erosional and climatic effects on long term chemical weathering rates in granitic landscapes spanning diverse climate regimes abstract we used cosmogenic nuclide and geochemical mass balance methods to measure long term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes our 42 study sites encompass widely varying climatic and erosional regimes with mean annual temperatures ranging from 2 to 25 c average precipitation ranging from 22 to 420 cm year 1 and denudation rates ranging from 23 to 755 t km 2 year 1 long term chemical weathering rates range from 0 to 173 t km 2 year 1 in several cases exceeding the highest granitic weathering rates on record from previous work chemical weathering rates are highest at the sites with rapid denudation rates consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock a simple empirical relationship based on temperature precipitation and long term denudation rates explains 89 95 of the variation in long term weathering rates across our network of sites our analysis shows that for a given precipitation and temperature chemical weathering rates increase proportionally with fresh material supply rates we refer to this as supply limited weathering in which fresh material is chemically depleted to roughly the same degree regardless of its rate of supply from breakdown of rock the temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter comparisons of catchment mass balance data from the field our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought at least in actively eroding unglaciated terrain similar to our study sites to the extent that chemical weathering rates are supply limited in mountainous landscapes factors that regulate rates of mineral supply from erosion such as tectonic uplift may lead to significant fluctuations in global climate over the long term

toward process based modeling of geochemical soil formation across diverse landforms a new mathematical framework we present a mathematical model that integrates geochemical and geomorphic processes responsible for soils elemental and mineralogical evolution on diverse landforms eroding to depositional to level grounds this new model combines a hillslope sediment mass balance and a soil geochemical mass balance we model soils as the sum of a physically disturbed zone pdz and the underlying physically undisturbed but chemically altered zone caz the model considers processes including the production of pdz weathering front propagation mineral specific dissolution colloidal translocation and colluvial transport thus this study serves to 1 review preexisting models of pedogenic processes and to 2 offer a framework to integrate the geochemical and geomorphic processes active in soils we further discuss the implications of our model in 1 scaling chemical weathering reactions from mineral grains to pedons to soil catenas 2 understanding the emergence of geochemical soil catenas and 3 coupling physical and chemical weathering processes

relict non glacial surfaces in formerly glaciated landscapes abstract relict non glacial surfaces occur within many formerly glaciated landscapes and contain important information on past surface processes and long term landscape evolution relict non glacial surfaces are distinguishable from glacial surfaces by large scale morphologies including rounded summits fluvial valleys and cryoplanation terraces and pediments and the presence of tors blockfields and or saprolites preservation during glaciation occurs either through coverage by non erosive cold based ice or as nunataks although surface morphologies and denudation rates indicate a continuous non glacial surface history since preglacial times relict non glacial surfaces are dynamic features that have evolved during the quaternary depending on spatial variables such as lithology slope regolith depth and the abundance of fine matrix and water some surfaces are denuding very slowly while others display more rapid denudation high spatial variability in denudation rates results in changing surface morphologies over time denudation rates also display high temporal variability with much surface evolution having perhaps occurred soon after the initial onset of glaciation or during paraglacial phases while some parts of non glacial landscapes are currently active others may be largely inactive relicts of past higher energy regimes although non glacial surfaces are dynamic much remains to be determined regarding surface denudation rates and the magnitude of morphological changes over time

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

patterns and rates of 103 105 yr denudation in carbonate terrains under subhumid to subalpine climatic gradient mount hermon israel

estimating millennial scale rates of dust incorporation into eroding hillslope regolith using cosmogenic nuclides and immobile weathering tracers 1 dust fluxes are of wide interest because of the effects of dust on climate oceanic primary productivity terrestrial biogeochemical cycles and regolith composition estimating long term dust deposition rates however can be difficult especially in steep eroding terrain here we present a geochemical mass balance method for estimating long term average rates of dust incorporation into regolith on steadily eroding hillslopes this method requires measurements of the local regolith production rate and the concentrations of two immobile elements in the regolith its parent rock and dust dust incorporation rates inferred with this method are averaged over the long timescale of regolith residence on the hillslope typically 103 105 years and thus may serve as long term averages against which modern day dust fluxes may be compared we apply this model to 17 field sites in the south fork of the salmon river in the idaho batholith where rock and regolith compositions imply that mafic rich material has been added to the otherwise granitic regolith we suggest that the most likely source of this mafic material is dust sourced from the same glacial outburst flood sediments that generated the palouse loess on the columbia plateau and we use the published composition of these sediments to infer dust incorporation rates of 3 13 t km 2 yr 1 at these sites comparable to modern day dust fluxes elsewhere in the western united states

mean mixed depth of sediments the wherefore and the why the activities and consequently the bioturbational effects of deposit feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide environmentally invariant mean of 9 8 cm with a standard deviation of 4 5 cm currently available theories of infaunal behavior cannot predict quantitatively this observation a new simple model that accounts for the feedback between resource food abundance its reactivity and the intensity of bioturbation leads to a quantitative estimate of 9 7 cm this model constitutes a fundamental advance in our understanding bioturbation

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

spatial patterns and controls of soil chemical weathering rates along a transient hillslope article i nfo hillslopes have been intensively studied by both geomorphologists and soil scientists whereas geomorphologists have focused on the physical soil production and transport on hillslopes soil scientists have been concerned with the topographic variation of soil geochemical properties we combined these differing approaches and quantified soil chemical weathering rates along a grass covered hillslope in coastal california the hillslope is comprised of both erosional and depositional sections in the upper eroding section soil production is balanced by physical erosion and chemical weathering the hillslope then transitions to a depositional slope where soil accumulates due to a historical reduction of channel incision at the hillslope s base measurements of hillslope morphology and soil thickness were combined with the elemental composition of the soil and saprolite and interpreted through a process based model that accounts for both chemical weathering and sediment transport chemical weathering of the minerals as they moved downslope via sediment transport imparted spatial variation in the geochemical properties of the soil inverse modeling of the field and laboratory data revealed that the long term soil chemical weathering rates peak at 5gm 2 yr 1 at the downslope end of the eroding section and decrease to 1 5gm 2 yr 1 within the depositional section in the eroding section soil chemical weathering rates appear to be primarily controlled by the rate of mineral supply via colluvial input from upslope in the depositional slope geochemical equilibrium between soil water and minerals appeared to limit the chemical weathering rate soil chemical weathering was responsible for removing 6 of the soil production in the eroding section and 5 of colluvial influx in the depositional slope these were among the lowest weathering rates reported for actively eroding watersheds which was attributed to the parent material with low amount of weatherable minerals and intense coating of the primary minerals by secondary clay and iron oxides we showed that both the morphologic disequilibrium of the hillslope and the spatial heterogeneity of soil properties are due to spatial variations in the physical and chemical processes that removed mass from the soil

interplay between soil formation and geomorphic processes along a soil catena in a mediterranean mountain landscape an integrated pedological and geophysical approach soil spatial distribution often shows high variability because of geo environmental settings affecting soil formation and geomorphic processes the paper describes a combined pedological geophysical approach for investigating the interplay among soil forming processes geomorphic dynamics and physico chemical soil properties in a soil catena in a mediterranean mountain landscape southern italy stocks of soil organic carbon soc and total nitrogen stn were determined and their interactions with slope position analyzed five slope units were defined along the soil catena summit shoulder back foot and toe slopes and in each of them a soil profile was dug and analyzed thickness of soil horizons and contact between soil and parent material were determined by a 2d electrical resistivity tomography survey results showed granitic bedrock is characterized by vary weathering degrees strongly affecting soil type development moreover soil properties variation is related to topographic features and water erosion processes soil chemical composition showed high values of sio2 and al2o3 probably due to inheritance from underlying granitic parent material particularly fe2o3 content increased with weathering conditions as shown by chemical index of alteration values along foot and toe slopes reworking and accumulation of colluvial soil lead to the formation of thicker accretionary a horizon consequently summit areas together with foot and toe slopes had greater soc and stn stocks than back and shoulder slopes that because soils are thicker and richer in soc on summit and valley bottom than in shoulder and back slope positions the proposed approach has showed the benefit of an integrated methodology for investigating spatial variations along soil catenas

silicate weathering of soil mantled slopes in an active alpine landscape abstract despite being located on high steep actively uplifting and formerly glaciated slopes of the swiss central alps soils in the upper rhone valley are depleted by up to 50 in cations relative to their parent bedrock this depletion was determined by a mass loss balance based on zr as a refractory element both holocene weathering rates and physical erosion rates of these slopes are unexpectedly low as measured by cosmogenic 10be derived denudation rates chemical depletion fractions cdf range from 0 12 to 0 48 while the average soil chemical weathering rate is 33 15 t km 2 yr 1 both the cosmogenic nuclide derived denudation rates and model calculations suggest that these soils have reached a weathering steady state since deglaciation 15 ky ago the weathering signal varies with elevation and hillslope morphology in addition the chemical weathering rates decrease with elevation indicating that temperature may be a dominant controlling factor on weathering in these high alpine basins model calculations suggest that chemical weathering rates are limited by reaction kinetics and not the supply rate of fresh material we compare hillslope and catchment wide weathering fluxes with modern stream cation flux and show that high relief bare rock slopes exhibit much lower chemical weathering rates despite higher physical erosion rates the low weathering fluxes from rocky rapidly eroding slopes allow for the broader implication that mountain building while elevating overall denudation rates may not cause increased chemical weathering rates on hillslopes in order for this sediment to be weathered intermediate storage for instance in floodplains is required

using chemical tracers in hillslope soils to estimate the importance of chemical denudation under conditions of downslope sediment transport 1 we present a model of hillslope soils that couples the evolution of topography soil thickness and the concentration of constituent soil phases defined as unique components of the soil with collective mass equal to the total soil mass the model includes both sediment transport and chemical denudation a simplified two phase model is developed the two phases are a chemically immobile phase which has far lower solubility than the bulk soil and is not removed through chemical weathering for example zircon grains and a chemically mobile phase that may be removed from the system through chemical weathering chemical denudation rates in hillslope soils can be measured using the concentration of immobile elements but the enrichment of these immobile elements is influenced by spatial variations in chemical denudation rates and spatial variations in the chemical composition of a soil s parent material these considerations cloud the use of elemental depletion factors and cosmogenic nuclide based total denudation rates used to identify the relationship between physical erosion and chemical weathering if these techniques do not account for downslope sediment transport on hillslopes where chemical denudation rates vary in space estimates of chemical denudation using techniques that do not account for downslope sediment transport and spatial variations in chemical denudation rates may be adequate where the chemical denudation rate is a significant fraction of the total denudation rate but are inadequate in regions where chemical weathering rates are small compared to the total denudation rate we also examine relationships between transient mechanical and chemical denudation rates soil particle residence times may affect chemical weathering rates and the relationship between total landscape lowering rates and soil particle residence times can thus be quantified

quantification of the effect of plants on weathering studies in iceland the weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric co 2 over geologic time and vascular plants may have played an important role in this process in western iceland we have examined the role of plants in weathering by measuring the chemistry of waters draining adjacent areas of basaltic rocks that are either barren having a partial cover of mosses and lichens or populated by trees the study area was chosen to maximize vegetational differences and to minimize differences in microclimate slope and lithology while avoiding hydrothermal waters and anthropogenic acid rain results including data on cation uptake by growing trees indicate that the rate of weathering release of ca and mg to streams and vegetation is two to five times higher in vegetated areas than the release of ca and mg to streams in barren areas this finding suggests a major role for vascular plants in accelerating weathering and thereby lowering atmospheric co 2 as they invaded upland areas of the continents between 380 and 350 ma

estimates of the rate of regolith production using and from an alpine hillslope abstract the production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material we use concentrations of the cosmogenic radionuclides crns 10 be and 26 al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the wind river range wy these calculations are based on a theoretical model which we develop here this model shows that it is important to consider dissolution of regolith in regolith production and in basin averaged erosion rate studies rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 be and 26 al are 14 3 4 0 and 13 0 4 0 m ma 1 respectively rates of production of regolith deduced from 10 be concentrations in regolith mantled bedrock support the rates deduced from regolith concentrations in the alpine environment examined here the rate of production of regolith beneath 90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces which small et al small e e anderson r s repka j l finkel r 1997 erosion rates of alpine bedrock summit surfaces deduced from in situ 10 be and 26 al earth and planetary science letters 150 413 425 previously documented rock mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope the regolith volume flux must be proportional to the local slope of the hillside therefore our results are consistent with gilbert s gilbert g k 1909 the convexity of hilltops journal of geology 17 344 350 steady state hillslope hypothesis if tor height and the difference between rates of weathering on bare and regolith mantled rock provide a fair estimate of the age of summit flats steady state hillslope conditions have been attained in less than several million years

slope gradient controls soil thickness and chemical weathering in subtropical brazil understanding rates and timescales of regional soilscape evolution through a combination of field data and modeling

chemical weathering in two adirondack watersheds past and present day rates rates of chemical weathering in two forested adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input output budgets based on precipitation and surface water chemistry long term rates of weathering have averaged about 500 to 600 eq ha yr for both watersheds since the glaciers retreated from the region about 14 000 yr ago present day denudation rates average 1679 eq ha yr in the panther lake watershed and only 198 eq ha yr in the woods lake watershed mineral weathering reactions in both watersheds involve primarily the dissolution of plagioclase potassium feldspar and hornblende mass balance calculations however indicate that hornblende weathers in disproportionately large amounts in the panther lake watershed relative to its abundance in soils and till cation exchange in the glacial till mantling the panther lake watershed may also play an important role in controlling surface water chemistry in the basin in the panther lake watershed the current rate of weathering which is about a factor of 3 greater than the long term average may reflect a recent adjustment for higher hydrogen ion fluxes brought about by acid deposition the capacity of the thick till deposits in the panther lake watershed to neutralize acidity is ultimately reflected by the circum neutrality of panther lake in the woods lake watershed the soils and thin till deposits cannot effectively neutralize current acid loadings and therefore woods lake is acidic with a ph below 5

sharp decrease in long term chemical weathering rates along an altitudinal transect we used cosmogenic nuclide and geochemical mass balance methods to measure long term rates of chemical weathering and physical erosion across a steep climatic gradient in the santa rosa mountains nevada our study sites are distributed along a 2 km ridgeline transect that spans 2090 to 2750 m in altitude and encompasses marked contrasts in both vegetative cover and snow depth but is underlain by a single roughly uniform granodiorite bedrock cosmogenic nuclides in colluvial soils reveal that denudation rates vary by less than a factor of 1 4 104 144 t km2 yr along this transect bulk elemental analyses indicate that relative to the parent rock soils are less intensively weathered with increasing altitude and show little evidence of weathering related mass losses near the top of the ridge chemical weathering rates decrease rapidly with increasing altitude both in absolute terms from 24 to 0 t km2 yr and as a fraction of total denudation rates from 20 to 0 thus these results indicate an increasing dominance of physical erosion with altitude the observed decrease in chemical weathering rates is greater than one would predict from the decrease in mean annual temperature using simple weathering kinetics suggesting that weathering rates along our transect may also be affected by the progressive decline in vegetative cover and increase in snow depth with increasing altitude these results considered together with weathering rate measurements for a wide range of climates in the sierra nevada usa suggest that chemical weathering rates may be particularly sensitive to differences in climate at higher altitude sites consistent with this hypothesis chemical weathering rates fall virtually to zero at the highest sites on our transect suggesting that sparsely vegetated high altitude crystalline terrain may often be characterized by extremely slow silicate weathering rates

ancient impact structures on modern continental shelves the chesapeake bay montagnais and toms canyon craters atlantic margin of north america abstract three ancient impact craters chesapeake bay 35 7 ma toms canyon 35 7 ma montagnais 51 ma and one multiring impact basin chicxulub 65 ma are currently known to be buried beneath modern continental shelves all occur on the passive atlantic margin of north america in regions extensively explored by seismic reflection surveys in the search for oil and gas reserves we limit our discussion herein to the three youngest structures these craters were created by submarine impacts which produced many structural and morphological features similar in construction composition and variability to those documented in well preserved subaerial and planetary impact craters the subcircular chesapeake bay diameter 85 km and ovate montagnais diameter 45 50 km structures display outer rim scarps annular troughs peak rings inner basins and central peaks similar to those incorporated in the widely cited conceptual model of complex impact craters these craters differ in several respects from the model however for example the montagnais crater lacks a raised lip on the outer rim the chesapeake bay crater displays only small remnants of a raised lip and both craters contain an unusually thick body of impact breccia the subtriangular toms canyon crater diameter 20 22 km on the other hand contains none of the internal features of a complex crater nor is it typical of a simple crater it displays a prominent raised lip on the outer rim but the lip is present only on the western side of the crater in addition each of these craters contains some distinct features which are not present in one or both of the others for example the central peak at montagnais rises well above the elevation of the outer rim whereas at chesapeake bay the outer rim is higher than the central peak the floor of the toms canyon crater is marked by parallel deep troughs and linear ridges formed of sedimentary rocks whereas at chesapeake bay the crater floor contains concentric faults and compression ridges formed in rocks of the crystalline basement the chesapeake bay crater is distinguished further by its cluster of at least 23 adjacent secondary craters the north american tektite strewn field a widespread deposit of distal ejecta is thought to be derived from the chesapeake bay impact perhaps with a small contribution from the toms canyon impact no ejecta field is known to be associated with the montagnais impact no immediate major extinction event is directly linked to any of these three impacts there is evidence however that the chesapeake bay and toms canyon impacts helped initiate a long term pulse of warm global climate whose eventual dissipation coincided with an early oligocene mass extinction event 2 ma after the impacts

long term base cation weathering rates in forested catchments of the canadian shield abstract a number of methods exist for estimating mineral weathering rates yet quantifying this crucial biogeochemical reaction in the field has always been a challenge because mineral weathering takes place in open systems whose initial state conditions are not well known and where material inputs and outputs vary in time and space here using the soil profile mass balance method we estimated long term weathering rates of base cations bcs for a set of 21 watersheds and we investigated the links between environmental variables and soil properties and the spatial variation in bc weathering rates in the study area the watersheds are located in southern quebec and vary with respect to hydro climatic conditions soil properties and forest cover average long term estimates of annual bc weathering rates for the study area were 0 16 0 10 0 09 and 0 06 kmolc ha 1 year 1 for ca mg na and k respectively overall redundancy analysis rda indicated that soil surface area percent deciduous canopy cover elevation as well as soil albite content were the most significant variables explaining together 56 of the total variation in bc weathering rates consistent with previous findings our results showed that beside soil properties climate related environmental variables played key roles in determining the variability of bc weathering rates in the study area in the context of global climate change more insights are needed for a better understanding of the effects of discrete climatic variables on bc weathering rates

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

erosional equilibrium and disequilibrium in the sierra nevada inferred from cosmogenic 26al and 10be in alluvial sediment we used cosmogenic 26 al and 10 be in stream sediment to measure landscape scale erosion rates for topographically diverse catchments at seven sierra nevada sites at three sites erosion rates and hillslope gradients are strongly correlated increasing with proximity to fault scarps and river canyons which appear to have accelerated local base level lowering rates and thus increased catchment erosion rates by up to 15 fold at four other sites far from fault scarps and river canyons erosion rates are much more uniform and less sensitive to average hillslope gradient our measurements show that contrasts in landscape erosion rates cannot be inferred from hillslope gradients alone because landscapes can evolve toward a state of erosional equilibrium in which steep and gentle slopes erode at similar rates

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

end cretaceous cooling and mass extinction driven by a dark cloud encounter abstract we have identified iridium in an 5 m thick section of pelagic sediment cored in the deep sea floor at site 886c in addition to a distinct spike in iridium at the k pg boundary related to the chicxulub asteroid impact we distinguish the contribution of the extraterrestrial matter in the sediments from those of the terrestrial matter through a co ir diagram calling it the extraterrestrial index f ex this new index reveals a broad iridium anomaly around the chicxulub spike any mixtures of materials on the surface of the earth cannot explain the broad iridium component on the other hand we find that an encounter of the solar system with a giant molecular cloud can aptly explain the component especially if the molecular cloud has a size of 100 pc and the central density of over 2000 protons cm 3 kataoka et al 2013 2014 pointed that an encounter with a dark cloud would drive an environmental catastrophe leading to mass extinction solid particles from the hypothesized dark cloud would combine with the global environment of earth remaining in the stratosphere for at least several months or years with a sunshield effect estimated to be as large as 9 3 w m 2 the dark cloud would have caused global climate cooling in the last 8 myr of the cretaceous period consistent with the variations of stable isotope ratios in oxygen barrera and huber 1990 li and keller 1998 barrera and savin 1999 li and keller 1999 and strontium barrera and huber 1990 ingram 1995 sugarman et al 1995 the resulting growth of the continental ice sheet also resulted in a regression of the sea level the global cooling which appears to be associated with a decrease in the diversity of fossils eventually led to the mass extinction at the k pg boundary

coupling chemical weathering with soil production across soil mantled landscapes soil covered upland landscapes constitute a critical part of the habitable world our understanding of how they evolve as a function of different climatic tectonic and geological regimes is important across a wide range of disciplines and depends in part on understanding the links between chemical and physical weathering processes extensive previous work has shown that soil production rates decrease with increasing soil column thickness but chemical weathering rates were not measured here we examine a granitic soil mantled hillslope at point reyes california where soil production rates were determined using in situ produced cosmogenic nuclides 10be and 26al and we quantify the extent as well as the rates of chemical weathering of the saprolite from beneath soil from across the landscape we collected saprolite samples from the base of soil pits and analysed them for abrasion ph as well as for major and trace elements by x ray fluorescence spectroscopy and for clay mineralogy by x ray diffraction spectroscopy our results show for the first time that chemical weathering rates decrease with increasing soil thickness and account for 13 to 51 per cent of total denudation we also show that spatial variation in chemical weathering appears to be topographically controlled weathering rate decreases with slope across the divergent ridge and increases with upslope contributing area in the convergent swale furthermore to determine the best measure for the extent of saprolite weathering we compared four different chemical weathering indices the vogt ratio the chemical index of alteration cia parker s index and the silicon aluminium ratio with saprolite ph measurements of the cia were the most closely correlated with saprolite ph showing that weathering intensity decreases linearly with an increase in saprolite ph from 4 7 to almost 7 data presented here are among the first to couple directly rates of soil production and chemical weathering with how topography is likely to control weathering at a hillslope scale copyright 2006 john wiley sons ltd

effects of physical erosion on chemical denudation rates a numerical modeling study of soil mantled hillslopes article i nfo many biogeochemical and earth surface processes depend critically on chemical weathering the immediate products of chemical weathering are present as solutes and secondary minerals in groundwater soils and streams and form the nutritional foundation for terrestrial biogeochemistry chemical weathering also contributes to physical erosion by weakening bedrock and producing easily erodible regolith and as the primary long term sink for atmospheric co2 it modulates earth s long term climate via the greenhouse effect long term chemical denudation rates on soil mantled hillslopes can be estimated from cosmogenic radionuclide crn concentrations in soil borne quartz and the enrichment of a chemically inert tracer in soil relative to its parent bedrock a technique that inherently assumes steady physical erosion over the timescale of crn accumulation we present a numerical model that computes changes in soil mineralogy and crn concentrations under time varying physical erosion rates and we use this model to assess the accuracy of the crn based technique for estimating chemical denudation rates in non steady conditions our modeling results suggest that crn based estimates of chemical denudation rates closely resemble actual chemical denudation rates averaged over the timescale of crn accumulation even during large amplitude and long period oscillations in physical erosion rates for example this model predicts that when physical erosion rates fluctuate sinusoidally by 50 of their mean over any period in time crn based estimates of chemical denudation rates should differ from actual chemical denudation rates by less than 15 our model also implies that chemical denudation rates should approach zero both when physical erosion rates approach zero because soluble minerals become depleted in the soil and when physical erosion rates approach the maximum soil production rate because soil thickness approaches zero modeled chemical denudation rates thus reach a maximum at intermediate physical erosion rates if this relationship holds in nature it implies that in rapidly eroding regions further increases in physical erosion rates e g due to increases in tectonic uplift rates may not necessarily lead to faster chemical denudation on soil mantled hillslopes published by elsevier b v

macrostratigraphy of north america abstract the geological record is a three dimensional mosaic of gap bound rock bodies that at any given scale of temporal resolution were emplaced more or less continuously at any geographic location the irregular alternation of processes responsible for the formation and destruction of rock bodies results in the preservation of hiatus bound rock packages that have a distinct time of first occurrence base or oldest portion a time of last occurrence top or youngest portion and a suite of defining characters e g lithologies thickness fossils etc many important aspects of the geologic record can be quantified by compiling the stratigraphic ranges of such gap bound rock packages these include the quantity and spatial and temporal distribution of preserved rock rates of rock formation sequence stratigraphic architecture and area weighted rates of expansion and contraction erosional truncation of rock emplacement settings this approach to characterizing the rock record is scalable per

ants as a powerful biotic agent of olivine and plagioclase dissolution the biotic enhancement of ca mg silicate weathering has helped maintain earth s habit ability over geological time scales by assisting in the gradual drawdown of atmospheric co 2 25 years of in situ measurements of ca mg silicate mineral dissolution by ants termites root mats bare ground and a control reveals ants to be one of the most powerful biotic weathering agents yet recognized six sites in arizona and texas usa indicate that eight different ant species enhance mineral dissolution by 50 300 over controls a comparison of extracted soil at a 50 cm depth in ant colonies and adjacent bare ground shows a gradual accumulation of caco 3 content for all eight ant species over 25 yr ants thus have potential to provide clues on how to enhance contemporary carbon sequestration efforts to transform ca mg sili cates and co 2 into carbonate given that ants underwent a great diversification and biomass expansion over the cenozoic a speculative implication of this research is that ant enhance ment of ca mg silicate dissolution might have been an influence on cenozoic cooling

k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia abstract woodleigh is a recently discovered impact structure with a diameter of 120 km and thereby represents the third largest proven phanerozoic impact structure known after morocweng and chicxulub k ar isotopic studies of fine grained authigenic illitic clay minerals

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

a granulometry and secondary mineral fingerprint of chemical weathering in periglacial landscapes and its application to blockfield origins abstract a review of published literature was undertaken to determine if there was a fingerprint of chemical weathering in regoliths subjected to periglacial conditions during their formation if present this fingerprint would be applied to the question of when blockfields in periglacial landscapes were initiated these blocky diamicts are usually considered to represent remnants of regoliths that were chemically weathered under a warm neogene climate and therefore indicate surfaces that have undergone only a few metres to a few 10s of metres of erosion during the quaternary based on a comparison of clay and silt abundances and secondary mineral assemblages from blockfields other regoliths in periglacial settings and regoliths from non periglacial settings a fingerprint of chemical weathering in periglacial landscapes was identified a mobile regolith origin under at least seasonal periglacial conditions is indicated where clay 0 5 silt 8 across a sample batch this contrasts with a mobile regolith origin under non periglacial conditions which is indicated where clay 0 5 silt 6 across a sample batch with clay 0 5 silt 8 in at least one sample a range of secondary minerals which frequently includes interstratified minerals and indicates high local variability in leaching conditions is also commonly present in regoliths exposed to periglacial conditions during their formation clay silt ratios display a threshold response to temperature related to the freezing point of water but there is little response to precipitation or regolith residence time lithology controls clay and silt abundances which increase from felsic through intermediate to mafic compositions but does not control clay silt ratios use of a sedigraph or coulter counter to determine regolith granulometry systematically indicates lower clay abundances and intra site variability than use of a pipette or hydrometer in contrast to clay silt ratios secondary mineral assemblages vary according to regolith residence time temperature and or precipitation a microsystems model is invoked as a conceptual framework in which to interpret the concurrent formation of the observed secondary mineral ranges according to the fingerprint of chemical weathering in periglacial landscapes there is generally no evidence of blockfield origins under warm neogene climates nearly all blockfields appear to be a product of quaternary physical and chemical weathering a more dominant role for periglacial processes in further bevelling elevated low relief non glacial surface remnants in otherwise glacially eroded landscapes is therefore indicated

a method to estimate the total magnetization direction from a distortion analysis of magnetic anomalies1 knowledge of the declination and inclination of the total and induced magnetization vectors is normally required for the interpretation and analysis of magnetic anomalies a new method of estimating the direction of the total magnetization vector of magnetized rocks from magnetic anomalies is proposed the unknown declination and inclination d t and i t can be found by applying a reduction to the pole operator to the measured anomalies for different couples of total magnetization direction parameters d t and i t and by observing the variation of the anomaly minimum as a function of both d t and i t and d t are estimated using the maximum of this function comparing our method to previous methods one advantage is that our estimates are not zero level dependent furthermore the method allows inclinations to be well estimated with the same accuracy as declinations finally declinations are not underestimated our method is applied to a real case and meaningful results are obtained it is shown that the feasibility of the method is improved by removing the low frequency components

evaluation of three methods for estimating the weathering rates of base cations in forested catchments abstract this paper investigates three techniques that are commonly used to generate estimates of base cation bc weathering rates i e the profile mass balance pedon the watershed input output budget watershed and the profile model model these methods were compared for their relative performance in estimating bc weathering rates for 21 watersheds located in southern quebec that vary with respect to hydro climatic conditions soil properties and forest cover average total bc weathering rates for the 21 watersheds were 0 41 0 09 se 1 20 0 17 and 1 71 0 22 kmol c ha 1 yr 1 for pedon watershed and model respectively passing and bablok regression analysis demonstrated good agreement between watershed and model regression formula watershed 0 08 0 74 model with 95 ci for intercept 1 13 0 25 and for slope 0 40 1 43 while poorer agreements were observed between these two methods and pedon contrary to the watershed and the model methods bc weathering rates obtained with pedon were not significantly associated with the spatial variation of the soil calcite content and of the size of the soil exchangeable bc pools we hypothesized that in the calcite containing watersheds the performance of pedon was negatively impacted by environmental conditions that favored the partial dissolution and leaching of the calcite contained in the initial parent material including in the material situated at the base of the profile c horizon

spatial variations in chemical compositions along langtang narayani river system in central nepal surface water samples were collected from langtang lirung glacier outlet point to the narayani river system in central nepal in order to investigate the role of elevation in the variation of chemistry along the drainage networks the chemistry of langtang narayani river system was dominated by sulfide oxidation coupled with carbonate dissolution and weathering of silicate minerals calcium and magnesium concentrations were relatively higher than other cations and the sum of both species strongly correlated with alkalinity supporting the dissolution of carbonate and dolomite as the dominant source for these ions aluminosilicate minerals primarily as albite and anorthite appeared as dominant silicate minerals within the drainage basin bisiallitization was the dominant type of weathering within the entire drainage system hydrogen ion concentration was lower in the low elevation sites than in high elevation sites reflecting the more consumption of carbon dioxide in the low elevation sites due to enhanced chemical weathering rates furthermore major solutes like sum of base cations silicon as well as alkalinity increased in concentration in the lower elevation sites all regulating factors appeared to be directly related to elevation and hence elevation appeared to be the prime factor for the variation in chemical species along the langtang narayani river system

weak influences of climate and mineral supply rates on chemical erosion rates measurements along two altitudinal transects in the idaho batholith 1 chemical weathering promotes regolith production physical erosion nutrient supply and drawdown of atmospheric co2 although mineral dissolution rates show dependences on temperature and moisture in theoretical models and laboratory experiments effects of climate on chemical erosion rates in nature remain uncertain in part because variations in physical erosion rates and lithology can obscure climatic signals in chemical erosion rate measurements here we present new estimates of millennial scale chemical and physical erosion rates along two altitudinal transects in the idaho batholith our measurements suggest that chemical erosion rates and the extent of regolith chemical alteration are insensitive to mean annual regolith temperature even though the study sites span a wide range in temperatures 4 8 c 10 9 c these data also suggest that rates of chemical erosion and the extent of regolith chemical alteration are weakly correlated with the annual duration of high regolith moisture conditions lastly our measurements show that the sampled regoliths tend to grow more chemically weathered with increasing regolith residence time implying that weathering at these sites is primarily limited by mineral dissolution kinetics rather than mineral supply rates to the regolith the lack of strong control by mineral supply means that chemical erosion rates at these sites could potentially vary in response to climatic factors such climatically driven variation in chemical erosion rates is not obvious in our data however instead our measurements suggest that chemical erosion rates at these sites are insensitive to mean annual regolith temperature and only weakly dependent on the length of the wet season

long term average mineral weathering rates from watershed geochemical mass balance methods using mineral modal abundances to solve more equations in more unknowns abstract the number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations sio 2 al na k mg and ca analyses of these species in water are common but following matrix algebraic methods limits the number of weathering rates that can be calculated to six for the brubaker run watershed located in the northern piedmont physiographic province of pennsylvania usa long term 10 3 10 6 year watershed chemical flux losses have been determined using 10 be derived total denudation rates and zirconium normalized total chemical concentrations from bedrock and soils chemical flux losses calculated from solid phase data have three advantages they 1 permit generation of a relatively large number of equations because both major and trace analyses are included 2 eliminate the need for many years of regular e g weekly sampling and chemical analyses of stream water and atmospheric precipitation and measurement of hydrologic parameters i e precipitation stream discharge etc and 3 long term weathering rate calculations need not address biomass for brubaker run eight minerals are involved in weathering the five primary minerals are ree rich epidote ankerite almandine spessartine garnet muscovite chlorite and the three secondary products are weathered muscovite kaolinite and gibbsite the long term average weathering rates of these minerals were calculated using the six major cations and two trace elements selected from rb sr ba la pr nd sm gd and dy despite having the eight equations needed geochemically reasonable weathering rates e g positive primary mineral rates that reflect destruction could not be achieved regardless of the two trace elements used in the mass balance calculations for brubaker run this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains with trace element stoichiometries in some minerals varying by as much as an order of magnitude because the trace elements are hosted by a relatively small number of minerals the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry ree rich epidote garnet and ankerite within the brubaker run watershed together host nearly all of the ca in the bedrock and completely dissolve at or near the weathering front consequently approximately all of the ca in bedrock is lost from the regolith in bedrock the mole percentages of ca hosted by ree rich epidote garnet and ankerite are 49 mol 4 mol and 43 mol respectively and are determined by the modal abundance of the mineral in the bedrock and its ca stoichiometry the weathering rates of ree rich epidote garnet and ankerite can be determined by distributing to each mineral that fraction of the total watershed ca flux loss for which it is responsible based on its mole percent ca in bedrock by using a base cation that is completely lost from the regolith and knowing the mole percentage of that element in the mineral s undergoing weathering additional equations may be added to the mass balance matrix we term this technique the flux distribution method the flux distribution method eliminates the need for additional equations established using trace elements based on the mineral weathering rates for the brubaker run watershed determined using the flux distribution method the rates at which the weathering front penetrated the bedrock the saprolitization rate are 4 5 m myr 1 and 6 5 m myr 1 for chlorite and muscovite respectively these measured long term average saprolitization rates compare very favorably with published theoretical values for the nearby northern maryland piedmont which range from 2 2 to 5 3 m myr 1

mineral specific chemical weathering rates over millennial timescales measurements at rio icacos puerto rico abstract mineral weathering plays a prominent role in many biogeochemical and geomorphological processes it supplies nutrients to soils and streams accelerates physical erosion by weakening bedrock and producing easily erodible soil and modulates earth s long term climate by drawing down atmospheric carbon dioxide we calculate mineral specific chemical weathering rates at two field sites in the rio icacos catchment puerto rico by combining new mineral abundance measurements from quantitative powder x ray diffraction xrd with existing measurements of i soil production rates from cosmogenic nuclides ii chemical alteration of the regolith from x ray fluorescence xrf and iii dust deposition rates the central purpose of this paper is to show that combining measurements of cosmogenic nuclides with xrf based geochemistry and xrd based mineralogy can in favorable cases provide weathering rates of abundant soluble mineral phases in actively eroding terrain to an accuracy of better than 20 of the mean even in places with high dust deposition rates mineral weathering at our two field sites is dominated by plagioclase at rates of 3274 575 mol ha 1 yr 1 and 3077 541 mol ha 1 yr 1 followed by hornblende at 187 71 mol ha 1 yr 1 and 308 93 mol ha 1 yr 1 within the uncertainty of our data all weathering of these primary minerals occurs below the saprolite soil interface our measurements imply that kaolinite production in saprolite is roughly 1 3 times faster than kaolinite weathering in the soil our results are the first to show that field measurements of cosmogenic nuclides xrf xrd and dust fluxes can be combined within the geochemical mass balance method to quantify long term mineral weathering rates even in locations with high dust deposition rates this implies that the mass balance method can be a valuable tool for quantifying the effects of climate vegetation tectonics and other factors on weathering rates of individual mineral phases

the chemical index of alteration cia as a proxy for climate change during glacial interglacial transitions in earth history abstract the glacial interglacial transitions of ice ages are associated with large rapid changes in climate which potentially can be reconstructed from stratigraphic profiles of the chemical index of alteration cia here we present a case study based on high resolution cia profiles of neoproterozoic glacial deposits from south china paleolatitude 30 n as a record of the climate transition at the termination of the sturtian glaciation 663 ma drillcore zk2115 gaodi deposit exhibits a progressive upsection increase in mean cia over a 50 m interval from 58 range 52 65 note all ranges are given as 16th 84th percentiles in the synglacial upper tiesi ao formation to 67 range 64 69 in the deglacial basal datangpo formation cap carbonates and to 68 range 66 70 in the postglacial datangpo formation black shales a coeval section from the lijiawan deposit also exhibits an increasing cia trend upsection within shallow water mn carbonate facies these cia patterns show no relationship to lithology and are interpreted to reflect climatic warming and intensified chemical weathering during the sturtian deglaciation similar large increases in postglacial cia values are seen in paleoproterozoic and late paleozoic successions and at least moderate increases in cia values are recorded during warm stages of the late quaternary e g the bolling allerod and holocene relative to the cold stages e g the last glacial maximum and younger dryas the rapidity of cia changes in quaternary systems 1 to 10 kyr suggests that weathering intensity may have changed at similarly rapid rates in more poorly dated deep time glacial successions with potential implications for snowball versus slushball models of cryogenian glaciations these examples show that cia can be a robust proxy for climate changes i e enhanced chemical weathering intensity during glacial interglacial transitions of both ancient and recent ice ages

quantifying effects of deep and near surface chemical erosion on cosmogenic nuclides in soils saprolite and sediment cosmogenic nuclides in rock soil and sediment are routinely used to measure denudation rates of catchments and hillslopes although it has been shown that these measurements are prone to biases due to chemical erosion in regolith most studies of cosmogenic nuclides have ignored this potential source of error here we quantify the extent to which overlooking effects of chemical erosion introduces bias in interpreting denudation rates from cosmogenic nuclides we consider two end member effects one due to weathering near the surface and the other due to weathering at depth near the surface chemical erosion influences nuclide concentrations in host minerals by enriching or depleting them relative to other more or less soluble minerals this increases or decreases their residence times relative to the regolith as a whole at depth where minerals are shielded from cosmic radiation chemical erosion causes denudation without influencing cosmogenic nuclide buildup if this effect is ignored denudation rates inferred from cosmogenic nuclides will be too low we derive a general expression termed the chemical erosion factor or cef which corrects for biases introduced by both deep and near surface chemical erosion in regolith the cef differs from the quartz enrichment factor of previous work in that it can also be applied to relatively soluble minerals such as olivine using data from diverse climatic settings we calculate cefs ranging from 1 03 to 1 87 for cosmogenic nuclides in quartz this implies that ignoring chemical erosion can lead to errors of close to 100 in intensely weathered regolith cef is strongly correlated with mean annual precipitation across our sites reflecting climatic influence on chemical weathering our results indicate that quantifying cefs is crucial in cosmogenic nuclide studies of landscapes where chemical erosion accounts for a significant fraction of the overall denudation copyright 2012 john wiley sons ltd

minimal climatic control on erosion rates in the sierra nevada california climate is widely thought to regulate erosion rates but the relationships among precipitation temperature and erosion rate have remained speculative because long term erosion rates have been difficult to measure we used cosmogenic nuclides to measure long term erosion rates at climatically diverse sites in the sierra nevada california spanning 20 180 cm yr in annual precipitation and 4 15 8c in mean annual temperature average erosion rates vary by only 2 5 fold across these sites and are not correlated with climate indicating that climate only weakly regulates nonglacial erosion rates in mountainous granitic terrain

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

direct effects of co2 and temperature on silicate weathering possible implications for climate control a critical uncertainty in models of the global carbon cycle and climate is the combined effect of organic activity temperature and atmospheric co2 on silicate weathering here we present new dissolution rates of anorthite and augite which indicate that silicate weathering in organic rich solutions is not directly affected by soil co2 but is very sensitive to temperature apparently co2 accelerates silicate weathering indirectly by fertilizing organic activity and the production of corrosive organic acids the weathering dependencies highlight the ability of silicate weathering to act as a global thermostat and damp out climate change when used as input in steady state carbon cycle and climate models

the role of climate and vegetation in weathering and clay mineral formation in late quaternary soils of the swiss and italian alps nteractions between climate and soil remain ambiguous particularly when silicate weathering and clay mineral formation and transformation rates are considered in relation to global climate changes recent studies suggest that climate affects weathering rates much less than previously thought here we show that the climate in the central european alps has a significant but indirect influence on the weathering of soils through vegetation the pattern of element leaching and mineral transformations is not only due to precipitation and temperature element leaching was greatest in subalpine forests near the timberline weathering is lessened at higher and lower altitudes vegetation therefore contributes significantly to weathering processes the highest accumulation of organic matter was found in climatically cooler sites subalpine range where the production of organic ligands which enhance weathering is greatest patterns of smectite formation and distribution had strong similarities to that of the elemental losses of fe and al r 0 69 p 0 01 or base cations r 0 58 p 0 05 higher precipitation rates and the production of organic chelating compounds in the soil promoted the appearance of smectites the relationship between climate element leaching fe al ca mg k na and smectite formation is strongly nonlinear and driven by the podzolisation process which is more pronounced near the timberline because of the bioclimatic constellation climate warming will probably in the future lead to a decrease in som stocks in the subalpine to alpine range because of more favourable conditions for biodegradation that would also affect weathering processes

integration of geochemical mass balance with sediment transport to calculate rates of soil chemical weathering and transport on hillslopes 1 we developed a process oriented hillslope soil mass balance model that integrates chemical and physical processes within hillslope soils the model explicitly factors that soil chemical weathering at any hillslope position is related to the flux of soil eroded from upslope as well as soil production from underlying bedrock the model was merged with measurements of soil elemental chemistry and cosmogenic radionuclide based saprolite to soil conversion rates along a 50 m transect of a semiarid granodiorite hillslope in the southeastern australian highlands inverse modeling results in the simultaneous quantification of the rates of soil chemical weathering and soil transport as a function of hillslope position soil chemical weathering rates per land surface area systematically varied along the transect from losses of 0 035 kg m 2 yr 1 on the ridge to gains of 0 035 kg m 2 yr 1 at the lowest slope position the mass loss via soil chemical weathering would have been overestimated by 40 if the impact of soil transport on soil chemistry was ignored the chemical mobility of elements combined with biological nutrient demand controlled the spatial redistribution of individual elements p and ca were preferentially retained relative to si al and fe within the hillslope base the calculated soil transport rate is linearly related to the product of soil thickness and slope gradient instead of slope alone soil residence time was determined by calculating the time length for a 3 dimensional box volume 1 m2 surface area soil thickness to be entirely removed by mass flux of soil transport 4 ka on the ridge to 0 9 ka at the hillslope base these soil residence times combined with soil chemical weathering rates indicate that a 1 m2 area of soil loses 1 800 kg via chemical weathering while passing through the upslope portion of the hillslope but that it regains 90 kg probably via clay precipitation and biological retention of cations during its passage through the lower segments of the transect this study provides a previously unrecognized linkage between physical soil transport and soil chemical weathering that have implications for hillslope evolution as well as biogeochemistry

assessment of shallow landslide susceptibility using artificial neural networks in jabonosa river basin venezuela landslides represent one of the most morphodynamic processes that affect the steep lands and may destroy croplands as well as urban and industrial development landslide risk analysis can help government agencies to select suitable locations for development schemes and plan mitigation measures in unstable landslide prone areas this study describes an approach for assessing the landslide risk potential mainly for shallow landslides with reference to jabonosa river basin in the venezuelan andes using artificial neural networks anns specifically a multilayer perceptron with backpropagation learning algorithm the approach developed uses a wide range of parameters of slope instability derived from digital elevation models dems remote sensing imagery and documentary data around 90 overall accuracy produced by the ann technique were found promising for future studies in landslide susceptibility zonation

cosmogenic nuclide derived rates of diffusive and episodic erosion in the glacially sculpted upper rhone valley swiss alps denudation rates of small tributary valleys in the upper rhone valley of the swiss central alps vary by more than an order of magnitude within a very small distance tens of kilometers morphometric data indicate two distinct erosion processes operate in these steep mountain valleys we determined the rates of these processes using cosmogenic beryllium 10 10be in pooled soil and stream sediment samples denudation in deep glacially scoured valleys is characterized by rapid non uniform processes such as debris flows and rock falls in these steep valleys denudation rates are 760 2100 mm kyr 1 in those basins which show minimal previous glacial modification denudation rates are low with 60 560 mm kyr 1 the denudation rate in each basin represents a binary mixture between the rapid non uniform processes and soil creep the soil production rate measured with cosmogenic 10be in soil samples averages at 60 mm kyr 1 mixing calculations suggest that the debris flows and rock falls are occurring at rates up to 3000 7000 mm kyr 1 these very high rates occur in the absence of baselevel lowering since the tributaries drain into the rhone trunk stream up stream of a knickzone the flux weighted spatial average of denudation rates for the upper rhone valley is 1400 mm kyr 1 which is similar to rock uplift rates determined in this area from leveling the pace and location of erosion processes are determined by the oscillation between a glacial and a non glacial state preventing the landscape from reaching equilibrium copyright 2010 john wiley sons ltd

solute geochemical mass balances and mineral weathering rates in small watersheds ii biomass nutrient uptake more equations in more unknowns and land use land cover effects abstract paired watersheds are used to develop a deciduous nutrient uptake stoichiometry the watersheds are those of the house rock run and the brubaker run located in the pennsylvania appalachian piedmont usa these two watersheds are nearly identical with respect to bedrock regolith climate geomorphology morphometry baseflow hydrology and type and successional stage of forest vegetation they only differ by the percentage of deciduous forest cover with house rock run having 59 and brubaker run having 76 from differences in their stream chemistries the biomass nutrient uptake stoichiometry of k1 0mg1 0ca1 4 was determined this stoichiometry applies to an aggrading deciduous biomass and differs from those previously used which were derived from net primary production npp data the difference may reflect that macronutrients in plant tissue may also originate from atmospheric inputs and or decomposing biomass although this stoichiometry may not be applied to all deciduous forest covered watersheds it is likely an improvement over a stoichiometry determined from npp data mass balance calculations of mineral weathering rates often suffer from the number of unknowns exceeding the number of equations to add equations to the mass balance matrices two methods are introduced the first method employs zr normalized bulk chemical compositions of bedrock and soil to calculate a mass transfer coefficient for chemical weathering the second approach uses chemical formulae and modal abundances of primary minerals undergoing complete dissolution during weathering both methods allow for calculation of weathering rate constraints without biomass and cation exchange influences these constraints serve as additional equations in the mass balance matrix this study finds that the watershed with the higher percentage of recently abandoned agricultural fields previously used for growing row crops has the higher chemical weathering rate the higher chemical weathering rate reflects greater runoff resulting from reduced evapotranspiration

quantitative geochemical approach to pedogenesis importance of parent material reduction volumetric expansion and eolian influx in lateritization abstract using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock through calculation of net mass fluxes through the laterite soils in mali west africa we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density contributes only a very minor fraction of the enrichment of al fe si and au instead we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features at the same sample depths that accumulation reaches extreme values we show that volumetric expansion in excess of 200 has occurred locally we infer that these spatially coincidental zones of mass influx of rock forming metals fe al si and also au with dilational hyperstrains result from a mutually reinforcing mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink swell cycles and root growth and decay we propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded we speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids consequently progressive hyperstrains accumulate above this critical depth by the long term influence of a proposed translocational wedge of chemically resistant minerals against which numerous generations of plant roots have exerted stresses eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted instead the source region is largely the existing regolith column itself which releases local material and in addition is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat

weathering the escarpment chemical and physical rates and processes south eastern australia differences in chemical weathering extent and character are expected to exist across topographic escarpments due to spatial gradients of climatic and or tectonic forcing the passive margin escarpment of south eastern australia has a debated but generally accepted model of propagation in which it retreated within 40 ma to near its current position following rifting between australia and new zealand 85 100 ma before present we focus on this escarpment to quantify chemical weathering rates and processes and how they may provide insight into scarp evolution and retreat we compare chemical weathering extents and rates above and below the escarpment using a mass balance approach coupling major and trace element analyses with previous measurements of denudation rates using cosmogenic nuclides 10be and 26al we find a slight gradient in saprolite chemical weathering rate as a percentage of total weathering rate across the escarpment the lowlands area encompassing the region extending from the base of the escarpment to the coast experiences a greater extent of chemical weathering than the highland region above the escarpment percents of denudation attributable to saprolite weathering average 57 6 and 47 7 at low and high sites respectively furthermore the chemical index of alteration cia a ratio of immobile to mobile oxides in granitic material that increases with weathering extent have corresponding average values of 73 7 3 9 and 65 5 3 4 indicating lower extents of weathering above the escarpment finally we quantify variations in the rates and extent of chemical weathering at the hillslope scale across the escarpment to suggest new insight into how climate differences and hillslope topography help drive landscape evolution potentially overprinting longer term tectonic forcing copyright 2009 john wiley sons ltd

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

competition between erosion and reaction kinetics in controlling silicate weathering rates article i nfo weathering rates of minerals may be limited by the reaction kinetics of the constitutive minerals or the supply of fresh minerals delivered to the near surface by denudation we use a model of denudation and reaction kinetics to analyze the relative importance of each in moderating silicate weathering fluxes in different erosional environments for different commonly occurring silicate minerals we find that minerals that reside in the near surface weathering zone for far longer than the reaction takes to complete as is the case with slow denudation produce weathering fluxes that respond most strongly to changes in denudation rate conversely when transit through the weathering zone is rapid relative to a given mineral s weathering timescale the weathering responds most vigorously to changes in the reaction kinetics based on field and laboratory measured values of silicate weathering kinetic constants observed weathering zone thicknesses in relation to denudation rates and denudation rates inferred from topography it appears that fresh mineral supply may play a sub equal or dominant role in moderating silicate weathering fluxes for four mineral phases considered if correct this suggests that the concentration of atmospheric co2 which is regulated by silicate weathering over geologic timescales may depend on those factors that control long term erosion rates across earth s surface

spatially averaged long term erosion rates measured from in situ produced cosmogenic nuclides in alluvial sediment spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16 000 years these two independent estimates agree within one standard error demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long term erosion rates using this technique we show that long term erosion rates are an exponential function of average hillslope gradient at these sites

spatial variations in chemical weathering and co2 consumption in nepalese high himalayan catchments during the monsoon season the major ion chemistry of the marsyandi basin and six of its tributaries in the nepalese himalaya have been investigated during the monsoon months of 2002 weekly water samples taken at 10 river monitoring stations in the annapurna watershed over the course of 4 months provide chemical weathering data for the region at an unprecedented temporal and spatial resolution the river chemistry of all but one basin is heavily dominated by carbonate weathering which compared to silicate weathering contributes 80 to 97 of the total solute load this prevalence is due to a combination of a intrinsically faster dissolution kinetics of carbonates b relatively high runoff and c glacial meltwater and low temperatures at high altitudes resulting in enhanced carbonate solubilities monitoring stations with headwaters in the tethyan sedimentary series tss are particularly carbonate rich and slightly supersaturated with respect to calcite through half of the monsoon season silicate weathering in the tss is driven largely by sulfuric acid and therefore does not contribute significantly to the drawdown of atmospheric co2 with respect to the tributaries in the greater himalayan sequence ghs carbonate weathering is practically as predominant as for the tss in spite of the largely felsic lithology of the ghs relative to the tss the primary proton source in the ghs has shifted with at least 80 of the protons derived from carbonic acid averaged over the whole field area the co2 fluxes based on silicate derived ca and mg are considerably lower than the global average assuming that this study area is representative of the entire range we conclude that in situ weathering of the high himalayas does not represent a significant sink of atmospheric carbon dioxide despite the presence of a watershed south of the ghs that is characterized by a four times higher co2 consumption rate than the global average silicate weathering rates of all basins appear to be climate controlled displaying a tight correlation with runoff and temperature given the extremely low chemical weathering under transport limited conditions in high altitude crystalline terrains outside of the monsoon season this would result in virtually no chemical exhumation for 2 3 of the year in such a cold and arid climate north of the rain shadow cast by the high himalayas 2009 elsevier ltd all rights reserved

resolving the integral connection between pedogenesis and landscape evolution abstract soil is central in the terrestrial ecosystem linking and providing feedback responses to the other components i e water atmosphere and vegetation however the role of soil in landscape evolution is usually not well acknowledged in modeling landscape evolution soil is only treated as a residue of weathering that is transported and redistributed along the hillslope weathering is considered as a process that produces clays and generates unconsolidated materials available for erosion while pedology has been debating the form of qualitative factorial models for 75 years models for soil water heat solute gas and chemical reactions in a profile have matured as soils are distributed continuously in three dimensions across landscapes the profile models need to consider lateral fluxes this review outlines the role of soil in landscape modeling first we review the role of soil in the current landscape evolution models we then review data and models on soil weathering rates and transport processes we discuss soil profile models that simulate soil formation processes and combined soil landscape evolution models finally we discuss how the models can be tested and validated in the real world and suggest how both soil scientists and landscape modelers can work together to address the grand challenges in modeling earth surface processes

chemical weathering in a tropical watershed luquillo mountains puerto rico i long term versus short term weathering fluxes abstract the pristine rio icacos watershed in the luquillo mountains in eastern puerto rico has the fastest documented weathering rate of silicate rocks on the earth s surface a regolith propagation rate of 58 m ma 1 calculated from iso volumetric saprolite formation from quartz diorite is comparable to the estimated denudation rate 25 50 ma 1 but is an order of magnitude faster than the global average weathering rate 6 ma 1 weathering occurs in two distinct environments plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith these environments produce distinctly different water chemistries with k mg and si increasing linearly with depth in saprolite porewaters and with stream waters dominated by ca na and si such differences are atypical of less intense weathering in temperate watersheds porewater chemistry in the shallow regolith is controlled by closed system recycling of inorganic nutrients such as k long term elemental fluxes through the regolith e g si 1 7 10 8 moles m 2 s 1 are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface 200 ma mass losses attributed to solute fluxes are determined using a step wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water pressure heads decrease with depth in the shallow regolith 2 03 m h2o m 1 indicating that both increasing capillary tension and graviometric potential control porewater infiltration interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr 1 at average field moisture saturation which is comparable with libr tracer tests and with base discharge from the watershed short term weathering fluxes calculated from solute chemistries and infiltration rates e g si 1 4 10 8 moles m 2 s 1 are compared to watershed flux rates e g si 2 7 10 8 moles m 2 s 1 consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation temperature and vegetation over the last several hundred thousand years have not significantly impacted weathering rates in the luquillo mountains of puerto rico this has important ramifications for tropical environments and global climate change

the relationship between tectonic uplift and chemical weathering rates in the washington cascades field measurements and model predictions tectonic uplift is one of the key factors controlling the supply of material to weathering environments at present however there is debate about whether tectonics or climate plays a larger role in controlling chemical weathering rates on a global scale over geologic time we measured riverine weathering fluxes from twelve catchments along the skykomish river in the washington cascades where long term exhumation rates have been constrained by u th he thermochronom etry to examine the effect of tectonic uplift on chemical weathering rates we show that in the western washington cascades dissolved si fluxes increase from approxi mately 1900 to 3000 mol ha 1 yr 1 from west to east across the range and are correlated with rock exhumation rates we use dissolved si flux data from these catchments depth to bedrock measurements and long term exhumation rates to test a steady state weathering model that describes the relative importance of reaction kinetics and rock supply on weathering rates characterization of study basins with respect to two key variables in this model weathering zone depth and exhumation rate allows us to examine the relative effects of climate and tectonics on chemical weathering by analyzing the ratio of the timescale of reaction kinetics and the residence time in the weathering zone evaluation of this model shows that chemical weathering rates in the cascades are controlled by the rate of rock erosion and factors that influence the depth of the active weathering zone however the model predicts that in tectonically inactive areas the supply of material limits the overall rate of chemical weathering while in active tectonic or erosional environments the rate of supply exceeds the ability of a system to weather that material resulting in a landscape in which weathering rates are expected to respond strongly to changes in climatic conditions

the critical role of climate and saprolite weathering in landscape evolution landscapes evolve in response to external forces such as tectonics and climate that influence surface processes of erosion and weathering internal feedbacks between erosion and weathering also play an integral role in regulating the landscapes response our understanding of these internal and external feedbacks is limited to a handful of field based studies only a few of which have explicitly examined saprolite weathering here we report rates of erosion and weathering in saprolite and soil to quantify how climate influences denudation by focusing on an elevation transect in the western sierra nevada mountains california we use an adapted mass balance approach and couple soil production rates from the cosmogenic radionuclide crn 10be with zirconium concentrations in rock saprolite and soil our approach includes deep saprolite weathering and suggests that previous studies may have underestimated denudation rates across similar landscapes along the studied climate gradient chemical weathering rates peak at middle elevations 1200 2000 m averaging 112 3 9 7 t km 2 y 1 compared to high and low elevation sites 46 8 5 2 t km 2 y 1 measured weathering rates follow similar patterns with climate as those of predicted silica fluxes modeled using an arrhenius temperature relationship and a linear relationship between flux and precipitation furthermore chemical weathering and erosion are tightly correlated across our sites and physical erosion rates increase with both saprolite weathering rates and intensity unexpectedly saprolite and soil weathering intensities are inversely related such that more weathered saprolites are overlain by weakly weathered soils these data quantify exciting links between climate weathering and erosion and together suggest that climate controls chemical weathering via temperature and moisture control on chemical reaction rates our results also suggest that saprolite weathering reduces bedrock coherence leading to faster rates of soil transport that in turn decrease material residence times in the soil column and limit soil weathering copyright 2009 john wiley sons ltd

cosmogenic nuclide evidence for low weathering and denudation in the wet tropical highlands of sri lanka 1 some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep humid tropical highlands of sri lanka the total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites bedrock from mountain crests and bedrock from inselbergs denudation rates are in the range of 5 30 t km 2 yr 1 2 11 mm ky 1 these rates average denudation over the last 50 250 ky weathering exports in rivers draining the mountainous central highlands show that silicate weathering rates are also low varying from 5 to 20 t km 2 yr 1 today 2 7 mm ky 1 but they represent a significant fraction of the total denudation all these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief temperatures and precipitation we speculate that the high relief in sri lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup it is possible that throughout this history hillslopes where not exposing bare bedrock were protected by thick weathered profiles such clay rich layers would inhibit silicate weathering by shielding bedrock from weathering agents in the absence of landscape rejuvenation physical erosion rates are low and fresh mineral surfaces are not being supplied the observation that wet steep tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long term controls of atmospheric co2 budgets high temperature and precipitation which are much invoked though controversial agents for silicate dissolution and co2 drawdown become ineffective in promoting weathering in areas that are not tectonically active

modulation of erosion on steep granitic slopes by boulder armoring as revealed by cosmogenic 26al and 10be cosmogenic 26 al and 10 be in quartz from boulders bedrock and sandy sediment from 21 small watersheds in the diamond mountains batholith ca usa and two small watersheds from the nearby fort sage mountains confirm that exposed granitic bedrock and boulders erode more slowly than the catchments in which they are found exposed bedrock and boulders are more abundant on steep slopes and may play an important role in regulating mountain erosion rates rapid transport of fine sediment on steep slopes exhumes resistant corestones which accumulate on the surface the resulting boulder lag apparently shields the underlying soil and bedrock from erosion even when the bedrock is deeply weathered and friable where steep slopes have an abundant boulder lag they erode as slowly as gentler slopes nearby in contrast steep slopes lacking a boulder lag erode much more quickly than gentle slopes boulder armoring can modulate hillslope erosion such that erosion rates of summits steep mountain flanks and gentle footslopes are indistinguishable thus permitting local relief and steep mountain slopes to persist for long periods of time fl 2001 elsevier science b v all rights reserved

estimating rates of denudation using cosmogenic isotope abundances in sediment we propose as a testable hypothesis a basin scale approach for interpreting the abundance of in situ produced cosmogenic isotopes an approach which considers explicitly both the isotope and sediment flux through a drainage basin unlike most existing models which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on earth s surface our model is designed for interpreting isotope abundance in sediment because sediment is a mixture of materials in favourable cases derived from throughout a drainage basin we suggest that measured isotope abundances may reflect spatially averaged rates of erosion we investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed

strong tectonic and weak climatic control of long term chemical weathering rates the relationships among climate physical erosion and chemical weathering have remained uncertain because long term chemical weathering rates have been difficult to measure here we show that long term chemical weathering rates can be measured by combining physical erosion rates inferred from cosmogenic nuclides with dissolution losses inferred from the rock to soil enrichment of insoluble elements we used this method to measure chemical weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates chemical weathering rates correlate strongly with physical erosion rates but only weakly with climate implying that by regulating erosion rates tectonic uplift may significantly accelerate chemical weathering rates in granitic landscapes

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

responses of chemical erosion rates to transient perturbations in physical erosion rates and implications for relationships between chemical and physical erosion rates in regolith mantled hillslopes abstract understanding the relationship between chemical erosion rates w and physical erosion rates e is of wide interest due to their roles in driving landscape evolution supplying nutrients to soils and streams and modulating the global carbon cycle measured relationships between w and e vary around the globe with some regions exhibiting positive correlations between w and e some negative correlations and others no correlation within uncertainty here we use a numerical model for mineral weathering in well mixed ridgetop regolith to explore how complex w e relationships can be generated by simple transient perturbations in e we show that a gaussian perturbation in e can produce positive or negative responses in w and can result in a variety of hysteresis loops clockwise counterclockwise or figure eight in plots of w against e the nature of the transient response depends on the shape of the steady state w e relationship which is set by regolith mineralogy and the ratio of e to the maximum possible regolith production rate the response time of w is controlled by the response time of soluble mineral concentrations at low e where soluble mineral concentrations are low and by the response time of regolith thickness at high e where regolith thickness is low these complex w e relationships arise in the absence of variations in climate and lithology which suggests that transients may account for some of the observed differences in w e relationships among field sites even among sites that share the same climate and lithology

quantification of chemical weathering rates across an actively eroding hillslope abstract chemical weathering rates for landscapes are difficult to quantify because the timescales over which weathering occurs are often unknown in this study we use timescales defined by prior cosmogenic nuclide analyses and a suite of geochemical measurements to calculate weathering rates in saprolite and soil and to determine how these rates vary across an eroding hillslope we also estimate the relative contributions of solute and erosional mass loss to landscape lowering analyses were conducted on a soil mantled hillslope developed on a granodiorite pluton in southern nsw australia mass loss in solution accounts for 35 to 55 of the total mass loss from the hillslope saprolite close to the soil saprolite boundary is less weathered at the ridge than at distance from the ridge the calculated flux of silica from the saprolite is 5 tons km 2 yr 1 and does not vary with distance from the ridge or overlying soil thickness the assumption of steady state but spatially variable erosion rates with a temporally constant thickness of soil and saprolite allows us to calculate the downslope variation in the rate of solute losses soil silica weathering rates initially increase from 12 tons km 2 yr 1 at the ridge crest to 20 tons km 2 yr 1 on the convex region of the hillslope and then decrease to 13 tons km 2 yr 1 by the time soils have been transported 45 50 m soils closer to the surface are more weathered than deeper soils this geochemical stratification suggests that the soil column does not become thoroughly vertically mixed as soils move down slope dependence of solute loss on transport distance may explain the previously noted discrepancy between the observed soil thickness and that predicted based on curvature variation of solute loss with transport distance may also contribute to the linear dependence of soil thickness on distance from the ridge

the convenient fiction of steady state soil thickness abstract steady state regolith or soil thickness ssst whereby surface removals are approximately balanced by production of new soil by bedrock weathering is a common assumption in most models of hillslope and landscape evolution ssst is also a fundamental assumption in the use of cosmogenic radionuclides crn to estimate erosion and weathering rates the steady state concept is based on feedbacks between soil thickness and weathering at the base of the regolith such that sometimes after an optimal or threshold thickness is achieved thicker soils lead to lower weathering rates and vice versa ssst is thus only applicable to soils formed chiefly from weathering of the underlying bedrock where sufficient time has elapsed for regolith accumulation and where effects of processes other than weathering and surface removals on thickness are negligible even within this domain the widespread occurrence of deep weathering profiles regolith stripping and inherited regolith features makes ssst problematic as a conceptual model for pedogenesis or weathering profile development the ratio of soil thickness to total weathering profile thickness is proposed as a simple index of steady state steady state profiles formed on weathered bedrock should exhibit ratios close to unity data from the cumberland plateau region of eastern kentucky show that soil weathering profile ratios in shallow 20 m and generally significantly greater than soil thicknesses in the region suggesting ratios convenient fiction facilitating the use of some models and tools the potential pitfalls arise from the possibility that the utility of ssst as a convenient fiction in some contexts may be mistaken for a realistic representation of the dynamics of pedogenesis and weathering profile evolution