collisional history of h chondrites the cosmic ray exposure histories of ordinary chondrites were recently reviewed marti and graf 1992 here we discuss in detail currently available cosmic ray exposure records and the adopted exposure ages of individual h chondrites we further present analyses of the distinct histograms of petrographic subgroups and the implications for orbital evolution and collisional records in addition to the stochastic events 7 to 8 m y ago a peak at 33 ma represents a statistically significant deviation from an exponential age distribution and signals a major collisional event at this time the h6 subgroup also recorded a similar event 24 m y ago these records present significant arguments against a continuous supply of h chondrites from the asteroid belt we show that the records of fall times for h5s are distinct from other subgroups and that a different orbit for the h5 parent is indicated in collisional events 7 to 8 m y ago the systematics of noble gas losses in the h5 subgroup are consistent with this interpretation the evidence for stochastic fragmentation and subsequent delivery to earth of h chondrites suggests that their present predominance among falls may represent a deviation from a representative population distribution

cosmogenic nuclides in the kirin chondrite abstract radioactive and stable cosmogenic nuclides have been determined in the various fragments of the kirin chondrite h5 experimental results obtained from our samples are described and compared with similar data obtained in other laboratories cosmogenic 54 mn 22 na 60 co 26 al 53 mn 40 k in the metal phase and light noble gases were measured based on these data the irradiation history of this meteorite can be explained in terms of 1 a multi stage exposure which involves a first stage irradiation for 10 7 years and a second stage for about 4 10 5 years and 2 depth effects in the productions of the nuclides in 4 second stage and 2 first stage geometries these conclusions are consistent with those of our previous work which was based on a limited number of samples

reply to comment of d w g sears f a hasan b m myers and h sears on update on terrestrial ages of antarctic meteorites

a model for meteoritic and lunar 40 ar 39 ar age spectra addressing the conundrum of multi activation energies abstract results of whole rock 40 ar 39 ar step heating analyses of extra terrestrial materials have been used to constrain the timing of impacts in the inner solar system solidification of the lunar magma ocean and development of planetary magnetic fields despite the importance of understanding these events the samples we have in hand are non ideal due to mixed provenance isotopic disturbances from potentially multiple heating episodes and laboratory artifacts such as nuclear recoil although models to quantitatively assess multi domain diffusive 40 ar loss have long been applied to terrestrial samples their use on extra terrestrial materials has been limited here we introduce a multi activation energy multi diffusion domain model and apply it to 40 ar 39 ar temperature cycling step heating data for meteoritic and lunar samples we show that age spectra of extra terrestrial materials the jilin chondrite k 4 and apollo 16 lunar breccia 67514 43 yielding seemingly non ideal behavior commonly interpreted as either laboratory artifacts or localized shock heating of pyroxene are meaningful and can be understood in context of the presence of multi diffusion domains containing multiple activation energies internally consistent results from both the meteoritic and lunar samples reveal high temperature short duration thermal episodes we interpret as due to moderate shock heating

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

noble gases in h chondrites and potential differences between antarctic and non antarctic meteorites abstract thirty one antarctic h chondrites from the allan hills have been analyzed for their noble gases seven of the specimens contain solar gases thirteen have a cosmic ray exposure age around 8 ma both fractions are as observed for h chondrites from the rest of the world scrutiny of the data for paired samples yields a minimum number of 17 independent falls with 6 falls required for the 8 ma peak of exposure age the distribution of exposure ages as well as that of radiogenic 4 he and 40 ar yield no evidence that h chondrites from the allan hills region represent a mixture of populations that is different from that brought to moderate latitudes of the earth in more recent times

response of late carboniferous and early permian plant communities to climate change abstract late carboniferous and early permian strata record the transition from a cold interval in earth history characterized by the repeated periods of glaciation and deglaciation of the south

spallogenic10be in the jilin chondrite abstract the 10 be contents measured in eight bulk samples of the jilin meteorite range from 5 05 0 55 to 16 3 1 7 dpm kg more than three fold and correlate with the 21 ne contents a least squares fit of these data has a positive intercept on the 10 be axis which can be explained in terms of a two stage irradiation the second more recent stage lasted 0 57 0 14 myr model calculations give first stage 10 be 21 ne ages ranging from 6 2 to 7 1 myr

light noble gases in jilin more of the same and something new results are reported for documented samples from two drill cores and for specimens from the strewn field of the largest known stone meteorite the h chondrite jilin core b was found to have been parallel to the surface of jilin during its first stage of irradiation in 2 geometry core a was normal to the 2 surface in it the mean attenuation length for the production by galactic cosmic rays of 38ar from metal and of 21ne from mg al si in silicates was found to be the same a numerical value for the mean attenuation length of 71 4 cm or 246 14 g cm 2 follows if corrections for the contribution from the second stage of exposure are based on t2 0 32 ma agreement with the lower values of 180 g cm 2 obtained from lunar studies and target data requires t2 to be about twice as long previous results are confirmed that in specimens with high contents of stable spallogenic gases the ratio 21nebulk 38armetal is low the suggestion had been and is mat this is a transition effect in near surface samples where the secondary cascade of nuclear active particles and hence the production of 21ne was not yet fully developed this suggestion is borne out by the present results on two samples that based on cosmic ray tracks and 60co content are certified near surface samples although strictly speaking this is true only for the depth of burial during the second stage of irradiation cosmic ray produced 60co is positively correlated with 4he content indicating that significant losses of 4he occurred when the jilin meteoroid had acquired already its final size and shape and that the losses were more severe for near surface samples than for such from the 4 interior presumably the losses were caused by a thermal spike associated with the excavation of jilin from its parent body the same event caused losses of part of the 3he produced during the first irradiation stage from the systematics of the 3he 21ne vs 4he correlation we derive for the second 4 irradiation a production ratio 3he 21ne 3 2 0 4

source mechanism of volcanic tremor low frequency 10 hz volcanic earthquakes originate at a wide range of depths and occur before during and after magmatic eruptions the characteristics of these earthquakes suggest that they are not typical tectonic events physically analogous processes occur in hydraulic fracturing of rock formations low frequency icequakes in temperate glaciers and autoresonance in hydroelectric power stations we propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low frequency volcanic earthquakes tremor the fluid dynamic source mechanism explains low frequency earthquakes of arbitrary duration magnitude and depth of origin as unsteady flow is independent of physical properties of the fluid and conduit fluid transients occur in both low viscosity gases and high viscosity liquids a fluid transient analysis can be formulated as generally as is warranted by knowledge of the composition and physical properties of the fluid material properties geometry and roughness of the conduit and boundary conditions to demonstrate the analytical potential of the fluid dynamic theory we consider a single phase fluid a melt of mount hood andesite at 1250 c in which significant pressure and velocity variations occur only in the longitudinal direction further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations for a simple constant pressure reservoir conduit orifice fluid system a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour fluid kinematic viscosity 0 412 m2 s is both included and neglected in the analysis to illustrate its effect upon system damping tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures the entire volcanic fluid system does not generally experience a transient flow condition at a given time closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system total fluid movement may not be accurately monitored by tremor data alone since steady state and slowly changing flows are aseismic

the l3 6 chondritic regolith breccia northwest africa nwa 869 ii noble gases and cosmogenic radionuclides abstract we measured cosmogenic radionuclides and noble gases in the l3 6 chondrite breccia northwest africa nwa 869 one of the largest meteorite finds from the sahara concentrations of 10be 26al and 36cl in stone and metal fractions of six fragments of nwa 869 indicate a preatmospheric radius of 2 0 2 5 m the 14c and 10be concentrations in three fragments yield a terrestrial age of 4 4 0 7 kyr whereas two fragments show evidence for a recent change in shielding most likely due to a recent impact on the nwa meteoroid approximately 105 yr ago that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic ray fluxes this scenario is supported by the low cosmogenic 3he 21ne ratios in these two samples indicating recent loss of cosmogenic 3he most nwa samples except for clasts of petrologic type 4 6 contain significant amounts of solar ne and ar but are virtually free of solar helium judging from the trapped 4he 20ne ratio of approximately 7 trapped planetary type kr and xe are most clearly present in the bulk and matrix samples where abundances of 129xe from decay of now extinct 129i are highest cosmogenic 21ne varies between 0 55 and 1 92 10 8 cm3 stp g 1 with no apparent relationship between cosmogenic and solar ne contents low cosmogenic 22ne 21ne c ratios in solar gas free specimens are consistent with irradiation in a large body combined 10be and 21ne concentrations indicate that nwa 869 had a 4 cosmic ray exposure cre age of 5 1 myr whereas elevated 21ne concentrations in several clasts and bulk samples indicate a previous cre of 10 30 myr on the parent body most probably as individual components in a regolith unlike many other large chondrites nwa 869 does not show clear evidence of cre as a large boulder near the surface of its parent body radiogenic 4he concentrations in most nwa 869 samples indicate a major outgassing event approximately 2 8 gyr ago that may have also resulted in loss of solar helium

isotopic composition of xenon and krypton in silicate graphite inclusions of the el taco campo del cielo iab iron meteorite noble gases extracted by temperature stepped pyrolysis from acid resistant silicate schreibersite and graphite residues of inclusions from the iab iron meteorite el taco campo del cielo have been analyzed for their isotopic composition the concentrations of spallogenic light gases agree with previous data on adjacent specimens nominal feldspar and pyroxene fractions have lost up to 80 of their 3he during or towards the end of the exposure of el taco to the cosmic radiation in olivine the 3ne21ne and nepne ratios are as low as 2 18 0 01 and 1 09 0 03 respectively presumably because the silicates were irradiated within a feni matrix under extremely heavy shielding argon kr and xe in the silicates are dominated by a trapped planetary component with 100 36ar132xe 700 and 1 7 84kr132xe 2 2 xenon released from the silicates at extraction temperatures 1000 c is isotopically close to ureilite xe while the high t xe from graphite and schreibersite is unlike any of the presently well established types of xe the isotopic composition of this xe can be generated as a mixture of mass fractionated u xe with light isotopes depleted by 2 2 0 1 per atomic mass unit and 244pu xe the fission component 10 in the case of 136xe is not due to in situ production it must have been added prior to incorporation of the xe into the el taco parent body and presumably underwent the mass fractionation process together with u xe from the silicates 127i n produced 128xe and radiogenic 129xe are released together with the same 128xe 129xe ratio in nominal olivine pyroxene and feldspar although the maximum release from pyroxene occurs about 300 c higher than from feldspar from this the cooling rate of silicates for the temperature range 1500 1200 c is deduced to be 100 c per million years the 80kr82kr ratio of 79 81br n produced 80 82kr is 2 86 0 04 in agreement with the value found in inclusions of the iab linwood but higher than observed in almost all stone meteorites

eccentricity paced late paleozoic climate change abstract cyclic sedimentary deposits characterize low latitude late paleozoic successions and preserve evidence of dynamic climate change on the pangaean supercontinent although their orbitally paced glacioeustatic origins are widely accepted their climatic signatures are open to interpretation in this study we utilize the genesis general circulation model gcm coupled to dynamic ice sheet and ecosystem components to explore the response of low latitude continental climate and high latitude ice sheets to orbital and atmospheric pco2 forcing our results suggest that atmospheric pco2 concentration exerts the primary control over low latitude continental climate and high latitude glaciation our experiments constrain the atmospheric pco2 window within which late paleozoic climate was amenable to orbitally driven glacial interglacial fluctuations the results suggest that both high latitude ice sheet accumulation and ablation and low latitude climate change were paced by the eccentricity of earth s orbit periods of high eccentricity amplified precession driven changes in insolation and promoted high latitude ice sheet volume fluctuations as well as increased low latitude precipitation variability when eccentricity was low the amplification of precessionally driven insolation fluctuations was reduced which promoted high latitude continental ice sheet stability and less variable low latitude precipitation based on these modeling results we discuss the implications of eccentricity paced precessional scale climatic changes on low latitude pangaean depositional environments

influence of high latitude vegetation feedbacks on late palaeozoic glacial cycles ice ages during the palaeozoic era are marked by glacial interglacial cycles thought to be driven by variations in the earth s orbit numerical simulations suggest that the response of vegetation to the varying insolation may be an important factor in the associated climate response

thorium and uranium abundances in the jilin h5 chondrite abstract thorium and uranium abundances have been measured in the jilin h5 chondrite by radiochemical neutron activation analysis although the abundances of 35 8 1 5 ppb th and 14 9 0 9 ppb u are within the range of literature values the ratioth u is about 25 lower than the average value for h chondrites terrestrial addition of uranium appears to be the most likely explanation

cosmic ray tracks and cobalt 60 in near surface samples of the jilin chondrite cosmic ray track densities crtds have been measured on several spot samples from two jilin specimens from the strewn field vi 21 057 and vi 42 04 along with 60co activity in three and four sections respectively of the same specimens the observed crtd gradient agrees remarkably well with that predicted by bhattacharya et al s calculations 1973 of crtd variation as a function of shielding depth which indicates a negligible contribution from the first 2 irradiation stage the shortest distance of each specimen to the preatmospheric surface is 3 6 cm and 6 8 cm respectively the distance scale deduced from these measurements allows the determination of the variation of 60co activity with shielding depth at low depth values which is found to be much less steep than model predictions two slightly different 60co profiles are proposed that correspond to the two specimens studied the mean preatmospheric radius of the jilin chondrite is 85 8 cm the uncertainty reflecting in part the meteoroid surface roughness

exposure history of the mocs l6 chondrite a study of strewn field samples we measured cosmogenic radionuclides 10be 26al and 36cl and noble gases he ne and ar in 10 specimens of the mocs l6 chondrite to determine the exposure history and preatmospheric relationship among fragments from known locations in the strewn field cosmogenic noble gas contents alone are consistent with a simple irradiation exposure of 15 2 ma however mocs has very low 22ne 21ne ratios indicative of deep burial in a large meteoroid but radionuclide levels at saturation values typical for much smaller meteoroids this paradox suggests a possible complex exposure for the latter case we propose a two stage exposure history in which mocs initially was deeply buried in a large object for 110 ma followed by exposure in a 65 cm object for 10 5 ma relative shielding was inferred from the measured 22ne 21ne ratios assuming constant 22ne 21ne production for all samples during the first stage these shielding levels which are supported by estimates based on 36cl production by neutron capture indicate a possible relationship between depth of samples in the mocs meteoroid and fall location in the strewn field

exposure history of the regolithic chondrite fayetteville ii solar gas free light inclusions abstract noble gases and cosmic ray tracks were determined on light solar gas free inclusions of the solar gas rich h4 chondrite fayetteville 10 be 26 a1 major and trace element concentrations as well as oxygen isotope ratios complete the data set one exceptional inclusion is an l chondrite xenolith identified by its oxygen isotope signature and the concentration of metallic fe ni oxidized fe of this inclusion equilibrated with the host early in the history of fayetteville s parent body track density variations indicate that the majority of fayetteville s inclusions were irradiated as pebbles in the parent body by solar flare particles for about 10 4 10 6 years the l chondritic inclusion contains an unequivocal excess of gcr produced he and ne relative to the remaining inclusions this clast as well as two more solar gas free samples suffered a gcr exposure on the parent body lasting some 10 million years or longer this exposure is comparable in length to the residence times of solar gas rich matrix samples in the gcr active zone of the fayetteville parent regolith probably the other inclusions also were irradiated by gcr during at least several million years prior to the separation of the meteoroid from the parent body

sequence stratigraphy and onlap history of the donets basin ukraine insight into carboniferous icehouse dynamics abstract the degree to which permo carboniferous cyclothemic successions archive evidence for long term variations in ice volume during the late paleozoic ice age is insufficiently resolved here we develop the sequence stratigraphy and onlap offlap history for a 33 my interval of the carboniferous using the u pb calibrated succession of the donets basin ukraine in order to assess the relationship between sea level high latitude changes in glacial extent and climate integrated subsurface and outcrop data permit meter scale correlation of 242 biostratigraphically constrained limestones and coals and in turn individual cyclothems across 250 km of the donets basin rapid uniform subsidence and basinwide continuity of marker beds indicate pennsylvanian deposition under relatively stable tectonic conditions three scales of sequences avg durations of 140 ky 480 ky and 1 6 my are recognized on the basis of stratigraphic stacking patterns and basinwide architecture of marine to terrestrial facies assemblages the hierarchy of sequences and the geographic and stratigraphic positions of shifts in base level sensitive facies across the donets ramp permit the construction of an onlap offlap history at a sub 400 ky scale major sea level lowstands occur across the mid carboniferous boundary and during the early moscovian these lowstands coincide with glacial maxima inferred from high latitude glacigenic deposits the middle to late pennsylvanian is characterized by a stepwise onlap culminating in an earliest gzhelian highstand suggesting contraction of carboniferous ice sheets prior to the initiation of early permian glaciation the stratigraphic position of climate sensitive facies within individual donets cyclothems indicates a turnover from seasonal sub humid or semi arid climate to everwet conditions during the late lowstand and maximum ice sheet accumulation comparison of the stratigraphic and aerial distribution of coals and evaporites in the donets basin with the onlap offlap history further indicates everwet conditions during lowstands and inferred glacial maxima and drier climate during onlap and inferred ice sheet contraction at the intermediate 0 8 to 1 6 my and long 106 yr time scales taken together the relationship between inferred climate and glacioeustasy suggests a likely teleconnection between high latitude ice sheet behavior and low latitude atmospheric dynamics

response of a volcanic conduit to step like change in magma pressure freely damped oscillations associated with strain offsets were observed by a nearby volumetric strainmeter during the 1986 izu oshima volcano eruption japan these events are interpreted as a response of the magma filled conduit to step like changes in magma pressure there were dramatic changes in the characteristic frequency of oscillation and the polarity of initial motion before and after the 6 hour interval of the enormous ground deformation due presumably to magma intrusion the frequency changed from 42 6 to 21 3 mhz and the polarity changed from contraction to expansion we propose a conceptual conduit model to explain this polarity reversal where magma is episodically supplied to the conduit before but is episodically drained back to the reservoir after an extensive magma escape away from the reservoir as dike intrusion stress caused by the dike intrusion is large enough to break the lid of the conduit thus to change a boundary condition this change in boundary condition reduces the frequency of oscillation by a factor of 2 in this model longitudinal wave speed of the magma fluid is 340 m s if the conduit length is taken to be 4 km

noble gases and the history of jilin meteorite abstract potassium and noble gases have been determined in more than twenty specimens from the largest known stone meteorite the h5 chondrite jilin thirteen specimens came from the surface of the present main mass the remainder from various locations in the strewn field the average k content is 802 ppm 29 samples from 23 specimens maximum deviations from the mean are 7 and 11 whole rock gas retention ages of different specimens are distinctly different they vary between 2 22 and 3 90 ae for40ar 40k and between 0 44 and 2 0 ae for4he u th severe losses of4he and40ar up to 95 and 80 respectively must have occurred up to less than 440 ma ago they cannot have happened during the fall of the meteorite however differences in concentration of cosmic ray produced3he 21ne and38arme by factors of five six and seven respectively reflect a complex irradiation history they are compatible with a short 4 irradiation and an extended one in 2 geometry at shallow depth top samples were most probably located near the transition maximum of nuclear active particles at around 15 cm depth they definitely cannot have been buried deeper than 4 m 3he 21ne ratios in bulk samples are lowered by diffusion losses of3he 25 61 while22ne 21ne ratios appear to be unaffected 22ne 21ne values range between 1 060 and 1 086 with a mean of 1 069 which is at variance with predictions for the particular irradiation conditions of jilin low36ar 38ar ratios down to 0 553 in clean metal samples are interpreted as the combined effect of large size and the transient lowering of this ratio because of a sudden increase in production rates upon going from 2 to 4 irradiation

nuclear track study of jilin chondrite twenty one specimens of jilin sampling the whole spectrum of21ne concentrations and coming from the main mass as well as from various locations in the strewn field were studied for their nuclear track record in olivines exceedingly low track densities were found in 19 of these specimens 300 cm 2 including 10 in which the low60co t1 2 5 3 years activities were seemingly indicative of a low shielding during the second 4 irradiation stage in two specimens vi 42 04 and vi 21 057 which have high21ne contents and the largest40ar and4he losses track densities of 750 and 1500 cm 2 clearly emerge from the background these tracks are attributed to very heavy cosmic ray nuclei 1 they were produced during the first 2 stage exposure it follows that the two samples were a the least shielded during the first irradiation period 28 cm of chondritic matter b not subjected to a last environmental temperature higher than 350 c for a few days in contradiction with the thermal model of muller and jessberger otherwise the cosmic ray tracks would have been totally erased c located at a depth of at least 20 cm within the meteoroid during the second 4 irradiation episode of 0 4 myr otherwise crtds produced during this time would have been detected this depth implies that the60co activities of these two specimens not known until now should be around 130 140 dpm kg it remains then to explain why 10 other specimens with60co activities in the range of 50 100 dpm kg defining depths of 6 15 cm within the meteoroid show no evidence at all of crtds above the track density background of 150 100 cm 2 2 the crtds were registered during the second 4 irradiation stage of 0 4 myr this requires the samples to have been located at depths of 15 and 12 cm from the pre atmospheric surfaces within a meteoroid with a radius of 85 cm at such depths the predicted60co activities for specimens vi 21 057 and vi 42 04 would range between 70 and 100 dpm kg the major problem with our results is to understand why in 10 locations with low 50 100 dpm kg 60co activitiesno crtds have been registered during the second irradiation stage a possible answer would be that depths inferred from60co activities are in error at least in the case of large meteoroids and for depths 15 20 cm this has indeed been found to be the case in allende where at shallow locations 15 cm depths of burial derived from crtds are larger by factors of 2 5 than those inferred from60co activities while at larger depths the agreement between the two methods appears to be good the ubiquitous presence of a track density background found in most samples is best explained by the spontaneous fission of238u since 4 0 gyr this explanation allows one to define a strict upper limit to the temperature of the last thermal event responsible for the losses of radiogenic and spallogenic gases a temperature of 500 c 1 hour or 465 c 24 hours would have annealed the internal fission tracks in olivines these limiting temperatures are not in real disagreement with the thermal model of muller and jessberger provided the crtds found in the key specimens vi 21 057 and vi 42 04 were registered during the second 4 irradiation stage and that shielding depths derived from crtds are right the jilin meteoroid had an original mass of about 15 tons r0 1 m of which about 4 tons were recovered

sea level curve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america at least 55 cycles of marine inundation and withdrawal are recognized in the mid desmoinesian to mid virgilian midcontinent outcrop sequence in north america they range from widespread major cycles classic cyclothems with deep water facies extending across the northern shelf through intermediate cycles persisting as marine horizons across the shelf to minor cycles developed on the lower shelf or as parts of major cycles biostratigraphic differentiation of the cycles should establish interbasinal correlation on a scale fine enough to allow evaluation of differential tectonics and sedimentation sequential groupings of cycles are more irregular than proposed megacyclothems or mesothems but they may be obscured by the distinctness of the major cyclothems estimates of cycle periods range from about 40 to 120 x 10 sup 3 yr for the minor cycles up to about 235 to 400 x 10 sup 3 yr for the major cyclothems the range for all cycles corresponds well to the range of periods of earth s orbital parameters that constitute the milankovitch insolation theory for the pleistocene ice ages and it further supports gondwanan glacial control for the pennsylvanian cycles even though the dominant period of the major pennsylvanian cyclothems is up to four times longer than the dominant 100 000 yr period inmore the pleistocene the shapes of both curves display rapid marine transgression rapid melting of ice caps and slow interrupted regression slow buildup of ice caps which suggest similar linkages between the climatic effects of the orbital parameters and ice cap formation and melting at the two different scales widely separated in time 28 references 3 figures less

tropical mountains and coal formation a climate model study of the westphalian 306 ma a global climate model has been used to study the seasonal sensitivity of tropical rainfall and soil moisture to the presence of himalayan scale mountains at tropical latitudes during the westphalian these mountains act to retard the northward progression of the intertropical convergence zone in their vicinity during the northern hemisphere summer such that there is converging low level flow from both the eastern and western panthalassa ocean and an eight fold enhancement of precipitation over the mountains the computed annual cycle of soil moisture for these tropical regions concurs with geologic evidence of extensive coal deposits formed in a broad area at tropical latitudes and only small seasonality in either temperature or precipitation in these areas

evidence for subsurface magma bodies beneath izu oshima volcano inferred from a seismic scattering analysis and possible interpretation of the magma plumbing system of the 1986 eruptive activity abstract observed seismic waveforms were analyzed to find the distribution of seismic scatterers beneath izu oshima volcano in an island located about 100 km south of tokyo japan using seismic waveforms generated by earthquakes whose epicentral distances are from several tens to about a thousand kilometers the method employed in the study was developed based on a prestack migration of shot gathers and is composed of the following procedure 1 static correction 2 the estimation of waveforms incident into a volume of interest i e subsurface structure of the volcano and 3 an inverse procedure using the developed migration scheme the location of scatterers and the order of scattering became clear with the present analysis the petrological implications and results from geoelectromagnetic studies by other researchers agree with the proposed model in which the existence of main and submagma reservoirs beneath the volcano were indicated these results are interpreted in terms of a model on the magma migration path during the 1986 eruptive activity of the volcano seismic scattering analysis played a significant and indispensable role in this study to give a direct view of a possible underground plumbing system and is considered a promising method for revealing the unknown structure of volcanoes in the future

strain offsets with monotonous damped oscillations during the 1986 izu oshima volcano eruption the sequential process of the 1986 izu oshima volcano eruption japan was recorded by a volumetric strain meter near the erupted summit the records show occasional strain offsets accompanying long period damped oscillations which we interpret as responses of the volcanic system to perturbations in magmatic pressure the polarities of strain offsets were contractive for all events before the enormous ground deformation of november 21 due presumably to the intensive dike intrusion on the contrary all events after that had expansive polarities the characteristic frequencies of oscillations were 42 6 mhz for the events before changing abruptly to half that after the ground deformation in our model which is a one dimensional magma filled conduit the boundary condition at the top of the conduit changed from a closed to an open end upon a void formation at the top of the magma head due to its descent immediately after the dike intrusion before the magma escape from the reservoir as a dike magma was episodically supplied from the reservoir to the conduit and thus contraction events were generated after the magma escape magma left in the conduit was episodically drained back to the reservoir to generate expansion events this episodic magma supply or drainage was modeled by a prescribed pressure change at the bottom of the conduit we solved the free oscillation problem of the conduit pipe with an elastic sidewall to calculate the resultant volumetric strain outside the pipe physical properties of the magma and the excitation mechanism are inferred from the application of our formulation to the observation the conduit length and fluid acoustic velocity are estimated to be 10 km and 430 m s respectively suggesting that the fluid is composed of a gas liquid mixture if the conduit radius is taken to be 40 m and if the damping of oscillation occurred by viscous dissipation the fluid viscosity is 108 109 pa s a reasonable estimate for the basaltic magma the applied pressure changes at the bottom of the conduit are in a range from 105 to 106 pa with risetimes of the order of 4 to 100 s

exposure history and terrestrial ages of ordinary chondrites from the dar al gani region libya we measured the concentrations of noble gases in 32 ordinary chondrites from the dar al gani dag region libya as well as concentrations of the cosmogenic radionuclides 14c 10be 26al 36cl and 41ca in 18 of these samples although the trapped noble gases in five dag samples show ratios typical of solar or planetary gases in all other dag samples they are dominated by atmospheric contamination which increases with the degree of weathering cosmic ray exposure cre ages of dag chondrites range from 1 myr to 53 myr the cre age distribution of 10 dag l chondrites shows a cluster around 40 myr due to four members of a large l6 chondrite shower the cre age distribution of 19 dag h chondrites shows only three ages coinciding with the main h chondrite peak at 7 myr while seven ages are 5 myr two of these h chondrites with short cre ages dag 904 and 908 show evidence of a complex exposure history five of the h chondrites show evidence of high shielding conditions including low 22ne 21ne ratios and large contributions of neutron capture 36cl and 41ca these samples represent fragments of two or more large pre atmospheric objects which supports the hypothesis that the high h l chondrite ratio at dag is due to one or more large unrecognized showers the 14c concentrations correspond to terrestrial ages 35 kyr similar to terrestrial ages of chondrites from other regions in the sahara but younger than two dag achondrites despite the loss of cosmogenic 36cl and 41ca during oxidation of metal and troilite concentrations of 36cl and 41ca in the silicates are also consistent with 14c ages 35 kyr the only exception is dag 343 h4 which has a 41ca terrestrial age of 150 40 kyr this old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr a possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods such as the current one which started 3000 years ago finally based on the 26al 21ne and 10be 21ne systematics in 16 dag meteorites we derived more reliable estimates of the 10be 21ne production rate ratio which seems more sensitive to shielding than was predicted by the semi empirical model of graf et al 1990 but less sensitive than was predicted by the purely physical model of leya et al 2000

deterministic non linear source processes of volcanic tremor signals accompanying the 1996 vatnajokull eruption central iceland summary observations and theoretical considerations have cast doubt on the suggestion that volcanic tremor source processes may be modelled by a linear oscillator that is set into resonance by a sustained disturbance volcanic tremor signals that accompanied the 1996 vatnajokull subglacial eruption central iceland have been analysed using methods from the discipline of non linear dynamics in order to investigate the possibility that they originated from a non linear source the volcano seismic phenomena associated with the eruption were recorded by a permanent network equipped with broad band seismometers hotspot using a sampling rate of 20 samples s 1 the eruption was preceded by increased seismic activity for a period of 2 days which also included a large earthquake with a moment magnitude of 5 6 the tremor during the first 2 days of the eruption has a high signal to noise ratio at the nearest station to the eruption site and starts as a continuous signal later evolving to low amplitude background tremor interrupted by high amplitude cigar shaped bursts having an average duration of 250 s the phase space which describes the evolution of the behaviour of a non linear system was reconstructed from the original tremor seismograms using the delay embedding theorem suggested by takens the delay time used for the reconstruction was selected after examining the autocorrelation function which showed a first zero crossing at a timelag of 4 samples and the average mutual information that showed no minimum indicating that the tremor process may have been undersampled based also on phase space portraits for different delay times a delay time of one sample interval 0 05 s was used the sufficient embedding dimension for phase space reconstruction was selected by applying the false nearest neighbours method which revealed complete unfolding of the tremor attractor at dimensions 7 8 implying upper bounds of its fractal dimension in the range 3 5 4 0 the phase space prediction errors of different segments of the tremor time series were compared in order to check whether the attractor dynamics change substantially with time it was found that for continuous tremor there was almost no dynamic variation in contrast to the background tremor and the superposed bursts that gave a maximum prediction error when the former was used to predict the latter this difference in dynamics also had an effect on their spectra the amplitude spectrum of a burst or continuous tremor has a much sharper decay at high frequencies than that of the background tremor a possible physical mechanism that may explain these observed characteristics involves turbulent slug flow of magma in a narrow cylindrical conduit generating the different dynamic regimes as the reynolds number varies

allan hills 88019 an antarctic h chondrite with a very long terrestrial age we have measured the concentrations of the cosmogenic radionuclides loge 26al and 36cl half lives 1 51 ma 716 ka and 300 ka respectively in two different laboratories by accelerator mass spec trometry ams techniques as well as concentrations and isotopic compositions of stable he ne and ar in the antarctic h chondrite allan hills alh 88019 in addition nuclear track densities were measured from these results it is concluded that the meteoroid alh 880 19 had a preatmospheric radius of 20 5 cm and a shielding depth for the analyzed samples of between 4 and 8 cm using calculated and experimentally deter mined production rates of cosmogenic nuclides an exposure age of 40 ma is obtained from cosmogenic 21ne and 38ar the extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 f 0 4 ma a similarly long terrestrial age was found so far only for the antarctic l chon drite lewis cliff lew 86360 such long ages establish one boundary condition for the history of mete orites in antarctica

glacial eustatic base level climatic model for late middle to late pennsylvanian coal bed formation in the appalachian basin abstract late middle late pennsylvanian middle allegheny and lower conemaugh marine units in the appalachian basin represent the distal edges of glacial eustatic marine incursions of the midcontinent sea onto the detrital apron of the appalachian highlands because these marine units typically overlie coal beds the main environment of formation of this type of coal bed appears to have been that of a coastal swamp migrating ahead of transgression as base level rose the water table continually rose and the swamp was further nourished and surrounding vegetation kept thick enough to inhibit detrital influx by rainfall from the expanding nearby source of moisture even in parts of the appalachian cyclic succession with no marine units upper allegheny monongahela dunkard the coal beds rep esent the wettest part of the climate cycle of cecil 1990 sandwiched between detrital units representing seasonal wet dry climates when sediment was most readily mobilized and antipodal to nonmarine limestones representing the driest climates throughout this time the appalachian basin was in the center of the tropical zone of a nearly accreted megacontinent quite distant 4000 km from permanent oceanic sources of moisture climate modeling without considering inland seas or orographic effects on winds shows that the tropical zone of such a large land mass would have suffered from a severe deficit in rainfall and soil moisture although further modeling shows that the appalachian orogenic highlands would have drawn in moisture to ncrease rainfall in their vicinity they also may have impeded the normal westward flow of moisture laden easterly winds the western source for enough moisture to form coal beds was most likely the north american midcontinent sea during major highstand this sea would have covered about 2 million km2 and provided both high base level and abundant nearby moisture for enough continual rainfall to keep the coastal peat swamps widespread fresh and relatively detritus free in the appalachian basin in contrast during major lowstand it would have covered only about 200 000 km2 with its shoreline about 1500 km away and it apparently provided less consistent moisture to the appalachian basin then because most nonmarine limestones and paleosols formed there during the e times contain evidence of drier climates therefore upper allegheny monongahela and dunkard coal beds most probably formed when the midcontinent sea was approaching highstand and stood just west of the presently preserved outcrop thus they likely represent the coastal lowland equivalent of a marine unit at highstand farther west this constrained time of peat formation and preservation in a variety of coastal environments prior to and during marine highstand is responsible for the discrete and widespread nature of the coal bed interval in a typical depositional cycle or cyclothem a pattern that stands in contrast to the complicated relations of overlying and underlying detrital units which reflect the segregation of sand and mud into a complex mosaic of local deltaic and ter estrial subenvironments that developed when sea level rainfall and the water table were lower

nitrogen in chondritic metal we report new nitrogen isotopic data in metals of h l and one ll chondrites with n abundances in the range of 0 3 to 3 3 ppm and half of these 1 ppm nitrogen isotopic signatures in metals with low indigenous n concentrations are modified by cosmic ray spallation components corrections are required to determine the indigenous n signatures the metals of type 4 and 5 show uniform indigenous nitrogen 15n 6 8 0 5 and confirm a reported possible genetic association of chondritic metal with metal in iie and iva iron meteorites distinct isotopic signatures are observed in two metal samples of the portales valley h6 meteorite which both are inconsistent with signatures in h4 and h5 chondrites but possibly reveal a record of impact induced melting and metamorphism on the parent asteroid anomalous nitrogen signatures in metals of type 3 chondrites on the other hand may reflect residues of surviving presolar isotopic signatures

the record of cosmogenic radiogenic fissiogenic and trapped noble gases in recently recovered chinese and other chondrites abstract we performed a comprehensive study of the noble gas isotopic abundances in thirty six chondrites including twenty seven chondrites recovered in china the large data base allows us to recognize some new characteristics of the nuclear record in chondritic matter the comparison of the trapped noble gas release pattern for ordinary and carbonaceous chondrites shows that the planetary trapped noble gases in ordinary chondrites are released mainly above 1200 c whereas 85 of these gases in carbonaceous chondrites are degassed at 200 c there exists a clear correlation of the fraction of trapped xe released at 1200 c and petrologic type of chondrites it thus appears that the carrier phases of the trapped noble gases in ordinary and in carbonaceous chondrites may not be the same the ngawi ll3 chondrite is solar gas rich the solar gases in meteorites and lunar surface material are mixtures of solar wind sw and solar energetic particles sep we show that the variations of the 3 he 4 he and 20 ne 22 ne ratios for solar gas rich meteorites and lunar surface material could be either due to preferential diffusive losses of the lighter isotopes of he and ne or due to a change of the sw sep flux ratio with time furthermore we demonstrate that the 81kr concentration is a function of the shielding depth of a sample within the meteoroid deviations from this depth dependency curve are observed for jilin h5 lishui l5 suizhou l6 and dongtai ll6 a complex exposure history for jilin is thus confirmed and is possible for the other three chondrites cosmic ray exposure ages are calculated based on six different nuclides 3he 21ne 38ar 83kr 126xe and 81kr kr in most cases good agreement is observed for the ages derived from the different methods quality classes are assigned to the exposure ages and the age distributions are discussed in some meteorites we observe effects induced by secondary cosmicray produced neutrons epithermal neutron fluxes jn 30 300 ev and fast neutron fluxes jn 5 mev are derived based on the reactions 79br n 80kr and 24mg n 21ne respectively and preatmospheric masses of the meteoroids are estimated we show that the ratio j n 30 300 ev j n 5 mev increases with increasing preatmospheric mass we introduce a 3he exposure age 21ne exposure age vs 4he gas retention age 40ar gas retention age diagram that is a powerful tool for distinguishing different thermal histories of meteoroids finally in some chondrites we observe xe produced by 244pu fission and calculate the time span between fission xe retention in these chondrites and that of the angra dos reis achondrite we find that the ordinary chondrites started to retain fission xe 48 30 ma earlier than angra dos reis we do not observe systematic differences between h l and ll or type 5 and 6 chondrites with respect to the time of fission xe retention

observations of volcanic tremor at mount st helens volcano digital recordings of ground motion during tremor episodes accompanying eruptions at mount st helens volcano in the state of washington on august 7 and october 16 18 1980 are studied the spectra of the vertical component waveforms contain at least two dominant peaks at 1 0 and 1 3 hz for all events recorded during both eruptions that were studied spectra of horizontal ground motion show peaks at 0 9 and 1 1 hz the relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time the unique waveform of tremor events observed at mount st helens must be due to a source effect since the relative amplitude of the two dominant peaks changes during tremor episodes the path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of mount st helens the consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant independent of the amplitude of motion at the source variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor amplitudes of ground motion varies between 0 11 and 4 7 m seismic moment rates during the two eruptions are calculated using the model of aki et al 1977 and found to vary between 6 1018 and 1 1020 dynes cm s 1 which are larger than values found by aki et al 1977 who studied amplitudes of shallow tremor events recorded during the october 1963 eruption of kilauea volcano in hawaii study of tremor amplitudes recorded at corvallis oregon leads to the conclusion that tremor accompanying the cataclysmic may 18 1980 eruption was at least one order of magnitude larger in amplitude than tremor during august and october

orbital and co2 forcing of late paleozoic continental ice sheets 1 contrasting views of the size duration and history of the gondwanan continental ice sheets have been proposed from late paleozoic glaciological and sedimentological evidence to evaluate these differing views a coupled ice sheet climate model is used to simulate continental ice sheets under a wide range of late paleozoic orbital and pco2 conditions the model experiments indicate that orbital variations at pco2 concentrations below 2x pre industrial atmospheric levels pal 280 ppm produce large changes in late paleozoic ice volume 1 3 108 km3 and sea level 20 to 245 m between 2 and 8x pal gondwana continental ice is simulated only under the most extreme southern hemisphere cold summer orbit but still produces significant ice volumes 8 12 107 km3 our results highlight the important role of atmospheric co2 in determining the distribution volume and stability of late paleozoic ice sheets factors that ultimately impacted sea level cyclothem deposition and global climate and reconcile disparate views of the late paleozoic ice age

conditions of the cosmic ray exposure of the jilin chondrite abstract nine documented samples from the main mass jilin 1 and three samples from different locations of the strewn field have been analyzed for cosmogenic 26 al t 1 2 0 716 myr 60 co t 1 2 5 27 yr and 22 na t 1 2 2 6 yr 53 mn t 1 2 3 7 myr has been determined in seven of the same documented samples and in two further documented samples the pre atmospheric size of jilin was reconstructed from the documented sample positions and from the measured 60 co activities jilin was almost spherical and had a pre atmospheric radius of about 85 cm the cosmogenic nuclides in jilin can be explained by a two stage exposure history the first stage occurred near the surface of the parent body in 2 geometry and lasted about 10 myr jilin was then expelled from its parent body and exposed to cosmic rays in a second 4 stage lasting approximately 0 4 myr the measured concentration profiles of spallogenic nuclides are compared to calculated production rates

thermoluminescence studies on jilin meteorite abstract the thermoluminescence tl of jilin meteorite was measured as a function of depth up to 21 7 mm below the fusion crust using the 50 100 m size fraction up to 6 mm depth the natural tl increases steeply due to thermal draining during atmospheric passage resulting in a temperature gradient of 45 c mm below 7 mm depth a constant natural tl signal is observed corresponding to an equivalent dose of 3300 gy this quantity represents a lower limit for the total natural dose received by the meteorite assuming a dose rate of 0 1 gy a a minimum age of 33 000 years is calculated for the last thermal event 7 cm below the fusion crust

co2 forced climate and vegetation instability during late paleozoic deglaciation the late paleozoic deglaciation is the vegetated earth9s only recorded icehouse to greenhouse transition yet the climate dynamics remain enigmatic by using the stable isotopic compositions of soil formed minerals fossil plant matter and shallow water brachiopods we estimated atmospheric partial pressure of carbon dioxide pco2 and tropical marine surface temperatures during this climate transition comparison to southern gondwanan glacial records documents covariance between inferred shifts in pco2 temperature and ice volume consistent with greenhouse gas forcing of climate major restructuring of paleotropical flora in western euramerica occurred in step with climate and pco2 shifts illustrating the biotic impact associated with past co2 forced turnover to a permanent ice free world

cosmic ray exposure ages of six chondritic almahata sitta fragments the almahata sitta strewn field is dominated by ureilites but contains a large fraction of chondritic fragments of various types we analyzed stable isotopes of he ne ar kr and xe and the cosmogenic radionuclides 10be 26al and 36cl in six chondritic almahata sitta fragments el6 breccia el6 el3 5 cb ll4 5 r like the cosmic ray exposure cre ages of five of the six samples have an average of 19 2 3 3 ma close to the average of 19 5 2 5 ma for four ureilites the cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with almahata sitta this corroborates that almahata sitta chondrite samples were part of the same asteroid as the ureilites however ms 179 has a lower cre age of 11 0 1 4 ma further analysis of short lived radionuclides in fragment ms 179 showed that it fell around the same time and from an object of similar size as almahata sitta making it almost certain that ms 179 is an almahata sitta fragment instead its low cre age could be due to gas loss chemical heterogeneity that may have led to an erroneous 21ne production rate or perhaps most likely ms 179 could represent the true 4 exposure age of almahata sitta or an upper limit thereof while all other samples analyzed so far experienced exposure on the parent body of similar lengths finally ms 179 had an extraordinarily high activity of neutron capture 36cl 600 dpm kg 1 the highest activity observed in any meteorite to date related to a high abundance of the cl bearing mineral lawrencite

cosmogenic radionuclides in l5 and ll5 chondrites from queen alexandra range antarctica identification of a large l ll5 chondrite shower with a preatmospheric mass of approximately 50 000 kg the collection of approximately 3300 meteorites from the queen alexandra range que area antarctica is dominated by more than 2000 chondrites classified as either l5 or ll5 based on concentrations of the cosmogenic radionuclides 10 be 26 al 36 cl and 41 ca in the metal and stone fraction of 16 que l5 or ll5 chondrites we conclude that 13 meteorites belong to a single meteorite shower que 90201 with a large preatmospheric size and a terrestrial age of 125 kyr members of this shower have properties typical of l e g pyroxene composition and ll chondrites e g metal abundance and composition as well as properties intermediate between the l and ll groups e g olivine composition and is thus best described as an l ll5 chondrite based on comparison with model calculations the measured radionuclide concentrations in the metal and stone fractions of que 90201 indicate irradiation in an object with a preatmospheric radius of approximately 150 cm representing one of the largest chondrites known so far based on the abundance of small l5 and ll5 chondrites at que and their distinct mass distribution we conclude that the que 90201 shower includes up to 2000 fragments with a total recovered mass of 60 70 kg 1 of the preatmospheric mass of approximately 50 000 kg the mass distribution of the que 90201 shower suggests that the meteoroid experienced catastrophic atmospheric fragmentation s either because it was a fragile object or it had a high entry velocity

a paleotropical carbonate dominated archive of carboniferous icehouse dynamics bird spring fm southern great basin usa abstract much of our far field knowledge of glaciation and climate during the late paleozoic ice age is built on decades of study of mixed carbonate siliciclastic cyclothemic successions from paleotropical euramerica far fewer carbonate dominated successions have been studied despite their high sensitivity to changes in accommodation space and environmental conditions here we present a sequence stratigraphic framework for the carbonate dominated bird spring formation southern great basin u s a and infer from it a relative sea level history for the latest mississippian through latest pennsylvanian correlation of four successions across the paleo platform documents changes in the lithofacies composition of meter scale cycles their cycle stacking patterns and cycle bounding surfaces that record substantial variation in relative sea level at several temporal scales during the pennsylvanian ten genetic sequences bounded by intervals of restricted peritidal lithofacies karst terra rossa rooted caliche and or protosols are recognized within sequences 126 parasequences stack into parasequence sets that in turn define lowstand transgressive and highstand systems tracts the stratigraphic cycle stacking patterns and across platform architecture of systems tracts suggest a mid carboniferous lowstand in relative sea level followed by a middle morrowan middle bashkirian highstand as previously suggested bishop et al 2010 subsequent progradation of the platform occurred during a gradual long term fall through the late morrowan and atokan late bashkirian early moscovian upon which repeated higher frequency events were superimposed culminating in widespread exposure across the platform proximal to the atokan desmoinesian boundary long term sea level increased into the mid desmoinesian end moscovian after which progradation late desmoinesian and aggradation missourian virgilian of shallow water and peritidal carbonates occurred across the bird spring platform the progradational to aggradational phase involved an abrupt shift from ramp to rimmed shelf geometry in the missourian kasimovian likely in response to progressively increasing sea level dampened high frequency sea level fluctuations and or to regional faulting along the bird spring platform margin inferred magnitudes of short term 105 yr fluctuations superimposed on the longer term rises and falls range from 20 to 70 m during the early to mid pennsylvanian and are substantially reduced 20 m during the late pennsylvanian the early to middle pennsylvanian sea level history inferred from the carbonate dominated bird spring platform corresponds well with a recently published pennsylvanian onlap offlap history defined from the cyclothemic succession of the donets basin and with independent near field reconstructions of glaciation during the pennsylvanian

paradox of late paleozoic glacioeustasy models of euramerican cyclothem deposition invoke orbitally driven glacioeustasy to explain widespread cyclic marine and nonmarine late paleozoic sedimentary sequences base level fluctuations of 100 m have been estimated for the deposition of mid continent north american subpycnoclinal black shales subaerial exposure relief of algal bioherms in the sacramento mountains and russian platform carbonates similar to the pleistocene these glacioeustatic fluctuations are thought to be driven by variations in orbital parameters to evaluate this hypothesis a coupled general circulation model ice sheet model was used to simulate the effects of both transient orbital changes and variable atmospheric p co 2 concentrations on late paleozoic continental ice sheets in our model large continental ice sheet inception is simulated at and below p co 2 levels of 280 ppm model results predict that while changing orbital parameters results in dynamic ice sheet behavior the maximum orbitally induced sea level fluctuation is 25 m the model also demonstrates that the complete ablation of ice sheets formed at 280 ppm 7 9 10 7 km 3 sea level change 135 m requires an increase in atmospheric p co 2 to levels 2240 ppm these results present a potential paradox while our model is able to simulate widespread gondwanan glaciation it is unable to reproduce significant orbitally driven glacioeustatic fluctuations without very large magnitude carbon cycle perturbations we discuss possible solutions to this paradox

mobile trace element contents in jilin chondrite abstract we determined ppm ppt levels of co se ga rb cs te bi ag in tl zn and cd arranged empirically in order of increasing mobility at 1000 c by radiochemical neutron activation analysis in eight jilin samples of known40ar content and a large vein from one of these the trace element contents of the vein do not differ markedly from those of jilin whole rock samples only rb correlates with40ar possibly due to chance all elements but cd are present at levels similar to those in h4 6 chondrites with longk ar age i e presumably mildly shocked these levels are low relative to those in analogous l4 6 chondrites suggesting that h chondrites formed and or evolved under higher pre shock temperatures than did mildly shocked l chondrites time temperature conditions during shock loading of jilin parent material were mild relative to those in strongly shocked l chondrites being sufficient at most only to mobilize cd

volcanic tremor at pavlof volcano alaska october 1973 april 1986 in thirteen years 1973 1986 of seismic monitoring of pavlof volcano 488 episodes of volcanic tremor have been recorded only 26 of which have been previously described in the literature this paper tabulates and describes all the tremor episodes and reports on the results of all analyses to date pavlof tremor durations range from 2 minutes to greater than 1 week episodes accompanying magmatic eruptions have durations greater than 1 hour and sustained amplitudes of greater than 6 mmp p 54 nanometers at 1 5 hz on station pvv 8 5 km from the vent digital data provide much better amplitude resolution than helicorders do helicorders however provide continuous coverage whereas digital data are intermittent correlations of tremor with visual eruption observations shows that tremor amplitudes are roughly correlated with heights of lava fountains but the correlation of tremor amplitudes with plume heights is more problematic fast fourier transform fft spectra show that pavlof tremor is quite statinary for the entire time period 1973 1983 all principal spectral peaks lie between 0 8 and 3 0 hz and may be caused by resonance of magma and gas and resonance of the volcanic pile preliminary analysis of 2 and 3 component data shows thatp s pl and rayleigh waves may be present in pavlof volcanic tremor other waveforms can be misidentified as tremor most commonly those caused by storms ors waves of regional earthquakes a strategy is proposed to distinguish tremor from noise using automatic seismic data acquisition and analysis systems pavlof s volcanic tremor is briefly compared with a preliminary sample of over 1100 cases of tremor from 84 volcanoes worldwide finally several recommendations for monitoring and reporting volcanic tremor are discussed

cosmic ray effects in the antarctic meteorite allan hills a 78084 abstract in the antarctic h4 chondrite alha 78084 measurements of 26 al 53 mn noble gases and cosmic ray tracks have been performed on documented samples to investigate depth dependences of cosmic ray induced effects 26 al activities vary between 224 and 246 dpm kg si equ 53 mn activities range from 299 to 359 dpm kg fe in agreement with the kohman and bender model 1 22 ne 21 ne ratios between 1 145 and 1 176 are observed the 3 he 21 ne vs 22 ne 21 ne correlation shows a trend line similar to those found for st severin and keyes track data indicate ablation depths between 1 3 and 6 5 cm with a mean ablation of 3 8 cm this leads to a preatmospheric mass of about 40 kg and an ablation loss of about 66 by mass preatmospheric size and shielding calculated from measured 22 ne 21 ne ratios using the semi empirical curves of bhandari and potdar 2 agree with those derived from track measurements the combined data of spallation nuclides and cosmic ray tracks indicate a simple one stage irradiation as a small object with a preatmospheric radius of 14 1 cm for about 32 m y and a terrestrial age of less than 170 000 years

noble gases 81krkr ages and10be of chrondrites from china abstract a comprehensive study of the cosmic ray exposure history of five ordinary chondrites from china was carried out using measurements of the noble gas isotopic abundances and 10 be concentrations the following average cosmic ray exposure ages based on cosmogenic 21 ne and on 81 kr kr dating were obtained zhaodong l4 15 7 3 0 m y nan yang pao l6 48 10 0 m y guangrao l6 16 8 3 5 m y and lunan h6 26 7 5 0 m y the h5 chondrite zaoyang was exposed for only 0 90 0 12 m y to galactic cosmic rays as calculated from the 10 be activity and from the low amounts of cosmic ray produced noble gases the zhaodong chondrite contains large amounts of 80 kr and 82 kr produced by neutron capture of bromine from the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

cosmic ray records in the ll chondrite dhurmsala abstract the radionuclides 10be 26al and 53mn the noble gas isotopic composition of he ne and xe and cosmic ray particle track densities were determined in six documented samples of a large fragment of the dhurmsala meteorite fall using the 22 ne 21 ne ratios and the 21ne concentrations the cosmic ray exposure age of dhurmsala is deduced to be 13 0 1 0 my the results suggest a simple exposure history the six samples covering a distance of 13 cm do not show any significant isotopic variations due to depth effects as expected in a large meteorite consistent with the model calculations of reedy 1985 and bhandari and potdar 1982 the results obtained in this study indicate that the dhurmsala samples investigated here originate at a depth of 20 to 24 cm in a meteoroid with a preatmospheric radius of r 50 cm

dating jilin and constraints on its temperature history abstract as part of the consortium effort to study the jilin h5 meteorite two documented samples from the main mass and one from a smaller strewn field fragment have been dated with the 40 ar 39 ar technique in addition 4 he and argon release patterns from a non irradiated sample were determined to obtain diffusion parameters concordant 40 ar 39 ar ages of 4 0 ae indicate that jilin was totally degassed at that time using also the results from other consortium studies 1 2 it is shown that largely varying secondary 40 ar and 4 he losses are related to the position of the samples within the pre atmospheric meteoroid this sets rather stringent limits to the temperature history of jilin the surface of the meteoroid was heated probably 400 000 years ago at the separation from its parent body model calculations suggest that the heating event was moderate initial surface temperature 500 600 c and brief surface cooling rate 1 2 c h heating of meteoroids at separation from their parent bodies might be a common phenomenon it could explain late disturbances in some isotope systems and could be the driving force for hydrothermal alterations the age of the reheating does not at all become apparent in the 40 ar 39 ar data calling in question the use of low temperature 40 ar 39 ar ages to date multi collisional events in meteorite parent bodies

an isotropic source of volcanic tremor observation with a dense seismic network at izu oshima volcano japan abstract volcanic tremor is frequently observed associated with activities of volcanoes in order to study the nature of the volcanic tremor in detail we installed a dense seismic network covering the area of the summit crater of izu oshima volcano we succeeded in observing volcanic tremors preceding a small explosive eruption on november 16 1987 the explosion was followed by a significant draining back of lava which had filled the summit crater of the 1986 eruption we analyzed the low frequency components of the tremor near 0 7 hz and found that it was generated by an isotropic volumetric source located 350 m above sea level assuming a cyclic pressure change at the source the amplitude of the seismic moment is 3 4 10 10 nm this corresponds to a pressure fluctuation of 3 10 3 pa 0 03 bar in the system containing the cavity of 10 7 m 3 which is comparable to the volume of subsidence at the 1987 eruption

cyclic changes in pennsylvanian paleoclimate and effects on floristic dynamics in tropical pangaea abstract wetland floras narrowly define perceptions of pennsylvanian tropical ecosystems the so called coal age such wetlands reflect humid to perhumid climate leading to characterizations of pennsylvanian tropics as everwet swampy these views are biased by the high preservation potential of wetlands sedimentation patterns paleosols and fossil floras indicate the presence of vegetation tolerant of subhumid to dry subhumid perhaps semi arid climate in basins between peat formation times understanding the significance of this seasonally dry vegetation has suffered from conceptual and terminological confusion a clearer view has emerged as models for framing the data have improved basinal floras typical of seasonally dry conditions relatively low soil moisture regimes are well documented but mainly from isolated deposits some of the earliest dominated by primitive pteridosperms flozfern floras occur in clastic rocks between european early pennsylvanian coal beds later early pennsylvanian fern cordaitalean vegetation different from coal floras is preserved in marine goniatite bullions conifers are first suggested by late mississippian potoniesporites pollen about the same time in north america broadleaf foliage lesleya and megalopteris occur in basin margin settings on drought prone limestone substrates the best known xeromorphic floras found between coal beds appear in the middle through late pennsylvanian containing conifers cordaitaleans and pteridosperms the middle pennsylvanian appearances of this flora are mainly allochthonous though parautochthonous occurrences have been reported parautochthonous assemblages are mostly late pennsylvanian the conifer flora became dominant in western and central pangaean equatorial lowlands in earliest permian location of the humid perhumid wetland flora during periods of relative dryness though rarely discussed is as or more perplexing than the spatial location of seasonally dry floras through time wetland plants had few migratory options and possibly survived in small refugia within and outside of basins coupled oscillations in climate sea level and vegetation were driven most likely by glacial interglacial fluctuations perhaps controlled by orbital cyclicity

excitation of a buried magmatic pipe a seismic source model for volcanic tremor recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so called long period or low frequency events accordingly we can view the long period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor generating system we present a seismic model in which the source of tremor is the acoustic resonance of a fluid filled volcanic pipe triggered by excess gas pressure the model consists of three elements namely a triggering mechanism a resonator and a radiator for simplicity we assume a hemispherical trigger cylindrial resonator and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk shaped radiator shutting off its bottom considering the simple case of a source buried in a homogeneous half space we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances the results demonstrate that the displacement attributed to the pipe dominates the near field motion while that due to the disk is representative of the intermediate and far fields the trigger itself has a smaller contribution mainly limited to the field in the proximity of the source the characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well developed harmonic wave train dominated by rayleigh waves at larger distances no clear shear arrival can be detected in the synthetic seismograms the displacement spectrum reflects the organ pipe modes of the conduit and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid in the case of the cylindrical magma column considered the radiation loss is proportional to the square of the pipe radius while the loss related to viscous damping is inversely proportional to the same factor indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir the relative importance of the pipe and disk elements likewise is a function of the conduit cross section this suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near and far field observations

magma chamber and eruptive processes at izu oshima volcano japan buoyancy control of magma migration underground magmatic processes are modeled for a sequence of eruptions that started on november 15 1986 at izu oshima volcano it is predicted from the seismic gap the seismic velocity structure the deformation center and the gravity minimum that there should be a magma chamber at a depth of about 4 km beneath the northwestern flank of the volcano the effusion of basalt that began at the summit crater established a well connected conduit system between the magma chamber and the summit crater the following eruption from new fissures a few days later produced more felsic and less homogeneous compositions basaltic andesite to andesite because of cooling and fractional crystallization of ascending magma subsequently four small eruptions were caused by gravitational sliding of sluggish semisolid magma into openings in and along the sides of the shallow conduit as magma was returning to the magma chamber systematic tilt changes due to transmission of pressure pulses from the summit crater to the magma chamber were observed during a small explosion on november 18 1987 and for many periods of episodic volcanic tremor the migration of magma is interpreted to have been driven by a buoyancy force associated with bubbles in the magma in this regard dominant ascent of magma beneath the summit crater and new fissures in november 1986 was triggered by vesiculation of volatiles in the magma and the subsequent return flow of magma resulted from the release of bubbles through degassing the observed response of volcanic processes following earthquake swarms northwest of the island is also explained by buoyancy controlled magma migration that could intensify the effect of small fluctuations in the crustal stress

the non trivial problem of meteorite pairing pairing is the procedure of identifying fragments of a single meteorite fall that were separated during atmospheric passage or during terrestrial history by establishing the similarity of two or more meteorite fragments we argue that pairing is governed by two principles that only a single mismatch of properties is required to refute a proposed pairing and that virtually all pairings bear some degree of uncertainty using data distributions for modern falls we take a probability approach to estimate degrees of certainty associated with proposed pairings emphasizing the importance of unusual features for new pairing criteria or new analytical additions to old criteria the degree of variation within individual meteorites must be delineated and the degree of variation within meteorite classes must be quantified criteria for pairing can be divided into 1 parent body history indicators 2 meteoroid space history indicators and 3 terrestrial history indicators included in these categories are 11 specific criteria including petrographic textures mineralogy and mineral composition terrestrial age estimates cosmic ray exposure ages and natural thermoluminescence tl levels not all criteria are applicable to all meteorite types about 2275 pairings suggested in the literature have been subjected to this analysis many literature pairings especially those involving common meteorite types bear large uncertainties due to lack of data

cosmic ray constancy and cosmogenic production rates in short lived chondrites abstract five chondrites with short cosmic ray exposure ages 26al and 53mn saturation values deduced from short lived chondrites alone agree within 20 with those deduced from all chondrites values of 21p based on 53mn and 26al are 0 30 0 03 and 0 46 0 05 respectively in h chondrites possible causes for the difference include half life errors data selection and a recent 2 5myr increase in the cosmic ray intensity

late paleozoic tropical climate response to gondwanan deglaciation coupled climate biome model simulations of the late paleozoic were developed to determine the response of pangean tropical climate to gondwanan deglaciation the model simulations predict substantial changes over equatorial pangea including continental drying a reversal of equatorial winds warming heavier 18 o values of meteoric precipitation and the expansion of deserts and the contraction of forests the magnitude of these tropical responses is sensi tive to the extent of gondwana continental ice and the deglacial rise in atmospheric pco 2 boundary conditions that are not well known for the late paleozoic nonetheless the model predictions are consistent with climatic and environmental trends determined from terrestrial proxy data implying that the deglaciation of gondwana was a transformational climate event in tropical pangea

stratigraphic imprint of the late palaeozoic ice age in eastern australia a record of alternating glacial and nonglacial climate regime stratigraphic and sedimentological data from new south wales and queensland eastern australia indicate that the late palaeozoic ice age comprised at least eight discrete glacial intervals each 1 8 ma in duration here termed glaciations separated by nonglacial intervals of comparable duration these events spanned an interval from mid carboniferous c 327 ma to the early late permian c 260 ma and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late early permian followed by an opposite trend towards the final demise of glaciation in the late permian the alternating glacial nonglacial motif suggests that the late palaeozoic ice age was considerably more dynamic than previously thought these patterns are remarkably consistent with recent interpretations of palaeofloral change eustatic sea level fluctuations and co 2 climate glaciation relationships for this interval of time the detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere notably in far field ice distal northern hemisphere successions

the magnitude of late paleozoic glacioeustatic fluctuations a synthesis a comprehensive literature review shows that the magnitude of eustatic fluctuations varied throughout the carboniferous and permian and that at least eight distinct phases can be recognized facies juxtapositions in carbonate successions and erosional relief in clastic successions indicate that glacioeustatic fluctuations of 20 25 m and occasionally as much as 60 m took place throughout the early mississippian tournaisian a widely recognized glacial period middle mississippian mid chadian through holkerian shallow marine carbonate and clastic successions indicate that eustatic fluctuations were 10 25 m a decrease that matches well with the paucity of coeval glacial deposits late visean asbian through mid brigantian glacioeustatic fluctuations of 10 50 m record the initial phases of ice accumulation in advance of the widespread mid carboniferous glacial event the latest mississippian earliest pennsylvanian mid brigantian through langsettian was a time of widespread glaciation and strata of this age commonly exhibit evidence of glacioeustatic fluctuations of as much as 40 100 m although middle pennsylvanian duckmantian through asturian glacial deposits are present in eastern australia paleovalley depths suggest that coeval glacioeustatic fluctuations were less than 40 m glacioeustatic fluctuations of as much as 100 120 m have been widely reported from late pennsylvanian earliest permian stephanian through mid sakmarian successions in north america an increase that corresponds to the growth of large ice sheets across much of gondwana and the accumulation of ice in the northern hemisphere incision and facies juxtaposition in early middle permian mid sakmarian through kungurian successions in eastern australia indicate that glacioeustatic fluctuations of 30 70 m occurred during the waning stages of major glaciation erosional relief in paleoequatorial carbonates and the presence of coeval glacial deposits in australia suggests that eustatic fluctuations of 10 60 m occurred during the final stages of glaciation in the middle to late permian roadian through capitanian but the modest size of most of these fluctuations makes it difficult to isolate the glacioeustatic signature this review demonstrates that far field cyclic successions record changing glacial conditions in gondwana that the magnitude of glacioeustatic fluctuations was directly related to the volume of glacial ice that carboniferous permian glacioeustasy was more variable than previously recognized and that generalizations from short temporal intervals are probably not representative of the late paleozoic ice age as a whole although any attempt to quantify the magnitude of ancient eustatic changes is based on caveats and assumptions this review incorporates the results of over 100 published papers on the topic in an attempt to minimize the errors inherent in any one study

high resolution geochemistry and sequence stratigraphy of the hushpuckney shale swope formation eastern kansas implications for climato environmental dynamics of the late pennsylvanian midcontinent seaway abstract the hushpuckney shale member of the swope formation missourian stage eastern kansas is the core shale of a kansas type cyclothem formed during the late transgressive to early regressive phases of a late pennsylvanian glacio eustatic cycle coeval high frequency climato environmental dynamics of the midcontinent seaway are preserved in the kgs orville edmonds no 1a study core as centimeter scale variation in major components organic carbon authigenic sulfides and phosphate detrital siliciclastics organic macerals trace element redox proxies mo u v zn and ichnofabric features benthic o 2 levels declined sharply from the base of the black shale 0 cm went sulfidic at 4 cm and reached a redox minimum at 21 cm above this level redox potential gradually rose fluctuating between sulfidic and nonsulfidic conditions from 35 cm to the black shale gray shale contact at 52 cm onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft bodied organisms comprised of deep tiered tracemakers tolerant of low o 2 conditions trichichnus and shallow tiered tracemakers favoring soupground helminthopsis or firmground substrates the zoophycos phycosiphon schaubcylindrichnus planolites association most geochemical records exhibit a low order cycle spanning the full 52 cm thickness of the black shale submember reflecting a dominant glacio eustatic control the base and top of the black shale submember record lateral migration of the pycnocline across the midcontinent shelf during the transgressive and regressive phases respectively the maximum flooding surface mfs is at 17 23 cm an interval containing the euxinic peak and characterized by high concentrations of illite representing a cratonic siliciclastic flux and terrestrial organic macerals the product of transient increases in humidity weathering rates and the export of coal swamp vegetation associated with the interglacial highstand of the swope cyclothem although a transgressive surface of maximum starvation sms cannot be confidently identified a phosphate rich regressive condensation surface at 28 34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth a correlative shift in dominance from terrestrial to marine derived organic macerals reflects increased upwelling of nutrient rich deepwaters and enhanced primary productivity penecontemporaneous paleogeographic and climatic factors e g semirestricted circulation monsoonal precipitation predisposed the midcontinent seaway toward sensitivity to high frequency climato environmental fluctuations which are preserved as 2 to 7 cm thick cycles in the study core the 52 cm thick black shale submember contains 12 such high order cycles which have an estimated average duration of 2 to 9 kyr implying a sub milankovitch band i e millennial scale climatic control the results of this study are relevant to the sequence stratigraphic interpretation of kansas type cyclothems the dynamics of gondwanan ice age glacio eustatic cycles and the development of boundary conditions for late pennsylvanian paleoclimate models

aluminum 26 sodium 22 and cobalt 60 in two drill cores and some other samples of the jilin chondrite two drill cores have been taken from the main mass of the jilin chondrite jilin i one was bored perpendicular to the 2 surface of the first exposure stage the other parallel to this surface and both intersected at the presumed 4 center of jilin s second exposure stage the probed depths were 65 cm and 105 cm respectively with a diameter of 26 5 mm fifteen drill core samples 2 documented samples and 14 strewn field samples have been analysed for 26al 60co and 22na the reconstruction of the preatmospheric jilin via60co resulted in slight deviations for the center position and for the radius now r 80 10 5 cm compared to an earlier study in 1985 the 60co production rate profile is now almost completely reproduced a constant production rate of 22na and of 26al during the 4 irradiation is confirmed also for the center region the good linear correlation of 26al and 21ne yields slightly revised irradiation ages for both stages of t1 7 1 ma and t2 0 32 0 06 0 03 ma

paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks climate is a primary control on sediment flux from continental sources into sedimentary systems in warm climates siliciclastic input is greatest under highly seasonal rainfall nonseasonal conditions favor formation of end member chemical rocks perennially wet climates are conducive to coal formation whereas dry climates produce carbonates and or evaporites stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive regressive events and tectonics

noble gases and cosmogenic radionuclides in the gold basin l4 chondrite shower thermal history exposure history and pre atmospheric size we measured the concentrations of the cosmogenic radionuclides 10be 26al 36cl and 41ca in the stone and metal fractions of 15 fragments of the gold basin l4 chondrite shower as well as noble gases in 18 gold basin fragments a comparison of 10be 26al and 41ca concentrations with calculated production rates from two different models indicates that the gold basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3 5 m as was predicted by recent model calculations the noble gases show a reversal of the 22ne 21ne ratio at very high shielding the 21ne 10be and 21ne 26al ratios in most samples are constant and correspond to a 4 exposure age of 18 2 myr however three gold basin samples show a 30 120 excess of 21ne implying that they were previously exposed close to the surface of the parent body whereas the other samples were buried several meters deeper concentrations of neutron capture 36ar in most samples are consistent with measured concentrations of neutron capture 36cl and an exposure age of 18 myr large excesses of neutron capture 36ar were found in those samples with an excess of 21ne providing additional evidence of a first stage exposure on the parent body the excess of spallation produced 21ne and neutron capture produced 36ar in these samples indicate a first stage exposure of 35 150 myr on the parent body the radiogenic 4he and 40ar concentrations indicate a major impact on the parent body between 300 and 400 myr ago which must have preceded the impacts that brought the gold basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 myr ago