formation and stability of transverse and longitudinal sand dunes the shape of dunes depends on the history of wind regimes and sand availability in deserts exposed to winds from two different directions but with comparable magnitude dunes are found to be linear ridges which are either perpendicular or parallel to the mean wind direction depending on the angle between the two wind directions these dunes respectively observed for small and large angles between winds are called transverse and longitudinal dunes in both cases their large width hundreds of meters and evolution time scale years strongly limit the investigation of their dynamics and thus our understanding of such structures here we show that under water similar structures can be obtained but at much smaller space and time scales performing controlled experiments together with numerical simulations we highlight the physical mechanisms at play in the formation and long term evolution of these structures we show in particular that while longitudinal dunes are stable and extend in time transverse dunes are unstable they evolve into wavy ridges and eventually break into barchans if the sand supply is too low this fundamental difference is understood through the study of single sand piles and bars exposed to two winds in the case of a large angle between winds a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune such an elongation does not occur for a small angle where a sand pile evolves into a barchan these results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field

planetary dune analogues a special issue on integrating models remote sensing and field data this article reports on the special issue of earth surface processes and landforms dedicated to planetary dune analogues and the advances that have been made through the integration of remote sensing and terrestrial field data to better model and understand aeolian systems the 11 articles presented demonstrate a spectrum of analytical techniques that are employed to investigate ripples dunes draa and ergs across a variety of spatial and temporal scales nine of the papers involve innovative research about aeolian bedforms and processes while the final two papers are examples of how vast quantities of data may be organized and scrutinized using a giscience approach based on these 11 papers and the many abstracts archived in a series of three planetary dunes workshops at the lunar planetary institute http www lpi usra edu publications abstracts shtml lpi contributions 1403 1552 and 1673 it is evident that there exists diverse and complementary approaches to the comparative study of landforms and processes for terrestrial and extra terrestrial aeolian landscapes that is driven by a vibrant international community of scientists and educators that engage in the cross fertilization of ideas copyright 2014 john wiley sons ltd

how do bedform patterns arise new views on the role of bedform interactions within a set of boundary conditions one explanation for bedform patterns is self organization in which the pattern emerges because of interactions among the bedforms themselves models remote images field studies and lab experiments have identified bedform interactions that involve whole bedforms only bedform defects or that are remote interactions between bedforms it is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development constructive interactions including merging lateral linking cannibalization and remote transfer of sediment push the system toward fewer larger more widely spaced bedforms regenerative interactions including bedform splitting defect creation and calving push the system back toward a more initial state other interactions including off center collision defect migration and bedform and defect repulsion cause pattern change but may not be strongly constructive or regenerative although bedform interactions are ubiquitous to any field of bedforms their dynamics flow field modification and impact upon measurable pattern parameters are yet poorly understood most bedform interactions span bedform types and fluids supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform flow and grain fluid interactions bedform interactions alone however cannot account for the rich diversity of bedform patterns in nature it is proposed that field diversity arises because of boundary conditions which are the environmental variables within which a field evolves conceptually boundary conditions modify the shape of the attractor toward which a field evolves possibly by altering the type and frequency of bedform interactions boundary conditions are broadly similar within system types but are unique for each bedform field so that no two are ever exactly alike although aeolian and fluvial systems share some types of boundary conditions flow depth is a unique boundary condition in shallow fluvial systems copyright 2009 john wiley sons ltd

equilibrium versus disequilibrium of barchan dunes abstract barchans are crescentic dunes which occur in mainly mono directional winds shape aspect ratios and velocities of these dunes have been studied as if they were in equilibrium however following a study of the shape and migration of 11 barchans of different sizes for 18 months in the field on moroccan atlantic sahara we show that they only appear to be in a stationary state if studied over a long timeframe at the scale of the year or several years but are never in equilibrium at the scale of weeks or months rather they are always trying to reach a possible equilibrium state but never have enough time to accomplish this this may be the main reason for the large variation observed in previous measurements and justifies some caution in what can be deduced from them

comparative methods for distinguishing flakes from geofacts a case study from the wenas creek mammoth site archaeologists have long struggled with distinguishing lithic materials modified by humans artifacts from natural objects e g geofacts or zoofacts this problem is especially difficult for finds of small numbers of flake like lithic specimens and particularly for very old finds we attempt to address the artifact versus geofact problem at a paleontology site by employing three systematic and objective tests on the two recovered possible artifacts first they are compared with debitage attributes typically expected of artifacts and geofacts based on published experimental and actualistic data second they are compared in terms of nine of these attributes with a toolstone sample from the site excavation matrix third the two possible artifacts are scored for these nine attributes and graphed against the toolstone matrix sample and two samples of flintknapped debitage assemblages in all three comparisons the two specimens are more like artifacts than geofacts while this does not prove the specimens are artifacts it at least shows they cannot be easily dismissed as the sort of geofacts typically expected in the site matrix we argue that this distinction is an important first step in the evaluation of possible lithic artifacts

latest pleistocene and holocene dune construction at the north eastern edge of the rub al khali united arab emirates abstract optical dating of two dune pro les developed in linear dunes in ras alkhaimah united arab emirates has been used to establish the timing and rateof dune accretion one section at awa was over 17 m high while that at idhnwas over 40 m high the awa dune appears to have accumulated very rapidly 10 000 years ago with a vertical accumulation rate of about 3ae3mka 1 theidhn dune appears to have accumulated over the past 1000 years with 20 m ofsediment accumulating in a time period of about 270 years the awa dunemay have accumulated in response to the transgression of the persian gulf byrising sea levels in late pleistocene and holocene times the idhn dune mayhave accumulated rapidly because of intensi ed human activity a short livedclimatic event or because it may periodically be reactivated after erosion by uvial action at its base keywords linear dune optical dating rub al khali sand accumulation introductionthere remains some uncertainty surrounding thestyle and timing of deposition of large aeoliandunes with authors supporting both gradual andpunctuated models of dune development for adiscussion see thomas 1997 this divergence ofopinion results from both the general absence ofdating control within dunes in most studies anddif culties in gaining exposures within some ofthe larger dune bedforms the purpose of thispaper is to establish a chronology of dunedevelopment for two thick 10 m dune sectionsexcavated within large complex linear dunesfrom within the rub al khali near ras alkhaimah in the north of the united arabemirates fig 1 the sites were selected becauseof the availability of thick fresh sections inrecently excavated moist sand with an area of around 600 000 km

dune field development interactions and boundary conditions for crescentic and stellate megadunes of the al liwa basin the empty quarter within the greater ar rub al khali empty quarter sand sea lies an internal depocentre the al liwa basin which comprises a variety of mega scale dune types crescentic dunes dominant the north of the basin while megadunes of stellate or star form are a major landform of the south eastern reaches their development into dune fields is determined by the style and rate of dune dune interactions the boundary conditions imposed by a multi modal wind regime fluctuating groundwater levels and sediment availability under an assortment of climatic conditions throughout the quaternary as a result dune field patterns are a collective response to these perturbations in space time and environment the r statistic is a collective measure of these responses and is a metric capable of identifying the degree of pattern maturity or self organization of the aeolian system and the pathways from which patterns evolve the spatial signature of the southerly located star dunes is characterized by two definitive patterns of organization the first one of complete spatial randomness the second a low degree of spatial uniformity in isolation these results appear to be unrelated to those for crescentic dunes of the region in which a significantly higher degree of pattern dispersion is the norm however when spatial statistical measures are integrated with the theoretical understanding of dune dune interactions and the involvement of environmental agents the complex morphodynamic pathways and linkages between regional dune fields is better understood in this case both constructive e g merging lateral linking and regenerative activity e g calving have played important roles in the development of dune size and associated adjustments in spacing and dune numbers and subsequently dune field patterns synergetic patterns are emblematic of this vast dunescape whereby transitional geographic morphologic dimensional and environmental modifications exist between the mega crescentic and mega stellate dunes of the empty quarter copyright 2012 john wiley sons ltd

factors determining desert dune type while most observers recognize four elemental types of desert dunes longitudinal transverse barchan and star1 3 there is little agreement about which factors determine these types the angular relationships between the resultant of sand shifting winds and both the crest and principal slipfaces of the elemental types have been discussed qualitatively for many decades these relationships have been quantified but the wide range of dune types that combine the elemental forms and the juxtaposition of different dune types show that wind regime is not the only determinant for example sand availability has been considered important we have now quantified the importance of both wind regime and sand availability as determinants of dune type we show here that the four elemental types occur in areas uniquely defined by both the equivalent sand thickness of sediments held in the dunes and a measure of directional variability of sand moving winds wind strength does not appear to be important vegetation has an ambiguous role and the particle size of dune sediments is unimportant

event stratigraphy paleoenvironment and chronology of se arabian deserts the geological record of the se arabian desert is exhibited in a variety of geomorphic features and their characteristic sediments ranging from alluvial fans to inland and coastal dunes and sabkhas together they suggest a large range of environmental and climatic variability between periods of relative aridity and humidity sedimentary events in the emirates appear to correlate broadly with high latitude glaciations which affected both the shamal winds and the extent of exposure of the arabian gulf in general the aeolian sands in the rub al khali responded to glacial events as a consequence of exposure of the floor of the arabian gulf during glacials when global sea level was low quartz sands from across the exposed floor of the arabian gulf were transported by the shamal winds south eastwards to the emirates and then south to the rub al khali the supply of shamal transported sand from the floor of the arabian gulf was cut off by high stands of sea level during interglacial periods the new coastal dunes miliolite rich in foraminifera and other cement assisting calcareous shell fragments were deflated down to the water table as their sand was transported further to the south leading to the creation of coastal sabkhas studies on aeolianites and the lacustrine deposits provide evidence of phases of enhanced humidity prior to the last glacial maximum and during the holocene in contrast the bedding attitudes of miliolites in se oman suggest that the wahiba sands were transported from south to north by a branch of the sw monsoon the depositional ages of these dunes indicate a close connection between the winds and coeval fluctuations of the sw monsoon the interdunal lakes and underlying dune sands reflect an active sw monsoon during the holocene this is also indicated by the aeolian and lacustrine records of the thar desert in india which likewise are controlled by the sw monsoon south and west of the oman mountains is a broad alluvial fan bajada that is mostly inactive today the oldest fluvial sediments are undated but preliminary age estimates suggest that some correlate with more humid earlier interglacials

spatial self organization in geomorphology from periodic bedforms and patterned ground to scale invariant topography a rich diversity of spatial order can be discerned in landforms and small scale features of landscapes with spatial order ranging in scale from 107 m to 10 2 m and in geometry from pronounced two dimensional periodicity to three dimensional scale invariance ripples of wind blown sand with their strikingly uniform spacing and mountainous topography with its fractal characteristics illustrate the range of geomorphic self organization patterns on the land surface are often very distinct easily observed and readily studied because of their accessibility and yet the systematic quantitative study of geomorphic self organization is only in its infancy it offers difficult challenges but also exceptional opportunities particularly for those versed in modern techniques used in studies of self organization in this overview diverse examples of spontaneous organization in geomorphic systems and recent pertinent studies are described starting with spatial structures that are generally most distinct and best understood they include 1 periodic structures that form on various substrates as water or air transports erodes and deposits sediments ripples and dunes meanders and bar pool topography of rivers and rhythmic shoreline features 2 polygonal patterns that result either from the growth cracks in such contrasting media as freezing lava and frozen ground or from freeze thaw alternations in soils 3 large scale periodic structures dictated by the deformation of magma or of the earth s crust linear arrays of evenly spaced volcanoes and sequences of alternating valleys and mountain ranges and 4 scale invariant structures revealed in drainage networks and topography

heterogeneous rare earth element ree patterns and concentrations in a fossil bone implications for the use of ree in vertebrate taphonomy and fossilization history abstract a bone fragment cgdq 3 of falcarius utahensis a therizinosaur from the early cretaceous cedar mountain formation utah contained within a carbonate nodule was analyzed by laser ablation inductively coupled plasma mass spectrometry la icp ms in order to investigate ree variability within a thin walled phalanx previous studies have found depth related ree pattern variations however in cgdq 3 variation occurs along the circumference of the bone nasc normalized ree patterns and concentrations vary between two apparent end members a light ree enriched lree pattern similar to solution icp ms analysis of this bone characterizes approximately two thirds of the bone fragment total ree concentrations are high and do not vary significantly from the periosteal surface to the medullar surface conversely the remaining one third of the bone has ree patterns that are mree depleted and low in total ree concentrations ree concentrations in this part of the bone do not vary significantly from the periosteal to the medullar surface a positive ce anomaly is found throughout the entire bone and is greatest within the lree enriched portion of the bone this in combination with the lree enrichment suggests that the bone fossilized under reducing conditions the distance between the lree enriched and mree depleted regions is less than 1 mm isotopic and petrographic analyses of the bone and surrounding carbonate matrix suggest the ree patterns in the bone were the result of partial fossilization incomplete filling of micro pore spaces around bone crystallites in an environment with changing redox conditions the lower mree depleted part of the bone fossilized contemporaneously with a pendant cement that formed on the underside of the bone in the vadose zone formation of the pendant cement restricted water flow through the bone isolating the lower portion which incorporated a mree depleted pattern the upper part of the bone lree enriched side fossilized under more reducing conditions than the lower part this resulted in reductive dissolution of minerals such as hydrous ferric oxides and manganese oxides which were lree enriched with positive ce anomalies these ree were then incorporated into the upper part of the bone this likely occurred after the bone entered the saturated zone below the water table the thinness of the bone and presence of a pendant cement helped facilitate partial fossilization of the bone preserving the ree signature of an earlier fluid as demonstrated by this case study differences in ree patterns within a fossil may record changes in geochemical conditions during fossilization particularly when bones are encased in a material that reduces fluid permeability such as micritic vadose calcite analysis of such bones may offer the opportunity to decipher complex fluid histories that occur during fossilization la icp ms should be utilized to confirm solution icp ms analysis if it yields high ree variability prior to an interpretation of reworking or time averaging

seismic ground roll absorption and reemission by sand dunes this work was performed at schlumberger cambridge research where m i arran was a case student intern and e muyzert is a senior research scientist

the last dicynodont re assessing the taxonomic and temporal relationships of a contentious australian fossil abstract dicynodonts a lineage of non mammalian therapsids who s derived taxa evolved edentulous beaked jaws sporting a pair of caniniform tusks dominated the herbivorous terrestrial vertebrate fauna for much of the permian and triassic periods long assumed to have met their demise during the end triassic extinction event the discovery of a fragmentary possible dicynodont in cretaceous rocks in queensland australia potentially extended the longevity of the lineage by nearly 100 million years this study reassesses the geological anatomical and historical aspects of this specimen through museum archival research detrital zircon geochronology trace element analysis and x ray synchrotron microtomography and present new knowledge regarding its temporal geographical and biological origins supporting a late cenozoic pliocene pleistocene mammalian megafaunal affinity for the specimen resulting in a lack of evidence for post triassic survival of dicynodonts

barchan shaped ripple marks in a wave flume barchans isolated crescent shaped bedforms are believed to be formed under almost unidirectional wind or water ows and limited sand supply the formation of barchan morphologies under the action of purely oscillatory wave motion has not yet been fully investigated the present study attempted to form barchan topography in a wave ume and to compare this with barchans in the eld barchan morphologies of ripple size called the barchan ripples were generated from a at bed by the action of waves the horn width the distance between horn tips of the barchan ripples increased linearly with an increase in the total length the overall length projected on the centre line of the barchan with a coef cient common to barchan dunes in deserts the ratio of horn length to horn width of the barchan ripples was smaller than that of barchan dunes but similar to that of subaqueous barchans in the eld the longer the wave period was the larger the ratio of the body length to horn width became most subaqueous barchans formed under waves in the laboratory and unidirectional ows in the eld had blunter horns than subaerial barchans the shape of the barchan ripples changed with wave period the outer rim became rounder with increasing wave period the relationship between the base area and the height of barchan morphologies seems to be linear with a constant coef cient for the scale from ripples to dunes the barchan ripples showed a linear relationship between the height and the horn width similar to that for barchan dunes the migration speed of the barchan ripples was proportional to the cube of the ow velocity and was inversely proportional to height the same relation with a different value of the coef cient was obtained for barchan dunes copyright 2004 john wiley sons ltd

late pleistocene and holocene paleoenvironments of chalco lake central mexico abstract in order to establish paleoenvironmental conditions during the late quaternary four cores from the basin of mexico central mexico were drilled in chalco lake located in the southeastern part of the basin the upper 8 m of two parallel cores were studied using paleomagnetic loss on ignition pollen and diatom analyses based on 11 14c ages the analyzed record spans the last 19 000 14c yr b p volcanic activity has affected microfossil abundances both directly and indirectly resulting in absence or reduction of pollen and diatom assemblages important volcanic activity took place between 19 000 and 15 000 yr b p when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry cold climate during this interval abrupt environmental changes with increasing moisture occurred from 15 000 until 12 500 yr b p the lake level increased and the pollen indicates wetter conditions the highest lake level is registered from 12 500 to ca 9000 yr b p the end of the pleistocene is characterized by an increase in humidity from 9000 until ca 3000 yr b p chalco lake was a saline marsh and the pollen record indicates warmer conditions after 3000 yr b p the lake level increased and human disturbance dominates the lacustrine record

the last glacial maximum in the basin of mexico the diatom record between 34 000 and 15 000 years bp from lake chalco lacustrine sediments from a section of a core extracted from the central part of the former lake chalco were analysed for diatom content radiocarbon dates and tephra correlations bracket the period under study between 34 and 15 ka bp according to the diatom stratigraphy chalco was an alkaline saline lake during the interval 34 31 5 ka bp after 31 5 ka bp it went through a shallower and more concentrated phase until ca 23 5 ka bp at ca 23 5 ka bp a major environmental change took place during which lake chalco was transformed from an saline alkaline pond into a freshwater lake previous archaeological work at tlapaocya indicates that humans entered the basin slightly after this environmental change by ca 22 ka bp at 19 ka bp another reduction in water level occurred with the onset of a period of quick fluctuations between slightly acidic and neutral water these variations were probably associated with intensified volcanic activity that affected the area between 19 and 15 ka bp

the triassic timescale based on nonmarine tetrapod biostratigraphy and biochronology abstract the triassic timescale based on nonmarine tetrapod biostratigraphy and biochronology divides triassic time into eight land vertebrate faunachrons lvfs with boundaries defined by the first appearance datums fads of tetrapod genera or in two cases the fads of a tetrapod species definition and characterization of these lvfs is updated here as follows the beginning of the lootsbergian lvf fad of lystrosaurus the beginning of the nonesian fad cynognathus the beginning of the perovkan lvf fad eocyclotosaurus the beginning of the berdyankian lvf fad mastodonsaurus giganteus the beginning of the otischalkian lvf fad parasuchus the beginning of the adamanian lvf fad rutiodon the beginning of the revueltian lvf fad typothorax coccinarum and the beginning of the apachean lvf fad redondasaurus the end of the apachean beginning of the wasonian lvf near the beginning of the jurassic is the fad of the crocodylomorph protosuchus the early triassic tetrapod lvfs lootsbergian and nonesian have characteristic tetrapod assemblages in the karoo basin of south africa the lystrosaurus assemblage zone and the lower two thirds of the cynognathus assemblage zone respectively the middle triassic lvfs perovkan and berdyankian have characteristic assemblages from the russian ural foreland basin the tetrapod assemblages of the donguz and the bukobay svitas respectively the late triassic lvfs otischalkian adamanian revueltian and apachean have characteristic assemblages in the chinle basin of the western usa the tetrapod assemblages of the colorado city formation of texas blue mesa member of the petrified forest formation in arizona and bull canyon and redonda formations in new mexico since the triassic lvfs were introduced several subdivisions have been proposed lootsbergian can be divided into three sub lvfs nonesian into two adamanian into two and revueltian into three however successful inter regional correlation of most of these sub lvfs remains to be demonstrated occasional records of nonmarine triassic tetrapods in marine strata palynostratigraphy conchostracan biostratigraphy magnetostratigraphy and radioisotopic ages provide some basis for correlation of the lvfs to the standard global chronostratigraphic scale these data indicate that lootsbergian uppermost changshingian induan and possibly earliest olenekian nonesian much of the olenekian perovkan most of the anisian berdyankian latest anisian and ladinian otischalkian early to late carnian adamanian most of the late carnian revueltian early middle norian and apachean late norian rhaetian the triassic timescale based on tetrapod biostratigraphy and biochronology remains a robust tool for the correlation of nonmarine triassic tetrapod assemblages independent of the marine timescale

rare earth element systematics of fossil bone revealed by la icpms analysis intra bone rare earth element ree fractionation trends were studied by la icpms analysis to put constraints on 1 the mechanisms controlling ree fractionation within fossil bones 2 the relative timing of ree uptake in various parts of fossil bone and 3 the origin of ree in fossil bones we have evaluated ree bone profiles across 54 fossil bones from a broad range of well characterised taphonomic settings ranging in age from triassic to early medieval ree patterns and concentration gradients are highly variable and intra bone fractionation trends in la yb n vs la sm n space of few specimens cover almost the entire range previously observed for bulk samples intra bone variability of ce anomalies as well as variable y ho and la la n is also observed sometimes diagenetic fluids with fractionated hree enriched compositions have entered the bone from the marrow cavity producing secondary ree uptake profiles theoretical intra bone fractionation trends modelled using lattice strain theory concur with most ree data but some trends cannot be modelled using realistic boundary conditions this fact as well as the occurence of positive and negative ce anomalies within the same fossil sample hint towards changing ree compositions of diagenetic fluids over the timescales of ree uptake because apparent ce anomalies frequently evolve over bone profiles and ce anomalies can be inherited from previous fractionation events ce anomalies are a highly ambiguous tracer for constraining ambient redox conditions in general bioapatite ree signatures may not always reliably reflect ambient taphonomic or redox conditions and diagenetic fluid compositions therefore ree patterns and ce anomalies of fossil bones must be interpreted cautiously as they vary spatially within skeletal remains and are affected by intra bone fractionation processes as well as changing compositions of the diagenetic fluid

barchan dune corridors field characterization and investigation of control parameters 1 the structure of the barchan field located between tarfaya and laayoune atlantic sahara morocco is quantitatively investigated and compared to that in la pampa de la joya arequipa peru on the basis of field measurements we show how the volume the velocity and the output sand flux of a dune can be computed from the value of its body and horn widths the dune size distribution is obtained from the analysis of aerial photographs it shows that these fields are in a statistically homogeneous state along the wind direction and present a corridor structure in the transverse direction in which the dunes have a rather well selected size investigating the possible external parameters controlling these corridors we demonstrate that none among topography granulometry wind and sand flux is relevant we finally discuss the dynamical processes at work in these fields collisions and wind fluctuations and investigate the way they could regulate the size of the dunes furthermore we show that the overall sand flux transported by a dune field is smaller than the maximum transport that could be reached in the absence of dunes i e in saltation over the solid ground

case for periodic colossal jokulhlaups from pleistocene glacial lake missoula two classes of field evidence firmly establish that late wisconsin glacial lake missoula drained periodically as scores of colossal jokulhlaups glacier outburst floods 1 more than 40 successive flood laid sand to silt graded rhythmites accumulated in back flooded valleys in southern washington hiatuses are indicated between flood laid rhythmites by loess and volcanic ash beds disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest slopes gentle and time between floods short 2 in several newly analyzed deposits of pleistocene glacial lakes in northern idaho and washington lake beds comprising 20 to 55 varves average 30 40 overlie each successive bed of missoula flood sediment these and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood they dispel the notion that the prodigious floods numbered only a few the only outlet of the 2 500 km 3 glacial lake missoula was through its great ice dam and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it like grimsvotn iceland lake missoula remained sealed as long as any segment of the glacial dam remained grounded when the lake rose to a critical level 600 m in depth the glacier bed at the seal became buoyant initiating underflow from the lake subglacial tunnels then grew exponentially leading to catastrophic discharge calculations of the water budget for the lake basin including input from the cordilleran ice sheet suggest that the lakes filled every three to seven decades the hydrostatic prerequisites for a jokulhlaup were thus re established scores of times during the 2 000 to 2 500 yr episode of last glacial damming j harlen bretz9s spokane flood outraged geologists six decades ago partly because it seemed to flaunt catastrophism the concept that lake missoula discharged regularly as jokulhlaups now accords bretz9s catastrophe with uniformitarian principles

tocuila mammoths basin of mexico late pleistocene early holocene stratigraphy and the geological context of the bone accumulation abstract we report new stratigraphic tephrochronology and dating results from the tocuila mammoth site in the basin of mexico at the site there is evidence for a thin meteorite airburst layer dated between 10 878 and 10 707 cal bc at the onset of the younger dryas yd cool period the upper toluca pumice utp tephra marker caused by a plinian eruption of the nevado de toluca volcano dated from 10 666 to 10 612 cal bc is above that layer the eruption must have caused widespread environmental disruption in the region with evidence of extensive reworking and channelling by the lake texcoco shoreline and contributed to the widespread death and or extinction of megafaunal populations as suggested by earlier authors but the new work reinforces the view that both catastrophic events must have caused large environmental disruption in a short time period of around two hundred years there is no evidence for megafauna mammoths sabre toothed cats camels bison glyptodonts after the utp volcanic event and subsequent lahars in the basin of mexico at tocuila although there are some in situ tephra markers in nearshore lake sediments such as the great basaltic ash gba and the utp ash there is evidence of much reworking of several tephra populations in various combinations the mammoth bone accumulation is reworked in a lahar sequence volcanic mudflow derived from several source sediments but associated with the major utp plinian eruption paleoindian populations were also present in the basin of mexico during the yd period where several paleoindian skeletons were found associated with the utp ash deposits e g metro man chimalhuacan man and tlapacoya man

nanodiamonds in the younger dryas boundary sediment layer we report abundant nanodiamonds in sediments dating to 12 9 0 1 thousand calendar years before the present at multiple locations across north america selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval cubic diamonds form under high temperature pressure regimes and n diamonds also require extraordinary conditions well outside the range of earth9s typical surficial processes but common to cosmic impacts n diamond concentrations range from 10 to 3700 parts per billion by weight comparable to amounts found in known impact layers these diamonds provide strong evidence for earth9s collision with a rare swarm of carbonaceous chondrites or comets at the onset of the younger dryas cool interval producing multiple airbursts and possible surface impacts with severe repercussions for plants animals and humans in north america

late pleistocene eolian history of the liwa region arabian peninsula the liwa region of the united arab emirates is one of the most distinctive geomorphological features of the rub al khali desert of southern arabia characterized by a sharp crescentic boundary coincident with a n s elevation drop of 65 m with the transverse ridges in al q fa to the north liwa is an area of some of the world9s largest megabarchan dunes deep drilling of continuous cores has provided an opportunity to observe the internal structure and age of these two ergs subsurface stratigraphy is complex reflecting rapid facies transitions between dune and interdune sub environments most of the cored sediment is dominated by cross bedded and structureless sand units of inferred eolian transverse dune depositional origin a total of 56 optical ages for sand sized quartz grains extracted from these cores provide a chronological framework for deposition of the late quaternary ergs a marine isotope stage mis 5 erg is identified north of the liwa crescent in the al q fa region which reaches vertical thicknesses 100 m a mis 1 erg in the form of megabarchan dunes is recorded south of the liwa crescent this was deposited on a pre mis 5 land surface since ca 6 ka over a period of just a few thousand years contemporary bypassing of eolian sands via superimposed dunes in both al q fa and liwa appears to be in equilibrium with the current wind regime a consideration of a variety of factors that control the availability mobilization and preservation of eolian sediments and resulting bedforms leads us to infer that the system is not sediment supply or transport limited instead the system is preservation limited being controlled by a correlated combination of sea level and precipitation both of these factors are strongly linked to global climate variations in the eccentricity ca 100 ka band paradoxically the bulk of the preserved record of eolian activity in the southern arabian peninsula occurs within relatively humid interglacial phases rather than arid intervals evidence of eolian deposition during arid phases may not for the most part be preserved in large areas of the sand sea observed contrasts in the preserved record of eolian activity between al q fa and liwa and the wahiba sand sea may in part relate to contrasting eolian bedform morphology

chronologies for late quaternary barchan dune reactivation in the southeastern arabian peninsula abstract the liwa region of the united arab emirates contains some of the largest and most areally extensive megabarchanoid sand dunes on a global scale here we present optical dating results on samples of aeolian sediment from deep drill cores extracted from the largest dune field of the liwa area optical dating of these core sediments using the single aliquot regeneration protocol indicates mid late holocene phases of rapid dune deposition the most recent period of reactivation began at ca 2 8 ka this event was preceded by a period of deposition at ca 5 ka these results suggest that the dune systems of the southeastern arabian peninsula are closely linked to changes in late quaternary global climate particularly linked to the intensity and spatial extent of palaeomonsoon rainfall since the last precessional maxima at ca 9 ka at which time a peak in monsoonal rainfall has been recognised a significant environmental transition to widespread desert conditions occurred in an apparently abrupt fashion during the initial period of aridification large quantities of sand were transported and deposited in the form of large and very large up to 160 m high scale aeolian bedforms following the initial phase of aeolian accumulation the system appears to have remained in stasis

vein quartz in lithic traditions an analysis based on experimental archaeology abstract worldwide vein quartz was a commonly used raw material for stone tools but this material has proved difficult for archaeologists to analyse because many quartz assemblages appear to be comprised of amorphous pieces not easily recognised as humanly modified or forming tools this paper discusses the analysis of the debitage focusing on the debitage fragmentation rate the debitage break and fragment types and the quantitative analysis of the complete flakes resulting from experimental knapping of quartz which formed part of a project which investigated the use of quartz in irish prehistoric lithic traditions the results have highlighted the complexity involved in analysing quartz assemblages and the significant differences between the debitage products of quartz and chert knapping assemblages while bipolar knapping is generally easy to differentiate from direct percussion it is harder to differentiate between soft and hard hammer percussion

sedimentological characteristics and morphology of the aeolian sand dunes in the eastern part of the uae a case study from ar rub al khali abstract the umm az zimul al wijan area is located in the southeastern part of the uae and covers about 1600 km 2 it is bounded to the east by the oman mountains and to the south and west by the extensive dune area of ar rub al khali field investigation revealed the common occurrence of parallel to sub parallel sand ridges with a general east west orientation also present but less frequent are barchan barchanoid and star dunes interdune areas are classified into four types that include dolomite mixed ancient sabkha and ophiolitic rock fragments mixed sand sheet and ophiolitic rock fragments and recent sabkha the textural characteristics of the sand forming the dunes are unimodal with a modal class in fine sand size moderately to very well sorted positively skewed and mesokurtic quartz grains are mainly subrounded to subangular the similarity in roundness properties in different dune types in the eastern province of the uae seems not to be influenced by dune form based on the heavy mineral suites and the angularity of quartz grains it is believed that the sand sediments are mostly derived from the ultrabasic rocks forming the ophiolitic sequence of the oman mountains acidic rocks of iran and from the coastal area

pattern analysis of dune field parameters as a basic form of pattern analysis the parameters of dune spacing defect density crest orientation and crest length are measured from remote images and treated statistically for dunes at white sands in new mexico the algodones in california the agneitir in mauritania and the namib in namibia statistical populations are identified from frequency plots of dune spacing and crest length field scale calculations of defect density and rose diagrams of crest orientation single populations characterize simple dune fields white sands whereas multiple populations characterize compound complex dunes algodones namib and complex dune fields agneitir as time increases dune fields show an increase in dune spacing and crest length a decrease in defect density more tightly clustered crest orientation and a reduction in the variance associated with measurements of these parameters the results are consistent with models of dune fields as self organizing complex systems in which a characteristic pattern emerges as a function of constructional time because pattern evolution is a function of time it may be possible to use pattern analysis to augment current methods of age determination statistically defined populations can be used in geomorphic backstripping to unstack generations of simple patterns that give rise to complex patterns and to reconstruct each generation in terms of construction time and palaeo wind regime copyright 2006 john wiley sons ltd

a 160 000 year record of dune development and atmospheric circulation in southern arabia aeolian deposits in the wahiba sands sultanate of oman reveal patterns of atmospheric circulation over the past 160 000 years luminescence dating indicates a correlation of dune activity with periods of low global sea level and decreased monsoon intensity evidence from dune orientation and sedimentary structures shows exclusively northbound transport of sand during times of high latitude glaciation these results are in contrast to the current paleocirculation model that assumes an increase of northwesterly winds because of a southward shift of the intertropical convergence zone our results indicate that the circulation pattern during glacial times was comparable to that of the present

growth increments in gomphotherium tusks and implications for late miocene climate change in north america abstract changes in mammalian faunas in north america during the late miocene are thought to have been caused by the replacement of woodland habitats with grassland or steppe the proposed cause of this habitat shift is the transition from the relatively low seasonality early miocene climate to a more highly seasonal climate regime by the end of the miocene tusks which are highly modified incisor teeth of late miocene gomphotherium mammalia proboscidea were sectioned to document the nature of tusk growth in this genus and to test for patterns of seasonal growth both the dentin and enamel of gomphotherium tusks preserve incremental growth lines dentin has increments on three scales inferred to represent annual first order weekly second order and daily third order periodicities luminance and second order increment thickness profiles were measured on transverse thin sections of tusk dentin viewed at low magnification and the data were examined with bivariate plots and autocorrelation autocorrelograms of luminance data support the consistent identification and measurement of second order increments within and among the tusks sampled profiles of second order increment thickness in three tusks from the barstovian north america land mammal age nalma show no indication of seasonal patterns of growth one tusk from the late clarendonian and one from the early hemphillian nalma have patterns of growth consistent with an increase in seasonality during the late miocene an interpretation supported by autocorrelation of the increment thickness data from these tusks the growth patterns are consistent with the hypothesized changes in north american climate during the late miocene and are suggestive of an increase in aridity and development of a wet season tests of this hypothesis are suggested

geomorphology solitary wave behaviour of sand dunes colliding dunes appear to traverse through one another and emerge unscathed barchan sand dunes are highly mobile crescent shaped dunes that occur in areas where sand is sparse and the wind is unidirectional here we show mathematically how two such loose grained dunes are able to pass through one another while still preserving their shape the crucial parameters for this solitary wave behaviour which is consistent with field observations are the heights of the two colliding dunes

analytic solutions for fresh groundwater lenses floating on saline water under desert dunes the kunin van der veer legacy revisited abstract a fully saturated lens of steady fresh groundwater floating in a homogeneous and isotropic desert sandy aquifer is analytically studied based on a hydrological model by kunin and interface solution by van der veer a static saline groundwater is beneath the lens a phreatic surface of moving fresh water inside the lens is partially recharged either naturally or by managed aquifer recharge from the vadose zone and partially exfiltrates to it a spatially focused recharge and intensive evapotranspiration preserve a steady downward upward topology of fresh water motion in terms of the 1 d dupuit forchheimer approximation in a horizontal in lens saturated flow a boundary value problem bvp for an ode for the strack potential is solved the shape of the water table and based on the ghijben herzberg assumption the interface are found the total volume of the positive pore pressure water flowing within the lens is evaluated constant infiltration and evaporation rates as well as evaporation linearly decreasing with depth of the water table counted from the ground surface are considered the case of 2 d flow is tackled by the toth model a triangular analytic element approximates a half of the flow domain and consists of an isobaric side and two no flow sides conformal mapping of this triangle onto a reference plane and solution of the dirichlet bvp in a half plane deliver the distribution of infiltration exfiltration intensity along the water table total flow rate and locus of the hinge point a mathematically more cumbersome approximation of the flow domain assumes the water table to be a tilted straight line but the interface to be found as a free boundary solution of the corresponding bvp uses a curvilinear triangle in the hodograph plane

seasonal variation of crescentic dune morphology and morphometry strzelecki simpson desert australia the general absence of contemporary dune building in the australian arid zone finds the occurrence of crescentic dune genesis and evolution at gurra gurra waterhole a somewhat enigmatic event amongst prevalent inactive linear dunescapes crescentic dune construction is geographically restricted to areas of dominant unidirectional high windiness and minimal vegetation parameters that are generally uncommon in arid australia today the influence of seasonal multidirectional winds can see dune form undergo continual transition between quasi equilibrium and disequilibrium such dynamic antithesis is depicted in seasonal mathematical comparisons of the planimetric attributes length and width dimensional equilibrium is a transient feature at gurra gurra waterhole and not characteristic of this dunescape copyright 2001 john wiley sons ltd

nearest neighbor methods applied to dune field organization the coral pink sand dunes kane county utah usa dune fields have recently come to be recognized as self organizing systems that can be seen progressing from states of disorganization or randomness to uniformity dune systems can be highly sensitive to changes in factors such as climate and sediment transport that determine system state changes in climate and sediment state can take time to work their way through a dune system this in turn leads to spatial heterogeneity in dune field organization using the coral pink sand dunes in southern utah as a model this study tests nearest neighbor analysis adapted as a method to objectively identify and characterize differences in two dimensional dune patterns within a dune field and to identify changes in dune patterns over time reducing transverse and barchanoid dunes from linear to three point features in planar space emphasizes the clustering that occurs when dune lengths and wavelengths are more disorganized or random this clustering may be in response to a system perturbation such as an influx of sediment and is reflected in lower nearest neighbor index r values as the system adjusts to the perturbation and moves towards steady state dune length and spacing increase through migration and coalescing of smaller dunes the resulting higher r values reflect this move towards greater uniformity in dune pattern with the organizational states of dune systems recording feedback to changes in extrinsic climate and sediment factors nearest neighbor analysis provides a proxy measure of system stability

collision of barchan dunes as a mechanism of size regulation 1 barchans are propagating crescentic dunes that form in arid regions where strong unidirectional winds blow across a firm soil lightly covered with sand recently solitary barchan dunes have been shown to be unstable towards sand flux variation either growing to form mega dunes or shrinking and disappearing when perturbed from equilibrium however observations of large corridors of barchan dunes in the field suggest that these bedforms are stable over long periods of time since dunes migration rates are inversely proportional to their size barchans of different size must collide and these collisions may be crucial in maintaining the stability of dunes in nature here we first explain the unstable behavior of solitary barchans and then illustrate using down scaled physical experiments the qualitative dynamics of binary collisions a stability analysis inspired by these experiments suggests that collisions may indeed regulate the size of barchans migrating in a corridor by redistributing sand from large dunes to smaller ones

morphology spatial pattern and sediment of nitraria tangutorum nebkhas in barchans interdune areas at the southeast margin of the badain jaran desert china abstract to understand the characteristics of the nebkhas in barchan interdune areas isolated barchan dunes at the southeast margin of the badain jaran desert in china and nitraria tangutorun nebkhas in the interdune areas were selected and the morphometric parameters spatial patterns and granulometric characteristics of the nebkhas in various interdune zones were compared according to the locations relative to barchan dunes the interdune areas were divided into three zones the windward interdune zone zw the leeward interdune zone zl and the horn interdune zone zh the zone that is proximal to barchan dunes and has never been disturbed by barchan dunes was also selected zi the morphometric parameters were measured through a satellite image and field investigation the population density and spatial patterns were analyzed using the satellite image and surface sediment samples of the nebkhas and barchan dunes were collected for grain size analysis the morphometric parameters of nitraria tangutorun nebkhas in the interdune zones differ significantly the nebkhas in zh are larger than those observed in the other zones and the nebkhas are the smallest in zl in all of the zones the long axis orientation of the nebkhas is perpendicular to the prevailing wind direction the population density of the nebkhas in zw is relatively higher whereas the density in zh and zl becomes obviously lower the spatial distribution of nebkhas in all of the zones can be categorized as a dispersed pattern the sediments of the nebkhas are coarsest in zh and finest in zl in addition the sediments of the nebkhas in all of the zones are finer than those of barchan dunes the amount of sand captured by the nebkhas in the interdune areas is approximately 20 of the volume of barchan dunes the variations of the nebkhas sizes spatial pattern and sediment are subjected to migration flow field and sand transport of barchan dunes and sand accumulation with plant growth in the interdune areas which suggest complex mutual interactions between barchan dunes and the nebkhas in the interdune areas

complex systems in aeolian geomorphology aeolian geomorphology provides a rich ground for investigating earth surface processes and landforms as complex systems sand transport by wind is a classic dissipative process withnon linear dynamics while dunefield evolution is a prototypical self organisation phenomenon bothofthesebroadareasofaeoliangeomorphologyarediscussedandanalysedinthecontextofcomplexityandasystems approach afeedbackloopanalysisoftheaeolianboundary layer flow sediment transport bedforminteractions basedoncontemporary physicalmodels revealsthatthesystemisfundamentallyunstable oratmostmeta stable andlikelytoexhibitchaoticbehaviour recent field experimental research on aeolian streamers and spatio temporal transport patterns however indicates that sand transport by wind may be wholly controlled by a self similar turbulence cascade in the boundary layer flow and that key aspects of transport event timeseriescanbefullyreproducedfromacombinationof self organised 1 fforcing motionthreshold andsaltationinertia theevolutionof various types of bare sand dunes and dune field patterns have been simulated successfully with self organising cellular automata that incorporateonlysimplifiedphysically basedinteractions rules becauseoftheirundefinedphysicalscale however itnotclearwhether they in fact simulate ripples bedforms or dunes landforms raising fundamental cross cutting questions regarding the difference between aeolian dunes impact ripples and subaqueous current ripples and dunes an extended cellular automaton ca model currently under development incorporates the effects of vegetation in the aeolian environment and is capable of simulating the development of nebkhas blow outs and parabolic coastal dunes preliminary results indicate the potential for establishing phase diagrams and attractor trajectories for vegetated aeolian dunescapes progress is limited however by a serious lack of appropriate concepts for quantifying meaningful state variables at the landscape scale state variables currently used in the bare sand models are far from capturing the rich 3d topography and patterns and are not sufficiently discriminative to distinguish different attractors the vegetationcomponentintheextendedmodel andindeedecogeomorphicsystemsingeneral poseevengraverchallengestoestablishing

the shape of the barchan dunes of southern morocco we present detailed shape measurements of several barchan dunes in southern morocco near la youne using these data we disprove the concept of shape invariance of barchan dunes of different sizes nevertheless some parts of the barchan dune scale and we try to distinguish these from non scaling ones furthermore we point out the importance of the exact position of the brink for the overall dune shape finally we propose a simple model for dune shape using our data to determine the model parameters despite its simplicity this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described

the nature and possible origin of mega dunes in liwa ar rub al khali uae abstract the liwa area is located in the south central part of the uae along the northeastern upwind margin of the ar rub al khali sand sea and covers about 3694 km 2 it has one type of dunes a mega barchan that is compound and complex in form characterized by the superimposition of smaller dunes due to the giant size of barchans interdune areas appear as closed depressions that vary in shape from irregular elongate triangular to circular or semi circular these depressions are mostly covered with small barchans or sabkha deposits with patches of gypcrete in their upper parts scattered flat top mounds are also observed in interdune areas the size of dunes in liwa is directly controlled by quaternary climatic changes multiple generations are superimposed on one another separated by calcretic horizons the climate during dune formation alternated between relatively arid to humid the dune sands are unimodal with a modal class of fine sand moderately to very well sorted and near symmetrical and mesokurtic quartz grains are mainly subrounded to subangular based on the heavy mineral suites the sand came mostly from the ultrabasic rocks that form the ophiolitic sequence of the oman mountains the acidic rocks of iran and from the arabian shield

evidence for deposition of 10 million tonnes of impact spherules across four continents 12 800 y ago airbursts impacts by a fragmented comet or asteroid have been proposed at the younger dryas onset 12 80 0 15 ka based on identification of an assemblage of impact related proxies including microspherules nanodiamonds and iridium distributed across four continents at the younger dryas boundary ydb spherule peaks have been independently confirmed in eight studies but unconfirmed in two others resulting in continued dispute about their occurrence distribution and origin to further address this dispute and better identify ydb spherules we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry morphologies origins and processes of formation we investigated 18 sites across north america europe and the middle east performing nearly 700 analyses on spherules using energy dispersive x ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization twelve locations rank among the world s premier end pleistocene archaeological sites where the ydb marks a hiatus in human occupation or major changes in site use our results are consistent with melting of sediments to temperatures 2 200 c by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere they are inconsistent with volcanic cosmic anthropogenic lightning or authigenic sources we also produced spherules from wood in the laboratory at 1 730 c indicating that impact related incineration of biomass may have contributed to spherule production at 12 8 ka an estimated 10 million tonnes of spherules were distributed across 50 million square kilometers similar to well known impact strewnfields and consistent with a major cosmic impact event

evidence from central mexico supporting the younger dryas extraterrestrial impact hypothesis we report the discovery in lake cuitzeo in central mexico of a black carbon rich lacustrine layer containing nanodiamonds microspherules and other unusual materials that date to the early younger dryas and are interpreted to result from an extraterrestrial impact these proxies were found in a 27 m long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial our attention focused early on an anomalous 10 cm thick carbon rich layer at a depth of 2 8 m that dates to 12 9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences collectively these changes have produced the most distinctive boundary layer in the late quaternary record this layer contains a diverse abundant assemblage of impact related markers including nanodiamonds carbon spherules and magnetic spherules with rapid melting quenching textures all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond n diamond i carbon and hexagonal nanodiamond lonsdaleite in order of estimated relative abundance this nanodiamond rich layer is consistent with the younger dryas boundary layer found at numerous sites across north america greenland and western europe we have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism it is however consistent with the younger dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst s and and or ground impact s at 12 9 ka

higher order neighbor analysis of the tartarus colles cone groups mars the application of geographical indices to the understanding of cone pattern evolution cone fields of ambivalent origin exist in the elysium amazonis cerberus isidis acidalia and cydonia regions of mars considerable interest exists in regard to their location and origin as their occurrence in many instances is most likely associated with near surface ground ice the presence of which has consequences for past and present climate astrobiological activity and future exploration this research has outlined an understanding of both spatial and geomorphic processes of cone development for a region within the tartarus colles mars as well as the manner in which spatial statistical indices can be affected by modification to the areal unit of study this study found that cone groups for the region of the tartarus colles are completely spatially random csr across all distances extending to their sixth order neighbor however significant statistical indices of aggregation were found for groups where post emplacement erosional modification may have occurred by solifluction like mechanisms or magmatic incursion into or over frozen ground such circumstances imply that r indices reflect both the underlying spatial and geologic processes of cone formation and also an overprint of erosional modification of the cone field within a glacial or periglacial setting in this instance the comparison of spatial indices can therefore offer a viewpoint from which geomorphic change can be recognized and quantitatively evaluated for cone fields

lithic taphonomy in desert environments contributions from fuego patagonia southern south america abstract this paper focuses on the contributions of taphonomic approaches to the study of lithic assemblages based on actualistic and archaeological case studies from fuego patagonia south america here i summarize some of the main taphonomic issues identified for the regional lithic record which are of relevance for the archaeological research in other desert environments specifically distributional patterns in lithic assemblages produced by wind are highlighted and main guidelines for pseudoartifact study are presented and discussed actualistic data show lithic artifacts up to 50 mm in size or weighting 13 g can be moved by winds blowing at 90 km h finally a methodological exercise banyai s zoom is proposed for lithic mimic recognition

tepexpan palaeoindian site basin of mexico multi proxy evidence for environmental change during the late pleistocene late holocene the tepexpan palaeoindian skeleton was discovered in 1947 close to the former lake texcoco margin in the basin of mexico the find has been the object of considerable interest and discussion over the last 60 years regarding its real age and archaeological interpretation here we report new ams radiocarbon dates associated with the sedimentary succession at tepexpan with ages between 19 110 90 and 612 22 14c years bp and a new uranium series date for the skeleton with an age of 4700 200 years bp that indicates a mid holocene age the sedimentary succession was studied in detail using stable isotopes diatoms organic geochemistry and tephrochronology the multi proxy evidence suggests large changes around the margins of lake texcoco in terms of the balance between aquatic and terrestrial plants c3 and c4 plants saline alkaline and freshwater conditions volcanic activity marginal reworking of lake sediments and input from the drainage basin through the late pleistocene late holocene these changes had large impacts on the prehistoric human populations living by the lake shores since the late pleistocene in the basin of mexico

late quaternary chronology of major dune ridge development in the northeast rub al khali united arab emirates the northeastern sector of the rub al khali desert in the eastern united arab emirates uae is dominated by large ne sw trending dune ridges orientated perpendicular to the currently prevailing northwesterly wind regime in this study extensive use has been made of artificially exposed sections through these major dune ridges that reveal internal sedimentary structures and allow an intensive high resolution sampling programme to be carried out here we present the optical dating results for samples from 7 sections the results indicate that dune activity and preservation occurred within the periods 7 3 ka 16 10 ka and 22 20 ka with evidence of earlier preservation during marine oxygen isotope stages mis 3 and 5 with net accumulation rates in the range 2 2 25 m ka 1 in several instances hiatuses in the preserved record of dune accumulation coincide with stratigraphic bounding surfaces visible in the exposed section profiles with associated truncation of internal sedimentary structures caution must be exercised when interpreting such gaps in the recorded accumulation chronologies of these dunes since these may simply constitute phases of low preservation potential rather than phases of low aeolian activity other factors such as sediment supply and availability in relation to sea level dynamics may be significant and are also considered

aeolian dune field self organization implications for the formation of simple versus complex dune field patterns abstract the interpretation of aeolian dune field patterns as self organizing complex systems is a new paradigm in which pattern evolution may be addressed computer simulations supported by field and experimental data indicate that a given wind regime produces a simple dune field pattern dune type and crest orientation are determined by wind regime and pattern ordering occurs through dune dune interactions over time because dunes reorient only at their crest terminations with a change in wind regime the rate of formation of a new pattern of small dunes is typically faster than the rate of reorientation of the existing pattern resulting in the superposition of simple patterns to give rise to complex patterns complex patterns are distinct from spatial changes in a simple pattern and from the type of superposition that characterizes compound complex dunes complex patterns necessarily indicate a rate of pattern formation that is rapid compared to the rate of sediment accumulation on the depositional surface

from rifted margins to foreland basins investigating provenance and sediment dispersal across desert arabia oman u a e arid arabia where chemical weathering and anthropic modifications are negligible is a huge natural laboratory for the study of sediment provenance mafic and ultramafic detritus from ophiolite belts is found along the oman mountains and on masirah island obduction orogen provenance dominant cellular serpentinite grains from residual mantle harzburgites of the sama il ophiolite contrast with gabbroic diabase and basaltic grains from igneous crustal rocks of the masirah ophiolite detritus from autochthonous basement and cover rocks is found along the arabian sea margin rift shoulder provenance transition from carbonate dominated lithic sands undissected stage to hornblende rich plagioclase arkoses dissected stage is best documented at the northern edge of the gulf of aden along the older strike slip owen basin margin only locally are feldspars hornblende and garnet derived from small outcrops of pan african basement modern eolian sands from yemen to the arabian persian gulf display marked enrichment in quartz low plagioclase feldspar ratios and depleted dense mineral assemblages with rounded epidotes hornblende garnet and ultrastables pointing to extensive recycling also fostered by quaternary changes the rub al khali and wahibah sand seas have distinct quartz feldspar ratios reflecting largely separate transport pathways dune fields fed chiefly from rift highlands in the west and south mix with orogenic detritus in the northeast carbonate dominated sands in coastal uae are mostly derived from the zagros mountains blown by northerly shamal winds across the gulf during pleistocene lowstands to reach the backbulge depozone and the foreland beyond contributions from the oman mountains reach only 5 10 of bulk sediment along their southern apron and mostly indirectly through deflation of plio pleistocene fans the sama il ophiolite supplies enstatite and diopside to the eastern uae and wahibah sands the masirah ophiolite supplies amphiboles and clinopyroxenes locally along the batain coast the composition of desert sediments faithfully mirrors tectonic history of source terranes deformation style during orogeny and unroofing level during extension bulk petrography coupled with dense mineral analysis thus represents a powerful tool to investigate not only provenance and sediment transport but also crustal dynamics and geologic evolution at continental scale

nearest neighbor analysis of mega barchanoid dunes ar rub al khali sand sea the application of geographical indices to the understanding of dune field self organization maturity and environmental change abstract the geographic signature of dune distribution and self organization as measured by the r statistic offers a viewpoint on the geography of crescentic aeolian systems and proposes an index from which to determine the degree of self organization maturity and possibly relative age across a variety of spatial scales and planetary contexts this analogue study examines and compares the geography of crescentic megadune patterns for the al liwa basin of the ar rub al khali sand sea on the arabian peninsula fields of mega barchanoid forms have shown the degree of self organization to be largely an association between morphological diversity and dimensional variability in short fields of transitional megadunes barchanoid seif are comparatively less regular in distribution than are those fields or part thereof that consist of ubiquitous compound barchanoid morphologies whose patterns are more highly regular the change in dune morphology dimensions and spatial signature can for the most part be associated with the directional differences and varying intensities of the north westerly shamal and reversing south westerly kharif wind regimes such associations have significant bearing on the maturity of dunefields and their response to sediment supply transport capacity and sediment availability within an aeolian system this study provides an analogue on which to base further comparative studies with other terrestrial dune fields as well as the ergs and intracrater fields of mars

sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt ellensburg formation late miocene central washington late miocene volcaniclastics of the ellensburg formation washington northwestern usa are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a cascade range source area two facies associations are recognized the first is composed of laterally restricted well sorted polymictic conglomerates representing a gravel bedload regime during inter eruption periods the second comprises laterally extensive sheets bounded by deeply developed paleosols composed of monomictic pebbly pumiceous dacitic sandstones with intercalated debris flow and hyperconcentrated flood flow deposits these sheets aggraded in response to eruptive events that are sometimes recorded by air fall tephras at the bases of sequences debris flow and hyperconcentrated flood flow deposits occur as far as 120 km from source but are uncommon beyond 50 km hyperconcentrated flood flow deposits initially increase in abundance away from source as debris flow deposits diminish suggesting formation of hyperconcentrated flood flows by dilution of debris flows sandy facies form broad sheets dominated by scour fill bedding in proximal and medial settings and grade to narrower trough cross bedded sheets in distal settings suggesting moderation of flood discharges with distance base level changes associated with episodic sediment influx caused incision aggradation cycles in mainstream settings and episodic impoundment of tributaries to form lakes or rapidly aggrading poorly drained floodplains although volcanism was the primary control on depositional style concurrent development of the yakima fold belt produced a structure consequent drainage pattern that determined sediment dispersal and basin subsidence permitted preservation of both syn eruption and inter eruption facies detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics but dominates deposits of small tributary streams flowing toward rather than from the cascades

a distribution free statistical method for the spatial characterization of dune granulometry an example from the strzelecki desert south australia distribution free statistical methods of comparative data analysis have identi ed subtle granulometric differences attributed to the evolution of barchan form at gurra gurra waterhole geomorphic locations on the barchan dunes display statistically signi cant grain size differences that assist in the interpretation of aeolian processes in summer very ne sands mantle the dunescape and are the fraction that most affects the parameters of sorting and skewness the sur cial sedimentological character is one of subtle contrasts between the processes of grain winnowing and intergranular protection the second and third moment measures are parameters that best demonstrate the spatial granulometric differences dune forming processes at gurra gurra have produced dune sands that have a very narrow range of grain size which in turn re ects textural and mineralogical maturity and hence an extensive transport history the statistical techniques employed in this study can also be used for the comparison of temporal seasonal sedimentological change and for the granulometric analysis and association of process for dunes of different morpho types copyright 2004 john wiley sons ltd

observation of the whole process of interaction between barchans by flume experiments 1 through laboratory experiments generating unidirectional water flows we examine the process of interaction between two moving barchans sandy bed configurations in a crescentic plan shape which may take dozens of years for barchan dunes in the nature three types of the interaction were observed the first type was absorption of two barchans into one the second was that the upstream fast barchan rode on the downstream slow barchan and simultaneously a newly born barchan was ejected from the lee side of the slow barchan the third was that the downstream slow barchan was split into two before the upstream fast barchan touched the downstream the type of the interaction was determined by both relative and absolute size of two barchans

recent archaeo palaeontological findings from barranca del muerto site santiago chazumba oaxaca mexico abstract this article presents the first results of an archaeo palaeontological study which began in 2007 at the sites of chazumba i and ii in the barranca del muerto located in the sierra madre del sur santiago chazumba oaxaca mexico the excavation work is part of a larger international cooperation project titled biodiversity and quaternary hunter gatherer societies from mexico led by the institut de paleoecologia humana i evolucio social iphes tarragona spain and the instituto nacional de antropologia e historia of mexico the main objective of the project is to study hunter gatherer groups and their relationships with pleistocene fauna and rock art to date five excavation campaigns have been carried out 2007 2008 2010 2013 2014 which have primarily yielded faunal remains from the late pleistocene and from within a still undetermined period during the rancholabrean age nalma the fauna is represented by megaherbivores giant sloth glyptodont and gomphothere among others and mesoherbivores deer prong horns horses and even small mammals reptiles and amphibians some lithic materials have been recovered in association with these remains and several fossils have been examined to determine the presence of cut marks there is an ams 14c date of 27 720 27 500 cal bp obtained from a charcoal sample from sediments on top of the bone layers further sample assays are warranted in order to confirm the antiquity of the archaeological assemblage

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

field evidence for surface wave induced instability of sand dunes barchans are crescent shaped dunes that move faster over desert surfaces than most other dune types a three year field study shows how they do it collisions and changes in wind direction destabilize the dunes generating surface waves that form new smaller barchans this prevents the formation of a single giant dune so plays a fundamental role in the development of dune patterns field studies of barchans crescent shaped dunes that propagate over solid ground under conditions of unidirectional wind1 have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography2 3 4 which are thought to control dune morphology and kinematics5 6 7 because of the long timescale involved however the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood8 here we combine data from a three year field study in the moroccan sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves as suggested previously9 10 11 12 we find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans because the resulting surface waves propagate at a higher speed than the dunes themselves they can produce a series of new barchans of elementary size by breaking the horns of large dunes the creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns

geochemical characterization of late pleistocene and holocene tephra layers from the basin of mexico central mexico in order to aid palaeoenvironmental research of late pleistocene and holocene deposits of central mexico tephra layers collected from the sediments of the texcoco and chalco sub basins in the southern part of the basin of mexico are geochemically characterized and used as stratigraphic markers the tephra layers range in composition from basaltic andesites to rhyolites and are calc alkaline the tephras range in age from 34 000 to ca 260014c yr b p new names are used informally to designate correlated tephras the tlahuac tephra is present in chalco at a depth of 18 m in the southeastern part of texcoco at a depth of around 10 m and at the tlapacoya archaeological site where it had been mistakenly described as basaltic this basalt andesite tephra is dated to at least 34 00014c yr b p the tlapacoya 1 tephra is dated to between 15 020 450 and 14 430 190 yr b p and is present in all chalco sections the tlapacoya 2 tephra corresponds to the previously described pomez gruesa con fragmentos de andesita ca 14 400 yr b p and is present in all chalco and texcoco sections the likely source of these three tephras is the volcano popocatepetl tephra ii at chalco dates to 12 520 135 yr b p and correlates with the upper toluca pumice from nevado de toluca volcano these represent the first geochemical glass shard analysis of tephras from the basin of mexico and so further research is necessary before a reliable tephrochronology can be established

paleoindian sites from the basin of mexico evidence from stratigraphy tephrochronology and dating we present new data on the stratigraphy dating and tephrochonology at the most important paleoindian sites in the basin of mexico these include a penon woman iii with the oldest directly radiocarbon dated human remains 10 755 75 bp b tlapacoya with two human crania dated to just over 10 ka bp c tocuila an important mammoth site with incorporation of fossils and suggested bone tools within the upper toluca pumice utp lahar volcanic mudflow the tocuila site also includes potential evidence for a layer associated with the younger dryas meteorite airburst with charcoal iron microspherules micro tektites melted glass and volcanic ash dated to 10 800 50 bp and d the santa isabel iztapan mammoths i and ii with lithics of scottsbluff lerma and angostura types and obsidian prismatic blades but lacking the characteristic fluted clovis type points normally associated with mammoth kills and butchering and dated after the pumice with andesite pwa layer between 14 500 bp and 10 900 bp before the younger dryas interval these results show that these lithic traditions in central mexico are older than in the great plains of usa several tephra markers are recognised in the sites that help to constrain the stratigraphy and dating of the archaeological sequences however tephra reworking in marginal lake sites is present and has been carefully considered especially for the pwa tephra