samarium neodymium and rubidium strontium systematics of nakhlite governador valadares abstract the sm nd systematics of whole rock and mineral separate samples from nakhlite governador valadares define a good 147sm 143nd mineral isochron age of 1 37 0 02 ga this age is in excellent agreement with the 39ar 40ar and rb sr ages obtained previously for this meteorite however the rb sr isotopic data for our sample show that the isotopic system is disturbed the lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases the whole rock and acid washed mineral data yield a rb sr age of 1 20 0 05 ga which probably represents a lower limit to the crystallization age of the rock the petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill mcsween 1994 thus the sm nd isotopic age probably represents the age of such a magmatic event the initial e143nd value determined for the rock at 1 37 ga is 17 1 indicating that the parent magma of the rock came from a light rare earth element depleted source of 147sm 144nd 0 237 based on a simple two stage evolution model results of the same model calculation for the initial 87sr 86sr ratio of the rock suggest that its source material was depleted in 87rb 86sr by 50 relative to the estimated martian value at 1 37 ga both the high sm nd and low rb sr values support a clinopyroxene rich cumulate source for the genesis of the nakhlite governador valadares furthermore our sm nd age and e143nd data and the previously published e142nd datum for the rock harper et al 1995 are consistent with early differentiation of the parent planet formation of cumulate sources 4 56 ga ago and late melting of the sources and formation of the rock 1 37 ga ago the good agreement of isotopic ages and petrographic features among governador valadares nakhla and lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories the nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to 1 37 ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history

aeolian sand drift from the intertidal zone on a temperate beach potential and actual rates this paper compares the amount of sand that is potentially moved into a coastal dune system from a beach using estimates based solely on the wind force with actual measured amounts the actual sand inputs are determined by measuring sand level changes each month over a period of eighteen months along a 1760 m section of foredune on braunton burrows southwest england the calculation procedure used to estimate the potential rates of sand drift is based on the capacity of the wind force to transport sand using a published transport equation white 1979 the results show that the actual movement rates are markedly smaller than the potential rates for all months except in the summer meteorological variables and the watertable level in the adjacent dune system are used in an empirically derived regression equation in an attempt to explain the large discrepancy between the potential and actual movement rates the resulting corrected potential movement rates are a closer estimate to the actual rates but there remains much unexplained variance

chronology and petrogenesis of young achondrites shergotty zagami and alha77005 late magmatism on a geologically active planet a study was undertaken to determine the chronology petrogenesis and relationships among the shergottites shergotty and zagami and the unique achondrite alha77005 these meteorites are the product of a variety of complex processes petrogenesis chondrite normalized abundance patterns of shergotty and zagami are very similar and show pronounced depletions of both the light ree la nd and heavy ree dy lu relative to sm gd these characteristic depletions are even more pronounced for alha77005 the light ree depletion is qualitatively consistent with the presence of cumulus pyroxene and or olivine in these meteorites but trace element models show that the parental magmas of all three meteorites were probably also light ree depleted both trace element model calculations and combined rb sr and sm nd isotopic systematics show that the meteorites could not have been co magmatic nor can alha77005 be representative of the source material of the shergottites light ree depletion of the parental magmas also implies light ree depletion of the source material the sm nd systematics of the shergottites require a time averaged sub chondritic light ree enriched sm nd ratio since 4 6 ae ago the sm nd systematics of alha77005 permit a time averaged super chondritic light ree depleted sm nd ratio if its crystallization age is less than tice 0 72 ae chronology rb sr internal isochrons for all three meteorites and a sm nd internal isochron for zagami are concordant at 180 myr 39ar 40ar plateau ages of shergotty and zagami maskelynite are 250 260 myr these ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite the concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity the time of this event is most precisely given by the rb sr age of 180 4 myr for zagami the crystallization ages of the meteorites were not precisely determined extreme upper limits are determined by sm nd model ages relative to an eucrite initial 143nd144nd 0 505835 at 4 6 ae ago these model ages for shergotty zagami and alha77005 are 3600 3500 and 2850 myr respectively the sm nd whole rock age of 1340 60 myr for the three meteorites gives the crystallization age if the sm nd ratios of the precursor materials were always the same we consider this 1340 myr age as a best estimate upper limit best estimate lower limits for shergotty and zagami are taken from the average 39ar 40ar ages of 1200 and 900 myr of pyroxene separates the average 39ar 40ar age of a whole rock sample of alha77005 was 1600 myr and can be partitioned between a low temperature feldspar phase and a high temperature olivine pyroxene inclusions phase the average apparent 39ar 40ar age of the low temperature phase is 1050 myr which is chosen as the best estimate lower limit to the age the crystallization ages of shergotty zagami and alha77005 probably lie within the ranges of 1200 1300 900 1300 and 1000 1300 myr respectively the rb sr whole rock age of 4400 400 myr and single stage babi model ages of 4800 5100 myr are interpreted as reflecting differentiation of the parent body at 4600 myr ago the complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history

theoretical and measured aeolian sand transport on a barrier island louisiana usa over the past 100 years the isles dernieres a low lying barrier island chain along the coast of central louisiana usa has undergone more than 1 km of northward beach face retreat with the loss of 70 of its surface area the erosion results from a long term relative sea level rise coupled with day to day wind and wave action that ultimately favours erosion over deposition at a site in the central isles dernieres 8 days of wind and beach profile measurements during the passage of one winter cold front documented aeolian erosion and deposition patterns under both onshore and offshore winds for offshore winds the theoretical erosion rate based on wind shear velocity closely matched the measured erosion rate for onshore winds the theoretical rate matched the measured rate only after being corrected by a factor that accounted for beach face morphology in late february 1989 a strong cold front moved into coastal louisiana that cold front stalled over the gulf of mexico resulting in 4 days of strong northerly winds at a study site on the isles dernieres during those 4 days the wind moved sand from the backshore to the upper beach face when the cold front finally moved out of the area the wind shifted to the south and decreased in strength the onshore wind then restored some of the upper beach face sand to the backshore while increased wave activity moved the rest into the nearshore the theoretical estimate of 1 28 m3 m 1 for the rate of sand transport by the northerly wind compares well with the measured backshore erosion rate of 1 26 m3 m 1 which was determined by comparing beach profiles from the start and end of the period of northerly winds the theoretical estimate of 0 04 m3 m 1 for the rate of sand transport by the southerly wind however is notably less than the measured rate of 0 45 m3 m 1 the large discrepancy between the two rates can be explained by a difference in the shear velocity of the wind between the beach face where the erosion occurred and the backshore where the wind stress was measured using an empirical relationship for the wind shear drag coefficient as a function of coastal environment the theoretical estimate for the rate of sand transport by the southerly wind becomes 0 44 m3 m 1

formation ages and evolution of shergotty and its parent planet from u th pb systematics the isotopic composition of lead was determined in different phases from shergotty zagami and eeta 79001 using phases with low 238 u 204 pb the initial leads of these meteorites could be well defined and are distinct from each other and have high 204 pb 206 pb from 0 0652 to 0 0739 these leads evolved in different reservoirs over most of solar system history in a low 238 u 204 pb 5 environment it follows that the parent planet of the shergottites has a high 204 pb concentration relative to u and must also be high in other volatiles if the parent planet of the shergottites is mars then this planet must be volatile rich compared to the earth by a factor of 1 8 this implies that mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed martian upper mantle the u th pb isotopic relationships involving phases with high 238 u 204 pb are quite regular and give ages for three shergottites which indicate an event of u th pb fractionation at 200 m y the initial leads at that time were highly evolved relative to the primordial lead the absence of substantial additions of radiogenic lead above the initial lead in high u phases precludes an older age for the u th pb fractionation however details of the data arrays are complex the young age is in general agreement with some of the ages obtained by other methods but precise concordance between the different methods is not established it is suggested that the shock event that produced maskelynite may possibly be associated with the late stage breakup at 2 5 m y and not necessarily related to the 180 m y record the major event of chronometer resetting is taken to be 180 m y this event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes

groundwater in hard rocks of benin regional storage and buffer capacity in the face of change summary groundwater plays a major role in supplying domestic water to millions of people in africa in the future the ability to increase reliable water supplies for domestic and possibly irrigation purposes will depend on groundwater development groundwater storage is a key property because it controls the buffering behavior of the aquifer as it is subjected to time varying conditions such as increased pumping or land use change however quantitative knowledge of groundwater storage in africa is very limited this lack of knowledge is a major concern in hard rocks which cover about 40 of the surface area of africa this paper presents a unique quantitative assessment of groundwater storage in different types of hard rocks and a first estimate of the capacity of hard rock aquifers to buffer changes in climatic and anthropogenic conditions our study area in benin west africa is composed of various grades of metamorphic rocks we used the latest developments in the application of the magnetic resonance geophysical method to confront the methodological difficulty of quantifying groundwater storage we successfully conducted 38 magnetic resonance measurements in eight 8 different geological units each measurement was quantitatively interpreted in terms of groundwater storage we determined the groundwater storage of our study area to be 440 mm 70 mm equivalent water thickness to assess the buffer capacity of aquifers we compared groundwater storage to groundwater discharge groundwater discharge is the sum of natural discharge plus human abstraction we estimated natural discharge i e deep drainage plus evapotranspiration from water table fluctuations monitored in six 6 piezometers human abstraction was calculated based on the number of operating boreholes and their average daily abstraction we found that human abstraction 0 34 mm year 0 07 mm is far less than natural discharge 108 mm year 58 mm we conclude that increased abstraction due to population growth will probably have a smaller impact on storage than observed land use change which may lead to a change in the evapotranspiration rate we calculated buffer capacity as the ratio of current storage to total discharge and obtained a result of 6 years 47 months this buffer capacity confirms groundwater s ability to buffer changes finally our study is intended to promote a more quantitative approach to assessing groundwater resources in africa and to support our ability to adapt to current and future changes

influence of averaging interval on shear velocity estimates for aeolian transport modeling abstract recent investigations of aeolian transport have focused on increasingly short time scales because of growing recognition that wind unsteadiness is a major factor in the dynamics of sediment transport however the statistical reliability of shear velocity u estimates becomes increasingly uncertain as averaging interval is decreased this study provides an empirical assessment of the influence of averaging interval on the reliability of u estimates the data consist of 15 min wind speed profiles 1 hz sampling collected at four coastal sites each profile was subdivided into progressively shorter fixed length time intervals and estimates of u and the 95 confidence interval for u were determined for each time block using standard statistical techniques the logarithmic model accurately represents the measured wind speed profiles even with relatively brief averaging intervals mean r 2 values remain robust down to block lengths as short as 10 20 s typically retaining better than 98 of the r 2 value found for the full length data sets fewer than 2 of the individual 10 s blocks had r 2 values less than 0 9 however mean confidence intervals typically expanded by 70 80 of the full record value as block length decreased from 900 to 10 s for highly log linear profiles this amounted to an absolute increase from about 8 to only 14 of u so that the additional information gained through the use of shorter averaging intervals may outweigh the increase in statistical uncertainty nevertheless given that rates of aeolian transport are generally modeled as a function of u 3 this increase in uncertainty may be significant for transport modeling thus very short averaging intervals should be used with caution when predicting aeolian sediment flux it is proposed that transport modeling should incorporate the shear velocity confidence interval as an indicator of the potential error associated with this source of uncertainty

gravity effect of water storage changes in a weathered hard rock aquifer in west africa results from joint absolute gravity hydrological monitoring and geophysical prospection advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding the evaluation of water storage changes wsc often involve point measurements observation wells moisture probes etc which may be inappropriate in heterogeneous media over the past few years there has been an increasing interest in the use of gravimetry for hydrological studies in the framework of the ghyraf gravity and hydrology inafrica project 3 yr of repeated absolute gravity measurements using a fg5 type gravimeter have been undertaken at nalohou a sudanian site in northern benin hydrological data are collected within the long term observing system amma catch once corrected for solid earth tides ocean loading air pressure effects polar motion contribution and non local hydrology seasonal gravity variations reach up to 11 gal equivalent to a wsc of 260 mm thick infinite layer of water absolute temporal gravity data are compared to wsc deduced from neutron probe and water table variations through a direct modelling approach first we use neutronic measurements available for the whole vertical profile where wsc occur the vadose zone and a shallow unconfined aquifer the rmsd between observed and modelled gravity variations is 1 61 gal which falls within the error bars of the absolute gravity data second to acknowledge for the spatial variability of aquifer properties we use a 2 d model for specific yield sy derived from resistivity mapping and magnetic resonance soundings mrs the latter provides a water content mrs known to be higher than the specific yield hence we scaled the 2 d model of mrs with a single factor wsc are calculated from water table monitoring in the aquifer layer and neutronic measurements in the vadose layer the value of is obtained with amonte carlo sampling approach minimizing the rmsd between modelled and observed gravity variations this leads to sy mrs 0 63 0 15 close to what is found in the literature on the basis of pumping tests experiments with a rmsd value of 0 94 gal this hydrogeophysical experiment is a first step towards the use of time lapse gravity data as an integrative tool to monitor interannual wsc even in complicated subsurface distribution

cold front driven storm erosion and overwash in the central part of the isles dernieres a louisiana barrier island arc abstract tropical and extratropical storms produce significant erosion on the barrier islands of louisiana over the past 100 years such storms have produced at least 2 km of northward beach face retreat and the loss of 63 of the surface area of the isles dernieres a low lying barrier island arc along the central louisiana coast elevations on the islands within the arc are typically less than 2 m above mean sea level the islands typically have a washover flat topography with occasional poorly developed dune terrace topography consisting of low lying and broken dunes the central part of the arc consists of salt marsh deposits overlain by washover sands along the gulf of mexico shoreline sand thicknesses range from zero behind the beach to less than 2 m under the berm crest and back to zero in the first nearshore trough the sand veneer is sufficiently thin that storms can strip all the sand from the beach face exposing the underlying marsh deposits the geomorphic changes produced by cold fronts a type of extratropical storm that commonly affect the isles dernieres between late fall and early spring are described between august 1986 and september 1987 repeated surveys along eleven shore normal transects that covered 400 m of shoreline revealed the timing and extent of cold front produced beach change along a typical section of the central isles dernieres during the study period the beach face retreated approximately 20 m during the cold front season but did not rebuild during the subsequent summer because the volume of sand deposited on the backshore 5600 m 3 was less than the volume of material lost from the beach face 19 200 m 3 approximately 13 600 m 3 of material disappeared assuming that underlying marsh deposits decrease in volume in direct proportion to the amount of beach face retreat an estimate of the mud loss during the study period is 14 000 m 3 thus the decrease in volume along the profiles can be accounted for without removing any sand from the area suggesting that a major effect of cold fronts is first to strip the sand from the beach face and then to erode the underlying marsh deposits after being eroded the mud is lost from the islands because currents transport it away from the islands

absolute gravity change associated with magma mass movement in the conduit of asama volcano central japan revealed by physical modeling of hydrological gravity disturbances the gravity signal originating from magma mass movement in a volcanic conduit is retrieved from the hydrologically disturbed absolute gravity data obtained at asama volcano central japan in 2004 using a three dimensional hydrological model we improve the hydrological model of the previous study using realistic soil parameters and boundary conditions to better estimate the spatiotemporal land water distributions and the consequent hydrological gravity disturbances the newly estimated gravity disturbances agree with the absolute gravity values observed by fg5 gravimeters in 2004 2009 within about 2 6 gal by additionally accounting for the excess discharge of groundwater mass associated with a sloping impermeable surface below the discharge area after the gravity disturbance of 20 gal amplitude is subtracted from the absolute gravity data observed during the 2004 eruptive event the gravity residual of 5 gal amplitude shows a significant decrease in synchronization with eruptions because the ascending magma mass in the conduit affects the upward attraction force to the gravimeters installed on the flank of asama volcano the magma head altitude to which the residual gravity is converted assuming a homogeneous linear density in the conduit shows a comprehensive agreement of the time variation in the magma head with those in other volcanic observations such as gas emission rate and earthquake frequency by correcting the hydrological gravity disturbances using this hydrological model and simultaneously obtained meteorological data in real time spatiotemporal variations in the magma mass can be instantaneously monitored at asama volcano even before eruptions during future volcanic events

rapid dune changes associated with overwash processes on the deltaic coast of south louisiana abstract the caminada moreau barrier headland of south louisiana is a low profile beach and dune coastline that is transgressing rapidly over the surfaces of the abandoned lafourche delta complex with the passage of cold fronts 10 30 times per year and hurricanes once every 4 yrs overwash events occur year round with varying degrees of frequency intensity and geomorphological modification this coastline consists of washover and dune surfaces that respond rapidly to overwash impact the fine sand stored in washover deposits is easily reworked by aeolian processes into a variety of dune forms in the shore zone that are vegetated rapidly geomorphological changes vary according to the position ground elevation and surface stability of the dunes independent factors are overwash surge elevations beach gradient and the presence of pre existing landforms for ten years detailed surveys supplemented by aerial photographs and videotape surveys have recorded these changes the analysis of these information sources provides insight into both the general evolution of this distinctive coastline and also the geomorphological interaction of dunes beaches and washover deposits in different physiographic settings

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

using time lapse gravity for groundwater model calibration an application to alluvial aquifer storage 1 the estimation of hydrological model parameters by calibration to field data is a critical step in the modeling process however calibration often fails because of parameter correlation here it is shown that time lapse gravity data can be combined with hydraulic head data in a coupled hydrogeophysical inversion to decrease parameter correlation in groundwater models this is demonstrated for a model of riverbank infiltration where combined inversion successfully constrains hydraulic conductivity and specific yield in both an analytical and a numerical groundwater model a sensitivity study shows that time lapse gravity data are especially useful to constrain specific yield furthermore we demonstrate that evapotranspiration and riverbed conductance are better constrained by coupled inversion to gravity and head data than to head data alone when estimating the four parameters simultaneously the six correlation coefficients were reduced from unity when only head data were employed to significantly lower values when gravity and head data were combined our analysis reveals that the estimated parameter values are not very sensitive to the choice of weighting between head and gravity data over a large interval of relative weights

towards a better estimate of storage properties of aquifer with magnetic resonance sounding summary providing people with fresh water is one of the greatest challenges of the century since most of the world s liquid fresh water is groundwater the knowledge of aquifer storage properties is essential moreover there is a need to focus research on poor aquifers i e capable of providing about 100 m 3 day which will play an increasing role for supplying many human communities this paper concerns a study carried out in a clayey sandstones aquifer in northern cambodia conventional hydraulic methods used to characterize aquifers are costly time consuming and thus they are usually not used in most of the water projects in developing countries therefore geophysical methods can be useful if they improve aquifer characterization as compared to other non invasive geophysical methods magnetic resonance sounding mrs is selective to groundwater mrs results are the distribution of both water content and pore size related parameters as a function of depth however relationships between the field scale mrs results and hydrogeological storage related properties have not been well established yet we present in this paper a comparison of mrs results with both specific yield calculated from pumping tests and effective porosity calculated from tracer tests we found that the mrs water content is equal or higher than the specific yield and the effective porosity thus indicating that mrs also measures capillary water in unsaturated zone and part of the bound groundwater attached to the aquifer solid matrix we also found that the mrs pore size parameter is linearly correlated with both the effective porosity and the specific yield thus suggesting that the hydrogeological storage properties are mainly controlled by the size of the pores of the aquifer consequently we adapted an approach used in the oil industry for differencing gravitational water from capillary water and from bound water based on the mrs pore size parameter in the clayey sandstones of cambodia our approach named mrs apparent cutoff time approach allowed calculating specific yield with an average error of 23 which is far less than the previous published results and for the first time it allowed calculating effective porosity with an average error of 11 we conclude that the mrs apparent cutoff time approach is useful for estimating aquifer storage properties down to 50 80 m deep in a single day and at an affordable cost

a review of meteorite evidence for the timing of magmatism and of surface or near surface liquid water on mars 1 there is widespread photogeological evidence for ubiquitous water flowing on the surface of mars however the age of surface and near surface water cannot be deduced with high precision from photogeology while there is clear evidence for old and young fluvial features in the photogeologic record the uncertainty in the absolute calibration of the martian crater flux results in uncertainties of 1 5 gyr in the middle period of martian geologic history aqueous alteration of primary igneous minerals produces secondary minerals in martian meteorites here we use the ages of secondary alteration minerals in martian meteorites to obtain absolute ages when liquid water was at or near the surface of mars aqueous alteration events in martian meteorites occurred at 3929 37 ma carbonates in alh84001 633 23 ma iddingsite in nakhlites and 0 170 ma salts in shergottites furthermore these events appear to be of short duration suggesting episodic rather than continuous aqueous alteration of the meteorites the martian meteorites appear to be contaminated by martian surface pb characterized by a 207pb 206pb ratio near 1 lead of this composition could be produced by water based alteration on the martian surface the high 129xe 132xe ratio in the martian atmosphere compared to martian meteorites indicates fractionation of i from xe within 100 myr after nucleosynthesis of 129i such fractionation is difficult to achieve through magmatic processes however water very efficiently fractionates i from xe raising the intriguing possibility that mars had a liquid water ocean within its first 100 myr 1

gravity profile interpretation using the r parameter imaging technique with application to ore exploration abstract this paper describes a fast imaging technique the so called here the r parameter imaging technique for the interpretation of gravity data measured along profile the technique calculates the r parameter a statistical parameter called the correlation coefficient between the analytic signal of the measured gravity field and that of the synthetic response of an assumed interpretive model in the restricted class of spheres and cylinders the characteristic source parameters depth shape origin point of the gravity profile and amplitude factor of the model correspond to the maximum r parameter value the method has been verified on noise free numerical examples following that the accuracy of the method has been carefully assessed on both noisy numerical examples and numerical examples with interference effects finally the technique has been successfully applied to five field data examples from canada sweden and cuba for mineral exploration and it is found that the obtained results are in good agreement with those reported in the published literature

the stabilizing role of bonding agents on the entrainment of sediment by wind well sorted fine sand pretreated with varying concentrations of two soluble salts mgcl2 and cacl2 was tested for threshold shear velocity in a recirculating wind tunnel these tests demonstrate that even relatively low surface salt concentrations can significantly increase threshold shear velocity however the relationships between surface salt concentration and threshold shear velocity for the two salts were not significantly different which is similar to the results for monovalent chlorides presented by nickling ecclestone a general relationship between threshold shear velocity and total surface soluble salt content is presented

estimating specific yield and storage change in an unconfined aquifer using temporal gravity surveys 1 two high precision gravity surveys were conducted to determine groundwater mass changes at a managed groundwater recharge site in northeastern colorado gravity data were collected during pumping and two months after pumping ceased during pumping gravity was lower by as much as 46 gal near the pumping wells and higher by as much as 90 gal near the recharge ponds in comparison to data collected after pumping had ceased these differences are attributed to changes in groundwater mass associated with drawdown and infiltration inverse modeling of the gravity data indicates a 5 1 105 m3 decrease in storage beneath the recharge ponds between the two surveys which we attribute to dissipation of the groundwater mound created by recharge during pumping this estimate of the change in groundwater storage is made independently of assumptions of physical properties of the aquifer dividing the change in water volume per unit area determined from the gravity modeling by the change in water level measured in wells provides an estimate of specific yield sy of 0 21 0 03 which is within the range of specific yield estimates derived from aquifer tests at the site water level changes predicted from the gravity data agree on average to within 0 45 m of those measured which we take to be an estimate of the uncertainty in water table depth predictions that can be obtained from gravimetric data in unconfined aquifers the study covers a 3 2 km2 area providing a prototype for similar studies at larger scales

constraints on the petrogenesis of martian meteorites from the rb sr and sm nd isotopic systematics of the lherzolitic shergottites alh77005 and lew88516 abstract detailed rb sr and sm nd isotopic analyses have been completed on the lherzolitic shergottites alh77005 and lew88516 alh77005 yields a rb sr age of 185 11 ma and a sm nd age of 173 6 ma whereas the rb sr and sm nd ages of lew88516 are 183 10 and 166 16 ma respectively the initial sr isotopic composition of alh77005 is 0 71026 4 and the initial end value is 11 1 0 2 these values are distinct from those of lew88516 which has an initial sr isotopic composition of 0 71052 4 and an initial end value of 8 2 0 6 several of the mineral and whole rock leachates lie off the rb sr and sm nd isochrons indicating that the isotopic systematics of the meteorites have been disturbed the sm nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143nd 144nd ratio that was probably added to the meteorites on mars tie lines between leachate residue pairs from lew88516 mineral fractions and whole rocks have nearly identical slopes that correspond to rb sr ages of 90 1 ma this age may record a major shock event that fractionated rb sr from lattice sites located on mineral grain boundaries on the other hand the leachates could contain secondary alteration products and the parallel slopes of the tie lines could be coincidental nearly identical mineral modes compositions and ages suggest that these meteorites are very closely related nevertheless their initial sr and nd isotopic compositions differ outside analytical uncertainty requiring derivation from unique sources assimilation fractional crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma if the assimilant has a sr nd ratio near 1 and a radiogenic sr isotopic composition further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it a formed at 4 5 ga b has a high la yb ratio c is an oxidant and d is basaltic in composition or is strongly enriched in incompatible elements the composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved martian crustal component or hydrous metasomatism within the martian mantle

evaluation of aeolian sand transport equations using intertidal zone measurements saunton sands england sand transport rates were measured using a vertical sand trap along the intertidal zone of a beach in north devon england together with simultaneous monitoring of the wind speed on the beach and moisture levels in the surface layers of sand the results of 88 sand trap samples in a wide range of wind speeds showed that moisture levels up to 14 in the top 1 2 mm of the beach sand have no discernible effect on the transport rates transport rates measured from areas of the beach where the moisture was below this level are compared with the rates predicted by seven expressions based on theoretical and wind tunnel research together with the empirical results of other published research measured transport rates range from 0 0001 to 0 22 kg m 1 s 1 the results indicate that expressions based on a power relation between the wind speed and the transport rate and which include a threshold velocity term provide the best estimates of the observed transport rates

aeolian transport rates across raked and unraked beaches on a developed coast wind characteristics and aeolian transport were measured on a naturally evolving beach and dune and a nearby site where the beach is raked and sand trapping fences are deployed the beaches were composed of moderately well sorted to very well sorted fine to medium sand the backshore at the raked site was wider and the foredune was more densely vegetated and about 1 m higher than at the unraked site wind speeds were monitored using anemometers placed at 1 m elevation and sand transport was monitored using vertical traps during oblique onshore alongshore and offshore winds occurring in march and april 2009 inundation of the low backshore through isolated swash channels prevented formation of a continuously decreasing cross shore moisture gradient the surface of the berm crest was dryer than the backshore making the berm crest the greatest source of offshore losses during offshore winds the lack of storm wrack on the raked beach reduced the potential for sediment accumulation seaward of the dune crest during onshore winds and the higher dune crest reduced wind speeds and sediment transport from the dune to the backshore during offshore winds accretion at wrack seaward of the dune toe on the unraked beach resulted in a wider dune field and higher narrower backshore although fresh wrack is an effective local trap for aeolian transport wrack that becomes buried appears to have little effect as a barrier and can supply dry sand for subsequent transport aeolian transport rates were greater on the narrower but dryer backshore of the unraked site vegetation growth may be necessary to trap sand within zones of buried wrack in order to allow new incipient foredunes to evolve copyright 2010 john wiley sons ltd

a sand budget for the alexandria coastal dunefield south africa the sand in the alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield sand is blown off the beach onto the dunefield by the high energy onshore directed dominant wind the dunefield has been forming over the past 6500 years sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m3 m 1 yr 1 in an ene direction the sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface aeolian sand movement rates of this order are typical of dunefields around the world the total volume of sand blown into the dunefield is 375 000 m3 yr 1 sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m3 yr 1 the dunefield thus gains 330 000 m3 of sand per year this results in dunefield growth by vertical accretion at about 1 5 mm yr 1 and landward movement at about 0 25 m yr 1 the dunefield is a significant sand sink in the coastal sand transport system the rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas the higher rate of deposition may result from the abundant sand supply on sandy beaches and the higher energy of coastal winds wind transport is slow and steady compared to fluvial or longshore drift transport of sediment and catastrophic aeolian events do not seem to be significant in wind laid deposits

modelling the effect of atmospheric pressure variations on gravity summary a model is developed to describe the effects of local and global atmospheric pressure fluctuations on local gravity the calculated admittance of gravity to the pressure tides s1 s4 at pinon flat california is compared to those measured using the superconducting gravimeter the theoretical and measured admittances are in good agreement for the s3 and s4 tides the difference between theory and observation at s2 can probably be explained by the ocean tide at that frequency while the discrepancy at s1 remains unexplained techniques for the removal of random pressure variations from gravity records are described using statistics derived from spherical harmonic decompositions of the earth s pressure field it is shown that more than 80 per cent of the pressure induced gravity variations should be removed from a gravity record by employing a single barometer and least squares procedure it is further shown that adding a ring of barometers should increase the effectiveness to about 90 per cent the amount of barometric pressure noise which remains after subtracting the simultaneous record of two gravimeters is calculated as a function of their separation and indicates that significantly less noise is present in the difference than in the individual records for separations of up to 17 of arc

origins of streamflow in a crystalline basement catchment in a sub humid sudanian zone the donga basin benin west africa inter annual variability of water budget during the last quarter of the 20th century west africa underwent a particularly intense and generalized drought during this period the biggest drops in streamflow were observed in the sudanian zone rather than in the sahelian zone but the reasons are still poorly understood in 2000 a meso scale hydrological observatory was set up in the sub humid sudanian zone of the upper oueme valley benin three embedded catchments of 12 586 km2 located on a crystalline bedrock were intensively instrumented to document the different terms of the water budget and to identify the main streamflow generating processes and base flow mechanisms at different scales geophysical hydrological and geochemical data were collected throughout the catchments from 2002 to 2006 crossing these data helped define their hydrological functioning the region has seasonal streamflow and the permanent groundwater in the weathered mantle does not drain to rivers instead seasonal perched groundwaters are the major contributor to annual streamflow the perched groundwaters are mainly located in seasonally waterlogged sandy layers in the headwater bottom lands called bas fonds in french speaking west africa of 1st order streams during the period 2003 2006 regolith groundwater recharge ranged between 10 and 15 of the annual rainfall depth depletion of permanent groundwater during the dry season is probably explained by local evapotranspiration which was seen not to be limited to gallery forests during the 4 year study period a reduction of 20 in annual rainfall led to a 50 reduction in streamflow this reduction was observed in the two components of the flow direct runoff and drainage of perched groundwater thanks to the comprehensive dataset obtained the results obtained for the donga experimental catchment are now being extrapolated to the whole upper oueme valley which can be considered as representative of sub humid sudanian rivers flowing on a crystalline basement for example the upper courses of several major west african rivers senegal niger bani and volta

hydrogeological investigations at the membach station belgium and application to correct long periodic gravity variations 1 a comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of membach belgium applying a regional water storage model the gravity contribution due to the elastic deformation of the earth was derived in addition the newtonian gravity effect induced by the local water mass variations was calculated using soil moisture observations taken at the ground surface about 48 m above the gravimeters the computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms the obtained results are compared with the observations of a superconducting gravimeter sg we find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local newtonian effects shorter period effects depend on the local changes in hydrology this result shows the sensitivity of sg observations to very local water storage changes

martian cratering vi crater count isochrons and evidence for recent volcanism from mars global surveyor this paper develops a methodology to establish absolute martian ages by deriving isochrons on a plot of martian impact crater density vs crater diameter calibrated by lunar crater age data the isochrons illustrated here are based on a mars moon cratering ratio of 1 6 at constant size but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages martian crater diameter distributions are determined in several areas down to diameter d 16 45 m the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by neukum the youngest areas studied here display the lunar like production function down to d 30 m where saturation equilibrium sets in model crater retention ages of several volcanic units are found to be a few hundred million years or less with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher the results are consistent with martian meteorite ages volcanism on mars has probably persisted into the last 10 to 15 of the planet s history and is likely ongoing because surfaces as young as a few hundred million years have reached crater saturation equilibrium at d 60 to 100 m mars is likely to have widespread impact produced regoliths at least a few meters deep and this may contribute to the widespread mobile dust and boulder fields of mars

absolute gravity monitoring of water storage variation in a karst aquifer on the larzac plateau southern france in this study we attempt to understand the water storage variations in a karst aquifer on the larzac plateau south of france using absolute gravimetry on this karst system water input is exclusively rainfall and draining occurs at the durzon perennial spring in a karstic valley our basis assumption is that water storage in the vadose zone epikarst and infiltration zone has a significant effect on surface gravity through newtonian attraction the karst aquifer is hence being monitored since january 2006 with monthly absolute gravity measurements at three sites the gravity measurements are corrected for regional scale gravity using water storage models in order to obtain gravity variations related exclusively to the local water storage variations the gravity variations exhibit an important seasonal component 10 15 mu gal corresponding to a variation of equivalent water stab thickness of similar to 24 36 cm water storage variation seen by gravimetry is not spatially uniform on the studied karst system corroborating current geomorphologic observations and interpretations finally we use a global mass balance consideration linking rainfall evapotranspiration and spring discharge to provide a conceptual framework for the understanding of observed gravity variations

local and global hydrological contributions to time variable gravity in southwest niger p advances in methods of observation are essential to ensure a better understanding of changes in water resources considering climate variability and human activities the ghyraf gravity and hydrology in africa experiments aim to combine gravimetric measurements with dense hydrological surveys to better characterize the annual water storage variability in tropical west africa the first absolute gravimetric measurements were performed in southwest niger near a temporary pond where rapid infiltration to an unconfined aquifer occurs as gravity is sensitive both to local and global variations of water mass distribution the large scale hydrological contribution to time variable gravity has been removed using either grace satellite data or global hydrology models the effect of the local water storage changes was modelled using in situ measurements of the water table soil moisture and pond water level the adjustment of these simulations to residual ground gravity observations helped to constrain the specific yield to a value ranging between 1 8 and 6 2 per cent this range of value is consistent albeit on the low side with the aquifer water content 6 12 per cent estimated by magnetic resonance soundings which are known to slightly overestimate the specific yield in this geological context the comparison of these two independent geophysical methods shows their potential to constrain the local hydrogeological parameters besides this study evidences the worth of correcting the gravity signal for large scale hydrology before recovering local water storage parameters