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

comparison of equivalent doses determined by thermoluminescence and infrared stimulated luminescence for dune sands in new zealand abstract a comparison is made between the use of thermoluminescence and infrared stimulated luminescence for determining the equivalent dose of sedimentary potassium feldspars results are presented for a suite of dune sands from raised marine terraces in south west north island new zealand with the exception of a modern dune sand the two methods yield very similar results in spite of the differences in the nature of the signals measured by the two methods

applications of network analysis for adaptive management of artificial drainage systems in landscapes vulnerable to sea level rise the vulnerability of coastal landscapes to sea level rise is compounded by the existence of extensive artificial drainage networks initially built to lower water tables for agriculture forestry and human settlements these drainage networks are found in landscapes with little topographic relief where channel flow is characterized by bi directional movement across multiple time scales and related to precipitation wind and tidal patterns the current configuration of many artificial drainage networks exacerbates impacts associated with sea level rise such as salt intrusion and increased flooding this suggests that in the short term drainage networks might be managed to mitigate sea level rise related impacts the challenge however is that hydrologic processes in regions where channel flow direction is weakly related to slope and topography require extensive parameterization for numerical models which is limited where network size is on the order of a hundred or more kilometers in total length here we present an application of graph theoretic algorithms to efficiently investigate network properties relevant to the management of a large artificial drainage system in coastal north carolina usa we created a digital network model representing the observation network topology and four types of drainage features canal collector and field ditches and streams we applied betweenness centrality concepts using dijkstra s shortest path algorithm to determine major hydrologic flowpaths based off of hydraulic resistance following this we identified sub networks that could be managed independently using a community structure and modularity approach lastly a betweenness centrality algorithm was applied to identify major shoreline entry points to the network that disproportionately control water movement in and out of the network we demonstrate that graph theory can be applied to solving management and monitoring problems associated with sea level rise for poorly understood drainage networks in advance of numerical methods

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

response of eolian geomorphic systems to minor climate change examples from the southern californian deserts abstract eolian processes and landforms are sensitive to changes in atmospheric parameters and surface conditions that affect sediment supply and mobility the response of eolian geomorphic systems to minor climate change can be examined through process response models based on a combination of relations between short term changes in climatic variables and eolian activity and the geologic and geomorphic record of holocene eolian activity at both time scales eolian activity in southern californian deserts is strongly controlled by variations in precipitation wind energy is not a limiting factor in this region formation of eolian deposits is a product of climatic changes that increase sediment supply from fluvial and lacustrine sources and may therefore be closely tied to periods of channel cutting and geomorphic instability during intervening periods eolian deposits migrate away from sediment source areas and are reworked modified and degraded remobilization of existing dormant dunes is a product of reduced vegetation cover and soil moisture in periods of drier climates the major control on these processes is decadal to annual changes in rainfall that determine vegetation cover and soil moisture content

a middle holocene vegetation record from the mojave desert of north america and its paleoclimatic significance abstract packrat middens from the mccullough range in the southeastern mojave desert of nevada record vegetation conditions during the periods 6800 to 5060 yr b p and 1250 yr b p to present their age and location are ideal to test for middle holocene aridity the period between 6800 and 5060 yr b p does appear to have been more arid than during the late holocene period of record this agrees with other evidence for middle holocene aridity from the southwest arguments for strengthened middle holocene monsoons in the region are based chiefly on packrat midden samples older than 7500 yr b p or on samples that are younger than 5500 yr b p simulations of 6000 yr b p climate that fail to show strong subtropical flow into the southwest are therefore supported by evidence for enhanced aridity in that region

discrimination of active and inactive sand from remote sensing kelso dunes mojave desert california abstract landsat tm images field data and laboratory reflectance spectra were examined for the kelso dunes mojave desert california to assess the use of visible and near infrared vnir remote sensing data to discriminate aeolian sand populations on the basis of spectral brightness results show that areas of inactive sand have a larger percentage of dark fine grained materials compared to those composed of active sand ehich contain less dark fines and a higher percentage of quartz sand size grains both areas are spectrally distinct in the vnir suggesting that vnir spectral data can be used to discriminate active and inactive sand populations in the mojave desert analysis of laboratory spectra was complicated by the presence of magnetite in the active sands which decreases their laboratory reflectance values to those of inactive sands for this application comparison of tm and laboratory spectra suggests that less than 35 vegetation cover does not influence the tm spectra

the distribution of apparent dose as determined by optically stimulated luminescence in small aliquots of fluvial quartz implications for dating young sediments abstract we noted recently murray et al 1995 that quartz samples taken from in channel fluvial deposits have apparent doses ranging over several orders of magnitude we suggested that these dose distributions are best explained by a mixture of a small number of poorly bleached grains and a larger number of well bleached grains in this paper we examine the distribution of apparent dose in small aliquots each composed of 2000 grains of different particle size fractions 63 90 m 90 125 m 125 180 m 180 212 m 212 250 m diameter from two samples collected from the murrumbidgee river new south wales australia the samples are of known burial age the first being 70 years old and the second being its modern equivalent 0 2 gy the difference in the lowest apparent dose in the two samples is considered to be the result of their different burial times we suggest a method for dating recently deposited

large scale stabilized dunes on the high plains of colorado understanding the landscape response to holocene climates with the aid of images from space principal component analysis of landsat thematic mapper images from eastern colorado reveals stabilized dune fields that are composed of single and compound parabolic dunes some longer than 10 km these dunes have been reactivated at least four times in the past 10 000 yr at ca 9500 to 5500 yr b p 5500 to gt 4800 yr b p 4800 to gt 1000 yr b p and lt 1000 yr b p during droughts that probably exceeded the dry conditions during the 1930s dune orientations indicate that these landforms were molded by winds from the northwest which now dominate during the winter and spring with the passage of pacific air masses this study underscores the sensitivity of the landscape of the high plains to drought conditions which may become more prevalent because of the anticipated effects of greenhouse warming

short duration holocene lakes in the mojave river drainage basin southern california stratigraphic sedimentologic and pedologic studies of beach ridge and lacustrine deposits indicate that up to five times during the holocene shallow lakes covered silver lake playa in southeastern california for periods of years to decades the two youngest lacustrine events at about 390 90 yr b p and 3620 70 yr b p coincide with the early and late neoglacial episodes of north america increasing evidence in recent years from other nonglaciated areas leads us to conclude that the effects of these climatic episodes were much more widespread than previously thought the climate during these episodes was characterized by an increased frequency of winter storms in the southwestern united states causing wetter conditions that affected diverse hyperarid environments in the mojave desert and adjacent regions we propose that this wide areal coverage was caused by large scale winter atmospheric circulation patterns which are probably related to changes in sea surface temperatures and oceanic circulation in the eastern north pacific ocean

stratigraphic evidence for late quaternary dune activity near hudson on the piedmont of northern colorado stabilized dune fields are common features near hudson on the piedmont of northern colorado exposures in dune and interdune sites expose a sequence of eolian sediments and paleosols that record a complex history of eolian activity during the late quaternary radiocarbon and thermoluminescence age estimates on a horizons buried by eolian sand indicate that dunes were reactivated sometime between 7 and 9 ka on the basis of morphology of surface soils the dunes were most recently stabilized 3 ka at present there are no data to indicate if there were multiple periods of dune movement and stabilization during the holocene the penultimate pre holocene dune forming episode probably terminated ca 13 ka and may be coincident with the pinedale glaciation the stratigraphy at interdune sites shows at least two eolian depositional events prior to the penultimate event they were separated by periods of pedogenesis one of which may have exceeded 40 ka this analysis indicates that dunes in northern colorado were active during both cold arid and warm arid periods in the late quaternary

atmospheric circulation during holocene lake stands in the mojave desert evidence of regional climate change it is commonly thought that the climate conditions that supported lakes over a period of years in the mojave desert in southern california only existed before 8 000 yr bp and that the environment has been arid since1 2 here we look at a drill core in the silver lake playa at the terminus of the mojave river and find holocene lake deposits which indicate that shallow lakes existed for at least a few decades these deposits were radiocarbon dated at 3620 70 and 390 90 yr bp corresponding to the early neo glacial and the little ice age respectively3 to identify the conditions necessary to produce these holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed available data indicate that there is a link between anomalous winter atmospheric conditions over the north pacific and mojave river floods that produced ephemeral lakes in the silver lake playa and that the mojave river filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions we suggest that the late holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods

infra red stimulated luminescence ages from aeolian sand and alluvial fan deposits from the eastern mojave desert california abstract infra red stimulated luminescence irsl from coarse grained 180 212 m potassium rich feldspars has been used to date deposits associated with the western piedmont of the providence mountains in the eastern mojave desert these deposits consist of alluvial fans aeolian sand which exists in the form of lenses within the fan matrices and sand from the kelso dune field which abuts the distal end of the fan sequence deposition and subsequent stabilisation of these sand units appears to have occurred in both aird and intermittent climates associated with pluvial phases during the holocene and late pleistocene ages of 6600 870 and 7840 1790 years were obtained from dune sand bracketing the mazama ash at skull creek dunes oregon giving an independent age control on the single aliquot irsl method which was used on the mojave samples

interpretations and common challenges of aeolian records from north american dune fields abstract geomorphic and chronologic data for dune fields are evaluated for three contrasting areas of north america 1 the prairie parkland and boreal ecozones of the northern great plains in canada 2 the central great plains of the usa and 3 the deserts of southwestern usa and northern mexico luminescence and radiocarbon ages for periods of dune accumulation and stability are compared with palaeoenvironment proxies to provide an assessment of the boundary conditions of dune system response to changes in sediment supply availability and mobility dune fields in the northern great plains were formed from sediment originating from glaciofluvial or glaciolacustrine sediments deposited during deglaciation 16 11 ka subsequent aeolian deposition occurred in parkland and prairie dune fields as a result of mid holocene 8 5 ka and late holocene despite spatial and temporal gaps in chronometric data as a result of sampling biases the record from north american dune fields indicates the strong influence of sediment supply on dune construction with changes in sediment availability as a result of drought episodes resulting in dune field reactivation and reworking of pre existing sediment

middle holocene drought on the southern high plains abstract the climate of the southern high plains during the middle holocene is indicated by data from a variety of sources stratigraphic geomorphic and pedologic research at six localities in draws several dune sites and one playa lake basin show that widespread eolian erosion and sedimentation began in some areas by 9000 yr b p and culminated 6000 4500 yr b p probably because of warmer drier conditions that reduced the vegetative cover archaeological investigations at three sites provide evidence for the excavation of wells at this time apparently because of a declining water table studies of a few vertebrate and invertebrate faunas also indicate warming and drying in the middle holocene climate models and very limited isotopic data from bison bone suggest that summers in the middle holocene were warmer than present with reduced effective precipitation all lines of evidence indicate that the southern high plains was subjected to prolonged drought in the middle holocene with a maximum between 6500 and 4500 yr b p conforming climatically and chronologically to the altithermal

eolian sand transport pathways in the southwestern united states importance of the colorado river and local sources geomorphologists have long recognized that eolian sand transport pathways extend over long distances in desert regions along such pathways sediment transport by wind can surmount topographic obstacles and cross major drainages recent studies have suggested that three distinct eolian sand transport pathways exist or once existed in the mojave and sonoran desert regions of the southwestern united states one hypothesized pathway is eolian sand transport from the eastern mojave desert of california into western arizona near parker and would require sand movement across what must have been at least a seasonally dry colorado river valley we tested this hypothesis by mineralogical geochemical and magnetic analyses of eolian sands on both sides of the colorado river as well as sediment from the river itself results indicate that dunes on opposite sides of the colorado river are mineralogically distinct eastern california dunes are feldspar rich whereas western arizona dunes are quartz rich derived from quartz rich colorado river sediments because of historic vegetation changes little new sediment from the colorado river is presently available to supply the parker dunes based on this study and previous work the colorado river is now known to be the source of sand for at least three of the major dune fields of the sonoran desert of western arizona and northern mexico on the other hand locally derived alluvium appears to be a more important source of dune fields in the mojave desert of california although many geomorphologists have stressed the importance of large fluvial systems in the origin of desert dune fields few empirical data actually exist to support this theory the results presented here demonstrate that a major river system in the southwestern united states is a barrier to the migration of some dune fields but essential to the origin of others published by elsevier science ltd

taphonomy palaeoecological implications and colouration of cambrian gogiid echinoderms from guizhou province china based on rich material 381 specimens examined from two cambrian echinoderm faunas the early cambrian balang fauna and middle cambrian kaili fauna in guizhou province south china the taphonomy of gogiid echinoderms is described in detail and the preservation of stereomic microstructure and organic remains of cambrian gogiid echinoderms is reported here for the first time taphonomic considerations include entombment patterns decay sequences individual specific diagenetic histories unusual burial postures selective disarticulation patterns and postmortem elongation in particular five categories of gogiid entombment patterns are proposed to describe the multi directional orientations recorded at the burial time of articulated gogiids gogiid bearing slabs of guizhou material primarily 70 display the type 2 entombment pattern articulated gogiids preserved with fan shaped brachioles thus most guizhou gogiids were buried with brachioles preserved in feeding posture during obrution events balang gogiid faunas contain the oldest evidence of palaeoecological interactions among echinoderms and other indigenous taxa in addition to pre burial and post burial decay other potential causes for unusual disarticulation patterns exhibited by the gogiids from the lower cambrian balang formation include pre burial bio disturbance and post burial bioturbation based on ichnogenera including rusophycus and planolites chemical analyses reveal that carbon calcium manganese and iron are the major elements responsible for the variety of colours exhibited by guizhou gogiids three dimensional stereomic microstructure mean stereom pore size 8 4 8 7 m average trabecular thickness 4 5 4 6 m occurs on the external surfaces of thecal plates in two gogiid species stereom preservation in calcite suggests that the dissolution of calcareous echinoderm plates yielding characteristic mouldic preservation is sub recent after lithificaiton and exposure of gogiid bearing marine sedimentary successions on or near the land surface

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

extracting holocene paleohydrology and paleoclimatology information from modern extreme flood events an example from southern california abstract the extraction of paleohydrological and paleoclimatological information from a modern hydrological system shown to represent unique and extreme hydroclimatological conditions is illustrated by an example from the mojave river drainage basin in southern california the mojave river allows only the most extreme floods to reach its terminal basin in the silver lake playa and to form ephemeral lakes all the other floods are lost by transmission into the alluvial aquifer along its 200 km channel this filtering out of regular floods by the river provides an essential tool in establishing a physical link between atmospheric and hydrologic conditions we demonstrate such a link between anomalous present day atmospheric circulation patterns over the north pacific ocean extreme storms in southern california that produced the heaviest precipitation on record the largest floods of record in the mojave river watershed and ephemeral lakes in its terminal playa this physical link determines the possible cause of the formations of perennial short duration shallow lakes in silver lake playa during the late holocene and characterizes the hydroclimatic conditions that prevailed during these lacustrine episodes hydrological simulations of this river and its filtering character demonstrate that these lakes could have formed only if the most extreme modern storms and floods were more frequent in at least an order of magnitude during specific time episodes we conclude that such extreme hydroclimatic conditions occurred more frequent in past episodes during which the holocene lakes formed in turn this conclusion indicates that the cause of these storms and floods i e the anomalous atmospheric circulation pattern must have been more frequent this research outlines a way to extract information on holocene climates in hydrologic settings that demonstrate a unique cause and effect relationship

flood frequency of the mojave river and the formation of late holocene playa lakes southern california usa perennial lakes lasting several decades occupied silver lake playa the terminus of the mojave river as recently as 390 90 and 3620 70 bp flood routings from the mojave river headwaters to its terminal playa and water balance calculations for the resulting lakes indicate that floods comparable in magnitude to the largest floods in the modern record must occur almost annually to generate and maintain perennial lakes in contrast the modern floods of this magnitude have recurrence intervals greater than 18 years such an increase in flood frequency is required even if lake evaporation is reduced by as much as 50 modern extreme floods of the mojave river are related to meridional atmospheric circulation and a southeastward shift in the winter low pressure center over the north pacific ocean causing vigorous storms over the headwaters in the san bernardino mountains of southern california this winter pattern probably occurred more frequently to induce the large number of storms and floods needed

a complex origin for the kelso dunes mojave national preserve california usa a case study using a simple geochemical method with global applications abstract the kelso dune field in southern california is intriguing because although it is of limited areal extent 100 km2 it has a wide variety of dune forms and contains many active dunes 40 km2 which is unusual in the mojave desert studies over the past eight decades have concluded that the dunes are derived primarily from a single source mojave river alluvium under a dominant westerly to northwesterly wind regime the majority of these studies did not however present data to support the mojave river as the only source we conducted mineralogical and geochemical studies of most of the 14 geomorphically defined dune groups of the kelso dune field as well as potential sand sources alluvial sediments from the surrounding mountain ranges results indicate that sands in the nine western dune groups have k rb and k ba primarily from k feldspar compositions that are indistinguishable from mojave river alluvium westerly northwesterly winds and budweiser wash alluvium southwesterly winds permitting an interpretation of two sources in contrast sands from the five eastern dune groups have k rb and k ba values that indicate significant inputs from alluvial fan deposits of the providence mountains this requires either rare winds from the east or southeast or more likely aeolian reworking of distal providence mountain fan sediments by winds from the west at a rate greater than input from the mojave river or other western sources the results indicate that even a small dune field can have a complex origin either from seasonally varying winds or complex alluvial fan dune interaction application of k rb and k ba in k feldspar as a provenance indicator could be used in many of the world s ergs or sand seas where dune origins are still not well understood or are controversial four examples are given from africa and the middle east where such an approach could yield useful new information about dune sand provenance

new airborne imaging radar observations of sand dunes kelso dunes california abstract high resolution multiple wavelength and polarization radar data of kelso dunes were acquired during the mojave field experiment they offer the opportunity to develop criteria for the selection of optimal wavelengths and polarizations for imaging of dune areas the backscatter from unvegetated dune surfaces at all wavelenghts c l and p band with direct hh vv polarization is largely controlled by dune topography and incidence angle these data especially p band provide the best images of dune topography the best discrimination between active and inactive dunes is obtained by cross polarized c band data cvh chv as a result of multiple scattering from vegetation and small scale topography e g shadow dunes

optical dating of sediments a new method for dating sediments is proposed which determines the time since the sediment was last exposed to sunlight an argon ion laser is used to excite electrons from thermally stable light sensitive traps and the subsequent luminescence used as a measure of the past radiation dose two sample sequences spanning the periods 0 700 kyr and 0 6 kyr show steadily increasing luminescence with age an age of 62 8 kyr is obtained for a silt radiocarbon dated at 58 8 0 3 kyr problems were found with two much younger samples

identification of sand sources and transport pathways at the kelso dunes california using thermal infrared remote sensing the kelso dune field is located in the eastern mojave desert california at the terminus of a sand transport pathway which has its primary source at the mojave river wash 50 km to the west initial examination of 1984 airborne thermal infrared multispectral scanner tims data showed significant spectral variations that indicate potential mineralogic heterogeneities within the active dunes this result prompted the collection of a suite of 48 sand samples in 1990 and the acquiring of new tims data in 1995 this new data set was used to test a newly developed linear spectral retrieval algorithm in conjunction with a spectral library of end member minerals results of this analysis produced images of end member minerals that showed marked variations within the dunes in addition standard petrographic techniques revealed that the dunes contain mineralogic variations and were much less quartz rich 42 than previously reported 70 90 point count results agreed with the spectral data to within an average of 5 3 for tims derived and 3 1 for laboratory derived mineralogic abundances high concentrations of several of the end member minerals on the surrounding alluvial fans indicate a potential nearby source for these minerals most evident is the presence of potassium feldspar from the fan that emanates from the providence mountains east of the dunes this previously unidentified potential sand input is not visible with other remote sensing techniques and was confirmed with additional field sampling much of the quartz and some plagioclase feldspar however appear to have been transported from the mojave river wash source as previously reported this study also validates the potential of using thermal remote sensing from future satellite based instruments to globally monitor desert fringe areas susceptible to the changing conditions of sand encroachment