evidence for muon induced production of 10be in near surface rocks from the congo distributions of 10be in vein quartz from a lateritic weathering profile from the congo provide evidence for significant in situ production of 10be by reactions other than neutron induced spallation after consideration of geomorphological and nuclear processes which might contribute to the observed depth variability we conclude that cosmic ray muon induced reactions are the most likely mechanism and that such reactions lead to 1 to 3 of 10be production at the surface this small muon induced component has only minor implications for the use of 10be for quantification of exposure histories of surficial rocks nevertheless the muogenic component has the potential to expand the utility of cosmogenic nuclides in examination of surficial processes overcoming some of the limitations associated with the use of the neutron produced component

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

complex multiple cosmogenic nuclide concentration and histories in the arid rio lluta catchment northern chile terrestrial cosmogenic nuclide tcn concentrations measured in river sediments can be used to estimate catchment wide denudation rates by investigating multiple tcn the steadiness of sediment generation transport and depositional processes can be tested measurements of 10be 21ne and 26al from the hyper to semi arid rio lluta catchment northern chile yield average single denudation rates ranging from 12 to 75 m myr 1 throughout the catchment paired nuclide analysis reveals complex exposure histories for most of the samples and thus the single nuclide estimates do not exclusively represent catchment wide denudation rates the lower range of single nuclide denudation rates 12 17 m myr 1 established with the noble gas 21ne is in accordance with palaeodenudation rates derived from 21ne 10be and 26al 10be ratio analysis since this denudation rate range is measured throughout the system it is suggested that a headwater signal is transported downstream but modulated by a complex admixture of sediment that has been stored and buried at proximal hillslope or terrace deposits which are released during high discharge events that is best evidenced by the stable nuclide 21ne which preserves the nuclide concentration even during storage intervals the catchment wide single 21ne denudation rates and the palaeodenuation rates contrast with previous tcn derived erosion rates from bedrock exposures at hillslope interfluves by being at least one order of magnitude higher especially in the lower river course these results support earlier studies that identified a coupling of erosional processes in the western cordillera contrasting with decoupled processes in the western escarpment and in the coastal cordillera copyright 2008 john wiley sons ltd

denudation rates and a topography driven rainfall threshold in northern chile multiple cosmogenic nuclide data and sediment yield budgets abstract the quantification of geomorphic process rates on the outcrop and the orogen scale is important to describe accurately the interaction between the relative effects of erosion tectonics and climate on landscape evolution we report single and paired cosmogenic nuclide 10 be 26 al and 21 ne derived erosion rates and exposure ages on hillslope interfluves from the tectonically active western central andes that show a distinct spatial variation a positive correlation of erosion rates with elevation and present day rainfall rates is observed erosion rates at lower altitudes the hyperarid coastal cordillera and the western escarpment with the northern part of the atacama desert are extremely low and of the order of 10 100 cm my nominal exposure ages 1 6 my in contrast erosion rates at higher altitudes the semiarid western cordillera range up to 4600 cm my nominal exposure ages 0 02 0 1 my this latter average long term bedrock erosion rate record suggested to be coupled to an orographically controlled pattern of rainfall is also reflected in the pattern of denudation rates derived from a short term decadal record of limited sediment yield data specifically denudation rates calculated from sediment flux data are of a similar order of magnitude as erosion rates deduced from long lived cosmogenic nuclides from bedrock hillslope interfluves of the western cordillera nevertheless the production and the supply of sediment from the western andean slope are very limited analysis of multiple cosmogenic nuclides allows simultaneous determination of erosion rates and exposure ages but also reveals complex exposure histories of non bedrock samples such as boulders or amalgamated clast samples notably this study shows that saturation of nuclides usually assumed in studies where only a single nuclide is analyzed is rather the exception than the rule as revealed by erosion island plots constant erosion that started much later than the formation age of the rocks or episodic erosion by spalling can partially explain non steady state concentrations and more complicated exposure scenarios furthermore the use of multiple nuclides with different half lives allowed us to infer that no significant variations in long term erosion rates have occurred and that at the western escarpment erosion rates have been low and constant for most of the late neogene nevertheless the time intervals necessary to reach steady state concentrations for cosmogenic nuclides can be quite different from those needed for landscapes to reach steady state

the cosmogenic 21ne production rate in quartz evaluated on a large set of existing 21ne 10be data abstract based on a compilation of published combined 10 be and 21 ne cosmogenic nuclide data sets from quartz samples obtained at eth zurich we assess the 21 ne 10 be p21 p10 production rate ratio with the goal to determine the 21 ne production rate p21 in quartz a variety of sliding erosion islands in a 21 ne 10 be versus 10 be diagram were evaluated to find the one that fits the data best which in turn yields the most probable p21 if the 10 be production rate is known the approach minimizes the influence of samples with a complex exposure history a best fit value for p21 p10 sp sp the 10 be fraction being produced by spallation as opposed to production by muons of 4 23 0 17 is obtained for a 10 be half life of 1 39 ma adopting a p10 sp value in quartz of 4 41 0 52 at g 1 yr 1 this yields a p21 of 18 7 2 3 at g 1 yr 1 it is possible that 2 of the 21 ne is produced by fast muons

late pleistocene landscape evolution in south central chile constrained by luminescence and stable cosmogenic nuclide dating landscapes in tectonically active regions develop in response to a complex interplay between rapid tectonic deformation and surface processes in order to understand the evolution of these environments and distinguish between the different factors it is fundamental to identify and quantify geomorphic processes and rates on different time scales the forearc of south central chile provides an ideal setting to elucidate the response of surface processes to climatic and tectonic forcing however in contrast to northern chile this region has attracted little attention due to difficulties in quantifying the age of geomorphic surfaces we use luminescence and stable cosmogenic nuclides 3 he 21 ne on conglomeratic units in the central depression and fluvial valley fills in the coastal cordillera to constrain exposure and burial ages of these units that form major landscape elements in the southern chilean forearc our data indicate that disturbances in the drainage network are controlled by local uplift which may be caused by deformation above an active blind thrust in the coastal cordillera local short term uplift rates reach a minimum of 0 27 mm a 1 and are on the same order as late quaternary minimum incision rates of 0 15 mm a 1 to 0 43 mm a 1 the valley fills in the coastal cordillera have depositional ages of 80 ka and 255 ka respectively exposure ages of the extensive alluvial surfaces in the central depression cluster between 135 ka and 175 ka and between 240 ka and 280 ka hence the deposition of these surfaces broadly coincides with marine isotope stages mis 6 and 8 and appears to be climatically driven

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

uplift of the altiplano puna plateau a view from the west 1 the western flank of the central andean plateau is a crustal scale monoclinal fold expressed in the geomorphology and in the westward tilt of fore arc basin strata data from three fore arc basins quantify the magnitude and time of displacement of the plateau system relative to the fore arc from 18 30 s to 22 s there is a single monocline strand there other authors documented 2000 m 500 m of early and middle miocene structural relief growth across small scale monoclines and our data reveal 810 m 640 m of 11 5 ma relief growth and 400 m 170 m relief growth since 5 ma across a long wavelength monoclinal fold limb this structural relief growth since 11 ma approximates the topographic relief growth between the fore arc and the altiplano plateau from 22 s to 24 s there are two subparallel long wavelength monoclines structural relief on the east side of the fore arc increased by 2840 m 2510 m during 17 10 ma and by 2320 m 1050 m since 10 ma some part of this monoclinal limb rotation led to topographic uplift of the adjacent puna plateau rotation across the western monocline could have increased the altitude of the fore arc basins by 900 m since 6 ma and may result from intense heating of the eastern margin of the cold and strong atacama lithosphere block although the wavelengths of the monoclines vary along strike and through time the monoclinal style of deformation dominated the uplift history of the eastern fore arc of the central andes throughout the neogene

review of erosion dynamics along the major n s climatic gradient in chile and perspectives abstract chile is an elongated country running in a north south direction for more than 30 along a subduction zone its climate is progressively wetter and colder from north to south this particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate land use slope tectonics volcanism etc here we review the erosion rates factors and dynamics over millennial to daily periods reported in the literature in addition 21 new catchment mean erosion rates suspended sediment and 10be are provided and previous suspended sediment derived erosion rates are updated a total of 485 local and catchment mean erosion rates are reported erosion rates vary between some of the smallest values on earth 10 5 mm a to moderate values 0 5 mm a compared to other active ranges this review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location slope and rainfall data for the mean erosion rates e for the millennial and decennial catchments a model of the form e s 1 s 0 6 2 r with 0 3 0 8 accounts for 40 to 70 of the erosion variance confirming a primary role of slope s compared to precipitation rate r over this time scale over the long term this review points to the long 5 to 10 ma response time of rivers to surface uplift in north central arid chile over millennia data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates as well as the link between glacier erosion and glacier sliding velocity in this period of time a discrepancy exists between the long term offshore sedimentological record and continental decennial or millennial erosion data for which no single explanation appears still little information is available concerning the magnitude of variation of millennial erosion rates over centuries data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years probably associated with historical harvesting

late cenozoic geomorphologic signal of andean forearc deformation and tilting associated with the uplift and climate changes of the southern atacama desert 26 s 28 s abstract we analyze remarkable examples of the large 10 000 km2 and local scale 100 km2 landscape forms related to late cenozoic geomorphologic evolution of the andean forearc region in the southern atacama desert we also consider the continental sedimentary deposits so called atacama gravels which are related to the degradation of the landscape during the neogene our analysis integrates 1 50 000 field cartography landsat tm images observations 1 1000 sedimentary logging data and 50 m horizontal resolution topographic data to reconstruct the late cenozoic geomorphologic evolution of this region and discuss the factors that control it i e miocene aridification of the climate and neogene central andean uplift we determine that the precordillera was already formed in the oligocene and most of the present day altitude of the precordillera was reached before that time afterward five episodes of geomorphologic evolution can be differentiated 1 the development of an oligocene deep incised drainage system cutting the uplifted precordillera up to 2000 m of vertical incision and connecting it to the ocean followed by 2 the infilling of deep incised valleys by up to 400 m of atacama gravels this infill started in the early miocene with the development of fluvial deposition and finished in the middle miocene with playa and playa lake depositions we propose that playa related deposition occurs in an endorheic context related to tectonic activity of the atacama fault system and coastal cordillera uplift however the upward sedimentologic variation in the atacama gravels evidences a progressive aridification of the climate subsequently we have identified the effects of the middle upper miocene slow tectonic deformation the neogene andean uplift is accommodated by a tilting or flexuring of the inner forearc central depression and precordillera related to some hundreds of meters of uplift in the precordillera this tilting or flexuring results in 3 the middle miocene re opening of the valley network to the pacific ocean upper miocene aridification from arid to hyperarid induces alluvial fans backfilling in the central depression 4 resulting in up to 50 m of atacama gravel deposition finally in response to an increase in the rate of tilting a new phase of vertical incision up to 800 m in the precordillera allows the development of the canyon that crosses the forearc 5

cosmic ray produced 21ne in terrestrial quartz the neon inventory of sierra nevada quartz separates the study of cosmic ray produced radioactive and stable nuclides on the surface of the earth can provide relevant geomorphological and glaciological information at present the cosmic ray production rates of stable 21ne are not well known this study attempts to remedy the situation by determining the production rate ratio of 21ne and 26al p21p26 in quartz 26al concentrations and p26 rates have previously been investigated for quartz separates of sierra nevada rocks which were brought to the surface by glacial scouring during the tioga period at the end of the last ice age 1 we used splits of the same samples for our studies and found that ne in these rocks represents a mixture of several components trapped ne nucleogenic 21ne and 22ne produced by n reactions in oxygen and fluorine respectively as well as cosmic ray produced ne which is the component of interest in this study the trapped component was substantially lost in one sample w86 12 by crushing and by a density separation of the grain sizes 38 90 m and 90 125 m permitting the resolution of the in situ produced 21ne into cosmic ray spallation and n produced components and the determination of a lower limit to p21p26 in a second sample w86 8 one split contained small enough amounts of nucleogenic 21ne to permit the determination of a reasonable upper limit to p21p26 the two ratio determinations are consistent within error limits and the value adopted 0 65 0 11 2 agrees with ratios observed in extraterrestrial matter apparently p21p26 is thus not very sensitive to the neutron spectrum however the observed production rate ratio is substantially larger than theoretical estimates for si targets reflecting poorly known neutron excitation functions the above p21p26 value coupled to the observed 26al production rate 1 corresponds to a 21ne production rate of p21 21 atoms g 1 a 1 in quartz or to p21 45 atoms g si 1 a 1 at sea level and high latitudes this rate is based on an adopted exposure age of 11 000 yr for our quartz samples

wind as the primary driver of erosion in the qaidam basin china deserts are a major source of loess and may undergo substantial wind erosion as evidenced by yardang fields deflation pans and wind scoured bedrock landscapes however there are few quantitative estimates of bedrock removal by wind abrasion and deflation here we report wind erosion rates in the western qaidam basin in central china based on measurements of cosmogenic 10 be in exhumed miocene sedimentary bedrock sedimentary bedrock erosion rates range from 0 05 to 0 4 mm yr although the majority of measurements cluster at 0 12570 05 mm yr these results combined with previous work indicate that strong winds hyper aridity exposure of friable neogene strata and ongoing rock deformation and uplift in the western qaidam basin have created an environment where wind instead of water is the dominant agent of erosion and sediment transport its geographic location upwind combined with volumetric estimates suggest that the qaidam basin is a major source up to 50 of dust to the chinese loess plateau to the east the cosmogenically derived wind erosion rates are within the range of erosion rates determined from glacial and fluvial dominated landscapes worldwide exemplifying the effectiveness of wind to erode and transport significant quantities of bedrock

landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 ma atacama desert chile the landscape of the hyperarid atacama desert in northern chile records extremely slow change on earth s surface disputed ages for the onset of hyperaridity range from the late paleogene through the pleistocene a long term paleoclimate record is recorded in a nonmarine basin whose fill is primarily alluvial strata for this setting the primary proxies for climate state are the mineralogical and chemical composition of soil which varies across a precipitation gradient and the landforms and deposits of alluvial fans during the most recent 15 million years five climate related landscape stages are resolved for the pampa del tamarugal sedimentary basin with each successively younger stage inset lower in the local topography than its predecessor the earliest landscape stage is expressed as a set of alluvial strata inherited from a time of arid or semi arid climate ca 14 12 ma the younger four landscape stages generated a composite long lasting exposure surface predominantly hyperarid conditions have persisted since ca 12 1 ma during which four intervals of arid to semi arid climate occurred each wet interval was short lived a million years or less whereas some of the hyperarid periods were lengthy 1 5 m y the hyperarid intervals were roughly 11 5 5 ma 4 5 4 ma 3 6 2 6 ma 2 2 1 ma and repeated intervals during the last 1 m y the onset of hyperaridity ca 12 ma likely reflects the growth of the andes mountains above a climate threshold in contrast sea surface temperature variability likely has controlled atacama paleoclimate changes since the late miocene

andean uplift and climate evolution in the southern atacama desert deduced from geomorphology and supergene alunite group minerals abstract supergene alunite group minerals from the late eocene el salvador porphyry cu district the el hueso epithermal gold deposit and the coya porphyry au prospect located in the precordillera of northern chile 26 to 26 30 lat s have been dated by the 40 ar 39 ar method and analyzed for stable isotopes these data support published geomorphologic and sedimentologic evidence suggesting that the precordillera in the southern atacama desert had been uplifted as early as the late eocene and thus significantly prior to the altiplano which attained its high elevation only in the late miocene the oldest supergene alunite from the damiana exotic deposit at el salvador was dated at 35 8 1 ma and yielded a d vsmow value of 74 which indicates elevations of the precordillera near el salvador of at least 3000 m in the late eocene in contrast miocene supergene alunite from el salvador el hueso and coya have less negative d signatures reaching values as high as 23 to 25 at el hueso and el salvador between about 8 2 and 14 ma late miocene to holocene supergene alunite jarosite and natroalunite ages are restricted to el hueso and coya located near 4000 m above sea level in the precordillera roughly 1000 m higher than the present elevation of el salvador the d values of samples younger than 5 ma vary between 57 and 97 the complex evolution of the d signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the central depression of the southern atacama desert after 15 ma which agrees well with published constraints from the atacama desert at 23 24 lat s our data also suggest that wetter climatic conditions than at present prevailed in the latest miocene and early pliocene in the precordillera the new and previously published age constraints for el salvador indicate that supergene mineralization at the damiana exotic cu deposit occurred periodically over 23 ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration

improving in situ cosmogenic chronometers new radiocarbon ages for sierra nevada deglaciation the first 10 be measurements from the laurentide terminal moraine and calculations based on paleomagnetic field strength have the potential to substantially improve the accuracy of cosmogenic age estimates specifically three new constraints apply to the interpretation of measured abundances of in situ produced cosmogenic 10be and 26al 1 a suite of minimum limiting radiocarbon dates indicates that the sierra nevada was deglaciated at least several thousand years earlier than assumed when nishiizumi et al 1989 first calibrated 10be and 26 al production rates based on polished bedrock surfaces in the range with retreat beginning by 18 000 cal yr b p and completed by 13 000 cal yr b p 2 concentrations of 10be in moraine boulders and glacier polished bedrock in new jersey show little variance 10 1 and can be used to calculate a preliminary 10be production rate integrated over the past 21 000 22 000 cal yr b p at 41 200 300 m altitude that is about 20 lower than currently accepted 3 calculations of the effect of past geomagnetic field strength variations on production rates suggest that the use of temporally averaged production rates may generate age errors of 20 however cosmogenic exposure ages can be corrected for this effect although the corrections currently are imprecise many previously reported late pleistocene 10be and 26al exposure ages are probably too young and are less accurate and less precise than implied by reported uncertainties the discrepancy between accepted production rates and those calculated from laurentide exposures when considered together with the sierran deglacial chronology and the model results suggest that correlations between cosmogenic and other numerical ages especially for brief events like the younger dryas and heinrich events will not be robust until temporal variations and the altitude latitude scaling of production rates are fully understood and quantified at levels comparable to current analytic uncertainties 3

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

paired cosmogenic nuclide paleoaltimetry abstract goal the reconstruction of past topographies remains challenging and only a few methods allow accurate determination of past surface elevations we propose here a new technique for deriving paleo elevations in which multiple cosmogenic nuclides are measured in the same geological sample exposed at the earth s surface this method relies on the altitude dependence of the cosmogenic nuclides production rates combined with the radioactive decays of nuclides with different half lives theory the position of the two cosmogenic nuclide exposure curves 26al 10be vs 10be or 10be 21ne vs 10be depends on the altitude of exposure if the studied surfaces have been exposed for sufficiently long durations 500 ka or have been affected by low erosion rates data comparison we tested the method using the multiple cosmogenic nuclides dataset available for the western arid tropical andes the altitudes computed using the cosmogenic nuclide concentrations agree within uncertainties with the reported sampling altitudes over a range of 0 to more than 4000 m illustrating the applicability of the method altitudes computed under the assumptions of continuous exposure or steady state erosion yields best fits that are statistically in agreement and close to the 1 1 line for both the 26al 10be and the 21ne 10be dataset the 21ne 10be inventories in samples that have been exposed for more than 5 ma yield elevations that are several hundreds of meters below their present day elevations 1000 m this may result from a post 10 ma uplift of the west andes or from an unrecognized exposure underwater or below a soil cover implications this study may also have implications in other fields that rely on multiple cosmogenic nuclide measurements the same approach might notably be used to compute the depth of exposure of samples located below the rock surface or underwater this study may also help to improve the accuracy of the common burial dating method that uses multiple radioactive cosmogenic nuclides for long pre burial exposures 500 ka or low erosion rates

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

sedimentation and preservation of the miocene atacama gravels in the pedernales chanaral area northern chile climatic or tectonic control in recent years longitudinal changes on the thin thick skinned tectonic styles of the central andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the peru chile trench the search for paleo climate records is therefore crucial for our understanding of the history of the central andes in this paper we focus on the atacama gravels an extensive blanket of miocene continental deposits filling a neogene paleo valley system along the southern atacama desert in northern chile an east west transect between pedernales and chanaral 26o30 s enabled us to carry out a sedimentological and tectonic study of the atacama gravels based on logging and field observations along the rio salado canyon new 39ar 40ar ages obtained on intercalated and overlying ignimbrites date the beginning of the atacama gravels sedimentation at around the oligocene miocene boundary and cessation of sedimentation in the late miocene thirteen lithofacies included within five facies associations a1 to a5 were identified depositional environments vary from proximal alluvial fan a1 a2 in the precordillera through ephemeral fluvial a3 a4 to distal playa lake a5 in the coastal cordillera no evidences of syn sedimentary deformation have been found showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes and climate change appears to be the dominant controlling factor of sediment preservation a progressive change from semi arid towards hyper arid climatic conditions during the miocene led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the oligocene

remnants of a fossil alluvial fan landscape of miocene age in the atacama desert of northern chile using cosmogenic nuclide exposure age dating we have used cosmogenic nuclides to estimate limits on the surface exposure durations and erosion rates of alluvial fans and bedrock surfaces in the atacama desert in chile the oldest landforms we studied are extensive alluvial fans referred to as atacama gravels with the exception of samples collected in antarctica the cobbles collected on these alluvial surfaces have the lowest erosion rates of any samples as determined by cosmogenic nuclides analyzed to date the oldest cobble has a model surface exposure age of 9 myr based on combined measurements of cosmogenic 10be 26al and 21ne concentrations cobbles from the alluvial fans are eroding slower than the surrounding steep mountainous bedrock surfaces maximum erosion rates for cobbles on alluvial surfaces are uniformly 0 1 m myr the survival of these gravels specifically and more generally the stability of landform features in this geographic area is made possible by the attainment of hyperarid conditions in the atacama desert resulting from global climatic cooling about 15 myr ago combined with the rain shadow effect caused by uplift of the central andes the landform features observed presently in the atacama desert are remarkably stable and despite the inevitable erosion that is detectable using cosmogenic nuclides undoubtedly bear considerable resemblance to conditions as they existed in the miocene over geologic time the atacama landscape is evolving in such a manner as to erode the higher bedrock ridges relative to the more stable but stratigraphically lower depositional surfaces through which clastic detritus now travels occasionally along the floors of incised drainage systems leaving the older permeable alluvial fan surfaces largely intact as widespread remnants of a miocene fossil landscape

recently active contractile deformation in the forearc of southern peru abstract geomorphic and structural features of southern peru 14 18 s provide strong evidence for distributed crustal deformation along range sub parallel contractile structures we use in situ produced cosmogenic radionuclides in conjunction with field and remote mapping to determine the ages of geomorphic features and find 1 ancient surfaces 1 ma preserved as a result of very low surface erosion rates 2 young 30 ka low relief pediment surfaces developed during recent landscape modifications 3 active tectonic structures accommodating compressional stresses and 4 pleistocene river incision rates of 0 3 mm yr consistent with longer term rates in this region of southern peru the steep western wedge trench to arc area of the andean margin presently maintains the high topography of the altiplano through a combination of uplift and contractile deformation along steep east dipping faults and isostatic responses to the focused removal of large amounts of crustal material through canyon incision

cosmogenic 26al 10be surface production ratio in greenland the assumed value for the cosmogenic 26al 10be surface production rate ratio in quartz is an important parameter for studies investigating the burial or subaerial erosion of long lived surfaces and sediments recent models and data suggest that the production ratio is spatially variable and may be greater than originally thought here we present measured 26al 10be ratios for 24 continuously exposed bedrock and boulder surfaces spanning 61 77 n in greenland empirical measurements such as ours include nuclides produced predominately by neutron induced spallation with percent level contributions by muon interactions the slope of a york regression line fit to our data is 7 3 0 3 1 suggesting that the 26al 10be surface production ratio exceeds the commonly used value of 6 75 at least in the arctic a higher 26al 10be production ratio has implications for multi nuclide cosmogenic isotope studies because it results in greater modeled burial durations and erosion rates

cosmic ray produced 10be and 26al in antarctic rocks exposure and erosion history we have measured cosmic ray produced t1 2 1 5 million years 10be and t1 2 0 705 million years 26al in purified quartz fractions of selected rock samples from antarctic mountains from these data we calculate 1 mean erosion rates for the limiting case of steady state surface exposure to cosmic rays and 2 minimum exposure ages for the limiting case of no erosion calculated mean erosion rates are very low on the order of a few times 10 5 cm yr we believe the sampling to be sufficient to generalize this result to exposed bedrock in antarctica in favorable cases it is possible to distinguish between the limiting cases steady state erosion seems a better description in such cases most samples including some taken a few meters above the present ice level seem to have been exposed for millions of years without major episodes of burial or abrasion by ice

chronology of exotic mineralization at el salvador chile by 40ar 39ar dating of copper wad and supergene alunite exotic copper mineralization is a complex hydrochemical process linking supergene enrichment lateral copper transport and precipitation of copper oxide minerals in the drainage network of a porphyry copper deposit at the el salvador porphyry copper deposit in northern chile the majority of the exotic ore comprised a mixture of copper bearing manganese oxyhydrates termed copper wad x ray diffraction scanning electron microscopy and electron probe microanalysis show that the copper wad is composed of copper bearing cryptomelane k1 2 mn3 mn4 8o16 x h2o and birnessite k0 33 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mn 7 3 9 end document o14 7h2o structures these natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits using recent advances in 40ar 39ar geochronology of supergene k mn oxides formed by weathering a suite of copper bearing cryptomelane and birnessite samples from exotic deposits within the el salvador district were characterized and dated by 40ar 39ar laser step heating supergene alunite kal3 so4 2 oh 6 found in paleospring feeder systems leading from the source zones of copper outward to the exotic mineralization was dated to independently constrain the age of exotic ore formation although the ar retentivity of the layered birnessite structure has been questioned by others the ar retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization ground water interaction in the hyperarid atacama desert the 40ar 39ar analytical results show that ar and or k losses after crystallization excess 40ar and 39ar recoil do not pose significant problems therefore in the context of exotic copper deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within copper wad the 40ar 39ar dating of exotic mineralization at el salvador indicates that supergene and exotic mineralization processes were active at 35 ma about 5 m y after the emplacement of hydrothermal mineralization and continued until the middle miocene the majority of exotic mineralization extends from the oligocene miocene 24 ma boundary through the middle miocene 11 ma and relates to supergene fluid emanating in multiple directions from the source of copper in indio muerto which represents a topographic high above the exotic deposits one reconnaissance date of exotic mineralization at the chuquicamata porphyry copper deposit yielded an age of 17 03 0 03 ma coincident with the known supergene alunite dates for this deposit reconnaissance dating at exotica mina sur and el abra proved problematic due to the presence of contaminant silicate minerals within the copper wad ages derived here for exotic mineralization are similar to the known supergene ages throughout northern chile the dates indicate that supergene exotic mineralization began at the eocene oligocene boundary and continued through the oligocene eventually ceasing in the middle miocene due to desiccation of the atacama desert a series of 10 cm thick supergene alunite veins were microsampled across their widths and dated to address the kinetics of alunite vein growth apparent growth rates of the veins vary from 71 to 100 mm m y in the horizontal direction to 24 mm m y in the vertical presumably caused by differences in pressure regimes due to their orientation

martian fan deposits insights on depositional processes and origin from mass balance survey abstract fan deposits located at the mouths of martian valleys have been interpreted as indicators of wet conditions during mars history however the processes time and amount of water needed to carve the valleys and form the fans are still debated here we present a detailed morphometric and mass balance analysis of valleys and fan shaped deposits using high resolution topography which provides new insights into past depositional environments based on morphometric and volumetric measurements we found that several martian fans previously interpreted as deltas can be categorized into two main types type i fans are more numerous relatively well preserved are associated with smaller and less mature drainage networks and were deposited on steeper gradients the balance between eroded and deposited materials indicates that this set of fans might have been formed mainly in subaerial settings by depositional processes other than typical deltaic sedimentation e g alluvial glacial mass wasting in general since a significant loss of sediment during deposition did not occur we obtained a volume ratio of eroded vs deposited materials of approximately 1 therefore we propose that the identified type i fans were not deposited in prevailing subaqueous deltaic settings but mainly in subaerial conditions with perhaps sporadic presence of ephemeral bodies of standing water within the basins in contrast type ii fans are less abundant highly eroded were deposited on flatter areas and formed downstream of more mature drainage networks with longer deeper and wider valleys the mass balance for this set of fans clearly shows that considerable amounts of sediment were not retained in the fans implying large offshore transport of sediment during their formation eroded vs deposited volume ratios between 3 and 10 are probable this evidence supports a fluvio deltaic origin for this class of fans indicating the existence of paleolakes over substantial periods of time based on the collected morphometric measurements and mass balance modeling we conclude that only a small percentage of the fans type ii fans correspond to 1 3 of the sampled areas are consistent with the occurrence of favorable and durable conditions for life i e long lived integrated fluvial deltaic and lacustrine environments whereas the majority of fans might have been formed in subaerial settings with significant contributions of alternative processes besides fluvial transport and deposition thus not necessarily requiring the occurrence of extended epochs of clement climatic conditions

andean growth and monsoon winds drive landscape evolution at sw margin of south america abstract in the atacama desert the driest place on earth located at the subduction margin of the andes the landscape evolves very slowly and changes in tectonic or erosion processes remain for a long time in the memory of topography at latitude 19 30 s a threshold between exoreic and endoreic drainage regimes is clearly associated with the latitudinal gradient imposed by the modern monsoon carrying humidity from the atlantic and disposed obliquely over catchments draining the andes to the pacific we summarize the geomorphic geological and climatic data in the threshold area we then use these data to constrain numerical experiments of drainage evolution data and experimental results are consistent with the development of a flat low energy morphology close to sea level interrupted at 10 ma by tectonic uplift prevailing to the present suggesting trench ward relief growth by incorporation of the coastal atacama region to the andes mountain belt

the sensitivity of hillslope bedrock erosion to precipitation decoupling the impacts of climate and tectonics on hillslope erosion rates is a challenging problem hillslope erosion rates are well known to respond to changes in hillslope boundary conditions e g channel incision rates through their dependence on soil thickness and precipitation is an important control on soil formation surprisingly though compilations of hillslope denudation rates suggest little precipitation sensitivity to isolate the effects of precipitation and boundary condition we measured rates of soil production from bedrock and described soils on hillslopes along a semi arid to hyperarid precipitation gradient in northern chile in each climate zone hillslopes with contrasting boundary conditions actively incising channels versus non eroding landforms were studied channel incision rates which ultimately drive hillslope erosion varied with precipitation rather than tectonic setting throughout the study area these precipitation dependent incision rates are mirrored on the hillslopes where erosion shifts from relatively fast and biologically driven to extremely slow and salt driven as precipitation decreases contrary to studies in humid regions bedrock erosion rates increase with precipitation following a power law from 1 m ma 1 in the hyperarid region to 40 m ma 1 in the semi arid region the effect of boundary condition on soil thickness was observed in all climate zones thicker soils on hillslopes with stable boundaries compared to hillslopes bounded by active channels but the difference in bedrock erosion rates between the hillslopes within a climate region slower erosion rates on hillslopes with stable boundaries decreased as precipitation decreased the biotic abiotic threshold also marks the precipitation rate below which bedrock erosion rates are no longer a function of soil thickness our work shows that hillslope processes become sensitive to precipitation as life disappears and the ability of the landscape to respond to tectonics decreases copyright 2010 john wiley sons ltd

source transport and fate of rhenium selenium molybdenum arsenic and copper in groundwater associated with porphyry cu deposits atacama desert chile we collected groundwaters in and around a large 313 mt at 1 08 cu and 0 3 cutoff undisturbed porphyry copper deposit spence in the hyperarid atacama desert of northern chile which is buried beneath 30 180 m of miocene piedmont gravels groundwaters within and down flow of the spence deposit have elevated se up to 800 g l re up to 31 g l mo up to 475 g l and as up to 278 g l concentrations compared to up flow waters interpreted to represent regional groundwater flow in contrast cu is only elevated up to 2036 g l in groundwaters recovered from within the deposit cu concentrations are low down gradient of the deposit the differential behavior of the metals metalloids occurs because the former group dissolves as anions enhancing their mobility whereas the base metals dissolve as cations and are lost from solution most likely through adsorption to clay surface exchange sites and through formation of secondary copper chlorides carbonates and oxides most groundwaters within and down flow of the deposit have eh ph values around the feii feiii phase boundary limiting the impact of fe oxyhydroxides on oxyanions mobility se re mo and as are all mobile with filtered unfiltered samples 1 to the limit of sampling 2 km down gradient from the deposit the increase in ore related metals metalloids and sulfate and decrease in sulfate s isotope ratios from values similar to regional salars 4 to 8 34scdt to lower values closer to hypogene sulfides 1 to 4 34scdt is consistent with active water rock reactions between saline groundwaters and the spence deposit it is likely that hypogene and or supergene sulfides are being oxidized under the present groundwater conditions and mineral saturation calculations suggest that secondary copper minerals antlerite atacamite malachite may also be actively forming suggesting that supergene and exotic copper mineralization is possible even under the present hyperarid climate of the atacama desert

neogene climate change and uplift in the atacama desert chile the relationship between andean uplift and extreme desiccation of the west coast of south america is important for understanding the interplay between climate and tectonics in the central andes yet it is poorly understood here we use soil morphological char acteristics salt chemistry and mass independent fractionation anomalies 17 o values in dated paleosols to reconstruct a middle miocene climatic transition from semiaridity to extreme hyperaridity in the atacama desert paleosols along the southeastern margin of the calama basin change from calcic vertisols with root traces slickensides and gleyed horizons to an extremely mature salic gypsisol with pedogenic nitrate we interpret this transition which occurred between 19 and 13 ma to represent a change in precipitation from 200 mm yr to 20 mm yr this drastic reduction in precipitation likely resulted from uplift of the central andes to elevations 2 km the uplift blocked moisture from the south american summer monsoon from entering the atacama the mid miocene gyp sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the cal ama basin significantly higher than modern nitrate soils which occur below 2500 m modern and quaternary soils in this elevation zone contain soil carbonate and lack ped ogenic gypsum and nitrate we infer that 900 m of local surface uplift over the past 10 m y displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the calama basin by decreasing local air temperatures and creating an orographic barrier to pacific air masses

two stage development of the late cretaceous to late eocene darende basin implications for closure of neotethys in central eastern anatolia turkey

slow cenozoic uplift of the western andean cordillera indicated by cosmogenic 3he in alluvial boulders from the pacific planation surface to determine the mechanisms responsible for mountain belt growth it is important to accurately establish the timing of surface uplift here we exploit the altitude control on the production rate of in situ cosmogenic nuclides to test the hypothesis that the andes was uplifted in the late miocene high concentrations of in situ cosmogenic 3he 3hecos have previously been measured in alluvial boulders on the western flank of the central andes northern chile evenstar et al 2009 these are consistent with deposition soon after formation of the surface 13 14 ma we have modeled the accumulation of cosmogenic 3he in several different surface uplift scenarios and compared them to the measured concentrations the measured 3hecos concentrations are too high to be produced by late miocene uplift and imply that the western flank of the andean cordillera attained a substantial part of its current elevation prior to 14 myr ago

active erosion deposition cycles in the hyperarid atacama desert of northern chile 10be 21 ne abstract there is significant debate over the rates and types of fluvial activity at the plio pleistocene boundary in the hyperarid atacama desert of chile to quantify fluvial processes and help resolve this debate we measure terrestrial cosmogenic nuclide tcn 10 be and 21 ne concentration depth profiles in three settings representing a chronosequence 1 a late pliocene alluvial fan representative of major regional deposits 2 a modern active channel and 3 an adjacent low terrace inset into the pliocene alluvium late pliocene deposits that are widely preserved in the region contain tcn profiles consistent with relatively rapid stripping of upland sediment at the plio pleistocene boundary deposits inset into these late pliocene features record cut and fill cycles that rework sediment throughout the quaternary the tcn profile in the modern channel is best explained by sediment aggradation at 2 1 m myr 1 during the last 250 000 yr similarly the adjacent low terrace sediments contain tcn concentration profiles consistent with aggradation of 2 0 m myr 1 over a period of 250 000 750 000 yr prior to the last 250 000 yr of stability in summary depth profiles of two tcns provide constraints on the rates of sediment deposition sources of sediment and transport history as well as the subsequent exposure conditions of the sediment following deposition our results are consistent with early quaternary initiation of hyperaridity for the region during the quaternary winter precipitation events experienced at our sites latitude 241s drive active erosion deposition cycles the northward migration of the subtropical front during quaternary glacial cycles may have enhanced precipitation at 241s leading to more active fluvial processes during cooler periods

geochronology of pediment surfaces in southern peru implications for quaternary deformation of the andean forearc abstract the geomorphology of the andean forearc has historically been viewed as an old remnant of a late miocene planar landscape with no significant active structures accommodating quaternary deformation however the well preserved sequence of planation surfaces and strath terraces developed within the forearc of southern peru provide evidence of recent uplift along the western margin previously the abandonment of these surfaces was attributed to uplift of late miocene age however abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation we use in situ produced 10be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity our new results in contrast to previous work yield pediment surface ages of 119 ka 203 ka 278 ka 549 ka and 1003 ka indicating a youthful morphology additionally these surfaces are affected by steeply dipping active faults producing localized deformation which can be quantified incision rates based on the exposure ages of abandoned strath surfaces are on the order of 0 04 0 3 mm yr the data set presented here not only suggests significant active deformation within the forearc but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the andes while the general notion is that active deformation is localized in the subandean fold and thrust belt our data support an emerging view where active deformation is occurring in the western margin as well

geomorphology on geologic timescales evolution of the late cenozoic pacific paleosurface in northern chile and southern peru abstract the atacama desert on the western margin of the central andes is one of the driest and oldest deserts in the world it is defined by a distinct and ancient surface known as the pacific paleosurface pps or atacama paleosurface the age of this surface is determined as the time at which sediment deposition ceased and the surface was effectively abandoned early studies suggested that this abandonment took place between 14 and 10 ma and was related to both the uplift of the andes and the onset of hyperaridity in the region here we provide a regional re examination of the pps compiling existing work on the underlying geology sedimentology surface exposure dating and seismic profiling we also present new multispectral satellite maps of the pps and 45 new cosmogenic 3he and 21ne surface exposure ages in order to constrain the formation age and the preservation and incision history of the paleosurface we conclude that the pps is not a single paleosurface but instead is a mosaic of smaller surfaces that were formed by aggradational and degradational processes over 19 million years or more and should be termed collectively as the pacific paleosurfaces the time at which individual paleosurfaces formed is related to regional climate where the location of each is controlled by regional tectonic activity cosmogenic surface exposure ages suggest that the surfaces are a record of regional scale climate events

climatic and geomorphic interactions on alluvial fans in the atacama desert chile abstract alluvial fan surfaces in the atacama desert of northern chile preserve evidence of recent precipitation driven surface flows determining the hydrologic characteristics of these flows is important for understanding the effects of rare yet significant storms in the region flow reconstruction runoff analysis and comparison with climatological data yield surface activation recurrence intervals of 1 20 years for three small fans and their associated catchments proximal to iquique and antofagasta chile modeled discharges provide minimum constraints on the precipitation rates required to resurface fans in the hyper arid region and suggest that relatively short lived and intense precipitation events 1 3 h 4 mm h 1 are capable of producing surface flows that mobilize and transport the largest grains found in channelized portions of the fan surface the results of this study provide an indication of the ability of a particular storm event i e precipitation rate to activate a geomorphic surface and contribute to our understanding of the controls on surface evolution in hyper arid environments

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

variability in erosion rates related to the state of landscape transience in the semi arid chilean andes we quantify erosion rates in the higher sectors of the huasco valley in the main cordillera of the semi arid andes of chile using elevation differences between three successive geomorphic markers pediments and paleo valleys and the present day valley available ar ar ages of neogene pediments are used to estimate mean erosion rates for the three periods 16 to 13 my 13 to 8 my and following 8 my the landscape of the huasco valley is in a transient state as indicated by well preserved pediment surfaces in interfluves valleys deeply incised by fluvial and glacial erosion and scarped head valleys that represent the current knickzones higher erosion rates 45 75 m my are calculated for the more recent period 8 my during which deep incision developed compared to previous periods 6 31 m my quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the main cordillera comparison with erosion rates calculated in other drainage basins of the chilean andes suggests that the variability of erosion rates depends on the landscape s transient erosive state the landscape s geomorphologic response to the uplift of the main cordillera results in the retreat of a knickzone for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity copyright 2011 john wiley sons ltd

slow rates of rock surface erosion and sediment production across the namib desert and escarpment southern africa slow erosion has characterized the namib desert the namibian escarpment and the adjacent namibian highlands over the pleistocene paired analyses n 66 of in situ produced 10be and 26al in quartz bearing samples of bedrock primarily from inselbergs of sediment from dry river and stream channels and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years bedrock samples n 47 collected in three transects from the coast across the escarpment and into the highlands show no spatial pattern in elevation normalized nuclide abundance despite a difference in mean annual precipitation map between sample sites at the coast map 400 mm yr 1 average model erosion rates inland of the escarpment 3 2 1 5 n 9 are indistinguishable from average rates seaward of the escarpment 3 6 1 9 n 38 indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands sediment samples n 3 from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster 16 m my 1 than either a basin in the highlands 5 m my 1 or a basin in the coastal plain 8 m my 1 data from large rivers n 4 constrain erosion rates averaged over 105 yrs and 104 to 105 km2 between 3 and 9 m my 1 small quartz clasts n 12 collected from four desert surfaces record extraordinarily long variable and in some cases complex exposure histories simple 10be model ages are as high as 1 8 my some minimum total histories considering both 10be and 26al and including both burial and exposure exceed 2 7 my as a group the namibian cosmogenic data do not support the model of significant and on going escarpment retreat the similarity of erosion rates calculated from 10be analysis of fluvial sediments and longer term 107 yr average mass removal rates estimated by others using fission track analysis of rock suggests that namibian erosion rates have reached a steady state and are changing little over time at outcrop scales the concordance of 10be and 26al in most bedrock samples suggests that the model of steady uniform bedrock erosion is valid there is no indication of intermittent burial shedding of thick rock slabs or stripping of previous cover at an intermediate scale a transect of bedrock samples north of gobabeb demonstrates that the northern boundary of the massive namib sand sea has been steady and unshifting similarly low cosmogenically estimated erosion rates across west and central namibia suggest that the landscape is in geomorphic steady state its overall appearance changing only slowly through time

production of 3he in crustal rocks by cosmogenic thermal neutrons accurate determination of cosmogenic 3he concentrations in minerals has to account for nucleogenic 3he produced from the reaction of thermalized neutrons with 6li thermal neutrons from the cosmic ray cascade are typically not considered however in exposed basalts and granites the flux of cosmogenic thermal neutrons ctn is generally 10 200 times higher than the thermal neutron flux produced by n reactions in the rock with the absolute values dependent on composition and geographic location the reaction of ctn with li can account for 5 50 of the cosmogenic 3he in minerals in typical granitiod rocks depending on the li content and distribution and petrography of the rock and may be considerably higher in special cases in basalts this contribution is usually lower 1 5 6 but may still warrant consideration the strong depth dependence of ctn produced 3he in rock may be used to assess erosion of landforms similar to ctn produced 36cl however as 3he is stable it has the potential to extend the useful time range to older surfaces

40ar39ar analysis of supergene jarosite and alunite implications to the paleoweathering history of the western usa and west africa supergene alunite kal sub 3 so sub 4 sub 2 oh sub 6 and jarosite kfe sub 3 so sub 4 sub 2 oh sub 6 are often precipitated during the oxidation of sulfide bearing rocks by meteoric solutions dating of these phases by the sup 40 ar sup 39 ar method allows timing of the progression of the oxidation front during chemical weathering fine scale laser heating sup 40 ar sup 39 ar dating of hypogene alunite and supergene jarosites allows to precisely and accurately time hydrothermal alteration and subsequent supergene oxidation in goldfield nevada the results indicate that pervasive weathering occurred in the western usa during the late miocene approximately 10 ma ago similar application of this technique to the study of weathering and laterite formation in west africa indicates that the last pervasive oxidation event recorded in the weathering profile in this area also occurred in the miocene approximately 13 ma ago the occurrence of a pervasive mid to late miocene oxidation event recorded in these weathering profiles in the western usa and africa and also previously measured in brazil and chile indicates that climatic conditions at that time were conducive to worldwide development of deep weathering sequences subsequent weathering processes have not been as pervasive as the late miocene event indicating a general climatic transition to cooler more drier climates in most of the areas studied the results indicate that deep weathering profiles reflect past climatic conditions and may not be directly linked to the present climates less

mapping the crustal thickness in southwest japan using moho reflected waves in this work we have tried to detect and collect later phases associated with the moho discontinuity and use them to study the lateral variations of the crustal thickness in southwest sw japan we first compute synthetic seismograms for local earthquakes taking into account the focal mechanism solutions estimated from first motion polarity data then we compare the synthetics with the observed waveforms to detect the major later phases in seismograms of shallow crustal earthquakes in sw japan taking the advantage of the high quality and great quantity of hi net waveform data now available throughout the entire japan islands we have detected 1659 moho reflected phases pmp and sms from shallow events in sw japan we estimated the crustal thickness in the study area using travel time differences between these later phases and the first p and s arrivals our results show that the moho is deep in chubu district and becomes shallower along the coastlines of the japan sea and the pacific ocean a thinner crust appears in osaka bay and awajii island where the 1995 kobe earthquake m7 2 occurred

timing and nature of alluvial fan development along the chajnantor plateau northern chile abstract alluvial systems in the atacama desert provide a unique opportunity to elucidate the sedimentary response to climate variability particularly changes in precipitation in hyperarid environments alluvial fans along the eastern margin of the salar de atacama adjacent to the chajnantor plateau in the atacama desert of northern chile provide an archive of climate modulated sediment transfer and erosion at an extreme of earth s climate three regional alluvial fan surfaces qf1 oldest to qf3 youngest were mapped along the western flank of the chajnantor plateau the alluvial fans were examined with geomorphic and terrestrial cosmogenic 36 cl surface exposure dating methods to define the timing of alluvial fan formation and to determine the role of climatic processes on fan development in a hyperarid environment alluvial fans in the study area are comprised of hyperconcentrated flow and boulder rich debris flow deposits that reflect deposition transitioning between cohesive and noncohesive regimes alluvial fan surfaces yield exposure ages that range from 49 6 4 4 to 194 12 ka while debris flow boulders yield exposure ages ranging from 12 4 2 1 to 229 53 ka cosmogenic 36 cl exposure ages indicate that abandonment of alluvial fan surfaces qf1 qf2 and qf3 date to 175 22 6 ka mis 6 134 5 9 18 ka mis 6 and 20 07 6 26 ka mis 2 respectively a 36 cl concentration depth profile through alluvial fan qf1 suggests a simple depositional history with minimal nuclide inheritance implying relatively rapid aggradation 6 m in ca 25 kyr followed by surface abandonment ca 180 200 ka our data support a strong climatic control on alluvial fan evolution in the region and we propose that the alluvial fans along the margins of the salar de atacama form according to the humid model of fan formation

complex exposure histories for meteorites with short exposure ages we report measurements of 26al and 10be activities in nine ordinary chondrites and of the light noble gas concentrations and 36cl and 41ca activities in subsets of those meteorites all but murray have low 21ne concentrations 1 0 10 8cm3stp g and have previously been used to estimate 21ne production rates ladder creek murchison sena and timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21ne production rates that are consistent with the standard l chondrite value of 0 33 10 8cm3stp g ma in contrast cullison guenie shaw and tsarev experienced complex irradiation histories they and several other meteorites with low nominal exposure ages also have lower 3he 21ne ratios than expected based on their 22ne 21ne ratios a general association between low 21ne contents and 3he losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia however meteorites with low 21ne contents one stage exposure histories and losses of cosmogenic 3he are rare possible explanations for the scarcity are 1 statistical 2 that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision and 3 that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia

age of supergene oxidation and enrichment in the chilean porphyry copper province twenty five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees s in northern chile yield k ar ages ranging from about 34 to 14 ma therefore supergene oxidation and enrichment processes were active from the early oligocene to the middle miocene a minimum of 20 m y supergene activity at individual deposits lasted for at least 0 4 to 6 2 m y the early oligocene supergene activity affected deposits in the paleocene porphyry copper belt whereas early and middle miocene supergene processes are documented in the early cretaceous paleocene and late eocene to early oligocene porphyry copper belts middle miocene oxidation also affected the oldest epithermal gold silver deposits in the maricunga belt farther east supergene activity commenced no less than 11 m y after generation of each porphyry copper deposit because of the time required to unroof the copper bearing parts of the system supergene activity throughout northern chile ceased at approximately 14 ma the geologic features of deposits and prospects and their morphotectonic positions present latitudes and present elevations display no obvious correlations with the supergene chronology exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels or ignimbrites of oligocene through middle miocene age unless their accumulation is demonstrably late in the documented history of supergene activity or in porphyry copper provinces such as those of central chile and northwestern argentina which formed after approximately 14 ma the uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern chile and contiguous areas unless their effects were much more prolonged intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern chile and tectonic uplift was its most probable cause however more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role

analysis of supergene ore forming processes and ground water solute transport using mass balance principles analysis of metal distribution in weathering profiles developed above ore deposits provides a simple and direct method to determine the geologic controls on surficial leaching and enrichment of metals by supergene descending fluids this approach offers an understanding of the interrelationships of metal concentration zone thicknesses and bulk rock density of the various zones within a weathered rock column analytical solutions to mass balance equations provide expressions describing the chemical evolution paths of enrichment blanket metal grade b in terms of original protore metal grade p leached zone metal grade 1 and the ratio of total leached zone column height to blanket thickness l o t b neglecting density terms for the sake of simplicity this function is expressed as b p l o t b p 1 linear data arrays in the coordinate system b versus p 1 for groups of drill holes in individual geologic domains e g wall rock types strongly suggest that enrichment blanket metal grades evolve from a protore metal grade p with a fairly constant slope of l o t b to a final value b determined by the extent of metal leaching in the zone of oxidation the constancy of the ratio l o t b within geologic domains indicates a simultaneous deepening of the zone of oxidation and thickening of the enrichment blanket under conditions of a descending ground water table analysis of metal mass balance in drilling profiles also provides a means of reconstructing paleosurficial topography at the time of active oxidation even in highly eroded terrains application of these methods using rock density and assay data from the butte district of montana shows the paleo oxidation surface to have been eroded by as much as 450 ft down to the present unconformity between weathered bedrock and overlying tertiary gravels geomorphic stability and slow erosion rates often thought necessary for effective chemical weathering are therefore questioned we propose instead a general balance of rates of erosion and descent of the ground water table for optimal enrichment preservation of the fossil weathering profiles occurs by submergence of the zone of oxidation below the paleoground water table resulting in cessation of supergene oxidation and preservation of the blanket the mass balance methods derived in this study also relate overall leaching efficiency to protore characteristics showing a strong hypogene alteration control at butte as described by mcclave 1973 lateral copper transport is evaluated by identifying local anomalies in the computed oxidation surface zones of copper introduction near faults and zones of high permeability appear as apparent topographic highs whereas zones of depletion appear as topographic depressions preliminary analysis of this type at the porphyry copper deposit at la escondida chile indicates deeper oxidation and leaching effects than at butte the assumption of vertically homogeneous protore copper grades leads to the conclusion that erosion of at least 200 m of leached capping has occurred throughout much of the la escondida district geologic domains defined by computed values of l o t b correlate well with hypogene alteration zoning computation of the magnitude and direction of overall subsurface lateral metal fluxes indicates transport distances of at least 1 km in directions consistent with present surface topography and fracture patterns there is a strong correlation of the thickest and highest grade enrichment zones with inferred positive lateral fluxes whereas the most thoroughly leached cap rocks make up zones of inferred negative lateral flux which are likely source regions alternative explanations to the lateral flux hypothesis are evaluated including assessment of variations in protore metal grade projections both in vertical and horizontal directions despite the approximations involved in the present calculations the lateral flux pattern appears to be the most likely interpretation of the present metal distribution in the supergene system at la escondida

evaluating the life expectancy of a desert pavement abstract this paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the sonoran desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements this synthesis identifies vital factors key factors and site specific factors promoting pavement stability hyperaridity is the vital factor in pavements surviving for 106 years or more aided by minimal bioturbation and clast size reduction three key factors aid in pavements surviving for 104 to 105 years accumulation of allochthonous dust underneath pavement cobbles a flat topography and a lack of headward retreating swales or gullies a unified explanation for pavement longevity however did not emerge from a literature review because a variety of site specific factors can also promote pavement antiquity including resistant bedrock beneath the pavement disk shaped cobbles to promote dust accumulation and microclimatological and ecological reasons for minimal bioturbation both key and site specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the sonoran desert central arizona the buildup of cosmogenic 10be and in situ 14c optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between 30 and 332 ka with conditions for longevity including flat surface topography pavements underlain by consolidated granitic bedrock a lack of headward retreating gullies and swales 87sr 86sr analyses indicating the infiltration of allochthonous dust floating disk shaped pavement cobbles and a quartzite lithology resistant to disintegration however 10be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate these weaker materials allowed the growth of headward retreating swales that in turn promoted exposure of newer gravels by surface erosion

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

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

evidence for active landscape evolution in the hyperarid atacama from multiple terrestrial cosmogenic nuclides the atacama desert is one of the driest places on earth multiple lines of evidence show that the atacama has been hyperarid since at least the late miocene among these are cosmic ray exposure ages indicating that individual clasts on some surfaces have been preserved for 9 ma and possibly since the oligocene although these remarkably old ages indicate slow landscape evolution it is not clear whether this pace is characteristic of the entire atacama or only of specific regions landforms or landscape elements to address this question we measured cosmogenic 10be 26al and 21ne from a wide variety of landscape elements in a transect across the central atacama where modern precipitation is at an extreme minimum but where the concentration of cosmogenic nuclides in stable landscape elements has not previously been recorded we find that the hyperarid core of the central atacama has substantially slower erosion rates than its eastern and western margins however even the driest part of this transect has erosion rates comparable to those of other deserts ranging from 0 2 0 4 m ma the most stable landscape elements are boulder fields with exposure ages of 1 5 2 6 ma the vast majority of samples in the central atacama desert however have cosmogenic nuclide concentrations corresponding to ages 5 ma documented elsewhere in the atacama were not found in our area and appear to be limited to exceptionally stable boulders or cobbles in either the northern or southern extremes of the atacama desert we suggest that the central atacama has been subject to episodic pliocene and pleistocene rainfall and geomorphic activity perhaps due to intrusion of pacific moisture

a combined u th he and cosmogenic 3he record of landscape armoring by biogeochemical iron cycling u th he geochronology and cosmogenic 3he in iron oxides reveal mineral precipitation ages as old as 55 ma and exposure ages greater than 5 ma for canga cemented plateaus in the quadrilatero ferrifero brazil showing that lateritic profiles overlying banded iron formation bif landscapes in tropical regions have a long history of surface exposure the long term erosion history obtained from cosmogenic 3he on bif plateaus confirms that relic surfaces persist in the landscape for millions of years combined 3he and u th he dating shows that cangas are preferentially goethite cemented by biogeochemical reactions in the subsurface importantly pebbles of hematite magnetite in colluvia or shallow creeks draining the canga cemented plateaus record a much longer exposure history than in situ canga blocks showing that even older duricrusts now eroded once blanketed these plateaus physically stable but biogeochemically dynamic cangas armor the landscape by pervasive and recurrent iron cycling and cementation slowing down the delivery of weathered bif or friable hematite magnetite ore to erosion

tectonic and climatic controls on the spatial distribution of denudation rates in northern chile 18 s to 23 s determined from cosmogenic nuclides in the arid region of northern chile the environmental conditions are favorable for measuring tectonic and climatic influences on catchment denudation rates in the absence of vegetation previous studies of denudation rates from cosmogenic 10be and 26al concentrations are limited to single drainages in this study we examine catchment to orogen scale spatial variation in denudation rates between 18 23 s in the coastal and western cordilleras of northern chile 10be and 26al data were obtained from 33 catchments to examine the relative roles of tectonics and climate on catchment averaged denudation rates at broader scales we examine whether denudation rates and orogen topography reflect the 3d plate geometry of the region cosmogenic nuclide derived denudation rates range from 0 4 0 5 to 20 6 1 5 m myr in the coastal cordillera and from 1 4 0 7 to 168 0 19 8 m myr in the western cordillera the controls on the denudation rates are evaluated using a statistical factor analysis of ten selected catchment parameters denudation rates indicate a strong linear relationship with channel steepness indices but insignificant correlations and covariation with mean annual precipitation rates drainage area stream order mean elevation mean local relief mean basin slope and analyzed grain size of the sampled sediments moreover denudation rates are better correlated with tectonic controls at catchment scale than orogen scale plate tectonics in the western and coastal cordillera

sedimentology based reconstructions of paleoclimate changes in the central andes in response to the uplift of the andes arica region between 19 and 21 s latitude northern chile we focus on the sedimentological record of the middle miocene to modern deposits in the andes of northern chile between 19 and 21 s these sediments deposited at the western escarpment of the central depression indicate successively more moisture on the western margin of the altiplano and the western cordillera where the sources are at the pacific coast 20 ma old exposure ages and salic gypsisols reflect an existing and ongoing hyperarid climate we interpret the increased divergence of climates between the coast and the altiplano as consequence of the andean rise to elevations higher than approximately 2 500 m a s l when the topography of the altiplano was sufficiently high and areally extensive to attract atlantic moisture accordingly the inferred general increase in run off was closely coupled with the uplift of the andes if the steady rise model applies in case that the rapid rise model for andean uplift is correct the inferred changes in sediment transport would have occurred independently of uplift requiring an alternative yet unknown driver

the topographic imprint of a transient climate episode the western andean flank between 15 5 and 41 5 s mountain range topography is determined by the complex interplay between tectonics and climate however often it is not clear to what extent climate forces topographic evolution and how past climatic episodes are reflected in present day relief the andes are a tectonically active mountain belt encompassing various climatic zones with pronounced differences in rainfall erosion and glacier extent under similar plate boundary conditions in the central to south western andes climatic zones range from hyperarid desert with mean annual rainfall of 5 mm a 22 5 s to year round humidity with 2500 mm a 40 s the andes thus provide a unique setting for investigating the relationship between tectonics climate and topography we present an analysis of 120 catchments along the western andean watersheds between 15 5 and 41 5 s which is based on srtmv3 90m data and new medium resolution rainfall tropical rainfall measurement mission trmm dataset for each basin we extracted geometry relief and climate parameters to test whether andean topography shows a climatic imprint and to analyze how climate influences relief our data document that elevation and relief decrease with increasing rainfall and descending snowline elevation furthermore we show that local relief reaches high values of 750 m in a zone between 28 s to 35 s during pleistocene glacial stages this region was affected by the northward shifting southern hemisphere westerlies which provided moisture for valley glacier formation and extended glacial coverage as well as glacial erosion in contrast the southern regions between 35 s to 40 s receive higher rainfall and have a lower local relief of 200 m probably related to an increased drainage density we distinguish two different climatically controlled mechanisms shaping topography 1 fluvial erosion by prolonged channel hillslope coupling which smoothes relief and 2 erosion by valley glaciers that generates relief finally our results suggests that the catchment scale relief of the andes between 28 s to 35 s is characterized by a pronounced transient component reflecting past climatic conditions copyright 2010 john wiley sons ltd

pleistocene uplift climate and morphological segmentation of the northern chile coasts 24 s 32 s insights from cosmogenic 10be dating of paleoshorelines we present new cosmogenic 10be exposure ages obtained on pleistocene marine abrasion shore terraces of northern chile between 24 s and 32 s in order to evaluate the temporal and spatial variability of uplift rates along the coastal forearc both the dispersion of cosmogenic concentrations in samples from the same terrace and data obtained in vertical profiles show that onshore erosion rates following emergence of paleoshorelines approached 1 m myr therefore minimum ages calculated without considering onshore erosion may be largely underestimated for middle pleistocene terraces the elevation of the last interglacial mis 5 paleoshoreline is generally between 25 and 45 m amsl suggesting that the entire coast of the study area has been uplifting during the upper pleistocene at rates approaching 0 3 mm yr available ages for middle pleistocene terraces suggest similar uplift rates except in the altos de talinay area where uplift may have been accelerated by the activity of the puerto aldea fault the maximum elevation of pleistocene paleoshorelines is generally close to 250 m and there is no higher older neogene marine sediment which implies that uplift accelerated during the pleistocene following a period of coastal stability or subsidence we observe that the coastal morphology largely depends on the latitudinal climatic variability north of 26 75 s the coast is characterized by the presence of a high scarp associated with small and poorly preserved paleoshorelines at its foot the existence of the coastal scarp in the northern part of the study area is permitted by the hyper arid climate of the atacama desert this particular morphology may explain why paleoshorelines evidencing coastal uplift are poorly preserved between 26 75 s and 24 s despite upper pleistocene uplift rates being comparable with those prevailing in the southern part of the study area

zebra stripes in the atacama desert fossil evidence of overland flow abstract some hillslopes in the hyperarid region of the atacama desert in northern chile have surface clasts organized into distinct contour parallel bands separated by bare soil we call the bands zebra stripes due to the contrast between the darkly varnished clasts and the light colored salt rich soil gravel that comprises the zebra stripes is sorted such that the coarsest clasts are at the downslope front and fine progressively upslope how and when the zebra stripes formed are perplexing questions particularly in a region experiencing prolonged hyperaridity using googleearth satellite imagery and field observations we report the first quantitative and qualitative observations of zebra stripes in order to test hypotheses of the mechanisms and timing of their formation we consider soil shrink swell seismic shaking and overland flow as possible formation mechanisms and find that overland flow is the most likely based on cosmogenic 10 be concentrations in surface clasts salt deposition rates from the atmosphere and content in the soils we propose that the salt rich soils began accumulating 10 6 y ago and the zebra stripes formed 10 3 10 4 y at the latest the zebra stripe pattern has been preserved due to the self stabilization of the clasts within the stripes and the continued absence of life which would disturb the surface as seen at a wetter site to the south we conclude that the occurrence of zebra stripes is diagnostic of a set of distinct characteristics of local and or regional precipitation soil hillslope form and bedrock type

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

more than ten million years of hyper aridity recorded in the atacama gravels abstract the atacama desert s hyper aridity is closely linked to the development of world class copper and nitrate iodine ores and to regional tectonics and global paleoclimate changes in the cenozoic era the timing when the hyper aridity commenced remains controversial with proposed ages ranging from late oligocene to pleistocene in this study we provide an independent constraint on the initiation of atacama hyper aridity utilizing a 100 m deep profile within the atacama gravels and underneath porphyry copper deposit in spence northern chile the overall high concentration of sulfate up to 10 wt and a multimodal distribution of water soluble salt sulfates chlorides and nitrates indicate multiple generations of sedimentation and salt accumulation events under semi arid to hyper arid climate conditions the multiple sulfate isotope compositions 17o 18o 34s of the upper section 15 0 to 34 5 m are close to those of modern hyperarid surface sulfates while the lower section 34 5 to 65 m displays a depth dependent isotope trend that is best interpreted as marking a period of climate change from semi arid to hyper arid when these data are combined with new chronological 40ar 39ar dates obtained from a volcanic ash layer at depth of 28 0 m our results show that hyper arid condition in the atacama desert was prevailing at least prior to 9 47 ma and may go back as old as the middle miocene

very slow erosion rates and landscape preservation across the southwestern slope of the ladakh range india erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes geomorphic and terrestrial cosmogenic nuclide tcn methods were used to determine erosion rates of the arid tectonically quiescent ladakh range northern india five different geomorphic domains are identified and erosion rates are determined for three of the domains using tcn 10be concentrations along the range divide between 5600 and 5700 m above sea level asl bedrock tors in the periglacial domain are eroding at 5 0 0 5 to 13 1 1 2 meters per million years m m y principally by frost shattering at lower elevation in the unglaciated domain erosion rates for tributary catchments vary between 0 8 0 1 and 2 0 0 3 m m y bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates 850 ky respectively these results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the ladakh range which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the greater himalaya copyright 2014 john wiley sons ltd

cosmogenic exposure and erosion history of australian bedrock landforms exceptionally high activities of in situ produced 1 0 be and 2 6 al indicate that australian bedrock surfaces are some of the most stable in the world these cosmogenic nuclides demonstrate that australian granite exposed and sampled at 10 sites along a transect from the eyre peninsula 33 s to the kakadu area in the northern territory 14 s has in some places eroded only decimeters over the past million years however nuclide activities are not saturated in any of the 61 exposed samples we analyzed mandating that the surfaces we sampled are losing mass albeit slowly over time such mass loss indicates that inselbergs are dynamic landforms confounding the assignment of discreet ages to inselberg surfaces nevertheless the differential between slow rates of erosion measured on the very stable uppermost parts of these landforms and the more rapid rates measured below their flanks and below the adjacent grus covered planes suggests that the forms of many large inselbergs which stand meters to tens of meters high must be at least tertiary in age and possibly much older activities of 1 0 be and 2 6 al in 61 exposed australian samples range from 5 1 0 1 10 6 to 5 3 0 6 x 10 5 atoms g 1 and 2 1 0 1 x 10 7 to 3 4 0 5 x 10 6 atoms g 1 respectively normalized to sea level at latitudes of 60 using only neutron scaling activities of 1 0 be and 2 6 al in seven shielded samples 3 5 13 m below the surface are much lower ranging from 3 4 0 2 x 10 4 to 7 0 0 1 x 10 4 atoms g 1 and 1 6 0 1 x 10 5 to 2 8 0 2 x 10 5 atoms g 1 respectively 2 6 al 1 0 be ratios of exposed samples range from 3 89 0 19 to 6 29 0 82 if 2 uncertainty ranges and a nominal sea level high latitude 1 0 be production rate of 5 17 atoms g 1 yr 1 are considered ratio data for all but one surface sample are consistent with continuous surface or near surface exposure and or erosion rock surfaces farther north in australia where the climate is more humid have lower nuclide activities thus we infer that surface stability is inversely related to mean annual precipitation from 61 exposed samples we calculated maximum limiting single nuclide model erosion rates of 0 3 0 1 to 5 7 1 0 m m y and minimum limiting single nuclide model exposure ages of 105 16 to 1310 190 ka by assuming a sea level high latitude 1 0 be production rate of 5 17 atoms g 1 yr 1 seven shielded samples in which the nuclide inventory is muon generated also have nuclide activities consistent with similarly low rates of erosion but over even longer time scales it appears that the eyre peninsula inselbergs have been and are eroding so slowly that they may well be direct descendents of cenozoic and perhaps even mesozoic forms

seismic origin of the atacama desert boulder fields abstract extensive fields of large boulders are common around the base of hills in the atacama desert how these boulders are transported from nearby hillslopes is unclear given the lack of rainfall of the region here we document the central role of seismicity not runoff in transporting and smoothing 1 ton boulders all across the hyperarid core of the atacama desert the generally granitoid boulders emerge as corestones on hillslopes at an erosion rate of 0 1 1 m ma 1 thereafter physical and cosmogenic isotopic evidence suggests that boulders slide and bounce rather than roll down hills and onto adjacent flats in the transport process the largest boulders are split and the smaller ones are weathered to grus narrowing average boulder mass to 2 tons 3 at the base of hills the boulders bunch together and rub during the frequent earthquakes in the region producing distinctive smoothing around boulder mid sections and silt moats around the boulder bases our measurements show a strong correlation between boulder field density and rubbing and only when the density exceeds 60 70 does rubbing become common except for slow removal by rubbing the boulders seem to undergo no further erosion while in the flats exposure times for some boulders are 12 ma making them among the oldest continuously exposed features on the earth boulder rubbing is just one geologic feature among many in the atacama that underscore the role that seismicity probably plays in shaping landscapes of the waterless worlds of the solar system