ar ar and i xe ages and thermal histories of three unusual metal rich meteorites abstract portales valley sombrerete and northwest africa nwa 176 are three unrelated meteorites which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies measured 39 ar 40 ar ages of these meteorites are 4477 11 ma and 4458 16 ma two samples of portales valley 4541 12 ma and 4524 13 ma respectively ma million years all one sigma errors the ar ar data for portales valley show no evidence of later open system behavior suggested by some other chronometers measured 129 i 129 xe ages of these three meteorites are 4559 9 0 5 ma 4561 9 1 0 ma and 4544 ma respectively relative to shallowater 4562 3 0 4 ma from stepwise temperature release data we determined the diffusion characteristics for ar and xe in our samples and calculated approximate closure temperatures for the k ar and i xe chronometers adopting results and interpretations about these meteorites from some previous workers we evaluated all these data against various thermal cooling models we conclude that portales valley formed 4560 ma ago cooled quickly to below the i xe closure temperature then cooled deep within the parent body at a rate of 4 c ma through k ar closure we conclude that sombrerete formed 4562 ma ago and cooled relatively quickly nwa 176 likely formed and cooled quickly 4544 ma ago or later than formation times of most meteorite parent bodies for all three meteorites the ar ar ages are in better agreement with i xe ages and preferred thermal models if we increase these ar ar ages by 20 ma such age corrections would be consistent with probable errors in 40 k decay parameters in current use as suggested by others the role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system

testing the validity of upslope migration in banded vegetation from south west niger abstract recent studies of banded vegetation have suggested a successional model in which bare bands are colonised in a pioneer front on the upslope side of a vegetation band vegetation patterns in south west niger have been interpreted to suggest that spatial transitions reflect this form of temporal succession and in these patterns there is corroborating evidence for slow up slope migration however given the inherent difficulties of long term field experiments there are few data to judge the validity of this model the use of the artificial radionuclide caesium 137 137 cs to provide information on net soil flux over the past ca 30 years offers potential in this regard furthermore the identification of various types of soil crust which can induce different types of hydrological behaviour provides valuable information for predicting soil evolution to test the hypothesis that banded vegetation migrates upslope a 70 m transect encompassing two vegetation bands and a single bare lane was sampled in south west niger the transect was aligned orthogonal to the bands and approximately parallel to the direction of water soil and nutrient flow soil samples for gamma ray spectrometry and particle size analysis were collected along the transect at 21 locations with 1 m intervals in the lower part and three samples were obtained on the upper part prior to collection the soil surface characteristics were examined to distinguish between crust types and to identify the presence of termite activity the results demonstrate the utility of these techniques for examining the net redistribution of soil over a period of three decades and its relations to vegetation succession the amount of soil eroded was found generally to decrease downslope whilst the proportion of fine silt in the soil generally increased downslope these patterns correspond with the location of the erosion and sedimentation crusts identified using a standardised classification the relations between microtopography and net soil flux may also explain some of the spatial variation in soil redistribution processes the intensity of crust and 137 cs measurements on the upslope edge of the lower vegetation band enabled the calculation of the upslope migration rate ca 0 19 0 27 m yr 1 which coincided with independent studies in the same region however because considerable spatial variations in topography and soil flux were found to occur over very small distances further detailed studies over larger areas will be needed

mound development as an interaction of individual plants with soil water erosion and sedimentation processes on slopes in the mediterranean region semi natural shrubland communities named matorral often present a discontinuous cover where isolated perennial plants alternate with bare inter plant areas in such ecosystems the patchy distribution of the vegetation is usually associated with microtopographic sequences of mounds that develop under isolated plants and break the overall slope continuity in this study the influence of three representative species of the mediterranean matorral rosmarinus officinalis stipa tenacissima and anthyllis cytisoides on slope microtopography is determined and the processes that take part in the development of microtopographic structures beneath the plant canopy are identified the influence of slope gradient plant species and plant parameters on the shape and height of microtopographic structures is also studied the shape of the microtopographic structures is described by using a two dimensional microprofilemeter and mound height is determined by measuring in the field a mound height index defined as the distance from the top to the bottom of the mound the results obtained show that plant species play a major role in the shape and height of the microtopographic structures whereas terrace type structures generally develop under anthyllis shrubs microtopographic forms associated with rosmarinus and stipa plants vary with slope gradient the almost symmetric mound type structures that develop under these two species on gentle slopes change into terrace type structures as slope gradient increases moreover statistically significant differences exist between the three species with regard to mound height mean values of mound height are 19 4 14 6 and 4 3 cm under the canopy of stipa rosmarinus and anthyllis respectively plant parameters essentially roughness and slope gradient have a significant influence on mound height index four main processes were identified as affecting mound development in the studied field site sedimentation differential interrill erosion differential splash erosion and bioturbation plant species interact in different ways with these processes according to their morphologies since stipa and rosmarinus plants are more efficient than anthyllis shrubs in controlling water erosion in retaining sediments and in modifying soil properties under their respective canopies they give rise to higher microtopographic structures that facilitate water and nutrient storage by plants on slopes copyright 2000 john wiley sons ltd

magnitude frequency analysis of water redistribution along a climate gradient in spain abstract banded patterns in soils and vegetation form part of important discontinuities on semi arid slopes in spain at fine scales

microbiotic soil crusts in the sahel of western niger and their influence on soil porosity and water dynamics abstract microbiotic soil crusts are common features of the surface of fallow land in western niger we investigated the interaction between these microbial covers and the porosity and water dynamics of soils at the surface of a sahelian landscape the soil pore system was examined by microscopic observations and mercury porosimetry the soil water retention capacity was measured using a richard pressure membrane apparatus runoff measurements were performed in situ at a 1 m2 scale under natural rainfall samples with dense cyanobacterial cover had microbially originated pores ranging from 0 04 and 50 m in size these samples also showed higher total soil porosity and retained two to four times more water than samples with thin microbial cover these properties are closely related to the hydrophobic nature of cyanobacterial components final runoff values obtained on densely covered surfaces were significantly higher compared to those measured on surfaces with thin microbial cover due to the geometry of the microbially originated pore system and its functioning

evaluating the information content of tomographic data application to mesoscale observations integrating tomography in the design of future experiments requires knowledge of the information content of this type of data the case of mesoscale observations is examined here the evaluation of the performances of tomography as the only data type available is done by simulating acoustic transmissions in a quasi geostrophic model during a period of 106 geophysical days a linear inversion is then applied to recover the stream function field the linear hypothesis is validated for that particular scale and type of oceanic perturbation as is the robustness to erroneous data the full three dimensional stream function fields are reconstructed information brought by tomography varies according to the mode the first baroclinic mode is well estimated the barotropic mode is partially estimated only one component of the current is felt and higher modes are not resolved the relative vorticity is assessed with horizontal resolution at the scale of the elementary cells defined by the tomographic array it is compared to direct triangular computations the original contribution of tomography is in the simultaneous time and space dimensions complementary to the classical point measurements

assessing mutuality of change in soil and vegetation patch pattern characteristics by means of cellular automata simulation mutual variations in soil and vegetation pattern characteristics have not received much attention despite their role in the evolution of eco geomorphic systems five landscape metrics were employed to characterize complementary soil and vegetation patterns of patches over a wide range of densities number density size heterogeneity shape and dominance these metrics were calculated for a large number of modeled patterns based on a cellular automata technique employing four separate vegetation spread strategies at different adhesive growth levels as indicated by the data mutual variations in the soil and vegetation metrics allow differentiation between the pattern evolution stages such as perforation dissection fragmentation shrinkage and attrition of one surface type due to the spread expansion and aggregation of a complementary surface type such information may be indicative of the hydro geomorphological processes taking place

biological soil crust effects must be included to accurately model infiltration and erosion in drylands an example from tabernas badlands in dryland ecosystems runoff is mainly generated in bare areas which are also more susceptible to water erosion than vegetated areas these bare areas are often covered and protected by biological soil crusts bscs which modify numerous physicochemical surface properties involved in runoff and erosion processes bscs are considered as one of the most important stabilizing factors in the soil surface but most previous research has concentrated only on patch or hillslope scale effects of bscs and their effect at coarser scales has rarely been studied in this article we present a new approach based on previous surface cover quantification for including the effects of bscs in physically based runoff and erosion modeling the limburg soil erosion model lisem was used to parameterize and simulate the effects of bscs on runoff and erosion in a small semiarid catchment characterized by fine textured soils and predominantly covered by bscs paired model simulations under two scenarios with and without including the effects of bscs were run under different rainfall intensities to evaluate the effect of bscs on runoff and erosion under different rainfall conditions runoff and erosion rates recorded in the field at the catchment outlet were predicted much more accurately when bscs were included because there was less overestimation of runoff rate maximum runoff peaks and erosion rates in the areas dominated by bscs the proposed approach enables bscs to be included in spatially distributed runoff and erosion models improving their predictions and may be used for evaluating how the effect of human activity on bscs affects catchment scale water erosion

39ar 40ar dating of the zagami martian shergottite and implications for magma origin of excess 40ar the zagami shergottite experienced a complex petrogenetic formation history mccoy et al 1992 1999 like several shergottites zagami contains excess 40ar relative to its formation age to understand the origin of this excess 40ar we made 39ar 40ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution surprisingly all these separates show similar concentrations of excess 40ar 1 x 10 6 cm 3 g we present arguments against this excess 40ar having been introduced from the martian atmosphere as impact glass we also present evidence against excess 40ar being a partially degassed residue from a basalt that actually formed 4 gyr ago we utilize our experimental data on ar diffusion in zagami and evidence that it was shock heated to only 70 c and we assume this heating occurred during an ejection from mars 3 myr ago with these constraints thermal considerations necessitates either that its ejected mass was impossibly large or that its shock heating temperature was an order of magnitude higher than that measured we suggest that this excess 40ar was inherited from the zagami magma and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old k rich crustal material similar concentrations of excess 40ar in the analyzed separates imply that this magma maintained a relatively constant 40ar concentration throughout its crystallization this likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released these concepts have implications for excess 40ar in other shergottites

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

mineralogical study of brown olivine in northwest africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles martian meteorites in particular shergottites contain darkened olivine so called brown olivine whose color is induced by iron nanoparticles formed in olivine during a shock event the formation process and conditions of brown olivine have been discussed in the northwest africa 2737 nwa 2737 chassignite however formation conditions of brown olivine in nwa 2737 cannot be applied to shergottites because nwa 2737 has a different shock history from that of shergottites therefore this study observed brown olivine in the nwa 1950 shergottite and discusses the general formation process and conditions of brown olivine in shergottites our observation of nwa 1950 revealed that olivine is heterogeneously darkened between and within grains different from brown olivine in nwa 2737 xanes analysis showed that brown olivine contains small amounts of fe3 and tem stem observation revealed that there is no sio rich phase around iron metal nanoparticles these observations indicate that iron nanoparticles were formed by a disproportionation reaction of olivine 3fe2 olivine fe0metal 2fe3 olivine volivine where volivine means a vacancy in olivine some parts of brown olivine show lamellar textures in sem observation and raman peaks in addition to those expected for olivine implying that brown olivine experienced a phase transition to e g ringwoodite in order to induce heterogeneous darkening heterogeneous high temperature of about 1500 1700 k and shock duration of at least 90 ms are required this heterogeneous high temperature resulted in high postshock temperature 900 k inducing back transformation of most high pressure phases therefore in spite of lack of high pressure phases nwa 1950 martian meteorites with brown olivine experienced higher pressure and temperature compared to other highly shocked meteorite groups

observations of deep convection in the gulf of lions northern mediterranean during the winter of 1991 92 abstract during december 1991 to april 1992 measurements with moored acoustic doppler current profiler adcp stations and shipboard surveys were carried out in the convection regime of the gulf of lions northwestern mediterranean first significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late december mixed layer deepening to about 400 m eroding the salinity maximum layer of saltier and warmer levantine intermediate water and causing temporary surface layer warming followed during a second cooling period of late january during a mistral cooling period from 18 to 23 february 1992 convection to 1500 m depth was observed where the size of the convection regime was 50 100 km extent vertical velocities 40 640 m deep recorded by four adcps of a triangular moored array of 2 km sidelength in the center of the convection regime exceeded 5 cm s 1 and were not correlated over the separation of the moorings hori

39ar 40ar age and thermal history of martian dunite nwa 2737 abstract we report an 39 ar 40 ar age determination of a whole rock sample of the olivine rich martian meteorite northwest africa nwa 2737 those extractions releasing 0 48 of the 39 ar define an 39 ar 40 ar isochron age of 160 190 ma when evaluated in various ways higher temperature extractions show increasing ages that eventually exceed the reported sm nd age of 1 42 ga at least part of this excess 40 ar may have been shock implanted from the martian atmosphere we considered two possible interpretations of the ar ar isochron age utilizing the measured ar diffusion characteristics of nwa 2737 and a thermal model which relates ar diffusion to the size of a cooling object after shock heating one interpretation that 40 ar was only partially degassed by an impact event 11 ma ago the cre age appears possible only if nwa 2737 was shock heated to temperatures 600 c and was ejected from mars as an object a few 10 s of cm in diameter the second interpretation which we prefer is that nwa experienced an earlier more intense shock event which left it residing in a warm ejecta layer and a less intense event 11 ma ago which ejected it into space our evaluation would require nwa 2737 to have been heated by this first event to a temperature of 300 500 c and buried in ejecta to a depth of 1 20 m these conclusions are compared to model constraints on meteorite ejection from mars reported in the literature the second mars ejection impact 11 ma ago probably heated nwa 2737 to no more than 400 c nwa 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space

the effect of landform variation on vegetation patterning and related sediment dynamics semi arid ecosystems are often spatially self organized in typical patterns of vegetation bands with high plant cover interspersed with bare soil areas also known as tiger bush in modelling studies most often straight planar slopes were used to analyse vegetation patterning the effect of slope steepness has been investigated widely and some studies investigated the effects of microtopography and hillslope orientation however at the larger catchment scale the overall form of the landscape may affect vegetation patterning and these more complex landscapes are much more prevalent than straight slopes hence our objective was to determine the effect of landform variation on vegetation patterning and sediment dynamics we linked two well established models that simulate a plant growth death and dispersal of vegetation and b erosion and sedimentation dynamics the model was tested on a straight planar hillslope and then applied to i a set of simple synthetic topographies with varying curvature and ii three more complex real world landscapes of distinct morphology results show banded vegetation patterning on all synthetic topographies always perpendicular to the slope gradient interestingly we also found that movement of bands a debated phenomenon seems to be dependent on curvature vegetation banding was simulated on the slopes of the alluvial fan and along the valley slopes of the dissected and rolling landscapes in all landscapes local valleys developed a full vegetation cover induced by water concentration which is consistent with observations worldwide finally banded vegetation patterns were found to reduce erosion significantly as compared to other vegetation configurations

secondary seed dispersal and its role in landscape organization 1 mathematical models of banded vegetation patterns predict rapid upslope migration of vegetated patches not realized in field observations a key point of disagreement between theory and observation it is shown that the disagreement between model results and field observations can arise from seed dispersal dynamics two representations of biomass movement are used to test the hypothesis that secondary seed dispersal in overland flow inhibits band migration the first is based on coupling down slope water transport and seed advection the second uses a kernel based representation of seed transport where an anisotropic dispersal kernel combines the effects of isotropic primary and downslope secondary seed dispersal and ensures that conclusions about secondary dispersal are independent of diffusive representations of biomass movement the analysis demonstrates that secondary seed dispersal can retard upward movement of banded vegetation irrespective of the precise representation of biomass movement as long as the anisotropic effects are accounted for

runoff generation fostered by physical and biological crusts in semi arid sandy soils abstract biological soil crusts occur extensively in semi arid regions in the western part of niger they are associated with various types of physical soil crusts in fallows and in the tiger bush ecosystem a landscape with a typical banded pattern consisting of densely vegetated bands of small trees and shrubs alternating with bare soil bands despite their widespread occurrence not much research has been done regarding their role on runoff generation in the sahelian environment the present study aims at i highlighting the relative contribution of biological soil crusts in infiltration and runoff on crusted soil in respect with other soil factors ii assessing the impact of underlying physical crusts associated with a wide range of microbial cover and iii gathering the data required for an evaluation of the potential of microbial cover in water redistribution in an arid environment and for modelling purposes the study was based on runoff measurements performed in situ at the level of ten plots 1 m 2 each under two simulated rainfalls those experiments were coupled with the visual characterization and image analysis of soil surface conditions as well as micromorphological examinations using a light microscope both on the field and at the laboratory visual estimation of crust cover was found consistent with the estimation based on the image analysis biological soil crusts capped the summit of pre existing physical soil crusts i e structural crusts formed in situ by the impact of water drop and erosion crusts resulting from water or wind erosion of structural crusts the percentage of microbial cover in the 1 m 2 plots range from 39 to 80 55 in average on structural crusts compared to 4 to 29 17 in average on erosion crusts runoff coefficient varied with both the level of microbial cover and the type of physical soil crust higher runoff coefficients were found where biological soil crusts had developed on erosion crusts as compared to structural crusts runoff coefficients recorded from erosion crusts increased with the level of microbial cover while negative correlations were found on structural crusts they show that the clogging effect of microbial cover for the plasmic layer of erosion crusts is higher than that of the sand layer of structural crusts separating biological soil crusts according to the types of the underlying physical crusts explain therefore the contradictory findings related to their role on infiltration and runoff generation those data will help to evaluate the potential role of biological soil crusts in the modelling of water redistribution in the arid environment of the western part of niger

a runoff capability classification system based on surface features criteria in semi arid areas of west africa abstract an analysis of the factors influencing infiltration and runoff was carried out using data obtained under simulated rainfall conditions on 87 plots in arid and semi arid areas of west africa arranged in a hierarchical sequence one of the key factors to emerge from this typological classification is the type of surface crust together with faunal activity vegetation cover and surface roughness this parameter permits the inclusion of a unit surface factor in the classification system each unit surface is characterized in terms of genetic morphological and hydrological properties at a higher level the combination of these unit surfaces allows a soil surface features unit to be defined based on an original mapping method the samples once defined in this system were found to correspond to homogeneous criteria both in terms of evolution dynamics and hydrological behaviour furthermore such cartography could be successfully extrapolated through remote sensing image analysis making possible a classification of the runoff capability of small watersheds based on objective and standardized criteria

observations with moored acoustic doppler current profilers in the convection regime in the golfe du lion abstract in the golfe du lion south of france favorable conditions for deep winter convection exist and were documented by the medoc experiments during 1969 75 a renewed investigation of that regime with upward looking moored acoustic doppler current profilers adcps was carried out during 24 january 5 march 1987 to record the three dimensional currents associated with the deep mixing while in the earlier studies initial deep convection did not begin until fairly late in the winter season a very strong mistral around 10 january 1987 had already generated a 1arge deep mixed patch homogeneous down to around 2000 m at deployment time three adcps two working at 150 khz and one at 75 khz were moored in a triangle of 15 km sidelength at 550 780 m depth full records at 1 hour ensemble time intervals 400 pings per ensemble 8 m bin lengths were obtained by the 75 khz and one of the 150 khz adcps in mid february a second mistral hit the region with the onset of strong winds and surface cooling the

lunar and martian meteorite delivery services launch mechanisms for lunar and martian meteorites have been investigated by integrating physical modeling constraints geochemical cosmic ray exposure cre constraints and petrologic constraints the potential source region for lunar meteorites is remarkably small compared to the final crater volume cre constraints indicate that most launches start at depths of 3 2 m and cratering theory implies derivation of suitably accelerated objects from a subvolume with diameter only about 0 3 the final crater diameter the shallow depth provenance is probably related to shock wave interference enhanced by the lunar regolith s extremely low compressional wave velocity cre constraints alone imply that four to five separate launch events are represented among the eight well studied lunar meteorites most of the lunar meteorites are regolith breccias which tend to show only limited compositional diversity within any kilometer scale region of the moon several others are polymict breccias which also show relatively subdued compositional diversity compared to igneous rocks the observed diversity among these samples in terms of abundances of mare basalt and kreep and in mg mg fe ratio implies that among eight well studied lunar meteorites only two potential source crater pairings are plausible between asuka 881757 y 793169 most probable and between y 793274 eet875721 altogether these eight lunar meteorites apparently represent at least six separate source craters including three in the past 105 years and five in the past 106 years cre constraints imply that snc meteorites are launched from systematically greater depths than lunar meteorites sncs are also systematically bigger and all nine well studied sncs are uncommonly young by martian standards mafic igneous rocks comparison between viking and apollo results reveals that rocks the size of common meteorites are remarkably scarce in the martian regolith probably due to pervasive weathering a plausible explanation for these trends is that most old small and shallow rocks on mars have been weakened by a two stage process of brecciation followed by pervasive weathering to the point where they seldom survive the stresses of spallation off the planet the scarcity of source crater pairing among the lunar meteorites implies that these objects can be launched from craters much smaller than previously estimated and tends to support the suggestion of rabinowitz 1993 astrophys j 407 412 427 that present day cratering rates for the earth moon region may be higher than previously estimated

shrub mound formation and stability on semi arid slopes in the northern negev desert of israel a field and simulation study in semi arid areas vegetation is scarce and often dominated by individual shrubs on raised mounds the processes of formation of these mounds are diverse and still debated often shrub mound formation is directly related to the formation of vegetation patterns thereby assuming that shrub mound formation is driven by biological interactions we hypothesize that water related erosion and sedimentation are also important drivers of shrub mound formation in the northern negev desert of israel we test this hypothesis by combining field observations with model simulations we studied shrub mounds in the semi arid catchment of sayeret shaked in the northern negev desert 200 mm annual precipitation height and diameter of shrub canopy and shrub mounds were measured and micro morphological techniques were used to reconstruct the formation of the shrub mounds we used landscape evolution model lapsus to simulate shrub mound formation at short single precipitation event and longer 100 years timescales at different slope angles both field and model results indicate that shrub mounds in sayeret shaked are at least partly formed by redeposition of eroded material below the shrubs and by erosion and lowering of the surrounding crust additional model simulations suggest that mounds are formed most under low shrub density and large shrub canopy diameter shrub mound formation increases with slope in dryer and wetter climates than the studied 200 mm rainfall semi arid climate zone shrub mound formation is less likely

lithologic structural and geomorphic controls on ribbon forest patterns in a glaciated mountain environment abstract so called ribbon forests have been attributed to snowdrift patterns and fire history without reference to geomorphology vegetatio 19 1969 192 this paper illustrates how site conditions of geomorphology and geology explain the origin of ribbon forests in glacier national park mt usa regional tectonic uplift associated with the laramide orogeny produced structural features that amplify lithologic differences pleistocene glaciation scoured deeply along the strike of bedding planes highlighting this pattern and in some cases producing fine scale parallel finger lakes between forested ribbon strips twelve ribbon forest sites on both sides of the continental divide were closely studied on stereoscopic aerial photographs and several of these sites were examined in the field or from helicopter overflights in all cases geologic and geomorphic conditions explain the location and distribution of the ribbon forests change detection of the distribution of trees versus nontree covered surfaces in an area of ribbon forest on flattop mountain a complex uplifted synclinal structure was undertaken using panchromatic low altitude aerial photographs from 1966 to 1991 areas changed from forest to meadow and from meadow to forest in roughly equal amounts in a generally random spatial pattern no evidence was seen to suggest that the creation of one ribbon eventually created another downwind as suggested by billings aerial photograph interpretation field examination and soils analyses of forest ribbons and adjacent unforested meadows clearly illustrated that trees occupy higher parallel to subparallel well drained sites where the spatial pattern is in turn a distinct reflection of the spatial pattern of structure and stratigraphy meadows occupy topographically lower positions between ridges where erosion along bedding plane strike was concentrated topography sets conditions that allow tree growth in certain locations while precluding it in immediately adjacent areas ribbon forests there are thus a spatial manifestation of the interaction between structure lithology and topography

nonlinearity of ecogeomorphic processes along mediterranean arid transect the study was conducted along a climatic transect extending from the judaean mountains mean annual rainfall 700 mm annual mean temperature 17 c to the dead sea mean annual rainfall 100 mm annual mean temperature 23 c a high correlation was found between climatic conditions and factors and processes of the ecogeomorphological system the values of certain factors and processes such as soluble salt content and overland flow increase with increasing aridity whereas those of others such as soil organic matter content clay content aggregate size and stability and infiltration rate diminish with increasing aridity these trends express the long term effect of the prevailing mean climatic conditions in the research stations conditions that led to the development of various types of soil and of plant communities it was also found that for many of the factors and processes examined such as organic matter content salt concentration aggregate stability infiltration rate and overland flow yield the rates of change with respect to position along the transect are not linear and that a climatic threshold value exists in the zone with a mean annual rainfall of about 300 mm and in which the p t ratio is about 17 which creates a sharp boundary between arid and mediterranean ecogeomorphological systems while in arid systems erosion occurs mainly through physical mechanical processes under mediterranean systems it is governed mainly by chemical and biological processes the significance of the existence of a sharp boundary between the arid and the mediterranean zones is that even a relatively slight increase in the annual mean temperature and or a slight decrease in the mean annual rainfall is liable to shift the boundary towards the mediterranean zone and accelerate desertification processes

petrography and geochemistry of the chassignite northwest africa 2737 nwa 2737 we report on the petrology and geochemistry of the northwest africa 2737 nwa 2737 meteorite that was recovered from the morrocan sahara in 2000 it is the second member of the chassignite subclass of the snc shergotitte nakhlite chassignite group of meteorites that are thought to have originated on mars it consists of black olivine and spinel cumulate crystals 89 7 and 4 6 wt respectively with intercumulus pyroxenes augite 3 1 wt and pigeonite orthopyroxene 1 0 wt analbite glass 1 6 wt and apatite 0 2 wt unlike chassigny plagioclase has not been observed in nwa 2737 olivine crystals are rich in mg and highly equilibrated fo 78 7 0 5 mol the black color of olivine grains may be related to the strong shock experienced by the meteorite as revealed by the deformation features observed on the macroscopic to the atomic scale chromite is zoned from core to rim from cr83 4uv3 6sp13 0 to cr72 0uv6 9sp21 1 pyroxene compositional trends are similar to those described for chassigny except that they are richer in mg compositions range from en78 5wo2 7fs18 8 to en76 6wo3 2fs20 2 for the orthopyroxene from en73 5wo8 0fs18 5 to en64 0wo22 1fs13 9for pigeonite and from en54 6wo32 8fs12 6 to en46 7wo44 1fs9 2 for augite bulk rock oxygen isotope compositions confirm that nwa 2737 is a new member of the martian meteorite clan 17o 0 305 0 02 n 2 ree abundances measured in nwa 2737 mineral phases are similar to those in chassigny and suggest a genetic relationship between these two rocks however the parent melt of nwa 2737 was less evolved and had a lower al abundance

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

concordant rb sr sm nd and ar ar ages for northwest africa 1460 a 346 ma old basaltic shergottite related to lherzolitic shergottites abstract multiple lines of evidence show that the rb sr sm nd and ar ar isotopic systems individually give robust crystallization ages for basaltic or diabasic shergottite northwest africa nwa 1460 in contrast to other shergottites nwa 1460 exhibits minimal evidence of excess 40ar thus allowing an unambiguous determination of its ar ar age the concordant rb sr sm nd and ar ar results for nwa 1460 define its crystallization age to be 346 17 ma 2 in combination with petrographic and trace element data for this specimen and paired meteorite nwa 480 these results strongly refute the suggestion by others that the shergottites are 4 1 ga old current crystallization and cosmic ray exposure cre age data permit identification of a maximum of nine ejection events for martian meteorites numbering more than 50 unpaired specimens as of 2008 and plausibly as few as five such events although recent high resolution imaging of the martian surface has identified limited areas of sparsely cratered terrains the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come or that the cratering chronology needs recalibration time averaged 87rb 86sr 0 16 for the mantle source of the parent magma of nwa 1460 480 over the 4 56 ga age of the planet is consistent with previously estimated values for bulk silicate mars in the range 0 13 0 16 and similar to values of 0 18 for the lherzolitic shergottites initial end for nwa 1460 480 at 350 16 ma ago was 10 6 0 5 which implies a time averaged 147sm 144nd of 0 217 in the martian mantle prior to mafic melt extraction similar to values of 0 211 0 216 for the lherzolitic shergottites these time averaged values do not imply a simple two stage mantle melt evolution but must result from multiple episodes of melt extractions from the source regions much higher late stage end values for the depleted shergottites imply similar processes carried to a greater degree thus nwa 1460 480 the lherzolitic shergottites and perhaps eet 79001 give the best albeit imperfect estimate of the sr and nd isotopic characteristics of bulk silicate mars

a methodology for assessing the postconvection mesoscale dynamics in the gulf of lion from composite datasets in order to access the statistical properties of the mesoscale dynamics in the western mediterranean and its associated transport and heat fluxes during the postconvection period the authors have applied data combination methods for analyzing a wide range of in situ measurements collected during the thetis 1 and convhiv experiments ctd and xbt profiles were merged with times series at a fixed or moving point and also with integral time series obtained from acoustic tomography data estimates of temperature and currents within a box of approximately one degree square over a time period of 35 days during the postconvection period were produced during this winter convection has been only partial rarely penetrating deeper than 1200 m the analysis concentrates on the upper 1000 m where most changes occur geostrophy is used as a dynamical constraint relating the parameters the time evolution is controlled by a kalman filter using simple persistence the contribution of the different datasets to the estimation indicates their complementarity in the time and space dimensions hydrography and eulerian measurements provide a major contribution to the estimation of the baroclinic modes tomography data complement the estimate at all horizontal scales float data bring some information on the barotropic mode but the major contribution on this mode comes from the reciprocal tomography data particularly at the largest scales the period analyzed mostly covers the postconvection estimation of the kinetic energy indicates that the barotropic contribution represents 85 of the total energy horizontal advection transfers heat toward the central area at a mean rate of 50 w m 2 compensating for the heat losses through the surface the mesoscale flow field observed is characterized by strongly barotropic coherent vortices with a size o 30 40 km these barotropic eddies are present during all phases of convection

dynamic crystallization of shock melts in allan hills 77005 implications for melt pocket formation in martian meteorites abstract a series of crystallization experiments have been performed on synthetic glasses matching the composition of a melt pocket found in allan hills alh 77005 in order to evaluate the heterogeneous nucleation potential of the melt and the effect of oxygen fugacity on crystallization the starting temperature of the experiments varied from superliquidus liquidus and subliquidus temperatures each run was then cooled at rates of 10 500 and 1000 c h at fmq the results of this study constrain the heating and cooling regime for a microporphyritic melt pocket within the melt pocket strong thermal gradients existed at the onset of crystallization giving rise to a heterogeneous distribution of nucleation sites resulting in gradational textures of olivine and chromite skeletal olivine in the melt pocket center crystallized from a melt containing few nuclei cooled at a fast rate nearer to the melt pocket margin elongate skeletal shapes progress to hopper and equant euhedral reflecting an increase in nuclei in the melt at the initiation of crystallization and growth at low degrees of supercooling cooling from post shock temperatures took place on the order of minutes an additional series of experiments were conducted for a melt temperature of 1510 c and a cooling rate of 500 c h at the fmq buffer as well as 1 and 2 log units above and below fmq variation in chromite stability in these experiments is reflected in crystal shapes and composition and place constraints on the oxygen fugacity of crystallization of the melt pocket we conclude that the oxygen fugacity of the melt pocket was set by the fe 3 fe 2 ratio imparted by melting of the host rock rather than external factors such as incorporation of co 2 rich martian atmosphere or melting and injection of oxidized surface e g regolith material comparison with previous crystallization experiments on melt pockets in martian basalts indicate that the predominance of dendritic crystal shapes reflects the likelihood that those melt pockets with lower liquidus temperatures will be more completely melted destroying most or all nuclei in the melt in this case crystal growth takes place at high degrees of supercooling yielding dendritic shapes it appears as though the melting process is just as important as cooling rate in determining the final texture of the melt pocket as this process controls elimination or preservation of nuclei at the onset of cooling and crystallization

localization of deep ocean convection by a mesoscale eddy abstract observations of open ocean deep convection indicate that it is a highly localized phenomenon occurring over areas of tens of kilometers in diameter the cause of this localization has been ascribed to preconditioning the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface intensified cyclonic circulation however most numerical and laboratory studies of localized convection have prescribed the localization artificially by confining the surface buoyancy loss to a circular disk in contrast in the numerical simulations described here deep convection forced by horizontally uniform buoyancy loss is localized within a region of initially weaker stratification than its surroundings the preconditioned region is associated with a cold core cyclonic eddy in geostrophic and cyclostrophic balance as in previous studies of disk shaped cooling the localized convection region undergoes baroclinic instability at late times causing the br

metal saturated sulfide assemblages in nwa 2737 evidence for impact related sulfur devolatilization in martian meteorites nwa 2737 a martian meteorite from the chassignite subclass contains minute amounts 0 010 0 005 vol of metal saturated fe ni sulfides these latter bear evidence of the strong shock effects documented by abundant fe nanoparticles and planar defects in northwest africa nwa 2737 olivine a ni poor troilite fe s 1 0 0 01 sometimes cr bearing up to 1 wt coexists with micrometer sized taenite tetrataenite type native ni fe alloys ni fe 1 and fe os ir ru alloys a few hundreds of nanometers across the troilite has exsolved flame like pentlandite fe fe ni 0 5 0 6 chalcopyrite is almost lacking and no pyrite has been found as a hot desert find nwa 2737 shows astonishingly fresh sulfides the composition of troilite coexisting with ni fe alloys is completely at odds with chassigny and nahkla sulfides pyrite metal deficient monoclinic type pyrrhotite it indicates strongly reducing crystallization conditions close to iw several log units below the fo2 conditions inferred from chromites compositions and accepted for chassignites fmq 1 log unit it is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures shock related s degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages with concomitant loss of magnetic properties as documented in some other martian meteorites

noble gases in the martian meteorite northwest africa 2737 a new chassignite signature we report noble gas data for the second chassignite northwest africa nwa 2737 which was recently found in the moroccan desert the cosmic ray exposure cre age based on cosmogenic 3he 21ne and 38ar around 10 11 ma is comparable to the cre ages of chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event or a suite of discrete events having occurred in a restricted interval of time in contrast u th he and k ar ages 0 5 ga are in the range of radiometric ages of shergottites despite a sm nd signature comparable to that of chassigny and the nakhlites misawa et al 2005 overall the noble gas signature of nwa 2737 resembles that of shergottites rather than that of chassigny and the nakhlites nwa 2737 does not contain in detectable amount the solar like xenon found in chassigny and thought to characterize the martian mantle nor apparently fission xenon from 244pu which is abundant in chassigny and some of the nakhlites in contrast nwa 2737 contains martian atmospheric noble gases trapped in amounts comparable to those found in shergottite impact glasses the loss of martian mantle noble gases together with the trapping of martian atmospheric gases could have occurred during assimilation of martian surface components or more likely during shock metamorphism which is recorded in the petrology of this meteorite

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

soil and water components of banded vegetation patterns abstract banded landscapes are comprised of alternating bands of vegetation and bare ground aligned along the contours in arid and semi arid regions 50 750 mm rainfall on very gentle and uniform slopes 0 2 2 vegetated bands can be perpendicular to the direction of the dominant wind or more frequently of the slope under given climatic conditions slope gradient is the controlling factor of the type of pattern spotted broadly or typically banded for a given slope gradient mean annual rainfall determines the contrast between the vegetated and bare phase as well as the band length and the interband width band width ratio a typical transect through such two phase mosaic includes a dynamic succession of surface crusts which generates a run off run on system this favours the capture of limited water resources and thus a biomass production greater for banded patterns than spotted patterns or uniformly scattered vegetation moreover vegetated bands act as natural bench structures that greatly limit soil erosion as suggested by modelling banded vegetation patterns can equally be derived from nearly bare areas or from dense vegetation patterns because banded vegetation patterns have often been considered as a form of degradation of previous continuous vegetation cover many attempts have been made by foresters to restore this initial cover by reforestation of the bare interbands however the numerous failures of reforestation of the bare interbands illustrate the key role of these mosaic components in the maintenance of the tiger bush ecosystem banded vegetation patterns are more resilient to climate change than to human disturbances the loss of landscape patchiness due to the clearing of the bands either for fuelwood or for ephemeral cropping induces a rapid decline in soil fertility and water infiltration potential long term monitoring shows that the interband width band width ratio adapts to rainfall variations that are intrinsic of semi arid zones moreover climatic changes might also induce a change in the type of patterning suggesting that banded vegetation patterns are highly self sustainable the ecological differences between the pioneer and the decaying edges of the vegetated bands the temporal spatial succession of soil crusts as well as other indices such as the spatial distribution of soil organic matter and termite nests suggest the upslope migration of the bands nevertheless field evidence of such a shift remains scarce profound lessons can be learnt from the study of banded landscapes in terms of ecosystems functions when designing water harvesting systems or restoring degraded arid and semi arid land banded vegetation patterns should be imitated to optimize biomass production and limit land degradation

morphology and microstructure of microbiotic soil crusts on a tiger bush sequence niger sahel abstract microbiotic crusts resulting from the colonization of soil surfaces by communities of micro organisms dominated by cyanobacteria eucaryotic algae or lichens are widespread in arid and semi arid regions in the tiger bush of niger sahel microbiotic crusts occur in bare bands between densely vegetated bands the tiger bush soils are sandy loams with ph 4 7 6 5 three different microbiotic crusts are recognised forming strips parallel to the vegetation banding they are all formed mainly by filamentous cyanobacteria dominated by schizothrix other important genera are scytonema lyngbya microcoleus nostoc and phormidium the filamentous micro organisms trap sand particles and finer particles stick on the filament surfaces these effects enhance soil cohesion and resistance to erosion soil porosity is also increased by the microbial cover with additional pores delineated by filaments on the surfaces of crusts and porous organic bodies derived from microbial remains at depth the microbiotic crusts are essential components of the tiger bush ecosystem water leaving the lower sides of vegetated bands results in plant decay and on the upslope sides vegetation grows because of increased water supply microbial covers help resist erosion and increase water and nutrient retention thus providing substrates for the expansion of plants from adjacent vegetated bands

mathematical models of vegetation pattern formation in ecohydrology 1 highly organized vegetation patterns can be found in a number of landscapes around the world in recent years several authors have investigated the processes underlying vegetation pattern formation patterns that are induced neither by heterogeneity in soil properties nor by the local topography are generally explained as the result of spatial self organization resulting from symmetry breaking instability in nonlinear systems in this case the spatial dynamics are able to destabilize the homogeneous state of the system leading to the emergence of stable heterogeneous configurations both deterministic and stochastic mechanisms may explain the self organized vegetation patterns observed in nature after an extensive analysis of deterministic theories we review noise induced mechanisms of pattern formation and provide some examples of applications relevant to the environmental sciences

the algerian eddies abstract this paper synthetizes various previous analyses of the algerian eddies these mesoscale anticyclonic eddies are of primary importance to the circulation of all the water masses in the western mediterranean they are generated by instability processes disturbing the flow of modified atlantic water maw shortly after it reaches the african coast near 0 e namely the algerian current young eddies which have diameters of 50 100 km and the upwelling generally attached to their southwestern edges drift eastward along the coast at a few km per day sometimes during several months as they grow older and larger the most vigorous ones leave the coastal zone and drift seaward some of them may reach the sardinian continental slope where a well defined vein of levantine intermediate water liw flows we suspect them to be capable to pull fragments of liw seaward and in fact the least modified liw we encountered in the middle of the algerian basin had the form of rings or filaments trapped by one eddy eddies several months old may have diameters as large as 200km they also entrain the mediterranean deep water mdw and they probably extend down to the bottom in 1984 two such eddies occupied most of the algerian basin the maw was deflected seaward by the westernmost one from the algerian coast at 3 e as far as the balearic islands and was then dispatched by other eddies through the whole basin as though by a set of paddle wheels such interactions are perhaps not as exceptional as was previously expected these mesoscale phenomena have strong biological implications the western mediterranean sea thus appears to be a very suitable place for the observation and modelling of such mesoscale coherent structures

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

ejection ages from krypton 81 krypton 83 dating and pre atmospheric sizes of martian meteorites cosmic ray exposure cre ages and mars ejection times were calculated from the radionuclide 81 and stable kr isotopes for seven martian meteorites the following 8lkr kr cre ages were obtained los angeles 3 35 0 70 ma queen alexandra range 94201 2 22 0 35 ma shergotty 3 05 0 50 ma zagami 2 98 0 30 ma nakhla 10 8 4 0 8 ma chassigny 10 6 2 0 ma and allan hills 84001 15 4 5 0 ma comparison of these ages with previously obtained cre ages from the stable noble gas nuclei 3he 21ne and 38ar shows excellent agreement this indicates that the method for the production rate calculation for the stable nuclei is reliable in all martian meteorites we observe effects induced by secondary cosmic ray produced epithennal neutrons epithermal neutron fluxes 30 300 ev are calculated based on the reaction 79br n yp gokr we show that the neutron capture effects were induced in free space during mars earth transfer of the meteoroids and that they are not due to a pre exposure on mars before ejection of the meteoritic material neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre atmospheric sizes are estimated we obtain minimum radii in the range of 22 25 cm and minimum masses of 150 220 kg these results are in good agreement with the mean sizes reported for model calculations using current semiempirical data

terrestrial and martian weathering signatures of xenon components in shergottite mineral separates abstract xenon isotopic ratios step heating release patterns and gas concentrations of mineral separates from martian shergottites roberts massif rbt 04262 dar al gani dag 489 shergotty and elephant moraine eet 79001 lithology b are reported concentrations of martian atmospheric xenon are similar in mineral separates from all meteorites but more weathered samples contain more terrestrial atmospheric xenon the distributions of xenon from the martian and terrestrial atmospheres among minerals in any one sample are similar suggesting similarities in the processes by which they were acquired however in opaque and maskelynite fractions martian atmospheric xenon is released at higher temperatures than terrestrial atmospheric xenon it is suggested that both martian and terrestrial atmospheric xenon were initially introduced by weathering low temperature alteration processes however the martian component was redistributed by shock accounting for its current residence in more retentive sites the presence or absence of detectable 129xe from the martian atmosphere in mafic minerals may correspond to the extent of crustal contamination of the rock s parent melt variable contents of excess 129xe contrast with previously reported consistent concentrations of excess 40ar suggesting distinct sources contributed these gases to the parent magma

the formation of labrador sea water part ii mesoscale and smaller scale processes abstract in a previous paper clarke and gascard argued that the formation of labrador sea water was taking place in a cyclonic gyre set up each winter in the western labrador sea within the gyre and at its boundaries a number of different scales of organization are believed to be important in the formation processes the longest of these scales is the mesoscale 50 km which appears to be related to topographic rossby waves generated in the labrador current and propagating offshore the next smaller scale is an eddy scale 20 km believed to arise because the mesoscale is baroclinically unstable as shown by applying a two layer model of tang this instability is believed to promote mixing by generating frontal structures and vertical motions along them thus bringing subsurface t s maxima nearer the surface then within the mesoscale and eddy scale structures intense vertical convective cells take place at scales which are probably of the order of 1 km in three dimensions these events are short liv

noble gases in mineral separates from three shergottites shergotty zagami and eeta79001 this study provides a complete data set of all five noble gases for bulk samples and mineral separates from three martian shergottites shergotty bulk pyroxene maskelynite zagami bulk pyroxene maskelynite and elephant moraine eet a79001 lithology a bulk pyroxene we also give a compilation of all noble gas and nitrogen studies performed on these meteorites our mean values for cosmic ray exposure ages from 3he 21ne and 38ar are 2 48 myr for shergotty 2 73 myr for zagami and 0 65 myr for eeta79001 lith a serious loss of radiogenic 4he due to shock is observed cosmogenic neon results for bulk samples from 13 martian meteorites new data and literature data are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic ray effects while a contribution of this low energy irradiation is strongly indicated for all of the shergottites spallation ne in chassigny allan hills alh 84001 and the nakhlites is fully explained by galactic cosmic ray spallation implanted martian atmospheric gases are present in all mineral separates and the thermal release indicates a near surface siting we derive an estimate for the 40ar 36ar ratio of the martian interior component by subtracting from measured ar in the k poor pyroxenes the small radiogenic component as well as the implanted atmospheric component as indicated from 129xe excesses unless compromised by the presence of additional components a high ratio of 2000 is indicated for martian interior argon similar to that in the martian atmosphere since much lower ratios have been inferred for chassigny and alh 84001 the result may indicate spatial and or temporal variations of 40ar 36ar in the martian mantle

a coupled vegetation crust model for patchy landscapes a new model for patchy landscapes in drylands is introduced the model captures the dynamics of biogenic soil crusts and their mutual interactions with vegetation growth the model is used to identify spatially uniform and spatially periodic solutions that represent different vegetation crust states and map them along the rainfall gradient the results are consistent extensions of the vegetation states found in earlier models a significant difference between the current and earlier models of patchy landscapes is found in the bistability range of vegetated and unvegetated states the incorporation of crust dynamics shifts the onset of vegetation patterns to a higher precipitation value and increases the biomass amplitude these results can shed new light on the involvement of biogenic crusts in desertification processes that involve vegetation loss

the role of surface water redistribution in an area of patterned vegetation in a semi arid environment south west niger the surface hydrology of a semi arid area of patterned vegetation in south west niger is described in this region alternating bands of vegetation and bare ground aligned along the contours of a gently sloping terrain give rise to a phenomenon known as brousse tigree tiger bush at the selected study site the vegetation bands are 10 30 m wide separated by 50 100 m wide bands of bare ground five species of shrub dominate guiera senegalensis combretum micranthum c nigricans acacia ataxacantha and a macrostachya herbaceous vegetation is generally limited to the upslope edges of vegetation bands a comprehensive field programme was undertaken to investigate the hydrology topographic vegetation and surface feature surveys were carried out in conjunction with the measurement of rainfall surface and subsurface hydraulic conductivity particle size and soil moisture content four types of vegetation class are recognised each tending to occupy a constant position relative to the others and to the regional slope in a downslope direction the classes are bare ground grassy open bush closed bush bare open bush bare ground etc the nature of the ground surface is closely linked to the vegetation class over the bare bare open and grassy open classes various types of surface crust are present with each type of crust tending to occupy a constant position on the regional slope relative to the vegetation class and other crust types below closed bush crusts are generally absent the typical downslope sequence from the downslope boundary of a vegetation band is structural sieving crust erosion crust gravel crust sedimentation crust microphytic sedimentation crust no crust sieving crust etc it is also shown that these crust types are dynamic and evolve from one to the other as hydrological conditions change hydraulic conductivities of surface crusts are low typically falling within the range 10 6 10 7 m s 1 the presence of large expanses of crust over bare regions tends to generate run off which moves down the regional slope to be intercepted and pond within and just upslope of vegetated areas such run off concentrates rainfall by a factor of up to 3 7 below vegetated areas this concentration combined with an absence of crust development in closed bush areas promotes rapid infiltration below and just upslope of vegetation bands in this way the hydrology of the area operates to ensure that the bulk of the rain which falls is directed as quickly as possible to the areas where it is most needed to support the existing vegetation

noble gases and nitrogen in martian meteorites dar al gani 476 sayh al uhaymir 005 and lewis cliff 88516 efa and extra neon meteorite finds from the terrestrial hot deserts have become a major contributor to the inventory of martian meteorites in order to understand their nitrogen and noble gas components we have carried out stepped heating experiments on samples from two martian meteorites collected from hot deserts we measured interior and surface bulk samples glassy and non glassy portions of dar al gani 476 and sayh al uhaymir 005 we have also analyzed noble gases released from the antarctic shergottite lewis cliff 88516 by crushing and stepped heating for the hot desert meteorites significant terrestrial ar kr xe contamination is observed with an elementally fractionated air efa component dominating the low temperature releases the extremely low ar kr xe ratios of efa may be the result of multiple episodes of trapping loss during terrestrial alteration involving aqueous fluids we suggest fractionation processes similar to those in hot deserts to have acted on mars with acidic weathering on the latter possibly even more effective in producing elementally fractionated components addition from fission xenon is apparent in dag 476 and sau 005 the ar kr xe patterns for lew 88516 show trends as typically observed in shergottites including evidence for a crush released component similar to that observed in eeta 79001 a trapped ne component most prominent in the surface sample of dag 476 may represent air contamination it is accompanied by little trapped ar 20ne 36ar 50 and literature data suggest its presence also in some antarctic finds data for lew 88516 and literature data on the other hand suggest the presence of two trapped ne components of martian origin characterized by different 20ne 22ne possibly related to the atmosphere and the interior caution is recommended in interpreting nitrogen and noble gas isotopic signatures of martian meteorites from hot deserts in terms of extraterrestrial sources and processes nevertheless our results provide hope that vice versa via noble gases and nitrogen in meteorites and other relevant samples from terrestrial deserts martian secondary processes can be studied

a four dimensional analysis of the thermal structure in the gulf of lion the theoretical and experimental tomography in the sea experiment thetis 1 took place in the gulf of lion to observe the evolution of the temperature field and the process of deep convection during the 1991 1992 winter the temperature measurements consist of moored sensors conductivity temperature depth and expendable bathythermograph surveys and acoustic tomography because of this diverse data set and since the field evolves rather fast the analysis uses a unified framework based on estimation theory and implementing a kaiman filter the resolution and the errors associated with the model are systematically estimated temperature is a good tracer of water masses the time evolving three dimensional view of the field resulting from the analysis shows the details of the three classical convection phases preconditioning vigourous convection and relaxation in all phases there is strong spatial nonuniformity with mesoscale activity short timescales and sporadic evidence of advective events surface capping intrusions of levantine intermediate water liw deep convection reaching 1500 m was observed in late february by late april the field had not yet returned to its initial conditions strong deficit of liw comparison with available atmospheric flux data shows that advection acts to delay the occurence of convection and confirms the essential role of buoyancy fluxes for this winter the deep mixing results in an injection of anomalously warm water t 0 03 to a depth of 1500 m compatible with the deep warming previously reported

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

local properties of patterned vegetation quantifying endogenous and exogenous effects dryland ecosystems commonly exhibit periodic bands of vegetation thought to form due to competition between individual plants for heterogeneously distributed water in this paper we develop a fourier method for locally identifying the pattern wavenumber and orientation and apply it to aerial images from a region of vegetation patterning near fort stockton tx usa we find that the local pattern wavelength and orientation are typically coherent but exhibit both rapid and gradual variation driven by changes in hillslope gradient and orientation the potential for water accumulation or soil type endogenous pattern dynamics when simulated for spatially homogeneous topographic and vegetation conditions predict pattern properties that are much less variable than the orientation and wavelength observed in natural systems our local pattern analysis combined with ancillary datasets describing soil and topographic variation highlights a largely unexplored correlation between soil depth pattern coherence vegetation cover and pattern wavelength it also surprisingly suggests that downslope accumulation of water may play a role in changing vegetation pattern properties

surface microtopography and soil penetration resistance associated with shrub patches in a semiarid rangeland the surfaces of hillsides in semiarid regions are characterized by vegetative patchiness and an associated pattern of soil characteristics in a semiarid patchy rangeland in the northern negev region of israel the pattern of ground surface microtopography and soil penetration resistance were investigated along transects centred on randomly selected shrubs of sarcopoterium spinosum each transect comprised five segments on the hillslope axis upslope us and downslope ds of the central stem cluster of the shrub patch immediately upslope of the shrub tr an upper intershrub ui segment above tr and a lower intershrub li segment below ds significant differences were found among the different segments with regard to both slope and soil penetration resistance segment us had a relatively shallow slope 9 0 in average and the deepest penetration depth 20 5 mm segment ds had the steepest slope 21 0 and a relatively deep penetration depth 16 4 mm segments ui and li had intermediate values for both slope 15 1 and 13 4 respectively and penetration depth 10 5 and 9 8 mm a spatial relationship between the shrub patches and grazing was found segment tr immediately above the shrub tended to be a flock trampling route and had the lowest values of both slope 5 1 and penetration depth 6 1 mm a conceptual model was proposed to explain these results it suggests that under the prevailing conditions of the study site these relationships regulate the redistribution of soil resources on the hillside

systematic variation of soil infiltration rates within and between the components of the vegetation mosaic in an australian desert landscape vegetation mosaics have commonly been thought to include two principal zones with distinctly different hydrology relatively bare and impermeable runoff source zones intergroves and more strongly absorbing vegetated runon zones groves however the data required to verify the internal uniformity of hydrologic response within these components of mosaic landscapes have been lacking as have data on the nature abrupt or gradational of the boundaries between them this study examines the degree of internal uniformity of key soil properties in the intergroves and groves of an australian vegetation mosaic infiltration rates soil water content shear strength bulk density and texture were determined at intervals of 1 5 2 5 m across several grove intergrove cycles of an australian banded shrubland results demonstrate that order of magnitude variability in soil infiltration rates can occur across intergroves with lesser variation in groves patterns of infiltration are systematically related to slope position rates are relatively high in the uppermost parts of the intergrove and fall to low values only in the lowermost intergrove where soils are mechanically strong infiltration rates increase rapidly from the lowermost intergrove to reach maxima within the upper to middle grove from where rates once again decline toward the next intergrove however there is only a gradational change in infiltration rates across the pioneer zone grove boundary which is the sharpest of the mosaic boundaries when identified using plant cover data hydrologic models built on the presumption that mapped plant cover units are equally distinct hydrologically may need to be refined to incorporate the presence of systematic internal variability of infiltration rates and gradational change in soil hydraulic properties copyright 2002 john wiley sons ltd

a three dimensional numerical study of deep water formation in the northwestern mediterranean sea abstract deep water formation dwf in the northwestern mediterranean sea and the subsequent horizontal circulation are investigated in a rectangular basin with a three dimensional primitive equation model the basin is forced by constant climatological heat and salt fluxes convective motion is parameterized by a simple nonpenetrative convective adjustment process plus richardson number dependent vertical eddy viscosity and diffusivity a homogeneous column of dense water is progressively formed in the forcing area meanders of 40 km wavelength develop at the periphery of the column these features agree with observations energy studies show that the meanders are generated mainly through a baroclinic instability process these meanders and the associated cells of vertical motion contribute to the process of dwf they generate vertical advection while the associated horizontal advection tends to restratify the surface water of the column and thus to inhibit very deep convection just before the end o

martian meteorite chronology and the evolution of the interior of mars abstract the earth moon and mars are the only three planets for which satellite and ground observations can be confronted with extensive chronological and geochemical observations of rock samples here we use new pb isotope data on shergottites nakhlites chassigny and the alh 84001 orthopyroxenite to reassess chronological evidence and discuss its bearing on the evolution of the interior of mars we find that the depleted shergottites formed at 4 3 ga i e 170 ma after the last equilibration of pyroxene rich cumulates with the magma ocean while the younger enriched and intermediate shergottites as well as alh 84001 formed 200 ma later at 4 1 ga after the residual part of the mantle had been largely mixed back into these cumulates we also demonstrate that the th u ratios inferred from pb isotopes require shergottite ages older than 4 0 ga we finally reexamine the implications of this new chronological framework for the interpretation of 142 nd and 182 w anomalies in martian meteorites we conclude that shergottites represent samples that derived from a volcanic basement subsequently shattered by the 3 9 ga late heavy bombardment lhb although the magmatic activity of mars had strongly declined by the time of the lhb the new chronology of martian meteorites proposed here allows the martian mantle to be convective until today the 1 3 ga old emplacement of nakhlites and chassignites corresponds to decompression melting of ilmenite rich mantle domains and not to renewed global magmatic activity of the planet with the exception of this event the history of the interior of mars therefore somewhat resembles that of the moon

vegetation arcs and litter dams similarities and differences abstract vegetation arcs are near parallel bands of denser vegetation aligned perpendicular to slope and separated by barer inter arc zones these patterns occur in arid and semi arid areas of gentle to very gentle gradient similar but much smaller scale patterns here referred to as litter dams and microterraces occur in a variety of climatic regions from tropical to temperate to arid on steeper slopes as well as on very gentle slopes we describe similarities between arcs and dams in attributes such as shape orientation cross sectional profile spatial organisation and surface features arrangement of plants surface crusts differences are in spatial and temporal scale range of wavelengths or spacing between bands is 25 300 m for arcs and 0 1 2 m for dams with regard to time arcs may be stable for tens to a hundred or more years while dams may be ephemeral or remain stable for more than a decade overland flow involving the transport of organic material leaves charcoal faecal pellets as floating load over comparatively bare surfaces is an important factor in the maintenance of vegetation arcs and formation of and maintenance of litter dams recognising the importance of gradient to transport by overland flow a preliminary investigation of the relationship between wavelength and gradient was conducted a combined analysis of a limited set of arc and dam data describes parallel lines which may indicate that similar processes are operating at the two scales this would allow the convenience of process studies on dams as an aid in understanding the origin of arcs however the difference in the intercept of the two lines with the ln wavelength axis may indicate changes in process that preclude extrapolations from one spatio temporal scale to the other

topographic controls on the spatial distribution of ground cover in the tabernas badlands of se spain abstract the relationships between the spatial distribution of ground cover and terrain attributes were examined in the tabernas badlands se spain in order to understand the terrain dependent driving forces of the spatially heterogeneous ground cover ground cover was mapped in the field and terrain attributes were derived from a 1 m resolution digital elevation model dem the association of spatial distribution of the landforms resulting from a regionalisation using a nonhierarchical classification of the topographic overlays and the ground cover pattern was proved from the analysis of relationships between terrain attributes and proportional abundance of ground cover types it was found that ground cover is arranged along topographic gradients plant covered surfaces are more abundant on low slope angles concave slopes relatively large contributing areas and with low length slope factor values unvegetated surfaces show contrary trends and lichens are associated with intermediate conditions relationships with local terrain attributes such as slope angle or elevation are more pronounced than those with terrain attributes related to sediment and water transfer such as contributing area wetness index or length slope factor which could be explained by the heterogeneity of runoff that is usually shorter than the hillslope length the relationships established between the spatial distribution of ground cover types and terrain attributes provide the basis for future development of a tool for mapping spatial distribution of ground cover in similar areas from only topographic information

the soil surface characteristics of vegetation stripes in northern mexico and their influences on the system hydrodynamics abstract vegetation stripes were described in arid areas of africa australia and mexico they depend on the local concentration of rain water after runoff in mexico we described the soil surface characteristics of vegetation stripes and performed rainfall simulations to analyze its hydrodynamic behaviour simulations were performed on plots characterized by the four soil surface features represented in the stripes four types of surface crusts were found in the stripes erosion sedimentation and two types of structural crusts the less vegetated plots had relatively impervious crusts with low preponding rains and high constant runoff rates on the contrary the highly vegetated plot had a thin crust and abundant litter which allowed a high infiltration rate runoff rates were positively related to rainfall intensity those relationships between surface characteristics and runoff determine the fate of rain water and may allow the prediction of the amount of runoff for a specific rain on a given plot

estimating hydraulic conductivity of crusted soils using disc infiltrometers and minitensiometers although soil crusting has long been recognized as a crucial runoff factor in the sahel very few field methods have been developed for the measurement of the crust hydraulic conductivity which is difficult to achieve because of the small thickness of most surface crusts a field method based on the simultaneous use of disc infiltrometers and minitensiometers is proposed for determining the crust hydraulic conductivity and sorptivity near saturation on crusted soils the classical analysis of the steady state water flow was found to be inadequate the proposed method is based on sorptivity measurements performed at different water supply potentials and uses recent developments of transient flow analysis a minitensiometer placed horizontally at the crust subsoil interface facilitated the analysis of the infiltration regime for the crust solely results are shown for representative soil units of the east central super site of the hapex sahel experiment fallow grasslands millet fields and tiger bush non crusted soils were also considered and validated the transient method as demonstrated by comparison with wooding s steady state solution this validation was obtained in the case of fallow grasslands soil but not for the millet fields in this latter case the persistent effects of localized working of the soil to remove weeds caused large variations in infiltration fluxes between the sampling points which tended to dominate over effects of differences in applied potential for the tiger bush crusted soils the ratio of the saturated hydraulic conductivity of the crust to that of the underlying soil ranges from 13 to 16 depending on whether the crust is of a structural st or sedimentation sed type the method also allows the estimation of a functional mean pore size consistent with laboratory measurements and 40 less for the crusts in comparison with the underlying soil the results obtained here will be used in hydrological models to predict the partition of rainfall between infiltration and runoff

hydrological cycle and water balance estimates for the megadune lake region of the badain jaran desert china the hydrology and water balance of megadunes and lakes have been investigated in the badain jaran desert of china field observations and analyses of sand layer water content field capacity secondary salt content and grain size reveal 3 types of important natural phenomenon a vegetation bands on the leeward slope of the megadunes reflect the hydrological regime within the sandy vadose zone b seepage wet sand deposits and secondary salt deposits indicate the pattern of water movement within the sandy vadose zone c zones of groundwater seeps and descending springs around the lakes reflect the influence of the local topography on the hydrological regime of the megadunes the seepage exposed on the sloping surface of the megadunes and gravity water contained within the sand layer confirm the occurrence of preferential flow within the vadose zone of the megadunes alternating layers of coarse and fine sand create the conditions for the formation of preferential flows the preferential flows promote movement of water within the sand layer water that leads to deep penetration of water within the megadunes and ultimately to the recharging of groundwater and lake water our results indicate that a positive water balance promotes recharge of the megadunes which depends on the high permeability of the megadune material the shallow depth of the surface sand layer affected by evaporation the occurrence of rainfall events exceeding 15 mm and the sparse vegetation cover water balance estimates indicate that the annual water storage of the megadunes is about 7 5 mm accounting for only 8 of annual precipitation however the shallow groundwater per unit area under the megadunes receives only 3 6 of annual precipitation but it is still able to maintain a dynamic balance of the lake water from a water budget perspective the annual water storage in the megadunes is sufficient to serve as a recharge source for lake water thereby enabling the long term persistence of the lakes overall our findings demonstrate that precipitation is a significant component of the hydrological cycle in arid deserts

zebra stripes in the atacama desert revisited granular fingering as a mechanism for zebra stripe formation abstract although a number of studies have pointed out the remarkable slowness of earth surface processes in the atacama desert process mechanisms under such extremely limited water availability are poorly understood and process rates remain unknown this paper revisits the discussion on the formation of the prominent atacama specific hillslope zebra stone stripes previously interpreted to result from palaeo overland flow owen et al 2013 compared to this study our data document different stripe characteristics with regard to stripe form and orientation as well as sorting and bedding patterns of stripe confining surface gravel units we found a remarkable form concordance between zebra stripes and deposits from experiments on segregation induced granular fingering hence we propose a combination of seismic shaking and instantaneous dry granular free surface flows as the key mechanism for zebra stripe formation our findings underline the potential significance of seismicity in shaping atacama landscapes which bear important analogies to extra terrestrial surfaces

ecohydrological interactions within fairy circles in the namib desert revisiting the self organization hypothesis vegetation patterns such as rings bands and spots are recurrent characteristics of resource limited arid and semiarid ecosystems one of the most recognizable vegetation patterns is the millions of circular patches often referred to as fairy circles within the arid grassland matrix extending over hundreds of kilometers in the namib desert several modeling studies have highlighted the role of plant soil interactions in the formation of these fairy circles however little is known about the spatial and temporal variability of hydrological processes inside a fairy circle in particular detailed field assessment of hydrological and soil properties inside and outside the fairy circles is limited we conducted extensive measurements of infiltration rate soil moisture grass biometric and sediment grain size distribution from multiple circles and interspaces in the namib desert our results indicate that considerable heterogeneity in hydrological processes exists within the fairy circles resulting from the presence of coarser particles in the inner bare soil areas whereas concentration of fine soil occurs on the vegetated edges the trapping of aeolian and water borne sediments by plants may result in the observed soil textural changes beneath the vegetation which in turn explains the heterogeneity in hydrological processes such as infiltration and runoff our investigation provides new insights and experimental data on the ecohydrolgical processes associated with fairy circles from a less studied location devoid of sand termite activity within the fairy circles the results seem to provide support to the self organization hypothesis of fairy circle formation attributed to the anti phase spatial biomass water distributions

factors related to gully erosion in woody encroachment in south eastern australia abstract gullies initiate when the equilibrium within a drainage line is upset by increased discharge or decreased soil resistance to detachment and transport of soil particles and is triggered and accelerated by human induced changes to vegetation cover woody encroachment is the increase in density cover extent and biomass of woody plants and can result in low ground cover which increases the potential of soil erosion this paper reports the relationships between gully erosion and site characteristics such as topography vegetation cover and road infrastructure in a 120 km study area of semi arid woodland subject to woody encroachment in south eastern australia this study characterised gully morphology and activity identified the factors related to gully volume and estimated the topographic thresholds for sub catchments subject to woody encroachment firstly relationships between gully volume activity topography vegetation cover and road infrastructure were explored secondly site characteristics of 32 sub catchments with and without gullies were compared finally the topographic thresholds for unstable and stable sub catchments with and without gullies respectively were estimated using the slope drainage area relationship s aa b all gullies were active and high gully volume was related to large sub catchments long gullies and short distances to roads stable sub catchments without gullies had gentler slopes higher foliage projective cover and ground cover than unstable sub catchments with gullies when data for sub catchments with and without gullies were combined the main factors related to gully volume were drainage area at the gully or stream heads and their local slope and ground cover the mean topographic threshold for gully development provided insights into the drainage area and slope conditions needed for gully development gully erosion in the study area is most likely the result of interactions among topography vegetation and human made structures roads in space and time the results of the study advance our understanding of gully erosion dynamics in areas of woody encroachment and can be used to identify areas susceptible to gullying

convection with rotation in a neutral ocean a study of open ocean deep convection abstract the intensity and scale of the geostrophically adjusted end state of the convective overturning of a homogeneous rotating ocean of depth h at a latitude where the coriolis parameter is f induced by surface buoyancy loss of magnitude b0 are studied by numerical experiment the experiments are related to observations and laboratory studies of open ocean deep convection a numerical model based on the nonhydrostatic boussinesq equations is used the grid spacing of the model is small enough that gross aspects of convective plumes themselves can be resolved yet the domain of integration is sufficiently large to permit study of the influence of plumes on the large scale and geostrophic adjustment of the convected water numerical simulations suggest that cooling at the sea surface is offset by buoyancy drawn from depth through the agency of convective plumes these plumes efficiently mix the water column to generate a dense chimney of fluid which subsequently breaks up through the mechanism of bar

a laser probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism spatially resolved argon isotope measurements have been performed on neutron irradiated samples of nw africa nwa 4797 shock heating of nwa 4797 completely melted and vesiculated precursor igneous plagioclase which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition ca k ratios using a focused ultraviolet laser beam is has been possible to distinguish between argon isotopic signatures from groundmass minerals igneous olivine pyroxene plagioclase and a shock vein this study focuses on the potential for this meteorite to shed light on shock ages of shergottites apparent 40 ar 39 ar ages of groundmass minerals show that there are large amounts of excess argon in this phase yielding a wide range of calculated ages from 690 30 ma to several apparent ages older than 4 5 ga a traverse of laser probe extractions across the 1 mm diameter shock vein in nwa 4797 yielded apparent 40 ar 39 ar ages younger than the groundmass a signature of the martian atmosphere identified by 40 ar 36 ar ratios of 1600 1900 was not found in the nwa 4797 shock vein this is distinct from other shergottites where the products of shock melting contain a nearly pure sample of martian atmosphere we attribute this to a distinct formation mechanism and hence gas trapping mechanism of the nwa 4797 shock vein we undertook 44 analyses of plagioclase areas identified by sem analysis ages ranged from 45 27 to 3771 109 ma and yield an average age of 375 77 ma considerably younger than ages obtained in this study from either the groundmass or the shock vein a plot of age v 37 ar 39 ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured older ages are correlated with higher ca k ratios of plagioclase indicating contamination from groundmass minerals rich in excess argon the youngest ages correlate to plagioclase extractions with the lowest ca k ratios interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a k rich mesostasis we see no evidence for multiple shock events in nwa 4797 rather we favour the interpretation that the cosmic ray exposure cre age of 3 0 0 5 ma obtained on nwa 4797 in this study using cosmogenic 38 ar approximates the timing of shock melting in this meteorite

three dimensional observations of a deep convective chimney in the greenland sea during winter 1988 89 abstract all available temperature data including moored thermistor hydrographic and tomographic measurements have been combined using least squares inverse methods to study the evolution of the three dimensional temperature field in the greenland sea during winter 1988 89 the data are adequate to resolve features with spatial scales of about 40 km and larger a chimney structure reaching depths in excess of 1000 m is observed to the southwest of the gyre center during march 1989 the chimney has a spatial scale of about 50 km near the limit of the spatial resolution of the data and a timescale of about 10 days the chimney structure breaks up and disappears in only 3 6 days a one dimensional vertical heat balance adequately describes changes in total heat content in the chimney region from autumn 1988 until the time of chimney breakup when horizontal advection becomes important a simple one dimensional mixed layer model is surprisingly successful in reproducing autumn to winter bulk temperatur

microstructural investigations on strongly stained olivines of the chassignite nwa 2737 and implications for its shock history abstract olivines from the meteorite northwest africa 2737 nwa 2737 show unique characteristics such as strong brownish staining the occurrence of perpendicular sets of bright lamellae and the exsolution of metallic nanoparticles these features were investigated by transmission electron microscopy and spectroscopic techniques to understand their formation and consequent implications for the shock history of nwa 2737 areas showing optically an intense brown staining are characterised by the occurrence of finely dispersed metallic nanoparticles an extreme high density 10 16 m 2 of polygonised dislocations with burgers vector 001 and a high abundance of trivalent iron 16 as determined by electron energy loss spectroscopy in contrast the bright lamellae contain individual dislocation lines with burgers vector 001 of a slightly lower density 10 14 m 2 almost no metallic nanoparticles and significantly lower trivalent iron content model mass balance calculations suggest that the high content of trivalent iron can be incorporated as a laihunite component into the cores of polygonised dislocations this is charge balanced by the formation of metallic nanoparticles trivalent iron is likely responsible for the intense brown colour whereas nanoparticles darken these areas considerably and cause an intense red slope in the infrared range the proposed diffusion of trivalent iron into the cores of polygonised dislocations and the associated exsolution of metallic iron nanoparticles are likely fundamental processes for intense olivine staining during meteoritic impacts in general detailed analyses of variously stained regions suggest that the bright lamellae have experienced slightly higher shock induced strain causing a recrystallisation of olivine which occurred simultaneously with the polygonisation and exsolution processes resulting in the olivine staining of adjacent areas the existence of individual dislocations can be interpreted as the result of a two stage shock history for nwa 2737 after a first impact the material of nwa 2737 was likely embedded in a hot ejecta blanket this would ensure sufficient time at elevated temperatures for the polygonisation of dislocations and the recrystallisation of bright lamellae a second impact resulted in the emission of individual dislocations into recrystallised olivine and the ejection of that meteorite from mars

observation of a deep convection regime with acoustic tomography in the winter of 1991 1992 a convection experiment was conducted in the western mediterranean gulf of lions combining a variety of observational techniques an essential component was an acoustic tomography array consisting of six moorings designed to observe the time evolution of the large scale processes believed to be relevant in a convection regime here two dimensional slice inversions in three directions from the central mooring are used to estimate the volume of convected water and the mean convection depth and to observe the preconditioning and restratification processes before and after the convection the near surface layer is well sampled by the acoustics which show cooling and subsequent entrainment of the warmer levantine intermediate water liw from below in agreement with mixedlayer calculations during the 2 months prior to the main convection event the total heat loss of the large scale field is in approximate agreement with the surface heat fluxes showing that little net warm water advection takes place from outside the convection region the implied confinement of water by the local circulation should be an important factor in setting the location and extent of the deep convection patch the volume of water modified by convection in this winter is estimated to correspond to an area of order 60 km radius and 1500 m depth it is argued that the homogenized area implies an annual mean deep water replenishment of 0 3 sv the restratification by the return of less dense water in the surface and liw layers occurs first by rapid capping in the near surface region thereafter the southern and eastern parts of the region restratify in the deeper layers on a 40 day timescale while a dense core of 50 60 km diameter remains in the northwest until the end of the experiment

exposure history of shergottites abstract the cosmogenic nuclides 36 cl t 1 2 3 0 10 5 years 26 al 7 05 10 5 years 10 be 1 6 10 6 years and 53 mn 3 7 10 6 years were measured in three shergottites alha 77005 eeta 79001 and shergotty by accelerator mass spectrometry and neutron activation the cosmogenic nuclide data constrain the history and origin of shergottites alha 77005 was ejected from its parent body about 2 7 my ago and was exposed to cosmic rays as a body of 5 6 cm radius solar cosmic ray produced 26 a1 and 53 mn were first clearly observed in this meteorite the exposure age of eeta 79001 was 0 6 my and the preatmospheric radius was about 15 cm the exposure age of shergotty was calculated to be 2 2 my and the preatmospheric size was about 10 15 cm in radius the terrestrial age of alha 77005 was 0 2 my and that of eeta 79001 was less than 0 06 my all cosmogenic nuclide data indicate that the three shergottites were never exposed to cosmic rays in their parent bodies whether that parent body was mars or an asteroid all shergottites were ejected from greater than a 3 m depth in their parent bodies the ejection of eeta 79001 was a different event from that of the other shergottites

dhofar 378 martian shergottite evidence of early shock melting 1 shock heating of the dhofar 378 dho 378 martian shergottite produced melting vesiculation and flow of the plagioclase which upon cooling recrystallized into complex textures heating experiments on the similar zagami shergottite indicate that dho 378 was shock heated to 1000 1100 c and was cooled at 2 5 c h an 39ar 40ar analysis of dho 378 plagioclase indicates different ar diffusion domains and k ca ratios the lower temperature phase defines an ar ar isochron age of 141 32 ma the higher temperature phase released more 40ar but does not define an age the meteorite s thermal history was examined by constructing a generic model to compare cooling rates for objects of different sizes against fractional diffusion loss of ar for different cooling times using gas diffusion parameter values measured for dho 378 this model indicates that it is improbable that the major shock heating event occurred at the time that dho 378 was ejected from mars 3 ma ago rather we suggest that the time of shock heating is probably given by its ar ar age for dho 378 to cool sufficiently fast not to lose most of its 40ar 3 ma ago would require it to have been ejected into space as an impossibly small object larger and more reasonable mars ejection sizes indicate that dho 378 should have lost most of its 40ar on the basis of plagioclase texture and ar data we suggest that a major impact event 141 ma ago melted dho 378 plagioclase degassed most of its 40ar and deposited it in crater ejecta to cool a smaller and later impact ejected it into space 3 ma ago

igneous and shock processes affecting chassignite amphibole evaluated using chlorine water partitioning and hydrogen isotopes amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma but also shock and postshock processes we have analyzed amphibole and other phases from nwa 2737 melt inclusions and we evaluate these data along with published values to constrain the crystallization cl and h2o content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains using a model for the cl oh exchange between amphibole and melt we estimate primary crystallization oh contents of chassignite amphiboles sims analysis shows that amphibole from nwa 2737 currently has 0 15 wt h2o it has lost 0 6 wt h2o from an initial 0 7 0 8 wt h2o due to intense shock chassigny amphibole had on average 0 3 0 4 wt h2o and suffered little net loss of h2o due to shock nwa 2737 amphibole has d 3700 it absorbed martian atmosphere derived heavy h in the aftermath of shock chassigny amphibole with d 1900 incorporated less heavy h low h2o cl ratios are inferred for the primitive chassignite magma which had significant effects on melting and crystallization volatiles released by the degassing of martian magma were more cl rich than on earth resulting in the high cl content of martian surface materials

morphology genesis and classification of surface crusts in loamy and sandy soils abstract in an attempt to organize the knowledge of soil crusting processes and to group soil crusts on the basis of common morphological features and physical properties we synthesized and reinterpreted information from previous publications as a result we propose a classification system for surface crusts formed by rainfall in loamy and sandy soils although this system does not aim to be comprehensive it was designed from the study of a great number of crust samples collected in the temperate and tropical zones thus corresponding to a wide range of environmental and land use conditions three main classes of crusts structural erosion and depositional were distinguished each with subclasses where possible correspondence has been made with former systems this system helps predict infiltrability in accounting not only for total porosity of surface crusts but also for pore shape and continuity since crust classes are genetically related this classification system also supplies some insight into predicting soil degradation and selecting the most suitable control techniques

martian dunite nwa 2737 integrated spectroscopic analyses of brown olivine received 5 october 2007 revised 13 december 2007 accepted 6 march 2008 published 18 june 2008 1 a second martian meteorite has been identified that is composed primarily of heavily shocked dunite northwest africa nwa 2737 this meteorite has several similarities to the chassigny dunite cumulate but the olivine is more mg rich and most notably is very dark and visually brown carefully coordinated analyses of nwa 2737 whole rock and olivine separates were undertaken using visible and near infrared reflectance

banded vegetation near broken hill australia significance of surface roughness and soil physical properties abstract selected physical properties of the soils developed within a strongly banded grassland in arid new south wales were investigated to reveal their possible significance for the hydrologic and erosional behaviour of the mosaic landscape detailed surface microtopography surface roughness soil bulk density and the unconfined compressive strength of the soils were determined using linear transects across the banded mosaic the landscape is shown to consist of a tier of concave upward microtopographic elements results indicate that the cross pattern downslope variation in the soil parameters is systematically related to position within a particular component grove intergrove etc of the mosaic compressive strength and bulk density increase downslope across intergroves peaking at very high levels within the zone of forbs while groves display lower but more uniform values surface roughness increases downslope through the intergrove and the zone of forbs at the upslope margin of a grove reaching its maximum within the grove mosaic components thus cannot be treated as uniform in their soil properties and single samples from within a component are shown in general to be inadequate the mapped pattern of soil properties implies a very stable configuration for banded mosaics surface runoff is increasingly hindered during flow from the intergrove onto the grove at the same time soil resistance to entrainment increases in opposition to the shear forces generated by the runoff in concert these tendencies imply that little sediment transport is possible across the mosaic the resulting landscape stability appears to confer robustness to the mosaic in the face of stresses such as drought and pastoralism when plant cover may be temporarily thinned or absent after drought for example water ponding would again begin at the downslope margin of the concave topographic elements fostering re establishment of the groves

landscape fragmentation and soil cover changes on south and north facing slopes during ecosystems recovery an analysis from multi date air photographs abstract a quantitative analysis is presented of soil and vegetation patch patterns from air photographs of three dates 1956 1976 and 1990 relationships between changes in soil cover and landscape fragmentation are assessed for north and south facing slope units in the semi arid zone of israel undergoing recovery following severe overgrazing and woodcutting phases soil loss accompanied by an increase in rock fragment exposure and an increase in soil erodibility due to a decrease in aggregate size and organic matter content are the two main characteristics of soil changes in this area changes in both of these characteristics are highly correlated with soil brightness properties as recorded from aerial photographs the methodology developed combines image processing techniques enabling differentiation between soil and vegetation patches with raster geographical information systems gis techniques allowing patch distribution fragmentation and form to be analyzed expansion of vegetation patches and contraction of soil patches were found to follow similar lines of spatial pattern evolution when parameterized according to their size form and density assessment of temporal changes in soil and vegetation patch composition indicated that there is a phase difference of 12 years between the processes taking place on the two slope units hypothetical descriptions of soil patch fragmentation and vegetation patch expansion processes between 1920 and 2010 were formulated following the analysis of data between 1956 and 1990 evidence of soil brightness changes provided indications for the continuation of soil loss mainly in large patches a few thousands of square meters in size undergoing relatively slow fragmentation

contrasting hydrological response of coastal and desert biocrusts biocrusts abound in southern israel covering the hallamish dune field near nizzana niz in the negev mean annual precipitation of 95 mm and the coast of nizzanim nim near ashdod mean annual precipitation of 500 mm while the hydrological response of the niz crust to natural rain events was thoroughly investigated no data is available on the hydrological response of the nim crust runoff was monitored in runoff plots during the years 2005 2008 and in addition sprinkling experiments were carried out on nim and niz crusts for the evaluation of the possible factors that may control runoff initiation fine content of the parent material crust thickness compressional strength hydrophobicity surface microrelief organic matter biomass chlorophyll a and total carbohydrates and the crust s species composition of nim were studied and compared to that of niz the data showed that in comparison to the niz crust that readily generated runoff no runoff was produced by the nim crust this was so despite the fact that 1 microculeus vaginatus predominated in both crusts 2 the substantially higher rain intensities in nim 3 the greater thickness and higher chlorophyll content and 4 the lower microrelief at nim in comparison to niz the lack of runoff in nim was explained by its low amounts of exopolysaccharides that did not suffice to affectively clog the surface and in turn to facilitate runoff initiation the absence of runoff and its consequences on the nim ecosystem are discussed copyright 2012 john wiley sons ltd

ejection times of martian meteorites noble gas isotopic analyses were made for four meteorites whose origin is mars basaltic shergottites queen alexandra range 94201 zagami the iherzolite lew88516 and orthopyroxenite allan hills 84001 the latter one was separated into orthopyroxene chromite and maskelynite fractions ejection times were calculated for all martian meteorites using cosmic ray exposure ages based on 3he 21ne and 38ar concentrations and literature data on the terrestrial ages we discuss the arguments for a scenario in which they were ejected from mars by asteroidal or cometary impact as small meteoroids and were delivered to earth within and up to 15 million years the basaltic shergottites que94201 shergotty and zagami show an ejection age cluster at 2 76 ma the fourth basaltic shergottite eet79001 being younger 0 82 ma lherzolites lew88516 and alh77005 were ejected by a later event 3 84 ma ago all nakhlites nakhla governador valadares and lafayette originate from an event 11 0 ma ago although chassigny is chemically distinct from the nakhlites its ejection age is 11 6 ma whether this is a separate event cannot be decided from the available data finally orthopyroxenite alh84001 was launched from mars 14 4 ma ago the distribution of the delivery times for martian meteorites to earth reflect individual impact events onto five or six different source terrains this result is significant in the context of a comprehensive knowledge of the martian crust composition finally we derived the k 40ar gas retention age of alh84001 mineral fractions adopting two different models assuming a component of martian atmospheric ar with 40ar36ar 1790 trapped by this meteorite we obtained an age of 3390 ma on the other hand assuming no contribution of martian atmospheric ar the resulting k 40ar age is about 4100 ma that is roughly 500 ma younger than the reported sm nd crystallization age

regime shifts in models of dryland vegetation drylands are pattern forming systems showing self organized vegetation patchiness multiplicity of stable states and fronts separating domains of alternative stable states pattern dynamics induced by droughts or disturbances can result in desertification shifts from patterned vegetation to bare soil pattern formation theory suggests various scenarios for such dynamics an abrupt global shift involving a fast collapse to bare soil a gradual global shift involving the expansion and coalescence of bare soil domains and an incipient shift to a hybrid state consisting of stationary bare soil domains in an otherwise periodic pattern using models of dryland vegetation we address the question of which of these scenarios can be realized we found that the models can be split into two groups models that exhibit multiplicity of periodic pattern and bare soil states and models that exhibit in addition multiplicity of hybrid states furthermore in all models we could not identify parameter regimes in which bare soil domains expand into vegetated domains the significance of these findings is that while models belonging to the first group can only exhibit abrupt shifts models belonging to the second group can also exhibit gradual and incipient shifts a discussion of open problems concludes the paper

a geomorphological based banded tiger vegetation pattern related to former dune fields in sokoto northern nigeria abstract a banded vegetation pattern has been observed on aerial photographs of sokoto province in northern nigeria it was successfully used as a land mapping characteristic in the soil and vegetation land unit reconnaissance survey see fao 1969 fao 1969 soils survey and land classification soils and water resources survey of the sokoto valley nigeria final report vol v fao sf 67 nr3 zonneveld et al 1971 zonneveld i s de leeuw p n sombroek w g 1971 an ecological interpretation of aerial photographs in a savanna region in northern nigeria publication of the international institute for aerial survey and earth sciences itc enschede the netherlands 41 pp the banded pattern is visible through vegetation density related to differences in surface hydrology the latter are caused by variation in soil sealing that is connected with a by sheet erosion pediplanisation levelled former early holocene to late pleistocene dune landscape sangiwa coversand landscape the difference in sealing is related to a very small difference in silt content between the levelled former dunes and the filled in valleys in combination with extreme low organic matter content the lower silt content in the valley filling is connected with the re sedimentation process a feature well known in coversand formation the sealing is a present day process fresh loosened soil material by ploughing and soil pit digging is after one rain shower already covered with a sealed crust of several millimetres through which no water penetrates the orientation of the bands about nnw sse is perpendicular to the prevailing wind direction ene wsw during the period of formation of the former dunes the genesis of this pattern of regional scale appears to be quite different from local scale banded patterns known to us in dry zones in africa and asia and described in the same period of our study as brousse tigree being moving vegetation arcs related to sheet runoff and by consequence oriented perpendicular to that water flow

the influences of ecological factors on tiger bush and dotted bush patterns along a gradient from mali to northern burkina faso seven catenas across contracted vegetation patterns including tiger bush and dotted bush were studied in the gourma region in mali and in northern burkina faso mean annual rainfall 200 550 mm the tiger bush encompasses an alternate succession of bands oriented nearly perpendicularly to the wind direction ne sw sandy grassy microdune sloped band of bare crusted ground and woody depressive band a similar succession without wind orientation is found for the dotted bush in this region tiger bush develops under specific conditions 1 same orientation but opposite direction of wind and slope 2 sedimentary and metamorphic rocks sandstone and schist ironcrust 3 shallow 1 m and impervious soils under such circumstances the dynamics of banded patterns result from alternate and successive action of water and wind erosion field evidences showed that vegetation bands migrate upwards with a velocity of 0 2 0 7 m yr 1