diagram 17o vs 18o compositions of chondrules measured follow approximately slope 1 line data for 19 magnesian cryptocrystalline chondrules from isheyevo 24 magnesian cryptocrystalline chondrules and 6 magnesian cryptocrystalline silicate inclusions inside chemically zoned fe ni metal condensates from cbb chondrites have nearly identical compositions 17o 2 2 0 9 2 3 0 6 and 2 2 1 0 2 respectively these observations and isotopically light magnesium compositions of cryptocrystalline magnesian chondrules in cbb chondrites gounelle et al 2007 are consistent with their single stage origin possibly as gas melt condensates in an impact generated plume in contrast 17o values for 11 type i and 9 type ii chondrules from isheyevo range from 5 to 4 and from 17 to 3 respectively in contrast to typical chondrules from carbonaceous chondrites seven out of 11 type i chondrules from isheyevo plot above the terrestrial fractionation line we conclude that i porphyritic chondrules in isheyevo belong to a unique population of objects suggesting formation either in a different nebular region or at a different time than chondrules from other carbonaceous chondrites ii isheyevo cb and ch chondrites are genetically related meteorites they contain non porphyritic chondrules produced during the same highly energetic event probably large scale asteroidal collision iii the differences in mineralogy petrography chemical and whole rock oxygen isotopic compositions between ch and cb chondrites are due to various proportions of the nebular and the impact produced materials

allan hills 85085 a subchondritic meteorite of mixed nebular and regolithic heritage abstract allan hills 85085 is a tiny 12 g meteorite whose closest relatives are the renazzo and al rais chondrites and the subchondritic host material in the bencubbin meteorite its bulk composition is generally chondritic but siderophile abundances are tens of per cent higher and abundances of moderately volatile elements factors of 2 4 lower than in known authentic chondrites in addition to this unusual composition alh85085 has an extremely small particle size metal that shows no evidence of metamorphic equilibration and a very high abundance of pyroxenitic lithic particles chondrules are rare and small most are pyroxenitic and chemically similar to the lithic particles previous workers have inferred that most or all of these strange features of alh85085 resulted from nebular processes but we suggest that melting vaporization outgassing condensation and size sorting in a cloud of impact ejecta offer a more viable alternative until additional similar materials have been discovered and characterized it seems safest to infer nebular properties and processes on the basis of the record in chondrites regarding which there is little doubt that nebular processes were dominant

symmetric instability in the gulf stream abstract analyses of wintertime surveys of the gulf stream gs conducted as part of the clivar mode water dynamic experiment climode reveal that water with negative potential vorticity pv is commonly found within the surface boundary layer sbl of the current the lowest values of pv are found within the north wall of the gs on the isopycnal layer occupied by eighteen degree water suggesting that processes within the gs may contribute to the formation of this low pv water mass in spite of large heat loss the generation of negative pv was primarily attributable to cross front advection of dense water over light by ekman flow driven by winds with a down front component beneath a critical depth the sbl was stably stratified yet the pv remained negative due to the strong baroclinicity of the current suggesting that the flow was symmetrically unstable a large eddy simulation configured with forcing and flow parameters based on the observations confirms that the observed structure of the sbl is consistent with the dynamics of symmetric instability si forced by wind and surface cooling the simulation shows that both strong turbulence and vertical gradients in density momentum and tracers coexist in the sbl of symmetrically unstable fronts si is a shear instability that draws its energy from geostrophic flows a parameterization for the rate of kinetic energy ke extraction by si applied to the observations suggests that si could result in a net dissipation of 33 mw m 2 and 1 mw m 2 for surveys with strong and weak fronts respectively the surveys also showed signs of baroclinic instability bci in the sbl namely thermally direct vertical circulations that advect biomass and pv the vertical circulation was inferred using the omega equation and used to estimate the rate of release of available potential energy ape by bci the rate of ape release was found to be comparable in magnitude to the net dissipation associated with si this result points to an energy pathway where the gs s reservoir of ape is drained by bci converted to ke and then dissipated by si and its secondary instabilities similar dynamics are likely to be found at other strong fronts forced by winds and or cooling and could play an important role in the energy balance of the ocean circulation

the i xe chronometer and the early solar system we review the development of the i xe technique and how its data are interpreted and specify the best current practices individual mineral phases or components can yield interpretable trends in initial 129i 127i ratio whereas whole rock i xe ages are often hard to interpret because of the diversity of host phases many of which are secondary varying standardizations in early work require caution only samples calibrated against shallowater enstatite or bjurble can contribute reliably to the emerging i xe chronology of the early solar system although sparse data for which i xe and mn cr can be compared suggest that the two systems are concordant among ordinary chondrite samples we derive a new age for the closure of the shallowater enstatite standard of 4563 3 0 4 myr from the relationship between the i xe and pb pb systems this yields absolute i xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system absolute i xe dates for aqueous and igneous processes are consistent with other systems consideration of the i xe host phases in cais and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre existing material the ranges of chondrule ages deduced from the al mg and i xe systems in semarkona ll3 0 and chainpur ll3 4 are consistent chainpur i xe data exhibit a greater range of ages than semarkona possibly reflecting a greater degree of parent body processing however individual chondrules show little or no evidence of such processing determining the host phase s responsible for high temperature correlations may resolve the issue

elevated mixing at a front 1 the mesoscale submesoscale and microscale structure of a front in the california current was observed using a towed vehicle outfitted with microconductivity sensors thirteen 60 km cross front sections from 0 to 350 m in depth were covered in 3 5 days objectively mapped data are fit via the omega equation to obtain vertical velocity a composite cross front section shows elevated mixing on the dense side within 10 20 km of the front water downwells and gradients are elevated there i e rossby number ro horizontal strain spice gradients and microscale thermal dissipation thermal eddy diffusivity kt reaches 10 3 m2 s 1 and increases 3 10 from the anticyclonic to the cyclonic side with a depth mean of 10 4 m2 s 1 the spatial structure of kt ro and are similar on the dense side suggesting an energy cascade from the mesoscale via the submesoscale to the microscale however it is unclear whether frontogenesis internal wave blocking by elevated vorticity or internal wave trapping by large produces the elevated mixing the mean turbulent heat flux opposes the mean restratifying mesoscale heat flux of 10 w m 2 and may allow the front to persist turbulent nitrate fluxes are 0 1 0 3 mmol m 2 s 1 chlorophyll fluorescence and beam transmission reveal a 6 km wide 100 km long alongfront streamer which is a deep biomass maximum time scales for mixing and nutrient fluxes are 0 3 3 days which are similar to phytoplankton growth rates and the time scale for frontal evolution

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

time dependent compaction as a mechanism for regular stick slips owing to their destructive potential earthquakes receive considerable attention from laboratory studies in friction experiments stick slips are studied as the laboratory equivalent of natural earthquakes and numerous attempts have been made to simulate stick slips numerically using the discrete element method dem however while laboratory stick slips commonly exhibit regular stress drops and recurrence times stick slips generated in dem simulations are highly irregular this discrepancy highlights a gap in our understanding of stick slip mechanics which propagates into our understanding of earthquakes in this work we show that regular stick slips emerge in dem when time dependent compaction by pressure solution is considered we further show that the stress drop and recurrence time of stick slips is directly controlled by the kinetics of pressure solution since compaction is known to operate in faults this mechanism for frictional instabilities directly relates to natural seismicity

ar ar and i xe ages and the thermal history of iab meteorites studies of several samples of the large caddo county iab iron meteorite reveal andesitic material enriched in si na al and ca which is essentially unique among meteorites this material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening such an origin implies extended metamorphism of the iab parent body new 39ar 40ar ages for silicate from three different caddo samples are consistent with a common age of 4 50 4 51 gyr less well defined ar ar degassing ages for inclusions from two other iabs eet elephant moraine 83333 and udei station are 4 32 gyr whereas the age for campo del cielo varies considerably over about 3 23 4 56 gyr new 129i 129xe ages for caddo county and eet 83333 are 4557 9 0 1 myr and 4557 4560 myr respectively relative to an age of 4562 3 myr for shallowater considering all reported ar ar degassing ages for iabs and related winonaites the range is 4 32 4 53 gyr but several iabs give similar ar ages of 4 50 4 52 gyr we interpret these older ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40ar diffusion loss the older ar ar ages for iabs are similar to sm nd and rb sr isochron ages reported in the literature for caddo county considering the possibility that iab parent body formation was followed by impact disruption reassembly and metamorphism e g benedix et al 2000 the ar ar ages and iab cooling rates deduced from ni concentration profiles in iab metal herpfer et al 1994 are consistent if the time of the postassembly metamorphism was as late as about 4 53 gyr ago however i xe ages reported for some iabs define much older ages of about 4558 4566 myr which cannot easily be reconciled with the much younger ar ar and sm nd ages an explanation for the difference in radiometric ages of iabs may reside in combinations of the following a i xe ages have very high closure temperatures and were not reset during metamorphism about 4 53 gyr ago b a bias exists in the 40k decay constants which makes these ar ar ages approximately 30 myr too young c the reported sm nd and rb sr ages for caddo are in error by amounts equal to or exceeding their reported 2 sigma uncertainties and d about 30 myr after the initial heating that produced differentiation of caddo silicate and mixing of silicate and metal a mild metamorphism of the iab parent body reset the ar ar ages

i xe systematics of the impact plume produced chondrules from the cb carbonaceous chondrites implications for the half life value of 129i and absolute age normalization of 129i 129xe chronometer it is inferred that magnesian non porphyritic chondrules in the cb bencubbin type carbonaceous chondrites formed in an impact generated plume of gas and melt at 4562 49 0 21 ma bollard et al 2015 and could be suitable for the absolute age normalization of relative chronometers here xenon isotopic compositions of neutron irradiated chondrules from the cb chondrites gujba and hammadah al hamra hh 237 have been analyzed in an attempt to determine closure time of their i xe isotope systematics one of the hh 237 chondrules 1 yielded a well defined i xe isochron that corresponds to a closure time of 0 29 0 16 ma after the shallowater aubrite standard release profiles and diffusion properties of radiogenic 129 xe and 128 xe extracted from this chondrule by step wise pyrolysis indicate presence of two iodine host phases with distinct activation energies of 73 and 120 kcal mol in spite of the activation energy differences the i xe isotope systematics of these two phases closed simultaneously suggesting rapid heating and cooling possibly quenching of the cb chondrules the release profiles of u fission xe and i derived xe correlate in the high temperature host phase supporting simultaneous closure of 129i 129xe and 207pb 206pb systematics the absolute i xe age of shallowater standard is derived from the observed correlation between i xe and pb pb ages in a number of samples it is re evaluated here using pb pb ages adjusted for an updated 238u 235u ratio of 137 794 and meteorite specific u isotope ratios with the addition of the new data for hh 237 chondrule 1 the re evaluated absolute i xe age of shallowater is 4562 4 0 2 ma the absolute i xe age of the hh 237 chondrule 1 is 4562 1 0 3 ma in good agreement with u corrected pb pb ages of the gujba chondrules bollard et al 2015 and hh 237 silicates krot et al 2005 all i xe data used here and in previous estimates of the absolute age of shallowater are calculated using 15 7 0 6 ma value for 129i half life the slopes of i xe pb pb correlation lines plotted for different sets of samples for shallowater normalization are always 1 assuming uranium half life values are correct this restricts the half life of 129i to 15 7 ma

old formation ages of igneous clasts on the l chondrite parent body reflect an early generation of planetesimals or chondrule formation abstract the barwell meteorite contains large abundant clasts that are igneous in nature we report iodine xenon ages of five clasts and one sample of host chondrite material the fragment of host chondrite material yielded the oldest age determined 4567 8 1 2 ma two clasts produced old well defined ages of 4564 96 0 33 ma and 4565 60 0 33 ma these and a third clast having a less precise old age of 4566 0 3 2 ma are interpreted as recording the timing of crystallisation of the samples they were incorporated into the barwell parent body before it underwent thermal metamorphism but the i xe ages survived secondary processing on the parent body and were not reset by metamorphism metasomatism or shock two further clasts record younger ages of 4560 96 0 45 ma and 4554 22 0 38 ma these samples contain a high abundance of albitic mesostasis and the most likely explanation of the ages is that they record the timing of metasomatism on the parent body we also analysed four host chondrite samples that do not give i xe ages in these samples the system appears to have been disturbed by shock it has been suggested previously that the igneous clasts are derived from an early generation of partially melted asteroids we do not have direct evidence that the clasts we examined were necessarily derived from a partially differentiated body only that they were derived from cooling of a silicate melt the clasts could thus be the products of any one of several proposed models for chondrule formation our results indicate that processes akin to chondrule formation in that they involve rapid cooling of a silicate melt were ongoing at the same time as cai formation lending support to the suggestion that al mg chondrule ages indicate either heterogeneous distribution of 26 al or resetting of the al mg system after chondrule formation

formation of metal and silicate globules in gujba a new bencubbin like meteorite fall gujba is a coarse grained meteorite fall composed of 41 vol large kamacite globules 20 vol large light colored silicate globules with cryptocrystalline barred pyroxene and barred olivine textures 39 vol dark colored silicate rich matrix and rare refractory inclusions gujba resembles bencubbin and weatherford in texture oxygen isotopic composition and in having high bulk 15n values 685 the 3he cosmic ray exposure age of gujba 26 7 ma is essentially identical to that of bencubbin suggesting that they were both reduced to meter size fragments in the same parent body collision the gujba metal globules exhibit metal troilite quench textures and vary in their abundances of troilite and volatile siderophile elements we suggest that the metal globules formed as liquid droplets either via condensation in an impact generated vapor plume or by evaporation of preexisting metal particles in a plume the lower the abundance of volatile elements in the metal globules the higher the globule quench temperature we infer that the large silicate globules also formed from completely molten droplets their low volatile element abundances indicate that they also formed at high temperatures probably by processes analogous to those that formed the metal globules the coarse grained bencubbin weatherford gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles a second class of bencubbin like meteorites represented by hammadah al hamra 237 and que 94411 may be a finer fraction derived from the same vapor cloud

buoyancy and wind driven convection at mixed layer density fronts in this study the influence of a geostrophically balanced horizontal density gradient on turbulent convection in the ocean is examined using numerical simulations and a theoretical scaling analysis starting with uniform horizontal and vertical buoyancy gradients convection is driven by imposing a heat loss or a destabilizing wind stress at the upper boundary and a turbulent layer soon develops for weak lateral fronts turbulent convection results in a nearly homogeneous mixed layer ml whose depth grows in time for strong fronts a turbulent layer develops but this layer is not an ml in the traditional sense because it is characterized by persistent horizontal and vertical gradients in density the turbulent layer is however nearly homogeneous in potential vorticity pv with a value near zero using the pv budget a scaling for the depth of the turbulent low pv layer and its time dependence is derived that compares well with numerical simulations two dynamical regimes are identified in a convective layer near the surface turbulence is generated by the buoyancy loss at the surface below this layer turbulence is generated by a symmetric instability of the lateral density gradient this work extends classical scalings for the depth of turbulent boundary layers to account for the ubiquitous presence of lateral density gradients in the ocean the new results indicate that a lateral density gradient in addition to the surface forcing can affect the stratification and the rate of growth of the surface boundary layer

new constraints on early solar system chronology from al mg and u pb isotope systematics in the unique basaltic achondrite northwest africa 2976 abstract we report elemental abundances and the isotopic systematics of the short lived 26 al 26 mg half life of 0 73 ma and long lived u pb radiochronometers in the ungrouped basaltic meteorite northwest africa nwa 2976 the bulk geochemical composition of nwa 2976 is clearly distinct from that of the eucrites and angrites but shows broad similarities to that of the paired nwa 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples the major and trace element abundances in nwa 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body the al mg and pb pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early solar system the absolute pb pb internal isochron age of nwa 2976 obtained from acid leaching residues of three whole rock samples and two pyroxene separates is 4562 89 0 59 ma mswd 0 02 this pb pb age is calculated using the measured 238 u 235 u ratio of a nwa 2976 whole rock of 137 751 0 018 2 which was determined relative to the recently revised value of 137 840 0 008 for the srm 950a u isotope standard the al mg systematics reveal the presence of 26 mg isotopic anomalies produced by the decay of 26 al with an 26 al 27 al 0 of 3 94 0 16 10 7 and indicate a time of formation of 0 26 0 18 ma after the d orbigny angrite using the revised pb pb age of 4563 36 0 34 ma for the d orbigny anchor corrected for its u isotopic composition we deduce an al mg model age of 4563 10 0 38 ma for nwa 2976 which is consistent with its pb pb internal isochron age the concordance of the pb pb and al mg chronometers when taking into account the differences in the u isotopic compositions of the d orbigny and nwa 2976 achondrites whose parent bodies likely formed in distinct regions of early solar system as indicated by their different oxygen isotopic compositions implies that 26 al was homogeneously distributed in the early solar system it also suggests that igneous processes on planetesimals as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies e g eucrites and angrites as well as ungrouped achondrites were widespread throughout the protoplanetary disk within the first 5 ma of the history of the solar system