long term talus flatirons formation in the hyperarid northeastern negev israel abstract colluvial sediments of talus relicts talus flatirons around mesas preserve a record that sheds light on slope forming processes at temporal scales 103 yr the sedimentology and soil stratigraphy of two groups of talus flatirons in the northeastern hyperarid negev desert reveal four deposition events in the younger talus and at least two in the older one numerical modeling of high resolution 10be depth profiles suggests that these taluses were deposited during the middle pleistocene the younger talus group first depositional event occurred at 551 142 80 ka and its abandonment occurred at 270 38 17 ka the abandonment of the older talus group and stabilization of its surface occurred at 497 114 176 ka these ages indicate that the development of the studied talus sequence is not specifically associated with pleistocene glacial interglacial cycles the 10be modeled concentrations indicate significant differences in the average inheritance of talus flatirons of different groups these differences can be attributed to variability in the transport distance and duration of gravel exposure during transport but could also reflect some temporal variability in cliff retreat our results also demonstrate that talus slopes in hyperarid areas despite their steepness can store sediment for long periods 500 ka and thus constitute a valuable archive

miocene gypcretes from the calama basin northern chile gypcretes of miocene age are preserved beneath a 9 53 0 36 ma ignimbrite along the eastern margin of the oligo pleistocene calama basin northern chile they are restricted to a single stratigraphic horizon developed within laterally extensive 35 km coalesced alluvial fan deposits developed along the margin of an endorheic basin two types of gypcrete are recognized type 1 comprises almost completely gypsum cemented sandstones containing alabastrine nodules and columns sub vertical and horizontal veins of fibrous gypsum and v shaped cracks infilled by clastic material and are interpreted as surface weathered gypsic crusts type 2 gypcretes are composed of massive reddened poikilitic and mesocrystalline gypsum up to 80 of the rock with isolated bedding parallel clast rich lenses 200 30 cm and sub vertical veins of fibrous gypsum the massive texture resembles that of well developed b horizons in quaternary alluvial desert soils the crystal forms suggest an origin as a subsurface gypsic crust formed by a combination of hydromorphic poikilitic and illuvial mesocrystalline processes with the fibrous gypsum veins suggestive of periodic surface exposure gypcrete horizons are up to 25 m thick and composed of both gypcrete types they represent superimposed phases of surface and subsurface gypcrete development quaternary gypcretes are developed in arid climatic regimes but are not considered to develop under hyper arid climates an arid climate is considered to have prevailed in the study area up to 9 5 ma after which a change to hyper aridity favoured gypcrete preservation

soil geomorphic research a selective overview abstract much soil geomorphologic research involves tracking the development of soils with time otherwise known as soil chronosequences most of these have been undertaken on landforms underlain by quaternary depsits of varying ages in general there is a predictable variation in both soil horizon sequence and amounts of constituents such as clay fe and caco 3 with time the rate at which these constituents accumulate in the soil is a function of parent material bioclimate and atmospheric dust influx and because of the latter soils in aridic regions can form at rapid rates profile properties can be used to estimate the ages of deposits and faulting events but the error on the estimated age can be 50 or greater the position of salt accumulations in soil profiles is one of the best indications of climatic change and these can be modelled to compare with various climatic parameters present and past profile characteristics due to pedogenic thresholds however can mimic those due to climatic change so close dating is necessary to differentiate the effects of both soil geomorphic data are essential to understand the origin of ancient paleosols in the sedimentary rock records but a major problem still is one of recognition of these features as soils and to separate the effects of pedogenesis from those of diagenesis

desert pavement coated surfaces in extreme deserts present the longest lived landforms on earth all exposed rocks on earth s surface experience erosion the fastest rates are documented in rapidly uplifted monsoonal mountain ranges and the slowest occur in extreme cold or warm deserts millennial submeterscale erosion may be approached only in the latter the oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings thus these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape while the surrounding landscape may erode faster and be younger we present erosion rate and exposure age data from the paran plains a typical environment in the near east where vast abandoned alluvial surfaces 10 2 10 4 km 2 are covered by well developed desert pavements these surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m y major factors that reduce erosion converge in these regions extreme hyperaridity tectonic stability fl at and horizontal surfaces i e no relief and effective surface armoring by a clast mosaic of highly resistant lithology the 10 be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern negev israel representing the sahara arabia deserts indicate simple exposure ages of 1 5 1 8 ma or correspond to maximum erosion rates of 0 25 0 3 m m y 1 the 36

estimating the age of desert alluvial surfaces with spaceborne radar data abstract we present a new dating approach for desert alluvial surfaces that utilizes radar image data to obtain regional scale correlations between dated surfaces and surfaces of unknown ages the study was carried out along the arava segment of the dead sea transform north of the gulf of elat aqaba using advanced land observation satellite alos phased array type l band synthetic aperture radar palsar data with 6 12 m pixel resolutions nineteen independently dated quaternary morphostratigraphic alluvial units 5 to 1910 ka from eight different sites with variable lithologies carbonate magmatic chert were examined in order to develop test and validate the proposed approach expanding on previous studies that established the characteristic smoothing of desert alluvial surface through time and the capability of measuring such roughness variations with radar we employed a pixel aggregation approach that yielded a robust r 2 0 95 power law relation between the average radar backscatter value from the surface ar and the abandonment age t of the unit as measured with independent methods such as luminescence and or cosmogenic radionuclide dating we found that 1 this ar t correlation can be inverted to obtain calibrated radar based t estimates as old as 1 5 ma with 35 uncertainty 2 alos palsar 6 m pixel images acquired at incidence angles of 34 38 yielded optimal results 3 a sample of at least 70 contiguous radar pixels was required to capture the natural roughness variability of the surfaces examined and 4 surface lithology did not exert a primary effect on t inversions validation experiments yielded radar based ages of 3 1 91 32 484 169 and 1604 561 ka that compare to holocene 56 14 540 60 and 1590 250 ka ages previously determined for the same surfaces respectively we propose that with region specific calibrations spaceborne radar data can be used to quantitatively estimate the age of abandoned alluvial units across regional scales

weathering of superficial limestone debris in a hot desert environment abstract weathering of superficial limestone debris is described from stone pavements on the hamada de meski in southeast morocco the dominant features are due to surface and subsurface solution mechanical weathering and aeolian action have only a limited effect microfeatures produced by solution include solution facetting microrills solution pits case hardening micropitting or vesicular weathering and possibly surface crazing such microforms are the most likely representatives of a truly arid karst

the micromorphology of gypsum and halite in reg soils the negev desert israel gypsum and halite are the most common salts in reg soils developed on alluvial parent material under extremely arid conditions in the negev desert israel the aim of this paper is to document the changes in the micromorphology of these salts at different stages of reg soil development on two alluvial fan chronosequences the micromorphological analyses included thin section observations and scanning electron microscope and electron microprobe analyses in this arid soil environment gypsum and halite possess a variety of crystal forms which may change with depth in a single profile and or between profiles of different ages the variety of crystal forms results from changes in the microenvironmental conditions that occur in desert reg soils over time poikilitic lenticular gypsum is found in all reg soils and is distributed throughout the profiles the conditions needed for such crystals to form are high ionic impurities and deposition in a void system where space is not limiting microcrystalline alabastrine gypsum is only found in mature reg soils and is crystallized when the profile has high amounts of fine material and a well developed desert pavement in a well developed reg soil profile indicators such as a well developed desert pavement and high amounts of fine earth limit the leaching depth and cause gypsum deposition from supersaturated soil solutions under high evaporation rates close to the surface prismatic and fibrous gypsum are less common low amounts of prismatic gypsum are found in young and mature soils while fibrous gypsum is found only in mature soils in re cemented shattered gravel the halite crystal form is mainly cubic with low amounts of host material incorporated into the crystal it occurs predominantly in mature reg soil profiles through the crystallization from supersaturated soil solutions at the depth of maximum water penetration although the alabastrine prismatic and fibrous gypsum and cubic halite are deposited in a displacive manner no correlation was found between their occurrence and the distribution of shattered gravel in the soil profile

revealing sediment sources mixing and transport during erosional crater evolution in the hyperarid negev desert israel abstract to better understand the sedimentary history of the erosional crater of makhtesh hazera in the hyper arid negev desert of southern israel we have measured concentrations of in situ 10 be in alluvial sediments from the active drainage system and from abandoned alluvial terraces and dated them using optically stimulated luminescence osl these sedimentary sequences suggest changes in the drainage system behavior over time and show a periodic pattern in which periods of sediment aggradation alternate with periods of degradation through incision and erosion in alluvial terrace sediments 10 be concentrations were combined with osl ages to gain insight of the temporal framework of sediment deposition and shed light on the process of sediment storage osl and simple 10 be exposure ages of terrace sediments give deposition ages between 340 ka to 50 ka at which time the youngest alluvial terrace was incised deposition was interrupted twice at 300 ka and 160 ka when periods of rapid incision caused the abandonment of the active fluvial surface and the stabilization of a younger and lower fluvial surface in the active drainage system 10 be concentrations suggest several possible quartz sources of which the lower cretaceous sandstone bedrock exposed at the base of the cliff enclosing the makhtesh is the most dominant the results suggest that sediment eroding form these cliffs are conveyed through the active alluvial channels without significant sediment contribution from alluvial terraces or from eroded bedrock exposed within the makhtesh in contrast to our measurements in the active drainage system 10 be concentrations in alluvial terrace sediments record significant storage within the makhtesh we suggest that the geometry of the makhtesh mainly the disproportion between the size of the makhtesh and its narrow outlet leads to significant accumulation of sediment within the makhtesh episodic breaching of the barrier is followed by rapid and short lived incision into the stored sediment the long residence time of sediment within the makhtesh results in high measured 10 be concentrations at the end of each of these short incision episodes when channels are already carved into the alluvial sediment and terraces are formed the channels only convey sediment directly eroded from the bounding cliff of the makhtesh a situation presently observed

runoff generation rill erosion and time scales for hyper arid abandoned alluvial surfaces the negev desert israel abstract sequences of abandoned alluvial surfaces are ubiquitous desert landscapes considered as responding to quaternary timescale climatic fluctuations and tectonic activity the post depositional evolution surficial properties of these alluvial surfaces change with time a surface clast breakdown and development of desert pavement b dust and salts accumulation c reg gravelly desert soil evolution in the present study we focus on the hydrological response and the resulting erosional processes to these changes in surface properties over a sequence of late pleistocene holocene abandoned alluvial surfaces in nahal evrona catchment at the hyperarid southern arava valley north of the towns of eilat israel and aqaba jordan the methods used in the study are a simulated rainfall experiments to determine hydrological characteristics b terrestrial laser scanning tls to determine surface roughness c osl dating for the chronology d surface and soil descriptions and measurements and grain size analyses to determine texture of the uppermost av horizon of the reg soil e gis analysis of the new drainage network developing over the older abandoned alluvial surfaces the results show that with time roughness decreases infiltration decreases to about 8 9 relative to present rates point runoff generation increases 5 fold and as a result rill and gully erosion initiated finally leading to the destruction of the alluvial surface under the current hyper arid climate in the southern negev desert between 30 and 50 ka of surface weathering and soil development are required before significant runoff is generated and 100 ka until effective runoff initiates rill and gully erosion over these abandoned alluvial surfaces this local source runoff and gully erosion essentially destroys the alluvial surfaces in the southern arava valley within a period of 400 500 ka roughness extracted from spaceborne lidar scanning which is inversely related to surface age directly reduces friction and resistance to runoff can provide an estimate for the hydrological characteristics of the surfaces this can support applications such as rainfall runoff models risk assessment etc

physical weathering in arid landscapes due to diurnal variation in the direction of solar heating despite the prominent role of physical weathering in arid and semi arid landscapes there has been little study of the specifi c processes responsible for the rapid breakdown of subaerially exposed rocks for example many boulders and cobbles in deserts exhibit fi ne near vertical cracks although workers have hypothesized that these and other cracks are initiated by diurnal heating and cooling no convincing specifi c mechanism for their formation has been proposed we have characterized these cracks at eight sites on surfaces of different ages in the mojave sonoran and chihuahuan deserts and the high desert of central new mexico our data reveal four basic types of cracks longitudinal surfaceparallel fabric related and meridional the orientations of the fi rst three types are associated with clast shape and rock fabric the azimuths of meridional cracks however are preferentially aligned north south typically with a nonrandom multimodal distribution we propose that these cracks are caused by tensile stresses that arise in the interior of clasts due to strong radial gradients in temperature that evolve and rotate in alignment with the sun s rays we suggest that the multimodal nature of crack orientations may be in part attributable to the seasonally varying latitude dependent solar elevation angle over millennial time scales we suggest that this thermal cracking is an effi cient weathering process that together with cumulic soil epipedon development creates the key attributes of most desert pavements in addition to individual clasts exposed on desert surfaces this mechanism of cracking is potentially signifi cant in other climates and on other planets as well as for rock outcrops and for man made structures

transition from arid to hyper arid environment in the southern levant deserts as recorded by early pleistocene cummulic aridisols abstract the time at which deserts established their current arid or hyper arid conditions remains a fundamental question regarding the history of earth cosmogenic isotope exposure ages of desert pavement and welded calcic gypsic salic reg soils that developed on relatively flat alluvial surfaces 2 ma ago in the negev desert indicate long geomorphic stability under extremely dry conditions over a short interval during their initial stage of development between 1 2 ma these cumulative soils are characterized by calcic soils reaching maximum stage iii of carbonate morphology this interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern negev desert since 1 ma pedogenesis changed toward more arid soil environment and the formation of gypsic salic soil horizons that were later followed by dust accumulation the dichotomy of only moderately developed calcic soil stages ii iii during a relatively long time interval 105 106 years indicates an arid environment that does not support continuous development but only occasional calcic soil formation the very low 18o and relatively high 13c values of these early pedogenic carbonates support soil formation under arid climatic conditions such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture this in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate at 1 ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern negev desert leading to deposition of soluble salts and dust deposited in the soils the combination of long term hyperaridity scarcity of vegetation and lack of bioturbation salts cementation dust accumulation and tight desert pavement cover has protected the surfaces from erosion forming one of the most remarkably stable landscapes on earth a landscape that essentially has not eroded but accumulated salt and dust for more than 106 yr

complex exposure histories of chert clasts in the late pleistocene shorelines of lake lisan southern israel activities of 26 al and 10 be in five chert clasts sampled from two beach ridges of late pleistocene lake lisan precursor of the dead sea in southern israel indicate low rates of chert bedrock erosion and complex exposure burial and by inference transport histories the chert clasts were derived from the senonian mishash formation a chert bearing chalk which is widely exposed in the nahal zin drainage basin the drainage system that supplied most of the material to the beach ridges simple exposure ages assuming only exposure at the beach ridge sampling sites range from 35 to 354 ky using the ratio 26 al 10 be total clast histories range from 0 46 to 4 3 my unrelated to the clasts current position and exposure period on the late pleistocene beach ridges 160 177 m below sea level optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20 0 s 1 4 ka and 36 1 s 3 3 ka these ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of lake lisan which suggest that the lake reached its highest stand around 27 000 cal years bp if the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50 1300 ky and 390 3130 ky respectively alternatively the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock back calculating such rates of rock erosion suggests values between 0 4 and 12 m my 1 t he relatively long burial periods indicate extended sediment storage as colluvium on slopes and or as alluvial deposits in river terraces some clasts may have been stored for long periods in abandoned pliocene and early pleistocene routes of nahal zin to the mediterranean before being transported again back into the nahal zin drainage system and washed on to the shores of lake lisan during the late pleistocene copyright 2003 john wiley sons ltd

stages and rate of the gravel shattering process by salts in desert reg soils abstract gravel shattering by salts is a common process in gravelly young to mature desert reg soils the shattered gravel is found on the surface and in the b and c horizons down to a maximum depth of 50 cm field studies on the process of salt weathering in deserts are scarce therefore the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the negev desert the studied reg soils were developed on two alluvial fan chronosequences in the negev desert the fan of nahal ze elim and the fan of nahal shehoret the development of the shattering process can be divided into five stages a e based on chemical physical and micromorphological features of the reg soils stages a d very young yo young reg soils represent the early holocene to the present some 14 000 years salinity values are between 0 27 16 mmho cm and the percentage of the shattered gravel is between 20 and 70 the shattered gravel however still maintains its original shape stage e mature reg soil represents the middle pleistocene to early holocene or 500 000 years salinity values at this stage reach 62 mmho cm although part of the gravel does not keep its original shape and starts to disperse in the profile the percentage of shattered gravel is only 75 80 it was found that the process of salt shattering in reg soils does not depend on concentrations or mineralogy of the salts but on the micro environmental conditions of the soil profile the process requires extreme and rapid variations of temperatures and moisture conditions such as found close to the soil surface to cause salt crystallization in a confined system with time the sealing of the soil surface through the development of desert pavement and a gravel free b horizon results in a change in the soil micro environment this change impedes the process irrespective of the amounts concentrations and types of salt minerals which exist in the profile the fact that in extreme arid regions the gravel shattering process develops primarily close to the reg soil surface suggests that it can be used to identify buried reg soils and to reconstruct climatic and micro environmental conditions of paleosols

late quaternary earthquake chronology from luminescence dating of colluvial and alluvial deposits of the arava valley israel abstract luminescence ages help provide a time scale for recurrent surface ruptures on a normal fault in the southern arava israel the dated samples come from alluvium and colluvium along a fault scarp north of elat the ages obtained mainly by infrared stimulation of sand size potassium feldspar grains accord with geomorphic age estimates from soils and degraded scarps four large earthquakes occurred between 37 000 and 14 000 yr ago five smaller earthquakes occurred thereafter continuing until recent times the mean recurrence interval for earthquakes of magnitudes greater than 6 is about 4000 yr

measuring the time and scale dependency of subaerial rock weathering rates over geologic time scales with ground based lidar the evolution of roughness as a function of surface age was used to quantify weathering rates on rocky desert surfaces surface topography on eight late quaternary alluvial terraces which record the weathering of holocene 5 1 ka boulder strewn deposits into mature 87 2 ka desert pavements in the negev desert of israel was measured with ground based lidar roughness on each terrace was characterized with power spectral density psd analysis and changes in psd as a function of length scale 0 04 1 50 m and surface age were used to estimate diminution weathering rates of the surface rocks we found psd values that systematically increase as a power law function of roughness exponent of 2 0 and decrease as an inverse power law function of surface age this psd evolution indicates a fragmentation rock weathering process driven by salt shattering throughout the 87 k y period examined psd analysis of the lidar data also revealed weathering rates that increase with rock size and decrease as an inverse power law function of time from initial values 20 mm k y to 1 mm k y within 60 k y

quaternary evolution of a hyperarid drainage under climatic fluctuations and rift margin base level fall ne negev israel abstract the long term bedrock incision of streams in the northeastern negev has been dictated since the early pliocene by the subsidence of the dead sea basin however this incision is accompanied by very long intervals of sediment aggradation here we document the quaternary scale fluvial response to changing base level lithology structure and climate along the hyperarid nahal zafit ne negev the oldest 1 2 ma alluvial remnants point to a widespread beveled bedrock pediment at the base of the hazera anticline this pediment like other stream pediment anticline relationships in the region provides the baseline framework into which nahal zafit incises during the early to middle pleistocene this pediment was the base level for all streams emerging out of the anticlines the tectonic subsidence of the regional base level at the rift margin farther downstream has caused incision and upstream migration of bedrock knickpoints triggering the development of a wide incised valley in the mazar syncline adjacent to the hazera anticline since 330 ka fluvial terraces landslides and taluses record intervals of basin wide fluvial deposition under prevailing hyperarid to arid climate despite the general arid climate prolonged fluvial deposition intervals occurred at ca 330 265 230 125 and 110 20 ka rapid and quite short duration bedrock incision occurred in between these much longer intervals of aggradation in addition to the proposed climatic imprint on fluvial aggradation incision cycles and the first order base level control of the rift margin fault the formation of strath terraces and distribution of stored sediments within the drainage basin point to the fundamental role that lithology and structure play in the quaternary evolution of the ne negev watersheds

the role of the nile in initiating a massive dust influx to the negev late in the middle pleistocene the late middle pleistocene initiation of loess accretion in the negev desert israel resulted from a chain of events starting with the exposure of shallow offshore nile delta sands beginning 180 ka during a period of glacially lowered eustatic sea level this exposure allowed the formation of the dunes of the sinai negev erg and the eastward movement of the dunes under the gusty glacial time winds in northern sinai this eastward dune advance occurred by eolian saltation that generated the coarse silts that accumulated downwind as loess we postulate that the absence of earlier negev loess is not a result of interglacial erosion as previously proposed but is due to the relatively recent convergence of the above conditions necessary for loess formation we based our interpretation on analyses of two long term natural dust traps and the association of coarse quartz silt production with the advancing quartz rich dunes over this carbonate terrain although the nile delta is considered an early to middle pleistocene feature and therefore could have been the source of sand throughout the quaternary it attained the shallow depth range of glacial interglacial sea level changes only during late marine oxygen isotope stage 7 to early stage 6 allowing its wide exposure in quaternary times prior to erg formation negev dust was finer composed of silt and clays derived from distal sources in the sahara and arabian deserts

optical dating of tsunami laid sands abstract the ages of some tsunami deposits can be determined by optical dating a key requirement being that the deposits are derived from sediment that was reworked and exposed to daylight by tidal currents waves wind or bioturbation during the last years before the tsunami measurements have been made using 1 4 ev infrared excitation of k feldspar grains separated from samples of prehistoric tsunami sand sheets and modern analogs of tsunami source sediments at four sites in washington state and british columbia source sands gave equivalent doses indicative of recent exposure to daylight tsunami sand at cultus bay washington yielded an optical age of 1285 95 yr calendric years before a d 1995 1 at 2 this age overlaps the range of from 1030 to 1100 yr determined through a combination of high precision radiocarbon dating and stratigraphic correlation tsunami sands at three sites near tofino and port alberni on vancouver island british columbia have optical ages of 260 20 325 25 and 335 45 yr historical records and radiocarbon dating show that the sand at each of the three sites is between 150 and 400 yr old these optical ages support the hypothesis that the vancouver island sands were deposited by a tsunami generated by a large earthquake on the cascadia subduction zone about 300 yr ago

indicators of faulting events and periods of quiescence in desert alluvial fans the main objective of the present paper is to present an integrated approach to the study of faulting stages in coarse alluvial deposits the study site is the alluvial fan of nahal shehoret presently under an extremely arid regime the exposure of fault scarps on the fan surfaces enables the identification of several geomorphic indicators employed in analysing and separating faulting events and periods of quiescence a the nature of fault scarp topography b soil catenary development on fault scarps c buried paleosols in alluvial deposits d colluvial bodies e rotated clasts along fault planes these indicators enable us to determine at all sites studied the total amount of displacement to separate faulting events and the amount of their displacement per event and to evaluate scarp stability and the duration of periods of tectonic quiescence

assessment of dam failure flood and a natural high magnitude flood in a hyperarid region using paleoflood hydrology nahal ashalim catchment dead sea israel 1 a dam failure flood in 1993 with a peak discharge of 600 700 m3 s 1 volume of 0 5 106 m3 and a natural rainfall runoff flood with a peak discharge of 420 m3s 1 volume of 2 0 106 m3 in 2004 in the nahal ashalim catchment 75 km2 in the hyperarid dead sea region were comparatively analyzed the two floods which caused serious damage to the dead sea works industries were analyzed for magnitude and frequency using regional analysis extrapolation from the short systematic record rainfall runoff modeling and paleoflood hydrology the 7600 year paleoflood record was reconstructed at the ashalim cave at the upper nahal ashalim peak discharges in this high gradient mountainous wadi are highly sensitive to the roughness coefficient and the flow regime the dam failure peak discharge is lower than the envelope curves of measured floods of the dead sea region and of paleofloods of the negev and the dead sea deserts the probability of the dam failure flood and the natural flood were estimated by various methods at 1 to 0 2 and 0 5 respectively adding the peak discharge of the october 2004 flood increased the probability from 0 2 to 0 5 0 2 the long paleoflood record encompasses different hydrological regimes related to climatic fluctuations which are not relevant to the present hydrological regime therefore the chosen period for frequency analysis was a d 1673 1996

on the formation of desert loess sequences of quartz rich coarse 20 63 m silt occur in many low and midlatitude unglaciated arid and semiarid areas and have been termed desert loess the processes by which these deposits are generated have been debated for decades all hypotheses to explain their origin seek to provide mechanisms for the generation of silt sized material without glacial grinding which is the main process involved in the production of coarse silt at high latitudes possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones mechanical weathering of silicate rocks and abrasion of sand grains in active dune environments during intense transport events examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt improvements in the characterization of desert loess particle size mineralogy and geochemistry are needed however to identify sources and sinks of coarse silt especially when combined with climatic back trajectory analysis properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations

morphology and significance of super mature calcretes from the upper old red sandstone of scotland thick calcrete profiles occur in the late devonian early carboniferous kinnesswood formation of the midland valley of scotland these profiles consist of highly brecciated peloidal and coated grain fabrics with laminar calcrete horizons and linings to fractures the profiles resemble supermature calcretes associated with late cenozoic surfaces in the southwestern united states and reflect pedogenesis over periods of several hundred thousand years or more the possible origins of the highly complex fabrics seen are reviewed and the polygenetic nature of such profiles is stressed it is suggested that brecciation takes place co incident with subaerial exposure of the calcrete mature profiles may be predisposed to brecciation and surficial weathering because of their susceptibility to soil erosion as a result of changes in vegetation cover the calcretes underwent phreatic cementation burial dolomitization and silicification

late pleistocene aeolian and fluvial interactions in the development of the nizzana dune field negev desert israel the interdunal areas in the nizzana linear sand dune field contain both sandy and silty sediments a series of trenches was excavated across the interdunal corridor exposing stacked sequences of silty and sandy units which are locally restricted to palaeodepressions the silty units contain fining upward sequences and are interpreted as overbank deposits from the nahal nizzana thermoluminescence dating and identification of buried palaeosols indicates that the silt and clay layers were deposited over a period of several thousands of years in the late pleistocene the sands between the silt layers have been fluvially reworked and are not primary aeolian deposits the stacked sequences of fluvial deposits indicate that the palaeodepressions persisted in the landscape for a significant time attesting to long term stability of the interdunal areas it also suggests that the linear dunes themselves have not moved laterally during this time despite climatic changes and devegetation since the end of the late pleistocene the nahal nizzana has downcut and overbank deposition no longer occurs within the interdunal corridors the playa deposits today are positive relief features indicating that topographic inversion has occurred and that the interdunal areas are geomorphically active

the evolution of holocene reg gravelly soils in deserts abstract reg soil is an aridisol developed upon a stable alluvial surface composed of sand to coarse gravel the objectives of the present study were 1 to examine the initial evolution of reg soil properties with time 2 to assess and explain the rates of development of different properties 3 to establish the suitability of different soil properties for relative age dating the time dependent soil properties examined were 1 mechanical weathering of surficial rocks and desert pavement evolution 2 chemical and biological weathering of surficial rocks 3 the thickness of soil profiles and soil horizons 4 soil texture in the different horizons 5 salts and gypsum distribution and concentration in different soil horizons 6 degree of mechanical weathering of gravel in c horizons fifteen successive surfaces of the nahal wadi ze elim sequence of alluvial fans were selected for study dead sea region israel under an extremely arid climate the sequence is of holocene age 14 000 years b p to recent the main conclusions are 1 reg soils develop rather fast in several 10 3 years since their initiation however the various soil properties developed at different rates 2 the rates of change of three soil properties decrease with time these are the texture in different horizons change in added airborne and rain washed dust salinity within the soil profile change in added salts and gypsum and the amount of mechanically shattered rocks 3 soil thickness is established rather rapidly young and older holocene soil have practically the same depth surficial weathering rates appear to remain fairly constant with time 4 in coarse gravel the evolution of reg soils is dependent on added fines and salts of airborne origin and the resultant changing hydrologic regime in the soil profile the run off infiltration ratio changes as more fines and salts are added to the soil profile and as desert pavement develops 5 surficial features and fine clay in the soil profile may be used for approximate dating of otherwise undatable holocene alluvial surfaces salts and gypsum content should be cautiously used in a rather general way

soils as a tool for estimating ages of quaternary fault scarps in a hyperarid environment the southern arava valley the dead sea rift israel abstract on the alluvial fan of nahal shehoret in the extremely arid region of the negev desert israel reg soils developed on stable alluvial surfaces in colluvium along terrace risers and in colluvial units on a complex fault scarp were compared the degree of soil profile development provided an age estimate of the surface that was faulted a maximum age estimate of the scarp or terrace riser and an estimate of the maximum recurrence intervals of faulting events irsl dates of the tectonically displaced alluvial surfaces and colluvial units at the same site enabled us to re evaluate the use of reg soils for relative dating the major faulting event started at 34 8 4 3 ka it displaced a late pleistocene surface of the nahal shehoret alluvial fan dated at 56 10 8 ka renewed fluvial activity and deposition of alluvium on the downfaulted block had almost terminated by 13 6 2 3 ka the scarp was formed during the latest pleistocene and was followed by smaller faulting events which had only minor effects on the fault scarp morphology the similarity of age estimates from irsl dating and the degree of soil development in this environment indicates that soils can be used to date surfaces and colluvial units

major ion chemistry in the headwaters of the yamuna river system chemical weathering its temperature dependence and co 2 consumption in the himalaya abstract the yamuna river and its tributaries in the himalaya constitute the yamuna river system yrs the yrs basin has a drainage area and discharge comparable in magnitude to those of the bhagirathi and the alaknanda rivers which merge to form the ganga at the foothills of the himalaya a detailed geochemical study of the yrs was carried out to determine i the relative significance of silicate carbonate and evaporite weathering in contributing to its major ion composition ii co2 consumption via silicate weathering and iii the factors regulating chemical weathering of silicates in the basin the results show that the yrs waters are mildly alkaline with a wide range of tds 32 to 620 mg l 1 in these waters the abundances of ca mg and alkalinity which account for most of tds are derived mainly from carbonates many of the tributaries in the lower reaches of the yamuna basin are supersaturated with calcite in addition to carbonic acid sulphuric acid generated by oxidation of pyrites also seems to be supplying protons for chemical weathering silicate weathering in yrs basin contributes on average 25 molar basis of total cations on a basin wide scale silicate weathering however does not seem to be intense in the basin as evident from low si na k in the waters 1 2 and low values of chemical index of alteration cia in bed sediments 60 co2 drawdown resulting from silicate weathering in the yrs basin in the himalaya during monsoon ranges between 4 to 7 105 moles km 2 y 1 this is higher than that estimated for the ganga at rishikesh for the same season the co2 consumption rates in the yamuna and the ganga basins in the himalaya are higher than the global average value suggesting enhanced co2 drawdown in the southern slopes of the himalaya the impact of this enhanced drawdown on the global co2 budget may not be pronounced as the drainage area of the yrs and the ganga in the himalaya is small the co2 drawdown by silicates in the yrs basin is marginally higher than the reported values of co2 release from oxidation of organic rich sediments estimated using re as a proxy this comparison shows the need to constrain co2 sources and sinks better to balance its budget in a regional scale the results also show that silicate weathering rate in the yrs basin is 10 mm ky 1 and on the ganga basin it is 5 mm ky 1 which are several times lower than the carbonate weathering rates the significantly higher silicate weathering rate observed in the yrs basin seems to be governed by rapid physical erosion in this region the apparent activation energy for overall silicate weathering in the yrs basin derived from na and si concentrations and water temperature ranges from 50 to 80 kj mol 1 these values are comparable to those reported for granitoid weathering in natural watersheds and feldspar weathering in laboratory experiments this study brings to light the sources contributing to major ions enhanced chemical weathering rates in the yamuna river basin and interdependence of silicate weathering on physical erosion and temperature

permanent quaternary hyperaridity in the negev israel resulting from regional tectonics blocking mediterranean frontal systems although various episodes of wet quaternary climates have been suggested in studies of the negev desert here we demonstrate that reg soils developed on flat alluvial surfaces and sensitive to minor changes in precipitation indicate that the southern negev has been permanently hyperarid at least since the middle pleistocene the wetter episodes were restricted to the northern negev currently mildly arid gypsic saline reg soils that de veloped on stable surfaces during 200 k y in the southern negev are cumulative and polygenetic but none exhibits even the weakest development of calcic horizons under current climate conditions in the negev calcic soils are widespread in areas with rainfall 80 mm yr 21 the fact that the hyperarid 50 mm yr 21 southern negev lacks any calcic horizons indicates that it never experienced an average rainfall of 80 mm yr 21 during the middle late pleistocene whereas the northern negev was wetter this negev dichotomy is explained by the main physical features controlling rainfall over the negev i e the southward decreasing depth of the atmospheric boundary layer with distance from the mediterranean and the altitude of the central negev highland 1000 m the interaction between these two features often prevents the passage of rain clouds into the southern negev the holocene and pleistocene gypsic salic soil distribution across the negev closely matches the current circumstances we suggest that hyperaridity has prevailed over the southern negev since the last stages of the uplift of the central negev highland in the late pliocene early pleistocene

debris flow dominance of alluvial fans masked by runoff reworking and weathering arid alluvial fan aggradation is highly episodic and fans often comprise active and inactive sectors hence the morphology and texture of fan surfaces are partly determined by secondary processes of weathering and erosion in addition to primary processes of aggradation this introduces considerable uncertainty in the identification of formative processes of terrestrial and martian fans from aerial and satellite imagery the objectives of this study are i to develop a model to describe the sedimentological and morphological evolution of inactive fan surfaces in arid settings and ii to assess the relative importance of primary processes of aggradation and secondary processes of weathering and reworking for surface morphology and sedimentology and for the stratigraphic record we studied an alluvial fan characterized by a recently active sector and a long abandoned inactive sector along the coast of the hyperarid atacama desert here rates of primary geomorphic activity are exceptionally low because of extreme aridity while weathering rates are relatively high because of the effects of coastal fogs long term processes of fan aggradation and reworking were determined through sedimentological facies analysis of stratigraphic sections ground surveys for textural and morphological patterns at the fan surface were integrated with remote sensing by an unmanned airborne vehicle uav discharges and sediment transport capacities were calculated to estimate the efficiency of secondary runoff in reshaping the inactive fan sector stratigraphic sections reveal that the fan was dominantly aggraded by debris flows whereas surface morphology is dominated by debris flow signatures in the active sector and by weathering and runoff on the inactive sector on the latter rapid particle breakdown prevents the formation of a coarse desert pavement furthermore relatively frequent local runoff events erode proximal debris flow channels on the inactive sector to form local lag deposits and accumulate fine sediment in low gradient distal channels forming a well developed drainage pattern that would suggest a runoff origin from aerial images nevertheless reworking is very superficial and barely preserved in the stratigraphic record this implies that fans on earth and mars that formed dominantly by sporadic mass flows may be masked by a surface morphology related to other processes

rb sr and sm nd geochronology of lower proterozoic granite greenstone terrains in french guiana south america abstract nine samples of metavolcanic rock from the lower parts of greenstone belts in central french guiana the paramaca series and 14 granitic samples from the intrusive gneisses the degrad roche and arawa gneisses were selected for sm nd and rb sr analysis the sm nd results from the metavolcanic series including two tholeiites five peridotitic komatiites and two andesites yield an isochron age of 2 11 0 09 2 ga with an initial 143 nd 144 nd ratio ind of 0 51002 9 2 corresponding to nd t 2 1 1 8 this isochron is interpreted as representing the age of initial volcanism of the paramaca series acid intrusives were dated by the rb sr method a whole rock rb sr isochron including data points from both the degrad roche and arawa gneisses yields an age of 2 00 0 07 2 ga with initial 87 sr 86 sr ratio isr value of 0 7019 4 2 this result is considered to be the time of emplacement of the orthogneiss protoliths the positive end value 2 1 1 8 obtained from the metavolcanic rocks of french guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in light rare earth elements lree this result confirms the possible existence of ancient lree depleted reservoirs within the lower proterozoic mantle moreover the high end t value for these rocks excludes any significant crustal contamination during magma genesis the french guianese orthogneisses yield a low isr value 0 7019 4 2 which together with geochemical considerations suggests that their granitic protoliths could have originated by partial melting of short lived crustal precursors of basaltic to granodioritic composition the present geochronological and isotopic study suggests that the guiana shield may represent a major continental accretion event during the lower proterozoic

evolution of reg soils in southern israel and sinai abstract reg soils mostly camborthids and gypsiorthids cover some 15 of the negev and sinai deserts analytical data of seven profiles on depositional surfaces of increasing age in the negev show clear relationships between ages of surfaces and soil profile features exemplified in the development of the cambic salic and gypsic horizons the youngest soil a coarse desert alluvium soil typic torriorthent of dry wadi beds up to a few thousand years old has little profile differentiation and is generally non saline or slightly saline on the higher terraces connected with the lisan formation about 70 000 12 000 years b p clear profile differentiation and the beginning of development of cambic horizons can be discerned evident in their colour textural p differences the soils are already saline and somewhat gypsiferous both soils are not yet defined as regs because of their negligible or restricted profile differentiation reg soils on the older and higher geomorphic surfaces in the arava rift valley several hundred thousand years old have well differentiated profiles with cambic salic and gypsic horizons below their stony desert pavements typical reg soils have light coloured vesicular horizons over mellow very pale brown loam frequently with laminar structure almost completely stone free layers the reddish brown loamy to clay loam b horizons are very saline and show intensive salt weathering of gravels gypsic and petrogypsic horizons occur at depth such reg soils represent the stable surfaces and soils of deserts and were developed over a long period of desert weathering

the history and nature of wind erosion in deserts recently the importance of wind erosion has been reevaluated many low angle surfaces developed on susceptible materials possess closed depressions pans and these often have a distinctive morphology and lunette dunes on their lee sides it has also become apparent that in drylands there are extensive areas of stone pavements some of which have been molded by deflation however it is now recognized that other horizontal sorting processes have played a role especially movement of fines by overland flow vertical sorting associated with wetting and drying salt heave bioturbation frost action and dust accretion have also played a role in hyperarid areas especially with unidirectional winds bedrock outcrops and old lake beds have been molded to give aerodynamic forms called yardangs the study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation especially in hyperarid areas with centripetal drainage deflation hot spots such as the bo

desert pavement characteristics on wadi terrace and alluvial fan surfaces wadi al bih u a e and oman abstract in arid mountain areas the dating and correlation of alluvial depositional surfaces is often uncertain especially in regions where the geomorphology is not well known surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages pleistocene wadi terraces and associated alluvial fans occur in wadi al bih u a e and oman for which correlations and age relationships are not known three age related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after miocene uplift of the region the younger two groups of terraces and fans are inset within the older group to identify the gross effects of pavement development comparisons have been made between terrace surface and subsurface particle size distributions the older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces this reflects the degree of pavement development particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces criteria for the classification of pavements were developed based on clast fracturing and angularity size sorting packing and surface texture from which a simple index of pavement development has been derived other properties rock varnish and weathering characteristics were also recorded but these proved to be less discriminatory than pavement characteristics the pavement data have been augmented by observations on soils detailed studies of pavements on terraces 8 sites 12 samples covering the three age groups and fans 5 sites 10 samples covering the three age groups allow differentiation between age groups the three terraces show three different age related pavement types expressed by differences in the pavement development index weakly developed pavements little fracturing sub rounded clasts some modification of the depositional fabric incipient soil development stage i caco3 accumulation occur on the youngest terrace and fan surfaces moderately developed pavements clast fracturing sub angular clasts moderate sorting and packing deeper soil development stage ii caco3 accumulation occur on the middle terrace and fan surfaces well developed pavements complete clast fracturing into small angular fragments mature sorting and packing of the pavement surface deep soil development with strong horizonation stage iii caco3 accumulation occur on the highest terrace and oldest fan surfaces there are minor differences between the youngest pavements on terraces and fans which reflect initial sedimentological differences these differences become less as the pavements develop on the basis of comparative studies the oldest terrace is estimated to date from sometime prior to ca 100 ka bp the second terrace and the most extensive fan surface from the late pleistocene and the youngest terrace and fan phase from the latest pleistocene or early holocene

late quaternary weathering erosion and deposition in nahal yael israel an impact of climatic change on an arid watershed in their seminal paper in 1979 bull and schick proposed a conceptual model for the geomorphic response to pleistocene to holocene climate change based on the hyperarid nahal yael watershed in the southern negev desert in this model the change from semiarid late pleistocene to hyperarid early holocene climates reduced vegetation cover increased the yield of sediment from slopes and accelerated aggradation of terraces and alluvial fans the model is now over 30 yr old and during this time chronologic paleoenvironmental and hydrogeomorphic research has advanced here we reevaluate the model using data acquired in nahal yael over the 30yr since the original model was proposed recent studies indicate that the late pleistocene climate was hyperarid and a transition from semiarid to hyperarid climates did not occur the revised chronology reveals a major 35 20 ka episode of accelerated late pleistocene sediment production on slopes with lower rates probably already at ca 50ka due to increased frequency of wetting drying cycles caused by frequent extreme storms and floods between 35 and 27 ka without lag time these sediments were transported and aggraded in depositional landscape components fluvial terraces and alluvial fans this intensified sediment production and delivery phase is unrelated to the pleistocene holocene transition the depositional landforms were rapidly incised between 20 and 18 ka since and or soon after this last glacial maximum lgm incision most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock using these new data we propose a revision to the bull and schick model in this hyperarid environment our revision suggests that the model should include the frequent storms and floods responsible for a late pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans we also discuss the common use and pitfalls of using the bull and schick conceptual model to explain observations in diverse arid environments usually without sufficient data on basin specific stratigraphic chronologic paleoenvironmental and paleoclimatic information

bathymetry of lake lisan controls late pleistocene and holocene stream incision in response to base level fall abstract this paper examines the millennial scale evolution of the longitude profile of nahal wadi zin in the dead sea basin in the northern arava valley israel nahal zin has incised 50 m into relatively soft late pleistocene lake lisan sediments incision was forced by the regressive 10 km lake level fall of a total of 200 m of lake lisan from its highest stand at 25 ka and exposure of the lake floor sediments to fluvial and coastal processes alluvial cut terraces of the incising channel are well preserved along the 17 5 km of the lowermost reach of nahal zin at its outlet into the dead sea basin nahal zin deposited a holocene alluvial fan at the base of a 10 80 m high escarpment in unconsolidated sediments the escarpment is associated with the amazyahu fault which forms the southern structural boundary of the present dead sea basin geomorphic mapping optically stimulated luminescence osl ages and soil stratigraphy allowed correlation of terrace remnants and reconstruction of several past longitudinal profiles of nahal zin and its incision history together with the published lake level chronology these data provide an opportunity to examine stream incision related to base level lowering at a millennial scale osl ages of the terraces fit relatively well with the established lake level chronology and follow its regression and fall for a few thousands of years the longitudinal profile response to the lake level fall was downstream lengthening onto the exposed former lake bed most of the incision 40 m occurred later when the lake level reached the top of the amazyahu fault escarpment and continued to drop the incision was a relatively short episode at about 17 ka and cut through this escarpment almost to its base the fast incision its timing and the profiles of the incising channels indicate that the escarpment was an underwater feature and was not formed after the lake retreated this fairly simple scenario of regressive lake level fall and knickpoint exposure and incision is modeled here using a one dimensional numerical incision model based on a linear diffusion equation the calculated diffusion coefficient fits earlier results and data obtained from other streams in the area and confirms the upscaling of this simple model to the millennial scale

the origin and distribution of salts on alluvial fans in the atacama desert northern chile the atacama desert has been predominantly hyperarid since the middle miocene combined ionic and x ray diffraction analysis shows that calcium sulphate is prevalent on three similar alluvial fans in salar basins across a transect of different environments in antofagasta province northern chile differences between the fans were largely due to the effect of local factors on salt input secondary redistribution and deposit preservation thus carbonate was notable in the high andes fan c and pre cordillera fan b whereas in the central valley fan a greater qualities of the more soluble salts chlorides and nitrates probably reflect the higher level of aridity and an oceanic source the camanchaca calcium sulphate distribution on the alluvial fans and on adjacent hilltops indicates an aeolian origin evidence from the pre cordillera fan b however suggests that salt input may have been episodic related to changes in conditions within the general aridity and possibly to stone pavement and soil development calcium sulphate sources range from an input in andean precipitation and the camanchaca to abundant regional evaporite deposits it is probable however that products of andean volcanism constitute the dominant primary source of calcium sulphate and that the salt has subsequently been distributed widely within the endoreic basins of the region by a combination of groundwater surface flow and the wind 1997 john wiley sons ltd

the role of topography and surface cover upon soil formation along hillslopes in arid climates abstract two north facing soil toposequences were selected from within the northern negev desert israel where average annual rainfall ranges from 70 to 200 mm both slopes are composed of an upper rocky and a lower colluvial section similar trends were found along both slopes a high salt content was characteristic of soils at the top of the slope salinity decreased downslope within the rocky slope section the opposite occurred along the colluvial slopes with salinity increasing sharply downslope at any location along the slopes the northernmost soil toposequence site 160 mm average annual rainfall represents from a pedological point of view an environment which is far more arid than its climatologically drier more southern counterpart the explanation provided for the variation of soil proporties at the scale of single hillslopes and at the regional scale is the same it is contended that water input into the soil and therefore leaching intensity is positively related to the ratio of bedrock soil cover rocky areas have limited infiltration thus yielding high runoff rates into adjoining soil covered areas and contribute to water concentration deeper infiltration and leaching intensity soil or sediment covered areas having relatively high absorption capacities will experience reduced runoff shallow infiltration and decreased water availability for leaching this leads over time to salt accumulation at a shallow depth the decrease in rock soil ratio downslope within the colluvium is therefore held responsible for the corresponding increase in salinity similarly the greater salinity of the soils in the northern site is explained by the fact that its rock soil ratio is lower than in the southern area the theoretical and practical implications regarding the relationship between climatic change and landscape evolution in arid areas are briefly discussed