documenting drainage evolution in bir kiseiba southern egypt constraints from ground penetrating radar and implications for mars 1 ground penetrating radar gpr helps to constrain the origin of relict and largely buried fluvial channels in the bir kiseiba region of southern egypt our results indicate that the trunk channel to a tributary system identified in shuttle imaging radar data is incised 10 12 m into bedrock was southwest draining in its final configuration and laterally migrating toward the northwest and accentuated relief along the nearby kiseiba escarpment alluvium partially filling the main channel likely reflects effects of increasing aridity and bed load combined with less frequent flashier precipitation in contrast to defining channel margins radar data do not indicate subtle stratigraphic changes in bedding related to fluvial aggradation but do highlight local reflections likely corresponding to relict alluvial bedforms our results support the contention that an impulse gpr system currently under development could be deployed on a future mars rover and assist in defining the regional evolution of the landscape and specific targets for sampling

a first look at the structure of the wave pouch during the 2009 predict grip dry runs over the atlantic abstractin support of the national science foundation pre depression investigation of cloud systems in the tropics nsf predict and national aeronautics and space administration genesis and rapid intensification processes nasa grip dry run exercises and national oceanic and atmospheric administration hurricane intensity forecast experiment noaa ifex during the 2009 hurricane season a real time wave tracking algorithm and corresponding diagnostic analyses based on a recently proposed tropical cyclogenesis model were applied to tropical easterly waves over the atlantic the model emphasizes the importance of a lagrangian recirculation region within a tropical wave critical layer the so called pouch where persistent deep convection and vorticity aggregation as well as column moistening are favored for tropical cyclogenesis distinct scenarios of hybrid wave vortex evolution are highlighted it was found that easterly waves without a pouch or with a shallow pouch did not develop although not all wav

rapid filamentation zones in intense tropical cyclones intense tropical cyclones often possess relatively little convection around their cores in radar composites this surrounding region is usually echo free or contains light stratiform precipitation while subsidence is typically quite pronounced in this region it is not the only mechanism suppressing convection another possible mechanism leading to weak echo moats is presented in this paper the basic idea is that the strain dominated flow surrounding an intense vortex core creates an unfavorable environment for sustained deep moist convection strain dominated regions of a tropical cyclone can be distinguished from rotationdominated regions by the sign of s 2 s 2 2 where s1 ux y and s2 x uy are the rates of strain and x uy is the relative vorticity within the radius of maximum tangential wind the flow tends to be rotation dominated 2 s 2 s 2 so that coherent structures such as mesovortices can survive for long periods of time outside the radius of maximum tangential wind the flow tends to be strain dominated s 2 s 2 2 resulting in filaments of anomalous vorticity in the regions of strain dominated flow the filamentation time is defined as fil 2 s 2 s 2 2 1 2 in a tropical cyclone an approximately 30 kmwide annular region can exist just outside the radius of maximum tangential wind where fil is less than 30 min and even as small as 5 min this region is defined as the rapid filamentation zone since the time scale for deep moist convective overturning is approximately 30 min deep convection can be significantly distorted and even suppressed in the rapid filamentation zone a nondivergent barotropic model illustrates the effects of rapid filamentation zones in category 1 5 hurricanes and demonstrates the evolution of such zones during binary vortex interaction and mesovortex formation from a thin annular ring of enhanced vorticity

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

surface processes deposits and development of star dunes dumont dune field california the dumont dune field in the northern mojave desert of california consists of a core of well developed star dunes flanked by smaller barchan crescentic ridge and linear dunes many of the small dunes on the margin of the dune field develop star dune like features such as a central peak and multiple radiating arms these incipient star dunes develop in the complex wind regime at dumont by the merging and modifying of small bed forms during the seasonal changes in wind directions but they typically lose their star dune like features a few weeks later the larger mature star dunes maintain their overall integrity throughout the seasons mapping of surface airflow and sedimentary processes on mature and incipient star dunes show that the arm trends correspond to directions of the primary winds over the seasons for any given time and wind direction however each arm behaves as an active transverse oblique or longitudinal element secondary airflow over star dunes is complex and the direction of the wind at the surface is markedly different over parts of the dune at a single time and at a high variance with the primary wind direction most of the depositional surface of star dunes is covered by wind ripples with slipfaces commonly confined to the upper portions of the dunes the strength direction and duration of the seasonal primary winds the size of the dune and its arms and the secondary airflow all play critical roles in the maintenance and growth of star dunes a multidirectional primary wind favors star dune formation but the persistence of winds from any one direction for a period of time favors some arms over others and hence loss of the star dune features a minimum survival size is necessary for incipient star dunes to maintain their star dune morphologies from season to season secondary airflow favors the maintenance of the arms as well as the accumulation of sand on the bedform thus allowing for growth the nature of star dune surface processes plus the internal structure revealed in shallow trenches suggests that the most common deposits of star dunes are low to moderate angle wind ripple laminae arranged in packages or sets separated by gently dipping truncational surfaces this may explain the rarity with which star dune deposits are identified in the rock record

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

subsurface imaging in south central egypt using low frequency radar bir safsaf revisited we present the capabilities of low frequency radar systems to sound the subsurface for a site located in south central egypt the bir safsaf region this site was already intensively studied since the sir a and sir b orbital radars revealed buried paleodrainage channels our approach is based on the coupling between two complementary radar techniques the orbital synthetic aperture radar sar in c and l bands 5 3 and 1 25 ghz for imaging large scale subsurface structures and the ground penetrating radar gpr at 500 and 900 mhz for sounding the soil at a local scale we show that the total backscattered power computed from l band sar and 900 mhz gpr profiles can be correlated and we combined both data to derive the geological structure of the subsurface gpr data provide information on the geometry of the buried scatterers and layers while the analysis of polarimetric sar data provides information on the distribution of rocks in the sedimentary layers and at the interface between these layers the analysis of 500 mhz gpr data revealed some deeper structures that should be detected by lower frequency sars such as a p band system

rapid filamentation zone in a numerically simulated tropical cyclone abstract in a recent study rozoff et al proposed a possible mechanism to explain the formation and maintenance of the weak echo annulus or moat outside of the primary eyewall of a tropical cyclone observed in radar images by this mechanism the moat is determined to be a region of the strain dominated flow outside of the radius of maximum wind in which essentially all fields are filamented and deep convection is hypothesized to be highly distorted and even suppressed this strain dominated region is defined as the rapid filamentation zone wherein the filamentation time is shorter than the overturning time of deep convection an attempt has been made in this study to test the hypothesis in a full physics tropical cyclone model under idealized conditions and to extend the concept to the study of the inner core dynamics of tropical cyclones the foci of this paper are the evolution of the rapid filamentation zone during the storm intensification the potential roles of rapid filamentation in the organiz

major bounding surfaces in aeolian sandstones a climatic model large areas of fixed vegetated dunes occur in the sahel and southern margin of the sahara these dunes were active during late pleistocene times and have been stabilized as a result of a change to more humid less windy conditions in the holocene the stabilized surfaces which are in part of erosional origin are of regional extent within aeolian sand sequences such surfaces form the highest order bounding surfaces since regional bounding surfaces of this type occur in all the world s major deserts it is suggested that they should also be present in some ancient aeolian sandstone sequences

unconfined alluvial flow processes recognition and interpretation of their deposits and the significance for palaeogeographic reconstruction abstract palaeogeographic interpretation of the sedimentary rock record depends on correct recognition from the preserved evidence of the processes responsible for transporting and depositing the sediment this in turn depends on robust knowledge transfer from previous workers and the successful exchange of ideas between workers requires consistent use of a well defined vocabulary we have identified serious breakdowns in all these interpretation steps in the case of terrestrial unconfined flow and its deposits and these failures are leading to unreliable environmental and climatic interpretation this is significant because such alluvial deposits commonly form a majority of the rock record of continental environments working from the basic principles of geomorphology and fluid dynamics we have undertaken a wide ranging analysis of the nature of out of channel flow and from this make predictions about the characteristics of its deposits we identify the range of possible locations and conditions that lead to the development of unconfined flow review the processes operating in each case and examine the range of lithological features that can be produced by these processes this allows us to evaluate the reliability of the criteria claimed for identification of out of channel flow deposits and examine how our new insights might alter palaeoclimatic and palaeogeographic reconstructions published previously by others the sedimentary record of unconfined flows is much more diverse and complex than usually portrayed the received wisdom that the record of unconfined flow consists solely of upwards fining thin beds produced from shallow waning flows is shown to be flawed a wide range of lithofacies are possible and the variation in both flow steadiness and uniformity needs to be taken into account the previously published criteria for recognition of flows of this type are not diagnostic of process or location unconfined flow deposits cannot reliably be identified from grain size or bed thickness similar lithofacies may develop in a wide range of geomorphic and climatic settings because the deposits solely reflect the local flow conditions and sediment availability we recommend that the terms sandflat and sheetflood should not be used in sedimentological accounts because there is no longer a safe informal usage for either contradictory application of these terms and lack of robust definitions is leading to significant misunderstanding of palaeogeography and process our analysis should improve reconstruction of past terrestrial environments because it reveals more clearly the true variety of possibilities for the occurrence of unconfined flow and the resultant deposits enhanced understanding of the inherent uncertainties and realisation of the wider range of plausible alternative explanations should help resolve apparent contradictions with independent indicators of climate or geographic position

ages of quaternary pluvial episodes determined by uranium series and radiocarbon dating of lacustrine deposits of eastern sahara abstract as documented by radiocarbon dating and geoarchaeological investigations the now hyperarid northwestern sudan and southwestern egypt experienced a period of greater effective moisture during early and middle holocene time about 10 5 ka we have used the uranium series technique to date lacustrine carbonates from bir tarfawi bir sahara east wadi hussein oyo depression and the great selima sand sheet localities results indicate five paleolakeforming episodes occurred at about 320 250 240 190 155 120 90 65 and 10 5 ka four of these five pluvial episodes may be correlated with major interglacial stages 9 7 5e and 1 the 90 65 ka episode may be correlated with substage 5c or 5a our results support the contention that past pluvial episodes in north africa corresponded to the interglacial periods farther north ages of lacustrine carbonates from existing oases and from the sand sheet fail to indicate pluvial conditions between about 60 and 30 ka age results and field relationships suggest that the oldest lake and ground water deposited carbonates were much more extensive than those of the younger period and that carbonate of the latest wet periods were geographically localized within depressions and buried channels

stratigraphy and sedimentology at bir sahara egypt environments climate change and the middle paleolithic abstract bir sahara situated in northeast africa contains a set of sedimentary sequences that imply episodic changes in climate and environment during the middle and late pleistocene some of the stratigraphic contexts are associated with middle paleolithic artifact assemblages the artifact assemblages are typically in sands that underlie deposits composed of high amounts of carbonate or fine clastics muds indicative of expanding lakes and wetter climates the wetter climates may have provided landscapes that were periodically inhabitable by pleistocene hominids

book reviewthe geomorphology of egypt landforms and evolution volume 1 the nile valley and the western desert nabil s embabi ed the egyptian geographical society special publication cairo egypt 2004 447 pp hardbound us 60 available from egyptian geographical society 109 qasr el aini street p o box mohamed farid cairo egypt isbn 977 5821 04 5

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

chevrons origin and relevance for the reconstruction of past wind regimes abstract since its first use in the late 80 s the term chevron has been employed in numerous studies to describe large u and v shaped ridges found in or near shorelines worldwide most studies have so far focused on bahamian chevrons that are exclusively of late pleistocene age and on the supposed holocene chevrons found in s madagascar and australia in the bahamas these deposits have been interpreted as the product of extreme storms at the end of the last interglacial lig warm period in contrast the extensive chevrons complex exposed in s madagascar and on the western coast of australia have been associated with a tsunami induced by a meteorite impact finally several authors have also proposed a non catastrophic i e eolian origin based on the recognized importance of wind related processes in these coastal areas and term such deposits parabolic dunes in this paper we collect and synthesize existing data on the morphology sedimentology and age of these chevrons and review the different interpretations proposed in the literature with the aim to lay out a consistent database to assist further investigations on these important coastal morphologies in addition we generated a synthesis of wind data at the three study areas which highlights the relationship between present wind regimes and chevrons morphologies the ubiquity of chevrons likely actually parabolic dunes in coastal areas around the globe and their relationship with coastal processes makes them crucial archives for reconstructing past wind regimes

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

the action of wind and water in a mid cretaceous subtropical erg margin system close to the variscan iberian massif spain aeolian processes and ephemeral water influx from the variscan iberian massif to the mid cretaceous outer back erg margin system in eastern iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements remains of aeolian dunes encased in ephemeral fluvial deposits aeolian pods demonstrate intense erosion of windblown deposits by sudden water fluxes the alternating activity of wind and water led to a variety of facies associations such as deflation lags desert pavements aeolian dunes pebbles scattered throughout dune strata aeolian sandsheets aeolian deposits with bimodal grain size distributions mud playa ephemeral floodplain pebble sand and cobble sand bedload stream pebble cobble sand sheet flood sand bedload stream debris flow and hyperconcentrated flow deposits sediment in this desert system underwent transport by wind and water and reworking in a variety of sub environments the nearby variscan iberian massif supplied quartzite pebbles as part of mass flows pebbles and cobbles were concentrated in deflation lags eroded and polished by wind driven sands facets and ventifacts and incorporated by rolling into the toesets of aeolian dunes the back erg depositional system comprises an outer back erg close to the variscan highlands and an inner back erg close to the central erg area the inner back erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits in the inner back erg area ephemeral wadis desiccated after occasional floods were mud cracked and overrun episodically by aeolian dunes subsequent floods eroded the aeolian dunes and mud cracked surfaces resulting in largely structureless sandstones with boulder size mudstone intraclasts floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes the remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands upon desiccation deflation resulted in lags of coarser grained sediments a renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions synsedimentary tectonics caused the outer back erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands ephemeral water flow to the outer back erg area supplied pebbles eroded aeolian dunes and produced hyperconcentrated flow deposits fluidization and liquefaction generated gravel pockets and recumbent folds dune damming after sporadic rains the case of the namib desert monsoonal water discharge thar desert and meltwater fluxes from glaciated mountains taklamakan desert are three potential non exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the cretaceous desert margin system an increase in relief driven by the aptian anti clockwise rotation of iberia led to an altitude sufficient for the development of orographic rains and snowfall which fed melt water fluxes to the desert margin system quartzite conglomerates and sands dominantly consisting of quartz and well preserved feldspar grains which are also observed in older cretaceous strata indicate an arid climate and the mechanical weathering of precambrian and palaeozoic metamorphic sediments and felsic igneous rocks unroofing of much of the cover of sedimentary rocks in the variscan iberian massif must therefore have taken place in pre cretaceous times

sand sheet dynamics and quaternary landscape evolution of the selima sand sheet southern egypt abstract the selima sand sheet occupies more than 120 000 km 2 of the hyperarid uninhabited darb el arba in desert centered at the border of egypt and sudan at latitude 22 n and is characterized by a featureless surface of lag granules and fine sand broken only by widely separated dune fields and giant ripples of varying height and wavelength monitoring of the largest of these chevron shaped ripples using repeat orbital images and field surveys indicates migration rates of 500 1000 m yr accompanied by 0 2 0 cm erosion or deposition of the youngest sand sheet stratigraphic units beneath this active surface several developmental stages of sand sheet sediments have undulatory upper contacts and varying degrees of pedogenic alteration the younger stages retain their horizontal lamination and have cracking patterns indicative of past wetter conditions while older stages have lost their laminar structure through pedogenesis historical remains in the desert as well as 14 c and uranium series dating indicate that the younger strata of the sand sheet have a very low accumulation rate despite the active movement of the surface the lower strata were extensively modified during mid and late pleistocene pluvials resulting in an initial undulatory surface that set the stage for later accumulation of sand sheet below these quaternary sediments lies irregular topography dissected by channels of mid tertiary drainage the selima sand sheet is neither the result of net aggradation nor degradation but results from inheritance of an initial fluvial landscape increasingly modified during climatic cycles wet periods led to local drainage and deposition while the increasingly severe arid periods of the late pleistocene and holocene resulted in deposition of the blanketing bimodal sediments of the sand sheet

facies control on seismites in an alluvial aeolian system the pliocene dunefield of the teruel half graben basin eastern spain abstract the recognition of seismically induced soft sediment deformation structures ssds in sedimentary successions characterized by different facies and hence by different rheology is challenging this is the case for high porosity and high permeability aeolian facies interbedded with muddy wet interdune deposits and alluvial conglomerates and sandstones several types of ssds have been studied in two exposures of the upper pliocene 2 9 2 6 ma sediments of a fault bounded intracontinental aeolian dune field in the teruel basin iberian chain eastern spain among ssds load and fluid escape structures apart from several animal tracks have been recognized those structures show an irregular distribution through the studied stratigraphic sections being scarce in homogenous aeolian sands and frequent in water related facies a detailed study of the distribution and geometry of ssds and their relationships with respect to the stratigraphic architecture and facies has allowed a critical discrimination of trigger mechanisms i e biological or physical overloading vs earthquakes the seismically induced structures are concentrated into seven deformed beds showing an uneven lateral distribution and geometry closely controlled by the hosting sedimentary facies and their rheology these seismites resulted from liquefaction during moderate earthquakes estimated magnitude from 5 0 to 6 8 the most probable seismogenic source was the sierra del pobo normal fault zone located 2 km to the east results show how an appropriate recognition of sedimentary facies is crucial to understand the lateral variability of seismites in sedimentary environments characterized by sharp facies changes

dune and dune field development on padre island texas with implications for interdune deposition and water table controlled accumulation abstract portions of the back island dune fields on padre island texas undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow bedforms develop through a series of morphologic and dynamic stages 1 irregular patches of dry sand 2 wind ripple protodunes with leeward flow expansion 3 grainfall protodunes with a range from leeward flow expansion to flow separation 4 barchan dunes with grainflow and 5 crescentic ridges that characterize the mature field the survival and further development of protodunes and dunes is f vored by increased size a more advanced stage of development unidirectional and moderate wind and a plentiful sand supply these factors can be viewed as the result of the interrelationship of aspects of eolian transport the nature of leeward secondary airflow patterns and sand supply dune field ordering occurs by repeated merging splitting cannibalization and lateral linking of dunes on padre island and other areas of dunes that we studied flow conditions across portions of interdune flats favor the transport of dry loose sand from interdune flats onto the dunes so that dunes grow at the expense of interdune flats dry surface deposits are therefore unlikely to accumulate on interdune flats but rather to characterize interdune depressions that may represent the minimum d stance to which an interdune flat can be closed otherwise accumulations on interdune flats most commonly should be formed on wet or damp surfaces or occur associated with some mechanism that protects the deposits from deflation padre island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water table rise with bypassing and deflation occurring as the water table becomes static or falls respectively

bagnold s barchan a 57 yr record of dune movement in the eastern sahara and implications for dune origin and paleoclimate since neolithic times abstract discovery in 1980 of camp 18 of the 1930 bagnold expedition in northwestern sudan has resulted in an exceptionally long record of barchan movement from which an average rate of 7 5 m yr has been calculated measurements of the rate of dune advance since 1980 have not varied significantly indicating a state of quasiequilibrium exists the dune field is moving over an undulating sand sheet exhibiting various stages of holocene pedogenesis and containing neolithic sites the sand sheet overlies pleistocene alluvium with stronger stages of pedogenesis all of these deposits may be a significant source of dune sand wind blown grass fragments trapped by advancing slipfaces mark former slipface positions and indicate late holocene rainfall events which can be radiocarbon dated parabolic forms revealed by spot images at the windward ends of the dune fields are transitional to barchans suggesting sand accumulation in vegetation germinated during a holocene wet period in addition sand streaks from sand sheet undulations suggest that active eolian erosion of sand sheets contributes to the sand supply of dune fields pedogenesis during pluvial periods contributes to silt and clay fractions of bimodal sand sheet sediments from wind streaks on landsat images and an estimated average rate of dune advance positions of dune fields during past hyperarid periods can be estimated

the formation process of a paleoindian open air site in central brazil integrating lithic analysis radiocarbon and luminescence dating abstract lithic analysis radiocarbon dating and luminescence dating are applied to a stratified open air paleoindian site in central brazil lending some balance to a published record dominated by rock shelter sites most lithics are found in two discrete occupation layers lithics found outside these layers are attributed to post depositional movement of small flakes mainly upwards single grain luminescence dating on quartz grains also suggest post depositional movement both wind and bioturbation are suspected mechanisms the luminescence analysis does not provide precise depositional dates in this situation but does support such dates suggested by radiocarbon

aeolian alluvial interactions at formentera balearic islands western mediterranean the late pleistocence evolution of a costal system abstract this study deals with the sedimentological and stratigraphic description of pleistocene coastal deposits of formentera island that show evidence of interference between processes characteristic of alluvial and aeolian environments here we analyze the sedimentology and stratigraphy of a pleistocene succession located at northeast of formentera and we constrain its chronology by optically stimulated luminescence osl dating main sedimentary facies representing the succession of aeolian alluvial marine and coastal environments are described in this study we conclude that although there were two prevailing environments such as coastal aeolian and alluvial with different processes and resulting forms the interaction resulted in deposits that share characteristics of both environments as well as maintain elements inherited from each environment in terms of sedimentary supply precipitation runoff or aeolian transport these coastal deposits represent a small alluvial apron covering the base of a vertical cliff most of the sediment that composes these deposits is marine bioclastic sand which was transported in the form of coastal dunes during periods of low sea level our results are a useful indicator of the geomorphological processes and changes that occurred during the pleistocene unraveling the environmental evolution and contributing to the growing knowledge on the western mediterranean aeolian alluvial interacting environments

late cretaceous climbing erg systems in the western xinjiang basin palaeoatmosphere dynamics and east asia margin tectonic forcing on desert expansion and preservation abstract this study reassesses the terrestrial deposits of the late cretaceous tangbian formation in the western xinjiang basin of the south china block originally considered to be aqueous deposits we reinterpret these strata as representing an extensive aeolian system in a desert environment this paper suggests that the development of aeolian deposits in the tangbian fm represents the culmination of the late cretaceous desertification that followed the relatively humid early late cretaceous epoch in the interior of south china the area was dominated by subtropical high pressure systems on a planetary scale combined with coastal mountains caused by sustained orogenic activity the interaction between subtropical high pressure systems and topographic conditions ultimately caused the development of a tropical desert climate characterized by an extensive dry climbing aeolian dune system we propose that the coastal mountains not only blocked moisture transport from the ocean and led to the aridification of south china interior but also exerted a control and transformed the original prevailing surface wind pattern

garnet polycrystals and the significance of clustered crystallization polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei if clustering is early or impingement of larger grains if clustering occurs later in the growth history the timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance if any of clustering electron backscattered diffraction ebsd analysis of garnet bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study the northern appalachians vermont maine new york usa north american cordillera north cascades range washington snake range nevada usa western rocky mountains british columbia canada southern menderes massif turkey santander massif colombia and the sanandaj sirjan zone hamadan iran in some samples polycrystals comprise 20 30 of garnets analyzed and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common some early coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal in thin section these garnets may be undetectable as polycrystals based on morphology or zoning in some polycrystals zoning is unrelated to the location of internal grain boundaries in others fe mn mg zoning has a different pattern than that of ca zoning patterns may vary on the scale of a single thin section in addition some polycrystals are characterized by high angle misorientation boundaries that may be in special non random orientations an observation that indicates that these polycrystals are not random clusters of grains the presence of internal grain boundaries may affect diffusion pathways and length scales and may facilitate communication of porphyroblast interiors with matrix phases thereby influencing reaction history of the rock and the composition zoning of garnet

relict drainages conical hills and the eolian veneer in southwest egypt applications to mars viking images indicate that no single process is sufficient to explain the origin and evolution of martian landforms landforms common to both earth and mars result from a complex interplay of geologic agents including water mass wasting wind and possibly ice the relict wadis of the gilf kebir plateau in southwest egypt strongly resemble fluvial channels on mars particularly those of the fretted terrain like the fretted channels on mars the egyptian wadis are dry flat floored and deeply incised in a plateau surface they have stubby theatre headed wide and deep tributaries that commonly join the trunk valleys at odd angles they apparently lack drainage basins and are separated by areas that appear on most of the available images to be undissected the wadis grade into adjacent low lying smooth plains mantled by eolian debris and dotted with myriads of conical and flat topped inselbergs that have obviously undergone mass wasting the unusual geomorphic characteristics of the gilf region are herein interpreted as the results of an extreme eolian and mass wasting overprint on a landscape originally carved by running water the relict networks of southwest egypt were initiated by streams that drained formerly extensive gilf highlands those highlands are inferred to have been a major tertiary watershed in north africa as quaternary dessication set in these rivers vanished and were replaced by ephemeral gullies cliff sapping wind abrasion and deflation have widened the gilf valleys and reduced many interfluves to clusters of flat topped and conical hills these now rise as inselbergs on the adjacent desert plains on which little evidence of former fluvial activity remains wind has deflated the surface and redistributed the finer particles from fluvial sediments and the debris of mass wasting into eolian sand sheets drifts and dunes dunes are migrating southward through the region along eolian thoroughfares that follow the apparent paths of ancient master streams toward distant basins of sand accumulation in the central sahara the fluvial and mass wasting origin of the particles in the eolian deposits the segregation of these materials according to grain size and the migration of those particles capable of saltation to areas of accumulation in lowland basins of the sahara are suggested as analogs for the formation and accumulation of sand sheets and dunes on the northern plains and in the polar erg on mars outliers of the martian plateau in the fretted terrain appear to have been dissected at least initially by channels whose upstream portions are incised in the uplands the martian wadis have many geomorphic peculiarities similar to those of the gilf wadis and like the egyptian features they have been attributed to mass wasting although basal sapping and removal of debris by wind have almost certainly modified the martian features their deep incision in the plateau and their inferred northward extensions on the northern plains seem to require not only initial downcutting by fairly energetic streams but also prolonged and long distance flow of water on mars as in southwest egypt running water is now nonexistent as suggested by milton 1973 the northern plains may be the most extensive of the landforms produced during a fluvial stage of martian history inferred integrated stream networks in the fretted terrain on mars as in the gilf plateau may be relics of earlier less arid climatic intervals when runoff collected and flowed with sufficient power to carve valleys in bedrock in this interpretation the uplands might once have stood as a watershed on a much less arid mars

geomorphic indicators of holocene winds in egypt s western desert abstract geomorphic mapping of egypt s western desert from landsat mss images reveals oriented aeolian landforms that record in part holocene winds wind directions reconstructed from these landforms indicate the dominance of n s airflow from 30 n to 20 n turning clockwise southward to ne sw conformable with modern circulation a second direction appears over western egypt w between 30 n and 26 n nw between 26 n and 20 n cross cutting aeolian landforms show that w nw winds are older than the n ne winds geomorphic evidence abundant south to 26 n and less abundant to 20 n also indicates that w and nw winds were early holocene palaeowesterlies some evidence also indicates that they extended eastward to at least 30 e perhaps to the red sea these winds steered moist atlantic mediterranean air masses to egypt sustaining early holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20 n upon aridification beginning after 5 kyr bp yardangs oriented west to east were eroded in early holocene basinal sediments in western egypt indicating that these winds continued there for 1 2 kyr until 3 4 kyr bp optically stimulated luminescence osl ages of surface sand sheet in southern egypt indicate that the present north south winds were established ca 3 4 kyr bp at the same time as the northern savanna boundary was stabilized at its present position

large scale low amplitude bedforms chevrons in the selima sand sheet egypt landsat images of the selima sand sheet in southwestern egypt display alternating light and dark chevron shaped patterns that occur downwind from low scarps and major dune fields images acquired between 1972 and 1988 indicate that these features move as discrete bedforms at a rate of up to 500 meters per year extremely long wavelength 130 to 1200 meters low amplitude 10 to 30 centimeters bedforms were measured in the field the light chevrons seen in the orbital data may be thin accumulations of active sand sheet deposits in the lee of these bedforms dark chevrons contain an admixture of coarse granule lag deposits that are continually winnowed by aeolian erosion on the windward sides of the large bedforms sediment transport budgets derived from orbital and field analyses suggest net movement of up to 83 000 cubic meters per year for a single light chevron such measurements can be used as a check on similar calculations from dunes and other smaller scale features to determine sand transport budgets for large areas of the eastern sahara

surficial geology of the safsaf region south central egypt derived from remote sensing and field data abstract we used a decorrelation stretched image of landsat thematic mapper tm bands 1 4 and 7 and field data to map and describe the main surficial units in the hyperarid safsaf region in south central egypt we show that the near infrared bands on landsat tm which are sensitive to very subtle changes in mineralogy common to arid regions significantly improve the geologist s capability to discriminate geologic units in desert regions these data also provide the spatial and spectral information necessary to determine the migration patterns and provenance of eolian materials the safsaf area was the focus of our post flight field studies using shuttle imaging radar sir data following the discovery of buried paleochannels in north africa most of the channels discernible on sir images are not expressed in tm data but traces of a few channels are present in both the sir and the tm data within the wadi safsaf area here we present a detailed digital examination of the sir and the tm band reflectance and reflectance ratio data at three locations of the more obvious surface expressions of the buried channels our results indicate that the tm expressions of the channels are not purely topographic but are more compositional in nature two possibilities may account for the tm expressions of the buried channels 1 concentrations of windblown iron rich materials that accumulated along subtle curvilinear topograpohic traps or 2 curvilinear exposures of an iron rich underlying unit of the flat sand sheet

middle pleistocene interglacial thames medway deposits at clacton on sea england reconsideration of the biostratigraphical and environmental context of the type clactonian palaeolithic industry abstract in 1987 an archaeological investigation was undertaken during redevelopment of the erstwhile butlin s holiday camp at clacton on sea on the grounds that the middle pleistocene clacton channel deposits containing the type clactonian palaeolithic industry were known to extend beneath the site excavations for a storm drain allowed sampling at points along a longitudinal traverse of the main clacton channel analysis of these samples has provided new palaeontological information including data pertaining to the rise in relative sea level during the interglacial represented systematic studies of molluscs and ostracods the latter undertaken at clacton for the first time have been particularly valuable information from the butlin s site supplements evidence previously gathered from the west cliff section and from other localities at clacton the calcareous clay marl that underlies the clacton golf course extends beneath butlin s where it was found to be part of the freshwater beds not the estuarine beds as hitherto supposed the clacton estuarine beds restricted to the eastern end of the site have their base just below 2 m o d implying that their superposition upon the clacton freshwater beds occurred when relative sea level in this area was close to present ordnance datum correlation of the clacton channel deposits with the interglacial immediately following the anglian elsterian stage appears secure equivalence with oxygen isotope stage 11 of the oceanic sequence is most probable a borehole survey and subsequent excavation revealed a holocene sequence of unlithified tufa and organic sediments beneath part of the site

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

evidence for pleistocene lakes in the tushka region south egypt space shuttle radar topography mission srtm data have revealed new details on the extent and geomorphic relations of paleodrainage in southern egypt following a period of late tertiary drainage from the red sea hills south through wadi qena and west across the tushka region the nile river as we now know it established its connections with central africa and the mediterranean in the middle pleistocene oxygen isotope stage ois 7 to ois 5 srtm topography reveals a lake level at 247 m that is coincident with the elevation of middle pleistocene fish fossils 400 km west of the nile and with the termination of shallow runoff channels in northern sudan that were active during the middle pleistocene and holocene pluvial periods an additional lake level at 190 m is based on the current elevation at wadi tushka and is consistent with paleolithic sites at bir kiseiba followed by neolithic sites at lower topographic levels overflow of the nile through wadi tushka during the wetter north african climate of the middle pleistocene coupled with limited local rainfall was the likely source of water for these lakes

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

on the dynamics of the formation of the kelvin cat s eye in tropical cyclogenesis part i climatological investigation abstractmotivated by dunkerton et al a climatological study of 54 developing easterly waves in 1998 2001 was performed time lagged composites in a translating reference frame following the disturbances indicate a weak meridional potential vorticity pv gradient of the easterly jet and a cyclonic critical layer located slightly to the south of the weak pv gradient consistent with previous findings in the marsupial paradigm using a closed pv contour as a criterion for the formation of the cat s eye it was shown that on average it takes 2 6 days for open pv contours to transform to a closed coherent structure bootstrap analysis was then applied to determine the reliability of the easterly wave like pattern in the composite perturbation pv analysis it is suggested that the coexistence of a nonlinear critical layer and a region of weak meridional pv gradient over several days found to occur in only 25 of the easterly waves might be a major factor to distinguish developing and nondeveloping distur

paleodrainages of the eastern sahara the radar rivers revisited sir a b implications for a mid tertiary trans afnrcan drainage system a complex history of cenozoic fluvial activity in the presently hyperarid eastern sahara is inferred from sir data and postflight field investigations in southwest egypt and northwest sudan sir images were coregistered with landsat and existing maps as a guide to exploration of the buried paleodrainages radar rivers first discovered by sir a field observations explain the radar responses of three types of radar rivers rr 1 rr 2 and rr 3 rr 1 broad aggraded valleys or basins filled with alluvium generally give a dark response relative to the surrounding radar bright tone of the wind roughened discontinuously sand covered bedrock of the remnant interfluves the alluvial fill in some of the rr 1 valleys gives a locally intermediate to bright response where cemented by diagenetic caco3 caliche the rr 2 type braided channels inset in the rr 1 valleys are filled with noncalichified loose sediment and are visible on sir because of the caliche effect the dark response of unconsolidated alluvium in the inset channels contrasts with the brighter response of calichified alluvium in the broad valleys the third type rr 3 narrow long bedrock incised channels which are partly visible on the ground and on landsat in areas where the sand sheet is patchy are conspicuous on sir because the dark response of their unconsolidated channel fill contrasts with the brighter response of the surrounding bedrock disconformably overlying and concealing the rr 1 valleys the rr 2 channels and parts of the rr 3 channels are eolian sand sheet sheet de deposits which in turn are locally overridden by trains of barchan dunes

conditions favourable for the formation of warm climate aeolian sand sheets aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent sand sheets are ubiquitous to modern warm climate sand seas generally occurring marginal to dune fields although they may exist within the interior of a sand sea or independent of a dune field sand sheet deposits are recognized in ancient aeolian sequences where they may account for significant accumulations of low angle aeolian stratification we suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets a study of six modern sand sheets in north america located at great sand dunes gran desierto dumont algodones padre island and colorado river delta indicates that the factors favourable for sand sheet development are 1 a high water table 2 surface cementation or binding 3 periodic flooding 4 a significant coarse grained sediment population and 5 vegetation these factors are reflected in the nature of stratification and the accessory features of sand sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the triassic dolores and pennsylvanian permian rico formations

aeolian construction and alluvial dismantling of a fault bounded intracontinental aeolian dune field teruel basin spain a continental perspective on late pliocene climate change and variability an aeolian dune field migrating to the east encroached on the toes of alluvial fans in the teruel basin eastern spain during a short interval in the late pliocene ca 2 9 to 2 6 ma when northern hemisphere glaciation and strong glacial interglacial cycles began preservation of the dune field was controlled by syn sedimentary activity of a normal fault ephemeral water discharge eroded aeolian sands and formed v shaped channels in which aeolian sandstone blocks accumulated the incorporation of loose aeolian sand in wadi waters modified the sediment water ratio changing the physical properties of the flows as they penetrated the aeolian dune field the erosion and cover of aeolian dune foresets by sheetflood deposits suggest that dune damming caused the intermittent ponding of water behind the dunes and its flashy release the arid climate in the late pliocene western mediterranean realm favoured the transport of windblown sediments from northern africa and western mediterranean land masses into the mediterranean the formation of the studied aeolian dune field 2 9 to 2 6 ma and possibly others for example the atacama namib and sahara deserts correlates with a strong increase of the influence of obliquity which can be attributed to the combination of a regional expression related to the reduced effect of precession due to a minimum in the long period 2 3 ma eccentricity cycle and a remote expression of the onset of the northern hemisphere glaciation

examining the roles of the easterly wave critical layer and vorticity accretion during the tropical cyclogenesis of hurricane sandy abstractthe tropical cyclogenesis sequence of hurricane sandy is examined it is shown that genesis occurs within a recirculating kelvin cat s eye flow of a westward propagating tropical wave the cat s eye flow is able to provide a protective environment for the mesoscale vortex to grow and is characterized by gradual column moistening and increased areal coverage of deep cumulus convection these findings are generally consistent with a recently proposed tropical cyclogenesis sequence referred to as the marsupial paradigm sandy s cyclogenesis provides a useful illustration of the marsupial paradigm within a partially open recirculating region with the opening located south of the pouch center it is suggested that the opening acts to enhance the genesis process because it is adjacent to an environment characterized by warm moist air conditions favorable for tropical cyclogenesis from a dynamical perspective accretion of low level cyclonic vorticity filaments into the developing vortex from sever

trans african drainage system of the sahara was it the nile an exciting result of the first spaceborne radar scans of the eastern sahara has been the recognition of drainage channels buried at depths of several metres below the dry desert sand we relate these observations to the megageomorphological evolution of africa and conclude that the radar river valleys may have been parts of an old nile river system rather than courses cut by a postulated westward flowing trans african drainage system tads extending from the red sea hills to the niger delta

sediment and pollen evidence for an early to mid holocene humid period in the eastern sahara a major problem in the study of holocene palaeoenvironments of the arid and wind deflated sahara is the low preservation potential of sediments from which a record of past climatic change can be established several indirect and imprecisely dated pieces of evidence have suggested humid episodes in the holocene that may have supported more productive ecosystems and denser human populations than today searches for reliable proxy data on past climates however have failed to yield satisfactory results we report here the discovery of buried lake muds in north west sudan in the hyperarid core of the eastern sahara which yield sedimentological and palynological data clearly interpretable as recording an early to mid holocene humid episode that supported a relatively deep stratified lake surrounded by tropical savanna woodland vegetation

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

great sand sea and selima sand sheet eastern sahara geochronology of desertification the relation of playa sediments and associated archeological sites with longitudinal dunes allows estimation of ages for the two uppermost strata of the great sand sea active dune formation corresponds with interpluvial periods of hyperaridity dune stability corresponds with semiarid pluvial periods archeological sites associated with truncated paleosols in the selima sand sheet suggest a similar climatic relation and indicate that the isohyets of central sudan shifted at least 400 kilometers northward during the peak of pluvials

influence of subaqueous processes on the construction and accumulation of an aeolian sand sheet in aeolian sand sheets the interaction between aeolian and subaqueous processes is considered one of the principal factors that controls this depositional environment to examine the role played by the subaqueous processes on the construction and accumulation of sand sheets surface deposits and subsurface sedimentary sections of a currently active aeolian sand sheet located in the upper tulum valley argentina have been examined on the sand sheet surface airflows enable the construction of nabkhas wind rippled mantles flattened accumulations of sand forming wind ripples megaripples and small transverse dunes subaqueous deposits consist of sandy current ripples covered by muddy laminae the latter are generated by annual widespread but low energy floods that emanate from the nearby mountains in the aftermath of episodes of heavy precipitations deposits of subaqueous origin constitute 5 of the accumulated sand sheet thickness the construction of the sand sheet is controlled by meteorological seasonal changes the source area the san juan river alluvial fan receives sediment by thaw waters in spring summer in fall winter when the water table lowers in the alluvial fan the sediment is available for aeolian transport and construction of the sand sheet area the flood events play an important role in enabling sand sheet accumulation the muddy laminae serve to protect the underlying deposits from aeolian winnowing incipient cement of gypsum on the sand and vegetation cover acts as an additional stabilizing agent that promotes accumulation episodic and alternating events of erosion and sedimentation are considered the main reason for the absence of soils and palaeosols results from this study have enabled the development of a generic model with which to account for i the influence of contemporaneous subaqueous processes on the construction and accumulation in recent and ancient sand sheets and ii the absence of developed soils in this unstable topographic surface copyright 2013 john wiley sons ltd

holocene palaeoecology of the eastern sahara selima oasis buried lake sediments and tufa strandlines of holocene age provide evidence of a former lake in the selima oasis depression northwest sudan near the hyperarid core of the eastern sahara desert a 1 6 m section through laminated lake muds and precipitates has been analysed in detail and 30 radiocarbon dates geochemical sedimentological diatom and pollen analyses indicate that the unit dated from approximately 8 4 ka bp to 6 ka bp and was deposited in an environment that passed through one arid to humid to arid cycle the arid episodes from prior to 8 4 ka bp and subsequent to 7 ka bp are characterized by high caco3 deposition abundant diatom indicators of relatively high salinity and well developed periphytic communities and pollen spectra dominated by desert grassland indicators a humid period centered on 8 ka bp is indicated by low caco3 deposition high organic rich mud and al and si oxide deposition diatoms indicating a fresh deep 20 m lake and a pollen assemblage with maximum values for sahelo sudanian tree taxa suggesting a thorn tree and shrub savanna vegetational mosaic this reconstruction concordant with higher groundwater table and precipitation between 9 3 and 7 ka bp agrees with palaeoecological inferences from other sites in north africa

dynamical properties of developing tropical cyclones using lagrangian flow topology the goal of this study is to characterize the dynamics and structure of tropical cyclone formation from global model analyses to determine thresholds marking the various stages of development that can be computed from the analysis data we introduce here a new methodology for identifying disturbances that show high likelihood of becoming a tropical depression or tropical storm we use lagrangian frame independent quantities to define intrinsic coordinates for candidate disturbances prior to and post tropical depression declaration in the best track data hurdat2 we use these lagrangian quantities also as metrics for quantifying the strength of these systems within global model analyses as they are declared depressions storms or hurricanes in hurdat2 the criteria proposed are more precise than similar eulerian criteria since the minimum thresholds for development in the best track data set are very close to the threshold at which no false alarms are produced in the global model analyses since only very loose thermodynamic thresholds are required these criteria can be considered dynamically based and require no statistical analysis to compensate for uncertainties in moisture or convection we describe further the structure of the developing systems using a set of objective profiles where level contours of the lagrangian averaged rotation rate are mapped to an equivalent radius composites of these profiles reveal that the transition to tropical storm strength vortices is marked by the existence of a notable shear sheath outside a region of enhanced solid body rotation

recent environmental change and prehistoric human activity in egypt and northern sudan abstract this paper reviews the various late quaternary records that are available from western egypt and northern sudan which includes more than 500 published radiocarbon dates and various sedimentary archives from local landscape components including palaeolakes soils drainages wadis and archaeological sites this palaeoenvironmental compilation frames the spatial and temporal context of local cultural activities when the region was most hospitable 9000 6000 bp at this time monsoonal weather influenced the portion of the african continental interior creating enough convective rainfall for occasional surface water storage in this part of the modern sahara rapid hydroclimatic changes play a key role in geomorphic evolution and resource availability as watering holes formed and dried up in the early to middle holocene neolithic people developed various subsistence strategies including opportunistic hunting of small animals e g gazelle and hare and food related e g wild sorghum millet and legumes activities gathering plant cultivation and livestock rearing during its wettest phases during the monsoonal maximum the area was drought prone sustaining a meager steppe shrub desert flora further desertification and aeolian deflation during the middle and late holocene fostered technological innovation migration and settlement as well as the further development of agrarian communities and complex culture

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

comparison of remote sensing observations of planetary wind streaks abstract wind streaks are common aeolian features observed on terrestrial planets they were first identified on mars thus the associated terminology is based on mars related observations usually describing surface patterns with distinct albedos however terms related to reflected tones are only relevant to past knowledge for mars and not necessarily suitable for description of radar visible streaks located on venus titan or earth furthermore the sensor type used to observe wind streaks can influence the subsequent data analysis the purpose of this study was to examine the effects of sensor type on wind streak identification and interpretation six case studies featuring wind streaks on earth were performed using imaging by both radar and optical sensors the results indicate that wind streak identification is constrained to a specific combination of sensor settings appropriate for the local surface properties only half the optically visible wind streaks considered in this study were also radar visible but all the wind streaks observable in the radar images also appeared in the optical images furthermore bright and dark reflectance and backscatter are relative terms and should be used with caution these results suggest that the venusian wind streak database is most likely far from complete and that many more streaks exist

palaeoenvironments and palaeotectonics of the arid to hyperarid intracontinental latest permian late triassic solway basin u k abstract the late permian to late triassic sediments of the solway basin consist of an originally flat lying laterally persistent and consistent succession of mature dominantly fine grained red clastics laid down in part of a very large intracontinental basin the complete absence of body or trace fossils or palaeosols indicates a very arid hyperarid depositional environment for most of the sediments at the base of the succession thin regolith breccias and sandstones rest unconformably on basement and early permian rift clastics overlying gypsiferous red silty mudstones very fine sandstones and thick gypsum were deposited in either a playa lake or in a hypersaline estuary and their margins these pass upwards into thick bedded multi storied fine to very fine grained red quartzo felspathic and sublithic arenites in which even medium sand is rare despite channels with clay pebbles up to 30 cm in diameter above thick trough cross bedded and parallel laminated fine grained aeolian sandstones deposited in extensive barchanoid dune complexes pass up into very thick multicoloured mudstones and gypsum deposited in marginal marine or lacustrine sabkha environments the latter pass up into marine lower jurassic shales and limestones thirteen non marine clastic lithofacies are arranged into five main lithofacies associations whose facies architecture is reconstructed where possible by analysis of large exposures the five associations can be compared with the desert pavement arid ephemeral stream sabkha saline lake and aeolian sand dune environments of the arid to hyperarid areas of existing intracontinental basins such as lake eyre and lake chad the accommodation space in such basins is controlled by gradual tectonic subsidence moderated by large fluctuations in shallow lake extent caused by climatic change and local variation and this promotes a large scale layer cake stratigraphy as exemplified in the solway basin here the dominant fine grained mature sandstones above the local basal reg breccias suggest water reworking of wind transported sediment as in the northern part of the lake chad basin growth faulting occurs in places in the solway basin caused by underlying evaporite movement but these faults did not significantly affect pre late triassic sedimentation and did not expose pre permian units above the basal breccias there is no evidence of post early permian rifting anywhere during deposition of the late permian to middle triassic british succession although the succession is often interpreted with a rift basin model the arid to hyperarid palaeoclimate changed little during deposition of the solway basin succession in contrast to lakes eyre and chad and this is attributed to tectonic and palaeolatitude stability unlike the later mesozoic cenozoic only limited plate movements took place during the triassic in western europe palaeolatitude changed little and the solway basin remained in the northern latitudinal desert belt from early to mid triassic times however the influence of the early triassic impoverished biota on environmental interpretations needs further study