accelerator mass spectrometry radiocarbon dating of rock varnish accelerator mass spectrometry ams 14c dating off rock varnish provides minimum limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods experiments on sample collection the effect of different individuals preparing samples the influence of different chemical extraction procedures the incorporation of carbonate detritus possible contamination from rock underlying varnish the role of bio geochemical erosion of varnish and other influences reveal procedures that yield reproducible results conventional radiocarbon dates from charcoal beneath lava flows of hualalai volcano hawaii and from arid sites in western north america provide controls to test varnish radiocarbon dating ams radio carbon ages on the very bottom layer of varnishes are typically 10 younger than the 14c ages of these controls applications are illustrated here for fluvial aeolian peri glacial hillslope lacustrine and glacial geo morphology as well as rock art research in archaeology

desert pavements and associated rock varnish in the mojave desert how old can they be desert pavements are common features of arid landscapes and have been widely used as a relative age indicator of the geomorphic surfaces upon which they are developed in this study i examined the patterns of pavement development as a function of elevation in the mojave desert as well as the causes for the gradual disappearance of pavement at high elevations pavement density as measured by percentage of pebble coverage decreases systematically with elevation gain by 3 per 100 m from 95 coverage below 500 m to less than 60 at 1700 m plants appear to be the main agent of pavement disruption plant density decreases as pavement density increases burrowing by rodents and crusting by cryptobiota also disrupt pavement development at higher elevation during the last glacial maximum plant communities were displaced 1000 1400 m downward in the mojave desert pavements today generally do not survive above the blackbush coleogyne ramossisma sagebrush artemisia tridentata zone evidence from packrat middens shows that these and other plants typical of high elevations today grew as low as 300 400 m during the last glacial maximum i suggest that during the last glacial maximum desert pavements were confined to the lowest alluvial fans of death valley and adjoining low valleys no alluvial desert pavements above 400 m in the region are older than the latest pleistocene by the same reasoning desert varnish on desert pavements above 400 m may all be holocene in age except where developed on stable boulders

infrared stimulated luminescence irsl dating of late holocene aeolian sands in the mojave desert california usa a new method of dating potassium feldspar grains from aeolian sands is demonstrated a luminescence signal is measured whilst the grains are stimulated in the near infrared at 880 nm a surface sand sample gave an age of 40 15 years showing that this method is able to provide ages for very young samples since the measurement of an infrared stimulated luminescence irsl signal can take place with a short infrared exposure the irsl signal of the grains is not depleted thus growth curves can be obtained for 5 mg sample aliquots this procedure was exploited in this paper dates were obtained for 10 samples from the kelso dune field as a demonstration of the applicability of the method in a desert environment where there is no material suitable for radiocarbon dating although the number of dates are limited sand from an area at the eastern end of the dune field with crescentic dunes showing a different wind direction from that of today had an age of 4115 335 years similar to the 3960 385 years fo

global climate change and active tectonics effective tools for teaching and research abstract multidisciplinary student teams foster innovative research projects that can discern the locations times and styles of late quaternary faulting field studies in the area of the 1992 landers magnitude 7 3 earthquake in southeastern california provide excellent opportunities for teaching and research in climatic and tectonic geomorphology graduate student work begins with descriptions of type sections for mappable units of quaternary alluvium climate change induced aggradation events these are stair stepped flights of alluvial fan and terrace surfaces topographic profiles along faulted stream channels and fans provide estimates of vertical tectonic deformation and insight into the responses of fluvial systems to wrench fault tectonics time lines in this tectonically deforming landscape are the result of global climate changes that caused synchronous pulses of alluviation in the mojave desert at about 125 55 and 10 12 and 8 ka these dated alluvial surfaces record tectonic deformation resulting from cumulative late quaternary surface ruptures tectonic geomorphologic analyses use topographic pedogenic stratigraphic and structural data but do not require expensive and time consuming trenching projects ages of faulted and unfaulted alluvium constrain the dates of two pre 1992 earthquakes the 1992 landers event is not characteristic of earlier faulting because it ruptured through adjacent segments of the emerson fault that previously ruptured in separate events one at 3 1 2 ka and the other before 8 ka styles of tectonic deformation and locations of active faults however may not have changed during the past 125 ka

importance of environment on the order of mineral weathering in olivine basalts hawaii examination of weathering rinds from lava flows on hawaii with backscatter electron microscopy and electron micro probe analysis reveals that olivine weathers more slowly than adjacent clinopyroxene and plagioclase in environments with a paucity of organic acids yet when weathering rinds are in contact with abundant organic acids secreted by lichens olivine weathers before clinopyroxene and plagioclase weathers last the exception to goldich s widely accepted mineral stability series in organic poor environments runs counter to a thermodynamics explanation for the order of mineral weathering and illustrates the importance of the biogeochemical environment

holocene rock varnish on the messak plateau libyan sahara chronology of weathering processes abstract rock varnish coats subaerially exposed rock surfaces on the messak settafet plateau se libya central sahara holocene surfaces host three microlayers formed by different accretion processes that record a climatic transition from a wet environment to progressively more arid conditions sem eds and optical investigations shed light on the processes leading to the formation of microlaminae while direct ams 14c dating on microlayers and relative radiocarbon dating on dark varnished pastoral neolithic stone monuments suggest a reliable chronology for varnish development the innermost microlayer includes illuvial clay and calcite incorporated during the early to middle holocene 9200 to 5500 uncal ybp under wet environmental conditions the second microlayer is more typical mn rich rock varnish biomineralized during a phase of growing aridity in the middle holocene 6000 to 4000 uncal ybp the upper microlayer is composed of mostly aeolian dust and it dates back to the last hyperarid millennia a positive correlation between varnish chronological development and climate changes in the sahara reflects also broad regional control on microlamination deposition

classification of large scale subaqueous bedforms a new look at an old problem sepm bedforms and bedding structures abstract a symposium entitled classification of large scale flow transverse bedforms was convened at the 1987 mid year meeting of sepm in austin texas with the purpose of examining the problems involved in classifying large subaqueous flow transverse bedforms developed in fluvial intertidal and marine environments and recommending changes in nomenclature the consensus of the participants is that despite the wide spectrum of morphologies of large scale flow transverse bedforms excluding antidunes they all occupy a similar position in the lower flow regime sequence between ripples and upper plane bed the wide variety of forms is a reflection of secondary effects such as channelization fluctuating water levels and unsteady and reversing flows the bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1 000 m the symposium panel proposes therefore that they should have only one name dune dune is preferred as it has historical precedence over other terms in use such as megaripple and sand wave the term dune should be modified by rimary descriptors of shape i e 2 d or 3 d and size based on spacing small 0 6 5 m medium 5 10 m large 10 100 m or very large 100 m and the adjective subaqueous when it is important to distinguish them from eolian dunes the panel recommends a morphologically based classification that is descriptive with an underlying genetic rationale second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature

extensive boulder erosion resulting from a range fire on the type pinedale moraines fremont lake wyoming abstract in july 1988 the flatfish range fire burned over the type pinedale moraines at fremont lake wyoming and caused extensive exfoliation of exposed boulder surfaces the mass of exfoliated material from 130 of 1030 boulders investigated was measured and recorded with information concerning factors that could influence the extent of fire induced exfoliation the range in thickness of material removed from 98 randomly selected boulders within the burn area averaged over the entire exposed boulder surface area is 6 1 to

manganese rich rock varnish does occur in antarctica abstract despite accounts to the contrary we have found that mn rich rock varnish is present in antarctica as others have before it is chemically and texturally similar to many varnishes found in lower latitudes antarctic varnishes show considerable potential as a dating and paleoenvironmental research tool

luminescence dating of mojave desert sands abstract infra red stimulated luminescence has been used to date periods of sand deposition in the mojave desert california dates from the cronese basins kelso dunes and silver lake are compared with published irsl ages to provide evidence for discrete pulses of sand accretion during the late pleistocene and holocene linked to both arid and pluvial climates a critical factor in aeolian deposition throughout this area is shown to be the availability of sand from existing source areas

flume experiments on the alignment of transverse oblique and longitudinal dunes in directionally varying flows for more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow whereas others are parallel or oblique to flow this problem of bedform alignment was studied experimentally using subaqueous dunes on a 3 6 m diameter sand covered turntable on the floor of a 4 m wide flume in each experiment two flow directions relative to the bed were produced by alternating the turntable between two orientations the turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms the resulting bedforms were in equilibrium with the time averaged conditions of the bimodal flows dune alignment was studied for five divergence angles the angle between the two flow directions 45 67 5 90 112 5 and 135 the flow depth during all experiments was approximately 30 cm mean velocity was approximately 50 cm s 1 and mean grain diameter was 0 6 mm each experiment continued for 30 75 min during which time the flume flow was steady and the turntable position changed every 2 min at the end of each experiment water was slowly drained from the flume and dune alignment was measured transverse dunes defined relative to the resultant transport direction were created when the divergence angle was 45 and 67 5 and longitudinal dunes were created when the divergence angle was 135 at intermediate divergence angles dunes with both orientations were produced but transverse dunes were dominant at 90 and longitudinal dunes were dominant at 112 5 one experiment was conducted with a divergence angle of 135 and with unequal amounts of transport in the two flow directions this was achieved by changing the orientation of the turntable at unequal time intervals thereby causing the amount of transport to be unequal in the two directions the dunes formed during this experiment were oblique to the resultant transport direction these experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments in both sets of experiments the bedforms developed with the orientation having the maximum gross bedform normal transport the orientation at which the sum of the bedform normal components of the two transport vectors reaches its maximum value in other words the bedforms develop with an orientation that is as transverse as possible to the two flows in those cases where the two flows diverge by more than 90 and transport equal amounts of sand bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors although such bedforms have been defined by previous work as longitudinal bedforms they are intrinsically the same kind of bedform as transverse bedforms

role of in situ cosmogenic nuclides 10be and 26al in the study of diverse geomorphic processes the central premises of applications of the in situ cosmogenic dating method for studying specific problems in geomorphology are outlined for simple and complex exposure settings in the light of these general models we discuss the information that can be derived about geomorphic processes utilizing concentrations of in situ produced cosmogenic radionuclides 10be half life 1 5 ma and 26a1 half life 0 7 ma in a variety of geomorphic contexts glacial polish and tills meteorite impact craters alluvial fans paleo beach ridges marine terraces sand dunes and bedrock slopes we also compare 10be 26al data with results obtained by other dating methods we conclude that the technique of measuring in situ cosmic ray produced nuclides holds promise for quantitative studies of processes and time scales in a wide range of geomorphological problems

understanding processes controlling sediment transports at the mouth of a highly energetic inlet system san francisco bay ca abstract san francisco bay is one of the largest estuaries along the u s west coast and is linked to the pacific ocean through the golden gate a 100 m deep bedrock inlet a coupled wave flow and sediment transport model is used to quantify the sediment linkages between san francisco bay the golden gate and the adjacent open coast flow and sediment transport processes are investigated using an ensemble average of 24 climatologically derived wave cases and a 24 8 h representative tidal cycle the model simulations show that within the inlet flow and sediment transport is tidally dominated and driven by asymmetry of the ebb and flood tides peak ebb velocities exceed the peak flood velocities in the narrow golden gate channel as a result of flow convergence and acceleration persistent flow and sediment gyres at the headland tips are formed that limit sediment transfer from the ebb tidal delta to the inlet and into the bay the residual transport pattern in the inlet is dominated by a lateral segregation with a large ebb dominant sediment transport and flow prevailing along the deeper north side of the golden gate channel and smaller flood dominant transports along the shallow southern margin the seaward edge of the ebb tidal delta largely corresponds to the seaward extent of strong tidal flows on the ebb tidal delta both waves and tidal forcing govern flow and sediment transport wave focusing by the ebb tidal delta leads to strong patterns of sediment convergence and divergence along the adjacent ocean beach

distribution and formation of rock varnish in southern tunisia field observations chemical analysis electron and light microscopy and radiolabelling experiments provide information on the characteristics formation and distribution of fe and mn rock varnishes in southern tunisia unstable rocks are not surface coated but when broken they commonly reveal internal microbial solution fronts stable rocks are coated in fe and more rarely mn rock varnishes radiolabelling indicates the presence of many respiring microbes on both fe and mn varnishes microscopy shows fungi and bacteria both on the fe varnishes and within the rock only rare microcolonial fungi occur on mn varnishes suggesting that the microbes are within or beneath the varnish the abundance of microorganisms supports a biogenic origin for rock varnishes different microbial communities appear to be involved in fe and mn varnish formation microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated apparently to avoid u v light and desiccation this explains the concentration of elements that form u v opaque minerals and clays the origin of these elements is external to the substrate these results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern tunisia

assessing ams 14c ages of detrital organics from holocene and late pleistocene moraines east central sierra nevada california usa obtaining age estimates of moraines which have insufficient carbon for conventional 14 c dating may be possible by ams 14 c dating of detrital organic material in the till matrix sufficient detrital carbon was available in the till matrix of holocene and late pleistocene moraines of upper laurel creek canyon east central sierra nevada california for ams 14 c measurements radiocarbon analysis of till matrix organics from the moraine closest to the headwall ml reveals an age of 2875 60 bp eth 13550 the next oldest moraine m2 yields 4450 60 bp eth 12810 m3 the oldest moraine in the bloody mountain cirque yielded organics with a radiocarbon age of 13 600 120 bp eth 12812 while results are preliminary these ages are among the first on till matrix in the sierra nevada and are consistent with geomorphic context other palaeoecologic records in the sierra nevada and palaeolake records in the western united states

cosmogenic chlorine 36 production rates in terrestrial rocks abstract chlorine 36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35cl spallation of 39k and 40ca and slow negative moun capture by 40ca we have measured the 36cl content of 14c dated glacial boulders from the white mountains in eastern california and in a 14c dated basalt flow from utah effective time intergrated production parameters were calculated by simultaneous solution of the 36cl production equations the production rates due to spallation are 4160 310 and 3050 210 atoms 36cl yr 1 mol 139k and 40ca respectively the thermal neutron capture rate was calculated to be 3 07 0 24 105 neutrons kg of rock 1 yr 1 the reported values are normalized to sea level and high geomagnetic latitudes production of 36cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates chlorine 36 dating was performed on carbonate ejecta from meteor crater arizona and late pleistocene morainal boulders from the sierra nevada california calculated 36cl ages are in good agreement with previously reported ages obtained using independent methods

lake size variations in the lahontan and bonneville basins between 13 000 and 9000 14c yr b p abstract recessions of lakes lahontan and bonneville that commenced 13 500 14 c yr b p were interrupted at 11 500 14 c yr b p in the lahontan basin and 12 200 14 c yr b p in the bonneville basin by relatively large perturbations in lake level that persisted for 2000 years minor glacial readvances in the sierra nevada and white mountains of california nevada occurred during the latter half of this interval between 11 000 and 9700 14 c yr b p the hydrologic response of lakes lahontan and bonneville and the mountain glacial advances were concurrent with the allerod younger dryas climatic intervals recorded in vegetational and glacial records of western and central europe

the rate of iron oxide enrichment in arid zone alluvial fan soils tunisian southern atlas measured by mineral magnetic techniques abstract rates of iron oxide enrichment of soils on a segmented alluvial fan in southern tunisia are examined mineral magnetic and chemical characteristics of soils developed on two dated relict segments and on contemporary alluvium suggest that the rate of fe oxide enrichment was more rapid in the holocene 2 6 10 8 g g 1 yr 1 over the last 5000 years than in the pleistocene 1 91 10 9 g g 1 yr 1 over the previous 45 000 years thus fe oxide concentration may be used to determine an ordinal chronology of some geomorphological surfaces in drylands but rates of enrichment cannot be assumed to be uniform over time

bedforms controlled by residual current vortices in a subtropical estuarine tidal channel abstract the objective of the present work was to analyze the bottom morphology the genesis of the bedforms and the patterns of current and sediment transport of the internal portion of the south mouth in paranagua bay parana brazil this region was used as a laboratory field to study the geomorphology of a dune field on a tide channel with features of flood delta in the inner portion and ebb delta in the direction of the continental shelf a sediment hydrodynamic model was used to calculate the direction and magnitude of the sediment transport and the residual current flows a map of apparent roughness was used in the numerical model obtained through the soundings of a sidescan sonar starfish sonar and estimated from the classification of the dune field so it is possible to obtain simulations with higher correspondence with reality it was observed two vortices of residual circulations near the shores of pontal do sul and ilha do mel with directions towards the continental shelf the residual circulation vortex is responsible for the triangular shape of the large dunes near the pontal do sul margin in the flood the sediment material is conducted inward forming small ripples and accumulating sediment material on large dunes during the ebb these ripples are supposedly destroyed and attached to the large dunes or to the water column on the other hand the bedforms of saco do limoeiro are formed due to the effects of the tide when entering the mouth and the confinement of the water mass in the region until the next moment of padding

asynchronous maximum advances of mountain and continental glaciers the last maximum glacier advance in many mountain ranges appears to have predated the last maximum advance of the wisconsinan continental ice sheets 20 000 years b p published evidence from widely spaced localities in north and south america europe asia and hawai i suggests that some mountain glaciers extended farther during one or more stades early in the last glaciation roughly 115 000 30 000 years b p than at its end when low global temperature and high precipitation presumably were most favorable for the growth of glaciers variations in the timing of glacier advances in different mountain ranges suggest that alpine glaciers are dependent on regional climate as well as global conditions their study might provide sensitive tests for theories of global climate change glacial deposits are often used as time markers in neotectonic and geomorphic studies and their ages sometimes are inferred from distant measurements or from proxy histories of global climate change undated geomorphic features such as moraines sharply incised streams and river terraces which commonly have been associated with the last maximum glacier advance 20 000 years b p or last deglaciation 13 000 years b p may actually be older by a factor of 2 to 4 or more conversely deposits assigned to the maximum advances of the penultimate global glaciation 130 000 years b p may actually date from early in the most recent glacial cycle and be younger by a comparable factor rates of faulting or folding estimated from displacements or deformation and the assumed ages of such geomorphic features might be too great or too small by corresponding factors

a 100 ka record of water tables and paleoclimates from salt cores death valley california abstract sedimentary and petrographic features of evaporites and associated sediments from a 185 m deep core taken in death valley ca together with uranium series dating have been used to reconstruct the history of water table fluctuations and climate changes in death valley for the past 100 ka death valley has been arid during the holocene 0 10 ka with predominantly mudflat and saline pan subenvironments a perennial lake up to 90 m deep existed in death valley from 10 to 35 ka saline pan and mudflat subenvironments dominated death valley from 35 to 100 ka the chronology of changing subenvironments and water table fluctuations in death valley generally correlates with other climate records in the western us owens lake and searles lake ca browns room cave calcite nv the marine oxygen isotope record and the vostok ice core record core intervals through saline pan sediments are composed of interbedded halite chaotic muddy halite and mud the halite contains abundant vertical dissolution pipes cemented with clear halite these sediments record repeated flooding by dilute waters dissolution of subaerially exposed surface salt crusts deposition of mud from suspension precipitation of halite during the saline lake phase and cementation by diagenetic halite mudflat sediments consist of clayey silt with sand patches and mud cracks which document long periods of desiccation and the formation of efflorescent salt crusts from the evaporation of groundwater brines saline pan and mudflat deposits formed during periods when death valley was relatively arid similar to the modern climate lacustrine deposits consist of mudhalite cycles accumulated during the early lake stage bedded thenardite na 2 so 4 and mud above and a cap of massive halite formed during the latest lake stage all of which record fluctuating salinities and lake levels in a perennial system such deposits document a relatively wet climate with a high ratio of water inflow to evaporation ostracodes in mud layers represent the least saline deepest lake phases halite layers are made of fine grained cumulates and clear vertically oriented crystals precipitated during shallower perennial lake stages of significance is the nearly complete absence of syndepositional dissolution of saline minerals in the lacustrine interval indicating that accumulated salts were permanently protected from dissolution by saline lake waters such evidence strongly suggests that lakes existed continually without desiccating for 25 ka between 10 and 35 ka b p

azimuthal anisotropy in the d layer beneath the caribbean 1 the lowermost mantle beneath central america has anisotropic seismic velocity structure manifested in shear wave splitting of signals from south american earthquakes recorded at north american broadband recording stations prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy vti which is sufficient to explain a general trend of early tangential sh component arrivals however vti models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region after accounting for splitting effects of upper mantle anisotropy beneath the recording stations we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic d layer this is the first attempt to quantitatively model a large data set including azimuthal anisotropy in d the models include transverse isotropy with either a vertical or tilted symmetry axis the latter resulting in azimuthal anisotropy for some initial shear wave polarizations tilted transverse isotropy tti produces small reversed polarity arrivals on the sv components at the arrival time of sh consistent with the data geographical variations in the azimuth of the tti symmetry axis are indicated by the data the lack of azimuthal coverage prevents unique resolution of the tti orientation and also precludes distinguishing between tti and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals nonetheless our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than vti for this region

magnetic techniques applied to the study of rock varnish the authors report the successful application of magnetic techniques to the study of rock varnish a surficial coating that forms on rocks in arid environments using thin wafers of varnished and unvarnished greywacke sandstone they have detected a stable remanent magnetization that can be unambiguously attributed to the rock varnish they have also measured several rock magnetic properties of the rock varnish and have shown that the primary magnetic carrier is either magnetite or maghemite these results demonstrate the feasibility of applying magnetic techniques to the study of geomorphic surfaces subsequent rock magnetic and paleomagnetic studies of rock varnish should provide new information about the geochemical processes involved in the formation of rock varnish and about the validity of a new chronological technique known as rock varnish dating

rock engraving chronologies and accelerator mass spectrometry radiocarbon age of desert varnish abstract it has been proposed that the minimum age of rock engravings can be established from dating organic carbon associated with desert varnish a naturally occurring coating on rocks in arid regions in this study five radiocarbon ages were obtained from organic carbon in previously undated varnishes from the pilbara western australia four varnish samples were scraped from dull black coatings and one from patchy shiny black varnish all radiocarbon ages were younger than 2800 years bp ages were younger at the rock varnish interface than in overlying varnish a reversal of expected stratigraphic relations these results highlighted a number of difficulties associated with the interpretation of varnish radiocarbon ages including the potential absence of a link between organic carbon age and varnish deposition age linkages are complicated by the time transgressive nature of varnish formation its weathering over time and the probable mixed ages of dated organic carbon engraving chronologies based solely on radiocarbon ages of varnish should be treated with extreme caution

ambiguities in direct dating of rock surfaces using radiocarbon measurements an attempt was made to date rock surfaces with accelerator mass spectrometry ams radiocarbon measurements of rock varnishes or rock weathering rinds in two case studies samples pretreated in the laboratory of dr ronald dorn prior to ams analysis have been found to contain significant quantities of carbon rich materials of two distinct classes type i material resembles bituminous coal whereas type ii material resembles pyrolized wood charcoal fragments in samples where these type i and type ii materials were separated and ams radiocarbon dated they were found to have widely differing radiocarbon ages in these cases the measurement of the radiocarbon age of the entire sample would yield results that are at best ambiguous neither type i nor type ii materials were found in comparable samples that were independently prepared

interactions among adriatic continental margin morphology deep circulation and bedform patterns abstract within the mediterranean region the adriatic basin is one of the key sites for the formation and spreading of dense shelf water once formed in the northern shallow region as a response to strong wintry cold air outbreaks dense water masses move southeastwards as bottom hugging gravity currents eventually reaching the south adriatic continental margin a fraction of the dense water mass is thus intercepted by the shelf break whence it flows down the continental slope driven by its negative buoyancy and interacting with ambient water and seafloor topography this process generates a wide variety of dynamics and morphological features such as erosional moats furrows giant comet marks sediment drifts and sediment waves in this study we merge the outcomes of a state of the art wave ocean coupled numerical model with the geological evidence from the analysis of southern adriatic bedform patterns the model was run to reproduce an exceptional cold event that occurred in winter 2012 and benefits from a high resolution bottom topography produced from data collected during recent bathymetric surveys model results allow the identification of flow pathways along and across the continental shelf and slope with particular emphasis on the role of large scale margin geometry in controlling morphodynamic processes and its implications in terms of resolution requirements for modelling purposes although modelled overall water fluxes and basin renewal ratios are mostly controlled by regional circulation and large scale morphology local topographic features and singularities appear crucial in triggering concentrating and modulating bottom currents dynamics and sediment transport towards the south adriatic pit as well as enhancing mixing and entrainment of ambient water as long as this degree of information is provided within the bathymetric constraint modelled hydrodynamic pathways in the study case are consistent with observed bedform morphology and their spatial pattern for different zones in the continental slope temporal variability of the flows has been investigated permitting to disentangle the relative contribution of dense water cascading and background circulation at the basin and sub basin scale in controlling bedform dynamics this study gives a first quantitative insight on the hydrodynamic mechanisms responsible for south adriatic margin reshaping suggesting a modelling approach to the interpretation of the high resolution bathymetric and stratigraphic data made available by recent survey technologies

a review of the history of dating rock varnishes the history of rock varnish dating spans almost 20 years from the initial suggestion to use uranium series isotopes through the use of a geochemical ratio cation ratio dating to the current application of accelerator mass spectrometry radiocarbon dating ams 14c uranium isotopes were ignored while other dating methods were tested exhaustively on several continents evidence of the unreliable dating of the formation of varnishes using cation ratios was demonstrated by examples of environmental and textural observations reflecting localised leaching and by chemical analyses that contradicted the fundamental assumptions of the method the radiocarbon dating of substances embedded in varnishes is also problematic where carbon compounds and sources are not identified the incompatible coexistence of different carbonaceous components a recent controversial issue is explained with reference to different environments of formation the future of ams 14c dating of rock varnishes centres around overcoming the practical limitations of sampling and analysing extremely small quantities of carbon bearing substances future directions for dating varnished rock surfaces may include innovative refinements to the u series cosmogenic isotope and luminescence methods

measured effects of desert varnish on the mid infrared spectra of weathered rocks as an aid to tims imagery interpretation the thermal infrared spectral properties 7 12 mu m of natural rock surfaces from silver lake ca are discussed although the reflectance of weathered rocks is largely a function of the quartz content in rocks the presence of desert varnish clay coating on rocks reduces the reflectance and spectral contrast with features unique to the rock spectra persisting if varnish is thin thick varnish has a spectrum with a reflectance peak near 9 6 mu m due to clays and resembles the spectra of clay rich playa surfaces comparison of laboratory reflectance spectra for varnish and weathered rock samples with thermal infrared multispectral scanner tims emissivity spectra for silver lake suggests that tims signatures for felsic rocks are dominated by weathered rock and rock debris in contrast it is likely that varnish plays an important role in the tims signatures of mafic rocks

rock varnish microlamination dating of late quaternary geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts in this study we establish a generalized late quaternary i e 0 300 ka varnish layering sequence for the drylands of western usa and tentatively correlate it with the specmap oxygen isotope record we then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late quaternary geomorphic features in the study region vml dating of alluvial fan deposits in death valley of eastern california indicates that during the mid to late pleistocene 5 15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation during the holocene interglacial time however 0 5 1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate vml dating of alluvial desert pavements in death valley and the mojave desert reveals that pavements can be developed rapidly

evolution of desert colluvial boulder slopes abstract we employed rhyolite domes formed during the past million years in eastern california to study the evolution of boulder dominated slopes the slopes studied are steep from 25 to 35 and are made of coarse boulder sized blocks these slopes include well varnished vertically oriented colluvial deposits that appear similar to relict periglacial stone stripes in this case they are the result of ongoing desert slope processes such deposits are common throughout the arid southwestern us but their morphometric character fabric and rates of formation have not been assessed systematically basic field and isotopic dating methods are combined with two relatively new surface dating techniques cosmogenic chlorine 36 and rock varnish microlaminae to produce a detailed million year long slope development history with broad implications for geomorphic theory grain size grain shape grain angularity and slope deposit area do not change significantly from genesis to 0 5 ma trends in the data change markedly after that time a geomorphic threshold occurs between 0 5 and 1 ma as indicated by significant changes in all parameters but grain orientation data on going erosion over the million year continuum is evidenced by active debris flow features by terminal pleistocene ages of microlaminae on all deposits and by 36cl ages that are progressively younger than potassium argon ages for slope genesis

bedform migration and bed load transport in some rivers and tidal environments reliable field data obtained by directly measuring bed load transport of fine to coarse grained bed material are extremely scarce mainly because of the difficulty of sampling accurately therefore the verification of bed load transport formulae is largely based on flume experiments which refer to unrealistic shallow water conditions in this study some bed load transport formulae were tested against data from natural environments as an alternative to ascertaining the bed load transport rate by sampling the bed load the transport rate was deduced from data on bedform height and bedform celerity for this purpose 43 sets of data from rivers representing a wide range of bed material bedform dimensions and hydraulic conditions were collected as were some sets of data from tidal settings two formulae were used for the prediction of the bed load transport the formula of van rijn 1981 and the kalinske 1947 formula as approximated by elzerman frijlink 1951 and in the present study slightly modified for application to tidal waters both the bed load function of van rijn and the modified formula of kalinske frijlink require data which are easily obtained and that can be measured accurately at those stages of the flow when bed load transport was high the van rijn function tended to overestimate that transport for flow stages when bed load transport was low the opposite was true the modified kalinske frijlink function gave consistently good results 86 of the transport rates predicted using the river data were within 0 5 2 0 times the values actually measured

nanometer scale layering in rock varnish implications for genesis and paleoenvironmental interpretation manganiferous rock varnish collected from death valley and antarctica contains the smallest known terrestrial sedimentary deposits with some layers only a few nanometers thick irregularities in these nanometer scale layers are consistent with shrinking cracking and weathering of clay minerals in the death valley rock varnish very different high resolution transmission electron microscope hrtem textures coexist that may be related to climatic change hrtem observations contradict previous microbial models of mn fe enhancement requiring a new three step model of biomineralization and diagenesis for varnish formation

unified view of sediment transport by currents and waves i initiation of motion bed roughness and bed load transport attention is given to the properties of sediment beds over the full range of conditions silts to gravel in particular the effect of fine silt on the bed composition and on initiation of motion critical conditions is discussed high quality bed load transport data sets are identified and analyzed showing that the bed load transport in the sand range is related to velocity to power 2 5 the bed load transport is not much affected by particle size the prediction of bed roughness is addressed and the prediction of bed load transport in steady river flow is extended to coastal flow applying an intrawave approach simplified bed load transport formulas are presented which can be used to obtain a quick estimate of bed load transport in river and coastal flows it is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions the proposed model is fully predictive in the sense that only the basic hydrodynamic parameters dep

granulometric study of the hanaupah fan death valley california we applied new granulometric techniques to the various surfaces of the hanaupah fan death valley california namely the q1 surface with an estimated age of 800 490 ka the younger q2 170 105 ka and q3 50 14 ka surfaces the 14 ka deposits of the incised channel and to a c 14 ka lake manly shoreline deposit at the northern periphery of the fan we used these techniques to generate quantitative information on surface clast grain size distributions clast sphericity roundness and clast orientation to provide a data set that could be used to define fan segment surfaces and to help interpret fan genesis grain size analyses were carried out by photo sieving of 139 surface pictures by petrographic identification of samples taken in the incised channel and by identification and measuring of the largest clasts 1452 measurements on the q3 surface the results show that all fan segment surfaces regardless of age have similar size distributions with a well defined gravel mode of 2 3 to 3 0 phi and are poorly to moderately sorted samples from the incised channel have distributions that are very similar to each other regardless of distance from the apex but display reduced sorting compared to the fan surfaces which largely lack fines perhaps from winnowing by secondary overland flow only the shoreline deposit is different from the other elements showing a much narrower well defined gravel mode 3 0 phi and is moderately well sorted sphericity and roundness of clasts on all surfaces show only minor differences similar to the other sedimentary parameters indicating a remarkable homogeneity of the surfaces of the sediment body in addition measurements of the largest clasts 100 cm long axis on the q3 surface showed no discernible trend either with radial distance or with rock type these data suggest large depositional episodes that produce extensive sedimentary units without differentiation relative to distance from the source of the examined parameters clast orientation is the best predictor of relative age of fan surfaces clast orientation in the main channel is bimodal i e the long axes of clasts are either at right angles or parallel to transport direction this bimodality disappears with increasing age and the preferred orientation becomes unimodal long clast axes normal to transport direction on the q1 surface although the causes of this change are still in debate use of this parameter as a relative age dating tool seems possible 1998 john wiley sons ltd

latest pleistocene lake shorelines and glacial chronology in the western basin and range province u s a insights from ams radiocarbon dating of rock varnish and paleoclimatic implications accelerator mass spectrometry radiocarbon dating of organic matter in the basal layer of rock varnish provides a minimum surface exposure age for the underlying landform varnishes 14c ages indicate that the latest pleistocene highstand of lake lahontan was abandoned before 12 7 ka searles lake before 13 6 ka lake manly before 13 0 ka and lake mojave before 11 6 ka the maxima of the tioga glaciation at pine creek in the eastern sierra nevada california and the middle creek glaciation in the adjacent white mountains of california nevada was just before 18 19 ka major recessional moraines were deposited in these ranges before 12 13 ka penecontemporaneous glacial retreat and high levels in closed basin lakes cannot be explained by glacial melt water but probably reflects higher snowlines and more precipitation a relatively minor glacial advance perhaps related to the younger dryas in europe may be penecontemporaneous with a slight transgression in lake levels about 10 11 ka in the western basin and range suggesting a temporary return to cooler or wetter conditions or both at the termination of the pleistocene

segmentation of alluvial fans in death valley california new insights from surface exposure dating and laboratory modelling laboratory experiments recent paleoenvironmental analyses of rock varnish and surface exposure dating of geomorphic units have led to new insights into the process of entrenchment and segmentation of alluvial fans and into the history of quaternary sedimentation in death valley entrenchment begins at the fanhead as the trench deepens its down slope end migrates down fan taking several tens of thousands of years to reach lower parts of the fan laboratory experiments suggest however that a new segment begins to grow at the toe long before the trench reaches this part of the fan furthermore the initial slope of the segment is not the equilibrium slope field evidence supports this model the tectonic tilting that caused entrenchment and segmentation in death valley may have been triggered by loading of the valley with water sedimentation on the salt pan in southern death valley is not at present in equilibrium with that on the fans rather it seems to be adjusting to an increase in the amount of fine material reaching the playa due in part to breaching of the outlet of lake tecopa somewhat after 600 ka bp and in part to subsidence of different parts of the valley at different rates failure to recognize this disequilibrium resulted in errors in earlier estimates of the age of the segmentation events

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

are the benches at mormon point death valley california usa scarps or strandlines the benches and risers at mormon point death valley usa have long been interpreted as strandlines cut by still stands of pluvial lakes correlative with oxygen isotope stage ois 5e 6 120 000 186 000 yr b p and ois 2 10 000 35 000 yr b p this study presents geologic mapping and geomorphic analyses gilbert s criteria longitudinal profiles which indicate that only the highest bench at mormon point 90 m above mean sea level msl is a lake strandline the other prominent benches on the north descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform the faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the mormon point turtleback fault implying late quaternary slip on this low angle normal fault our geomorphic reinterpretation implies that the ois 5e 6 lake receded rapidly enough not to cut strandlines and was 90 m deep consistent with independent core studies of the salt pan no evidence of ois 2 lake strandlines was found at mormon point which indicates that the maximum elevation of the ois 2 lake surface was 30 m msl thus as measured by pluvial lake depth the ois 2 effective precipitation was significantly less than during ois 5e 6 a finding that is more consistent with other studies in the region the changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at mormon point are uninterpretable with respect to lake history

radiocarbon dating of prehistoric rock paintings by selective oxidation of organic carbon dating of prehistoric rock paintings pictographs has traditionally relied on indirect evidence this includes inferences based on the archaeological context such as superpositions of pictorial styles1 and the depiction of images that constrain their ages1 2 as well as dating of deposits that either cover the art in situ3 4 or contain separated fragments of the painted surface5 migration of ions between the bulk rock and the natural coatings that form on a newly exposed surface has also been exploited to date petroglyphs rock carvings in desert regions6 13 until recently14 17 however direct dating by radiocarbon techniques of pictographs has not been possible18 19 mainly because of the problem of separating inorganic carbon from the organic material in the pigments here we report on a new technique which allows this separation to be effected by using a low temperature low pressure oxygen plasma to oxidize selectively the organic component this may then be analysed using standard14c methods we have applied this technique to a portion of a pictograph from the lower pecos region of southwest texas fig 1 the date obtained 3 865 100 yr bp before present is consistent with that expected on the basis of archaeological inference20 as organic carbon is a ubiquitous component of pictograph paints this technique should be applicable to rock paintings throughout the world

holocene rock varnish microstratigraphy and its chronometric application in the drylands of western usa abstract analyses of hundreds of rock varnish samples from latest pleistocene and holocene geomorphic features in the drylands of western usa reveal a regionally replicable holocene microlamination sequence this sequence consists of 12 approximately evenly spaced weak dark layers intercalated with 13 orange yellow layers preliminary radiometric age calibration indicates that six dark layers in the upper portion of the sequence were deposited during the last 6000 yr diagnostic of the little ice age and late holocene wet events five dark layers in the lower portion of the sequence were deposited after the termination of the pleistocene but slightly before 7000 ybp indicative of the early holocene wet events and one dark layer in the middle portion of the sequence was deposited around 6500 ybp suggestive of the middle holocene wet phase our age calibration further indicates that the holocene wet events represented by the dark layers largely correlate in time with the millennial scale holocene cooling events in the north atlantic region this radiometrically calibrated and climatically correlated holocene microlamination sequence was then used as a unique correlative dating tool to determine surface exposure ages of geomorphic and geoarchaeological features in western usa deserts the varnish microlamination vml dating of debris flow fan deposits in death valley california yields minimum ages of 12 500 12 500 11 100 11 100 10 300 9400 and 2800 ybp for six debris flow fan building events suggesting that such events were more likely to have occurred during relatively wet periods of the holocene the vml dating of a prehistoric grinding stone from chili of northern new mexico yields a minimum age of 900 1100 ybp for the abandonment of this occupation site by the anasazi indians the vml dating of a prehistoric flaked stone a primary core from ocotillo southern california yields a minimum age of 12 500 ybp for the flaking of this stone artifact suggesting at least a paleo indian human occupation at ocotillo during the terminal pleistocene these results indicate that when properly applied the vml dating technique has the great potential to yield numerical age assignments for surface stone tools petroglyphs and geoglyphs of prehistoric age in the drylands of western usa

blind testing of rock varnish microstratigraphy as a chronometric indicator results on late quaternary lava flows in the mojave desert california rock varnish is a manganiferous dark coating ubiquitous in desert landscapes to test the validity of varnish microstratigraphy as a chronometric indicator varnish samples were collected from radiometrically dated and undated late quaternary lava flows in amboy cima and pisgah volcanic fields avf cvf pvf in the mojave desert of california western united states varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the younger dryas and heinrich events in the north atlantic region microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western us sites with available radiometric age constraints based on this regional chronology varnishes from the a flow h flow and a stone pavement surface in the cima volcanic field were estimated to be 16 5 24 74 85 and 74 85 ka respectively these ages are consistent with previously published cosmogenic 3 he ages of 18 20 72 74 and 80 85 ka for these geomorphic surfaces varnishes from the i flow at cima yielded a puzzling age estimate of 39 ka which is consistent with an older 3 he age of 37f6 ka reported for the i flow but inconsistent with a younger 3 he age of 31f7 ka and a cosmogenic 36 cl age of 27f1 3 ka for the same flow reinterpretation of the original varnish age data with knowledge of then available field mapping results of the i flow suggests that the i cone is polycyclic and different flow units were probably unintentionally sampled in the field the revised varnish ages of 30 and 39 ka for the i flow thus may be in good agreement with their corresponding 3 he and 36 cl ages in a blind test of the method varnishes from the phase 1 flow at pisgah an unnamed flow called here the ivflow at cima and the amboy flow were estimated to be 24 30 46 60 and 74 85 ka respectively these ages agree well with 36 cl ages of 22 5f1 3 46f2 and 79f5 ka reported for the same flows by phillips geomorphology 2002 these test results provide convincing evidence that varnish microstratigraphy once radiometrically calibrated can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western us drylands d 2002 elsevier science b v all rights reserved

age and geomorphic history of meteor crater arizona from cosmogenic 36cl and 14c in rock varnish abstract using cosmogenic 36 cl buildup and rock varnish radiocarbon we have measured the exposure age of rock surfaces at meteor crater arizona our 36 cl measurements on four dolomite boulders ejected from the crater by the impact yield a mean age of 49 7 0 85 ka which is in excellent agreement with an average age of 49 3 ka obtained from thermoluminescence studies on shock metamorphosed dolomite and quartz these ages are supported by undetectably low 14 c in the oldest rock varnish sample

the genesis of an inter field marine sandwave and the associated anti asymmetry migration of neighboring crests 1 a series of high resolution multibeam echo sounding surveys near san francisco ca suggests the occasional migration of marine sandwave crests in the direction opposite to that indicated by the shape asymmetry of the individual sandwaves in contrast to all previous observations of the relationship between bedform shape asymmetry and migration the anti asymmetry migration occurs over approximately the same time period that a new sand wave crest is formed within the large and relatively stable field the new sandwave crest appears approximately midway between two larger crests in the vicinity of structural defects in a nearly two dimensional portion of the field the sandwaves in the vicinity of the new crest are found to have migrated away from the new crest regardless of their shape asymmetry later a large section of the new crest disappears and the neighboring sandwaves migrated back toward the vacated crest location this migration occurs for sandwaves up to at least ten wavelengths away from the new crest