modelling post glacial rebound with lateral viscosity variations summary observations of isostatic adjustment of the earth s surface due to glacial loading provide important constraints on mantle viscosity structure we solve the forward problem of glacial isostatic adjustment in two complementary ways a spectral method for strictly 1 d spherically symmetric earth models and a finite element method that can accommodate 3 d viscosity structure we discuss how each method may be augmented in three ways to accommodate motion of the centre of mass to implement a gravitationally self consistent ocean load via the sea level equation and to include the influence of polar wander with all these effects implemented the two methods are benchmarked against each other we also study the influence of lateral viscosity variations upon measurements of post glacial rebound pgr in two ways first by observing the effect of viscosity perturbations in an idealized model and second by developing a realistic 3 d viscosity model and comparing it with results of related 1 d spherically symmetric models the 3 d viscosity structure is derived starting from seismic tomography models we conclude from both approaches that pgr observations are sensitive to both the local viscosity structure and to the viscosity structure beneath the loaded region even if it is removed from where the observations are made in particular pgr measurements made at hudson bay tend to reflect the local viscosity structure beneath hudson bay pgr measurements made along the east coast of north america being sensitive to both the local east coast viscosity structure as well as the loaded continental viscosity structure are not reproducible with a 1 d viscosity model

fixation of radionuclides in the 238u decay series in the vicinity of mineralized zones 1 the austatom uranium prospect northern territory australia abstract the minimum age of a zone of secondary uranium mineralization located at the austatom prospect in the alligator rivers region of australia is estimated to be 3 6 10 5 y this is derived from a geochronological model based on retarded leaching of 234 u with respect to 238 u and on ratios within the ore of these members of the 238 u decay series although kaolinite is a dominant mineral in the weathered schist host rocks retarded dissolution of 234 u occurs only in the presence of the clay minerals illite and montmorillonite in their absence the reverse occurs a model is proposed to explain the results ratios of 230 th to 238 u indicate that the mineralization has probably remained stationary within the weathered schist for at least 1 to 2 10 5 y future use of clay minerals as buffers in radioactive waste repositories is supported by the excellent long term retention obtained for oxidized uranium probably due in part to isomorphic substitution into the clay crystal lattice

lateral variations in mantle rheology implications for convection related surface observables and inferred viscosity models summary over the past decade numerous analyses of convection related observables such as horizontal surface divergence geoid or gravity anomalies and dynamic surface topography have been carried out in the context of tomography based mantle flow models one of the major objectives of this modelling has been the inference of the rheological structure of the mantle with few exceptions these studies have been conducted in the framework of a viscous flow theory which assumes that the mantle rheology may be represented in terms of an effective viscosity which varies with depth only here we present a detailed assessment of the impact of lateral variations in viscosity on global convection related observables using forward modelling of buoyancy induced flow in a 3 d spherical shell we find that the resulting dynamic topography at the surface and the core mantle boundary as well as the gravitational response of the earth are affected relatively little by the inclusion of lateral viscosity variations lvv when compared with results for a purely 1 d radial viscosity model in particular we found that the effect of lvv on the global observables is significantly smaller than the variability due to uncertainties in the current seismic tomography models we also quantify the effect of lvv in the context of the viscosity inverse problem using two synthetic data sets generated with a 1 d viscosity profile and with a fully 3 d viscosity model in which the lvv span across three orders of magnitude we compared the 1 d viscosity profiles recovered from the inversions and found that lvv have virtually no effect on our inversion results the synthetic viscosity inversion further revealed that the effect of lvv is small in comparison to the uncertainties arising from the seismic tomography models the inversions also suggest that the 1 d viscosity profiles derived from actual surface data represent the depth variation of the horizontally averaged logarithm of the 3 d viscosity distribution in the mantle intr o duction constraining the rheological properties of the mantle using surface geophysical data has traditionally involved separate treatments of short seismic intermediate solid earth tides long postglacial rebound and very long mantle convection timescale processes in each case the initial focus historically speaking has been on the inversion of the data to determine a one dimensional 1 d purely radial variation of the relevant rheological parameter e g elastic moduli intrinsic attenuation and viscosity which provides the best fit to the data it has long been recognized particularly in the context of the seismic inverse problem that the search for a 1 d earth model is subject to possible bias due to the presence of lateral variations in structure which may be as significant as the radial variations especially at shallow depths this paper is concerned with the mantle viscosity field the earliest inferences of the radial i e horizontally averaged profile of viscosity based on glacial isostatic adjustment gia data e g mcconnell 1968 parsons 1972 were characterized by a pronounced increase of viscosity with depth in the top half of the mantle however these were followed by two decades of gia research that argued for an essentially isoviscous

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

comparison of 226ra 238u and 228th 228ra disequilibrium with weathering indices in beach sand sediments from greece weathering indices can be used to quantify weathering of rocks at various stages of weathering processes and provide a better understanding of element mobility during weathering various weathering indices have been proposed in the literature worldwide in the present work radioactive secular disequilibrium has been used to determine weathering extent thirty seven samples from kavala and thirty from sithonia beach sands all of them being the weathering products of the local granitic plutons are studied for their secular disequilibrium in both 238u and 232th series 226ra 238u and 228ra 228th respectively the results are compared to weathering indices already in use worldwide and suggest a consistent correlation with wip alkaline and alkaline earth element leaching and pwi silica leaching for the well and very well sorted sands and for the heavy minerals of kavala this study suggests that with an appropriate choice of the weathering index and samples both u series and weathering index can recover the extent of chemical weathering

u th ra fractionation during weathering and river transport the potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960 s e g rosholt et al 1966 hansen and stout 1968 this interest results from the dual property of the nuclides of the u and th radioactive series 1 to be fractionated during water rock interactions and 2 to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters therefore the study of radioactive disequilibria in surface environments should help to bring information about the nature the intensity but also the time scale of the water rock interactions produced during weathering and related chemical transfers these different properties have justified many of the studies on u th series in weathering profiles and river waters rosholt 1982 scott 1982 and osmond and ivanovich 1992 gave a synthesis of the studies of u series in weathering and surface hydrology up to the 1980 s and the 1990 s respectively here we present the main directions taken in these domains over the last decade they were partly stimulated by the analytical developments made in the measurement of the medium half life nuclides of 238u series i e 234u 230th 226ra in the mid 1980 s namely the use of thermal ionisation mass spectrometry tims e g chen et al 1986 edwards et al 1987 cohen et al 1991 chabaux 1993 chabaux et al 1994 and more recently the use of mc icp ms turner et al 2001 robinson et al 2002 for u and th isotope analysis details of these developments are given in this volume goldstein and stirling 2003 compared to the radioactive counting methods previously used the new techniques permit 1 a reduction by one order of magnitude or more of the size of the sample required for

short lived u and th isotope distribution in a tropical laterite derived from granite pitinga river basin amazonia brazil application to assessment of weathering rate we have analyzed samples of a 15 m thick profile weathered from the madeira granite located in the pitinga basin river north of manaus in the state of amazonia brazil this profile consists essentially of a yellow red saprolite covered by a soil u and th concentrations are particularly high in the granite 20 and 80 g g respectively normalized element to th concentrations indicate that th is most resistant to chemical weathering except to some extent in the top soil higher concentrations in the saprolite compared to the granite comprise a relative enrichment resulting from a loss of mass the saprolites are initially generated by a descending weathering front which alters the granite to a yellow red saprolite a second front close to the top turns the saprolite into a soil weathering has led to leaching of u the 234u 238u and 230th 238u isotopic ratios are in radioactive disequilibrium numerous nodules are present and apparently started to form at the base of the saprolite these nodules achieve more developed form during their relative ascent until they are reached by the descending top front where they undergo dissolution the th and pb are concentrated in the nodules close to the top front the u being more mobile is strongly leached by the first front and most of the remainder freed by the second engages in a descending flux which supplies the underlying saprolite using the data an attempt is made to model the isotopic distribution in the profile we conclude that the first front has descended at a rate of 5 cm 1000 yt and that the time needed to create the saprolite must have been around 300 000 yr

solid liquid ree fractionation in the lateritic system of goyoum east cameroon the implication for the present dynamics of the soil covers of the humid tropical regions the ree th weathering geochemistry and mineralogy has been investigated in a lateritic soil cover in relation to the close hydrographical system at goyoum east cameroon the stutied area composed of gneissic hills covered by humid tropical forest belongs to the sanaga river basin and corresponds to a transition vegetation zone between rainforest and savannah a representative soil catena was taken as a reference zone close to the sanaga river the gneissic parent rock soil and groundwater samples have been cut off in pits in the lower part of the catena waters were also sampled both in brooks rich in organic matter and in the sanaga river all the waters were sampled seasonally during the rainy and dry periods the water rock interactions and the exportation of ree and th have been discussed as a function of the evolution of specific ratios considering th as the less mobile element during laterization variation of parent rock pr normalized ratios ln th pr vs depth show that the upper ferruginous horizons are depleted in ree while the basal saprolite presents accumulation zones which are differently located for ce and the other lrees the hrees are also depleted the la accumulations are located near the saprolite gneiss weathering front associated with negative ce anomalies ce ce 0 4 the ce accumulations inducing strong positive ce anomalies ce ce 4 are located in the upper saprolite beneath the ferruginous horizons all along the soil catena the secondary lree bearing minerals which reflect lree fractionation during incipient weathering of the gneiss are hydrous phosphates rhabdophane lreepo4 nh2o and cerianite ceo2 whose precipitation depends on the eh ph changes in the soil profiles on the other hand hree depletion is controlled by the dissolution of xenotime the major hree bearer in the parent gneiss in the 0 45 m filtered acidic groundwater ph 5 5 which have flooded the saprolite for a long residence time th contents are extremely low 0 th 10 ppt while lree contents are high 90 la 450 ppt in these filtered waters th seems to be strongly controlled by minerals i e thorianite which lead to its immobilization in the soil while lree remain in the soluble and or colloidal pools moreover the groundwaters show significant negative ce anomalies throughout the year 0 1 ce ce 0 4 the mobilization of lree except redox active ce results from the dissolution and or the mobilization in colloidal form of the secondary phosphates which may present major negative ce anomalies as indicated by the bulk soil analyses at the saprolite gneiss weathering front therefore the groundwaters have the signature of saprolitic materials we propose that the strong positive ce anomalies of the upper saprolite result from ancient weathering processes these processes would have led to much stronger ce iii ce iv dissolution reprecipitation conditions than those occurring at the present time in the saprolite in the 0 45 m filtered brook acidic blackwaters lree and th contents are high 100 th 400 ppt 400 la 1700 ppt in this organic matter rich environment lree and th are highly mobilized the colloial pool controls half of the whole lree th load the geochemical signature of these waters is independent of the gneissic parent rock and soil lree contents there is no ce anomaly this signature is similar to those of other organic rich rivers draining humid tropical ecosystems in the world in the 0 45 m filtered sanaga waters lree and th contents are lower than in the brooks 15 th 40 ppt 40 la 300 ppt these concentrations are dependent on the time and on the colloidal composition however 90 of these elements are mainly controlled by the suspended pool

thermochemical structure and dynamics of the african superplume 1 we present a new three dimensional 3 d model of the thermochemical structure of the african superplume region obtained from simultaneous inversion of global seismic and convection related data convection related observations include the global free air gravity field tectonic plate motions dynamic surface topography and the excess ellipticity of the core mantle boundary a 3 d image of the chemically induced density perturbations provides direct evidence that intrinsically dense material is entrained within the superplume and concentrated into a rounded structure 1000 km above the core mantle boundary the thermally induced density perturbations are greater in magnitude than the chemically induced implying overall positive buoyancy throughout the superplume the observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally distinct pile at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy citation simmons n a a m forte and s p grand 2007 thermochemical structure and dynamics of the african superplume geophys res lett 34 l02301 doi 10 1029 2006gl028009

evaluating u series tools for weathering rate and duration on a soil sequence of known ages four soil profiles of known age 40 250 600 and 3000 ka from the merced soil chronosequence in california were analysed for u and th isotopes and for major and trace element compositions to test the assumption that leaching of u series isotopes is a first order process with an invariant rate constant as frequently assumed when applying this isotopic tool to determine weathering duration and sediment residence time average 230 th 238 u values for each profile decrease from a high of 1 28 in the youngest soil 40 ka to 1 09 0 99 and 0 98 in the 250 ka 600 ka and 3000 ka soils respectively 234 u 238 u values also show variation across the 4 soils the simultaneous variation in 230 th 238 u and 234 u 238 u cannot be explained by a first order kinetic model with invariant rate constants and instead requires that leaching coefficients for the u series isotopes change with time the observed data is consistent with a two stage model that reflects an initial period of fast leaching of u and th from a readily weathered soil pool such as grain boundaries or defects followed by slow leaching from a more resistant soil pool application of such a two stage model may lead to significantly shorter calculated timescales of weathering than those obtained by the widely applied single stage model of u series weathering which assumes no change in leaching coefficients over time the results of this study have wide implications for published and future use of u series tools in weathering studies

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

rivers turned to rock late quaternary alluvial induration influencing the behaviour and morphology of an anabranching river in the australian monsoon tropics abstract late quaternary alluvial induration has greatly influenced contemporary channel morphology on the anabranching gilbert river in the monsoon tropics of the gulf of carpentaria the gilbert one of a number of rivers in this region has contributed to an extensive system of coalescing low gradient and partly indurated riverine plains extensive channel sands were deposited by enhanced flow conditions during marine oxygen isotope oi stage 5 subsequent flow declined probably associated with increased aridity however enhanced runoff recurred again in oi stages 4 3 65 50 ka aridity then capped these plains with 4 7 m of mud a widespread network of sandy distributary channels was incised into this muddy surface from sometime after the last glacial maximum lgm to the mid holocene during a fluvial episode more active than the present but less so than those of oi stages 5 and 3 this network is still partly active but with channel avulsion and abandonment now occurring largely proximal to the main gilbert flow path a tropical climate and reactive catchment lithology have enhanced chemical weathering and lithification of alluvium along the river resulting in the formation of small rapids waterfalls and inset gorges features characteristic more of bedrock than alluvial systems thermoluminescence tl and comparative optically stimulated luminescence osl ages of the sediments are presented along with u th ages of pedogenic calcrete and fe mn oxyhydroxide oxide accumulations they show that calcrete precipitated during the late quaternary at times similar to those that favoured ferricrete formation possibly because of an alternating wet dry climate intense chemical alteration of the alluvium leading to induration appears to have prevailed for much of the late quaternary but probably due to exceptional dryness not during the lgm the result has been restricted channel migration and a reduced capacity for the channel to adjust and accommodate sudden changes in bedload consequent avulsions have caused local stream powers to increase by an order of magnitude inducing knickpoint erosion local incision and the sudden influx of additional bedload that has triggered further avulsions the gilbert river while less energetic than its pleistocene ancestors is clearly an avulsive system and emphasizes the importance in some tropical rivers of alluvial induration for reinforcing the banks generating nickpoints reworking sediment and thereby developing and maintaining an indurated and anabranching river style

soil formation rates on silicate parent material in alpine environments different approaches different results abstract high mountain soils develop in particularly sensitive environments consequently deciphering and predicting what drives the rates of soil formation in such environments are a major challenge in terms of soil production or formation from chemical weathering the predominating perception for high mountain soils and cold environments is often that the chemical weathering portion of soil development is temperature inhibited often to the point of non occurrence several concepts exist to determine long term rates of soil formation and development we present three different approaches 1 quantification of soil formation from minimally eroded soils of known age using chronosequences known surface age and soil thickness sast 2 determination of soil residence times srt and production rates through chemical weathering using un stable isotopes e g 230 th 234 u activity ratios and 3 a steady state approach using cosmogenic isotopes e g 10 be for each method data from different climate zones and particularly from high mountains alpine environment are compared the sast and steady state approach give quite similar results for alpine environments european alps and the wind river range rocky mountains usa using the srt approach soil formation rates in mountain areas but having a temperate climate do not differ greatly from the sast and steady state approaches independent of the chosen approach the results seem moderately comparable soil formation rates in high mountain areas alpine climate range from very low to extremely high values and show a clear decreasing tendency with time very young soils have up to 3 4 orders of magnitude higher rates of development than old soils 10 5 to 10 6 yr this apparently is a result of kinetic limits on weathering in regions having young surfaces and supply limits to weathering on old surfaces due to the requirement for chemical weathering to occur soil production rates cannot be infinitely high consequently a speed limit must exist in the literature this limit has been set at about 320 to 450 t km 2 a our results from the sast approach show however that in alpine areas soil formation easily reaches rates of up to 800 2000 t km 2 a these data are consistent with previous studies in mountain regions demonstrating that particularly young soils intensively weather even under continuous seasonal snowpack and thus that the concept of temperature controlled soil development soil forming intervals in alpine regions must be reconsidered

time scales of sedimentary transfer and weathering processes from u series nuclides clues from the himalayan rivers abstract in order to define time scales of erosion and sedimentary transfer in the himalaya 238 u 234 u 230 th disequilibria have been analysed in river bank sediments and in bedloads collected along the kali gandaki river one of the main nepalese rivers and in the ghaghara and gandak rivers two major plain tributaries of the ganges the th activity ratios and u th ratios in river sediments of the two ganges tributaries constantly decrease from upstream to downstream this is related to the maturation of sediments by weathering during their transfer to the plain the u series data allow to calculate a transfer time for the sediments in the alluvial gangetic plain from the chain front to the confluence with the ganges of about 100 kyr for both rivers the kali gandaki river sediment data highlight a decrease of both the th isotopic and u th ratios which is explained by a mixing between two sources with similar u th ratios but having suffered a different u th fractionation history interpretation of the u series data in the frame of this scenario gives long time scales of weathering of several 100 s kyr for the himalayan terranes the results imply that himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers in addition occurrence of similar u sr signatures in dissolved i e

geochemistry of laterites stability of al goethite al hematite and fe super 3 kaolinite in bauxites and ferricretes an approach to the mechanism of concretion formation

formation and evolution of lateritic profiles in the middle amazon basin insights from radiation induced defects in kaolinite abstract the content of radiation induced defects rids in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle amazon basin brazil is investigated using electron paramagnetic resonance the paleodose registered by kaolinites ranges from 80 to 900 kgy present day dose rates of radiation determined from the whole rock u and th content range between 4000 and 40 000 mgy ka in most samples u and th concentrations are correlated suggesting that u has not been remobilized by lateritization this observation is consistent with the fact that 80 of the total u content is incorporated in resistant minerals such as zircon and ti oxides the heterogeneous distribution of u observed by induced fission tracks mapping makes it possible to neglect the radiation contribution of the u decay chains in the dose rate calculation the interpretation of the measured content of rids in kaolinite is then performed using the calculated present day dose rate and assuming equilibrium in the radioactive decay chains for the sedimentary samples the amount of rids is broadly correlated to the dose rate and provides apparent absolute ages older than 20 ma the rid contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 ma the high rid content of these lateritic kaolinites shows that their chemical isotopic and crystallographic properties are not representative of present day weathering conditions models assuming the dynamical equilibrium of kaolinites with local physical chemical conditions prevailing in lateritic soils are thus questionable alternatively our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo climates

determination of recent 238u 234u and 230th mobility in granitic rocks application of a natural analogue to the high level waste repository environment abstract the isotope activity ratios 234u 238u and 230th 234u have been investigated in drill core samples up to 1300 m depth in three granitoids from australia to identify the nature of possible recent 1 0 which has generated excess 230th these enriched ground waters do not appear to have penetrated below 530 m depth and 230th excess indicates bulk loss of u the observed disequilibria indicate that significant radionuclide mobilisation has occurred in these rocks during the recent past

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

regolith formation rate from u series nuclides implications from the study of a spheroidal weathering profile in the rio icacos watershed puerto rico abstract a 2 m thick spheroidal weathering profile developed on a quartz diorite in the rio icacos watershed luquillo mountains eastern puerto rico was analyzed for major and trace element concentrations sr and nd isotopic ratios and u series nuclides 238 u 234 u 230 th 226 ra in this profile a 40 cm thick soil horizon is overlying a 150 cm thick saprolite which is separated from the basal corestone by a 40 cm thick rindlet zone the sr and nd isotopic variations along the whole profile imply that in addition to geochemical fractionations associated to water rock interactions the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts the mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile the upper part of the profile is probably associated with a different weathering system lateral weathering of upper corestones than the lower part which consists of the basal corestone the associated rindlet system and the saprolite in contact with these rindlets consequently the determination of weathering rates from 238 u 234 u 230 th 226 ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile i e the rindlet zone and the lower saprolite similar propagation rates were derived for the rindlet system and the saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of u series nuclides in the rindlet saprolite subsystem the consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that in the rio icacos watershed the weathering system has reached a geomorphologic steady state our study also indicates that even in environments with differential weathering such as observed for the puerto rico site the radioactive disequilibrium between the nuclides of a single radioactive series here 238 u 234 u 230 th 226 ra can still be interpreted in terms of a simplified scenario of congruent weathering incidentally the u th ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered

the delicate balance between soil production and erosion and its role on landscape evolution abstract the diversity in landscapes at the earth s surface is the result amongst other things of the balance or imbalance between soil production and erosion while erosion rates are well constrained it is only recently that we have been able to quantify rates of soil production uranium series isotopes have been useful to provide such estimates independently of erosion rates in this study new u series isotope are presented data from weathering profiles developed over andesitic parent rock in puerto rico and granitic bedrock in southeastern australia the site in australia is located on a highland plateau neighbouring a retreating escarpment where soil production rates between 10 and 50 mm kyr have been determined the results show that production rates are invariant in these two regions of australia with values between 15 and 25 mm kyr for the new site andesitic soils show much faster rates about 200 mm kyr overall soil production rates determined with u series isotopes range between 10 and 200 mm kyr this is comparable to erosion rates in soil mantled landscapes but faster than erosion in cratonic areas and slower than in alpine regions and cultivated areas this suggests that soil mantled landscapes maintain soil because they can there is a balance between production and erosion similarly thick weathering profiles develop in cratonic areas because despite slow erosion rates soil production is still significant bare landscapes in alpine regions are probably the result of the inability of soil production to catch up with fast erosion rates although this needs testing by u series isotope studies of these regions finally the range of production rates is up to several orders of magnitude lower than erosion rates in cultivated areas demonstrating quantitatively the fast depletion of soil resources with common agricultural practices

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

regolith production and transport in the susquehanna shale hills critical zone observatory part 1 insights from u series isotopes 1 to investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect we measured u series isotopes in regolith profiles from two hillslopes north facing versus south facing within the east west trending shale hills catchment in pennsylvania this catchment is developed entirely on the fe rich silurian rose hill gray shale hillslopes exhibit a topographic asymmetry the north facing hillslope has an average slope gradient of 20 slightly steeper than the south facing hillslope 15 the regolith samples display significant u series disequilibrium resulting from shale weathering based on the u series data the rates of regolith production on the two ridgetops are indistinguishable 40 22 versus 45 12 m ma however when downslope positions are compared the regolith profiles on the south facing hillslope are characterized by faster regolith production rates 50 15 to 52 15 m ma and shorter durations of chemical weathering 12 3 to 16 5 ka than those on the north facing hillslope 17 14 to 18 13 m ma and 39 20 to 43 20 ka the south facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry despite lower extents of chemical depletion these results are consistent with the influence of aspect on regolith formation at shale hills we hypothesize that aspect affects such variables as temperature moisture content and evapotranspiration in the regolith zone causing faster chemical weathering and regolith formation rates on the south facing side of the catchment the difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at shale hills at least 15 ka before present at that time the erosion rates may also have been different from those observed today perhaps denuding the south facing hillslope of regolith but not quite stripping the north facing hillslope an analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at shale hills is not in geomorphologic steady state i e so called dynamic equilibrium but rather is likely still responding to the climate shift from the holocene periglacial to the modern temperate conditions

effect of lateral viscosity variations in the top 300 km on the geoid and dynamic topography summary we have investigated the impact of lateral viscosity variations lvv in the top 300 km of the mantle on the long wavelength gravitational response of the earth in contrast to the previous studies we demonstrate that the lvv may play a crucial role if a model with imposed plate velocities and partial layering is considered assuming lvv in the top mantle associated with the continental roots and the oceanic asthenosphere we are able to explain a significant portion of the geoid 85 per cent and of the free air gravity data 55 per cent without invoking a complex radial viscosity profile or assuming a lithosphere with an unrealistically low uniform viscosity the best fit to the data is found for models in which mass anomalies located at the boundary between upper and lower mantles reduce the mass flux across the 660 km discontinuity by a factor of 3 these models are characterized by a low average value of viscosity in the asthenosphere below the oceans 10 19 pa s and a large viscosity contrast 100 between the young suboceanic mantle and the deep continental roots the viscosity in the deep upper mantle is found to be around 3 10 20 pa s while a value higher by at least two orders of magnitude is typical for the lower mantle the predicted dynamic surface topography is influenced by the surface plate velocities and its amplitude amounts to a few hundred metres besides investigating the viscosity structures that reflect the different thicknesses of the lithosphere in continental and oceanic regions we have searched for a large scale pattern of lateral viscosity variation in the top 300 km that yields the best fit to the geoid the pattern obtained by this inversion shows a good correlation with the ocean continent distribution and 90 per cent of the geoid and 67 per cent of the free air gravity can be explained

age and origin of terra rossa soils in the coonawarra area of south australia abstract the famous terra rossa soil in the coonawarra area south australia is dominated by locally derived aeolian detritus which probably accumulated over the last 120 130 ka four soil profiles and associated limestone and lunette deposits were investigated using the following methods mass balance geochemistry of bulk soil samples major and trace elements quantitative x ray diffraction xrd mineralogy strontium isotopes 87 86 as well as grain size analysis and cation exchange capacity these data show that the terra rossa soil from the coonawarra has a thick clayey b horizon which is geochemically homogeneous and dominated by smectite and kaolinite mass balance calculations show unrealistic weathering scenarios when plotted using silicate residuum from the underlying limestone as parent realistic weathering scenarios are produced with fine grained silicate material from local lunette deposits as parent strontium isotopes of silicate residuum from gambier limestone 0 78 contrast strongly with the clayey b horizon 0 726 strontium isotope ratios of silicate material from a local lunette 0 725 are similar to the b horizon soil values strontium isotope ratios from regional geological units indicate that the strontium signature in the lunette and soil b horizon is dominated by weathering products from the palaeozoic kanmantoo shales extensively exposed upwind to the west on kangaroo island and the fleurieu peninsula optical optically stimulated luminescence osl dating of 61 individual quartz grains single aliquot from three samples in the coonawarra soil profile one from the a horizon and two from the b horizon shows that most of the quartz sand grains have been buried for only a few thousand years many of the grains however have been buried for tens of thousands of years with three grains having exposure ages of between 105 and 109 ka the large population of young exposure dates represents quartz sands recently exposed in the a horizon and which have been translocated down to the b horizon the older exposure dates are interpreted as representing grains that were buried during or soon after the accumulation of wind blown silt and clay our current model concerning the timing and conditions of aeolian deposition of the coonawarra soil is that much of it accumulated during the relatively wet last interglacial period around 120 130 ka during that time span it is thought that the playa lunette systems in the low lying areas to the west were particularly active and generated a significant local dust flux

rapid regolith formation over volcanic bedrock and implications for landscape evolution abstract the ability to quantify how fast weathering profiles develop is crucial to assessing soil resource depletion and quantifying how landscapes evolve over millennia uranium series isotopes can be used to determine the age of the weathering front throughout a profile and to infer estimates of regolith production rates because the abundance of u series isotopes in a weathering profile is a function of chemical weathering and time this technique is applied to a weathering profile in puerto rico developed over a volcaniclastic bedrock u series isotope compositions are modelled revealing that it takes 40 60 kyr to develop an 18 m thick profile this is used to estimate an average regolith production rate of 334 46 mm kyr this value is higher by a factor of up to 30 when compared to production rates estimated for weathering profiles developed over granitic or shale lithologies this quantitatively underpins the lithological control on rates of regolith production in a neighbouring watershed but over a granitic bedrock production rates are only 30 40 mm kyr moreover by comparing these results to a compilation of soil erosion rates it is clear that landscapes are controlled by the balance or imbalance between regolith production and erosion soil mantled landscapes are the result of a relative balance between production and erosion whereas in cratonic areas thicker weathering profiles are generated because erosion fails to match regolith production rates

late quaternary sediment fluxes from tropical watersheds abstract inherited saprolite stores and continued weathering in quaternary time juxtapose abundant clay and fresh rock in tropical landscapes this influences sediment fluxes and affects the interpretation of sediment sequences derived from tropical watersheds detrital kaolinites derive from inherited saprolite sources as well as from soil clays and appear in delta and ocean sediments these sediments appear to correspond with sub milankovitch millennial scale cycles of climate change but may also record century scale episodes of rapid warming dansgaard oeschger events destabilisation of sediment sources and increased sediment fluxes in the late quaternary followed millennia of climatic deterioration cooling aridity and vegetation change and led to altered patterns of sedimentation during the last glacial maximum lgm sediment yield from slopes increased 10 around the lgm when rainfall was reduced by 30 60 and led to fan building and braided channels rainfall increased 40 80 from the lgm to the early holocene maximum and this led to channel cutting and major sediment fluxes to delta and ocean sinks vegetation recovery lagged the rapid warming by several millennia and was interrupted by younger dryas yd aridity influencing slope and stream behaviour holocene sedimentation has been by both vertical and lateral accretion increasing floodplain sediment stores

rare earth and trace element geochemistry of termite mounds in central and northeastern namibia mechanisms for micro nutrient accumulation abstract rare earth element ree and trace element concentrations of ten termite mounds and adjacent topsoil from central and northeastern namibia were used to investigate the processes underlying the alteration of soil chemical and physical properties by termites macrotermes spp accumulation of micro nutrients in the mounds was of particular interest because of the ecological implications of the enhanced availability of these scarce elements the absolute concentrations of ree and trace elements including nine micro nutrients b fe mn ni cu zn se mo and cd were greater in termite mounds compared to topsoil suggesting a possible external supply of enriched materials or accumulation of in situ weathering products of the underlying bedrock similarities between the chondrite normalized ree patterns of the samples and those of the average upper continental crust ucc and post archaean average australian shale compositions indicated that the mounds and topsoil at each site originated from the same host bedrock coherent behavior between two incompatible elements th and la in both topsoil and mound samples suggested that the two sets of samples were exposed to two different weathering processes the relatively low zr sc ratios fine particle sizes and the persistence of positive eu anomalies of the mounds relative to ucc were interpreted as reflecting 1 a lack of grain sorting effects and 2 active transport of materials rich in primary clay minerals and trace elements from the deeper regolith to the surface by termites lower permeability of mound materials may also prevent leaching of primary minerals and trace elements titanium and zr enrichment in most mound samples 8 samples was attributed to the decomposition of biotite by macrotermes with the concomitant release of ti and zr into the mounds suggesting the termites ability to transform soil minerals and enhance chemical weathering this was further corroborated by a substantial increase in th u ratios in most mounds relative to topsoil because of the oxidizing conditions of the mounds ti and zr are likely to be adsorbed onto fe oxides positive ce anomalies relative to ucc and the high ph values in the mounds indicate calcrete aggradation or mound aeration as a result of burrowing activities of the termites and evaporation from mounds the impacts of the changes in environmental conditions and the termite activities on the mound geochemistry are highlighted by heavy ree hree as well as fe and mn retentions in the mounds heavy ree and light ree lree are likely to coprecipitate with carbonate complexes and fe and mn oxyhydroxides and in the process scavenge a series of micro nutrients such as zn cu cd and co a substantial enrichment of se in mounds despite the oxidizing conditions of the mounds could be attributed to active transport of se enriched materials from deep soil horizons the study demonstrated the suitability of ree and trace element geochemistry for assessing the influence of macrotermes on the physico chemical properties of semi arid soils

regolith production rates calculated with uranium series isotopes at susquehanna shale hills critical zone observatory abstract in the critical zone where rocks and life interact bedrock equilibrates to earth surface conditions transforming to regolith the factors that control the rates and mechanisms of formation of regolith defined here as material that can be augered are still not fully understood to quantify regolith formation rates on shale lithology we measured uranium series u series isotopes 238 u 234 u and 230 th in three weathering profiles along a planar hillslope at the susquehanna shale hills observatory ssho in central pennsylvania all regolith samples show significant u series disequilibrium 234 u 238 u and 230 th 238 u activity ratios range from 0 934 to 1 072 and from 0 903 to 1 096 respectively these values display depth trends that are consistent with fractionation of u series isotopes during chemical weathering and element transport i e the relative mobility decreases in the order 234 u 238 u 230 th the activity ratios observed in the regolith samples are explained by i loss of u series isotopes during water rock interactions and ii re deposition of u series isotopes downslope loss of u and th initiates in the meter thick zone of bedrock that cannot be augered but that nonetheless consists of up to 40 clay silt sand inferred to have lost k mg al and fe apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m myr to 17 m myr with increasing regolith thickness from the ridge top to the valley floor with increasing distance from the ridge top toward the valley apparent equivalent regolith residence times increase from 7 kyr to 40 kyr given that the ssho experienced peri glacial climate 15 kyr ago and has a catchment wide averaged erosion rate of 15 m myr as inferred from cosmogenic 10 be we conclude that the hillslope retains regolith formed before the peri glacial period and is not at geomorphologic steady state both chemical weathering reactions of clay minerals and translocation of fine particles colloids are shown to contribute to mass loss of u and th from the regolith consistent with major element data at ssho this research documents a case study where u series isotopes are used to constrain the time scales of chemical weathering and regolith production rates regolith production rates at the ssho should be useful as a reference value for future work at other weathering localities

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

oxygen isotope geochemistry of kaolinite in laterite forming processes manaus amazonas brazil abstract mineralogic crystallographic and oxygen isotopic analyses were made of kaolinites from a 20 m thick lateritic profile developed on the detrital sands and clays of the mid cretaceous to tertiary alter do chao formation central amazon basin brazil a detailed 18o profile of soil water from the upper 4 m of the same section was also measured kaolinite separates were prepared from different pedogenic facies of small hand specimens most of these kaolinite separates exhibit polymodal size distributions the separates were further divided into individual particle size modes for subsequent isotopic and crystallographic analysis in general hinckley index and 18o values of different size fractions of the same sample differ hinckley index values decrease strongly and fe content increases in the upward direction in the profile we conclude that 1 the kaolinite in the profile has progressively evolved in response to changing microenvironments as weathering fronts have moved progressively downward 2 different kaolinite fractions even within a single facies of a single sample a few cm3 in size typically formed under somewhat different conditions 3 the bulk of the fine grained kaolinite in the lateritic profile does not constitute a continuous inverted time record that is lower levels do not represent more recent kaolinite formation and higher levels do not represent older kaolinite formation and 4 isotopic compositions of kaolinites from lateritic paleosols should provide an indication of mean climatic conditions and some measure of seasonality during the interval preceding the removal of the laterite from the active weathering environment we speculate that the different conditions under which the different kaolinite fractions have formed may be related at least in part to variations in soil microenvironments that are ultimately derived from seasonal and year to year variations in the chemistry and isotopic composition of infiltrating rainfall

thorium uranium disequilibrium dating of late quaternary ferruginous concretions and rinds radioactive ferruginous nodules from the alligator rivers uranium province northern territory australia were systematically examined by optical microscopy xrd alpha and fission track autoradiography and analysed for u th ra and major stable elements correlation of autoradiographs with microscopic structure and analyses of selective extractions confirmed that u and th were strongly associated with fe oxides fission track autoradiography showed no concentration gradient evidence for postdepositional leaching of uranium strong acid leaching studies showed that irreversibly adsorbed u and authigenic 234u and 230th but not 226ra are quantitatively retained by accumulated oxide oxyhydroxide correlation of the groundwater activity ratio with oxidic 234u 238u activity ratios indicated the latter was radiogenically consistent with oxidic 230th 234u activity ratios 1 dense accumulated fe mn oxide matrices are apparently capable of forming radiogenic closed systems in respect of the 230u 234u 230th decay set 234u 238u and 230th 234u activity ratios were used to derive similar mean ages of rinds for four nodules in the same horizon th u ages were also determined using a th index detrital correction method previously used for impure carbonates of pedogenic fe mn accumulations having background concentrations of u and th from other locations in australia these ages were compared with tl and 14c ages of the host sediments the comparisions suggest that th u dating could be used to age indurated layers of fe mn oxides in soil horizons 350 ka old

direct u th dating of organic and carbonate rich lake sediments from southern scandinavia u th ages of holocene lacustrine sediments from southern sweden have been determined to assess the reliability of this technique for dating climatic events from the last interglacial glacial interglacial cycle and for calibration of the14c dating method the sediments are organic and carbonate rich and they are shown to be very well suited to high precision238u 234u 230th dating by thermal ionisation mass spectrometry tims twenty one samples from 2 58 m of sediment from lake igelsjon southern sweden yield ages between 3058 89 and 9426 262 yr 2 error the sediments consist of hydrogenous calcium carbonate 50 90 organic matter 5 50 and siliceous residue 5 the high u contents of the lake sediments between 6 and 76 ppm are in contrast to the low concentrations in the lake and groudwater 1 9 and 3 2 ppb respectively and mass balance calculations based on u content of water and sediments can be used to calculate a lakewter residence time of 24 days which is close to the residence time calculated using hydrologic data recent ground and lakewater 234u 238u 0 920 is enriched in 234u compared to soils from the catchment area 234u 238u 0 587 and 234u 238u in the lake sediments shows a marked decrease with time from 0 958 at 9426 yr bp to 0 919 at 3058 yr bp this is attributed to progressive leaching of u from soils which have experienced a previous history of recoil related u loss and it indicates that the assumption of constant initial 234u 238u ratios may be a source of significant error in the calculation of 234u 238u ages for continental deposits

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

trace element geochemistry in brazilian cerrado soils the cerrado region on the central plateau of brazil contains some of the oldest soils on earth with some soils weathering in place since the late cretaceous the region is currently undergoing rapid agriculturalization and industrialization the objectives of this paper are 1 to inventory the native trace element contents of pristine cerrado soils and 2 to better understand which trace elements accumulate and which are depleted in well drained soils over extremely long periods of pedogenic weathering we sampled the 0 20 and 80 100 cm layers of 45 pedons located in three widely separated areas on three different geomorphic surfaces and determined total elemental contents by x ray fluorescence analysis in comparing the contents of cerrado soils to soils worldwide the contents of sc v cr ga zr nb ce and th are 1 2 times the world average y la and u are about the same and mn co ni cu zn rb sr ba and pb are 0 4 times the world average trace elements that accumulated during weathering are either pentavalent nb5 tetravalent ti4 zr4 u4 th4 or trivalent sc3 v3 cr3 ga3 y3 la3 ce3 at the acid ph of these soils while trace elements that were depleted are either divalent or monovalent rb mn2 co2 ni2 cu2 zn2 sr2 ba2 and pb2 at acid ph thus trace element geochemistry over long periods of pedogenic weathering at acid ph mirrors major element geochemistry in that mono and divalent cations are leached and depleted while tri tetra and pentavalent elements accumulate

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

mantle heterogeneities geoid and plate motion a monte carlo inversion seismic tomography in both the upper and the lower mantle as well as subducting oceanic slabs defined by seismicity has been translated into density heterogeneities to generate models of mantle circulation these models can predict both the surface velocities and the geoid which can be compared with plate tectonics and gravity data a given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs the values of these parameters are chosen at random within an acceptable range each model is submitted to an appropriate test comparing observations with predictions the results of the most successful models selected by this monte carlo inversion are displayed they yield preferred mantle viscosity structures exhibiting large variations at depth with a physical interface between upper and lower mantle i e with the possibility for the circulation to penetrate the 650 km discontinuity two classes of viscosity profiles stand out the first one implies a regular increase of the viscosity in the sublithospheric mantle with reasonable values for the density parameters the second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle it also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography which would thus have to be of lithological rather than thermal origin with a chemical interface at 650 km the outcome is very similar the same two classes of viscosity structures do yield a satisfactory geoid prediction however only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities altogether the best models out of some 60 000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6 nevertheless the main features of these two observed patterns are present in the computed maps and 4 out of 6 correlation coefficients lie close to the 90 confidence level this is true for the geoid as well as for the surface divergence of the displacement velocity however as the internal viscosity structure has been assumed to have spherical symmetry the rotational component of the surface velocities cannot be predicted

inferring radial models of mantle viscosity from gravity grace data and an evolutionary algorithm convective flow in the mantle can be thought of and modeled as exclusively driven by density heterogeneities in the mantle itself and the resulting lateral variations in the earth s gravity field with this assumption and a model of mantle rheology a theoretical relationship can be found between 3d mantle structure and flow related quantities that can be measured on the earth s surface like free air gravity anomalies this relationship can be used to set up an inverse problem with 1d mantle viscosity as a solution in the assumption that seismic velocity anomalies be of purely thermal origin and related to density anomalies by a simple scaling factor we invert the large scale length component of the above mentioned measurements jointly with seismic observations waveforms and or travel times to derive an accurate 5 layer spherically symmetric model of upper and lower mantle viscosity we attempt to account for non uniqueness in the inverse problem by exploring the solution space formed of all possible radial profiles of earth viscosity by means of a non deterministic global optimization method the evolutionary algorithm ea for each sampled point of the solution space a forward calculation is conducted to determine a map of gravity anomalies whose similarity to grace gravity recovery and climate experiment is then measured the procedure is iterated to convergence according to ea criteria the robustness of the inversion is tested by means of synthetic tests indicating that our gravity data set is able to constrain less than 6 radial layers each with uniform viscosity independently of the tomographic model or the scaling factor adopted to convert seismic velocity into density structure the ea optimization method finds viscosity profiles characterized by low viscosity in a depth range corresponding to the transition zone and relatively uniform elsewhere

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

temporal and spatial evolution of dynamic support from river profiles a framework for madagascar 1 we present a strategy for calculating uplift rates as a function of space and time from large sets of longitudinal river profiles this strategy assumes that the shape of a river profile is controlled by the history of uplift rate and moderated by the erosional process we assume that upstream drainage area is invariant the algorithm was tested on a set of 100 river profiles which were extracted from a digital elevation model of madagascar this set of profiles was simultaneously inverted to obtain uplift rate as a smooth function of space and time the fit between observed and calculated profiles is excellent and suggests that madagascar was uplifted by 1 2 km at rates of 0 2 0 4 mm yr during the last 15 myrs the location of madagascar suggests that its topographic elevation is maintained by convective circulation of the sub lithospheric mantle residual depth anomalies of oceanic fragments encompassing the island show that the island straddles a dynamic topographic gradient which generates asymmetric neogene uplift volcanism warped peneplains and uplifted marine terraces corroborate the existence of youthful uplift we suggest that sets of longitudinal river profiles contain useful information about the history of regional uplift which can be extracted by inverse modeling and calibrated by independent geologic observations

new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

on recent seismic tomography for the western united states 1 in recent years a number of relatively high resolution seismic tomography models for the uppermost mantle underneath the western united states have been published and vigorous debate has ensued about their tectonic interpretation i present a straightforward yet quantitative comparison between models in order to help establish a framework for geodynamic interpretation and to help judge the role of tomographic theory vs data selection mapped s and p wave anomalies are found to be remarkably consistent between models which implies that seismologists are beginning to narrow down the structure underneath active continental margins to scales of 200 km large discrepancies between published anomaly amplitudes exist however and the models on the high end of the spectrum raise questions as to how they are to be interpreted in terms of temperature composition and melting

on the accuracy of digital bathymetric data the global 5 arcmin gridded topography data etopo 5 are based on contour maps rather than original soundings and have large errors the artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth age variations their amplitude spectrum is bounded by a frequency 4 power law so that they should not be used when the gravity topography transfer function is important such as in studies of flexural isostasy and mantle convection i assess the accuracy of 14 491 069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the lamont doherty earth observatory on line data base by analyzing 329 058 crossover errors coes at intersecting ship tracks five percent of cruises with internal coes yield root mean square coe amplitudes exceeding 500 m all of these have errors in digitizing two way travel time from analog precision depth recorder traces twenty eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths two nominal sound velocities in common use differ by 2 5 an amount which is often undetectable producing uncertainties in depth of this magnitude ship data have been acquired at different rates over time with the peak of activity in the early 1970s although present technologies can yield very accurate data these are acquired at a rate which is small with respect to the total available data the cumulative median global coe has remained constant at 26 m since the late 1970s most recent data acquisition has been in the northern hemisphere oceans and the oldest and least accurate data are in the southern oceans where median coes are 100 250 m the majority of the data in the south pacific were acquired before the advent of satellite navigation

using short lived nuclides of the u and th series to probe the kinetics of colloid migration in forested soils abstract the recent chemical dynamics of a podzolic forest soil section from the strengbach watershed france was investigated using u and th series nuclides analyses of 238 u 230 th 226 ra 232 th 228 ra and 228 th activities in the soil particles the seepage waters and the mature leaves of the beech trees growing on this soil were performed by tims or gamma spectrometry the simultaneous analysis of the different soil sl compartments allows to demonstrate that a preferential th leaching over ra must be assumed to explain the 226 ra 230 th 228 ra 232 th and 228 th 228 ra disequilibria recorded in the soil particles the overall ra and th transfer schemes are entirely consistent with the prevailing acido complexolysis weathering mechanism in podzols using a continuous open system leaching model the 226 ra 230 th and 228 ra 232 th disequilibria measured in the different soil layers enable dating of the contemporary processes occurring in this soil in this way we have determined that a preferential th leaching from the shallow ah horizon due to a strong complexation with organic colloids began fairly recently 18 years ago at most the continual increase in ph recorded in precipitations over the last 20 years is assumed to be the cause of this enhanced organic complexation a lower soil horizon 50 60 cm is also affected by preferential th leaching though lasting over several centuries at least with a much smaller leaching rate the migration of th isotopes through this soil section might hence be used as a tracer for the organic colloids migration and the induced radioactive disequilibria demonstrate to be useful for assessing the colloidal migration kinetics in a forested soil ra and th isotopic ratios also appear to be valuable tracers of some mineral water plant interactions occurring in soil the 228 ra 226 ra ratio enables discrimination of the ra flux originating from leaf degradation from that originating from mineral weathering in shallow 10 cm seepage soil waters it appears that at least in some cases the ra isotopic ratio measured in forest soil seepage waters may not be representative of the ra isotopic ratio released from mineral weathering indicating that the different origins of the dissolved 226 ra and 228 ra must be taken into account

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

effects of lateral viscosity variations on long wavelength geoid anomalies and topography models for the earth s geoid and dynamically supported topography usually account only for radial vertical variations in mantle rheology and flow structure however expected lateral variations in viscosity due to temperature or stress dependent rheology can degrade our ability to infer mantle structure from geoid models based on seismically determined density contrasts in the mantel theoretically the main difficulty is that the harmonic components of the flow stress deformation field are no longer independent and exact analytical methods cannot account for this mode coupling for large thermal density contrasts of small lateral dimension e g subducted slabs it is difficult to form any general theory for these effects however we believe that the largest geoid anomalies which occur at the lowest harmonic degrees l 2 3 are due to a global pattern of relatively mild temperature variations for convection with mainly broadscale thermal and rheological changes some relatively straightforward ideas can be developed combining results from simple perturbation theory and numerical models for convective flow we find the following 1 if we are correct in inferring viscosity variations of less than one order of magnitude globally the longest wavelength geoid anomalies l 2 3 are not seriously contaminated by lateral viscosity variations 2 contamination of the higher degrees l 4 is expected and in particular self coupling between l 2 heterogeneity and viscosity variations may be observable at degree 4 the most severe contamination occurs at the doubled harmonic l 4 of the dominant heterogeneity harmonic l 2 3 perturbation techniques are adequate to model global scale lateral viscosity variations of an order of magnitude or less 4 the steady state surface deformation problem at the heart of our geoid models may be more robust with respect to broadscale lateral viscosity variations than are models for transient deformations e g postglacial rebound 5 for loading by broadscale density heterogeneity the effects of stress dependent rheology are probably secondary to the effects of radial viscosity stratification

equatorial weathering landform development and the formation of white sands in north western kalimantan indonesia abstract the morphology and geochemistry of weathering profiles in miocene granodiorites from the coastal lowlands of nw kalimantan indonesia 1 n lat indicate long term weathering rates landform development and mechanisms for the formation of white sands found widely in valleys and depressions samples are analysed from convex hill piedmont glacis and valley slope profiles all profiles above the saturated zone are strongly gibbsitic but kaolinite minerals are dominant below the water table the profiles are interpreted as expressions of the perhumid equatorial climate rainfall 3000 mm y 1 acting within a dynamic regime of landscape lowering during the neogene the current profiles being largely quaternary 0 4 0 6 ka possible effects of quaternary tectonic and environmental changes are considered including those caused by a late pleistocene drier climate when there was greater water table fluctuation and surface erosion landform evolution is a form of dynamic etchplanation with a long term rate of landscape and weathering profile lowering of ca 50 mm ka 1 within this regime some of the white sands may be residual weathering products locally redistributed by colluvial or alluvial processes they originated in the podzolising environment of the weathering profile and later experienced post depositional diagenetic changes within the hydromorphic regime of the valley floor

gravity field over northern eurasia and variations in the strength of the upper mantle the correlation of long wavelength gravity anomalies in northern eurasia with seismic velocity anomalies in the upper mantle reverses in sign between western and eastern eurasia the difference between western and eastern eurasia can be explained by the presence of a low viscosity zone in the uppermost mantle beneath eastern eurasia that is absent to the west the location of the lateral change in viscosity corresponds with the geologic boundary between the older shields and platforms of the baltics russia and siberia and the younger geologically active mountain belts of eastern asia this relation provides evidence that differences in the strength of the upper mantle control the locus of intracontinental deformation

subducted slabs and lateral viscosity variations effects on the long wavelength geoid summary the characteristic broad local maxima exhibited by the long wavelength geoid over subduction zones are investigated with a numerical model of mantle flow in a spherical axisymmetric geometry a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle by the viscosity stratification and by lateral viscosity variations lvv in the lithosphere upper and lower mantle the presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle the viscosity of the lower mantle controls the long wavelength geoid to the first order ensuring a clear positive signal when it is at least 30 times greater than the upper mantle viscosity the presence of lvv in the lithosphere in the form of weak plate margins helps to increase the contribution of the surface topography causing a pronounced reduction of the geoid localized lvv associated with the cold slab play a secondary role if they are in the upper mantle on the other hand highly viscous slabs in the lower mantle exert a large influence on the geoid they cause its amplitude to increase dramatically way beyond the values typically observed over subduction zones long wavelength flow becomes less vigorous as the slab viscosity increases deformation in the upper mantle becomes more localized and power is transferred to short wavelengths causing the long wavelength surface topography to diminish and the total geoid to increase slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid it is shown that a phase change from perovskite to post perovskite above the core mantle boundary can cause the geoid to reduce significantly thereby helping to reconcile models and observations

the geomorphological and geological importance of palaeosurfaces this special publication is a collection of papers which identifies describes and interprets the occurrence of palaeosurfaces in the geological record the concept of a palaeo surface is one which is generally understood across the geomorphological and geological sciences but according to discipline and application often differs in the emphasis and detail of its interpretation in order to encompass the widest range of contributions and views upon the subject of palaeosurfaces a broad remit was deliberately adopted during the preparation of this volume cations of ancient palaeosurfaces the research areas represented are geographically widespread and include examples from australia africa south america india and europe

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

effects on the long wavelength geoid anomaly of lateral viscosity variations caused by stiff subducting slabs weak plate margins and lower mantle rheology abstract instantaneous flow numerical calculations in a three dimensional spherical shell are employed to investigate the effects of lateral viscosity variations lvvs in the lithosphere and mantle on the long wavelength geoid anomaly the density anomaly model employed is a combination of seismic tomography and subducting slab models based on seismicity the global strain rate model is used to represent weak low viscosity plate margins in the lithosphere lvvs in the mantle are represented on the basis of the relation between seismic velocity and temperature i e temperature dependent rheology when highly viscous slabs in the upper mantle are considered the observed positive geoid anomaly over subduction zones can be accounted for only when the viscosity contrast between the reference upper mantle and the lower mantle is approximately 1 0 3 or lower and weak plate margins are imposed on the lithosphere lvvs in the lower mantle exert a large influence on the geoid pattern the calculated geoid anomalies over subduction zones exhibit generally positive patterns with quite high amplitudes compared with observations even when the low activation enthalpy of perovskite in the lower mantle is employed inferred weak slabs in the lower mantle may be explained in terms of recent mineral physics results highlighting the possibility of grain size reduction due to the postspinel phase transition

234u238u and 230th234u activity ratios in mineral phases of a lateritic weathered zone abstract a selective phase extraction procedure was developed for the identification of the significant phases of a typical deep soil profile sampled in the vicinity of the ranger no 1 uranium ore body alligator rivers region n t australia the significant phases were identified as amorphous iron oxide crystalline iron oxide and a clay quartz resistate the distribution of 238 u 234 u 230 th and 226 ra between the phases was measured the results indicated that the amorphous iron oxide phase is in adsorption desorption equilibrium with the ground water the crystalline iron oxide phase contains a chemical control the kinetics of which are commensurate with or less than the half life of 230 th 7 52 10 4 y the clay quartz resistate is enriched in 238 u descendants in such a way that they are not readily accessible to the ground water

dating of chemical weathering processes by in situ measurement of u series disequilibria in supergene fe oxy hydroxides using la mc icpms abstract constraints on the timing of weathering processes in the northern australian regolith have been obtained by in situ measurement of u series disequilibria in u rich supergene fe oxy hydroxides using a laser ablation mc icpms technique this approach has permitted the measurement of 234 u 238 u and 230 th 238 u activity ratios in finely crystalline fe oxy hydroxides from the ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes 230 th 238 u systematics in the finely crystalline fe oxy hydroxides yield 230 th ages ranging from 60 to 350 ka the most reliable 230 th ages cluster between 122 and 216 ka suggesting fe oxy hydroxides formation and associated weathering peaked during the previous 2 interglacial periods this is supported by the 230 th ages and isotope composition of pisolith fe oxy hydroxide pedogenic nodules cores which are demonstrated to behave as closed systems the u isotopic composition of the fe oxy hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite secondary overprinting is evident in some samples as a large range in 234 u 238 u our results suggest that weathering intensity varies with global climate cycles and that together with weathering events dated by 40 ar 39 ar of mn oxides elsewhere in northern australia feng y x and vasconcelos p 2001 quaternary continental weathering qeochronology by laser heating 40 ar 39 ar analysis of supergene cryptomelane geology 29 7 635 638 weathering rates in this region are orbitally forced

coupling uranium series and 10be cosmogenic radionuclides to evaluate steady state soil thickness in the betic cordillera abstract on earth the critical zone is the near surface environment where interactions between the atmosphere lithosphere hydrosphere and biosphere take place the development of the regolith mantle is controlled by the downwards propagation of the weathering front into the bedrock and denudation at the surface of the regolith by mass movements water and wind erosion when the removal of surface material is approximately balanced by the production of weatherable material at the bottom of the regolith profile the soil system is assumed to be in steady state although recent literature on chemical weathering often assumes that soils have a steady state thickness the concept has rarely been validated with empirical field data in this study we present and compare analytical data from two independent isotopic techniques in situ produced cosmogenic nuclides and u series disequilibria to constrain soil development under semi arid climatic conditions three soil profiles were sampled across the betic ranges at the ridge crest of zero order catchments with distinct topographic relief hillslope gradient and 10be derived denudation rate soil production rates determined based on u series isotopes 238u 234u 230th and 226ra are of the same order of magnitude as 10be derived denudation rates suggesting steady state soil thickness in two out of three sampling sites our results show the potential of a combined u series disequilibria and in situ 10be isotope approach to evaluate steady state soil thickness our study also illustrates the frontiers in applying u series disequilibria to track soil production in environments characterized by weak soil development

slow advance of the weathering front during deep supply limited saprolite formation in the tropical highlands of sri lanka abstract silicate weathering initiated by major mineralogical transformations at the base of ten meters of clay rich saprolite generates the exceptionally low weathering flux found in streams draining the crystalline rocks of the mountainous and humid tropical highlands of sri lanka this conclusion is reached from a thorough investigation of the mineralogical chemical and sr isotope compositions of samples within a regolith profile extending 10 m from surface soil through the weathering front in charnockite bedrock a high grade metamorphic rock corestones formed at the weathering front as well as from the chemical composition of the dissolved loads in nearby streams weatherable minerals and soluble elements are fully depleted at the top of the profile showing that the system is supply limited such that weathering fluxes are controlled directly by the supply of fresh minerals we determine the weathering rates using two independent means 1 in situ produced cosmogenic nuclides in surface soil and creek sediments in the close vicinity of the regolith combined with immobile element mass balance across the regolith and 2 river dissolved loads silicate weathering rates determined from both approaches range from 16 to 36 t km 2 y 1 corresponding to a weathering front advance rate of 6 14 mm ky 1 these rates agree across the 10 1 to 10 4 y time scales over which our rate metrics integrate suggesting that the weathering system operates at steady state within error these rates are furthermore compatible with those obtained by modeling the advance rate of the weathering front from chemical gradients and mineral dissolution rates the silicate weathering flux out of the weathering profile measured on small creeks amounts to 84 of the profile s export flux the remaining 16 is contributed by non silicate atmospheric derived input the silicate weathering flux as measured by dissolved loads in large catchments amounts to ca 50 of the total dissolved flux the remainder being contributed by dust rain and weathering of local marble bands spheroidal weathering is the key processes of converting the fresh bedrock into saprolite at the weathering front the mineralogical composition of weathering rinds shows that the sequence of mineral decomposition is pyroxene plagioclase biotite k feldspar observable biotite alteration does not appear to initiate spheroidal weathering within corestones therefore we infer that other processes than biotite oxidation like pyroxene oxidation clay formation from pyroxene and plagioclase decomposition the development of secondary porosity by plagioclase dissolution or even microbiologic processes at depth enable the coupling between slow advance of the weathering front and slow erosion at the surface the comparison to tectonically more active tropical landscapes lets us conclude that the combination of hard rock with tightly interlocked mineral grains and slow erosion in the absence of tectonically induced landscape rejuvenation lead to these exceptionally low weathering rates

systematics of u th disequilibrium in calcrete profiles lessons from southwest india pedogenic carbonates calcretes have often been proposed as possible markers of the pedogenetic processes in soils under semi arid climate however precise chronological constraints on their formation are required in order to investigate the climatic and paleo environmental conditions that prevailed during and after their formation and to improve our understanding of the physical and chemical conditions that promoted their development and preservation moreover these authigenic calcium carbonate precipitates provide us with an interesting test of the u th radioactive disequilibrium dating method the reliability of which has been demonstrated extensively in aragonitic corals and calcitic continental speleothems but which remains much more questionable in highly porous chemically and mineralogically complex media such as soil profiles we report here 238u 234u 230th radioactive disequilibrium analyses measured by thermo ionization mass spectrometry in calcretes from two sites in southwest india since calcretes are impure carbonates mixed with various amounts of parent rock and weathered minerals all ages must be computed using the isochron technique involving the extraction of several coeval subsamples from the same soil horizon and u th analyses after total dissolution tsd method in order to correct for the detrital component 232th is usually used as index of the detrital contamination and the isochron age is derived from the isotopic composition 230th 238u 234u 238u of the pure authigenic carbonate end member of the mixing line our results show that each set of samples taken from decimetric blocks is characterized by a well defined isochron line in a 3 d osmond diagram 232th 238u 230th 238u 234u 238u the regression lines obtained for most of the samples show probabilities of fit satisfying the mathematical assumptions of the isochron model in most cases 0 05 and mean square weighted deviations mswd values around 1 the compositions of the end members are highly variable between the different isochrons both for the carbonate phase indicating various ages of cristallisation and for the detrital phase indicating various origins and compositions notably our data demonstrate that the detrital component is in all cases out of secular equilibrium by contrast with the standard paradigm of dirty carbonate dating which is based on the assumption of average continental crust th u elemental ratio and secular equilibrium values of the 230th 238u and 234u 238u ratios at the first site coimbatore we obtain u th ages between 304 38 ka and 44 2 9 ka 2 from the base to the top of the profile these suggest discontinuous precipitation of the different facies in good agreement with micromorphological observations at the second study site gundlupet the carbonate nodules sampled from two different profiles yield consistent ages of 200 ka which suggests that precipitation of nodular carbonate occurred across the entire pediment toposequence during a single event surprisingly in mere contradiction with the standard model of impure carbonate dating two puzzling samples display vertical isochrons i e variable 232th 238u without any variation of the 230th 238u and 234u 238u ratios such patterns require extremely high 232th 238u ratios in an as yet unidentified detrital phase taken at face value these results suggest that no clear correlation can be inferred between the precipitation of these pedogenic carbonates and major orbitally driven climate variations recorded in other archives during the last glacial interglacial cycles

influence of variable uncertainties in seismic tomography models on constraining mantle viscosity from geoid observations the radial viscosity structure of the earth is explored on the basis of the geoid observations the variations of uncertainty in seismic tomography models are accounted for when finding the radial viscosity structure the new methodology we propose attempts to fit more closely those features of the geoid that are better constrained by tomography models and avoids to fit those features that are poorly constrained this approach is particularly important because the error of geoid predictions caused by uncertainties in seismic tomography models is overwhelmingly larger than the noise in the geoid measurements the synthetic tests indicate that the viscosity structures obtained by disregarding the uncertainty variations in seismic tomography models can be biased depending on the geoid spectral band and on the input seismic tomography model when the uncertainty variations in seismic models are considered in the inversion process results do not indicate a viscosity in the transition zone lower than in the upper mantle a robust feature found with the new method is a viscosity in the upper mantle two orders of magnitude smaller than in the lower mantle the error covariance of seismic tomography models is critical for the method we suggest a covariance matrix rigorously derived by seismologists should help to even more reliably infer the viscosity structure and relation between anomalies in density and seismic velocities from surface observations such as the geoid and thus lead to a better knowledge of the earth interior

major element geochemistry and geomorphic relationships in brazilian cerrado soils abstract a thorough understanding of major and trace element geochemistry is important in assessing the impacts of rapid agriculturalization and increased human occupation of the cerrado region of central brazil a region with some of the oldest soils on earth the objectives of this paper are to summarize the major element composition of cerrado soils in the context of their morphology and geomorphology to describe impacts of biogeochemical cycling on the chemical properties of cerrado soils and to use elemental data to provide insights on the possible origin of soil parent materials on the south american erosion surface and on the serra geral basalts we sampled three major subregions selected to be representative of the main areas of the cerrado namely i eastern goias ii northwestern minas gerais and iii the tri ngulo mineiro area of western minas gerais five soils were sampled in each subregion each represented by three pedons sampled at 0 0 2 and 0 8 1 0 m depths geomorphologically the soils were on the south american velhas i and velhas ii erosion surfaces all sites were carefully selected to minimize potential anthropogenic contamination general chemical characterization was by standard procedures and total major and trace elemental contents were quantified using wavelength dispersive x ray fluorescence spectroscopy the soils ranged from loamy sands to clays with si 142 947 g kg 1 sio 2 al 14 439 g kg 1 al 2 o 3 fe 10 331 g kg 1 fe 2 o 3 and ti 3 101 g kg 1 tio 2 contents closely following the clay contents organic c averaged 17 g kg 1 for the surface and 7 g kg 1 for the subsurface horizons the na ca mg and k contents were very low tended to be higher in surface than in subsurface horizons because of recycling by vegetation and addition due to atmospheric deposition and were higher in soils containing hard plinthite nodules the th zr ratios suggest that the parent materials of the clayey soils on the south american erosion surface have a common origin in sediments from rocks in the central plateau of brazil that were transported eastwards to cover the underlying sandstone trace element signatures indicate that basalt derived soils occurring at different elevations in the tri ngulo mineiro area formed from distinctly different magmas that may have been deposited in different basalt flow events

learning to read the chemistry of regolith to understand the critical zone the base of the critical zone includes the mantle of altered soil and rock regolith that changes in response to chemical physical and biological processes occurring at earth s surface these processes are recorded in the chemistry of the regolith and this long term record can often be deciphered for example on eroding ridgetops where flows are generally downward for water and upward for earth material element concentrations vary with depth to constitute depletion addition depletion enrichment and biogenic profiles models can be used to explore the records of mineral dissolution atmospheric input coupled dissolution precipitation and biolifting documented in these profiles these models enable interpretation of the effects of time climate rates of erosion and human and other biotic impacts on the profile patterns by testing quantitative models against the long term record of information in regolith we will learn to project changes arising from human and natural perturbations of the critic

localization of the gravity field and the signature of glacial rebound the negative free air gravity anomaly centred on hudson bay canada shows a remarkable correlation with the location of the laurentide ice sheet suggesting that this gravity anomaly is the result of incomplete post glacial rebound this region however is also underlain by higher than average mantle seismic velocities suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings here we analyse the global gravity field as a simultaneous function of geographic location and spectral content we find that the hudson bay gravity low is unique with anomalously high amplitude in the spectral band where the power from the laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest we estimate that about half of the hudson bay gravity anomaly is the result of incomplete post glacial rebound and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central laurentide and fennoscandian regions this model has a jump in viscosity at 670 km depth comparable to that in dynamic models of the geoid highs over subducted slabs but lacks a low viscosity asthenosphere consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions

applications and limitations of u th disequilibria systematics for determining ages of carbonate alteration minerals in peridotite abstract 238 u 234 u 230 th dating was conducted on carbonate alteration minerals in the peridotite layer of the samail ophiolite sultanate of oman in order to assess the applicability of u series dating techniques to these types of quaternary terrestrial carbonates and also to further constrain natural rates of carbonation of the peridotite due to their low u concentrations and relatively high th u ratios samail carbonates are challenging to date with the 230 th technique because of the sensitivity of ages to corrections for initial 230 th uncorrected 230 th ages for ca rich travertines are consistently older than previously obtained 14 c ages however geologically reasonable initial 230 th corrections bring the two sets of ages into concordance this age concordance suggests that the travertines are generally closed systems adding a level of credence to the reliability of previously obtained 14 c ages in contrast uncorrected 230 th ages for mg rich carbonate veins are generally younger than previously obtained 14 c ages these young ages are interpreted in terms of remobilization of hexavalent u which is subsequently deposited as tetravalent u by reduced serpentinization fluids two mg rich carbonate veins sampled at a roadcut have near equilibrium 230 th 238 u and 234 u 238 u values which indicate that these veins are 375 000 years in age consistent with their 14 c dead 50 000 years bp ages the variable young and old ages for these mg rich carbonate veins indicate that carbonation of the peridotite layer of the samail ophiolite is an ongoing process and that there have been multiple generations of subsurface carbonate vein formation overall this study provides insights into some of the challenges associated with applying u series dating methods to quaternary terrestrial carbonates in particular carbonate alteration minerals in peridotites and highlights some areas where there is room for improvement such as obtaining better constraints on the isotopic composition of admixed detritus and also some advantages such as the ability to identify open system behavior not apparent from 14 c dating and stable c and o isotopic analysis alone

fossil andisols identified with mass balance geochemistry oligocene john day formation oregon u s a mass balance geochemistry of thick sequences of fossil soils oligocene john day formation central oregon show trends similar to those produced by modern andisol soil forming processes these andic trends include phosphorus accumulation potassium retention and alumina and iron depletion identification of these trends benefited from the geochemical comparison of numerous fossil soil horizons from two paleosol sequences one andic and one non andic the comparison of two paleosol sequences is justified because both sequences developed from the same type of parent material in nearly identical tectonic depositional settings the two sequences are 1 the early oligocene big basin member dominated by clayey alfisol like paleosols and 2 the mid oligocene turtle cove member dominated by ash rich andisol like paleosols with calcareous horizons the change in paleosol type occurred circa 30 ma throughout the john day basin and is most likely due to global cooling which changed climatic conditions in central oregon from humid to subhumid drier climate is interpreted to have caused a decrease in weathering intensity so that amorphous weathering products of volcanic glass accumulated in soils to form andisols prior to this circa 30 ma transition weathering produced crystalline clays from volcaniclastic detritus to form alfisols rather than amorphous weathering products

determination of chemical weathering rates from u series nuclides in soils and weathering profiles principles applications and limitations the development of u series nuclides for investigating weathering processes has been significantly stimulated by the analytical improvement made over the last decades in measuring the 238 u series with intermediate half lives i e 234 u 230 th 226 ra it is proposed in this paper to present principles and methods that are now being developed to determine weathering rates from the study of u series nuclides in soils and weathering profiles mathematical approaches developed to calculate such rates are based on some implicit assumptions that are also presented and must be kept in mind if one wants to correctly interpret the obtained ages

the geoid constraint in global geodynamics viscosity structure mantle heterogeneity models and boundary conditions summary the earth s non hydrostatic gravity field or geoid provides a first order constraint on mantle density structure and dynamics geodynamic models for the geoid have proliferated since the advent of seismic mapping of mantle heterogeneity structure tomography because the geoid offers perhaps the best measured independent constraint on mantle density heterogeneity however dynamic geoid models involve a number of questionable physical assumptions and uncertainties whose effects need to be evaluated before geodynamic inferences based upon the geoid can be considered sound troubling issues include the appropriate surface boundary conditions free slip no slip plates and parametrization of radial viscosity variations how many layers can be resolved in addition to lateral viscosity variations possible chemical layering of the mantle phase transitions etc there are also uncertainties in the density heterogeneity models used as input to dynamic geoid models most of which are derived from seismic tomography and require weakly constrained empirical conversion factors to go from seismic velocity variations to density variations here we address several of the most straightforward problems inherent in geoid modelling namely the issues of viscosity structure resolution uncertainties in appropriate boundary conditions and differences among mantle heterogeneity models a robust feature of all models is a lower mantle viscosity at least a factor of 30 greater than that of the upper mantle but there is little resolution with regard to finer details such as lithospheric or uppermost mantle low viscosity zone viscosity ironically free slip boundary conditions result in the best fits to the geoid in all cases but all boundary conditions exhibit predictable trade offs with the uppermost mantle viscosity models with a single viscosity layer representing the lower mantle yield similar dynamic topography estimates of the order of 700 1000 m in amplitude regardless of the finer details of upper mantle viscosity structure boundary conditions or input heterogeneity models comparing mantle heterogeneity models based on two independent seismological determinations harvard and berkeley models and on the history of subduction we find that these models are virtually indistinguishable regarding inferences of mantle viscosity structure and amplitude of dynamic topography and in terms of the effects of different boundary conditions uncertainties concerning which type of boundary condition is appropriate are much more important than which mantle heterogeneity model is chosen given other uncertainties in modelling the geoid particularly the strong effects due to lateral viscosity variations for intermediate 10 000 km wavelengths we conclude that the class of dynamic geoid models explored so far cannot reliably elucidate the details of upper mantle viscosity structure

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

spherical dynamic models of top down tectonics 1 we use the multipole boundary element method mp bem to simulate regional and global geodynamics in a spherical 3 d setting we first simulate an isolated subducting rectangular plate with length llitho and width wlitho varying between 0 5 and 2 times the radius of the earth rearth and with viscosity litho varying between 100 and 500 times the upper mantle um sinking in a layered mantle characterized by lower upper mantle viscosity ratio lm um varying between 1 and 80 in a mantle with small upper lower viscosity contrast 1 trench and plate motions are weakly dependent on wlitho plate motion is controlled by slab pull if llitho rearth while for longer plates plate speed strongly decreases because of the plate basal friction and flow reorganization an increasing viscosity ratio gradually breaks this pattern and for 10 combined with wlitho rearth and greater trench advance and retreat are simultaneously observed these results offer a first order explanation of the origin of the size llitho wlitho rearth of the largest plates observed over the past 150 myr finally two global plate tectonic simulations are performed from reconstructed plates and slabs at 25 ma before present and before 100 ma respectively it is shown that mp bem predicts present plate kinematics if plate mantle decoupling is adopted for the longest plates llitho rearth models for 100 ma show that the slab slab interaction between india and izanagi plates at 100 ma can explain the propagation of the plate reorganization from the indian to the pacific plate

evolution of rhonegletscher switzerland over the past 125 years and in the future application of an improved flowline model to study the past and future evolution of rhonegletscher switzerland a flowline model was developed to include valley shape effects more accurately than conventional flowband models in the model the ice flux at a gridpoint was computed by a two dimensional ice flow model applied to the valley cross section the results suggested the underestimation of the accumulation area which seems to be a general problem of flowline modelling arising from the model s one dimensional nature the corrected mass balance was coupled with the equilibrium line altitude ela change which was reconstructed for the period 1878 2003 from temperature and precipitation records to run the model for the past 125 years the model satisfactorily reproduced both changes in the terminus position and the total ice volume derived from digital elevation models of the surface obtained by analyses of old maps and aerial photographs this showed the model s potential to simulate glacier evolution when an accurate mass balance could be determined the future evolution of rhonegletscher was evaluated with three mass balance conditions the mean for the period 1994 2003 and the most negative 2003 and positive 1978 mass balance values for the past 50 years the model predicted volume changes of 18 58 and 38 after 50 years for the three conditions respectively

mantle viscosity derived by genetic algorithm using oceanic geoid and seismic tomography for whole mantle versus blocked flow situations abstract we have applied the genetic algorithm ga technique a nonlinear global optimization method to determine the radial viscosity structure of the mantle from regional geoidal patterns from numerical simulations of 2 d mantle convection we examine the horizontal spectra of the vertical mass flux at 660 km depth and find that for long wavelengths there are minor differences between partially layered convection induced by the phase transitions and mantle convection without any phase transition the differences in the spectra of the vertical mass flux become more prominent at shorter wavelengths this result has led us to study mantle viscosity for the intermediate wavelength geoid from the whole mantle and blocked flow situations in which the appropriate boundary condition is imposed on the radial velocity at 660 km depth in order to confirm the robustness of this study two different density models have been used which were constructed from three tomographic models and appropriate velocity to density scaling relations based on recent results from mineral physics we have analyzed only oceanic geoid spanning between spherical harmonic degree l 12 25 the correlation of the predicted geoid with the observations over the atlantic indian and pacific oceans have been employed as the fitting function in our ga approach which has been modified from the common algorithm in constructing the families of suitable viscosity profiles we have used 100 parents which have been iterated for 100 generations and have been started with 10 different sets of initial parents convergence to acceptable viscosity solutions is obtained for all the three oceans and for both the whole mantle and layered models in some cases multiple viscosity solutions are found acceptable by using the correlation criteria the outstanding feature of these models is the nearly ubiquitous presence of two low viscosity zones one lying under the lithosphere the other right under the bottom of the spinel to perovskite phase change the solutions for the whole mantle model can fit better and are preferred over the solutions with the layered boundary condition which generally result in unrealistic viscosity profiles our results would suggest a more complex mantle viscosity structure which has not been detected previously from geoid signals with longer wavelengths and also reveal the potential difficulties in treating the dynamical boundary condition at the 660 km discontinuity

226ra 238u and 228th 228ra disequilibrium as weathering indices in beach sand sediments associated with granitoids from cyclades greece abstract radioactive secular disequilibrium measured as the absolute difference from 1 of the 226ra 238u and 228ra 228th ratios have been used to determine weathering extent twenty eight samples from beach sands of cyclades islands all of them being the weathering products of the local granitic plutons are studied for their secular disequilibrium in both 238u and 232th series 226ra 238u and 228th 228ra respectively the results are compared to weathering indices already in use worldwide and suggest a significant linear correlation of u series disequilibrium with wip alkaline and alkaline earth element leaching and pwi silica leaching for the beach sands studied and significant non linear correlations with cia chemical index of alteration and pia plagioclase index of alteration considering only the heavy mineral enriched samples significant correlations exist between u series disequilibrium and cia ciw chemical index of weathering and pia therefore with an appropriate choice of the weathering index and samples both u series and weathering index can recover the extent of chemical weathering on the other hand the use of th series disequilibrium as a weathering index is limited due to 228th and 228ra geochemistry and especially due to their short radionuclide half lives u series disequilibrium can therefore be used as a relatively sensitive weathering index in sediments consistent with cia ciw and pia indices however the mineralogical composition must be considered more specifically the mineral content of the sediments should involve considerable amounts of sensitive to weathering minerals e g fe mg minerals as well as feldspars

riverine li isotope fractionation in the amazon river basin controlled by the weathering regimes we report li isotope composition 7li 7li of river borne dissolved and solid material in the largest river system on earth the amazon river basin to characterize li isotope fractionation arl a continental scale the 7li 7li in the dissolved load 1 2 to 32 is fractionated toward heavy values compared to the inferred bedrock 1 to 5 and the suspended sediments 6 8 to 0 5 as a result of the preferential incorporation of 66li into secondary minerals during weathering despite having very contrasted weathering and erosion regimes both andean headwaters and lowland rivers share similar ranges of dissolved 7li 7li 1 2 to 18 correlations between dissolved 7li 7li and li na and li mg ratios suggest that the proportion of li incorporated in secondary minerals during weathering act as the main control on the 7lidiss 7lidiss across the entire amazon basin a batch steady state fractionation model for andean and lowland rivers satisfactorily reproduces these variations with a fractionation factor between weathering products and dissolved load sec dis sec dis of 0 983 two types of supply limited weathering regimes can be identified for the lowlands clearwaters with dominant incorporation of li in secondary minerals and black waters e g rio negro where dissolution of secondary minerals enhanced by organic matter produces low 7li 7li apart from the black waters the 7li 7li of andean and lowland rivers is negatively correlated to the denudation rates with the lowest 7li 7li corresponding to the rivers having the highest denudation rates in contrast the main tributaries draining both the andes and the lowlands have higher 7li 7li compared to other rivers we propose that part of the dissolved li derived from weathering in the andes is re incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved 7li 7li along the course of these rivers unlike other rivers the dissolved 7li 7li in the main tributaries is best described by a rayleigh fractionation model with a fractionation factor sec dis sec dis of 0 991 altogether the control imposed by residence time in the weathering zone and floodplain processes results in i a non linear correlation between dissolved 7li 7li and the weathering intensity defined as w d and ii a positive relationship between the dissolved li flux and the denudation rate these results have important implications for the understanding of past ocean 7li 7li and its use as a paleo weathering proxy

sediment production and delivery in the amazon river basin quantified by in situ produced cosmogenic nuclides and recent river loads we use cosmogenic nuclide derived denudation rates from in situ produced 10 be in river sediment to determine sediment production rates for the central amazon river and its major tributaries recent developments have shown that this method allows calculating denudation rates in large depositional basins despite intermediate sediment storage with the result that fluxes of the sediment producing hinterland can now be linked to those discharged at the basins outlet in rivers of the central amazonian plain sediment of finer grain sizes 125 500 m yields a weighted cosmogenic nuclide derived denudation rate of 0 24 0 02 mm yr that is comparable to the integrated rate of all main andean draining rivers 0 37 0 06 mm yr which are the beni napo mamore ucayali and maranon rivers coarser grained sediment 500 m of central amazonian rivers is indicative of a source from the tectonically stable cratonic headwaters of the guyana and brazilian shields for which the denudation rate is 0 01 0 02 mm yr respective sediment loads can be calculated by converting these cosmogenic nuclide derived rates using their sediment producing areas for the amazon river at obidos a sediment production rate of 610 mt yr results non andean source areas contribute only 45 mt yr a comparison with published modern sediment fluxes shows similarities within a factor of 2 with an average gauging derived sediment load of 1000 mt yr at obidos for example we attribute this similar trend in cosmogenic versus modern sediment loads first to the absence of long term deposition within the basin and second to the buffering capability of the large amazon floodplain the buffering capability dampens short term high amplitude fluctuations climatic variability in source areas and anthropogenic soil erosion by the time the denudation rate signal of the hinterland is transmitted to the outlet of the basin

joint modeling of lithosphere and mantle dynamics sensitivity to viscosities within the lithosphere asthenosphere transition zone and d layers abstract although mantle rheology is one of the most important properties of the earth how a radial mantle viscosity structure affects lithosphere dynamics is still poorly known particularly the role of the lithosphere asthenosphere transition zone and d layer viscosities using constraints from the geoid plate motions and strain rates within plate boundary zones we provide important new refinements to the radial viscosity profile within the key layers of the lithosphere asthenosphere transition zone and d layer we follow the approach of the joint modeling of lithosphere and mantle dynamics ghosh and holt 2012 ghosh et al 2013b 2019 wang et al 2015 to show how the viscosities within these key layers influence lithosphere dynamics we use the viscosity structure sh08 steinberger and holme 2008 as a starting model the density variations within the mantle are derived from the tomography models which based on prior modeling had provided a best fit to the surface observables wang et al 2015 our results show that narrow viscosity ranges of moderately strong lithosphere 2 6 5 6 1022 pa s and moderately weak transition zone 5 9 3 1020 pa s as well as slightly large ranges of moderately weak asthenosphere 5 34 1019 pa s and d layer 4 8 18 1020 pa s are necessary to match all the surface observables we also find that a very strong lithosphere 8 6 1022 pa s along with a weak asthenosphere

understanding the effects of mantle compressibility on geoid kernels summary we develop a quasi analytical solution to compute geoid kernels for a compressible mantle with newtonian rheology by separating the stresses induced by self gravitation from the stresses resulting from viscous flow we simplify the equations and gain some insight for realistic variations in the background density field po r the solution obtained using propagator matrices converges rapidly compressibility enters into the flow problem directly through the continuity equation and indirectly by influencing parameters such as gravitational acceleration g r and density contrasts across compositional boundaries in order to understand all these effects we introduce them sequentially starting with an incompressible earth model and ending up with a realistic compressible model that includes a compressible inner and outer core phase changes in the transition zone and an ocean the largest effects on geoid kernels are from different assumptions for g r possible effects of transformational superplasticity and differences in assumptions for density contrasts at the surface and at the core mantle boundary are next in importance the effects of compressibility on the flow itself are somewhat smaller followed by the effect of compressibility of the outer core a gravitationally consistent treatment of the ocean layer yields geoid kernels that are very similar to those for a dry planet the compressibility of the inner core has a negligible impact on the geoid kernels the largest effects from compressibility are comparable to the effects of a moderate 40 per cent change in the viscosity contrast between the upper and lower mantle

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

weathering flux and co2 consumption determined from palaeosol sequences across the eocene oligocene transition abstract rates of base cation depletion are determined for late eocene and early oligocene alluvial palaeosols pacific northwest of north america from pedogenic mass balance geochemistry and time estimates of palaeosol sequences pedogenic strain volume change and mass transfer of base cations silica and other pertinent elements are calculated from the geochemical differences between palaeosol horizons and parent material parent material compositions are determined by identification of the least altered alluvial strata in the sequence time spans of sequences of palaeosols are estimated from 40ar 39ar age dates of interbedded tuff beds and from time of formation estimates of palaeosols based on their degree of development palaeosol sequences from the mid latitude setting of central oregon document a significant decrease in the degree of weathering of individual palaeosols across the eocene oligocene transition as the climate became cooler and drier this decrease in the degree of weathering is accompanied by an increase in the accumulation rate of alluvial soils as indicated by sedimentation rates of stratigraphic packages according to these calculations and estimates the weathering flux of base cations in the john day basin measured in grams per 1000 years per square centimetre increased across the eocene oligocene transition these results when attributed to climate change indicate that weathering flux at this local increased as climate changed from warm humid late eocene conditions to cooler drier oligocene conditions furthermore the extrapolation of these weathering flux results to other mid latitude settings with similar climate change history may help to explain the severe climate change of the eocene oligocene boundary it is proposed that in some mid latitude settings greater quantities of less weathered silicate material were exposed during the early oligocene than during the late eocene thus causing an increase in flux of weathered product increased weathering flux during this time in certain regions of the mid latitudes may have increased both co2 consumption and bicarbonate transport to the ocean and contributed to atmospheric pco2 drawdown and global cooling

uranium series isotopes in river materials insights into the timescales of erosion and sediment transport abstract the uranium series isotope signatures of the suspended and dissolved load of rivers have emerged as an important tool for understanding the processes of erosion and chemical weathering at the scale of a watershed these signatures are a function of both time and weathering induced fractionation between the different nuclides provided appropriate models can be developed they can be used to constrain the residence time of river sediment this chronometer is triggered as the bedrock starts weathering and the inferred timescale encompasses the residence time in the weathering profile storage in temporary sediment deposits e g floodplain and transport in the river this approach has been applied to various catchments over the past five years showing that river sediments can reside in a watershed for timescales ranging from a few hundreds of years iceland to several hundreds of thousands of years lowlands of the amazon various factors control how long sediment resides in the watershed the longest residence times are observed on stable cratons unaffected by glacial cycles or more generally climate variability and human disturbance shorter residence times are observed in active orogens andes or fast eroding recently glaciated catchments iceland in several cases the residence time of suspended sediments also corresponds to the time since the last major climate change the u series isotope composition of rivers can also be used to predict the river sediment yield assuming steady state erosion is reached by comparing this estimate with the modern sediment yield obtained by multi year sediment gauging it is clear that steady state is seldom reached this can be explained by climate variability and or human disturbance steady state is reached in those catchments where sediment transport is rapid iceland or where the region has been unaffected by climate change and or human disturbance u series are thus becoming an important tool to study the dynamics of erosion