a probabilistic model of debris flow delivery to stream channels demonstrated for the coast range of oregon usa debris flows are important geomorphic agents in mountainous terrains that shape channel environments and add a dynamic element to sediment supply and channel disturbance identification of channels susceptible to debris flow inputs of sediment and organic debris and quantification of the likelihood and magnitude of those inputs are key tasks for characterizing spatial and temporal patterns found in channel conditions and associated habitats in a river network widely available digital elevation and land cover data 10 m dems and 25 m satellite imagery offer the potential to assess debris flow runout paths over regional extents this paper presents a model for using these data to calculate empirical probabilities for debris flow runout over dem determined flow paths and shows how these probabilities can be combined over all sources to estimate the potential for debris flow delivery to stream reaches throughout entire channel networks the model is calibrated and model predictions are compared to field mapped debris flow travel paths from study sites in the coast range of oregon usa this model predicts debris flow probability over channel reach scales that can be aggregated to basin scale measures of debris flow potential it offers unprecedented ability to characterize debris flow effects over channel networks providing a tool for risk assessment and for generating hypotheses that relate topographic and forest cover controls on debris flow runout to the types and abundance of channel habitats in a river basin

the kerogen type depositional environment and maturity of the irati shale upper permian of parana basin southern brazil abstract two samples from the upper and lower horizons of the irati oil shale of the parana basin brazil were sampled in a single borehole and analysed using organic petrography and geochemistry the results are interpreted in terms of the kerogen type maturity and depositional environment of the two horizons organic petrography shows the oil shales to be composed of a mineral groundmass mainly clay minerals carbonate and pyrite associated and sometimes impregnated with fluorescing organic material and disseminated phytoclasts humic material is fairly rare and mostly present as very small particles the liptinitic particles are mostly alginite a and b sporinite and more rarely resinite reflectance measurements upper seam 0 34 r0 lower seam 0 40 r0 indicate an equivalent rank of lignite sub bituminous coal astm i e immature with respect to oil and gas generation different organic geochemical methods rock eval pyrolysis solvent extraction gc and gc ms demonstrate both samples to be immature rich oil shales 100 114 kg ton containing type i kerogen of a dominantly bacterially degraded algal origin deposited in a lacustrine environment the presence of botryococcus suggests deposition under fresh brackish water conditions a tentative interpretation of the extract and vitrinite reflectance data suggests a maximum paleo burial of between 1 3 and 2 8 km for the analysed section of the irati formation

strontium isotopes as tracers of ecosystem processes theory and methods abstract the strontium sr isotope method can be a powerful tool in studies of chemical weathering and soil genesis cation provenance and mobility and the chronostratigraphic correlation of marine sediments it is a sensitive geochemical tracer applicable to large scale ecosystem studies as well as to centimeter scaled examination of cation mobility within a soil profile the 87sr 86sr ratios of natural materials reflect the sources of strontium available during their formation isotopically distinct inputs from precipitation dryfall soil parent material and surface or groundwater allow determination of the relative proportions of those materials entering or leaving an ecosystem the isotopic compositions of labile soil exchange complex and soil solution strontium and sr in vegetation reflect the sources of cations available to plants strontium isotopes can be used to track the biogeochemical cycling of nutrient cations such as calcium the extent of cation contributions from in situ weathering and external additions to soil from dust and rain can also be resolved with this method in this paper we review the geochemistry and isotopic systematics of strontium and discuss the use of this method as a tracer of earth surface processes

introducing a terrestrial carbon pool in warm desert bedrock mountains southwestern usa growth of the phoenix metropolitan area led to road cut or house platform exposures of the internal bedrock material of surrounding semiarid mountain ranges similar exposures in the surrounding sonoran and mojave deserts reveal the presence of sedimentary calcium carbonate infilling the preexisting fracture matrix of the bedrock field surveys at 31 sites with bedrock fractures filled with carbonate referred to as bffc in the following text reveal an average of 0 079 0 036 mtc m2 stored in the upper 2 m of analyzed bedrock exposures backscattered electron microscopy images indicate the presence of carbonate at the micron scale not included in this estimate of carbon storage pilot radiocarbon and sr isotope analyses suggest that one of the surveyed bffc veins was flushed into the bedrock from a nonbedrock source during the wetter last glacial period in the late pleistocene

slip rate of the calico fault implications for geologic versus geodetic rate discrepancy in the eastern california shear zone 1 long term 10 5 years fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the eastern california shear zone ecsz the calico fault is one of a family of dextral faults that traverse the mojave desert portion of the ecsz its slip rate is determined from matching and dating incised pleistocene alluvial fan deposits and surfaces displaced by fault slip a high resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features the oldest geomorphically preserved alluvial fan unit b is displaced 900 200 m from its source at sheep springs wash in the northern rodman mountains this fan deposit contains the first preserved occurrence of basalt clasts derived from the pipkin lava field and overlies quaternary conglomerate deposits lacking these clasts the 40 ar 39 ar dating of two flows from this field yields consistent ages of 770 40 ka and 735 9 ka an age of 650 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3 he exposure date of 653 20 ka on a basalt boulder from the surface of unit b this assigned age and offset together yield a mid pleistocene to present average slip rate of 1 4 0 4 mm yr ayounger fan surface unit k records 100 10 m of dextral displacement and preserves original depositional morphology of its surface granitic boulders and pavement samples from this surface yield an average age of 56 4 7 7 ka after taking into account minimal cosmogenic inheritance of granitic clasts the displaced and dated k fans yield a slip rate of 1 8 0 3 mm yr distributed deformation of the region surrounding the fault trace if active could increase the overall displacement rate to 2 1 0 5 mm yr acceleration of slip rate from an average of 1 4 mm yr prior to 50 ka to 1 8 mm yr since 50 ka is possible though a single time averaged slip rate of 1 6 0 2 mm yr satisfies the data these rates are faster than any other paleoseismic or long term slip rate yet determined for other dextral faults in the mojave desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ecsz total displacement across the calico fault diminishes northward as shear is distributed into folding and sinistral faults in the calico mountains this pattern is consistent with an approximately threefold drop in geologic slip rate as the calico fault steps over onto the blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present day strain accumulation in the ecsz

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

dating climatic change in hot deserts using desert varnish on meteorite finds abstract a thin coating of desert varnish occurs on forrest 009 and nurina 004 both equilibrated ordinary chondrite l6 finds from the nullarbor plain australia this finely laminated deposit is chemically and petrographically comparable to the varnish found on terrestrial rocks forrest 009 which has a terrestrial age of 5 9 kyr has a 100 130 m thick coating of desert varnish that has a laterally consistent chemical microstratigraphy comprising a narrow ba and mn poor lower region a thick ba and mn rich central area and a narrow outer zone almost devoid of both cations the interior of the meteorite contains fe oxide and oxyhydroxide veins that have formed by chemical weathering of metals and sulphides as these veins do not cross cut the varnish it must have accreted rapidly relative to the weathering rate of the meteorite the 70 m thick varnish on nurina 004 which has a terrestrial age of 33 4 kyr lacks a consistent chemical microstratigraphy but it is cross cut by fe oxide and oxyhydroxide veins some of which have supplied fe to the varnish this implies that the chemical weathering rate of nurina 004 s interior was slow in comparison to the accretion rate of the varnish the petrography and chemical composition of varnish on forrest 009 indicates that this meteorite may have resided in a relatively humid environment for most of its 5 9 kyr terrestrial history and that the nullarbor recently became more arid this conclusion supports results from an analysis of fe bearing weathering products in the interior of the meteorite by mossbauer spectroscopy which also indicate that forrest 009 experienced an early period of rapid weathering under relatively humid conditions the petrography of varnish on nurina 004 shows that the interior of the meteorite weathered relatively slowly probably because it fell during an arid time which is again in agreement with previous mossbauer spectroscopy results results from both meteorites are in agreement with palaeoclimate data derived from a number of other proxies the implications of this work are that the large number of meteorites that have been collected from several hot deserts of the world may be a powerful source of information on climate change over the last 30 35 kyr

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

chronology of rock falls and slides in a desert mountain range case study from the sonoran desert in south central arizona abstract in order to respond to the general paucity of information on the chronology of ubiquitous small rock falls and slides that litter the slopes of desert mountain ranges a case study in the sonoran desert reveals new insight into the desert geomorphology of mountain slopes rock falls and rock slides in the mcdowell mountains that abut metropolitan phoenix usa fall in three chronometric groupings dated by conventional radiocarbon and rock varnish microlamination methods first the oldest events are 74 ka and take the form of stable colluvial boulder fields positive relief features that are tens of meters long and a few meters wide second randomly sampled slides and falls of various sizes and positions wasted during wetter periods of the terminal pleistocene and holocene third an anomalous clustering of slides and falls occurred during the late medieval warm period medieval climatic anomaly when an extreme storm was a possible but unlikely trigger one speculative hypothesis for the cluster of medieval warm period events is that a small to moderate sized earthquake shook heavily shattered bedrock close to failure just enough to cause a spate of rock falls and slides a second speculative hypothesis is that this dry period enhanced physical weathering processes such as dirt cracking however the reasons for the recent clustering of rock falls remain enigmatic while the temporal distribution of slides and falls suggests a minimal hazard potential for homes and roads on the margins of the mcdowell mountains this finding may not necessary match other desert ranges in metropolitan phoenix or mountains with different rock types and structures that abut other arid urban centers

identification of debris flow hazards in warm deserts through analyzing past occurrences case study in south mountain sonoran desert usa abstract after recognition that debris flows co occur with human activities the next step in a hazards analysis involves estimating debris flow probability prior research published in this journal in 2010 used varnish microlamination vml dating to determine a minimum occurrence of 5 flows per century over the last 8100 years in a small mountain range of south mountain adjacent to neighborhoods of phoenix arizona this analysis led to the conclusion that debris flows originating in small mountain ranges in arid regions like the sonoran desert could pose a hazard two major precipitation events in the summer of 2014 generated 35 debris flows in the same study area of south mountain providing support for the importance of probability analysis as a key step in a hazards analysis in warm desert settings two distinct mechanisms generated the 2014 debris flows intense precipitation on steep slopes in the first storm and a firehose effect whereby runoff from the second storm was funneled rapidly by cleaned out debris flow chutes to remobilize pleistocene debris flow deposits when compared to a global database on debris flows the 2014 storms were among the most intense to generate desert debris flows indicating that storms of lesser intensity are capable of generating debris flows in warm desert settings the 87 sr 86 sr analyses of fines and clasts in south mountain debris flows of different ages reveal that desert dust supplies the fines thus wetter climatic periods of intense rock decay are not needed to resupply desert slopes with fines instead a combination of dust deposition supplying fines and dirt cracking generating coarse clasts can re arm chutes in a warm desert setting with abundant dust

isotopic evidence for eolian recycling of pedogenic carbonate and variations in carbonate dust sources throughout the southwest united states using isotopic ratios of sr c and o we trace calcium carbonate through surface systems across a wide region of semi arid terrain in arizona southwestern usa in order to evaluate the contribution of cations from silicate weathering to soil carbonate we present 87sr 86sr ratios of soil carbonate parent rock dry river course silts floodplain playa dust and rain samples as well as 18o and 13c values of selected samples results show that both parent rock and dust are important sources of cations for soil carbonate in this inland setting where bedrock is dominated by silicate lithologies dust in southeast arizona has higher 87sr 86sr ratios 0 7100 0 7123 than phanerozoic sea water 0 7070 0 7096 these high ratios derive ultimately from silicate rocks our 18o and 13c data show clearly that the dominant source of carbonate dust is eroded older soil carbonate not bedrock limestone because dust contributes significantly to newly forming soil carbonate some products of silicate weathering may reside in soil carbonate two or more times before being removed from the region and this recycling retards the rate at which the products of silicate weathering enter the sedimentary system comparison of the 87sr 86sr ratios of carbonate dust from southeast arizona with those from surrounding regions shows that dust 87sr 86sr ratios and by inference carbonate dust sources vary on a scale of 200 300 km in the southwest united states

cosmogenic 3he age estimates of plio pleistocene alluvial fan surfaces in the lower colorado river corridor arizona usa abstract plio pleistocene deposits of the lower colorado river lcr and tributary alluvial fans emanating from the black mountains near golden shores arizona record six cycles of late cenozoic aggradation and incision of the lcr and its adjacent alluvial fans cosmogenic 3he 3hec ages of basalt boulders on fan terraces yield age ranges of 3 3 2 2 ma 2 2 1 1 ma 1 1 ma to 110 ka

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

dendrogeomorphic approaches for identifying the probable occurrence of debris flows and related torrential processes in steep headwater catchments the hrubý jesenik mountains czech republic abstract culminating parts of mid mountain ranges in central europe are interwoven by a network of high gradient streams these parts together with the surrounding slopes form a coupling system in which hazardous geomorphic processes such as landslides debris flows or flash floods can occur in the case of the hrub jesenik mountains eastern sudetes czech republic we present an evaluation of the debris flows and related torrential processes in the high gradient streams of the selected catchment based on dendrogeomorphic methods in addition to the classical dendrogeomorphic approaches we refined the procedure of identifying certain and probable events using new weighted indexes for event reconstruction additionally because of the steep narrow channels without a characteristic accumulation cone a new spatial approach is defined based on the spacing of disturbed trees for each event random distribution or clustering and location of disturbed trees in an individual section of the valley floor using kernel density analysis for each event and calculating the particular grids in arcgis software we are able to describe the spatial reach and probable nature of an event debris flow or hyperconcentrated flow dendrogeomorphic analysis of 397 sampled trees predominantly coniferous trees revealed 24 torrential events 15 certain nine probable since 1928 with peaks in 1991 1997 and 2010 according to index values in addition the spatial distribution of disturbed trees grouped in clusters in these years indicates debris flow events in the upper parts of the catchment it seems that new dendrogeomorphic approaches should be valuable in remote steep headwater catchments where several types of processes affect the valley floors

revisiting dirt cracking as a physical weathering process in warm deserts abstract a half century ago c d ollier proposed that insolation driven temperature changes expand and contract fill in fissures enough to widen cracks a process that would permit progressively deeper penetration of fissure fills that would in turn generate a positive feedback of greater and greater strain until desert boulders and bedrock shatters although desert physical weathering by dirt cracking has occasionally been cited this hypothesized process remains without support from subsequent research here field observations electron microscopy x ray powder diffraction particle size analysis and laboratory experiments shed new light on dirt cracking little clear evidence supports the original notion of expansive pressures from thermal fluctuations however mineralogical high resolution transmission electron microscopy back scattered electron microscopy and experimental evidence support two alternative processes of widening fractures wetting and drying of fills inside fissures and the precipitation and remobilization of calcium carbonate a re envisioned dirt cracking wedging process starts with calcium carbonate precipitating in fissures less than 5 m wide first precipitation and then ongoing dissolution of this laminar calcrete opens enough space for dust to penetrate into these narrow fractures wetting of expansive clays in the fissure fill exerts enough pressure to widen and deepen the fissure allowing the carbonate precipitation process to penetrate even deeper and allowing even more dust to move into a fracture as the dust infiltrates its texture changes from a chaotic mix of particles to an alignment of clays parallel to fissure sides this parallel alignment could increase the efficiency of fill wedging ollier s concept of a positive feedback remains supported each increment of fracture deepening and widening permits more even deeper infiltration of laminar calcrete and dust field and electron microscope observations of rock spalling in the winter of 2010 are consistent with ollier s hypothesis that dirt cracking is a common physical weathering process in deserts that splits rocks of all different sizes

quantitative strontium isotope models for weathering pedogenesis and biogeochemical cycling abstract isotopes of strontium sr are a useful tracer for weathering atmospheric fluxes cation biocycling and pedogenesis we present basic models for application of strontium isotopes to the soil vegetation atmosphere system the mathematical formulations fall into the general categories of 1 steady state models in which isotopic ratios remain constant over the time scale of interest and 2 time dependent models in which isotope ratios change through time in the steady state models fluxes of sr and other elements to the system are constant steady state models can be used to infer short term weathering rates from river and stream isotope compositions to determine fluxes to a single or multiple layer soil exchange solution system and to quantify nutrient fluxes to vegetation time dependent models involve a change in isotopic ratio from some initial value to a new value over the time period of interest in some cases the change may represent a shift from one steady state situation to a new one after a shift in one or more of the fluxes feeding the system examples of applications of time dependent models include identifying the dominant cation sources to an evolving soil exchange solution system and calculating weathering rates by measuring the isotopic compositions of primary soil minerals we use time dependent models to explain differences in the isotopic ratios of labile and carbonate sr from arid sites in hawaii with a basalt parent material isotopic signature and new mexico with an atmospheric isotopic signature these models suggest that the difference is due to a combination of low atmospheric strontium fluxes and high weathering rates in the hawaiian profile compared to the new mexico calcrete profile

initiation conditions for debris flows generated by runoff at chalk cliffs central colorado abstract we have monitored initiation conditions for six debris flows between may 2004 and july 2006 in a 0 3 km 2 drainage basin at chalk cliffs a band of hydrothermally altered quartz monzonite in central colorado debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14 to 45 the availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes rainfall exceeding i 6 61 d 0 77 where i is rainfall intensity mm h and d is duration h was required for the initiation of debris flows in the drainage basin the approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0 15 m 3 s colluvium in the basin was unsaturated immediately prior to antecedent and during debris flows antecedent volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4 9 and 4 7 respectively during debris flows peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10 20 and 4 12 respectively channel sediment at a depth of 45 cm was unsaturated before and during debris flows antecedent moisture ranged from 20 22 and peak moisture ranged from 24 38 although we have no measurements from shallow rill or channel sediment we infer that it was unsaturated before debris flows and saturated by surface water runoff during debris flows our results allow us to make the following general statements with regard to debris flows generated by runoff in semi arid to arid mountainous regions 1 high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff 2 locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows and 3 rainfall and simulated surface water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment

impact of consecutive extreme rainstorm events on particle transport case study in a sonoran desert range western usa abstract quantifying erosion rates in different landscape settings provides insight into how landforms change under different climatic tectonic and anthropogenic influences sediment traps designed to capture grus detached from granitic hill crests of an arid sonoran desert mountain range were placed prior to every precipitation event over a three year period just above rills that drain areas between 18 and 68 m 2 the slopes are underlain by moderately to strongly weathered granitic rocks to a depth of about a meter within this 3 year window a 1000 year precipitation event followed 27 days later by a 500 year event detached granitic grus in amounts far greater than previous storms capturing between 22 and 63 the average amount transported in the previous 59 rain events indicating the non linear nature of the response of grus detachment to precipitation intensity considering every precipitation event over a 3 year period no detachment occurred from events with less than 2 mm of total rainfall and only minimal erosion occurred from rainfall events with totals between 2 and 10 mm with durations typically less than 30 min detachment increased greatly with rain intensities of 36 mm h or more grus detachment from these arid crests increases with drainage area a higher percentage of exposed soil and steeper slopes 87 sr 86 sr ratios reveal that suspended sediment transported from hill crest to trap derives from recycled dust and not the local granite bedrock

debris flows from small catchments of the ma ha tuak range metropolitan phoenix arizona abstract debris flows debauch from tiny but steep mountain catchments throughout metropolitan phoenix arizona usa urban growth in the past half decade has led to home construction directly underneath hundreds of debris flow channels but debris flows are not recognized as a potential hazard at present one of the first steps in a hazard assessment is to determine occurrence rates the north flank of the ma ha tuak range just 10 km from downtown phoenix was selected to determine the feasibility of using the varnish microlaminations vml method to date every debris flow levee from 127 catchment areas only 152 of the 780 debris flow levees yielded vml ages in a first round of sampling this high failure rate is due to erosion of vml by microcolonial fungi the temporal pattern of preserved debris flow levees indicates anomalously high production of debris flows at about 8 1 ka and about 2 8 ka corresponding to northern hemisphere climatic anomalies because many prior debris flows are obliterated by newer events the minimum overall occurrence rates of 1 3 debris flows per century for the last 60 ka 2 2 flows century for the latest pleistocene and 5 flows century for the last 8 1 ka has little meaning in assessment of a contemporary hazard this is because newer debris flows have obliterated an unknown number of past deposits more meaningful to a hazards analysis is the estimate that 56 flows have occurred in the last 100 years on the north side of the range an estimate that is consistent with direct observations of three small debris flows resulting events from a january 18 22 2010 storm producing 70 mm of precipitation in the ma ha tuak range and a 500 m long debris flow in a northern metropolitan phoenix location that received over 150 mm of precipitation in this same storm these findings support the need for a more extensive hazard assessment of debris flows in metropolitan phoenix

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

mid holocene geochronology palaeoenvironments and occupational dynamics at quebrada de amaicha tucuman argentina this paper focuses on three interconnected issues lithic technology geochronology and climate change based on rock varnish proxies and artifact reclamation processes ongoing research along the western the semiarid slope of cumbres calchaquies aconquija ranges and centered in the amaicha del valle archaeological locality has focused on lithic surface scatters these are the product of multiple behavioral events an accretion phenomena a palimpsest areas that were repetitively visited used and occupied over the long run for procurement of lithic raw material manufacture of diverse types of artifacts and specific activities research results suggest based on typological analysis of lithic surface scatters a multi component occupation that may have included site specific activities based on rock varnish microlaminations vml studies a correlation dating technique that is a reliable time clock for establishing chronological control human occupation was already in place prior to 6500 5900 years further rock varnish carries a past climate signal indicating eight regional wet events for the last 7000 years four of which are mid holocene rock varnish differential formation on rocks and artifacts provide additional insights into the occupational dynamics and functionality of sites

orographic influences on a great salt lake effect snowstorm abstractalthough several mountain ranges surround the great salt lake gsl of northern utah the extent to which orography modifies gsl effect precipitation remains largely unknown here the authors use observational and numerical modeling approaches to examine the influence of orography on the gsl effect snowstorm of 27 october 2010 which generated 6 10 mm of precipitation snow water equivalent in the salt lake valley and up to 30 cm of snow in the wasatch mountains the authors find that the primary orographic influences on the event are 1 foehnlike flow over the upstream orography that warms and dries the incipient low level air mass and reduces precipitation coverage and intensity 2 orographically forced convergence that extends downstream from the upstream orography is enhanced by blocking windward of the promontory mountains and affects the structure and evolution of the lake effect precipitation band and 3 blocking by the wasatch and oquirrh mountains which funnels the flow into the sal

sources of strontium and calcium in desert soil and calcrete the carbon cycle significance of soil carbonate fluxes is subject to large uncertainties because it is not clear precisely how much calcium is derived from atmospheric sources compared with that from the chemical weathering of silicate minerals in the petrocalcic horizon calcrete of a pleistocene soil from the usda scs desert project area near las cruces nm approximately 1 5 g ca cm 3 has been added with an associated expansion of the profile of200 strontium isotope values for the labile cations and carbonate from the a b and k soil horizons have 87 sr 86 sr values that range from 0 7087 to 0 7093 similar to the values for easily soluble local dust and rain the parent material non calcareous camp rice alluvial sediment has a 87 sr 86 sr ratio of0 7165 mixing calculations indicate a minimum atmospheric contribution to soil carbonate calcium of 94 the more likely scenarios indicate at least 98 of the ca originated from atmospheric input the variations in 87 sr 86 sr ratios of soil silicate 0 7131 to 0 7173 are consistent with weathering of volcanogenic sediments and neoformation of clay minerals in the petrocalcic horizon moreover the sr isotope data suggest that 50 70 of silicate in the uppermost 25 cm of the profile could be atmosphere derived the isotopic composition of labile strontium in the a horizon and the mass distribution of silicon and calcium indicate that the uppermost portion of the profile is the present zone for the release of cations due to silicate weathering steady state models of the whole profile yield a sr weathering flux ranging from200 to 400 m gc m 2 ma 1 the results indicate that both the present day and long term contribution of calcium from silicate weathering is less than 2 of that supplied from the atmosphere and confirm that desert soil formation is not a significant sink for atmospheric carbon 1999 elsevier science b v all rights reserved

dating of debris flow fan complexes from lantau island hong kong china the potential relationship between landslide activity and climate change five debris flow fan complexes bordering the coastal foothills of western lantau island hong kong have been the subject of a stratigraphic and dating study to explore the potential relationship between past periods of climate change and landslide activity luminescence osl ages from the fan complexes supported by a few calibrated radiocarbon 14c ages on duplicate samples suggest six main periods of accumulation 28 000 20 000 ka 14 500 10 500 ka 5800 5000 ka 4370 4230 ka 3560 3320 ka and 1350 1050 ka the younger periods 10 000 ka appear to be dominated by relatively thin 0 5 1 5 m units deposited mainly by debris flood events whereas the older periods are dominated by thicker 1 0 3 5 m units deposited by debris flow events we hypothesise that immediately prior to the last glacial maximum lgm i e 21 000 ka the climate was cool and dry upland areas were probably devoid of vegetation providing favourable conditions for sediment production in source areas these debris laden source areas are likely to have been mobilised during periodic rainstorm activity rapidly channelised over relatively short drainage courses and then deposited as thick sediment rich debris flow deposits along the coastal foothills during the early to middle holocene the climate was warmer and more humid pluvial conditions dominated forests occupied the upland source areas leading to possibly reduced sediment yields more frequent flash floods and deposition dominated by debris floods compared with recent studies on the weathering and erosion history of the pearl river delta our landslide age data are indicative of a potential link between increased landslide activity and intensification of the east asian monsoon during the early to middle holocene

spatial variations in slip rate along the death valley fish lake valley fault system determined from lidar topographic data and cosmogenic 10be geochronology 1 the death valley fish lake valley fault zone dv flvfz is a prominent dextral fault system in the eastern california shear zone ecsz combining offset measurements determined with lidar topographic data for two alluvial fans with terrestrial cosmogenic nuclide 10be ages from the fan surfaces yields a late pleistocene slip rate of 2 5 to 3 mm yr for the northern part of the dv flvfz in fish lake valley these rates are slower than the late pleistocene rate determined for the system in northern death valley indicating that slip rates decrease northward along this major fault zone when summed with the slip rate from the white mountains fault the other major fault in this part of the ecsz our results suggest that either significant deformation is accommodated on structures east of fish lake valley or that rates of seismic strain accumulation and release have not remained constant over late pleistocene to holocene time

crustal structure from three dimensional gravity modeling of a metamorphic core complex a model for uplift santa catalina rincon mountains arizona gravity modeling shows that the observed fluctuations in gravity over the catalina rincon metamorphic core complex of southeastern arizona are caused primarily by shallow crustal density contrasts the wilderness suite granites the probable source of a 20 mgal low over the central part of the core complex may extend to depths of 7 12 km below the surface seismic refraction information is consistent with the existence of a deep crustal root under the catalina rincon core complex we hypothesize that the voluminous wilderness suite granites are related to a crustal thickening event that built the root in early tertiary time modeling of flexural isostatic uplift resulting from an excess crustal root predicts two episodes of uplift for the catalina rincon metamorphic core complex 1 low relief flexural uplift in the middle tertiary and 2 complete uplift precipitated by high angle basin and range faulting at 10 15 ma the uplift and exposure of core complexes therefore can be an isostatically driven process that occurs in regions of locally thickened crust

reconstructing the timing of flash floods using 10be surface exposure dating at leidy creek alluvial fan and valley white mountains california nevada usa large alluvial fans characterize the piedmonts of the white mountains california nevada usa with large boulders strewn across their surfaces the boulders are interpreted as flash floods deposits with an unclear trigger for the transport process several triggers are possible including glacial lake outburst floods glofs thunderstorms or rainfall on snow cover from a paleoenvironmental perspective the origin of the flash floods is of fundamental importance the alluvial fans that flank the white mountains at leidy creek display particularly impressive examples of these deposits the boulder deposits and the source catchment at leidy creek were examined using 10be terrestrial cosmogenic nuclide tcn surface exposure dating to help elucidate their age and origin all boulders dated on the alluvial fans date to the holocene this is in accordance with the geomorphic analyses of the leidy creek catchment and its terraces and sediment ridges which were also dated to the holocene using optically stimulated luminescence osl and 10be surface exposure the results suggest that the boulders on the alluvial fan were deposited by flash floods during thunderstorm events affecting the catchment of the leidy creek valley paleomonsoonal induced mid holocene flash floods are the most plausible explanation for the discharges needed for these boulder aggradations but a regional dataset is needed to confirm this explanation

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

millennial scale varnish microlamination dating of late pleistocene geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a climate based correlative age determination technique used to correlate and date various geomorphic features in deserts in this study we establish a generalized late pleistocene 18 74 ka millennial scale microlamination sequence in fine grained fast accumulating rock varnish for the drylands of western usa radiometrically calibrate the sequence and correlate it with the 18 o record in the gisp2 greenland ice core we then use this climate correlated varnish microstratigraphy to estimate surface exposure ages for radiometrically dated late pleistocene geomorphic features in the study region the vml dating of debris flow deposits on the sehoo recessional shorelines of lake lahontan at the jessup embayment of central nevada yields a minimum limiting age of 14 95 15 95 ka in good agreement with a calibrated 14 c age of 15 22 0 12 ka for the timing of the lake recession the vml dating of a giant ejecta block on the rim of meteor crater in northern arizona yields a minimum limiting age of 49 15 ka closely matching a thermoluminescence tl age of 49 3 ka and slightly younger than a recently updated cosmogenic 36 cl age of 56 0 2 4 ka for the meteor impact event the vml dating of distal q2c fan surfaces on hanaupah canyon alluvial fan in death valley california yields a minimum limiting age of 73 55 ka in accord with cosmogenic 36 cl depth profile ages of 66 22 14 ka and 72 24 20 ka for the same fan deposits the close agreement between the vml age estimates and the independently derived radiometric ages for these geomorphic features attests to the validity and reliability of millennial scale vml dating to further assess its potential in desert geomorphological research we use the vml method to study alluvial fan responses to millennial scale climatic changes the vml dating of a small tributary fan in death valley reveals two episodes of fan aggradation one ceasing at 73 55 86 75 ka during the dry period of the last interglacial mis 5a and the other finishing at 66 15 ka during the wet period of the last glacial mis 4 the vml and 36 cl dating of the distal q2c fan surfaces on hanaupah canyon fan reveal two episodes of large scale fan aggradation ended at 72 24 20 ka and 73 55 ka during the wet period of mis 4 fanhead incision and associated within channel or fantoe aggradation are found to take place during the relatively dry period of the glacial to interglacial climatic transition 12 24 ka and the holocene interglacial dry period 0 12 ka these data indicate that on the millennial to sub milankovitch timescale 10 3 10 4 years fan aggradation is a discrete sedimentational process under various climatic conditions because fan aggradation is ultimately controlled by the intensity and frequency of precipitation events which in turn are modulated by major climatic oscillations such as heinrich events dansgaard oeschger do events and glacial interglacial shifts these major climatic changes could be the pacemaker of regionally contemporaneous large area fan segmentation