the record of sea water 187os 186os variation through the cenozoic the variation of sea water 187os 186os during the cenozoic has been determined by analyzing the hydrogenous os released by acidic hydrogen peroxide leaching of layers in an abyssal north pacific pelagic clay core overall sea water 187os 186os has increased from 3 2 at 58 ma to the present day value of 8 6 this pattern is similar to the well known 87sr 86sr evolution of sea water the increase in the proportion of continental os 187os 186os 10 could be due to enhanced weathering rates of black shales associated with the uplift of the himalayas other elements associated with black shale weathering such as u and p should show effects in the sedimentary record

extraterrestrial iridium sediment accumulation and the habitability of the early earth s surface we have measured the concentrations of iridium and platinum in ancient metasediments from akilia island southern west greenland to obtain the first direct constraints on the extraterrestrial mass flux at the earth s surface before 3 8 gyr the lunar impact record indicates this was a time of heavy bombardment the average lunar cratering rate between 4 15 and 3 8 gyr was elevated 150 times the modern value hence heightened ir and pt abundances are possible in sediments from this time however the observed concentrations of ir and pt in the akilia metasediments are extremely low in most samples ir 3 ppt and pt 20 ppt in contrast the average modern crustal abundances of ir and pt are 50 ppt and 2 ppb respectively the very low concentrations observed in the akilia metasediments cherts and banded iron formations can be reconciled with the lunar record by quantifying the effects of sedimentation rate on the efficiency with which the extraterrestrial mass flux is sampled the observations are explained if rapidly accumulating sediments sampled stochastic bombardment by an impactor population governed by a power law mass distribution such that exogenous ir and pt are concentrated in rare horizons the required sedimentation rates are geologically plausible this explanation has two consequences first evidence of massive impact events may be rare in sediments older than 3 8 gyr despite heavy bombardment second conditions at the earth s surface may have been conducive for extensive accumulation of sedimentary sequences and for microbial life during most of the period 4 15 3 8 gyr it is possible that the earth s surface hosted microbial habitats during most of this time if microbes living in deep sea or deep crustal environments survived photic zone vaporizing impacts that would have occurred every 30 myr hence evidence of life is expected even in the earliest sedimentary sequences

osmium isotope variations in the oceans recorded by fe mn crusts this study presents osmium os isotope data for recent growth surfaces of hydrogenetic ferromanganese fe mn crusts from the pacific atlantic and indian oceans in general these data indicate a relatively uniform os isotopic composition for modern seawater but suggest that north atlantic seawater is slightly more radiogenic than that of the pacific and indian oceans the systematic difference in the os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high re os ratio in the north atlantic ocean consistent with the distribution of nd and pb isotopes this spatial variation in the os isotope composition in seawater is consistent with a residence time for os of between 2 and 60 kyr indian ocean samples show no evidence of a local source of radiogenic os which suggests that the present day riverine input from the himalaya tibet region is not a major source for os recently formed fe mn crusts from the tag hydrothermal field in the north atlantic yield an os isotopic composition close to that of modern seawater which indicates that in this area the input of unradiogenic os from the hydrothermal alteration of oceanic crust is small however some samples from the deep pacific 4 km possess a remarkably unradiogenic os isotope composition 187 os 186 os ratios as low as 4 3 the compositional control of os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles rather than indicating the presence of an unradiogenic deep water mass moreover this unradiogenic signal appears to be temporary and local and has had little apparent effect on the overall evolution of seawater these results confirm that input of continental material through erosion is the dominant source of os in seawater but it is not clear whether global os variations are due to the input of mantle or meteoritic material or simply indicate that the continental source itself is not uniform 1999 elsevier science b v all rights reserved

glacial to interglacial changes in carbonate and clay sedimentation in the atlantic ocean estimated from 230th measurements abstract the cause of the climatically controlled fluctuations in the carbonate content of deep sea sediments remains the subject of uncertainty and debate three variables are involved supply of biogenic carbonate loss by dissolution and dilution by non carbonate phases it is suggested that 230 th which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale results of a preliminary analysis based on published data indicate that for depths at and above the lysocline the carbonate fluctuations observed in cores from the north atlantic ocean are due primarily to variations in the terrigenous clay input which was 2 5 times higher during glacials than during interglacials carbonate deposition appears to have been somewhat reduced during glacials but probably not by more than a factor of 2 from published 230 th 232 th profiles it appears that the south atlantic ocean also received increased inputs of terrigenous clay during glacial periods

the osmium isotopic composition of organic rich marine sediments abstract the osmium os concentration and 187 os 186 os ratio of several recent marine organic rich sediment samples from three widely separated sites have been measured os concentrations range from 0 095 to 0 212 ppb and 187 os 186 os ratios range from 8 2 to 8 9 the calculated fraction of hydrogenous os exceeds 78 in all samples thus the 187 os 186 os ratio of these samples reflects os isotopic composition of seawater the small range in measured 187 os 186 os ratio indicates that the os isotopic composition at these sites is fairly homogeneous the large magnitude of the os burial flux at these sites indicates the os burial in association with organic rich sediments is an important sink in the marine cycle of os these data also suggest that ancient organic rich sediments may provide a record of past variations in the os isotopic composition of seawater

extraterrestrial 3he in seafloor sediments evidence for correlated 100 kyr periodicity in the accretion rate of interplanetary dust orbital parameters and quaternary climate we have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the ontong java plateau western equatorial pacific ocean odp site 806 these results provide a two million year record of the burial flux of extraterrestrial 3he which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles the 3he burial flux prior to 700 ka was relatively low 0 5 pcc cm 2 kyr 1 but from 700 ka to the present the burial flux gradually increased to a value of 1 0 pcc cm 2 kyr 1 100 kyr periodicity in the 3he burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate with high 3he burial fluxes associated with interglacial periods this periodicity and phase are consistent with previous 3he measurements in north atlantic sediments although 100 kyr periodicity in 3he burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris the measurements and predictions differ by one half cycle in phase nevertheless our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and or inclination relative to the solar system invariable plane such control is a necessary but not sufficient condition for the hypothesis of muller and macdonald 1995 that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period we also identify several brief 25 kyr intervals of strongly enhanced 3he burial possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles

a comparative study of accumulation rates derived by he and th isotope analysis of marine sediments abstract we present a detailed down core analysis of helium isotope ratios and concentrations for bulk sediments from the central equatorial pacific that span the last two glacial interglacial cycles measured 3 he 4 he ratios range from 1 0 10 5 to 2 1 10 4 or 7 4 to 149 times the atmospheric ratio the 3he from interplanetary dust particles idps constitutes virtually all of the 3he measured within the sediment because carbonate accumulation rates are high in the equatorial pacific the measured 3he concentrations are lower than have been measured elsewhere and range from 4 7 10 13 to 3 0 10 12 cm3stp g 1 if the cosmic dust 3he flux is constant with time sediment mass accumulation rates can be determined from the 3he concentration in sediments the excess 230th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in situ decay of 234u in seawater to first order initial excess 230th activities correlate with 3he concentrations within this core based on the 230th results we estimate the 3he flux to the earth s surface as 9 6 2 0 10 16 cm3stp cm 2 a 1 if this flux has remained constant over extended periods of time it can be used to determine sediment accumulation rates beyond the 230th range 300 000 yr

post depositional mobility of platinum iridium and rhenium in marine sediments the discovery of high concentrations of iridium in cretaceous tertiary boundary sediments engendered the hypothesis1 that a meteorite collided with the earth 65 million years ago coincident with the mass extinction that occurred at that time iridium spikes of various magnitudes have subsequently been reported at more than 10 other extinction horizons2 11 it has been suggested on the other hand that geochemical processes might create or modify many of these spikes5 11 4 but a critical evaluation of these suggestions has been hindered by incomplete understanding of low temperature iridium geochemistry other platinum group elements ru rh pd re os pt au are often found to be associated with ir spikes and inter element ratios have been used to assess the cosmic or terrestrial nature of the enrichments5 7 9 28 29 but the geochemical influences on these relative abundances are also poorly constrained here we describe analyses of recent abyssal marine sediments which allow us to characterize the behaviour of pt re and ir during early diagenesis these elements are redistributed by changes in sedimentary redox conditions such changes can probably account for many of the small platinum group element spikes found in the geological record and may render ambiguous attempts to interpret inter element ratios

meteoric smoke fallout over the holocene epoch revealed by iridium and platinum in greenland ice an iridium anomaly at the cretaceous tertiary boundary layer has been attributed to an extraterrestrial body that struck the earth some 65 million years ago1 it has been suggested that during this event the carrier of iridium was probably a micrometre sized silicate enclosed aggregate2 or the nanophase material of the vaporized impactor3 but the fate of platinum group elements such as iridium that regularly enter the atmosphere via ablating meteoroids remains largely unknown here we report a record of iridium and platinum fluxes on a climatic cycle timescale back to 128 000 years ago from a greenland ice core4 we find that unexpectedly constant fallout of extraterrestrial matter to greenland occurred during the holocene whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust laden atmosphere of the last glacial age we suggest that nanometre sized meteoric smoke particles5 6 formed from the recondensation of ablated meteoroids in the atmosphere at altitudes 70 kilometres are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps this implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the holocene

variations of the187os 186os ratio of seawater over the past 28 million years as inferred from metalliferous carbonates abstract the os concentration and isotopic composition of metalliferous carbonates deposited on the east pacific rise over the past 28 ma are reported with complimentary sr isotope data variations in the os isotopic composition of these samples are interpreted as a record of past changes in the os isotopic composition of seawater these results are consistent with isotopic analyses of leachable os in pelagic clays which have also been interpreted as a record of the187os 186os ratio of seawater through time 1 the metalliferous carbonate record clearly shows that seawater os and sr isotope systems are partially decoupled from one another over the past 28 ma accelerated weathering of ancient organic rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the187os 186os ratio of seawater over the past 15 ma this rapid increase suggests that the seawater os record can potentially be used as a stratigraphic tool in some neogene marine deposits

petrology and geochemistry of antarctic micrometeorites abstract the petrology and geochemistry of twentythree chondritic dust particles with masses of 1 47 g sizes 100 400 m were recovered from blue ice near cap prudhomme antarctica and studied by inaa asem empa and optical microscopy sample selection criteria were irregular shape and for a subsample black color with the aim of studying as many unmelted micrometeorites mms as possible of thirteen unmelted mms six were phyllosilicate dominated mms and seven were coarsegrained crystalline mms consisting mainly of olivine and pyroxene the remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases scoriaceous mms thus of the black particles selected an astonishing portion 40 by number consisted of largely unmelted mms although unmelted most phyllosilicate mms have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed but not melted the original preterrestrial mineralogy is occasionally preserved and consists of serpentine like phyllosilicates with variable amounts of cronstedtite tochilinite like oxides olivine and pyroxene the crystalline mms consist of olivine low ca pyroxene tochilinite like oxides and occasional ni poor metal relics in scoriaceous mms consist of the same phases mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of cm and cr type carbonaceous chondrites however the olivine pyroxene ratio 1 and the lack of carbonates sulfates and of very fe poor refractory element rich olivines and pyroxenes sets the mms apart from cm and cr chondrites the bulk chemistry of the phyllosilicate mms is similar to that of cm chondrites however several elements are either depleted ca ni s less commonly na mg and mn or enriched k fe as br rb sb and au in mms as compared to cm chondrites similar depletions and enrichments are also found in the scoriaceous mms we suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted mms the overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere most mms are coated by magnetite of platy or octahedral habit which is rich in mg al si mn and ni we interpret the magnetites to be products of recondensation processes in the high 90 km atmosphere which are therefore probably the first refractory aerominerals identified

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

extraterrestrial 3 he as a tracer of marine sediment transport and accumulation the deposition rate of deep sea sediments and their focused redeposition by deep sea currents can be evaluated from analyses of sedimentary 230th with a temporal resolution limited only by bioturbation6 7 10 11 230th is produced uniformly throughout the ocean by radioactive decay of dissolved 234u and is removed sufficiently fast by sorption onto sinking particles to act as a constant flux tracer of sedimentation rates but the half life of 230th 75 kyr limits its use for this purpose to the past 200 250 kyr here we explore the use of extraterrestrial 3he from interplanetary dust particles1 4 idps as a constant flux proxy that is free from this limitation a comparison of 3he with 230th in two cores from the equatorial pacific ocean indicates that the variability in the mean flux of idps over the past 200 kyr is less than 75 but in contrast to this relatively constant rate of supply of 3he to the deep sea the local burial rates of 3he and 230th have varied by a factor of five over the past 450 and 200 kyr respect ively we interpret this variability as reflecting sediment focusing with a temporal pattern that suggests regular cycles of climate driven reorganization of near bottom currents in the deep pacific ocean

do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific 1 the paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing bioturbation and horizontal sediment transport diagenesis and bioturbation are relatively well studied but the effects of physical sedimentation have been largely ignored modeling u series isotopes e g 230th can potentially quantify horizontal sediment movement but model horizontal sediment focusing often equals or exceeds the vertical particle rain we find no evidence of this level of sediment focusing in the equatorial pacific from geophysical data or sediment core comparisons the overestimate of sediment focusing by 230th is probably caused by poor model assumptions that sediment does not fractionate does not sort according to size during transport and that 230th cannot leak from slowly accumulating sediments both assumptions are weak u series methods do hold promise to quantify sediment focusing if properly calibrated with calibration the trade offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed

osmium in seawater vertical profiles of concentration and isotopic composition in the eastern pacific ocean os concentrations 6 6 to 9 8 pg l and isotopic compositions mean 187os 188os 1 067 0 011 2 error were measured by a new method in seawater samples collected from the eastern pacific ocean in contrast to recent work in the indian ocean measured os concentrations display a 30 depletion coincident with both the core of the oxygen minimum zone and a local minimum in light transmission whilst we cannot preclude the possibility that an oxidation resistant form of os in seawater prevented isotopic equilibration between the enriched isotope spike and naturally occurring os which would bias measured concentrations to low values we interpret these data as evidence of non conservative behaviour of os in seawater in this zone the data may be explained by two possible processes the horizontal advection of a water mass of low os concentration from the continental slope and or the in situ adsorption of os onto sinking particles to what extent each process is contributing to the os minimum is unclear from the data the os isotopic composition of these eastern pacific samples are within 2 of those recently reported from the indian ocean which indicates that the isotopic composition of seawater os in these two ocean basins is indistinguishable at this level of precision neutrally buoyant hydrothermal plume samples collected in close proximity to active vents at 9 46 n on the east pacific rise have os concentrations and isotopic compositions similar to ambient deep water this shows that the flux of os from high temperature hydrothermal vents is not large compared to the inventory of os in ambient seawater comparisons between filtered and unfiltered samples show that the majority of seawater os in these samples is in the dissolved phase

quaternary time scale for the ontong java plateau milankovitch template for ocean drilling program site 806 a 2 m y oxygen isotope record of globigerinoides sacculifer from the ontong java plateau based on cores from ocean drilling program leg 130 is dated by matching variations to an orbital template the procedure allows us to present the most complete quaternary record available for the western equatorial pacific the template generating algorithm describes a balance between growth and melting of ice following basic milankovitch theory ice growth is taken as constant while melting is taken to depend on summer insolation current ice mass and average past ice mass template settings must be changed once between 1 and 1 2 ma to reflect a major shift in climate template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances one result of template dating is an exact age for the brunhes matuyama magnetic reversal boundary at 790 5 ka as well as several other precise dates 900 ka for the middle pleistocene climate shift 1070 1240 and 1450 ka for isotope stages 31 37 and 47 respectively sedi mentation rates fluctuate between 18 and 28 m m y a ca 400 ka cycle being the most prominent major anomalies arise within the transitional regime 1 2 to 1 ma the origin of the cycles is unknown we propose productivity variations in the western equatorial pacific

miocene seawater 187os 188os ratios inferred from metalliferous carbonates abstract seawater 187os 188os ratios for the middle miocene were reconstructed by measuring the 187os 188os ratios of metalliferous carbonates from the pacific dsdp 598 and atlantic dsdp 521 oceans atlantic and pacific 187os 188os measurements are nearly indistinguishable and are consistent with previously published os isotope records from pacific cores the atlantic data reported here provide the first direct evidence that the long term sedimentary 187os 188os record reflects whole ocean changes in the os isotopic composition of seawater the pacific and the atlantic os measurements confirm a long term 0 01 myr increase in marine 187os 188os ratios that began no later than 16 ma the beginning of the os isotopic increase coincided with a decrease in the rate of increase of marine 87sr 86sr ratios at 16 ma a large increase of 1 in benthic foraminiferal 18o values interpreted to reflect global cooling and ice sheet growth began approximately 1 million years later at 14 8 ma and the long term shift toward lower bulk carbonate 13c values began more than 2 myr later around 13 6 ma the post 16 ma increase in marine 187os 188os ratios was most likely forced by weathering of radiogenic materials either old sediments or sialic crust with a sedimentary protolith we consider two possible miocene specific geologic events that can account for both this increase in marine 187os 188os ratios and also nearly constant 87sr 86sr ratios 1 the first glacial erosion of sediment covered cratons in the northern hemisphere 2 the exhumation of the australian passive margin new guinea arc system the latter event offers a mechanism via enhanced availability of soluble ca and mg silicates in the arc terrane for the maintenance of assumed low co2 levels after 15 ma the temporal resolution three samples myr of the 187os 188os record from site 598 for which a stable isotope stratigraphy was also constructed is significantly higher than that of previously published records these high resolution data suggest oscillations with amplitudes of 0 01 to 0 02 and periods of around 1 myr although variations in the 187os 188os record of this magnitude can be easily resolved analytically this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent however the short term 187os 188os variations may correlate inversely with short term benthic foraminiferal 18o and bulk carbonate 13c variations that reflect glacioeustatic events

the marine osmium isotope record over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research several analytical developments particularly in the past few years have significantly increased our ability to study the behaviour of osmium in the surficial environment the 187os 188os and osmium concentration of seawater river water rain and hydrothermal vent fluids have been measured directly recently the behaviour of osmium in estuaries critical for estimating the marine residence time of osmium has been studied our knowledge of the surficial osmium cycle has thus significantly improved in addition reconstructions of past variations in the marine 187os 188os recently have been extended back into the mesozoic this review attempts to summarize our current understanding of the marine osmium system present and past the 187os 188os of seawater during the cenozoic to first order mimics the marine 87sr 86sr record it is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic tectonic processes however the marine osmium isotope system differs fundamentally from the marine strontium isotope system this review emphasizes three important differences first large impacts are capable of resetting the 187os 188os to unradiogenic values without significantly affecting the marine strontium system second organic rich sediments are characterized by high 187re 188os resulting 187os 188os ingrowth trajectories are similar to the average slope of the cenozoic 187os 188os seawater record trends towards more radiogenic 187os 188os seawater therefore can be caused by weathering of organic rich sediments at a constant rate third the marine residence time of osmium is sufficiently short to capture short periodic glacial interglacial fluctuations that are inaccessible to the buffered marine strontium isotope system this offers the opportunity to discriminate between high frequency climatic and low frequency tectonic forcing

accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record the flux of particulate extraterrestrial et matter to the deep sea has been calculated using a four component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the pacific ocean nineteen published bulk leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation the calculated annual particulate flux averaged over several 100 ka by slow accumulation and bioturbation ranges from 18 000 to 67 000 tons with a mean value of 37 000 13 000 tons the data indicate no significant variability in the flux with time except at the k t boundary and are thus compatible with results based on ir accumulation in deep sea sediments the inferred constancy of the et matter flux is in contrast to recent results based on the et 3he flux that integrates only a small size fraction of the entire et matter flux 3he data indicate variability by a factor of 6 over the past 70 ma and a fivefold increase in the et flux between 3 ma and 1 ma that is not seen in the bulk et matter flux based on osmium isotope data the apparent constancy of the cosmic matter flux over the past 80 ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record except at the k t boundary a comparison between osmium isotope and ir data from two pelagic sediment cores from the pacific dsdp site 596 and ll44 gpc3 indicates that the seawater soluble fraction of et os cannot exceed 36 kg a and most likely is significantly smaller the maximum value of 36 kg a can account for approximately one half of the seawater soluble et matter flux necessary to balance the radiogenic continental runoff of os without any additional unradiogenic source simple mass balance calculations indicate that an additional unradiogenic source of os to the oceans most likely alteration of oceanic crust is required to balance the present day seawater osmium isotopic composition this source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

osmium isotopic variations in metalliferous sediments from the east pacific rise and the bauer basin the osmium concentration and isotopic composition of metalliferous sediments from the east pacific rise epr and bauer basin have been determined 187os186os ratios range from 7 9 to 8 6 with epr samples between 8 4 and 8 6 and bauer basin samples between 7 9 and 8 2 the osmium isotopic composition of epr samples is indistinguishable from the inferred osmium isotopic composition of seawater requiring that nearly all the osmium in these sediments is hydrogenous in origin this result is used to place broad constraints on the behavior of osmium in ridge crest hydrothermal systems while bauer basin samples also contain predominantly hydrogenous osmium an additional source of osmium with low 187os186os is required to account for the lower measured ratios calculation of total osmium burial fluxes in the bauer basin show that the background flux of cosmic osmium is adequate to produce the observed depression in bauer basin 187os186os ratios burial flux calculations also suggest that metalliferous sediment deposition is a significant sink in the marine osmium cycle

accretion rate of extraterrestrial particles determined from osmium isotope systematics of pacific pelagic clay and manganese nodules abstract pelagic clay and mn nodules from domes sites in the north pacific and a varved glacial lake deposit from connecticut were analysed for os concentration and isotopic composition by isotope dilution secondary ion mass spectrometry after treatment by nis fusion or oxalic acid leaching bulk pelagic clay from domes site c has 187 os 186 os 6 5 and os 0 14 ng g oxalic acid leaches of this same sediment and of mn nodules from domes sites a and c have more radiogenic 187 os 186 os ratios which average 8 3 bulk glacial lake hitchcock sediment has 187 os 186 os 12 5 and os 0 06 ng g the total os flux to north pacific pelagic clay is 7 to 10 ng os cm2 106 y lake hitchcock sediment provides an integrated value for the local crustal 187 os 186 os ratio the oxalic acid leaches are assumed to attack hydrogenous phases selectively a simple model in which the only sources of os to the ocean are continental crust with the isotopic composition of lake hitchcock 187 os 186 os 12 5 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of osmium to the sediment of 4 9 ngos cm2 106 y of which 20 is hydrogenous a model in which the sources of os to the ocean are continental crust with an 187os 186os ratio of 30 from the model of palmer and turekian 1986 oceanic mantle or crust with 187 os 186 os 1 04 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of os to the sediment of 5 7 ngos cm2 106 y of which none is hydrogenous these extraterrestrial os fluxes correspond to maximum c 1 chondrite accretion rates of between 4 9 104 and 5 6 104 tonnes y

platinum group elements pge and rhenium in marine sediments across the cretaceous tertiary boundary constraints on re pge transport in the marine environment the nature of re platinum group element pge pt pd ir os ru transport in the marine environment was investigated by means of marine sediments at and across the cretaceous tertiary boundary ktb at two hemipelagic sites in europe and two pelagic sites in the north and south pacific a traverse across the ktb in the south pacific pelagic clay core found elevated levels of re pt ir os and ru each of which is approximately symmetrically distributed over a distance of 1 8 m across the ktb the re pge abundance patterns are fractionated from chondritic relative abundances ru pt pd and re contents are slightly subchondritic relative to ir and os is depleted by 95 relative to chondritic ir proportions a similar depletion in os 90 was found in a sample of the pelagic ktb in the north pacific but it is enriched in ru pt pd and re relative to ir the two hemipelagic ktb clays have near chondritic abundance patterns the 1 8 m wide re pge peak in the pelagic south pacific section cannot be reconciled with the fallout of a single impactor indicating that postdepositional redistribution has occurred the elemental profiles appear to fit diffusion profiles although bioturbation could have also played a role if diffusion had occurred over 65 ma the effective diffusivities are 10 13 cm 2 s much smaller than that of soluble cations in pore waters 10 6 cm 2 s the coupling of re and the pges during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances if redistribution was caused by diffusion then the effective diffusivities are the same fractionation of os from ir during the ktb interval must therefore have occurred during aqueous transport in the marine environment distinctly subchondritic os ir ratios throughout the cenozoic in the south pacific core further suggest that fractionation of os from ir in the marine environment is a general process throughout geologic time because most of the inputs of os and ir into the ocean have os ir ratios 1 mass balance calculations show that os and re burial fluxes in pelagic sediments account for only a small fraction of the riverine os 10 and re 0 1 inputs into the oceans in contrast burial of ir in pelagic sediments is similar to the riverine ir input indicating that pelagic sediments are a much larger repository for ir than for os and re if all of the missing os and re is assumed to reside in anoxic sediments in oceanic margins the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes however putting all of the missing os and re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of os at pelagic ktb sites where at most 25 of the k t impactor s os could have passed through estuaries if os is preferentially sequestered in anoxic marine environments it follows that the os ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition there is thus a clear fractionation of os and re from ir in precipitation out of sea water in pelagic sections accordingly it is inferred here that re and os are removed from sea water in anoxic marine depositional regimes

variations in seawater osmium isotope composition since the last glacial maximum a case study from the japan sea abstract concentrations of re and os and the isotopic composition of os have been measured in the japan sea sediments to assess the response of the japan sea to glacial interglacial climate change and associated weathering fluxes the osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg g and 187os 188os from 0 935 to 1 042 only 187os 188os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of re and os and with 80 hydrogenous os are explained in terms of seawater composition lower 187os 188os were observed for sediments deposited during the last glacial maximum lgm when planktonic foraminifera from the japan sea recorded lighter oxygen isotopic composition decrease in dissolved os fluxes from continents and or change in the composition of the dissolved load to the japan sea are suggested as the driving mechanisms for the observed lower lgm 187os 188os the results of this study coupled with lower 187os 188os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates suggest that this trend is characteristic of the global oceans data from this study show that the japan sea recorded higher 187os 188os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values this is explained in terms of preferential release of 187os during deglacial weathering and or higher continental os flux driven by warm and wet climate this study demonstrates that os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition in addition 187os 188os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi enclosed basins such as the japan sea

the osmium riverine flux and the oceanic mass balance of osmium the osmium concentration os and isotopic composition were determined in a set of 17 of the largest rivers of the world os varies between 4 6 and 52 1 pg kg and the 187os 188os ratio varies between 0 64 and 2 94 measurement of rainwater samples shows that there is no input of oceanic os to the continent through rain assuming a negligible anthropogenic os input in the dissolved load the natural average river water has an os concentration of 7 9 pg kg and a mean 187os 188os ratio of 1 54 the total riverine flux of os to the ocean is estimated to be 295 kg yr the dissolved os flux from island arcs and oceanic islands represents less than 5 of the total riverine flux and is not further considered the continental os flux to the ocean is then represented by the riverine flux as dissolved os from eolian dust and glacial sediments is negligible assuming steady state it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg yr cosmic dust or mantle derived and an oceanic residence time between 2 5 104 and 5 4 104 with a mean of 3 5 104 year the estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater os would be 14 of the contribution of the unradiogenic component which means that the mantle derived flux should contribute a major part the first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and or of the ultramafic exposures contributes dominantly to the input of unradiogenic os to the seawater we show that it would be necessary to leach 1 3 of the os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic os to the ocean in which case the riverine input represents 94 of the total dissolved flux to the ocean instead of 70 the modern global dissolved os flux to the ocean would then have a 187os 188os ratio of 1 44 instead of 1 06 and the system would be far from steady state

elastic parameter estimation for the identification of sweet spots vaca muerta formation neuquen basin argentina abstract early identification of zones with possible good production sweet spots in shale plays is important to optimize the return on capital investment analyzing elastic rock properties helps in the identification of sweet spots in the vaca muerta formation the vertical and lateral heterogeneity of total organic carbon toc and carbonate content are two critical factors that partly control the geomechanical response of the rock unconventional reservoirs have low permeability making it necessary to define brittle zones to stimulate the rock and induce fractures to produce this leads to the definition of mineralogy ranges that help discriminate brittle and ductile rock based on rock properties this study is based on a 3d seismic survey and several vertical pilot wells with full log suites horizon interpretations and logs are used to build a low frequency model that together with prestack seismic data is input for seismic inversion inversion results predict poisson s ratio v directly and to

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

osmium isotopic composition of corals evidences for multiple sources osmium concentrations in corals are very low 20 ppt and large isotopic variations between 2 92 0 05 and 9 01 0 05 are observed indicating sample heterogeneity these variations are interpreted as a two component mixing for each sample the unradiogenic component has about the same value for all samples and is of mantle meteorite composition and concentration the radiogenic component is interpreted as a composite signal different for each sample and resulting from the mixing of seawater osmium and continental detrital sediment osmium simple calculations show that the micrometeoritic flux of 40 20 106 kg yr is too small by about two orders of magnitude to explain the composition of these samples in the absence of a potential mantle source for the corals it is considered that corals as they filter water for feeding scavenge meteoritical osmium from seawater

the marine 187 os 188 os record of the eocene oligocene transition the interplay of weathering and glaciation abstract osmium os isotope analyses of bulk sediments from the south atlantic equatorial pacific and the italian apennines yield a well dated and coherent pattern of 187os 188os variation from the late eocene to the early oligocene the resulting composite record demonstrates the global character of two prominent features of the low resolution ll44 gpc3 os isotope record pegram and turekian geochim cosmochim acta 63 1999 4053 4058 these are 1 a pronounced minimum in 187os 188os 0 22 0 27 in the late eocene between 34 and 34 5 ma and 2 a subsequent rapid increase in 187os 188os to approximately 0 6 by 32 ma an ultramafic weathering event and an increased influx of extraterrestrial particles to the earth are discussed as alternative explanations for the late eocene 187os 188os minimum comparison of the 187os 188os to benthic foraminiferal oxygen isotope records demonstrates that the nearly three fold increase in 187os 188os from the late eocene minimum coincides with the growth and decay of the first large ice sheet of the oligocene oi1 miller et al j geophys res 96 1991 6829 6848 the fine structure of the os isotope record indicates that enhanced release of radiogenic os unrelated to the recovery from late eocene minimum lagged the initiation of the oi1 event by roughly 0 5 myr this record in conjunction with weathering studies in modern glacial soils blum in w f ruddiman ed tectonic uplift and climate change plenum press new york 1997 pp 259 288 peucker ehrenbrink and blum geochim cosmochim acta 62 1998 3193 3203 suggests that exposure of freshly eroded material during deglaciation following oi1 enhanced chemical weathering rates and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide the improved temporal resolution and age control of the refined eocene oligocene os isotope record also makes it possible to illustrate the late eocene os isotope excursion as a tool for global correlation of marine sediments

evaluation of osmium isotopes and iridium as paleoflux tracers in pelagic carbonates new osmium os isotope and platinum group element pge concentration data are used in conjunction with published 3 he and th isotope data to determine the relative proportions of lithogenic extraterrestrial and hydrogenous iridium ir in a pacific pelagic carbonate sequence from the ocean drilling program odp site 806 on the ontong java plateau ojp these calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary ir budget are minor while hydrogenous ir accounts for roughly 85 of the total ir application of analogous partitioning calculations to previously reported data from a north pacific red clay sequence ll44gpc3 yields very similar results total ir burial fluxes at site 806 and ll44 gpc3 are also similar 45 and 30 pg cm 2 kyr 1 respectively average ir 3 he and ir xs 230 thinitial ratios calculated from the entire site 806 data set are similar to those reported earlier for pacific sites in general down core profiles of ir 3 he and xs 230 thinitial are not well correlated with one another however all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two while these results indicate that ir concentration has potential as a point paleoflux tracer in pelagic carbonates ir based paleoflux estimates are likely subject to uncertainties that are similar to those associated with co based paleoflux estimates consequently local calibration of ir flux in space and time will be required to fully assess the potential of ir as a point paleoflux tracer measured 187 os 188 os of the ojp sediments are systematically lower than the inferred 187 os 188 os of contemporaneous seawater and a clear glacial interglacial 187 os 188 os variation is lacking mixing calculations suggest os contributions from lithogenic sources are insufficient to explain the observed 187 os 188 os variations the difference between the 187 os 188 os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic os carried by particulate extraterrestrial material down core variations of 187 os 188 os with pt ir and os ir also point to contributions from extraterrestrial particles mixing calculations for each set of several triplicate analyses suggest that the unradiogenic os end member cannot be characterized by primary extraterrestrial particles of chondritic composition it is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions 187 os 188 os of pelagic carbonates has greater potential compared to abundances of pge an attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial os in the ojp samples and previously reported extraterrestrial os flux throughout most of the ojp record os isotope based paleoflux estimates are within a factor of two of those derived using other constant flux tracers meaningful flux estimates cannot be made during glacial maxima because the ojp sediments do not record the low 187 os 188 os reported previously we speculate that this discrepancy may be related to focusing of extraterrestrial particles at the ojp as has been suggested to explain down core 3 he variations

a high pressure experimental study on a magnesian rich leucite lamproite from the west kimberley area australia petrogenetic implications liquidus and subliquidus phase relations of a leucite lamproite wolgidite from the west kimberley area australia have been studied experimentally under the volatile conditions of 3 22 wt h2o x co 2 0 11 and 13 0 wt h2o x co 2 0 03 between 10 to 40 kbar under these conditions liquids are vapour undersaturated in experiments with 13 0 wt h2o olivine is the liquidus phase up to 24 kbar and orthopyroxene above 24 kbar phlogopite and rutile occur close to the liquidus above 16 kbar crystallization temperatures of clinopyroxenes are 50 120 c below the liquidus

the late eocene 187os 188os excursion chemostratigraphy cosmic dust flux and the early oligocene glaciation abstract high resolution records ca 100 kyr of os isotope composition 187os 188os in bulk sediments from two tropical pacific sites odp sites 1218 and 1219 capture the complete late eocene 187os 188os excursion and confirm that the late eocene 187os 188os minimum earlier reported by ravizza and peucker ehrenbrink earth planet sci lett 210 2003 151 165 is a global feature using the astronomically tuned age models available for these sites it is suggested that the late eocene 187os 188os minimum can be placed at 34 5 0 1 ma in the marine records in addition two other distinct features of the 187os 188os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca 0 1 myr we propose a speculative hypothesis that higher cosmic dust flux in the late eocene may have contributed to global cooling and early oligocene glaciation oi 1 by supplying bio essential trace elements to the oceans and thereby resulting in higher ocean productivity enhanced burial of organic carbon and draw down of atmospheric co2 to determine if the hypothesis that enhanced cosmic dust flux in the late eocene was a cause for the 187os 188os excursion can be tested by using the paired bulk sediment and leachate os isotope composition 187os 188os were also measured in sediment leachates results of analyses of leachates are inconsistent between the south atlantic and the pacific sites and therefore do not yield a robust test of this hypothesis comparison of 187os 188os records with high resolution benthic foraminiferal 18o records across the eocene oligocene transition suggests that 187os flux to the oceans decreased during cooling and ice growth leading to the oi 1 glaciation whereas subsequent decay of ice sheets and deglacial weathering drove seawater 187os 188os to higher values although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187os 188os records are obscured by recovery from the late eocene 187os 188os excursion evidence of the global influence of glaciation on supply of os to the ocean is robust as it has now been documented in both pacific and atlantic records

some comparative marine chemistries of platinum and iridium abstract iridium has seawater concentrations about one hundred times less than those of platinum as a consequence of its weaker complexing abilities with halides and of a stronger tendency to hydrolyze iridium like platinum is enriched in some ferromanganese minerals compared to palladium platinum and iridium are probably oxidized during mineral formation from the divalent and trivalent states respectively to the tetravalent ones the pelagic sediment contents of irridium as well as those of platinum are related to the manganese contents for higher platinum contents of the authigenic ferromanganese minerals the pt ir ratios approach the seawater ratio suggesting little dilution by platinum and iridium in detrital minerals the marked enrichments of platinum and iridium in the ferromanganese concretions enhance their value as potential ores for heavy metals

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

the glacial interglacial variation of seawater osmium isotopes as recorded in santa barbara basin abstract the pleistocene deposits of santa barbara basin sbb show laminations during interglacial periods typical of an anoxic or suboxic depositional environment and more sporadically laminated deposits during glacial times indicating a more oxic environment of deposition the 187os 188os value of sediment leaches varies as a function of climate consistent with a more radiogenic os signal during interglacial times and less radiogenic os values more typical of glacial times the signal is confounded by contributions from a high os detrital component with a variable 187os 188os value but generally agrees with prior result from deposits from other marine environments

variations in the osmium isotope composition of sea water over the past 200 000 years the first osmium isotope data are presented for marine sediments spanning the past 200 000 years two full glacial cycles and establish that there have been ocean wide variations in sea water osmium isotope composition over this time while the osmium isotope composition of sea water has been relatively invariant over much of this interval 187os 186os of 8 5 to 8 6 excursions to lower isotope ratios 187os 186os 8 1 occur late in the last two glacial periods at 20 ka and 160 ka these are times of high global aridity low global temperature and maximum ice extent while various interpretations are possible a likely explanation is that the low sea water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity there is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation the response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12 000 years