terrestrial manganese 53 a new monitor of earth surface processes we report the first systematic study of the terrestrial cosmogenic radionuclide manganese 53 t1 2 3 7 ma measured in thirteen samples from nine dolerite surfaces in the dry valleys antarctica the terrestrial manganese 53 concentrations correlate well with cosmic ray produced helium 3 and neon 21 concentrations in the same samples implying that the manganese 53 is produced continuously in situ and retained quantitatively over millions of years the terrestrial manganese 53 production rate determination normalized to iron the only important target element and to high latitude and sealevel yields a value of p53 103 11 atoms yr 1 g fe 1 this is consistent with the theoretical value of 120 18 atoms yr 1 g fe 1 obtained from modeling calculations our results show that the manganese 53 concentrations in bulk terrestrial rocks can be used to monitor earth surface processes on time scales exceeding 10 my 2006 elsevier b v all rights reserved

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

cosmogenic 3he concentrations in ancient flood deposits from the coombs hills northern dry valleys east antarctica interpreting exposure ages and erosion rates abstract in situ produced cosmogenic 3 he analyses provide independent support for the model of a stable hyper arid polar climate persisting in east antarctica since the mid miocene and provide quantitative constraints on long term rates of erosion within the dry valleys in the coombs hills area a series of cobble size boulders form mega ripples with wavelengths of approximately 50 m their topographic position and association with features characteristic of scabland such as stripped corrugated bedrock surfaces indicate the boulders were deposited by subglacial floodwaters such outburst flooding could only have occurred during overriding of the northern dry valleys by a greatly expanded east antarctic ice sheet timing of the overriding episode has been previously assigned to 14 8 to 13 6 ma by correlation with volcanic ash deposits dated by 40 ar 39 ar in the asgard range of the dry valleys cosmogenic 3 he concentrations in clinopyroxene from ferrar dolerite boulders are consistent with 8 6 to 10 4 ma exposure calculated using scaling factors appropriate for antarctica and assuming zero erosion these are among the oldest surface exposure dates yet measured on earth but are not however consistent with the 40 ar 39 ar chronology used to define the age of the landscape due to unconstrained levels of erosion erosion rates of 0 03 0 06 m ma 1 are necessary to have produced the measured boulder exposure age if they were deposited at 14 8 ma these are less than half the steady state erosion rate derived from cosmogenic 3 he in the nearby bedrock surfaces 0 17 m ma 1 and testify to the extreme stability of the landscape

cosmogenic 3he in himalayan garnets indicating an altitude dependence of the 3he 10be production ratio to investigate the potential of using cosmogenic helium in garnet as a dating tool 3hec and 10bec have been measured within garnets and coexisting quartz respectively sampled in the himalayan range at various elevations comparison and correlation of 3hec and 10bec concentrations demonstrate that cosmogenic 3he is well retained in himalayan garnets and can thus be used in this mineral to quantify precisely earth s surface processes this in the case of recently cooled orogenic samples which contain limited amounts of non cosmogenic helium for these high elevation samples the exposure ages derived from 3hec using classical scaling factors lead however to an overestimation of 3hec ages compared to 10bec ages 0 1 to 28 ky by up to a factor of 2 additionally over the samples altitude range 3000 4622 m the theoretically invariant polar 3hec 10bec production ratio 22 5 increases with increasing elevation challenging classical models of production rate evolution to explain this observation we propose a new 3hec production mechanism based on the altitudinal dependence of the energy spectrum of cosmic rays and according to which neutrons or protons resulting from a first spallation reaction within the rock tertiary particles may have sufficient energy to induce an additional in situ spallation reaction producing a second 3h or 3he but insufficient energy to produce 10be the probability of such a mechanism increasing with altitude according to these observations we propose for future and ongoing studies to use the empirically determined attenuation length of 121 g cm2 for helium production at high altitude

cosmogenic 36cl accumulation in unstable landforms 1 effects of the thermal neutron distribution cosmogenic nuclides produced in situ within minerals at the surface of the earth are proving to be an effective means of assessing geomorphic histories the use of multiple cosmogenic nuclides permits both exposure times and erosion rates to be determined however if two nuclides are produced only by spallation reactions the systematic differences in their accumulation rates depend only on the differences in their production rates and half lives the relatively small differences that result require a high degree of analytical precision to yield useful results in contrast to other spallogenic nuclides sup 36 cl is also produced by low energy neutron absorption which creates a different pattern of production as a function of depth we have measured the thermal flux with depth in a concrete block using sup 3 he filled neutron detectors the measured thermal neutron profile agrees well with predictions from a simple diffusion based thermal neutron distribution model calculations of sup 36 cl production using the model suggest that the use of sup 36 cl along with a purely spallogenic nuclide to determine erosion rates and exposure times should be less sensitive to analytical error than are determinations from two purely spallogenic nuclides 31 refs 7 figs 3 tabs

long term cosmogenic 3he production rates 152 ka 1 35 ma from 40ar 39ar dated basalt flows at 29 n latitude a set of time integrated cosmogenic 3he production rates in olivines for the last 1 35 ma are presented we investigated three basaltic lava flow tops from lanzarote canary islands spain the 40ar 39ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152 26 ka 281 19 ka and 1 35 0 01 ma 2 three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3he abundances the resulting 3he production rates in olivine at sea level at 29 latitude are 82 14 and 82 8 atoms g 1 a 1 as obtained from the 152 ka and 281 ka old flows respectively considering effects of erosion on the 1 35 ma old flow we find that the production rate of 82 8 atoms g 1 a 1 is consistent with the cosmogenic 3he production during the last 1 35 ma there appears to be a 14 discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of lal are used to compare results this discrepancy is greatly reduced however if the revised scaling factors of dunai this issue are applied using the new scaling factors we derive a production rate for cosmogenic 3he in olivine at sea level and high latitudes 60 of 118 11 atoms g 1 a 1 2 the correspondingly revised value of cerling and craig and ackert et al is 123 6 atoms g 1 a 1 2 the mean value of these two calibrations is 121 6 atoms g 1 a 1 2 we suggest that the production rate of 121 6 atoms g 1 a 1 at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long term averaged intensity variation in the earth s magnetic field i e over the last 10 ma

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

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

the use of magnetic susceptibility and trace element geochemistry for the correlation of fine grained siliciclastic sequences a late llandovery example from northwest england

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

spatial relationship between earthquakes and volcanic vents in the central northern main ethiopian rift abstract during the breakup of continents extension is commonly accommodated by connected networks of fluid filled fractures dykes and by faults despite the importance of these two extension mechanisms their spatial relationship in three dimensions is poorly understood primarily because it is difficult to quantify the subsurface distribution of faulting and intrusion in order to address this problem we conduct a quantitative analysis of the spatial distribution and clustering of earthquakes and volcanic vents in the main ethiopian rift in east africa in order to understand how extension by faulting and intrusion is distributed throughout the volcanic rift we use fractal analysis of earthquake epicentres in order to infer the 2d characteristics of the subsurface fault network and directly test our model results against the 3d distribution of earthquake hypocentres our results show that fractal analysis of these features is a reliable means to characterise the 3d properties of the fault network in addition the strong similarity between the properties of the fault network derived from earthquakes and properties of the magma filled fracture network derived from fractal analysis of volcanic vents strongly suggests that these are genetically linked we then explore their spatial link using computation of earthquake and vent density which shows that the zone of seismicity is generally around 20 30 km wide while the zone of vents is narrower and centred within the zone of seismicity this spatial relationship suggests that the faults which form rift axial grabens are induced above a narrower and central zone of diking we also demonstrate significant along rift variation in degree of magmatism and faulting with regions of increased degree of diking inferred from a higher cone density characterised by reduced degree of faulting

lithium isotope geochemistry of the hawaiian plume results from the hawaii scientific drilling project and koolau volcano 1 we determined lithium isotopic compositions of mauna loa and mauna kea basalts from the 3 1 km drill hole of the hawaiian scientific drilling project hsdp for comparison li isotopic ratios were also determined for basalts from koolau volcano these two suites of samples define geochemical extremes in the range of hawaiian shield lavas the 400 ka record of mauna kea in the hsdp core shows temporal fluctuations between low 7li 4 relative to the l svec standard and high 7li 5 6 suggesting that the source components in the hawaiian plume are heterogeneous in li isotopic composition based on sio2 content and isotopic ratios of he li nd hf and pb three geochemical groups are identified in mauna kea lavas mauna kea basalts between 1900 and 2500 mbsl have relatively low 7li of about 4 they are low sio2 lavas distinguished by the highest 3he 4he and 208pb 204pb and low 176hf 177hf and 143nd 144nd like basalt from loihi seamount this mauna kea group is considered to originate from the core of the plume above 1900 mbsl high 7li lavas with high sio2 contents appear in both the submarine and subaerial sections they are marked by low 3he 4he and high 176hf 177hf the 7li rich signature of some samples 7li up to 5 7 is indicative of recycled oceanic crust in the plume this magma group defines the kea component the low sio2 lavas in the subaerial section have low 7li 4 3he 4he and 208pb 204pb their 7li values overlap the range of 7li in unaltered mid ocean ridge basalt morb and are consistent with upper mantle material entrained by the plume or contamination of plume derived magmas by the pacific lithosphere the 7li of koolau lavas mostly fall within the range of 4 5 0 3 exceptions are two samples that have 7li of 2 3 the lightest isotopic values may indicate subducted li that was isotopically fractionated during slab dehydration in contrast to other isotopic systems most koolau samples however resemble mauna kea samples in li isotopic composition mauna loa samples have 7li values of 3 5 to 4 9 within the range of the koolau and mauna kea lavas based on these data the loa trend volcanoes and kea trend volcanoes have largely overlapping li isotopic compositions in summary the hawaiian plume is not highly variable in li isotopic composition 7li is typically 4 with perturbations by subducted components to lower and higher ratios 2 5 to 5 7 the overlap of most hawaiian basalt and morb in their range of li isotopic ratios suggests minor influence of recycled oceanic crust in the plume and perhaps similar li isotopic ratios in the upper and lower mantle

long term low latitude and high elevation cosmogenic 3he production rate inferred from a 107 ka old lava flow in northern chile 22 s 3400 m a s l abstract available geological calibration sites used to estimate the rate at which cosmogenic 3he is produced at the earth s surface are mostly clustered in medium to high latitudes moreover most of them have exposure histories shorter than tens of thousands of years this lack of sites prevents a qualitative assessment of available production models used to convert cosmogenic 3he concentrations into exposure ages and or denudation rates it thus limits our ability to take into account the atmospheric geomagnetic and solar modulation conditions that might have affected the production of cosmogenic nuclides in the past for longer exposure histories and in low latitude regions we present the cosmogenic 3he production rate inferred from a new geological calibration site located in northern chile five samples were collected on the surface of the largest and best preserved lava flow of the san pedro volcano 21 934 s 68 510 w 3390 m a s l which displays pristine crease structure features 40ar 39ar dating yields a reliable plateau age of 107 12 ka for the eruption of this lava flow eight pyroxene aliquots separated from the surface samples yield a weighted average cosmogenic 3he concentration of 99 3 1 2 mat g 1 from which a local cosmogenic 3he production rate of 928 101 at g 1 yr 1 is calculated the local production rate is then scaled to a sea level high latitude slhl reference position using different combinations of geographic spatialization schemes atmosphere models and geomagnetic field reconstructions yielding slhl production rates between 103 11 and 130 14 at g 1 yr 1 consistent with the most recent estimates available from the literature finally we use the same scaling frameworks to re evaluate the mean global scale cosmogenic 3he production rate in olivine and pyroxene minerals at 120 16 at g 1 yr 1 from the compilation of previously published calibration datasets

cosmogenic 3he production rates revisited from evidences of grain size dependent release of matrix sited helium measurements of the cosmogenic 3 he 3 hec content of various size aliquots of exposed olivines show that the fine fraction 140 m has 3 hecconcentrations between14and100 lowerthanthatofthecoarsefractions 0 14 1mm suchdifferencesattest to a grain size dependent partial release of 3 hec from the phenocrysts matrix during the preliminary in vacuo crushing this result mighthave importantimplications since most 3 hecmeasurements haveusedfor 20yrastandardroutine basedonthe fusion ofbulk powdered phenocrysts whatever their grain size a suite of new data obtained from coarse olivine grains yielded a mean sea level high latitude 3 hec production rate slhl p3 of 128 5 and 136 6 at g 1 yr 1 depending on the scaling factors used this new value which is 15 higher than previously published rates is obtained from 5 ropy flow surfaces of mt etna 38 n and hawaiian 19 n volcanoes at elevations between sea level and 870 m and ranging in age from 1 47 0 05 to 149 23 ka according to independent 14 c or k ar dating 3 he loss during the crushing step might account for the discrepancy between the standard reference valueof110 115at g 1 yr 1 andthehigherslhlp3proposedhere moregenerally removal ofthe powderedfraction beforefusion is an important point to consider in further studies in order to avoid any 3 hec systematic underestimates

in situ cosmogenic 10be and 21ne in sanidine and in situ cosmogenic 3he in fe ti oxide minerals abstract we report concentrations of in situ cosmogenic 10 be and 21 ne from coexisting quartz and sanidine separates and of cosmogenic 3 he in coexisting fe ti oxide minerals from ignimbritic successions of northern chile oxaya and lauca ignimbrites new mineral isotope pairs such as sanidine and fe ti oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz pyroxene or olivine production rates in sanidine and fe ti oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz the experimentally determined production rates are compared to model production rates calculated with new cross sections for 3 he 21 ne and 10 be production from the individual target elements the mean experimental 21 ne production rate for five sanidine samples is 30 4 3 7 atoms g 1 yr 1 30 4 5 4 atoms g 1 yr 1 including the uncertainty of the 21 ne production rate in quartz p 21 ne qtz this is in excellent agreement with the modelled value of 28 3 atoms g 1 yr 1 which has an estimated uncertainty of 20 the 21 ne production rate in sanidine is thus about 50 higher than that in quartz the cosmogenic neon in sanidine is entirely released in low temperature steps 400 600 c and no signs of an interfering nucleogenic neon component were observed this is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies 3 he diffuses out of the sanidine structure preliminary results also indicate that sanidine is well suited for 10 be studies the mean experimental 10 be production rate from two sanidine samples is 4 45 0 38 atoms g 1 yr 1 4 45 0 42 atoms g 1 yr 1 including uncertainty of p 10 be qtz very close to the modelled value of 4 55 atoms g 1 yr 1 which has an estimated uncertainty of 20 we emphasize that 10 be and 21 ne production in sanidine is composition dependent therefore major element analyses should be carried out and production rates calculated on a sample by sample basis fe ti oxide minerals retain 3 he quantitatively experimentally derived production rates are in excellent agreement with new values derived from physical modelling the mean experimental 3 he production rate for five fe ti oxide minerals samples is 120 11 atoms g 1 yr 1 120 12 atoms g 1 yr 1 including the uncertainties of p 21 ne qtz compared to a mean modelled value of 124 atoms g 1 yr 1 which has an estimated uncertainty of 20 the variable chemical and structural composition of the solid solution lines of fe ti oxide minerals has little effect on the total 3 he production rate cosmogenic 21 ne is not produced in significant quantities in fe ti oxide minerals due to the absence of suitable target elements

monsoonal forcing of holocene glacier fluctuations in ganesh himal central nepal constrained by cosmogenic 3he exposure ages of garnets abstract in the himalayas the late pleistocene glacier oscillations have produced spectacular glacial landforms detailed reconstructions of the chronology and extent of these oscillations are essential to document the sensitivity of the himalayan glaciers to past and future climatic changes in this paper we present a new cosmogenic helium 3 3hec dating on garnets that were sampled on moraine blocks and ice scoured surfaces in a small glaciated valley of the central nepal the mailun valley and that provided a detailed chronology of himalayan glacier fluctuations during the holocene soon after the younger dryas the glacier of the mailun valley underwent a significant retreat around 10 ka this retreat was followed by relative stability of the extent of the glacier between 8 5 and 7 5 ka a second phase of rapid retreat occurred at 7 ka but rapidly slowed down at 5 6 ka finally a last phase of re advance occurred between 0 and 1 ka the interpretation of the equilibrium line altitude ela variation deduced from this chronology for the holocene period and carbon 14 14c dating for the pleistocene period shows that the early history of the mailun valley deglaciation late pleistocene is in good agreement with the global paleoclimatic records the main extent of the glacier and the major ice volume drop are in phase with the global last glacial maximum 25 17 ka and with the major worldwide temperature increase following the younger dryas respectively indicating that the mailun glacier was primarily driven by temperature oscillations during the late pleistocene in contrast the glacier chronology during the holocene suggests that the mailun glacier was modulated by the variation in annual precipitation and is asynchronous relative to most glaciers of the northern hemisphere the significant sensitivity of the himalayan glaciers to precipitation might explain the striking lack of synchronism of the himalayan glaciers both along and across the himalayan arc

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

cosmogenic 10be production rate calibrated against 3he in the high tropical andes 3800 4900 m 20 22 s abstract many geomorphologic applications notably glacier chronologies require improvements in both the precision and the accuracy of the cosmogenic dating tool of particular importance is the need to better constrain the spatial variability of the cosmogenic nuclides production rates at high elevation and low latitudes one strategy that can be adopted for this is to couple absolute calibrations from independently dated surfaces with cross calibration studies performed by measuring several cosmogenic nuclides in the same rock in the present study we report the highest elevation 4800 m cross calibration published to date comprising measurements of cosmogenic 3 he and 10 be in cogenetic pyroxene and quartz the samples were collected from six dacitic moraine boulders exposed from 32 to 65 ka at 4820 m on the flanks of the uturuncu volcano 22 s 67 w southern lipez bolivia the samples yield a remarkably tight cluster of 3 he 10 be production ratios with a weighted mean of 33 3 0 9 1 this production ratio is undistinguishable within uncertainties from the 3 he 10 be production ratio of 32 3 0 9 determined in the same mineral pair at low elevation 1333 m by amidon et al 2009 these results agree at the 1 level and suggest that any hypothetical increase of the 3 he 10 be production ratio in pyroxene and quartz is likely to be lower than 5 over this elevation range 1000 5000 m moreover the production ratio is almost insensitive to the li content of the pyroxene 20 to 50 ppm li suggesting that the cosmogenic thermal neutron production of 3 he is very low in this setting the high elevation 3 he 10 be production ratio is used in combination with a local determination of the 3 he production rate in the high central altiplano 3800 m blard et al 2013 to establish a local 10 be production rate of 30 0 1 4 at g 1 yr 1 at 3800 m and 20 s after scaling to sea level high latitude with the time dependent lal stone model this yields a 10 be production rate in quartz of 3 63 0 17 at g 1 yr 1 importantly this rate can be used for high precision geomorphologic dating for example for determining glacial chronologies 1 4 through 10 be dating of moraines in the high tropical andes any inaccuracy attached to the scaling model is canceled out when the calibration site is located close to the dated object the same experiment was also undertaken in pyroxene and plagioclase from two andesitic boulders from the tunupa volcano moraines exposed at 3800 m and 4200 m a 3 he 10 be production ratio of 35 9 1 3 1 is obtained for this mineral pair indicating that the 10 be production rate in plagioclase is about 8 lower than in quartz

cosmogenic 3he production rate in ilmenite and the redistribution of spallation 3he in fine grained minerals cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3he in olivine or pyroxene however geochemical investigations using 3he have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals or where such crystals were present but have been chemically weathered fine textured basalts often contain small grains of ilmenite a weathering resistant mineral that is a target for cosmogenic 3he production with good he retention and straightforward mineral separation but with a poorly constrained production rate here we empirically calibrate the cosmogenic 3he production rate in ilmenite by measuring 3he concentrations in basalts with fine grained 20 m cross section ilmenite and co existing pyroxene or olivine from the columbia river and snake river plain basalt provinces in the western united states the concentration ratio of ilmenite to pyroxene and olivine is 0 78 0 02 yielding an apparent cosmogenic 3he production rate of 93 6 7 7 atom g 1 yr 1 that is 20 30 greater than expected from prior theoretical and empirical estimates for compositionally similar minerals the production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3he and the associated long stopping distances that cause them to redistribute within a rock fine grained phases with low cosmogenic 3he production rates like ilmenite will have anomalously high production rates owing to net implantation of 3he from the surrounding higher 3he production rate matrix semi quantitative modeling indicates implantation of spallation 3he increases with decreasing ilmenite grain size leading to production rates that exceed those in a large grain by 10 when grain radii are 150 m the modeling predicts that for the ilmenite grain size in our samples implantation causes production rates to be 20 greater than expected for a large grain and within uncertainty resolves the discrepancy between our calibrated production rate theory and rates from previous work the redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number as they do between whole rock basalt and ilmenite

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

long term low latitude cosmogenic 3he production rate determined from a 126 ka basalt from fogo cape verdes an ankaramite lava from fogo cape verdes has been used to determine the long term cosmogenic 3he 3hecos production rate at low latitude 14 9 n 40ar 39ar incremental heating yield an eruption age of 125 8 7 ka cosmogenic 3he abundances yield local production rates of 75 6 7 0 atoms g year in olivine 1 n 10 and 70 1 2 9 atoms g yr in pyroxene 1 n 11 the lower 3hecos production rate in pyroxene is consistent with compositional control on production when scaled to sea level high latitude using available algorithms the olivine 3hecos production rate ranges from 86 109 atoms g yr these values overlap the long term production rate determined at the canary islands 99 atoms g yr they are however lower than previously reported long term production rates from hawaii and southern south america 129 152 atoms g yr and the range of 20 ka calibration sites 110 130 atoms g yr erosion burial and high atmospheric pressure appear to be of negligible importance the likeliest explanation is that the geomagnetic field strength was higher in the past the agreement between the fogo geomagnetic field strength corrected production rate scaled using various published scaling algorithms suggests that differences in incorporation of the non dipole component of the earth magnetic field in the scaling algorithms are negligible for long term production rate determinations at low elevation and low latitude longitudinal variation in long term cosmogenic nuclide production rates between the central atlantic ocean and the americas hawaii and patagonia may exist this has implications for exposure dating of old surfaces it will require that locally determined production rates should be used where available and that scaling schemes incorporating longitudinal variations should be used if nearby calibration sites are not available furthermore if long term longitudinal production rate variations exist it implies that variation in the geomagnetic field may not average out after around 20 ka but may require significantly longer periods up to 100 ka

beryllium 10 dating of the foothills erratics train in alberta canada indicates detachment of the laurentide ice sheet from the rocky mountains at 15 ka the foothills erratics train consists of large quartzite blocks of rocky mountains origin deposited on the eastern slopes of the rocky mountain foothills in alberta between 53 5 n and 49 n the blocks were deposited in their present locations when the western margin of the laurentide ice sheet lis detached from the local ice masses of the rocky mountains which initiated the opening of the southern end of the ice free corridor between the cordilleran ice sheet and the lis we use 10 be exposure dating to constrain the beginning of this decoupling based on a group of 12 samples well clustered in time we date the detachment of the western lis margin from the rocky mountain front to 14 9 0 9 ka this is 1000 years later than previously assumed but a lack of a latitudinal trend in the ages over a distance of 500 km is consistent with the rapid opening of a long wedge of unglaciated terrain portrayed in existing ice retreat reconstructions a later separation of the western lis margin from the mountain front implies higher ice margin retreat rates in order to meet the younger dryas ice margin position near the boundary of the canadian shield 2000 years later

a test of the cosmogenic 3he production rate in the south west pacific 39 s calculation of surface exposure ages using in situ cosmogenic nuclides requires accurate knowledge of local production rates here we report the first attempt to calibrate cosmogenic 3he production in the south west pacific region we present a new radiocarbon chronology that precisely dates the emplacement of the murimotu formation a large debris avalanche deposit in central north island new zealand ca 830 m asl 39 s which occurred 10 4 10 6 cal ka bp measurements of cosmogenic 3he in pyroxene separated from large andesitic blocks exposed during this event yield a sea level high latitude production rate of 120 12 atoms g 1 a 1 lm scaling this is consistent with a recent global compilation comprised predominantly of calibration sites located in the northern hemisphere thus we conclude that the globally compiled cosmogenic 3he production rate is valid in the south west pacific using independent proximal calibrations of cosmogenic isotopes 10be and 14c from quartz in new zealand we derive cosmogenic 3he 10be and 3he 14c production ratios of 32 2 3 2 and 10 6 1 6 respectively

middle pleistocene formation of the rio grande gorge san luis valley south central colorado and north central new mexico usa process timing and downstream implications abstract the rio grande is the fourth longest river in north america extending over 3 000 km from the rocky mountains to the gulf of mexico the pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated herein we constrain the middle pleistocene evolution of the northernmost and largest rio grande basin the san luis basin and the timing of incision of the rio grande gorge based on new geologic mapping 3he surface exposure dating and u series dating on pedogenic carbonates 3he dating of shoreline deposits of closed basin lake alamosa and fluvially scoured bedrock along the rio grande gorge rim indicate the san luis basin was connected with southerly rio grande basins and progressively incised since 400 ka waning tectonic activity coupled with the hydrologic response to intensified middle pleistocene glaciation drove southward spillover of lake alamosa 400 ka expanding the river s drainage basin by nearly 24 000 km2 and adding recharge areas in the high altitude glaciated san juan tusas and sangre de cristo mountains geologic barriers to streamflow on the taos plateau were breached and basin tributary drainages were integrated and inset into their present canyons 250 200 ka with approximately 120 m of incision during 400 200 ka previous work demonstrates that southerly streamflow through basins south of the slb terminated into a large rift related bolson complex until the middle pleistocene and most axial rio grande river incision has occurred since 630 ka we propose that middle pleistocene integration of the slb was the major geomorphic event that eroded plio pleistocene tectonic and volcanic barriers and re established flow of the rio grande from the rocky mountains to the gulf of mexico

oligocene miocene age of aridity in the atacama desert revealed by exposure dating of erosion sensitive landforms the age of onset of hyperaridity in the atacama desert chile which is needed to validate geological and climatological concepts has been heretofore uncertain measurement of cosmogenic 21 ne in clasts from erosion sensitive sediment surfaces in northern chile shows that these surfaces have been barely affected by erosion since 25 ma surface exposure ages of sediment clasts give replicate values at 25 20 and 14 ma and individual values at 37 and 9 ma predominantly hyperarid conditions are required to preserve these oldest continuously exposed surfaces on earth our findings are compatible with the hypothesis that the onset of aridity in the atacama desert could be the reason for rather than the consequence of uplift of the high andes

scaling factors for production rates of in situ produced cosmogenic nuclides a critical reevaluation abstract new scaling factors are presented describing the altitude and latitudinal dependence of production rates for in situ produced cosmogenic nuclides the new factors incorporate the influence of the non dipole contributions to the geomagnetic field on the cosmic ray flux the currently used scaling factors of lal earth planet sci lett 104 1991 424 439 are based on the assumption of a dipole field the overall strategy used in deriving the new scaling factors is however very similar to that of lal and relies on the same and or similar nuclear data in this reevaluation data are only used if the effective geomagnetic parameters inclination horizontal field strength can be reconstructed for the time of measurement the absorption free pathlengths for cosmic rays selected for this study are based on observational data at altitudes relevant for exposure age dating sea level to 7000 m at sea level and latitudes between 20 and 40 the new factors are up to 18 lower and at high altitudes more than 30 higher than those of lal in addition to accounting for the influence of the effective geomagnetic field and providing more applicable estimates of the new factors also allow for correction for the significant deviations from the standard pressure altitude relationship that exist in the atmosphere 10

cosmogenic and nucleogenic 3he in apatite titanite and zircon cosmogenic 3he was measured in apatite titanite and zircon and cosmogenic 21 ne in quartz at 13 depth intervals in a 2 7 m long drill core in a miocene ignimbrite from the altiplano of bolivia all three 3he depth profiles as well as the 21 ne profile attenuate exponentially with depth indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources the attenuation lengthscale for 3he production of 180 11 g cm 2 is consistent with expectations for neutron spallation and is identical to that found for the cosmogenic 21 ne in quartz by normalizing the measured 3he concentrations to 21 ne and using the independently known cosmogenic 21 ne production rate the apparent cosmogenic 3he production rates in apatite titanite and zircon were respectively found to be 112 97 and 87 atoms g yr at sea level and high latitude the formal uncertainty on these estimates is 20 2 and arises in equal parts from uncertainties in the measured 3he 21 ne ratios and the uncertainty in the 21 ne production rate however an additional factor affecting the apparent 3he production rate in these phases arises from the long stopping range of spalled 3he and tritium which decays to 3he because all three accessory phases have higher mean atomic number than major rock forming minerals they will have lower 3he production rates than their surroundings as a consequence the long stopping ranges will cause a net implantation of 3he and therefore higher apparent production rates than would apply for purely in situ production thus these apparent production rates apply only to the specific grain sizes analyzed analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size from 50 to 100 m cross section yields a 10 decrease in apparent production rate while this effect warrants further study the grain sizes analyzed here are typical of the accessory phases commonly encountered so the apparent rates provide an appropriate starting place for surface exposure dating using 3he in these minerals