chemical weathering rates of silicate dominated lithological classes and associated liberation rates of phosphorus on the japanese archipelago implications for global scale analysis abstract lithology is an important characteristic of the terrestrial surface whose properties influence chemical weathering rates specifically non silicate minerals may contribute significantly to the weathering derived fluxes from silicate dominated lithological classes the japanese archipelago consists of predominantly silicate dominated lithologies with a high proportion of volcanics however the spatially explicit representation of chemical weathering rates remains difficult for such a large region because many controlling factors on chemical weathering rates are correlated with each other due to the spatial heterogeneity of lithology a multi lithological model approach to estimate spatially explicit chemical weathering rates for unmonitored areas is applied here to achieve this hydrochemical data of 381 catchments are used to train a set of models recognizing the contribution of a variety of proposed factors influencing chemical silicate rock weathering rates csrwr cations plus dissolved silica flux the monitored catchments cover 44 of the japanese archipelago cation chemical weathering rates excluding si are linearly correlated with csrwr and show outliers if basic volcanics or pyroclastic flows are present due to increased silica release rates lithology and runoff are identified as the strongest predictors for chemical weathering rates temperature and gradient of slope are of less relevance for the regional scale prediction while further proposed factors like soil properties or land cover are not identified as major predictors latter findings are partly attributed to geodata quality low variability of parameter values as well as spatial correlations of proposed controlling factors with lithology or runoff the calculated average csrwr of the archipelago is 25 t km 2 a 1 and ranges from 5 9 to 107 t km 2 a 1 in monitored catchments weathering rates per lithological class as a function of runoff can be grouped into three classes a pyroclastic flows showing the highest chemical weathering rates b alluvial deposits mixed sediments and basic to intermediate volcanics with medium rates and c metamorphics siliciclastic sediments acid volcanics acid plutonics and unconsolidated sediments other than alluvial deposits showing the lowest rates the recognition of lithogenic sulfur would add 9 7 to csrwr of considered catchments results suggest that the lithological classes acid volcanics and unconsolidated sediments contribute above average to the sulfur fluxes possible biases of this observation are discussed the contribution of ca fluxes from non silicate calcic minerals named ca excess ca fluxes in addition to silicate ca fluxes is about 10 of the csrwr on average and is attributed by a wide value range the calculated ratio ca excess to total ca fluxes from chemical weathering averages around 62 75 56 83 and 84 for the lithological classes acid plutonics metamorphics siliciclastic sediments mixed sediments and acid volcanics respectively this suggests a major ca contribution from non silicate calcic minerals for these lithological classes phosphorus release from rocks due to chemical weathering is estimated to be between 1 kg p km 2 a 1 and 390 kg p km 2 a 1 the p release patterns in dependence of runoff per lithological class are different from csrwrs due to differences of applied p content in rocks the identified spatial p release patterns suggest that the consideration of dynamic and spatially resolved p release rates by chemical weathering might improve ecosystem studies later findings may be of importance for analysing the influence of p release from rocks on the climate system via ecosystem functioning on geological time scales a first application of the p release model to the global scale suggests an annual release of 1 6 mt p 13 8 kg p km 1 a 1 by chemical weathering of silicate dominated lithological classes excluding carbonate sedimentary rocks

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

modeling of water rock interaction in the mackenzie basin competition between sulfuric and carbonic acids the chemical weathering of continental surfaces depends on many factors including lithology climate tectonics erosion soil or vegetation and also on the availability of acidity in the weathering zone in addition to atmospheric soil co2 sulfide oxidation may represent a significant source of protons for weathering reactions as shown previously for the rivers draining the mackenzie basin canada here a numerical model describing continental weathering reactions based on laboratory kinetic laws is coupled to a dynamic global vegetation model the b witch model in order to estimate the impact of sulfide oxidation on chemical weathering and associated atmospheric co2 consumption fluxes throughout the mackenzie river basin the model is first calibrated with available field data for the various sub basin of the mackenzie watershed and then a simulation is performed removing the contribution of sulfuric acid to the weathering system the main outcomes of the present study are that 1 the dissolved silica export is controlled by secondary minerals reactivity in soils and the calcium and magnesium fluxes are driven by the dissolution of calcite and dolomite 2 the atmospheric co2 consumption by weathering processes in the mackenzie river is limited accounting for only 24 of the total bicarbonate flux to the ocean confirming a previous study and suggesting that weathering processes in the mackenzie watershed acts as a source of carbon to the atmosphere at the geological timescale and 3 the presence of sulfide minerals within a river basin decreases the apparent net consumption of atmospheric co2 through chemical weathering we show that the actual production of sulfuric acid in the weathering zone of the mackenzie basin reduces the net atmospheric co2 consumption by about two compared to what the atmospheric co2 consumption should be in the absence of sulfide minerals sulfuric acid driven chemical weathering has thus to be considered at both short and long timescales when looking at the link between chemical weathering and the global carbon cycle

the role of arsenic bearing rocks in groundwater pollution at zimapan valley mexico interaction of groundwater with as bearing rocks has been proposed as one of three main sources of arsenic at zimapan valley mexico the complexity of the geology and hydrogeology of the valley make it difficult to identify the natural causes of arsenic poisoning samples from the different rock outcrops and water from wells tapping various rock formations were analyzed the rocks from mineralized areas contained higher concentrations of arsenic with respect to the same formations in non mineralized areas the arsenic minerals arsenopyrite scorodite and tennantite were identified in some rock samples higher temperature and lower eh values were found for those wells containing more arsenic the physicochemical characteristics of these naturally polluted well waters could be produced by arsenopyrite oxidation the geochemical model phreeqci was used to perform the inverse modeling of two wells located along the same fault arsenopyrite oxidation and scorodite dissolution appear to be the geochemical processes producing the natural pollution according to the model the release and transport of arsenic mainly occur through fractures within the cretaceous limestones where the most productive wells are drilled the presence of arsenic should be expected also in other formations near mineralized zones in the zimapan valley field determinations of eh and t could be used to detect potentially polluted wells

rates of weathering rind formation on costa rican basalt weathering rind thicknesses were measured on 200 basaltic clasts collected from three regionally extensive alluvial fill terraces qt 1 qt 2 and qt 3 preserved along the pacific coast of costa rica mass balance calculations suggest that conversion of unweathered basaltic core minerals plagioclase and augite to authigenic minerals in the porous rind kaolinite allophane gibbsite fe oxyhydroxides is iso volumetric and ti and zr are relatively immobile the hierarchy of cation mobility ca na k mg si al fe p is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering anorthite albite hypersthene orthoclase apatite alteration profiles across the cm thick rinds document dissolution of plagioclase and augite and the growth of kaolinite with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age the rate of weathering rind advance is evaluated using a diffusion limited model which predicts a parabolic rate law for weathering rind thickness rr as a function of time t rr t and an interface limited model which predicts a linear rate law for weathering rind thickness as a function of time rr kappt in these rate laws is a diffusion parameter and kapp is an apparent rate constant the rate of advance is best fit by the interface model terrace exposures are confined to the lower reaches of streams draining the pacific slope near the coast where the stream gradient is less than 3 m km and terrace deposition is influenced by eustatic sea level fluctuations geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest assigning the most weathered regionally extensive terrace qt 1 mean rind thickness 6 9 0 6cm to oxygen isotope stage ois 7 ca 240 ka and assuming that at time 0 rind thickness 0 it is inferred that terrace qt 2 rr 2 9 0 1 cm is coincident with stage 5e ca 125 ka and that qt 3 rr 0 9 0 1 cm is consistent with ois 3 ca 37 ka these assignments yield a value of kapp of 8 6 10 13 cm s 1 r 2 0 99 only this value satisfies both the existing age controls and yields ages coincident with sea level maxima using this value elemental weathering release fluxes across a weathering rind from qt 2 range from 6 0 10 9 mol si m 2 s 1 to 2 5 10 11 mol km 2 s 1 the rate of rind advance for the costa rican terraces is 2 8 10 7 my r 1 basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface controlled weathering with an apparent activation energy of about 50 kj mol 1 rates of rind formation in costa rica are an order of magnitude slower than reported for global averages of soil formation rates copyright 2004 elsevier ltd

weathering rock to regolith the amount of pore space in most unweathered granite is too small to support a plant based ecosystem but porosity grows as intact rock interacts with surface waters beneath the soil

detection of carbonates in martian weathering profiles noachian surfaces on mars exhibit vertical assemblages of weathering horizons termed as weathering profiles this indicates that surface water caused alteration of the rocks that required a different warmer climate than today evidence of this early martian climate with co 2 vapor as the main component causing greenhouse warming has been challenged by the lack of carbonate in these profiles here we report the analysis of compact reconnaissance imaging spectrometer for mars l detector data leading to the detections of carbonates using a spectral signature exclusively attributed to them the carbonates are collocated with hydroxylated minerals in weathering profiles over the martian surface the origin of co 2 for the formation of carbonates could be the atmosphere the widespread distribution of weathering profiles with carbonates over the surface of the planet suggest global interactions between fluids containing carbonate bicarbonate ions with the surface of mars in the presence of atmospheric water until around 3 7 billion years ago plain language summary the oldest surface of mars witnessed an impact of dense and humid atmosphere leading to the formation of hydrated minerals the analysis of remote sensing data allows identifying carbonates mixed with the hydrated minerals these associations of carbonates and hydrated minerals are widespread on the surface of the planet this indicates a planetary scale process of formation involving the presence of fluids containing inorganic carbon this finding allows a better understanding of the environment of mars during the period suspected to be the most likely to have host habitable environment on the planet

constancy of silicate mineral weathering rate ratios between natural and experimental weathering implications for hydrologic control of differences in absolute rates abstract inverse models which apportion watershed effluxes geochemical mass balance over estimated mineral surface area give weathering rates for individual silicate minerals e g feldspar 1 3 orders of magnitude slower than laboratory rates comparisons of rates calculated from several recent watershed geochemical mass balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data thus the coefficient which corrects the weathering rate for any mineral from a laboratory data set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets the correction factor linking experimental and natural rates must be independent of composition physical e g hydrologic controls rather than compositional or chemical controls cause the difference between weathering rates in nature and the laboratory because flow in natural weathering profiles is spatially heterogeneous not all of the potentially available surface in natural systems actually participates in reactions with pore fluids

a spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady state denudation abstract spheroidal weathering a common mechanism that initiates the transformation of bedrock to saprolite creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets in the spheroidally weathering rio blanco quartz diorite puerto rico diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides a positive v of reaction results in the build up of elastic strain energy in the rock formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy the rate of spheroidal weathering is thus a function of the concentration of reactants the reaction rate the rate of transport and the mechanical properties of the rock substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the rio icacos bedrock 2 3 cm and a time interval between fractures 200 300 a based on an assumption of steady state denudation at the measured rate of 0 01 cm a averaged over times longer than this interval the rate of advance of the bedrock saprolite interface during spheroidal weathering the weathering advance rate is constant with time assuming that the oxygen concentration at the bedrock saprolite interface varies with the thickness of soil saprolite yields predictive equations for how weathering advance rate and steady state saprolite soil thickness depend upon atmospheric oxygen levels and upon denudation rate the denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite in our model for spheroidal weathering of the rio blanco quartz diorite fractures occur every 250 yr ferric oxide is fully depleted over a four rindlet set in 1000 yr and saprolitization is completed in 5000 yr in the zone containing 20 rindlets spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures but for the same kinetics and transport parameters the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the rio blanco system the spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time

clay mineralogy of weathering rinds and possible implications concerning the sources of clay minerals in soils weathering rinds on volcanic clasts in quaternary deposits in the western united states contain only very fine grained and poorly crystalline clay minerals rinds were sampled from soils containing well developed argillic b horizons in deposits about 105 yr old or more the clay size fraction of the rinds is dominated by allophane and iron hydroxy oxides whereas the b horizons contain abundant well crystallized clay minerals the contrast between the clay mineralogy of the weathering rinds in which weathering is isolated from other soil processes and that of the associated soil matrices suggests a need to reassess assumptions concerning the rates at which clay minerals form and the sources of clay minerals in argillic b horizons this study suggests that crystalline clay minerals form more slowly in weathering rinds than is generally assumed for soil environments and that the weathering of primary minerals may not be the dominant source of crystalline clay minerals in middle to late pleistocene soils

the effect of curvature on weathering rind formation evidence from uranium series isotopes in basaltic andesite weathering clasts in guadeloupe abstract to quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions we analyzed uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from basse terre island in guadeloupe u th and ti elemental profiles reveal that th and ti behave conservatively during rind formation but that u is added from an external source to the rind in the rind weathering reactions include dissolution of primary minerals such as pyroxene plagioclase and glass matrix as well as formation of fe oxyhydroxides gibbsite and minor kaolinite rare earth element ree profiles reveal a significant eu negative anomaly formed during clast weathering consistent with plagioclase dissolution significant porosity forms in the rind mostly due to plagioclase dissolution the new porosity is inferred to allow influx of soil water carrying externally derived dissolved u due to this influx u precipitates along with newly formed clay minerals and oxyhydroxides in the rind the conservative behavior of th and the continuous addition of u into the rind adequately explain the observed systematic trends of 238 u 232 th and 230 th 232 th activity ratios in the rind rind formation rates determined from the measured u series activity ratios with an open system u addition model increase by a factor of 1 3 0 18 0 24 mm kyr from a low curvature to a high curvature section 0 018 0 12 mm 1 of the core rind boundary revealing that curvature affects rates of rind formation as expected for diffusion limited rind formation u series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale the weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300 2200

extreme element mobility during transformation of neoarchean ca 2 7ga pillow basalts to a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports petrographic major and trace element and oxygen isotopic data for a subaerial weathering profile at schreiber beach ontario canada the weathering profile developed on neoarchean ca 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic ca 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation rocks at schreiber beach have been classified as pillow cores pillow rims hyaloclastites weathered red basalts and weathered brown to green basalts there are gradual textural mineralogical and geochemical transitions from unweathered basalts to intensely weathered hematite bearing basalts with stratigraphic height rocks at schreiber beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the neoarchean following the subaerial weathering they were buried and re metamorphosed to greenschist facies in the paleoproterozoic given the preservation of primary igneous textures and trace element patterns in pillow cores they are used as a protolith to estimate element mobility during seafloor alteration metamorphism and subaerial weathering chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major si ca na fe ti k and trace lile ree hfse and transition metals elements during neoarchean seafloor alteration alteration of these rocks at relatively low temperatures is indicated by enrichment in 18 o 18 o 10 4 to 12 7 relative to fresh basalt 5 5 subaerial weathering resulted in significant textural mineralogical e g hematite precipitation and geochemical variations in metamorphosed basalts causing additional redistribution of most major and trace elements and further enrichment in 18 o the highest 18 o values 16 1 to 17 1 occur at the silica rich base of the weathering profile many elements including lile ree and hfse are strongly enriched 100 or depleted up to 96 relative to their original abundances alkaline metasomatism which is a characteristic of precambrian paleosols is also evident at schreiber beach where extreme enrichment in k 280 to 1000 rb 390 to 660 and cs 1160 to 2560 occurs in the weathering profile

metal release from limestones at high partial pressures of co2 abstract co 2 leakage from underground co 2 sequestration and storage poses potential risks to degradation of water quality in shallow aquifers increased co 2 concentrations can result in decreased ph and lead to subsequent metal release from mineral dissolution or desorption from mineral surfaces dissolution of carbonate minerals present in aquifer sediments or rocks will buffer ph and is generally thought to reduce the potential risk of metal release in the event of a co 2 leak as a result much of the research on geochemical impacts of co 2 leakage has focused on siliciclastic aquifers with little to no carbonate minerals present however carbonate minerals contain trace amounts of metals in their crystal structure that will be released into solution with dissolution and may pose a risk to drinking water quality here we perform laboratory water rock experiments to analyze the potential for metal release due to carbonate mineral dissolution in limestone aquifers rock samples from three limestone aquifers were dissolved in batch reactors with varying partial pressures of co 2 from 0 01 to 1 bar in the headspace as co 2 dissolved into the fluid and decreased the ph the carbonate minerals dissolved and released metals into solution the concentrations of calcium magnesium strontium barium thallium uranium and cobalt increased but remained below any regulatory limits the concentrations of arsenic and nickel increased and exceeded primary drinking water standards set by the usepa and the state of california respectively potential sources of metals in the rocks were determined through detailed sample characterization using sequential extractions laser ablation inductively coupled mass spectrometry and high resolution mineralogical mapping with qemscan we found that calcite dissolution released more metals to solution than pyrite dissolution or metal desorption from mineral surfaces in these experiments geochemical models based on the experimental data were used to evaluate the relative importance of calcite dissolution versus pyrite dissolution over a 30 year time frame under both oxic and sub oxic conditions calcite dissolution is the dominant source of metals to solution immediately after exposure to co 2 pyrite dissolution becomes the dominant source at later times as the fluid reaches equilibrium with respect to calcite for all model scenarios the cumulative contribution of metals to solution was dominated by calcite dissolution results from this study suggest that the ph buffering benefit of carbonate mineral dissolution in the event of a co 2 leak may be offset by the potentially negative effect of trace metal release from the crystal structure this study highlights the need for detailed sample characterization at individual sites to identify sources of metals when assessing the potential risk of co 2 leakage into shallow aquifers

controls on ecohydrologic properties in desert ecosystems differences in soil age and volcanic morphology abstract research examining the influence of parent material on soil and landscape development is limited in dryland environments in volcanic soils most studies examining soil development and succession have been conducted in humid environments where weathering occurs rapidly this study was conducted in a semi arid environment to examine the influence of volcanic parent material morphology on soil development and ecohydrologic properties that may regulate primary succession four study sites were selected across young 2 0 ka and old 12 1 ka end members of two different parent materials cinder cones and pahoehoe lava flows we hypothesized that soils and associated hydraulic properties would evolve more rapidly on the cinder cones compared to the pahoehoe flows owing to greater surface area on the cinder for each site we quantified plant cover soil organic matter bulk density texture soil water retention and hydraulic conductivity the old cinder cone soils accumulated approximately 9 soil organic matter and increased in silt plus clay fraction by 23 relative to the young soil this increased water retention and decreased hydraulic conductivity similarly plant cover increased by 47 the old pahoehoe site remained dominated by unweathered basalt where silty eolian material accumulated preferentially in crevices soil organic matter and silt plus clay fraction increased by 18 and 49 respectively in old pahoehoe soils resulting in increased water retention but little change in hydraulic conductivity plants were restricted to crevices and cover increased by only 34 we observed the cinder cone sites had extensive soil and plant community development from the young to old sites the pahoehoe soils and plant communities established earlier than on cinder cones but showed only modest functional changes between young and old sites the combined characterization of soil development hydraulic properties and plant community dynamics highlights potential ecohydrologic mechanisms controlling successional processes in young semi arid landscapes

scale issues in weathering studies a review of the major scale issues in weathering studies reveals concerns over the fundamental spatio temporal distributions of weathering phenomena as well as issues of upscaling microscope based observations and linking different scales of observations in explanations of landform development various strategies are proposed which can be used to tackle these issues many rooted in non linear dynamical systems ideas as an initial step spatio temporal scale distributions are estimated here for weathering processes landforms and controls based on a range of empirical data two case studies of phytokarst in grand cayman and blistering and scaling of building stones are presented to illustrate the types of data that might be used to establish more convincing scale linkages in weathering investigations

groundwater arsenic variations the role of local geology and rainfall high concentrations of as were detected in the aquifer system of zimapan valley mexico arsenic is related to a multisource system both natural and anthropogenic the main source is as bearing rocks groundwater as variations were detected in the more polluted urban deep well variations were correlated with the precipitation regime and with the hydrogeologic and geologic framework an induced as transport mechanism related mainly with the rainfall regime is proposed

role of hydraulic diffusivity in the decrease of weathering rates over time summary springs emerging within massifs of crystalline rocks were monitored for discharge rate q and the q values combined with geomorphic and hydrographic parameters in a hydrologic model to calculate hydraulic conductivity k and effective porosity n e of the spring watersheds the spring waters several borehole waters and rain water were analyzed for major dissolved compounds strontium and isotopes 18 o 2 h 13 c and 87 sr 86 sr with a shift to less negative values 18 o and 2 h were fitted by a line approximately parallel to the gmwl but no significant dependence on altitude was found the 18 o and 2h values correlate better with those of precipitation amount the 87 sr 86 sr ratios in drilled well waters correlate positively with the depth of water circulation reported in the borehole logs the corresponding regression equations were used to extrapolate the depth of hydraulic circuits within the spring watersheds the previous data together with groundwater travel times calculated by a water balance model and with reactions of granite metassediment plagioclase and biotite precipitating halloysite gibbsite and vermiculite were assembled in a mass balance model to calculate solute flux weathering rates of plagioclase w pl the w pl s were described as a function of d k n e where d is the hydraulic diffusivity the discrepancies between the w pl values and solid state rates based on the differences between elemental isotopic and mineral compositions measured in present day regoliths and in the assumed protolith were assigned to a decrease in d over time from values in the protolith to values in the weathered aquifer

determining mineral weathering rates based on solid and solute weathering gradients and velocities application to biotite weathering in saprolites abstract chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths an increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change the solid state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration these relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients contemporary weathering rates based on solute residence times can be directly compared to long term past weathering based on changes in regolith composition both rates incorporate identical parameters describing mineral abundance stoichiometry and surface area weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the piedmont of northern georgia usa and in luquillo mountains of puerto rico solid state weathering gradients for mg and k at panola produced reaction rates of 3 to 6 10 17 mol m 2 s 1 for biotite faster weathering rates of 1 8 to 3 6 10 16 mol m 2 s 1 are calculated based on mg and k pore water gradients in the rio icacos regolith the relative rates are in agreement with a warmer and wetter tropical climate in puerto rico both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering

evidence for weathering on early mars from a comparison with terrestrial weathering profiles abstract phyllosilicates on mars are widespread in the ancient crust suggesting the presence of liquid water at the martian surface and therefore warmer conditions during its early history however the role of the ancient climate in the alteration process which produced these phyllosilicates remains under debate because similar mineral assemblages can be produced by hydrothermal alteration at depth this paper focuses on the origin of coincident outcrops of fe mg bearing phyllosilicates and al bearing phyllosilicates which are observed in several regions of mars we performed a detailed mineralogical comparison between a section in nili fossae mars and a weathering profile located at murrin murrin western australia the murrin murrin profile is developed in archaean serpentinized peridotite massifs over a 40 m thick sequence it has three alteration zones the serpentine mineral saprolite is found at the bottom immediately overlain by fe mg bearing smectites and then al bearing phyllosilicates kaolinite mixed with iron hydroxides this example illustrates how al dominated minerals can derive from the alteration of initially al poor ultramafic rocks by the intense leaching of mg2 this mineralogical sequence is very similar to that detected locally in nili fossae by orbital spectroscopy by analogy we propose that the mineral assemblage detected on mars is the result of long term weathering and thus could be the best evidence of past weathering as a direct result of a climate significantly warmer than at present

conditions of subaerial weathering of basalts in the neoarchean and paleoproterozoic abstract precambrian weathering profiles or regoliths were formed on the earth s surface in contact with meteoritic water this signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering the aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation we have demonstrated that in a wide range of modeling conditions weathering profiles of basalts consist mostly of clay minerals smectites and illite carbonate minerals may also be deposited within the profile at the initial stage of weathering which are later dissolved during a further process the time necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 kyr modeling results indicate that neoarchean and paleoproterozoic 2 8 2 0 ga weathering profiles were formed under an atmosphere with pco2 not much higher than 25 pal and climatic conditions close to those found in modern times weathering under co2 rich pco2 1 and 10 bar and ch4 rich atmospheres as well as high temperature 50 c and 75 c conditions do not produce compounds corresponding to the analyzed samples

element mobilization and redistribution under extreme tropical weathering of basalts from the hainan island south china abstract chemical weathering of rocks has substantial influence on the global geochemical cycle in this paper the geochemical profile of a well developed basalt weathering profile 15 m thick including soil saprolite semi weathered rock and fresh basalt on the island of hainan south china was presented the soil and saprolite samples from this profile are characterized by high al2o3 and fe2o3 concentrations up to 32 3 and 28 5 respectively the mineral assemblage is dominated by kaolinite fe oxides hydroxides and gibbsite or boehmite indicating extensive desilicate and ferrallitic weathering the acidic and organic rich environment in the soil horizon may have promoted elemental remobilization and leaching the strongest sio2 depletion and al2o3 enrichment at about 2 4 m deep indicate that the main kaolinite hydrolysis and gibbsite formation occurred near the soil saprolite interface the mild sr reconcentration at about 3 9 m and 7 1 m deep may be attributed to secondary carbonate precipitation mn oxides hydroxides precipitated at 6 1 m deep accompanied by the strongest enrichment of ba and co uranium is mildly enriched in the middle part about 7 1 m and 9 1 m deep of the weathering profile and the enrichment may have been caused by the decomposition of uranyl carbonates or the accumulation of zircon immobile element i e zr hf nb ta th and ti distributions at different depths are mainly controlled by secondary fe oxides hydroxides and follow the stability sequence of nb ta th zr hf ti the limited thickness 15 cm of the semi weathered basalt horizon at the rock regolith interface 15 28 m deep suggests that plagioclase and pyroxene are readily altered to kaolinite smectite and fe oxides under tropical climate the marked enrichment of transitional metals such as cu zn ni and sc along the rock regolith interface may have associated mainly with increasing ph values as well as the dissolution of primary apatite and formation of secondary phosphates our findings highlight the importance of secondary phosphates in the redistribution of transition metals and in the possible mg cu and ni isotopic fractionation under extreme weathering of basalt in tropic climate

chemical weathering in a tropical watershed luquillo mountains puerto rico ii rate and mechanism of biotite weathering abstract samples of soil saprolite bedrock and porewater from a lower montane wet forest the luquillo experimental forest lef in puerto rico were studied to investigate the rates and mechanisms of biotite weathering the soil profile at the top of a ridge in the rio icacos watershed consists of a 50 100 cm thick layer of unstructured soil above a 600 800 cm thick saprolite developed on quartz diorite the only minerals present in significant concentration within the soil and saprolite are biotite quartz kaolinite and iron oxides biotite is the only primary silicate releasing significant k and mg to porewaters although biotite in samples of the quartz diorite bedrock is extensively chloritized chlorite is almost entirely absent in the saprolite phyllosilicates phyllosilicate grains are present as 200 1000 m wide books below about 50 cm depth x ray diffraction xrd and electron microprobe analyses indicate that the phyllosilicate grains contain a core of biotite surrounded by variable amounts of kaolinite lattice fringe images under transmission electron microscope tem show single layers of biotite altering to two layers of kaolinite suggesting dissolution of biotite and precipitation of kaolinite at discrete boundaries some single 14 a layers are also observed in the biotite under tem the degree of kaolinitization of individual phyllosilicate grains as observed by tem decreases with depth in the saprolite this tem work is the first such microstructural evidence of epitaxial growth of kaolinite onto biotite during alteration in low temperature environments the rate of release of mg in the profile calculated as a flux through the soil normalized per watershed land area is approximately 500 mol hectare 1 yr 1 1 6 10 9 molmg msoil 2 s 1 this rate is similar to the flux estimated from mg discharge out the rio icacos 1000 mol hectare 1 yr 1 or 3 5 10 9 molmg msoil 2 s 1 indicating that scaling up from the soil to the watershed is possible for mg release the rate of mg release from biotite normalized to brunauer emmett teller bet surface area is calculated using a mass balance equation which includes the density and volume of phyllosilicate grains porewater chemistry and flux and soil porosity the mean rates of biotite weathering calculated from k and mg release rates are approximately 6 and 11 10 16 molbiotite mbiotite 2 s 1 respectively significantly slower than laboratory rates 10 12 to 10 11 molbiotite mbiotite 2 s 1 the discrepancy in scaling down from the soil to the laboratory is probably explained by 1 differences in weathering mechanism between the two environments 2 higher solute concentrations in soil porewaters 3 loss of reactive surface area of biotite in the saprolite due to kaolinite and iron oxide coatings and or 4 unaccounted for heterogeneities in flow path through the soil

chromium fluxes and speciation in ultramafic catchments and global rivers abstract the role of ultramafic rock weathering in global chromium cr budgets and how these may be linked to changes in earth s climate over geological time was investigated by compiling dissolved cr speciation and fluxes in i 73 of the worlds large rivers representing 35 of global river discharge to oceans and in ii an ultramafic catchment putah creek watershed in the california coast range mountains usa weathering of ultramafic rocks creates ideal conditions for cr release and redox cycling alkaline river water draining ultramafic catchments is naturally enriched in cr up to 582 nmol l chromium concentrations increase with the extent of rock water interaction as indicated by correlations with mg and hco3 concentrations and ph whereas cr cycling depends on the availability of electron donors and acceptors e g fe ii organics mn iii iv oxides thus dissolved cr is exported from ultramafic watersheds as both the soluble hexavalent cr vi species mgcro4 cro42 and also as trivalent cr iii species croh co3 22 cr oh 3 the latter cr iii species were previously thought to have low solubility ultramafic catchments have higher area normalized cr and major ion fluxes for a given runoff when compared to global rivers and may thus have a disproportional impact on global cr budgets long term carbon cycling and moderation of the earth s climate over million year timescales riverine export fluxes of cr are linearly correlated with fluxes of ca mg and alkalinity in both ultramafic catchments and global rivers this suggests that silicate weathering is a key control on cr fluxes and that major element weathering fluxes may be a useful proxy for estimating cr and other trace element fluxes globally present day riverine dissolved cr fluxes to oceans are spatially variable and estimated to be 1 7 109 mol yr three times higher than previously reported however when geochemical reactions in estuaries are considered riverine cr fluxes may be lower throughout earth history cr weathering fluxes also may have varied with changes in the global distribution of ultramafic rocks cr concentrations in continental rocks or climactic conditions e g atmospheric co2 levels or runoff such variation should be considered when interpreting both modern and past cr seawater isotopic compositions and residence times

vegetation succession and climate change across the plio pleistocene transition in eastern azerbaijan central eurasia 2 77 2 45 ma abstract the plio pleistocene transition marked a key moment in global climate history characterised by the onset of major glaciations in the northern hemisphere the palaeoenvironmental history of the plio pleistocene transition is not well known for the caspian sea region despite its importance for global climate dynamics here we present an independently 40ar 39ar dated high resolution terrestrial palynological record spanning the plio pleistocene boundary based on a lacustrine marine sedimentary sequence from eastern azerbaijan despite complex pollen transport pathways and the proximity of closely stacked mountain vegetation belts in the greater and lesser caucasus the record shows that regional vegetation responded to milankovitch forced glacial interglacial cycles tentatively correlated with global climatic records spanning mis g8 to 98 2 77 2 45 ma the persistence of mesophilous forests during glacial times indicates that some settings in the south caspian basin acted as glacial refugia and that vegetation response to glaciations was muted by increased moisture availability linked to caspian transgression the palynological record shows a relationship with global 18o stacks and specifically to the obliquity record we anticipate that precise correlation with the global climatostratigraphic timescale will allow better understanding of the nature and timing of important transgressive events in the caspian sea and their relevance on a global scale

the fractured moon production and saturation of porosity in the lunar highlands from impact cratering we have analyzed the bouguer anomaly ba of 1200 complex craters in the lunar highlands from gravity recovery and interior laboratory observations the ba of these craters is generally negative though positive ba values are observed particularly for smaller craters crater ba values scale inversely with crater diameter quantifying how larger impacts produce more extensive fracturing and dilatant bulking the bouguer anomaly of craters larger than 93 47 19 km in diameter is independent of crater size indicating that there is a limiting depth to impact generated porosity presumably from pore collapse associated with either overburden pressure or viscous flow impact generated porosity of the bulk lunar crust is likely in a state of equilibrium for craters smaller than 30km in diameter consistent with an 8km thick lunar megaregolith whereas the gravity signature of larger craters is still preserved and provides new insight into the cratering record of even the oldest lunar surfaces

response to b morse 2001 on the role of arsenic bearing rocks in groundwater pollution at zimapan valley mexico by armienta and others environmental geology 40 4 5

determining rates of chemical weathering in soils solute transport versus profile evolution abstract sio 2 fluxes associated with contemporary solute transport in three deeply weathered granitoid profiles are compared to bulk sio 2 losses that have occurred during regolith development climates at the three profiles range from mediterranean to humid to tropical due to shallow impeding alluvial layers at two of the profiles and seasonally uniform rainfall at the third temporal variations in hydraulic and chemical state variables are largely attenuated below depths of 1 2 m this allows current sio 2 fluxes below the zone of seasonal variations to be estimated from pore water concentrations and average hydraulic flux densities mean annual sio 2 concentrations were 0 1 1 5 mm hydraulic conductivities for the investigated range of soil moisture saturations ranged from 9 to 10 6 m s 1 estimated hydraulic flux densities for quasi steady portions of the profiles varied from 6 10 9 to 14 10 9 m s 1 based on darcy s law and field measurements of moisture saturations and pressure heads corresponding fluid residence times in the profiles ranged from 10 to 44 years total sio 2 losses based on chemical and volumetric changes in the respective profiles ranged from 19 to 110 kmoles sio 2 m 2 of land surface as a result of 0 2 0 4 ma of chemical weathering extrapolation of contemporary solute fluxes to comparable time periods reproduced these sio 2 losses to about an order of magnitude despite the large range and non linearity of measured hydraulic conductivities solute transport rates in weathering regoliths can be estimated from characterization of hydrologic conditions at sufficiently large depths the agreement suggests that current weathering rates are representative of long term average weathering rates in the regoliths

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

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

widespread surface weathering on early mars a case for a warmer and wetter climate abstract early mars 3 ga underwent aqueous alteration as evidenced by fluvial lacustrine morphologies and the recent discovery of widespread hydrous clays despite compelling evidence for diverse and localized aqueous environments the possibility for sustained liquid water globally on the martian surface and over geological timescales is still highly debated instead a durably cold and dry mars scenario is often proposed by studying specific fe mg and al rich clay stratigraphies on mars by means of a planetary scale orbital investigation we present new evidence that mars experienced an early era 3 7 ga of widespread aqueous alteration consistent with surface weathering

a reactive transport model for weathering rind formation on basalt abstract saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution here we investigate the chemical and physical transformations that occur during weathering by studying small scale saprolites in the form of weathering rinds which form on rock in soil or saprolite and grow in thickness without physical disturbance with time we compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in costa rica to diffusion reaction simulations of rind formation using the fully coupled reactive transport model crunchflow the four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include 1 the mineralogy of weathering products 2 weathering rind thickness 3 the coincidence of plagioclase and augite reaction fronts and 4 the thickness of the zones of mineral reaction i e reaction fronts four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations to fit the model to all four criteria however it was necessary to 1 treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9 and 2 treat mineral surface area as a fitting factor this latter approach was presumably necessary because the mineral water surface area of the connected accessible porosity in the costa rica samples is much less than the total porosity navarre sitchler et al 2009 the model fit surface area here termed reacting surface area was much smaller than the bet measured surface area determined for powdered basaltic material in the parent basalt reacting surface area and diffusivity are low due to low pore connectivity and early weathering is therefore transport controlled however as pore connectivity increases as a result of weathering the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics the transition point between transport and kinetic control appears to be related to a critical porosity 9 at which pore connectivity is high enough to allow rapid transport based on these simulations we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface and that this porosity increases because of mineral dissolution following a rate that is largely surface reaction controlled

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

nontronite dissolution rates and implications for mars abstract the fe rich smectite nontronite m 1 05 si6 98al1 02 al0 29fe3 68mg0 04 o20 oh 4 has been detected using orbital data at multiple locations in ancient terrains on mars including mawrth vallis nilli fossae north of the syrtis major volcanic plateau terra meridiani and the landing site of the mars science laboratory msl gale crater given the antiquity of these sites 3 0 ga it is likely that nontronite has been exposed to the martian environment for long periods of time and therefore provides an integrated record of processes in near surface environments including pedogenesis and diagenesis in particular nontronite detected at mawrth vallis is overlain by montmorillonite and kaolinite and it has been previously suggested that this mineralogical sequence may be the result of surface weathering in order to better understand clay mineral weathering on mars we measured dissolution rates of nontronite in column reactors at solution ph values of 0 9 1 7 and 3 0 and two flow rates 0 16 ml h and 0 32 ml h solution chemistry indicates stoichiometric dissolution at ph 0 9 and non stoichiometric dissolution at ph 1 7 and 3 0 mineral dissolution rates based on elemental release rates at ph 1 7 and 3 0 of ca si and fe follow the order interlayer tetrahedral octahedral sites respectively the behavior of all experiments suggests far from equilibrium conditions with the exception of the experiment performed at ph 3 0 and flow rate 0 16 ml h a ph dependent dissolution rate law was calculated through si release from experiments that showed no dependence on saturation far from equilibrium conditions under both flow rates and is r 10 12 06 0 123 10 0 297 0 058 ph where r has the units mol mineral m 2 s 1 when compared to dissolution rates from the literature our results indicate that nontronite dissolution is significantly slower than dissolution of the primary phases present in basalt under acidic conditions suggesting that once nontronite forms it could remain stable at or near the surface of mars for extended periods of time nontronite dissolution rates are faster than dissolution rates of montmorillonite rozalen et al 2008 and kaolinite huertas et al 1999 suggesting that chemical weathering of a mixed clay deposit would enrich the proportions of montmorillonite and kaolinite through the preferential dissolution of nontronite vis nir analyses of our reacted products and thermodynamic modeling of our experimental conditions both indicate the precipitation of amorphous silica within columns and amorphous silica has also been observed in association with phyllosilicates on the martian surface mustard et al 2008 ehlmann et al 2009 murchie et al 2009a therefore chemical weathering of strata containing mixtures of montmorillonite nontronite and kaolinite provides a potential formation mechanism for the mineralogic stratigraphy observed at mawrth vallis and other locations on mars

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

weathering dynamics reflected by the response of riverine uranium isotope disequilibrium to changes in denudation rate abstract the ratio between the activity of dissolved 234 u and 238 u in river water 234 u 238 u riv shows promise for tracing weathering processes it has been suggested that preferential 234 u release generates a high 234 u 238 u riv under low weathering intensity while congruent weathering as expected for high weathering intensity would release uranium with a secular equilibrium 234 u 238 u riv of 1 however weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate complicating the use of 234 u 238 u riv as a tool in weathering studies this work attempts to resolve the weathering dynamics reflected by 234 u 238 u riv using catchment scale denudation rate d as a first order approximation for the age of weathering interfaces a global dataset n 653 including 132 new measurements from chinese catchments indicates changing responses of 234 u 238 u riv to d higher values 1 5 and larger variability of 234 u 238 u riv are observed in catchments both with high 3 mm yr and low d 234 u 238 u riv are associated with catchments of medium to high d 0 3 2 mm yr the observed pattern can be explained by a model that involves competing uranium release from active and inactive weathering interfaces in response to a shifting weathering regime from kinetically limited to supply limited 234 u 238 u riv decreases with decreasing d under a kinetically limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of 234 u from radioactively damaged sites as d decreases in contrast 234 u 238 u riv increases with decreasing d under a supply limited weathering regime because the minerals that remain under these conditions continue to release 234 u to solution by recoil while contributing little to the dissolved 238 u because of their slow dissolution rate a much deeper depth of the weathering interface is suggested for catchments with high d due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith a young age of 10 kyrs is constrained by the model for complete dissolution of exposed weathering active minerals which implies a rapid shifting from kinetically limited to supply limited weathering regime and limited influence of weathering on the calculation of comminution age based on 234 u 238 u in sediments a site specific dissolution rate that is consistent with those estimated from weathering profiles is also constrained which suggests that the discrepancy between field and laboratory weathering rate may largely be controlled by the difference in chemical affinity associated with stagnant micro pores low permeability and high rock fluid ratios rather than the passivation of weathering interfaces

kinetics of weathering and diagenesis weathering and diagenesis are discussed from a kinetic standpoint such a discussion depends upon an intimate interplay between field measurements laboratory experiments and mathematical theory because of the accessibility of soils and sediments models based on laboratory measurements and theoretical calculations can be used along with field data to calculate rate parameters which are otherwise difficult to measure in the laboratory also theoretical models can be tested in the field ways in which chemical kinetics can be applied to geological problems in the surficial environment are discussed and the author recomends extensive future study of such problems jmt

modeling rock weathering in small watersheds summary many mountainous watersheds are conceived as aquifer media where multiple groundwater flow systems have developed toth 1963 and as bimodal landscapes where differential weathering of bare and soil mantled rock has occurred wahrhaftig 1965 the results of a weathering algorithm pacheco and van der weijden 2012a 2014 which integrates topographic hydrologic rock structure and chemical data to calculate weathering rates at the watershed scale validated the conceptual models in the river sordo basin a small watershed located in the marao cordillera north of portugal the coupling of weathering groundwater flow and landscape evolution analyses as accomplished in this study is innovative and represents a remarkable achievement towards regionalization of rock weathering at the watershed scale the river sordo basin occupies an area of approximately 51 2 km 2 and was shaped on granite and metassediment terrains between the altitudes 185 1300 m the groundwater flow system is composed of recharge areas located at elevations 700 m identified on the basis of 18 o data discharge cells comprehend terminations of local intermediate and regional flow systems identified on the basis of spring density patterns infiltration depth estimates based on 87 sr 86 sr data and spatial distributions of groundwater ph and natural mineralization intermediate and regional flow systems defined where infiltration depths 125 m develop solely along the contact zone between granites and metassediments because fractures in this region are profound and their density is very large weathering is accelerated where rocks are covered by thick soils being five times faster relative to sectors of the basin where rocks are covered by thin soils differential weathering of bare and soil mantled rock is also revealed by the spatial distribution of calculated aquifer hydraulic diffusivities and groundwater travel times

effect of porosity on the increase in weathering rind thicknesses of andesite gravel abstract factors affecting the increase in weathering rind thickness i e weathering rates were investigated using andesite gravel in fluvial terrace deposits with a known emergence time the time between the age of each terrace formation 20 130 290 and 660 ka and the present was assumed to be the weathering period three zones were classified from measurements of the vickers hardness number vhn across weathering rinds from the rock surface to the inner fresh part 1 zone i the outermost brown layer which has low vhn values of 10 to 80 gf m2 2 zone ii the inner gray layer which has increasing vhn values 100 to 500 gf m2 with increasing depth 3 the innermost fresh part which has large vhn values of 500 gf m2 in this paper zones i and i ii are defined as the brown layer and the weathering rind respectively diffusion theory was applied to the relationship between the thickness of the brown layer or the weathering rind and time weathering periods the results show that the thickness of the brown layer depends mainly on time whereas the thickness of the weathering rind is controlled by the porosity of the host rock as well as time consequently rocks with higher porosity have thicker weathering rinds than brown layers

diffusive transport of water in porous fresh to altered mid ocean ridge basalts abstract studying diffusive transport in porous rocks is of fundamental importance in understanding a variety of geochemical processes including element transfer primary mineral dissolution kinetics and precipitation of secondary phases here we report new findings on the relationship between diffusive transport and textural characteristics of the pore systems on the example of mid oceanic ridge basalts having different degree of alteration but very similar bulk pore volume diffusion processes in porous basalts were studied in situ using h 2 o d 2 o exchange experiments the effective diffusion coefficients of water molecules increase systematically from 5 05 10 11 to 1 19 10 10 m 2 s for fresh and moderately altered basalts and from 2 40 10 11 to 6 72 10 11 m 2 s for completely altered basalt as temperature increases from 5 to 50 c the activation energy of the diffusion process increases from 12 29 0 71 kj mol for fresh and moderately altered basalts to 14 3 1 33 kj mol for completely altered basalt the results indicate that neither the bulk porosity nor the degree of alteration can be used as proxies for the efficiency of element transport during morb water interaction the formation of secondary phases that replace primary minerals and fill the pore space in the rock leads to the formation of tiny pores and phases with large specific surface area these factors might have a dominant control on the transport properties of altered basaltic rocks

a reactive transport model for marcellus shale weathering abstract shale formations account for 25 of the land surface globally and contribute a large proportion of the natural gas used in the united states one of the most productive shale gas formations is the marcellus a black shale that is rich in organic matter and pyrite as a first step toward understanding how marcellus shale interacts with water in the surface or deep subsurface we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions constrained by soil and water chemistry data the simulation was carried out for 10 000 years since deglaciation assuming bedrock weathering and soil genesis began after the last glacial maximum results indicate weathering was initiated by pyrite dissolution for the first 1000 years leading to low ph and enhanced dissolution of chlorite and precipitation of iron hydroxides after pyrite depletion chlorite dissolved slowly primarily facilitated by the presence of co2 and organic acids forming vermiculite as a secondary mineral a sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases co2 and o2 specific surface area and flow velocity of infiltrating meteoric water the soil chemistry and mineralogy data could not be reproduced without including the reactive gases for example pyrite remained in the soil even after 10 000 years if o2 was not continuously present in the soil column likewise chlorite remained abundant and porosity remained small if co2 was not present in the soil gas the field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1 3 orders of magnitude smaller than surface area values measured for powdered minerals small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings in addition some mineral surface is likely interacting only with equilibrated pore fluids an increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far from equilibrium conditions we conclude from these observations that availability of reactive surface area and transport of h2o and gases are the most important factors affecting rates of marcellus shale weathering of the in the shallow subsurface this weathering study documents the utility of reactive transport modeling for complex subsurface processes such modelling could be extended to understand interactions between injected fluids and marcellus shale gas reservoirs at higher temperature pressure and salinity conditions

reconciling chemical weathering rates across scales application of uranium series isotope systematics in volcanic weathering clasts from basse terre island french guadeloupe abstract to elucidate the factors that contribute to the orders of magnitude difference generally observed in apparent weathering rates across scales we utilized a u series isotopic technique to directly determine the duration and rates of chemical weathering recorded in weathering clasts in this study we systematically compiled 15 individual volcanic weathering clasts collected from eight andesitic watersheds along a gradient of mean annual precipitation ranging from 1600 mm to 6400 mm on the tropical volcanic basse terre island of french guadeloupe we measured u series isotope compositions to quantify weathering advance rates along the core rind transects in these weathering clasts total rind formation ages the length of time it took for these weathering clasts to form from andesitic rock fragments or volcanic clasts in the soil zones range from 60 kyr to 300 kyr as expected for each individual core rind weathering transect the rind ages generally increase almost linearly with distance away from the core rind boundary the derived clast weathering rates range from 0 08 0 04 to 0 34 0 01 mm kyr 1 and the rates exhibit a strong positive correlation r 2 0 74 with the annual runoff values of these watersheds this correlation documents the first direct evidence that weathering over geological timescales on the island is controlled by the amount of precipitation and corroborates the relationship that was previously reported for the riverine weathering fluxes at basse terre island in guadeloupe but measured with much shorter time scales the clast scale rates inferred for the characteristic length scale of mm are compared to the watershed scale rates of basse terre island length scale of km constantly higher rates are observed at the watershed scales than the clast scale and the discrepancy equals 1800 400 times despite changes in watershed characteristics such as watershed size relief and runoff values the discrepancy is attributed to the fractal nature of the roughness and surface area across scales for example the presence of undulations and fractures e g roughness at the bedrock saprolite contact of the watershed which is generally not assessed in the watershed surface area estimates is most likely the main contributor to the constant rate difference of 1800 times

models of transport and reaction describing weathering of fractured rock with mobile and immobile water we propose models for calculating the rate of chemical weathering of minerals as a function of depth in the weathering zone a macropore network is assumed to be responsible for the transport of mobile water which removes soluble weathering products at the interface of that network and the matrix conditions of infrequent rainfall a and of very frequent rainfall b are separately modeled but both lead to a volumetric weathering rate with the general form 0exp z where the amplitude 0 and the equilibration length depend on the pore geometry of regolith and on parameters describing transport across the macropore matrix interface a or mineral dissolution b this is obtained with no assumption of steady state for regolith evolution extrapolating these end members into intermediate conditions the model is consistent with the exponential decay of regolith production rate versus depth reported by several authors and yields in a variety of regolith types in the range from centimeters to meters the velocity of the regolith bedrock interface also shows an exponential decay with weathering zone thickness and is enhanced by bedrock fractures when compared to models of unfractured bedrock when the residence time of fluid in the weathering zone f is large the bulk weathering rate rb is inversely proportional to this time and for small f rb reverts to the laboratory rate application to compiled data from several sites leads to estimates of a dissolution rate constant and specific surface area consistent with those of albite

accretion of asian dust to hawaiian soils isotopic elemental and mineral mass balances hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs including sea salt aerosols and asian dust we use soil mineralogy and radiogenic isotope geochemistry sr and nd to evaluate the impact of asian dust on a chronosequence of hawaiian soils dust becomes an important constituent of soils 20 ky and older near surface 50 cm depth horizons contain as much as 30 quartz a mineral absent from local parent material basaltic sr and nd isotope signatures in these horizons have been completely overprinted by asian dust signatures with 87sr 86sr ratios as high as 0 723 and end values as low as 7 ree patterns in these soils are indistinguishable from that of average upper continental crust quartz abundance and nd isotopes provide two independent tracers of long term dust additions to the chronosequence soils the two tracers indicate a minimum long term average dust accretion rate of 125 mg cm 2 ky 1 at the 150 ka chronosequence site roughly a factor of three higher than holocene dust accumulation rates estimated from north pacific sediment cores we find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older requiring a loss mechanism for accreted dust on the basis of the geomorphic stability of these sites observed preferential loss of dust derived mica relative to quartz and estimates of soil si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils dust has a profound effect on the budgets of elements that are susceptible to leaching losses dust becomes the dominant source of soil nutrients si and p in the oldest most intensely weathered soils we calculate a dust derived p input flux of 0 8 mg cm 2 ky 1 and a dust derived si input flux of 35 mg cm 2 ky 1 si leaching fluxes are high 1400 mg cm 2 ky 1 in the youngest 2 ka soils and drop systematically with soil age to a value that closely balances the dust derived si input flux by 4100 ka extremely refractory elements such as nb which are concentrated in soils by residual enrichment processes are much less readily impacted by dust addition although dust has had a pronounced impact on sr and nd isotopic budgets and on soil mineral composition at the 150 ka site dust cannot have contributed 4 of the total nb contained in this soil profile

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

biogenic silica storage in soils the role of plants in soil development has become of crucial interest in quantifying global weathering processes on the volcanic reunion island indian ocean a 15 cm thick phytolith rich horizon biogenic opal a developed at the expense of trachytic ashes between 3820 85 and 335 90 yr b p then 97 138 t km 2 yr of sio 2 were biogeochemically recycled from the weathering of parent rocks through a past forest of bamboos nastus borbonicus this rate is in the same range as the present chemical weathering rates of silica in the area considered to be among the highest in the world these results demonstrate that the storage of biogenic silica in soils may be significant and may retard the output of silica to rivers and ocean

slow chemical weathering in a semiarid climate changes in the mineralogy and geochemistry of subaerial lava flows in the deschutes river basin central oregon abstract the progression of mineral alteration and chemical changes of thirteen pairs of fresh and weathered surficial lava samples from the deschutes river basin of central oregon were studied to understand incipient weathering in a semiarid climate samples range from basalt to dacite in composition and quaternary to late miocene in age only the tertiary samples showed significant chemical weathering based on element mobility and mineral alteration the slight climatic variations between tertiary outcrop locations map 38 74 cm yr and mat of 7 2 7 6 c did not correlate with degree of weathering the susceptibility sequence of phases was determined to be glass olivine plagioclase orthopyroxene clinopyroxene fe ti oxides the older weathered samples have experienced significant leaching of mobile cations e g ca mg sr resulting in a lower calculated weathering index the tertiary weathered samples are enriched in ree particularly lree which is attributed to a lree phosphate found in association with allophane clay mineralization in plagioclase these samples also have a negative ce anomaly likely due to its tetravalent state in oxidizing conditions and subsequent lack of mobility in weathering solutions although there is a clear petrographic and chemical distinction between fresh and weathered lavas 2 ma the distinction between fresh and weathered samples is not evident in the quaternary lavas significant element mobility between sample pairs is closely correlated to the breakdown of plagioclase and orthopyroxene and the subsequent secondary mineralization of clays and or lree rich phosphates with the exception of plagioclase all the mineral phases in the deschutes lavas have persistence times or stability of at least one and possibly several orders of magnitude greater than the same phases in soil chronosequences or weathering rinds in more temperate and humid climates as a result elemental release rates and silicate fluxes are much lower under semiarid conditions and their role in the global carbon cycle should be examined more closely

using a reactive transport model to elucidate differences between laboratory and field dissolution rates in regolith the reactive transport model flotran was used to forward model weathering profiles developed on granitic outwash alluvium over 40 3000 ka from the merced california usa chronosequence as well as deep granitic regolith developed over 800 ka near davis run virginia usa baseline model predictions that used laboratory rate constants km measured fluid flow velocities v and bet volumetric surface areas for the parent material a o m were not consistent with measured profiles of plagioclase potassium feldspar and quartz reaction fronts predicted by the baseline model are deeper and thinner than the observed consistent with faster rates of reaction in the model reaction front depth in the model depended mostly upon saturated versus unsaturated hydrologic flow conditions rate constants controlling precipitation of secondary minerals and the average fluid flow velocity va unsaturated hydrologic flow conditions relatively open with respect to co2 g resulted in the prediction of deeper reaction fronts and significant differences in the separation between plagioclase and potassium feldspar reaction fronts compared to saturated hydrologic flow relatively closed with respect to co2 g under saturated or unsaturated flow conditions the rate constant that controls precipitation rates of secondary minerals must be reduced relative to laboratory rate constants to match observed reaction front depths and measured pore water chemistry additionally to match the observed reaction front depths va was set lower than the measured value v for three of the four profiles the reaction front gradients in mineralogy and pore fluid chemistry could only be modeled accurately by adjusting values of the product kma o m by assuming km values were constrained by laboratory data field observations were modeled successfully with tst like rate equations by dividing measured values of a o m by factors from 50 to 1700 alternately with sigmoidal or alinhibition rate models this adjustment factor ranges from 5 to 170 best fit models of the wetter hydrologically saturated davis run profile required a smaller adjustment to a o m than the drier hydrologically unsaturated merced profiles we attributed the need for large adjustments in va and a o m necessary for the merced models to more complex hydrologic flow that decreased the reactive surface area in contact with bulk flow water e g dead end pore spaces containing fluids that are near or at chemical equilibrium thus rate models from the laboratory can successfully predict weathering over millions of years but work is needed to understand how to incorporate changes in what controls the relationship between reactive surface area and hydrologic flow

pyrite oxidation in the tertiary sands of the london basin aquifer abstract possible groundwater quality changes related to pyrite oxidation during artificial groundwater recharge and its storage in the tertiary sands of the london basin are investigated pyrite textures in the tertiary sands are examined by scanning electron microscopy while an experimental approach is used to study mechanisms of pyrite oxidation and of some associated chemical reactions in the tertiary sands of the london basin aquifer pyrite occurs as aggregates made of discrete individual crystals 0 5 5 m in size or in a cryptocrystalline form often as pseudomorphs of biogenic debris it can expose a very large specific surface area to porefluids although ferric iron which can be an oxidising agent of pyrite is abundant in the solid phase of the tertiary sands it does not appear to take a significant part in this case pyrite oxidation seems to rely on a supply of oxygen leaching experiments using a 0 001 m h2so4 solution were carried out to examine interactions between mildly acidic groundwater resulting from pyrite oxidation at a moderate rate and the host sediment in the presence of caco3 in the solid phase h is rapidly buffered by caco3 dissolution oscillations of this reaction around equilibrium appear to trigger cation exchange reactions on clay mineral surfaces resulting in the release of major cations e g k and mg into solution in the absence of caco3 in the solid phase h buffering occurs less efficiently solely through exchange of cations for h on clay minerals surfaces if the rate of pyrite oxidation in the tertiary sands becomes high enough for the buffering capacity of the system to be exceeded the groundwater ph begins to fall interactions between low ph 2 groundwaters and the host sediments were examined by leaching solid material in 0 01 m and 0 1 m h2so4 solutions concentrations of fe mg and k increase in solution throughout the experiment indicating partial dissolution of clay minerals the composition of the porefluid thus depends on the geochemical composition and surface area of the different clay minerals present

basalt weathering across scales abstract weathering of silicate minerals impacts many geological and ecological processes for example the weathering of basalt contributes significantly to consumption of atmospheric carbon dioxide co2 and must be included in global calculations of such consumption over geological timeframes here we compare weathering advance rates for basalt wd where d and indicate the scale at which the rate is determined and surface area measured respectively from the laboratory to the watershed scales data collected at the laboratory weathering rind soil profile and watershed scales show that weathering advance rate of basalt is a fractal property that can be described by a fractal dimension dr 2 3 by combining the fractal description of rates with an arrhenius relationship for basalt weathering we derive the following equation to predict weathering advance rates at any spatial scale from weathering advance rates measured at the bet scale w d k 0 a d r 2 e e a r t here k0 is the pre exponential factor 1 29 107 mm3 mm 2 yr 1 ea is the activation energy 70 kj mol 1 and a is a spatial constant related to the scale of measurement of bet surface area 10 7 mm the term a d r 2 is the roughness the roughness fractal dimension can be conceptualized as a factor related to both the thickness of the reaction front and the specific surface area within the reaction front however the above equation can also be written in terms of a surface fractal dimension and the hypothetical average grain radius these fractal dimensions provide insight into reaction front geometry and should vary with lithology once the surface area discrepancy has been accounted for using this method we find a one to two order of magnitude range in weathering advance rates measured at any scale or temperature that can be attributed to factors such as changes in erosional regime parent lithology mechanism climate composition of reacting fluid and biological activity our scaled equation when used to predict global basalt co2 consumption based upon global lithologic maps yields an uptake flux 1 75 1013 mol co2 yr 1 within the predicted error of fluxes estimated based upon riverine measurements

pore scale heterogeneity in the mineral distribution and reactive surface area of porous rocks abstract the reactive surface area is an important control on interfacial processes between minerals and aqueous fluids in porous rocks spatial heterogeneity in the surface area can lead to complications in modelling reactive transport processes but quantitative characterisation of this property has been limited in this paper 3d images obtained using x ray micro tomography were used to characterise heterogeneity in surface area in one sandstone and five carbonate rocks measurements of average surface area from x ray imagery were 1 2 orders of magnitude lower than measurements from nitrogen bet a roughness factor defined as the ratio of bet surface area to x ray based surface area was correlated to the presence of clay or microporosity co registered images of berea sandstone from x ray and energy dispersive spectroscopy imagery were used to guide the identification of quartz k feldspar dolomite calcite and clays in x ray images in berea sandstone clay and k feldspar had higher average surface area fractions than their volumetric fractions in the rock in the edwards carbonate however modal mineral composition correlated with surface area by sub sampling digital images statistical distributions of the surface area were generated at various length scales of subsampling comparing these to distributions used in published modelling studies showed that the common practice of leaving surface area and pore volume uncorrelated in a pore has lead to unrealistic combinations of surface area and pore volume in the models we suggest these models adopt a moderate correlation based on observations in berea sandstone constraining ratios of surface area to pore volume to a range of values between that of quartz lined and five times that of clay lined spheres appeared sufficient

a mathematical model for steady state regolith production at constant erosion rate it has been hypothesized that many soil profiles reach a steady state thickness in this work such profiles were simulated using a one dimensional model of reaction with advective and diffusive solute transport a model rock is considered consisting of albite that weathers to kaolinite in the presence of chemically inert quartz the model yields three different steady state regimes of weathering at the lowest erosion rates a local equilibrium regime is established where albite is completely depleted in the weathering zone this regime is equivalent to the transport limited regime described in the literature with an increase in erosion rate transition and kinetic regimes are established in the transition regime both albite and kaolinite are present in the weathering zone but albite does not persist to the soil air interface in the weathering limited regime here called the kinetic regime albite persists to the soil air interface the steady state thickness of regolith decreases with increasing erosion rate in the local equilibrium and transition regimes but in the kinetic regime this thickness is independent of erosion rate analytical expressions derived from the model are used to show that regolith production rates decrease exponentially with regolith thickness the steady state regolith thickness increases with the darcy velocity of the pore fluid and in the local equilibrium regime may vary markedly with small variations in this velocity and erosion rate in the weathering limited regime the temperature dependences for chemical weathering rates are related to the activation energy for the rate constant for mineral reaction and to the h of dissolution while for local equilibrium regimes they are related to the h only the model illustrates how geochemical and geomorphological observations are related for a simple compositional system the insights provided will be useful in interpreting natural regolith profiles copyright 2010 john wiley sons ltd

the chemical composition of rivers and snow affected by the 2014 2015 barðarbunga eruption iceland abstract the 2014 15 bar arbunga volcanic eruption was the largest in iceland for more than 200 years this eruption released into the atmosphere on average 60 000 tonnes day of so2 30 000 tonnes day of co2 and 500 tonnes day of hcl affecting the chemical composition of rain snow and surface water the interaction of these volcanic gases with natural waters decreases fluid ph and accelerates rock dissolution this leads to the enhanced release of elements including toxic metals such as aluminium to these waters river monitoring including spot and continuous osmotic sampling shows that although the water conductivity was relatively stable during the volcanic unrest the dissolution of volcanic gases increased the so4 f and cl concentrations of local surface waters by up to two orders of magnitude decreasing the carbon alkalinity in addition the concentration of sio2 ca mg na and trace metals rose considerably due to the water molten lava and hot solid lava interaction the presence of pristine lava and acidic gases increased the average chemical denudation rate calculated based on na flux within jokulsa a fjollum catchment by a factor of two compared to the background flux melted snow samples collected at the eruption site were characterised by a strong dependence of the ph on so4 f and cl and metal concentrations indicating that volcanic gases and aerosols acidified the snow protons balanced about half of the negatively charged anions the rest was balanced by water soluble salts and aerosols containing a variety of metals including al fe na ca and mg the concentrations of f al fe mn cd cu and pb in the snowmelt water surpassed drinking and surface water standards snowmelt river water mixing calculations indicate that low alkalinity surface waters such as numerous salmon rivers in east iceland will be more affected by polluted snowmelt waters than high alkalinity spring and glacier fed rivers

modelling soil thickness in the critical zone for southern british columbia abstract the critical zone cz is defined as the outer layer of the solid earth extending from the vegetation canopy through the pedosphere and down to the bottom of the weathered bedrock zone most biological chemical and physical interactions take place in the cz and it is here that most terrestrial life is found being able to predict the thickness of the pedosphere i e soil thickness from the mineral soil surface down to bedrock in the cz can lead to a better understanding of rates of physical chemical change such as carbon sequestration soil erosion and water storage the objective of this study was to accurately map the thickness of the pedosphere on a landscape scale for a ca 3400 km 2 landscape in southern british columbia bc the data inputs used were exposed bedrock eb points identified from orthophotos well record wr data and in situ observations of soil thickness for which conditioned latin hypercube sampling clhs was used to define a set of locations where soil thickness was determined four methods were then used to model soil thickness as a function of environmental data layers derived from a digital elevation model and satellite imagery generalized linear model glm random forest rf glm residual kriging glmrk and rf residual kriging rfrk an equal weighted random sampling scheme of 100 eb wr and in situ soil thickness points was used with each model a second sampling scheme was used with the same wr and in situ soil thickness points and an additional 5000 randomly sampled eb points used to improve prediction accuracy with limited data sources of the modelling methods used glmrk proved to be the best method for shallow thicknesses 0 to 2 m as assessed by root mean square error rmse values 1 87 m with the equal weighted sample scheme the addition of the 5000 eb points substantially improved predictions for shallow soil thickness rmse 0 9 m as well as soil thickness in the 2 5 m range while having negligible impact on predictions of thicker soils this demonstrates that an exposed bedrock layer can help constrain shallow soil thickness predictions when used in conjunction with geostatistical approaches such as glmrk and rfrk for mapping soil thickness

weathering and erosion of fractured bedrock systems we explore the contribution of fractures joints in controlling the rate of weathering advance for a low porosity rock by using methods of homogenization to create averaged weathering equations the rate of advance of the weathering front can be expressed as the same rate observed in non fractured media or in an individual block divided by the volume fraction of non fractured blocks in the fractured parent material in the model the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material the low porosity rock weathers by reaction transport processes as observed in field systems the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non fractured system because the volume fraction of blocks is 1 the increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density and to the increase in exposure of surface of low porosity rock to reaction transport for constant fracture aperture the weathering advance rate increases when the fracture spacing decreases equations describing weathering advance rate are summarized in the list of selected equations if erosion is imposed at a constant rate the weathering systems with fracture bounded bedrock blocks attain a steady state in the erosional transport limited regime bedrock blocks no longer emerge at the air regolith boundary because they weather away in the weathering limited or kinetic regime blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate for convex hillslopes the block size exposed at the surface increases downslope this model can explain observations of exhumed rocks weathering in the luquillo mountains of puerto rico published 2017 this article is a u s government work and is in the public domain in the usa

reconstruction of a toposequence on volcanic material in the honje mountains ujung kulon peninsula west java abstract soil formation on weathered acid tuffs was studied in the national park of the ujung kulon peninsula west java indonesia in a toposequence in the honje mountains five principal types of materials layers were distinguished based on micromorphology mineralogy and chemistry the deepest part of all profiles corresponds to a saprolite derived from tertiary and pleistocene volcanic deposits locally showing the influence of pedoplasmation the overlying layer also contains saprolite fragments but later pedogenic processes are indicated by striated b fabrics and strong clay illuviation in most profiles formation of plinthite partly postdates clay illuviation a transition layer separates this part of the profile from the top layer which is characterised by a more open microstructure an undifferentiated b fabric and the presence of pumice gravel feldspars and pyroxenes originating from the krakatau ash fall after the uplift of the mio pliocene banten tuffs deep ferrallitic weathering was followed by a period of erosion during which stone lines were formed hitherto these have been attributed to occasional coarser tephra falls in a subsequent period of stability weathering and processes of pedoplasmation clay illuviation and plinthitisation were active during the eruption of the nearby mount krakatau in 1883 the area was covered by a layer of fresh ash which affected the composition and microfabric of the top layer leading to more andic properties no important erosion event followed the ash fall

the impact of climate on the biogeochemical functioning of volcanic soils abstract rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering secondary mineral synthesis and soil chemical properties along an arid to humid climosequence on kohala mountain hawaii we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall the lavas are influenced by a strong rain shadow with mean annual precipitation map averaging 160 mm near the coast and rising to 3000 mm near the summit a temperature decline from 24 to 15 c with increasing elevation is matched by lower potential evapotranspiration et a water balance model monthly precipitation minus monthly et defines three broad climate zones along the sampling transect an arid zone with moisture deficit in every month an intermediate zone with moisture deficit during low rainfall summer months and moisture surplus during high rainfall winter months and a humid zone with moisture surplus during every month the annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index the index reaches 1 total filling of the pore space on an annual basis at about 1400 mm map index values 1 imply intense leaching conditions because of pore water replacement in these 170 ka soils leaching losses of soluble base cations and si are nearly complete at index values 1 whereas only 60 of al has been lost at index values 1400 mm soil buffering capacity has been totally exhausted leading to low ph and low ecec the nonlinear decline in ecec is irreversible under natural conditions base cation depleted soils will remain so even if the climate shifts to drier conditions in contrast a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals elements are lost to leaching and surface chemistry is modified there is a time dimension implied in climate gradient studies soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system the details of this transfer depend on present and past climate and the age and erosional stability of landscapes

chemical weathering in the krishna basin and western ghats of the deccan traps india rates of basalt weathering and their controls rates of chemical and silicate weathering of the deccan trap basalts india have been determined through major ion measurements in the headwaters of the krishna and the bhima rivers their tributaries and the west flowing streams of the western ghats all of which flow almost entirely through the deccan basalts samples n 63 for this study were collected from 23 rivers during two consecutive monsoon seasons of 2001 and 2002 the total dissolved solid tds in the samples range from 27 to 640 mg l 1 the rivers draining the western ghats that flow through patches of cation deficient lateritic soils have lower tds average 74 mg l 1 whereas the bhima except at origin and its tributaries that seem to receive na cl and so4 from saline soils and anthropogenic inputs have values in excess of 170 mg l 1 many of the rivers sampled are supersaturated with respect to calcite the chemical weathering rates cwr of selected basins which exclude rivers supersaturated in calcite and which have high cl and so4 are in range of 3 to 60 t km 2 y 1 this yields an area weighted average cwr of 16 t km 2 y 1 for the deccan traps this is a factor of 2 lower than that reported for the narmada tapti wainganga ntw systems draining the more northern regions of the deccan the difference can be because of i natural variations in cwr among the different basins of the deccan ii selection of river basin for cwr calculation in this study and iii possible contribution of major ions from sources in addition to basalts to rivers of the northern deccan traps silicate weathering rates swr in the selected basins calculated using dissolved mg as an index varies between 3 to 60 t km 2 y 1 nearly identical to their cwr the ca mg and na mg in these rivers after correcting for rain input are quite similar to those in average basalts of the region suggesting near congruent release of ca mg and na from basalts to rivers comparison of calculated and measured silicate ca in these rivers indicates that at most 30 of ca can be of nonsilicate origin a likely source being carbonates in basalts and sediments the chemical and silicate weathering rates of the west flowing rivers of the deccan are 4 times higher than the east flowing rivers this difference is due to the correspondingly higher rainfall and runoff in the western region and thus reemphasises the dominant role of runoff in regulating weathering rates the silicon weathering rate swr in the krishna basin is 15 t km 2 y 1 within a factor of 2 to those in the yamuna bhagirathi and alaknanda basins of the himalaya suggesting that under favourable conditions intense physical weathering high runoff granites and the other silicates in the himalaya weather at rates similar to those of deccan basalts the co2 consumption rate for the deccan is deduced to be 3 6 105 moles km 2 y 1 based on the swr the rate though is two to three times lower than reported for the ntw rivers system it still reinforces the earlier findings that in general basalts weather more rapidly than other silicates and that they significantly influence the atmospheric co2 budget on long term scales

chemical denudation rates of the western canadian orogenic belt the stikine terrane we report in this paper major element and sr isotopic ratio analyses for 19 rivers draining the stikine province in the western canadian cordillera dissolved solutes suspended sediments and river sands were analyzed the aim of the paper is to calculate chemical denudation rates for this uplifted accretionary prism mainly dominated by volcanics and volcanic derived sedimentary rocks the major chemical features of the stikine rivers are their dilute character and the excess of ca mg and sr in the dissolved load with regards to na molar ca na ratios from 2 to 14 mean value of 5 4 are observed although the region is dominated by silicate rocks suspended sediments show ca na ratios close to 0 4 isotopically most of the rivers investigated display lower sr isotopic ratios in the dissolved load compared to the suspended load which is very unusual to account for the excess of cations three different hypotheses are tested by an inversion method allowing us to determine the origin of each major solute the first scenario assumes that the excess of ca and mg is due to the dissolution of unradiogenic carbonates the second scenario supposes that the excess of base cations is derived from the preferential dissolution of ca and mg rich minerals such as amphiboles in the third scenario all solutes are assumed to derived from silicate weathering the cationic weathering rates of silicates although imprecise are in the order of 2 5 tons km2 year for the two first hypotheses if all solutes are taken into account denudation rates are between 9 and 15 tons km2 year on a global scale these numbers are among the lowest found for rivers draining basaltic rocks they are however found to be consistent with the global law for chemical weathering rates established for basaltic rocks showing that chemical denudation rate are linearly correlated to runoff and depend on temperature according to an arrhenius law the rivers draining the complex assemblage of volcanic and volcanic derived rocks in the subduction collision geological context of the stikine province thus respond similarly to the other volcanic provinces of the world this does not seem to be true for associated co2 consumption rates the abundance of sulfate ions in the dissolved load possibly derived from oxidative weathering of sulfides may indeed provide a non negligible part of the protons necessary for chemical weathering within canada the stikine geologic province is one whose silicates weather at a high rate the order of chemical weatherabilty of canadian geological province from the lowest to the highest is established as follows granites shales of the rockies volcanics of the western cordillera shales of the interior platform carbonates

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

geochemical anatomy of a spheroidally weathered diabase major trace element concentrations and nd sr isotope ratios were measured in micro drilled samples of a 237 ga old hand specimen sized spheroidally weathered diabase from southern india a sample of the un weathered diabase dike was also analyzed x ray micro ct imaging of the weathered sample shows three dominant mineral phases which are plagioclase pyroxene and a fe bearing phase possibly hematite and or ilmenite this imaging documents the pervasive nature of two generations of ribbon like cross cutting fractures the older fracture is sealed while the more recent fracture is open without any in filling the values of the chemical index of alteration cia of the samples show a wide range but are less than 50 despite being a relatively less weathered rock we observe that concentrations of major minor and trace elements vary significantly with the percentage relative standard deviation rsd for the elements ranging from 10 2 41 8 the cia of the samples do not show any trend with the position of the sample in the hand specimen barring ca and li whose concentrations decrease from the core to the rim of the sample there is no significant spatial trend in the concentrations of the elements concentrations of na al and sr increase with increasing cia values while concentrations of mg fe and sc decrease with increasing cia the strong positive correlations of na and al as well as na and sr indicates preferential weathering of plagioclase in the diabase na ca increases while mg al mg na mg ca fe al and sc sr decrease with increasing cia values and the un weathered rock plots in the middle of these trends such variations are explained in terms of differential weathering of plagioclase in samples with lower cia than the un weathered rock w1 type and pyroxene in samples with higher cia than the un weathered rock w2 type which have varying resistance to weathering at the hand specimen scale the variability in the weathering indices like cia are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering chondrite normalized la sm and gd lu co vary with cia values indicating mobility of the rees during spheroidal weathering even at the hand specimen scale the eu anomaly also increases with increasing cia values which is explained by differential weathering of pyroxene and plagioclase we observe large percentage deviations of the nb normalized concentrations of elements from the un weathered rock in specific samples but no spatial trend is observed overall the variations in element concentrations can be explained by varying fluid mobility of the elements selective weathering of the minerals in the diabase and ambient environmental conditions considerable nd and sr isotopic variability is observed at the hand specimen scale and is explained in terms of weathering related fractionation of parent daughter ratios this elemental fractionation must have happened long time ago to allow for radiogenic decay of the long lived isotopes of rb 87 and sm 147 the spread rsd in the initial sr and nd isotope compositions of the weathered samples reach a minimum value around 12 13 ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent daughter ratios for nd isotopes the average epsilon nd 1 2 ga of the weathered samples coincides with the epsilon nd 1 2 ga of the unweathered rock the timing of the weathering event coincides with the timing of the breakup of the columbia supercontinent and follows wide spread alkaline volcanism in the indian subcontinent this is the first such attempt to determine the timing of a weathering event in rocks using long lived radioactive isotopes c 2016 elsevier b v all rights reserved

development of a fracture network in crystalline rocks during weathering study of bishop creek chronosequence using x ray computed tomography and 14c pmma impregnation method the progressive development of porosity during subsoil weathering of granodiorite clasts was studied at the bishop creek moraine chronosequence in east central california fractures and other pores were examined using two complementary imaging techniques x ray computed tomography xrct and a 14c pmma 14c polymethylmethacrylate method the well known xrct method allows the investigation of three dimensional 3 d pore space 14c pmma is a less known method based on the complete impregnation of pore space with 14c doped pmma and subsequent autoradiograph of a rock section these imaging methods allow us to decipher the evolution of pore space in the granodiorite during the 120 k y weathering period the 14c pmma imaging technique was found to be more suitable for following the evolution of the whole sequence from intact bedrock to saprock in terms of crack density porosity and aperture working with hand specimens this method was adapted to detect both the low aperture fractures microcracks and macrocracks only a slight and progressive increase in total fracture density was observed during the whole weathering period however this trend does not hold if macrocracks and microcracks are separated microcrack density slightly decreases whereas macrocrack density increases due to a progressive expansion of microcracks the total porosity of the rock increases during weathering and is correlated to the progressive aperture increase of all types of cracks this evolution is accompanied by a change of crack morphology and connectivity and an overall increase in intragranular porosity of biotite and plagioclase aggregates

lattice boltzmann based approaches for pore scale reactive transport important geoscience and environmental applications such as geologic carbon storage environmental remediation and unconventional oil and gas recovery are best understood in the context of reactive flow and multicomponent transport in the subsurface environment the coupling of chemical and microbiological reactions with hydrological and mechanical processes can lead to complex behaviors across an enormous range of spatial and temporal scales these coupled responses are also strongly influenced by the heterogeneity and anisotropy of the geologic formations reactive transport processes can change the pore morphology at the pore scale thereby leading to nonlinear interactions with advective and diffusive transport which can strongly influence larger scale properties such as permeability and dispersion therefore one of the greatest research challenges is to improve our ability to predict these processes across scales doe 2007 the development of pore scale experimental and modeling methods to study reactive processes involving mineral precipitation and dissolution and biofilm dynamics allows more fundamental investigation of physical behavior so that more accurate and robust upscaled constitutive models can be developed for the continuum scale a pore scale model provides fundamental mechanistic explanations of how biogeochemical processes and pore scale interfacial reactions alter flow paths by pore plugging and dissolving under different geochemical compositions and pore configurations for example dissolved co2 during geological co2 storage may react with minerals in fractured rocks confined aquifers or faults resulting in cementation and or dissolution and altering hydrodynamics of reactive flow this can be observed in a natural analogue where primary porosity in sandstone is cemented by carbonate precipitates affecting dissolved co2 flow paths at the little garde wash fault utah e g fig 1a b several other examples demonstrating macroscopic characteristics of calcium carbonate caco3 precipitation in figure 1 include an elongated concretion along the groundwater flow direction caco3 precipitation along the vertical pathway sealed

quantifying chemical weathering rates along a precipitation gradient on basse terre island french guadeloupe new insight from u series isotopes in weathering rinds abstract inside soil and saprolite rock fragments can form weathering clasts alteration rinds surrounding an unweathered core and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates recently uranium series u series disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds to further investigate whether the u series isotope technique can document differences in long term weathering rates as a function of precipitation we conducted a new weathering rind study on tropical volcanic basse terre island in the lesser antilles archipelago in this study for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient electron microprobe emp point measurements bulk major element contents and u series isotope compositions were determined in two weathering clasts from the deshaies watershed with mean annual precipitation map 1800 mm and temperature mat 23 c on these clasts five core rind transects were measured for locations with different curvature high medium and low of the rind core boundary results reveal that during rind formation the fraction of elemental loss decreases in the order ca na k mg si al zr ti fe such observations are consistent with the sequence of reactions after the initiation of weathering specifically glass matrix and primary minerals plagioclase pyroxene weather to produce fe oxyhydroxides gibbsite and minor kaolinite uranium shows addition profiles in the rind due to the infiltration of u containing soil pore water into the rind as dissolved u phases u is then incorporated into the rind as fe al oxides precipitate such processes lead to significant u series isotope disequilibria in the rinds this is the first time that multiple weathering clasts from the same watershed were analyzed for u series isotope disequlibrian and show consistent results the u series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a u series mass balance model the weathering advance rates generally decreased with decreasing curvature 0 17 0 10 mm kyr for high curvature 0 12 0 05 mm kyr for medium curvature and 0 11 0 04 0 08 0 03 0 06 0 03 mm kyr for low curvature locations the observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models i e that the curvature of the rind core boundary controls the porosity creation and weathering advance rates at the clast scale at the watershed scale the new weathering advance rates derived on the low curvature transects for the relatively dry deshaies watershed average rate of 0 08 mm kyr map 1800 mm and mat 23 c are 60 slower than the rind formation rates previously determined in the much wetter bras david watershed 0 18 mm kyr low curvature transect map 3400 mm and mat 23 c also on basse terre island thus a doubling of map roughly correlates with a doubling of weathering advance rate the new rind study highlights the effect of precipitation on weathering rates over a time scale of 100 kyr weathering rinds are thus a suitable system for investigating long term chemical weathering across environmental gradients complementing short term riverine solute fluxes

mineral neoformation in pore spaces during alteration and weathering of andesitic rocks in humid tropical indonesia abstract the morphology chemistry and mineralogy of minerals newly formed in voids products of weathering of andesitic rocks were studied using optical and submicroscopic techniques on thin sections combined with scanning electron microscopy sem energy dispersion x ray analysis edxra and x ray diffraction analysis on subsamples samples were taken from the core the outer core and the soft outer layers of weathered andesitic boulders in the cigudeg area west java about 75 km south of jakarta indonesia with udic moisture and isohyperthermic temperature regime the neoformed minerals observed in planar voids and cavities comprise 1 1 phyllosilicates goethite and gibbsite and juxtaposed coatings of halloysite mixed layer phyllosilicates goethite and gibbsite the mineral sequences observed confirm former studies on the groundmass of the boulder in the first weathering layer an absolute accumulation of gibbsite takes place further away from the core the gibbsite is redissolved and replaced by illuvial phyllosilicate clay the possibility of inheritance of the 2 1 layer silicates from a hydrothermal alteration is discussed the sharp boundaries in the juxtaposed coatings point to a stepwise sequential rather than a gradual evolution of the microenvironment

direct evidence of the feedback between climate and weathering long term climate moderation is commonly attributed to chemical weathering the higher the temperature and precipitation the faster the weathering rate weathering releases divalent cations to the

quantitative model prediction of the combined effect of moisture content and temperature on purple mudstone decay in south western china abstract rock decay is mainly the result of the combined effects of moisture content and temperature but little is known about the quantitative relationship between these variables and the rate of rock decay in this study we develop quantitative calculation models of rock decay rate under laboratory conditions and validate the efficiency of these models by comparing the predicted rock decay mass and that measured for rock exposed in the field rainfall and temperature data in the field were standardised to a dimensionless moisture content and temperature variables respectively and then the predicted rock decay mass was calculated by the models the measured rock decay mass was determined by manual sieving based on our previously determined relationship between a single factor moisture content or temperature and the rate of rock decay in the laboratory power function models are developed results show that the rock decay mass calculated by the model was comparable with field data with averaged relative errors of 1 53 9 00 and 11 82 for the tuodian group j 3 t matoushan group k 2 m and lufeng group j 1 l respectively which are mainly due to inaccurate transformation of field rainfall into the rock moisture content and artificial disturbance when the samples were sieved in the field our results show that the developed models based on laboratory derived rates can accurately predict the decay rates of mudstones exposed in the field

uranium thorium chronometry of weathering rinds rock alteration rate and paleo isotopic record of weathering fluids abstract the potential of 238u 234u 230th chronometry for constraining the formation rate of weathering rinds developed on fresh rocks is assessed by analyzing a weathering rind on a basaltic clast from a 125 kyr old costa rican alluvial terrace eighteen subsamples were collected from one section of the clast by drilling cores 4 mm in diameter and 5 mm depth along two transects straddling the core rind boundary variations of major and trace element concentrations along the two transects point out a intense loss of alkaline and alkaline earth elements b conservative behaviour of elements such as zr hf and th and c external input of u into the rind without any evidence of u loss during basalt weathering in addition variations in u concentrations along the transects show that the main u th fractionation process associated with the weathering of the basaltic clast is an external input of u without addition nor loss of th in the basalt rind transition zone and that once deposited u is immobile in the weathering rind in the frame of this interpretation scheme a weathering rate of 0 5 0 2 mm kyr can be calculated for the studied clast which is consistent with geological and isotopic evidence constraining the depositional ages of the terraces in addition the variations in the 234u 238u ratio along the analysed transects as well as the increase in sr isotopic ratios within the weathering rind are best explained by temporal variation of the u activity ratios and sr isotope ratios of the soil solutions brought into the rind this work highlights how well detailed u th chronological studies of weathering rinds can 1 constrain the formation rates of weathering systems and 2 record the time variation of isotopic composition of weathering fluids

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