african laterite dynamics using in situ produced 10be abstract we have investigated the development of iron crust lateritic systems and rain forest soils in tropical environments using in situ produced 10be in quartz veins and cobbles the variability of its concentration as function of subsurface depth in soil profile provides criteria for distinguishing between allochthonous elsewhere formation and autochthonous in situ formation processes and hence for determining whether colluvial transport or in situ chemical weathering dominates soil development in these environments in the stable west african craton in southwest burkina faso at larafella the exponential decrease in 10be concentration observed along a quartz vein from the lowland lateritic system corresponds to the decrease in its production rate associated with attenuation of cosmic rays and thus indicates an autochthonous development models of the 10be depth profile in this unit suggest that this surface has been subject to erosion at a mean rate of 2 m my 1 and that 1 5 of the surface 10be production rate is associated with muon induced reactions in a contrasting environment tropical rain forest malemba congo 10be concentrations have been measured in quartz vein and in round and angular quartz cobbles incorporated in a downslope stone line comparison with data from larafella suggests that the angular cobbles have been transported a few meters from the quartz veins by downslope lateral creeping whereas the round cobbles have an allochthonous origin models of 10be distributions as this site yield an erosion rate of 12 m my 1 and a rate of lateral creeping on the order of 60 m my 1

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

cosmogenic 3 he in hematite and goethite from brazilian canga duricrust demonstrates the extreme stability of these surfaces helium isotopes were measured in hematite and goethite samples from several lateritiric duricrusts canga developed on banded iron formations these samples uniformly have high 3 he concentrations which must arise from long periods of cosmic ray exposure from coexisting phases from the quadrilatero ferrifero in east central brazil we determined the ratio of cosmogenic 3 he in hematite to that of 21 ne in quartz to be 3 96 0 19 combined with best current estimates of the 21 ne production rate in quartz this ratio implies a sea level high latitude slhl 3 he production rate in hematite of 68 1 8 1 atoms g yr from the chemical composition we estimate the 3 he production rate in goethite to be 5 higher we use these production rate estimates to interpret 3 he concentrations measured in goethite and hematite from a 10 m depth profile collected from a surface canga in carajas in the amazon basin of brazil we find that the carajas canga has experienced a very low rate of surface erosion 0 16 0 54 m myr over at least the last few millions of years this iron rich canga surface is remarkably resistant to erosion despite its location in a wet tropical environment details of the depth profile suggest that despite its stability the canga has also been internally dynamic translocation of material solution and reprecipitation over million year timescales

palaeoecology and survival strategies of an impoverished benthic chalk community in a mid danian deep shelf setting abstract upper cretaceous chalk is characterized by a rich and diverse benthic fauna followed by a poor benthic fauna in the lowermost danian immediately after the k pg extinction event in the early to mid danian diversity increased and rich bryozoan and cold water coral mound ecosystems were established in localized areas in the contemporary mid danian dalbyover deep shelf chalk the benthic fauna is markedly impoverished the danian macrofauna from bulk samples at dalbyover comprises 22 species and several bryozoan morphogroups and is of low diversity and density compared to latest maastrichtian and mid danian shelf faunas average fischer alpha index is 2 34 and shannon index 1 38 with minor stratigraphic variations guild analysis of the macrofauna indicates that the vast majority of species show special adaptations to survival at and in the uppermost few centimetres of the soft substrate with dominance of root attached species attaching to small substrates and free lying suspension feeders further the morphology of the serpulid tubes and the occurrence of crinoids in situ reflect a stable substrate with little overturning by the infauna the benthic foraminiferal assemblages are dominated by low trochospiral and planispiral epifaunal morphogroups whereas elongated forms with an infaunal mode of life are rare the combined macrofauna and foraminiferal data indicate deposition in a relatively stable environment where food was a limiting factor for the macrofauna it is evident that species occurring in small numbers are long ranging and also known from the cretaceous whereas species occurring in high numbers are known solely from the danian evidently the mesozoic survivors had the strategy to survive in this special chalk environment where only few danian species were successful

two million years of river and cave aggradation in ne brazil implications for speleogenesis and landscape evolution abstract this study characterizes and provides ages for an extensive sedimentary record occurring in lapa doce and torrinha caves ne brazil with 40 km of surveyed passages these caves integrate a distributary cave system fed by allogenic recharge from the surrounding sandstone plateaus sediment petrography together with descriptions of depositional facies and architectural elements shows four depositional units related to fluvial and standing water environments these include from bottom to top 1 a channel unit including lateral bars deposited during an ordinary flood regime 2 a sandy flood unit including minor channel and scour fills derived from bank full equivalent flood events 3 mud caps deposited in standing water that often reach the ceiling and 4 intraclast breccias associated with collapse of the mud caps under saturated conditions the deposits were dated using a combination of cosmogenic nuclide burial dating and u series dating of flowstone cosmogenic nuclide data point to fluvial aggradation being active since 1 91 0 12 my and extending until 0 36 0 08 my with intensive cave and valley aggradation events between 0 78 0 10 my and 0 44 0 12 my long term alluviation of the cave system seems to be important in forming passages determining their configuration and setting up a general distributary pattern evident in passage morphology and sedimentary sequences mud caps overlapping the fluvial deposits are interpreted as the products of successive rising and lowering of the water table static level radiometric ages of interstratified flowstones and speleothems show that these oscillations were active at least since 115 ky ago and finally ceased around 12 ky ago indicated by the recurrent age obtained from uneroded capping flowstones these long term water table oscillations may drive paragenetic expansion of the whole cave system valleys caves and other landforms in our study area are part of an ancient landscape with multiple episodes of burial and exhumation despite the possibility of much older sediment being preserved somewhere in the cave system our data indicate a younger quaternary age for the bulk of the sediment filling the study caves

stone line formation processes documented by in situ produced 10be distribution jardim river basin df brazil abstract concentrations of in situ produced cosmogenic 10 be t 1 2 1 5 myr were measured in quartz samples from a quartz vein and its connected stone line from the jardim river basin distrito federal brazil in order to quantify the processes involved in the landscape evolution of that region the exponential decrease of the 10 be concentrations along the quartz vein as well as their systematic increase along the stone line away from the quartz vein suggests an autochthonous development two models allow to estimate the lateral displacement rate a plugs model assuming that the sample depth and hence the production rate is constant and a burial model assuming that the sample depth and hence the production rate varies through time yield lateral displacement rate of 37 5 and 68 6 m myr 1 respectively

erosion rates and landscape evolution of the lowlands of the upper paraguay river basin brazil from cosmogenic 10be abstract the importance of earth s low sloping areas in regard to global erosion and sediment fluxes has been widely and vigorously debated it is a crucial area of research to elucidate geologically meaningful rates of land surface change and thus the speed of element cycling on earth however there are large portions of earth where erosion rates have not been well or extensively measured for example the tropical lowlands the cuiabana lowlands are an extensive low altitude and low relief dissected metamorphic terrain situated in the upper paraguay river basin central west brazil besides exposures of highly variable dissected metamorphic rocks flat residual lateritic caps related to a late cenozoic planation surface dominate interfluves of the cuiabana lowlands the timescale over which the lowlands evolved and the planation surface developed and the rate at which they have been modified by erosion are poorly known here we present measurements of in situ produced cosmogenic 10 be in outcropping metamorphic bedrock and clastic lateritic caps to quantify rates of erosion of the surface and associated landforms in order to better understand the quaternary landscape evolution of these lowlands overall slow erosion rates mean 10 m ma suggest a stable tectonic environment in these lowlands erosion rates vary widely between different lithologies range 0 57 to 28 3 m ma consistent with differential erosion driving regional landform evolution the lowest erosion rates are associated with the low relief area irregular plains where clastic laterite mean 0 67 m ma and quartzite mean 2 6 m ma crop out whereas the highest erosion rates are associated with dissection of residual hills dominated by metasandstone mean 11 6 m ma and phyllite mean 27 6 m ma these data imply that the cuiabana lowland is comprised of two dominant landform sets with distinct and different dynamics because the planation surface mostly lowlands is lowering and losing mass more slowly than associated residual hills regional relief is decreasing over time and the landscape is not in steady state the extremely slow erosion rates of the clastic laterite are similar to the slowest outcrop erosion rates reported worldwide these slow rates are due to the material s properties and resistance being comprised of quartzite fragments cemented by an iron rich crust and reflecting long term weathering with iron chemical precipitation and ferricrete formation at least since the middle pleistocene the lateritic caprock appears to be a key factor maintaining hilltop summits of the planation surface over long timescales

a paleotemperature record derived from dissolved noble gases in groundwater of the aquia aquifer maryland usa low 14c activities in groundwater of the confined part of the aquia aquifer in southeastern maryland suggest that most of this water infiltrated at least 30 000 years ago however radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange possibly with secondary calcite deposits in the formation leading to overestimated 14c ages whereas the geochemistry of the aquia aquifer complicates the application of the widely used 14c dating method the accumulation of radiogenic he seems to provide a viable alternative for establishing a chronology the quasi linear increase of he concentrations with flow distance observed in the aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic he u and th concentrations in aquia sand were measured in order to determine the accumulation rate of 4he with sufficient confidence to establish a he time scale concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration these noble gas temperatures ngts clearly show the presence of water that infiltrated under much cooler conditions than at present ngts are correlated with chloride concentrations corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal in contrast the stable isotope ratios 180 and d do not provide a clear record of past climatic changes in the aquia aquifer and the correlation between ngts and stable isotope ratios is weak the ngt record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum lgm were 9 0 0 6 c colder than during the holocene this difference is slightly lower than estimates derived from pollen data for this region but considerably larger than the rather uniform cooling of about 5 c indicated by noble gas studies in more southern locations of north america the larger cooling is ascribed to the influence of the laurentide ice sheet which at its maximum extension came as close as 250 km to our study site

long term fluvial incision rates and postglacial river relaxation time in the french western alps from 10be dating of alluvial terraces with assessment of inheritance soil development and wind ablation effects studying river long profile development as a response to tectonic and climatic controls requires reliable age dating of paleo profile remnants preserved as river terraces cosmic ray exposure cre dating often represents the only method available to date river terraces but the interpretation of cosmogenic nuclide concentrations is complicated by pre depositional inheritance and post depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation here we use cosmogenic 10be measurements to date alluvial terraces in the french western alps in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing we assess inheritance by constructing 10be concentration vs pebble depth profiles and use a monte carlo technique to estimate terrace ages we find that inheritance is negligible on all terraces enabling us to date terraces as young as 5 ky terraces that predate the last glaciation experienced intense pedogenesis and wind ablation which led to significant scatter in the 10be concentrations of surface samples we assess these effects using a model of 10be ingrowth and show that the oldest cre ages of surface clasts are close to the probable terrace age we study two catchments which have undergone varying degrees of glacial disruption the buech river experienced variations in runoff and sediment flux during the last glaciation as well as occasional ice dammings of its outlet its upper terraces record incision rates averaged over 190 ky of 0 8 mm yr consistent with denudation rates estimated in the surrounding areas and suggesting long term stability of river incision rates climatic forcing is well documented for the drac river which was repeatedly dammed by glaciers during cold periods its postglacial incision history was triggered by an 800 m drop in base level following ice dam disappearance long profile development by knickpoint propagation explains the time lag of 2 5 ky between this base level drop and terrace abandonment upstream as well as subsequent peak incision rates of 6 cm yr followed by a gradual decrease in incision rates the present knickpoint location 55 km upstream from the glacial damming site enables us to calculate a fluvial response time of 15 20 ky controlled by knickpoint propagation rates of several meters per year within the drac river

gold a tracer of the dynamics of tropical laterites laterites have developed essentially from long term exposure of cratonal rocks to the atmosphere and hydrosphere the properties of gold density and relative insolubility make residual gold particles tracers of surficial processes causing mineralized laterites gold particles are useful in distinguishing between in situ chemical weathering and physical transport in african lateritic systems developed on slowly eroding stable landforms in an equatorial rainforest setting chemical dissolution and translocation of fine particles are the dominant processes episodic alluvial and or colluvial transport appears to contribute to laterite formation in the subsahelian steppe the use of gold particles coupled with the examination of in situ 10 be from auriferous quartz indicates the applicability of such methods for examination of lateritic weathering dynamics of the tropical transcontinental lithosphere

rates and timing of earth surface processes from in situ produced cosmogenic be 10 beryllium 10 is the longest lived of the seven known unstable isotopes of be it results mostly from the interaction of cosmic radiation primarily neutrons with a variety of target atoms by spallation the splitting of nuclei lal 1988 it can also be produced at very low levels by radio disintegration of u and th sharma and middleton 1989 although all radioactive be isotopes are produced in the atmosphere via cosmic ray reactions arnold 1956 lal and peters 1967 morris et al chapter 5 this volume in this paper we are most interested in 10be produced in rock which we refer to as in situ produced such in situ produced 10be is a cosmic ray dosimeter functioning as a quantitative monitor of near surface residence time in some geologic situations 10be can be used as a chronometer allowing one to estimate the duration of surface exposure in other settings 10be can be used as a tracer allowing one to estimate the rate distribution and behavior of earth surface processes rates of 10be production are very low in rocks exposed at earth s surface 102 atoms g 1 y 1 resulting in typical in situ produced 10be concentrations termed activities of 103 to 107 atoms g 1 the development of accelerator mass spectrometry ams in the 1980s allowed routine measurement of 10be at levels typical of rocks exposed near earth s surface elmore and phillips 1987 analytical refinements in the 1990s lowered detection limits by an order of magnitude allowing dating of exposure periods 103 years in favorable geologic and geographic situations simple exposure histories at high altitude and latitude most in situ 10be is measured in quartz mineral separates where the primary spallation target is o lal and arnold 1985

influence of matrix diffusion and exchange reactions on radiocarbon ages in fissured carbonate aquifers the parallel fissure model coupled with the equation of diffusion into the matrix and with exchange reaction equations has been used to derive a simple formula for estimating the influence of matrix porosity and reaction parameters on the determination of radiocarbon ages in fissured carbonate rocks examples of evidently too great radiocarbon ages in carbonate formations which are not explainable by models for the initial sup 14 c corrections can easily be explained by this formula parameters obtained for a chalk formation from a known multitracer experiment combined with a pumping test suggest a possibility of sup 14 c ages more than three orders of magnitude greater than the ages which would be observed if the radiocarbon transport took place only in the mobile water in the fissures it is shown that contrary to the solute movement on a small scale and with a variable input the large scale movement characteristic for the sup 14 c dating does not necessarily require the knowledge of kinetic parameters because they may be replaced by the distribution coefficient discordant tritium and sup 14 c concentrations are commonly interpreted as a proof of mixing either in the aquifer or at the discharge site for fissured carbonate formations however anmore alternative explanation is given by the derived model showing a considerable delay of sup 14 c with respect to nonsorbable tracers less

use of a new 10 be and 26 al inventory method to date marine terraces santa cruz california usa comment perg et al 2001 1 present ages for five uplifted marine terraces near santa cruz california based on soil column inventories of in situ produced 10be these ages are consistent with terrace formation at marine oxygen isotope stages 3 5a 5c 5e and 7 and imply an uplift rate of 1 1 mm

long term soil erosion derived from in situ 10be and inventories of meteoric 10be in deeply weathered soils in southern brazil abstract meteoric 10 be is commonly used as a geochemical tracer of soil erosion and regolith residence time over long time scales 10 3 yr the acidic conditions of tropical soils are less favorable for meteoric 10 be retention and make a translation of meteoric 10 be inventories into surface erosion rates complex in this empirical study we evaluate the potential loss of meteoric 10 be in the weathering zone along a toposequence in southern brazil three regolith profiles were sampled to explore the mobility of be in the soil system the sequential chemical extractions of four reactive fractions indicate that the amorphous oxy hydroxide and crystalline oxide fractions are the main carriers of meteoric 10 be our data from the brazilian toposequence reveal significantly higher inventories of meteoric 10 be in the slope concavities compared to the upslope positions this demonstrates that chemical leaching and deep percolation of meteoric 10 be within the soil profile leads to vertical and lateral redistribution of meteoric 10 be across the hillslope our data show that more than 50 of the delivered meteoric 10 be is leached from the upslope regolith profiles after correcting the inventories for incomplete retention of meteoric 10 be surface erosion rates are derived for the slope convexities upslope erosion rates are below 5 mm kyr and agree with the in situ 10 be derived denudation rates our results suggest that soil and sediment fluxes can accurately be derived from meteoric 10 be inventories in well developed soils when the meteoric 10 be mobility is constrained using differential mass balances

combined dating of goethites and kaolinites from ferruginous duricrusts deciphering the late neogene erosion history of central amazonia abstract this study focuses on the dating of millimetric pisoliths from ferruginous duricrusts located in central amazonia brazil by coupling detailed mineralogy and two relevant dating methods i u th he analysis of goethites by mass spectrometry ii analysis of radiation induced defects of kaolinites embedded in duricrusts by electron paramagnetic resonance spectroscopy epr three samples collected at different elevations in the landscape were selected the goethite corrected ages range from 1 1 15 2 ma and consistently increase with elevation epr was performed on kaolinites after several cycles of a deferration procedure considering extreme geochemical scenarii the ages of kaolinites are 1 7 7 1 ma for a closed system and 4 0 16 7 ma for an open system with 100 rn loss ages of goethites and kaolinites are close in the limit of the uncertainty of the methods they show important periods of duricrust formation at middle miocene and at late miocene pliocene data allow the estimation of lowering rates between 3 and 8 m ma in agreement with independent data available in the literature for the central amazonia region or even for erosion of other cratons in the world the two dating methods can be compared on common duricrust samples by revealing single or multi step weathering erosion episodes related to relictual paleosurfaces they also bring promising contributions to an advanced knowledge of ancient and complex lateritic geosystems

carbon isotope exchange rate of dic in karst groundwater abstract the kinetics of isotopic exchange between dissolved inorganic carbon dic of groundwater and calcite of the matrix of karst aquifers of cyrenaica libya can be deduced from 13 c and 14 c data the aquifers are mostly confined and the majority of the wells do not show any occurrence of modern recharge in 1976 1980 in fact the tritium content was below 1 tritium unit tu in most sites assuming that the isotopic exchange takes place through a first order reaction such as 14 c radioactive decay it can be shown that a linear correlation occurs between ln a and ln m e p where a is the 14 c activity m and are the 13 c contents of matrix calcite and dic respectively and e p is the 13 c enrichment in caco 3 precipitation the slope of the correlation provides the half life of the isotopic exchange process for cyrenaica karst groundwater a half life of about 11 000 years is obtained i e about double that of 14 c radioactive decay the isotopic exchange kinetics also depends on the ratio between groundwater volume and the calcite surface exposed to the exchange process thus other aquifers will show different exchange half life values the cretaceous chalk aquifers of the paris basin france and lagerdorf germany give a half life of about 4000 years much shorter than that of cyrenaica which may be due to the high porosity i e to the large surface available for the isotope exchange process the berkshire chalk aquifer uk gives a half life of about 10 000 years much higher half lives above 20 000 years are obtained for two sandy aquifers in flevoland the netherlands and texas usa which could be explained by the low caco 3 content of the aquifer matrix the highest half life value about 40 000 years is obtained in an artesian limestone aquifer in florida usa

rate and processes of river network rearrangement during incipient faulting the case of the cahabon river guatemala deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics although the reorganization of deeply incised drainage systems has been documented the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood to address this problem we have explored the rapid drainage reorganization that affected the cahabon river in guatemala during the quaternary sediment provenance analysis field mapping and electrical resistivity tomography ert imaging are used to reconstruct the geometry of the valley before the river was captured dating of the abandoned valley sediments by the be 10 al 26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes such as the rate of tectonic deformation of the paleovalley the rate of propagation of post capture drainage reversal and the rate at which canyons that formed at the capture sites have propagated along the paleovalley transtensional faulting started 1 to 3 million years ago produced ground tilting and ground faulting along the cahabon river and thus generated differential uplift rate of 0 3 0 1 up to 0 7 0 4 mm y 1 along the river s course the river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence then the river experienced two captures and one avulsion between 700 ky and 100 ky the captures breached high standing ridges that separate the cahabon river from its captors captures occurred at specific points where ridges are made permeable by fault damage zones and or soluble rocks groundwater flow from the cahabon river down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges valley fill formation and capture occurred in close temporal succession suggesting a genetic link between the two we suggest that the aquifers accumulated within the valley fills increased the head along the subterraneous system connecting the cahabon river to its captors and promoted their development upon capture the breached valley experienced widespread drainage reversal toward the capture sites we attribute the generalized reversal to combined effects of groundwater sapping in the valley fill axial drainage obstruction by lateral fans and tectonic tilting drainage reversal increased the size of the captured areas by a factor of 4 to 6 at the capture sites 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm y 1 deepening at a rate of 0 7 to 1 5 mm y 1 at this rate 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley it is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field which involved transient sedimentation along the river s course if the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced

studies of isotopic hydrology of the basin of mexico and vicinity annotated bibliography and interpretation abstract this study deals exclusively with the behavior of the oxygen 18 and deuterium composition of precipitation surface water and groundwater within and near the basin of mexico it reviews 16 studies from 1974 to 1993 reporting new measurements related to the study area location maps and the summary of original conclusions are included parts of the documents whose circulation is restricted were collected from private files and government libraries these data which were not previously available to the scientific community represent a total of 791 oxygen 18 and 703 deuterium analyses most of them 18 o d pairs the discussion is an overview of the whole data set with interpretation as inferred from the data regional precipitation follows the regression d 7 97 18 o 11 03 n 85 r 2 0 97 being slightly different from a local meteoric line previously published variations in 18 o are about 21 7 to 1 3 following a gaussian distribution f 18 o 0 036 exp 0 5 18 o 9 97 4 43 groundwater from springs and deep production wells fitted the new regional meteoric line variations in 18 o are about 13 5 to 6 5 and their distribution is also gaussian f 18 o 0 248 exp 0 5 18 o 10 09 0 81 shallow piezometer and pore saline waters in the sediments of the former terminal marsh of the study basin showed a trend of isotopic alteration this feature is interpreted by the corresponding source study as mixing of in situ post 1953 infiltration with older evaporated waters this mixing trend intersects the regional meteoric line close to 18 o 8 0 isotope data and our interpretation are consistent with the current concept of a closed groundwater basin associated with the closed surface water basin of mexico

climatic solar oceanic and geomagnetic influences on late glacial and holocene atmospheric 14c 12c change late glacial and holocene 14c12c ratios of atmospheric co2 vary in magnitude from a few per mil for annual decadal pertubations to more than 10 for events lasting millennia a data set illuminating 10 to 104 yr variability refines our understanding of oceanic climatic versus geomagnetic or solar forcing of atmospheric 14c12c ratios most of the variance in the holocene atmospheric 14c12c record can be attributed to the geomagnetic millennia time scale and solar century time scale influence on the flux of primary cosmic rays entering the atmosphere attributing the observed atmospheric 14c12c changes to climate alone leads to ocean circulation and or global wind speed changes incompatible with proxy records climate ocean related 14c redistribution between carbon reservoirs while evidently playing a minor role during the holocene may have perturbed atmospheric 14c12c ratios measurably during the late glacial younger dryas event first order corrections to the radiocarbon time scale 12 000 30 000 14c yr b p are calculated from adjusted lake sediment and tree ring records and from geomagnetically defined model 14c histories paleosunspot numbers 100 9700 cal yr b p are derived from the relationship of model 14c production rates to sunspot observations the spectral interpretation of the 14c12c atmospheric record favors higher than average solar activity levels for the next century minimal evidence was found for a sun weather relationship

effective attenuation lengths of cosmic rays producing 10be and 26al in quartz implications for exposure age dating we have measured cosmic ray produced 10be t 1 5 my and 26al t 0 72 my as functions of depth in a core of quartz sandstone bedrock collected in south victoria land antarctica these data were used to place limits on the exposure age and erosion rate of the exposed surface and to calculate the effective attenuation lengths of cosmic rays producing these nuclides these latter results indicate that the production attenuation length for 26al is comparable to that for 10be this suggests that the production ratio 26al 10be does not vary substantially with depth in a rock and that its value at the surface of exposed rock is not strongly affected by erosional losses allowing wider use of the ratio in determining exposure ages

examination of surface exposure ages of antarctic moraines using in situ produced 10be and 26al concentrations of 10be t12 1 5 106 y and 26a1 t12 0 72 106 y have been determined by accelerator mass spectrometry ams in a suite of quartz samples taken from sandstone boulders in several moraines in arena valley a dry valley adjacent to the taylor glacier in the quatermain mountains southern victoria land east antarctica these isotopes are produced in surficial quartz by cosmic ray spallation of o and si the concentrations in these samples ranged from 6 1 105 to 3 0 107 at g 1 for 10be and from 9 4 106 to 1 2 108 at g 1 for 26a1 depending upon the extent of exposure at the surface analyses of 10be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons the value of 142 96 41 g cm 2 is consistent with other published values production rates of 17 6 4 at g 1 y 1 for 10be and 113 54 16 at g 1 y 1 for 26a1 at 1300 m and 87 s and a 26al 10be production ratio of 6 5 1 3 1 3 were calculated from the data these values correspond to sea level production rates at high geomagnetic latitude of 6 4 at g 1 y 1 and 41 7 at g 1 y 1 for 10be and 26a1 respectively consistent with determinations based on 11 ky glacially polished surfaces in the sierra nevada in california these production rates imply exposure ages for the various moraines ranging from 50 ky to 2 5 my in accordance with other geological evidence the 10be and 26al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3he examination of the 3he concentrations in conjunction with the present data set yields an estimate of 3he production of 230 8540 at g 1 y 1 corresponding to 85 31 15 at g 1 y 1 at sea level this is lower than previously reported values of 240 60 at g 1 y 1 and 190 at g 1 y 1 scaled to sea level and high geomagnetic latitude based respectively on volcanic flows less than 2000 years old at low latitude and samples collected from 14 400 year old volcanic flows this discrepancy is likely due to diffusive losses which may become significant up to 50 for surface exposure ages on the order of 2 0 my nevertheless the general correlations among the three isotopes suggest that 3he does not suffer gross losses through diffusion

statistical geochemical and morphological analyses of stone line formation in uganda abstract the resolution of conflicting fluvial allochthonic erosion and deposition and bioturbation autochthonic faunal turbation theories of stone line formation has important implications for tropical geomorphology with the former indicating dynamic punctuated landscape development and the latter implying relative landscape stability with ongoing moderate erosion on a gentle hill in central uganda we quantitatively characterized and statistically modeled gravel distributions for 93 profiles elemental analysis of k selected rare earth elements ree and appropriate index elements were employed to test for compositional differences between soils above and saprolite below hilltop stone lines we also excavated and carefully described four indicative profiles the results of this study were difficult to reconcile with the bioturbation theory of stone line formation important findings included i the lack of a consistent gravel free biomantle with a weak connection between biomantle thickness and stone line depth ii highly variable stone line depth with a strong inverse relationship between stone line depth and maximum gravel content iii unweathered and rounded cobbles in a stone line over saprolite with angular highly weathered quartz and iv a statistically significant geochemical difference between soils and saprolites with evidence of a less weathered surface mantle over more weathered saprolite these findings indicated a sedimentary origin for soils at this site even for what is presently a flat geomorphically isolated hilltop evidence from this study and previous work on the geochemistry of this site geochim cosmochim acta 67 2003 2711 suggests that the contemporary hilltop once occupied a lower landscape position

weathering geochronology by u th he dating of goethite nine samples of supergene goethite feooh from brazil and australia were selected to test the suitability of this mineral for u th he dating measured he ages ranged from 61 to 8 ma and were reproducible to better than a few percent despite very large variations in u and th in all samples with internal stratigraphy or independent age constraints the he ages corroborated the expected relationships these data demonstrate that internally consistent he ages can be obtained on goethite but do not prove quantitative 4he retention to assess possible diffusive he loss stepped heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3he the 3he release pattern indicates the presence of at least two diffusion domains one with high helium retentivity and the other with very low retentivity at earth surface conditions the low retentivity domain which accounts for 5 of 3he contains no natural 4he and may represent poorly crystalline or intergranular material which has lost all radiogenic 4he by diffusion in nature diffusive loss of 3he from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate so probably represents diffusion from individual micrometer size goethite crystals the 4he 3he evolution during the incremental heating experiments shows that the high retentivity domain has retained 90 95 of its radiogenic helium this degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to earth surface temperature and held for the appropriate duration considering both the high and low retentivity domains these data indicate that one of the samples retained 90 of its radiogenic 4he over 47 5 ma and the other retained 86 over 12 3 ma thus while diffusive loss corrections to supergene goethite he ages are required these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton irradiation 4he 3he method

an isotopic investigation of groundwater in the central san juan basin new mexico carbon 14 dating as a basis for numerical flow modeling carbon 14 dating of groundwater can be used to help determine the transmissivity distributions of aquifers this method may offer significant advantages in certain respects over traditional aquifer pumping test techniques we have applied 14c dating to a hydraulic analysis of a multilayer aquifer system in the central san juan basin of new mexico after corrections for geochemical evolution of the solutes the influence of dispersive processes on the 14c distribution was investigated a model incorporating stochastic dispersion theory indicated that macroscopic dispersion exerted only a small influence on the measured 14c activities the 14c derived transmis sivity distribution was used to construct a numerical flow model which was applied to an analysis of interaquifer leakage the model showed that even though vertical flow between aquifers was significant in this case it did not cause the 14c distribution to differ significantly from that predicted by a simple piston flow model carbon 14 can be used as a basis for detailed hydraulic evaluations of groundwater flow in areas where traditional well hydraulics methods are not practical

brazilian laterite dynamics using in situ produced 10be abstract concentrations of in situ produced cosmogenic 10 be t 1 2 1 5 myr were measured using accelerator mass spectrometry ams in quartz samples from three brazilian lateritic systems in order to characterize and quantify processes involved in their development results from a bedrock embedded quartz vein outcropping at gentio do ouro lead to a calculated mean erosion rate of 2 5 m myr 1 for a muon participation to the total 10 be production set at 1 5 at itaberaba the 10 be content of the surficial sample yields a mean erosion rate of 9 0 0 5 m myr 1 in addition the systematic increase in 10 be concentrations of stone line quartz with distance from the quartz vein strongly suggests an autochthonous development the data constrain a model of stone line emplacement and imply a rate of lateral displacement on the order of 70 m myr 1 by contrast the constant 10 be concentration measured within the rounded quartz cobbles all along the cuiaba stone line suggests that it most likely results from almost contemporaneous rapid events that occurred at least 500 kyr ago the proposed scenario involves sheet washing at the surface of the lateritic paleolandscape leading to surficial dispersion of allochthonous quartz cobbles immediately followed by colluvial deposition of weathered lateritic material

spatial distribution of denudation in eastern tibet and regressive erosion of plateau margins article i nfo the longmen shan range is one of the major topographic and structural features of the eastern margin of the tibetan plateau with an impressive topographic gradient and low convergence rates across the range this region has raised important questions concerning the dynamics of plateau margin settings such as the long term mechanisms of topographic evolution the investigation of the distribution in space and time of denudation can provide critical insight into such dynamics and shed light on still unresolved controversies for that purpose we present a new dataset that documents the intensity and distribution of denudation processes across this plateau margin through field survey of fluvial incision markers quantitative geomorphology and cosmogenic nuclide derived basin wide erosion rates erosion is b0 5 mm year in the frontal region of the longmen shan and between 0 5 and 1 mm year further west with a gradual decrease when reaching the northern headwaters of the min jiang watershed adjacent to the beginning of the tibetan plateau the spatial distribution of denudation inferred from the various methods we use suggests that most of the differential uplift in the central longmen shan is accommodated by the beichuan fault and frontal structures located in the foothills the denudation pattern seems also to reflect the large scale propagation of erosion from the sichuan basin toward the plateau this suggests that the longmen shan range is submitted to the combined influences of slow thrusting activity on the frontal structures and progressive westward regressive erosion as a probable response to a pulse of uplift of the tibetan plateau eastern margin that

groundwater chemistry and mass transfers in the independence aquifer central mexico by using multivariate statistics and mass balance models in the light of progressive depletion of groundwater reservoir and water quality deterioration of the independence aquifer an investigation on chemical data of dissolved major and minor constituents in 246 recent groundwater samples was performed the main objective was the detection of processes responsible for the geochemical evolution and mineralization throughout the area multivariate techniques revealed different sources of solutes a dissolution of calcium and magnesium carbonate minerals b weathering of acid volcanic minerals c alteration of manganese containing alkaline silicates d leaching of halite deposits of meteoric origin e contamination from agricultural and urban wastewaters and f evaporative effects due to intensive irrigation although nitrate contamination is associated with pollution from intensive cultivated areas natural contamination plays an important role in the study area the investigation reveals that weathering of acid volcanic rocks rhyolite and oxidation of arsenic bearing sulfide minerals are the responsible processes for high fluorine up to 16 mg l and arsenic up to 0 12 mg l contents respectively exceeding the mexican maximum admissible concentration for drinking water except for kaolinite all recharge processes are dissolution oriented co2 calcite dolomite k feldspar plagioclase silicate precipitation amorphous silica and chalcedony is of growing importance in discharge zones cation exchange is not an important issue in the whole study area

cosmogenic 10be in zaire alluvial diamonds implications for 3he contents of diamonds to determine the amounts of cosmic ray produced cosmogenic 3he which could be created in diamonds during their post eruptive residence in near surface regions we have measured the concentration of 10be half life 1 6 myr in an industrial diamond sample the motivation to study cosmogenic 3he in diamonds comes from the recent observations1 2 of 3he 4he ratios in some diamonds that are even higher than meteoritic values so called planetary helium with 3he 4he 1 4 10 4 we measured 10be in an alluvial diamond sample using radiochemical and accelerator mass spectrometry ams techniques the observed 10be concentration 2 46 0 39 106 atoms g 1 leads to an effective surface exposure of 3 3 105 yr for these diamonds this corresponds to a substantial cosmogenic production of 3he in the alluvial diamonds 29 of the total 3he we conclude that the high 3he 4he ratios in at least some diamonds are due to cosmogenic 3he production in near surface regions during their residence in alluvial or weathering deposits thus in order to establish that such high 3he 4he ratios existed in the mantle at one time it is a priori necessary to study diamonds collected by underground mining methods

the application of quartz grain morphology measurements to studying iron rich duricrusts abstract iron rich duricrust profiles are commonly located on old flat surfaces they promote landscape stability by the protective action of ferricrete providing important records of multiple soil and geomorphic processes as well as paleoclimatic events in the southern serra do espinhaco sse in brazil iron rich duricrust profiles occur on flat to slightly convex hills of quartzite at elevations of mostly 1200 1400 m a s l some of these profiles may be remnants of old paleosurfaces here we describe an image analysis technique to measure morphological attributes of quartz grains in thin sections of iron rich duricrusts we present a way to investigate the genesis of these materials size and shape variations of quartz grains were measured for iron rich duricrust profiles from four different locations gs2 pf1 pf2 and pf3 thin sections of 5 7 5 cm from surface horizons and iron rich duricrusts were studied distinct morphological regions iron concretion and matrix of iron rich duricrusts were separately analyzed to identify variations in regolith morphology a tescan vega3 scanning electron microscope sem was used to produce backscattered electron bse images for regions of sufficient area to enable the use of image analysis software imagej 1 50i bse images provide a clear contrast between quartz grains and iron rich duricrust matrix allowing resolution of quartz grains for specific investigation the size circularity and roundness of many quartz grains were easily determined this procedure enables the quantitative analysis of soil materials allowing comparisons of soils and the testing of hypotheses for soil genesis quartz grains within iron concretions of pf3 had different morphological properties from quartz in the surrounding ferricrete indicating that different processes had occurred during the genesis of the two materials in contrast pf1 which showed no evidence of parent material diversity contained quartz grains that are indicative of a polycyclic character we observed fragments of iron concretions with pores previously occupied by quartz grains indicating that extensive quartz dissolution had occurred during duricrust evolution

in situ produced 10be measurements at great depths implications for production rates by fast muons in situ cosmogenic 10be values have been used to investigate a brazilian quartz vein from the surface to a depth of 15 m at depths greater than 1000 g cm2 deep enough for neutron induced reactions to be insignificant there is only a slight decrease in 10be concentration with increasing depth our results are consistent with deep production of 10be by a mechanism presumably induced by fast muons with an attenuation length of 5300 950 g cm2 and a contribution of 0 65 0 25 to the total surface production results are compared with values from the literature and implications of this re evaluation are discussed

the development of iron crust lateritic systems in burkina faso west africa examined with in situ produced cosmogenic nuclides we have investigated the development of iron crust laterites on the stable west african craton in northern burkina faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles lateritic soils develop in tectonically stable slowly eroding tropical environments and are a major component of the earth s surface to examine processes affecting laterite formation we determined 10be and 26al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3 8 m myr 1 in addition data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years quartz cobbles presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial colluvial material have10be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode presumably associated with a wetter climate roughly 300 kyr bp these results illustrate the practicality and the potential of the use of in situ produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in situ chemical weathering and mechanical transport

chronology of quaternary glaciations in east africa abstract a new glacial chronology for equatorial east africa is developed using in situ cosmogenic 36 cl measured in 122 boulders from moraines on mount kenya and kilimanjaro the oldest deposits sampled on kilimanjaro yield a limiting 36 cl age of 360 calendar kyr all 36 cl ages are in calendar years cal kyr or cal yr on mount kenya the oldest moraines give ages of 355 420 kyr liki i and 255 285 kyr teleki given the uncertainty in our 36 cl ages the liki i moraine may correspond to either marine isotope stage 10 or 12 whereas the teleki moraine correlates with stage 8 there is no evidence for stage 6 on either mountain the liki ii moraines on mt kenya and moraines of the fourth glaciation on kilimanjaro give ages of 28 3 kyr and 20 1 kyr respectively they represent the last glacial maximum lgm and correlate with stage 2 of the marine isotope record a series of smaller moraines above the lgm deposits record several readvances that occurred during the late glacial on mt kenya these deposits date to 14 6 1 2 kyr liki iia 10 2 0 5 kyr liki iii 8 6 0 2 kyr liki iiia and 200 yr lewis the corresponding deposits on kilimanjaro have mean ages of 17 3 2 9 kyr fourth glaciation saddle 15 8 2 5 kyr little glaciation saddle and 13 8 2 3 kyr fourth glaciation kibo these data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle

geochemical and isotopic investigations on groundwater residence time and flow in the independence basin mexico abstract the independence basin in the semi arid guanajuato state of central mexico is facing serious groundwater resources deficiency due to an increasing demand linked to a rapid population growth and agricultural development this problem is aggravated by an inadequate evaluation of groundwater resources in the region geochemistry and isotopic tracers were used in order to investigate the groundwater flow system and estimate the groundwater residence time the groundwater is characterized by low salinity with some exceptions associated to a contribution of more saline groundwater from deep formations the predominant reactions are co 2 gas dissolution carbonate dissolution albite weathering kaolinite and chalcedony precipitation six principal hydrochemical zones were recognized which provided information on plausible recharge sources and groundwater chemical evolution the 14 c concentration varies between 19 and 94 pmc the high 14 c values indicating recent recharge are observed at the basin margins and a trend to lower 14 c values is observed along the modern groundwater flow paths the groundwater residence time according to radiocarbon estimations ranges between recent and 11 ka the residence time distribution matches the regional important discharge zones west in the basin center from dolores hidalgo and southwest from doctor mora hydrochemical tracers are in general agreement with the predeveloped and current hydraulic head configuration however show some inconsistencies with the predeveloped head in the downgradient areas which means that the impact by gradually increasing groundwater extraction during the last decades is reflected on radiocarbon age distribution geochemical evidences imply that the recharge input from the northern basin area is insignificant

weathering and the formation of hillslope deposits in the tropical highlands of itatiaia southeastern brazil abstract deposits of cobbles and boulders found hanging on midslopes of the itatiaia plateau reflect tectonic reactivation and a large range of climate controlled processes probably including frost weathering these processes more efficient in detaching debris from freefaces were replaced at the end of the pleistocene by colluviation on the lower slopes of the plateau less coarse ci and cii colluvia evince the reworking of upslope materials freed in former phases of weathering or erosion interpretation of hillslope processes was based upon the sedimentological mineralogical and micromorphological characteristics of colluvia ci may have originated in mass movements reaching deep into the regolith cii colluvia were probably deposited by shallow processes which reworked initially highly evolved materials including on the way downslope poorly weathered rock fragments changes in weathering characteristics and hillslope processes suggest variations in environmental conditions of the itatiaia plateau since the late pleistocene coarser deposits would be associated to the wet and cold conditions of the last glacial maximum in the end of the pleistocene warmer and still humid conditions would have favored weathering and mass movement processes triggering the first generation of colluvia ci discontinuous stone lines found on mid slopes would represent a short return to colder and eventually drier conditions in the last 8 000 years prevailing humid climates with temperature excursions larger than those observed at present encompassing phases of more intense gelifraction would explain the deposition of cii sequences however such variations were not sufficient to interrupt the geochemical weathering trend toward kaolinitic and gibbsitic materials that persisted after the last glacial maximum environmental fluctuations of small amplitude and a decrease in the intensity of slope denudation during the holocene seem to follow the general trend previously recognized in the neighboring highlands of campos do jordao

carbon 14 dating of groundwater in confined aquifers implications of aquitard diffusion the influence of diffusive losses into fine grained aquitards on the 14c age interpretation of groundwater in long thin confined aquifers is examined an analytical model is developed by coupling two one dimensional steady state differential equations one representing advective dispersive transport in the aquifer and the other representing diffusion into the aquitards the results indicate that aquitard diffusion is a mechanism that can cause a significant reduction in 14c concentrations in a confined aquifer groundwater velocities derived from 14c measurements that are unadjusted for diffusion would therefore be underestimated the error that ensues is highly sensitive to the aquifer thickness and the magnitude of the diffusion coefficient in contrast the steady state distribution of 14c is shown to be relatively insensitive to the magnitude of the longitudinal dispersivity value of the aquifer dispersivities representative of the upper range of values that have been reported in the literature produced a negligible difference in steady state 14c concentration compared to results obtained using small dispersivities an equation is presented which permits the adjustment of 14c data to account for diffusion providing that the aquifer aquitard system is not stratigraphically complex and heterogeneous the analysis can be extended to include age dating isotopes that have half lives different from that of 14c the results of this study also apply to age dating in fractured porous media

termites and stone lines traps for the unwary archaeologist abstract an unproven assumption in the case of prehistoric archaeological sites has always been that where the optical and or radiocarbon ages are in stratigraphic order becoming older with depth the ages obtained provide unambiguous ages for any artefacts within the same stratigraphic horizon this is not necessarily so a growing body of evidence gathered by dating practitioners soil scientists and geomorphologists during the past few decades in many parts of the world has shown that post depositional and syn depositional disturbance is the norm rather than the exception field observations in tropical northern australia show that termite activity operating in conjunction with soil creep subsurface lateral eluviation and slope wash can generate a buried stone layer 30 50 cm thick within 10 000 15 000 years and can replenish the topsoil to a depth of 15 30 cm thick within the same time termites can cause differential displacement of artefacts and can seriously interfere with attempts to obtain a reliable chronology based on radiocarbon and or optical ages for the sediments in which the artefacts occur

cosmic ray exposure cre dating in a wet tropical domain late quaternary fan emplacements in central sulawesi indonesia accurate chronological records are critical for studying the alluvial fan sequences emplaced during local global climatic changes but in general there are no satisfactory quantitative dating methods available in situ produced be 10 concentrations have been measured in quartz boulders exposed on sulawesian late quaternary fan surfaces these concentrations suggest that the two fan units evidenced by geomorphic study were emplaced during two distinct major climatic events the calculated minimum exposure ages imply abandonment ages of 11 kyr and 120 kyr for the younger and older units respectively this study demonstrates that using both the neutron and muon components implicated in the in situ be 10 production surficial erosion rates can be estimated and alluvial fans dated ln particular it shows that in situ be 10 may be used to date fan emplacement during the last 120 kyr under humid tropical conditions significantly helping to constrain continental palaeoclimatology