chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

major and trace element abundances and strontium isotopes in the nyong basin rivers cameroon constraints on chemical weathering processes and elements transport mechanisms in humid tropical environments this paper aims to improve our understanding of chemical weathering processes and element transport mechanisms in the humid tropical environments we studied the nyong river basin 27 800 km2 located on the northwestern part ntem complex of the congo craton central africa the dissolved concentrations i e 0 20 m of major and trace elements dissolved organic carbon doc and the 87sr 86sr ratios have been measured in more than 20 rivers draining watersheds with various surface areas 1 to 28 000 km2 all these rivers exhibit low major cations concentrations i e na mg k ca but high concentrations of some trace elements al fe th zr y ree silica and doc the total dissolved load tds is low 20 mg l 1 and dominated by silica and organic matter the comparison of different watersheds shows us that chemical weathering is more efficient in the small unit watersheds all the sampled rivers exhibit a wide range of 87sr 86sr ratios and high ca na ratio that can be explained by the heterogeneity of the crystalline rock constituting the cratonic basement four selected rivers mengong awout soo and nyong having different drainage areas and hydrological parameters were sampled over a 2 year period collected data show that all these rivers present the same monthly seasonal variations with higher concentrations during rainy season and lower concentrations during dry season this implies that the weathering and transport mechanisms of small watersheds can be extended to the whole nyong basin in the small unit watersheds chemical weathering mainly occurs in swamp zones where mineral dissolution is enhanced by humic substances these swamp zones constitute a pool of organic rich water which can be quickly mobilized during rain seasons in these waters doc and insoluble element concentrations e g al fe and th were strongly correlated which show the key role of organic colloidal matter in the transport of some insoluble elements some other relationships al vs fe ree vs al were also examined in order to get information about chemical weathering or element transport based on these data it has been concluded that the chemical composition of these river waters is controlled by geomorphic and historic factors e g thick cation poor soil in contrast the present day climatic parameters high rainfall and temperature play a minor role in water chemistry regulation even though they are likely to enhance mineral dissolution even if organic matter favors mineral dissolution chemical weathering in this area is low compared to other world regions which suggest on a global scale a relatively small effect of these environments on the co2 consumption

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

geochemistry of daihai lake sediments inner mongolia north china implications for provenance sedimentary sorting and catchment weathering abstract to advance the understanding of sediment distribution catchment weathering hydraulic sorting and sediment provenance in a tectonically stable basin the geochemistry of surface sediment samples from daihai lake in north china is presented mud bulk sediments were analyzed for 10 major and 30 trace elements organic carbon and nitrogen and for 87sr 86sr ratios in silicate fraction acid insoluble ai and carbonates acid soluble as enrichment in fe2o3 cao mno mgo k2o and p2o5 and their positive correlation with al2o3 rb sr ratios and bivariate discrimination diagrams of k vs rb and th u vs th suggest low chemical weathering intensity under semiarid conditions in the daihai catchment accumulation of clays in deeper water sites by sedimentary sorting results in an evident distribution of sediment composition with the lake water depth although similar chondrite normalized ree rare earth element patterns of lake sediment samples with lree light rare earth element enrichment negative eu anomalies and an almost flat hree heavy rare earth element pattern may suggest similar sedimentary source rocks original sediment provenance can be recognized on the basis of their distinctive 87sr 86sr ai ratios and immobile th zr ti and nb trace element signature separating from weathering and sedimentary processes as well as from grain size effect

weathering processes and the composition of inorganic material transported through the orinoco river system venezuela and colombia abstract the composition of river borne material in the orinoco river system is related primarily to erosion regime which in turn is related to tectonic setting especially notable is the contrast between material derived from tectonically active mountain belts and that from stable cratonic regions for a particular morpho tectonic region the compositional suites of suspended sediment bed material overback deposits and dissolved phases are fairly uniform are are typically distinct from whose of other regions for each region a consistent set of chemical weathering reactions can be formulated to explain the composition of dissolved and solid loads in developing these formulations erosion on slopes and storage of solids in soils and alluvial sediments are important considerations compositionally verymature sediment is derived from areas of thick soils where erosion is transport limited and from areas where sediments are stored for extended periods of time in alluvial deposits compositionally immature sediments are derived from tectonically active mountain belts where erosion is weathering limited weathering limited erosion also is important in the elevated parts of the guayana shield within areas of sleep topography compared to the mountain belts sediments derived from elevated parts of the shield are more mature a greater degree of chemical weathering seems to be needed to erode the rock types typical of the shield the major element chemistry and mineral composition of sediment delivered by the orinoco river to the ocean are controlled by rivers that have their headwaters in mountain belts and cross the llanos a region of alluvial plains within the foreland basin the composition of sediments in rivers that drain the shield seems to be established primarily at the site of soil formation whereas for rivers that drain the mountain belts additional weathering occurs during s episodes of storage on alluvial plains as sediments are transported across the llanos to the main stem of the orinoco after mixing into the main stem there seems to be little subsequent alteration of sediment

mineralogy and geochemistry of late permian coals from the taoshuping mine yunnan province china evidences for the sources of minerals abstract this paper describes the mineralogy and geochemistry of a batch of 17 coal samples covering the late permian longtan formation from a drill core from the taoshuping mine eastern yunnan province china the results show that minerals in the coals include quartz kaolinite calcite chamosite pyrite mixed layer i s and minor marcasite siderite anatase ankerite and dolomite modes of occurrences of authigenic quartz and chamosite suggest that they were mainly precipitated from fe mg rich siliceous solution derived from the weathering of emeishan basalt which was induced by mantle plume eruption during late middle permian and covers most places of southwest china kaolinite and mixed layer i s are mainly of detrital origin correspondingly the elevated trace elements including v cr co and ni in the coals are closely related to emeishan basalt trace amounts of detrital calcite ankerite and siderite in the coals probably originated from the maokou limestone the frequent presence of high temperature quartz indicates that felsic volcanic eruption took place through the entire late permian epoch mineral matter in the coals was not only derived from emeishan basalt in the kangdian oldland but also felsic volcanic ashes mafic volcanic ashes and limestone of the maokou formation

base saturation nutrient cation and organic matter increases during early pedogenesis on landslide scars in the luquillo experimental forest puerto rico we evaluate data from a chronosequence study of landslide scars ranging in age from 1 55 yr in the luquillo experimental forest lef a subtropical lower montane wet forest sensu holdridge in eastern puerto rico surface mineral soil 0 10 cm base saturation index bsi values increase during primary succession in the lef r 0 85 p 0 001 both bsi values and major nutrient cation concentrations are extremely low on new landslide scars during the course of the 55 yr chronosequence major nutrient cation concentrations are positively correlated with soil organic matter som content p 0 079 and not with clay content p 0 794 when data collected from plots representing the two dominant late succession vegetation associations are added to the analysis nutrient cation concentrations correlate significantly with both som p 0 001 and clay content p 0 033 our data show that when initial conditions are oligotrophic both nutrient cation pools and bsi values can increase in the surface mineral soil during early pedogenesis we discuss exogenic litter input substrate weathering and precipitation as potential sources for nutrient cations in these soils we further suggest that production and decomposition of som is the dominant process controlling capture retention and intra ecosystem cycling of nutrient cations in these forests

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

controls on major solutes within the drainage network of a rapidly weathering tropical watershed 1 surface water chemistry in the main stem and source points of the rio icacos basin luquillo experimental forest puerto rico was studied to investigate the factors regulating spatial variability in major solutes in a rapidly weathering landscape we sampled along the main stem as well as at small source points at high elevation where fresh bedrock is frequently exposed and at low elevation in the floodplain colluvial plain of the main stem concentrations of silicon alkalinity and the sum of base cations were lower at the source points than in the main stem and were lowest in low elevation source points calcium and sodium were the dominant cations at all sampling points after sea salt correction reflecting the weathering of plagioclase feldspar throughout the basin the partial pressure of carbon dioxide pco2 tended to be higher and hco3 concentrations were lower in the low elevation source points than at other positions in the landscape when coupled with the relatively low concentrations of si and base cations this suggests that the availability of primary reactive minerals rather than carbonic acid concentrations limits weathering in these low elevation sources mechanical denudation appears to enhance chemical weathering rates not only by refreshing reactive mineral surfaces but also by contributing carbon dioxide from the decomposition of organic rich material in landslides which occur frequently the spatial variability of major solutes appears to depend primarily on the availability of fresh primary reactive minerals carbon dioxide concentrations and hydrolysis conditions

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

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

contrasting zinc isotopic fractionation in two mafic rock weathering profiles induced by adsorption onto fe hydr oxides abstract zinc isotopes have been increasingly utilized as a proxy for bio geochemical cycles of nutrient zn in the ecosystem like soils but the behavior and mechanism of zn isotope fractionation during weathering and soil formation remain to be constrained here we reported zinc contents and isotope data for saprolites and laterites from two well characterized mafic rock weathering profiles formed under a subtropical climate south carolina usa and a tropical climate hainan island china respectively leaching experiments with basalt and andesite as the starting materials were also performed for comparison the leaching results revealed no resolvable zn isotope fractionation during dissolution of silicate minerals in the absence of organic ligands in solutions the hainan laterites show strong zn depletion and have lower 66znlyon values 0 10 to 0 27 relative to the parent rock 0 31 0 04 which are coupled with light copper isotope compositions in the same samples these observations suggest that zn mobilization was achieved via metal organic complexation and fe colloids transportation by contrast saprolites from the south carolina are extremely zn rich ten fold higher concentration than the parent rock and most samples have higher 66zn values 0 10 to 0 65 compared with the parent rock 0 24 0 04 zinc isotope ratios of the south carolina profile are positively correlated with iron isotope ratios 56fe and fe3 fetotal the latter of which is related to secondary fe hydr oxide formation these relationships link the high zn contents and 66zn values of saprolites from the south carolina profile to the substantial adsorption of zn onto or isomorphic substitution into fe hydr oxides that prefers heavier zn isotopes our study highlights that fe hydr oxides exert a strong influence on the re distribution of nutrient zn in natural soils and the bio geochemical cycling in natural soil systems

evidence of zn isotopic fractionation in a soil plant system of a pristine tropical watershed nsimi cameroon abstract the objective of this study is to determine for the first time the zinc zn isotopic composition of different soil horizons parent rocks litter and plants from a single pristine watershed three soil profiles from nsimi zoetele site south cameroon africa have been investigated the 66zn 66zn 66zn 64zn sample 66zn 64zn jmc 3 0749l 1 103 measured in the soils and rocks range from 0 05 to 0 64 in a typical hillslope soil profile developed in situ from a granitic parent rock we observe two important and systematic trends i a weathering regime in the saprolite horizon that involves strong zn depletion with no or slight fractionation compared to the fresh igneous rock and ii a shallower weathering regime with significant depletion in heavy zn isotopes in the most superficial soil horizons in contrast the soil profile of the swamp zone does not exhibit isotopic fractionation between the soil and the parent rock six plants and two litter samples were analyzed for their zn isotopic compositions 66zn varied from 0 75 to 0 91 among all samples plant roots and shoots are generally enriched in heavy isotopes 66zn of 0 42 to 0 76 relative to the litter layer 0 12 to 0 25 and most superficial soils similar to the controlled plant growth study of weiss et al weiss d j mason t f d zhao f j kirk g j d coles b j and horstwood m s a 2005 isotopic discrimination of zinc in higher plants new phytologist 165 703 710 negative 66zn values were obtained for tree leaves the most aerial part of the tree based on biomass data for the tropical forest vegetation is on average enriched in light isotopes compared to the litter and the superficial soil horizons which are the nutrients reservoirs due to the age of these soils and the complexity of pedogenetic processes it is difficult to explain the isotopic composition of the soils at present however the transformation of the saprolite horizon into a ferruginous horizon is associated with an enrichment in light zn isotopes this observation may be due to the adsorption of zn onto clays surfaces the uptake of zn by roots in the soils leads to an enrichment in heavy isotopes once the zn is in the xylem it will be transported within the plant during this transport both diffusion and active uptake of heavy isotopes by cells out of the xylem favour the mobility of light isotopes to the most aerial parts of the plants the difference in zn isotopic composition between the deep horizons and the superficial horizons of soils suggests that zn isotopes can be used to discriminate the source of zn and other metals leaving the catchment by river water

weathering and the global carbon cycle geomorphological perspectives abstract the links between the weathering of silicate minerals the global carbon cycle and climate change are complex and involve a whole range of often inter related tectonic climatic geomorphological and biological factors silicate weathering rates are known to have varied through geological time and increasingly sophisticated models now link such variation to the long term carbon cycle and climate change in order to improve the modelling further one key research need is for more and better datasets on weathering and the factors that control it as a contribution to this we review the nature of the weathering carbon cycle system the sources of evidence used to understand it and the challenges that remain in quantifying the feedbacks between weathering and the global carbon cycle we propose that increased understanding of geomorphological diversity of landscapes and processes and especially the dynamic relationships between weathering and erosion will aid clearer portrayal of the links between weathering and the long term carbon cycle

elemental weathering fluxes and saprolite production rate in a central african lateritic terrain nsimi south cameroon abstract the comparison between contemporary and long term weathering has been carried out in the small experimental watershed sew of nsimi south cameroon in order to quantify the export fluxes of major and trace elements and the residence time of the lateritic weathering cover we focus on the hillside system composed of a thick lateritic weathering cover topped by a soil layer this study is built on the recent improvements of the hillside hydrological functioning and on the analyses of major and trace elements the mass balance calculation at the weathering horizon scale performed with the parent rock as reference indicates i strong depletion profiles for alkalis na k rb and alkaline earths mg ca sr ba ii moderate depletion profiles for si p cd cu zn ni and co iii depletion enrichment profiles for al ga ge sn pb lree hree y u fe v cr mn it is noteworthy that i mn and ce are not significantly redistributed according to oxidative processes as it is the case for fe v and cr and ii ge is fractionated compared to silica with enrichment in fe rich horizons the calculations performed for the topsoil with iron crust as parent material reference reveal that the degradation of the iron crust is accompanied by the loss of most of the constituting elements among which are those specifically accumulated as the redox sensitive elements fe v cr and iron oxide related elements like th the overall current elemental fluxes from the hillside system at the springs and the seepage zones are extremely low due to the inert lateritic mineralogy ninety four percent of the whole na flux generated from the hillside corrected from atmospheric deposits 77 mol ha yr represents the current weathering rates of plagioclase oligoclase in the system the other remaining 6 may be attributed to the dissolution of hornblende the silica hillside flux is 300 mol ha yr and can be mostly attributed to the plagioclase and kaolinite dissolution al and ga are exported from the lateritic regolith and maybe due to the dissolution of kaolinite crystals compared to the other immobile elements zr hf nb and th ti is significantly exported among redox sensitive elements fe v cr mn ce only ce and mn are exported out of the hillside system the other elements fe v cr are likely able to be mobilized but over a short distance only rb sr ba ni cu zn are affected by export processes lree and y are exported but in very low amounts in the range from mol ha yr to mmol ha yr while hree and u are exported in negligible quantities a first attempt is carried out to compare the mature ridge top profile from nsimi sew with the immature ridge top weathering profile from the mule hole sew south india developed on similar granodioritic basement in order to get deeper insight into i the contemporary saprolite production rates and ii the combined effect of precipitation in terms of mean annual rainfall mar and evapotranspiration on the aggressiveness of the draining solutions considering i the contemporary na flux as representative of the dissolution of plagioclase crystals and conservative during saprolitization processes and ii steady state of the inter annual recharge r over a 10 years period the current saprolite production rates r are of 22 mm kyr for mule hole sew and 2 mm kyr for nsimi sew respectively even with a very low r mar ratio 0 04 compared to nsimi the chemical weathering at mule hole is active and related to the groundwater exports at mule hole plagioclase crystals are still present in the saprolite and the soil cover leading to a diffuse weathering front the high nsimi r mar ratio 0 2 allows the solution to be still aggressive with respect to the plagioclase and other weatherable minerals at the bedrock interface resulting in their complete breakdown in a few centimetres sharp weathering front leading to a mature saprolite for the nsimi sew if we consider i the low contemporary saprolite production rate 2 mm kyr ii the miocene age average 15 myr of the south cameron plateau landscape and ii the limited movement of africa continent since eocene the long term saprolite production rate should have remained in its lower range from 2 to 10 m myr this suggests that for thick weathering profiles the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions climate variation leading to the alternation of setup of savanna or humid forest will have an effect on physical erosion rather than chemical erosion for such deep weathering profiles

the provenance of sediments in the gulf of lions western mediterranean sea in this study we undertook a reconnaissance study of sediments provenance in the gulf of lions focusing over the last 16 ka we used geochemical and isotopic tracers to determine the source of sediments and give insight into the weathering conditions prevailing sediments samples were selected both onshore and offshore from the western eastern and central part of the gulf of lions we analyzed bulk sediments coarse and fine silt and clay fractions elemental and nd isotope compositions appeared to differ from one grain size fraction to another one these are interpreted in terms of zircon addition in the coarse silt fraction for the elemental concentrations and variable sources influences for the nd isotope compositions our results indicate that sediments in the gulf of lions mainly originated from the rhone river watershed although a contribution of saharan dust is seen in one sample influence of pyrenean small rivers is minor in these samples some sr isotope compositions shifts are interpreted as reflecting variable amounts of chemical weathering that are consistent with published paleoclimatic reconstructions

how coriolis forces can limit the spatial extent of sediment deposition of a large scale turbidity current abstract the rotation rate of the earth controls the deposition scale of turbidity currents when their flow transit time is comparable to one day using laboratory experiments it is shown that the maximum length scale l of deposition is set by the scale where the rossby number of the current is equal to one the dimensionless rossby number is defined as ro u fl where u is a depth averaged velocity of the turbidity current and f is the coriolis parameter in these new laboratory experiments the coriolis parameter is varied by changing the platform rotation rate and there is good agreement between experimental observations and the prediction that the extent of the deposition scales as l 4 1 4 g v f 1 4 where g is the reduced gravity and v the initial volume of the sediment laden material this scaling implies that the depositional patterns will vary with latitude and that at higher latitudes one would expect smaller thicker deposits using this scaling implies that the 200 400 km spatial extent of the turbidite arising during the 1929 grand banks earthquake had ro 1 and so coriolis forces controlled the length scale of the turbidite deposition

the significance of biogenic element cycling in ancient tropical soils many lateritic tropical soils on stable cratonic areas have developed over millions to 10 s of millions of years some elements e g alkalies alkaline earths si are extremely depleted by leaching and others a1 fe ti are greatly enriched commonly this enrichment is regarded as merely the accumulation of insoluble constituents but inspection of the soil shows development of pisolites nodules and other features indicating extensive dissolution precipitation of feand al oxides and hydroxides this paper suggests that cycling of elements through vegetation is an important part of soil development in these ancient soils this project was initiated to investigate the processes forming lateritic gold deposits in which a relatively small primary vein or similar au deposit in bedrock mushrooms in the soil to become a significant au enrichment in the laterite zone davy and e1 ansary 1986 lecomte and colin 1989 the site of our investigation is the gold deposit of igarape bahia in the serra carajas mining region para province brazil in this region a very ancient plateau surface furnishes the environment for major fe mn and a1 deposits of the carajas district beisiegel 1982 machamer et al 1991 as well as a lateritic gold deposit at igarape bahia the plateau surface is interpreted to have been developed in cretaceous or early tertiary beisiegel 1982 kotschoubey and lemos 1985 the entire region is covered by dense tropical rain forest at the igarape bahia au deposit a narrow primary zone of au cu mineralization at depth gives rise to a relatively rich au deposit in lateritic soil the deposit appears analogous to other lateritic au accumulations in which the initial primary agbearing gold grains are etched and become smaller and new grains of ag poor gold grow in the laterite lecomte and colin 1989 a preliminary study indicated that considerable au was present in vegetation over the deposit so in a later sampling expedition an extensive group of vegetation samples was collected along with soils and waters andrade et al 1991 bliss 1991 machesky et al 1993 vegetation sampling was concentrated along a traverse across the unmined f30 deposit a satellite au deposit 1 km from the main igarape bahia deposit additional samples were collected at three background sites 1 10 and 30 km from the deposit at the f30 deposit and at the 1 km background site all species of tree and shrub were collected with leaves twigs and bark being collected from most species five specific species a large tree two bamboo species and two bushes were sampled along the traverse and at the background sites samples were washed dried chopped homogenized and analyzed for 40 elements by neutron activation activation laboratories ltd ancaster ontario and xray fluorescence materials characterization lab penn state university

improvement to resistivity pseudosection modelling by removal of near surface inhomogeneity effects application to a soil system in south cameroon near surface inhomogeneities nsis can lead to severe problems in the interpretation of apparent resistivity pseudosections because their effects significantly complicate the image aspect in order to carry out a more efficient and reliable interpretation process these problematic features should be removed from field data we describe a filtering scheme using two sided half schlumberger array data the scheme was tested on synthetic data generated from a simple 2d resistivity model contaminated by nsis and is shown to be suitable for eliminating such contaminations from apparent resistivity data furthermore the original model without nsis can be recovered satisfactorily from the inversion of filtered apparent resistivity data the algorithm is also applied efficiently to a real data set collected at nsimi in southern cameroon along a 200 m shallow depth profile crossing a complex transitional zone for this case the filtering scheme provides accurate structural and behavioural interpretations of both the geometry of the major soil constituents and the groundwater partitioning

controls on intense silicate weathering in a tropical river southwestern india the silicate weathering rate swr and associated carbon dioxide consumption rate ccr in tropical silicate terrain is assessed through a study of the major ion chemistry in a small west flowing river of peninsular india the nethravati river the specific features of the river basin are high mean annual rainfall and temperature high runoff and a precambrian basement composed of granitic gneiss charnockite and minor metasediments the water samples n 56 were collected from three locations along the nethravati river and from two of its tributaries over a period of twelve months chemical weathering rate cwr for the entire watershed is calculated by applying rainwater correction using river chloride as a tracer chemical weathering rate in the nethravati watershed is estimated to 44 t km 2 y 1 encompassing a swr of 42 t km 2 y 1 and a maximum carbonate contribution of 2 t km 2 y 1 this swr is among the highest reported for granito gneissic terrains the assessed ccr is 2 9 10 5 mol km 2 y 1 the weathering index re calculated from molecular ratios of dissolved cations and silica in the river suggests an intense silicate weathering leading to kaolinite gibbsite precipitation in the weathering covers the intense swr and ccr could be due to the combination of high runoff and temperature along with the thickness and nature of the weathering cover the comparison of silicate weathering fluxes with other watersheds reveals that under similar morpho climatic settings basalt weathering would be 2 5 times higher than the granite gneissic rocks c 2012 elsevier b v all rights reserved

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

the effect of organic matter on chemical weathering study of a small tropical watershed nsimi zoetele site cameroon abstract the effect of organic matter during soil water interaction is still a debated issue on the controls of chemical weathering in a tropical environment in order to study this effect in detail we focused on the weathering processes occurring in a small tropical watershed nsimi zoetele south cameroon this site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry the lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones unsaturated slope soils on the hills and or elevated areas and water saturated soils in the swamp zone which represent 20 of the basin surface the study present chemical analysis performed on water samples collected monthly from different localities between 1994 1997 and on soil samples taken during a well drilling in december 1997 the results suggest the existence of chemical and spatial heterogeneities of waters in the basin colored waters flooding the swamp zone have much higher concentrations of both organic matter i e doc and inorganic ions e g ca mg al fe th zr than those from springs and groundwater from the hills nevertheless these organic rich waters present cation concentrations na ca mg k which are among the lowest compared to that of most world rivers the main minerals in the soils are secondary kaolinite iron oxi hydroxides quartz and accessory minerals e g zircon rutile we mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through sem observations the textural characterization of weathered minerals such as kaolinite zircon rutile and the secondary recrystallization of kaolinite microcrystals within the soil profile water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin thus in order to explain the increase of element concentration in the organic rich waters we suggest that organic acids enhance dissolution of minerals such as kaolinite goethite and zircon and also favors the transport of insoluble elements such as al fe ti zr and ree by chemical complexation sio2 aq concentrations in these waters are above saturation with respect to quartz dissolution of phytholithes amorphous silica may be responsible for this relatively high sio2 aq concentration al mg ratios obtained for the soil and the mengong river waters show that a significant amount of al does not leave the system due to kaolinite recrystallisation in the swamp zone soils geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone quantitatively we propose that the increasing amount in total dissolved solid tds due to organic matter and vegetation effect is about 35 in summary this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements thus the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils

the effects of lithology on trace element and ree behavior during tropical weathering abstract the thick regolith developed in the humid tropics represents an endmember of critical zone evolution where shallow and deep biogeochemical cycles can be decoupled in terms of the predominant source of trace elements atmospheric input at the surface weathering at depth and of the processes that control their cycling to investigate the influence of lithology on trace element behavior and in this potential decoupling we studied two deep 9 3 and 7 5 m highly leached ridgetop regolith profiles at the luquillo critical zone observatory puerto rico these profiles have comparable internal degree of weathering topography and external vegetation climate characteristics but differ in their underlying bedrock andesitic volcaniclastic and granitic at these two sites we analyzed a large suite of trace elements and used the rare earth elements and yttrium rey as tracers of critical zone processes because they are fractionated by the chemical reactions involved in weathering and pedogenesis e g sorption dissolution colloidal transport and by redox fluctuations we found that both regolith profiles show atmospheric inputs of trace elements at the surface and evidence of bedrock dissolution at depth as expected we also found noticeable differences in the re distribution of trace elements and rey within the profiles indicative of different geochemical environments with depth and lithology in the volcaniclastic profile trace element and rey behavior is controlled mainly by redox mediated sorption desorption reactions whereas ph controlled dissolution precipitation and sorption reactions predominate in the granitic profile the most noticeable difference between the two regolith profiles is in the long term redox conditions inferred from redox sensitive elements and ce anomaly variations which are more variable and stratified in the volcaniclastic profile and change gradually with depth in the granitic profile the contrasting redox conditions and the different sources of elements dust vs bedrock produce a decoupling between the surface and deep geochemical environments of the volcaniclastic regolith the difference in redox conditions between the two lithologies likely stems from the finer grain size and higher clay content of the volcaniclastic regolith

estuarine distributions of zr hf and ag in the hudson river and the implications for their continental and anthropogenic sources to seawater 1 zirconium and hf in seawater are derived from continental weathering and are transported to the oceans by rivers but the modification of this flux with estuaries is largely unknown furthermore there is no information as to whether there is a pollutant source of these metals that might affect their budget calculations in the modern ocean to address the riverine flux and potential anthropogenic sources the distributions of dissolved zr and hf have been measured in surface and bottom water through the hudson river estuary the distribution of ag is included because it is sensitive to urban sources and has partial similarities in its distribution to those of zr and hf there is about 50 removal of zr and hf in surface water relative to their freshwater end member concentrations of 243 and 3 1 pmol kg as salinity starts to increase but an urban source of these metals midestuary may obscure greater removal silver concentrations in surface water are below the detection limit until salinity of 3 5 is reached the concentrations of zr and hf 0 5 m above the bottom are 1 5 to 3 times higher than their surface water concentrations throughout the estuary and may result from input of groundwater diffusion from pore fluids or inclusion of colloidal phases in our analyses downstream of the turbidity maximum bottom water ag concentrations are high due to release from particles that originally formed within the estuary and were moved inland following the spring freshet this process may also affect zr and hf the increase in the freshwater atomic zr hf ratio from 70 to 80 characteristic of terrestrial rocks to the elevated zr hf 110 ratio of seawater takes place at middle to high salinity due to enhanced release of zr from resuspended particles and slower removal rates compared to hf a main implication of this study is that estuaries are a sink for zr and hf the reduction of the predicted export of continental zr and hf to the oceans increases their previously calculated seawater residence by as much as 2 to 4 times assuming rivers dominate the supply of zr and hf to seawater

the role of soil processes in determining mechanisms of slope failure and hillslope development in a humid tropical forest eastern puerto rico abstract translational failures with associated downslope earthflow components and shallow slides appear to be the primary mechanism of hillslope denudation in the humid tropical forests of the mountains of eastern puerto rico in situ weathering of quartz diorite and marine deposited volcaniclastics produces residual soil saprolite up to 21 m deep weathered rock profiles discontinuous zones of contrasting density and permeability particularly in quartz diorite slopes at 0 5 m and between 3 and 7 m create both pathways and impedances for water that can result in excess pore pressures and ultimately aid in determining the location of failure planes and magnitudes of slope failures in combination with relict fractures which create planes of weakness within the saprolite and the potential significance of tensile stresses in the upper zone of saprolite hypothesized to be caused by subsurface soil creep shear failure can then occur during or after periods of heavy rainfall results of in situ shear strength testing show negative y intercepts on the derived mohr coulomb failure envelopes approximately 50 of all tests that are interpreted as apparent tensile stresses observation of tension cracks 1 2 m deep support the test data subsurface soil creep can cause extension of the soil and the development of tensile stresses along upper slope segments shear strength data support this hypothesis for both geologic types apparent values of maximum and mean tensile stress are greatest along upper slopes 16 5 and 6 29 kpa previously documented maximum rates of downslope movement coincided with local minima of shear strength and the shear strength minimum for all tests was located near 0 5 m below land surface the shallow zone of contrasting permeabilities these results indicate that subsurface soil creep a slow semi continuous process may exert a profound influence on rapid shallow slope failures in saprolitic soils data indicate that cove slopes in quartz diorite tend to be the most unstable when saturation levels reach 75 deep failures 7 m deep appear the most critical but not the most frequent because pore pressure build up will occur more rapidly in the upper perched zone of translocated clays before reaching the lower zone between 3 and 7 m frequent shallow failures could reduce the probability of deeper failures by removing overburden and reducing shear stress at depth deep failures are more likely to result from storm events of great duration and intensity sixty six naturally occurring and more than 100 road related landslides were mapped forest elevations exceed 1000 m but the majority of these failures were found between 600 and 800 m in elevation this appears to be the area where there is sufficient concentration of subsurface water to result in excess pore pressures the high percentage of slope failures in the 600 800 m range relative to the percentage at higher elevations suggests that differences in soil water processes are responsible for the form of these mountain slopes steep linear segments are maintained at higher elevations slope angles are reduced in the 600 800 m range by frequent shallow slides creating a largely concave surface in combination slope segments above 800 m and those between 600 and 800 m produce the characteristic form of the mountains of eastern puerto rico

rainfall patterns and the distribution of coals and evaporites in the mesozoic and cenozoic abstract paleoclimatologists have adequately demonstrated over the last twenty years that the distribution of climatically sensitive sediments can be related to present day climatic zones that are roughly parallel to latitude for example coals were deposited at paleolatitudes that correspond to the present day rainy zones at the equator and at about 55 n and 55 s indicating that those zones were present in the past from this it can be concluded that atmospheric circulation has not been radically different from its present configuration despite some apparently great differences in some climatic parameters such as the equator to pole temperature gradient nevertheless the zonal model for predicting the distribution of climatically sensitive sediments is not wholly adequate although general atmospheric circulation may not have been radically altered from its present pattern it would have differed in some respects because the paleogeography was different from the present geography therefore we have constructed what we believe are more realistic maps of the distribution of rainfall through time these maps are based on maps of atmospheric circulation that have been successfully used to predict the distribution of some petroleum source beds and phosphorites we have compared the predicted distribution of rainfall to the distribution of coals and evaporites for each of seven geologic stages the circulation rainfall maps were more successful than the zonal model for predicting the distribution of evaporites and coals for times when the paleogeography would have maximized the development of a monsoonal circulation which most effectively disrupts the general zonal pattern for times when a monsoonal circulation would not have been important the zonal model adequately predicted the distribution of these rocks

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

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

denudation rates on cratonic landscapes comparison between suspended and dissolved fluxes and 10be analysis in the nyong and sanaga river basins south cameroon south cameroon is located in a tropical and tectonically quiescent region with landscapes characterized by thick highly weathered regolith indicative of the long term predominance of chemical weathering over erosion currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources we analysed two of the main south cameroon rivers the nyong river and sanaga river the sanaga catchment undergoes a contrasted tropical climate from sub humid mountainous and humid climate and is impacted by deforestation agriculture damming mining and urbanization especially in the mbam sub basin draining the highly populated volcanic highlands by contrast the nyong catchment only under humid tropical climate is preserved from anthropogenic disturbance with low population except in the region of yaounde mefou sub basin moreover the nyong basin is dam free and less impacted by agriculture and logging we explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchment averaged erosion and denudation datasets the first one consists of an 11 year long gauging dataset while the second one comes from cosmogenic radionuclides crns here beryllium 10 be 10 from sand sampled in the river mainstreams timescale of tens to hundreds of thousands of years modern fluxes estimated from gauging data range from 5 to 100m ma 10 to 200t km 2 yr our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60 and 40 respectively of the total denudation beryllium 10 denudation rates and sediment fluxes range from 4 8 to 40 3m ma or 13 to 109t km 2 yr respectively after correction for quartz enrichment these fluxes are slightly less than the modern fluxes observed in cameroon and other stable tropical areas the highest be 10 derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact copyright c 2016 john wiley sons ltd

experimental evidence for mobility of zr and other trace elements in soils a soxhlet extraction was carried out over a period of 27 d on a column comprising 3 cm of quartz overlain by 4 cm of soil from the b horizon and then 1 cm of soil from the a horizon of a granitic podzol major and trace elements were leached from the column and accumulated in a reservoir at the base of the column total loss of elements from the soil over the course of the experiment ranged from 0 002 to 1 wt with major elements and the light and heavy rare earth elements ree showing the largest percentage losses zirconium 0 002 and then al 0 008 showed the lowest percentage loss the light ree were leached out of the soil preferentially to the mid ree all elements showed accumulation by a factor of 2 to 11 in the quartz layers at the base of the column particularly in the upper first 1 cm of the quartz major elements were leached from the column at a rate of 0 02 to 0 59 mol h 1 whereas zr nd sm gd dy rb and sr were leached at the rate of 0 5 to 30 10 6 mol h 1 concentrations of other ree in the reservoir increased over the duration of the experiment but they were poorly correlated with time so leaching rates were not calculated normalization of the major element leaching rates to take into account the constant flushing of water through the column the average annual rainfall in the allt a mharcaidh catchment in scotland from where the soil was sampled and the cross sectional area of the soil in the column together with the temperature of the soil in the column 70 c compared with the average annual temperature of the allt a mharcaidh catchment 5 7 c gave major element release rates from the soil of 0 002 to 0 97 meq m 2 yr 1 depending on the choice of ea the dissolution activation energy which are generally less than those measured in the field of 0 1 to 40 9 meq m 2 yr 1 calculations showed that despite the redistribution and loss of zr from the column assumptions of zr mobility would have had a negligible effect on calculated element release rates of na ca fe and mg however significant underestimates of the release of k 5 ti 57 al 5 and si 10 as well as some trace elements e g nd 23 rb 54 sr 24 would have occurred concentrations of ca and sr leached from the column correlated well rsq 0 93 p 0 01 supporting the idea of the use of sr release as a proxy for ca release in weathering rate calculations the release rates and percentage loss of ree from the soil varied between elements indicating that ree distribution patterns of rocks and soils may not be preserved in drainage waters

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

volumetric changes in weathered profiles iso element mass balance method questioned by magnetic fabric abstract a major issue in understanding weathering processes is to determine to what extent fabrics structures and volumes from the parent rock are preserved through the weathering profile iso element mass balance methods imb are often used to estimate volume changes during progressive weathering based on the assumption that a given element either ti zr or th is not mobile the petrofabric of the weathered material is often characterized by a mimetic replacement of primary minerals that fully preserve the primary fabric this suggests an isovolume weathering as volume change must be associated with shape change due to boundary conditions not allowing horizontal strains so collapse or dilation should induce vertical compaction or constriction respectively the weak petrofabric of weathered materials may be precisely quantified using the anisotropy of low field magnetic susceptibility technique ams this paper reports for the first time a combined imb and ams study of two different environments a complex lateritic sequence on precambrian metamorphic rocks in cameroon and a weathering profile on a pleistocene basaltic flow in morocco the lateritic profile divided into a lower saprolite zone 12 m and an upper nodular iron rich unit 4 m is characterized by neoformed magnetic minerals goethite hematite spinels and a weak but rather consistent magnetic fabric saprolitization which induces low susceptibility values 50 10 9 m3 kg preserves the linear tectonic fabric of the parent gneiss ams evidence for isovolumetric weathering agrees with th based imb while ti and zr partly indicate apparent collapse and dilation up to 50 respectively ferralitization in the nodular iron crust which enhances susceptibilities 500 to 700 10 9 m3 kg due to spinel phases induces drastic reductions in anisotropy due to multiple generation of neoformed minerals destroying primary fabric however a weak horizontal planar magnetic fabric suggests compaction in agreement with th and ti based imb while zr would indicate isovolume weathering conversely in the one meter thick weathering profile on basalt the strong susceptibility 1 5 to 3 10 6 m3 kg is dominated by inherited titanomaghemite grains which totally preserve the weak magmatic fabric of the fresh basalt this again suggests that volume change is minor while ti and zr based imb indicate 20 to 50 of dilation in both examples large volume changes inferred from the imb in units showing preserved primary ams fabric appear unrealistic our ams study together with the large discrepancies between the imb results produced by various elements indicate that the assumption of immobility of a given element is probably not fulfilled all along these profiles isovolumetric weathering in the saprolitic zone may be more widespread than suggested by imb results

a numerical study of seafloor scattering the processes which result in backscattered acoustic energy from the seafloor are of primary interest for interpreting sonar signals measured in the deep ocean basins this complex problem can be approached from a realistic perspective with the employment of modern high resolution seafloor bathymetry measurements and viscoelastic wave simulation techniques capable of modeling highly heterogeneous anelastic media a numerical study of low frequency 100 200 hz backscattering processes from rough seafloor models is presented and several scattering hypotheses are proposed in particular the effects that low q sediments have on the backscattered field from a rough seafloor and the effects that perturbations in the different material properties have on the backscattered energy are investigated the principal conclusions reached are that anelastic sediments on the seafloor can have a significant impact on the backscattered field from low grazing angle scattering experiments and that small perturbations in the

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

limited iron isotope variations in recent lateritic soils from nsimi cameroon implications for the global fe geochemical cycle laterites are oxidized fe rich soils covering one third of the continents and are drained by half of the continental waters they therefore represent a key component of the iron geochemical cycle at the earth s surface yet no iron isotope study has been conducted on recent laterites so far building on previous integrated morpho pedological studies of soils located in an equatorial rainforest southern cameroon we have undertaken a mineralogical elemental and isotopic study of iron in two lateritic profiles one is a borehole 3620 cm deep going to the parent granodioritic rock located at the top of a hill whereas the other is 675 cm deep and is located downhill mossbauer spectroscopy reveals that typically geq 96 of the soil s iron is held in nanocrystalline hematite and goethite iron isotope measurements performed by plasma source mass spectrometry show that fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size overall it is found that most samples display iron isotope signatures very close to the mean crustal value with a maximum range of 0 2 in delta 57 fe given that lateritic soils evolve over millions of years show large variations in fe concentrations and mineralogical abundances this range is surprisingly small when compared to other fe isotope studies of soils from different climatological contexts that can easily show delta 57 f e ranges in excess of 1 at the bulk sample scale the most likely explanation of this finding is that despite notable vertical and lateral fe mobility in the studied laterites from cameroon as computed using the open system mass fraction transport function tau fe w iron remained mostly in the oxidized form as shown by mossbauer pectroscopy this probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon this study therefore reveals that soils that remained as an open system for iron do not necessarily show large fe isotopic variations it is suggested that the fe drained from such lateritic soils should have delta 57 fe values within sim 0 1 of that of the continental crust this is in contrast with previous soil studies notably from sites located at higher latitudes that imply that these soils should release isotopically more variable fe to surface waters such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of fe in seawater

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

plant impact on the biogeochemical cycle of silicon and related weathering processes abstract the contribution of plants to the biogeochemical cycle of si and related weathering processes was studied in an equatorial rainforest ecosystem congo where the biologic turnover of si is high 58 to 76 kg ha y litterfall leaves a soil profile and groundwaters were analysed phytoliths and organic matter have a similar distribution with depth in the soil profile the model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92 of the biogenic silica input is rapidly recycled while about 8 of the biogenic silica input supplies a stable pool of phytoliths with a lower turnover reprecipitation of silica was observed at the base of the soil profile indicating a local geochemical environment that is oversaturated with respect to amorphous silica a balance in biogeochemical cycle of si requires that the vegetation absorb dissolved silicon released from weathering of minerals which otherwise would be available for mineral neoformation or export from the profile towards regional drainages plant uptake of si increases the chemical weathering rate without increasing the denudation rate this study shows that the uptake storage and release of si by the vegetation have to be taken into account when using dissolved si for tracing chemical weathering dynamics

mobilization and re distribution of major and trace elements during extreme weathering of basalt in hainan island south china major trace and organic elements of a laterite profile developed on neogene basalts in northern hainan island south china were reported in this paper the aim of which was to investigate element mobilization and re distribution during extreme weathering the results indicate that most of the elements have been mobilized and transferred downwards along the profile by aqueous solution organic matter om can significantly improve the transport of insoluble elements among all the elements th is the least mobile as for the general conservative elements during incipient chemical weathering such as fe ti zr hf nb and ta the removals are up to 20 40 in the upper profile however these elements behave as conservatively as th in the lower profile in the middle profile oxic environment occurs accompanied with significant om decomposition the mn and ce transferred downward are readily oxidized into insoluble mn iv and ce iv and precipitate in the oxic front important om decomposition decreases the capacity of transfer of insoluble elements in aqueous solution consequently al significantly precipitates in the oxic front and rees with the exception of ce precipitate largely in the omdepleted layers co and u are also concentrated in the oxic front in association with mn and ce respectively however cr shows a negative correlation with mn because its response to redox condition changes is reversed from that of mn mn oxides hydroxides fe oxides hydroxides and secondary phosphate minerals other than clay minerals are potential hosts for rees except for ce in the profile rees with high concentrations in the profile seem closely associated with mn oxides hydroxides remarkable highly correlated ce and gd anomalies are observed in the profile ce anomalies are caused by ce precipitation in the oxic environment and successive decomposition of organic matter gd anomalies are likely to have resulted from lower stability constants of gd om complexes compared to those of neighboring rees the overall elemental behaviors in this profile suggest that organic matter plays a very important role in the mobilization and re distribution of the elements during extreme weathering

evidence for an aeolian origin of the holocene lateritic surface cover of gabon central africa abstract field granulometric and geochemical data are reported for a set of twenty one samples collected at six sites spread over gabon in the extensive lateritic clayey to sandy layer cover horizon which constitutes the upper part of the profiles of surficial formations granulometric analyses show that this material of homogeneous aspect is a mixture of two fractions a clay fraction and a medium to coarse grained sandy fraction relative proportions of these two constituents show strong variations from one site to another but are almost constant from the bottom to the top of a given profile the coarse grained fraction includes lithic elements of local provenance irrespective of their underlying basement the samples display many common geochemical features including a lack of ca na and mg their major element compositions may be accounted by the combination of a discrete number of mineralogical phases namely quartz kaolinite gibbsite fe oxides hydroxides and subordinate k minerals micas the normative contents of aluminous and ferrous phases regularly increase with the weight percent of clays conversely the quartz regularly decreases this inverse relationship is consistent with mixing between an al and fe bearing clay fraction and a coarse grained fraction dominated by quartz correlations between most immobile trace elements and the weight percent of clays indicate a control of those elements by al and fe bearing phases the main exception is for zr and hf as those elements are probably associated with zircons in the coarse fraction because of the control of the trace element distribution by secondary phases the geochemical signatures of all samples are clearly distinct from those of the upper continental crust and any basement rocks thus the trace element characteristics of the cover horizon carry no sign of any source material being the sole consequence of mineralogical sorting prior to deposit from field granulometric and geochemical data an aeolian origin appears as the most probable for the cover horizon geochronological constraints including archaeological data and 14 c dating indicate that this aeolian event took place during the holocene and probably occurred in less than 3000 y additional works will be necessary to assess the global context which made possible this very fast and extended event over large areas of central africa

chemical weathering in the drainage basin of a tropical watershed nsimi zoetele site cameroon comparison between organic poor and organic rich waters abstract this study deals with the weathering processes operating at the scale of a small catchment nsimi zoetele cameroon and is focused on the role of organic colloids on mineral weathering and transport of elements in natural waters samples of river spring and groundwaters from nsimi zoetele were filtered through membranes of decreasing pore size 0 22 m 0 025 m or 300 000 da 5000 da to separate colloidal fractions from the truly dissolved one major and trace elements and dissolved organic carbon doc were analysed in each fraction two kinds of waters can be distinguished in the catchment clear and coloured waters clear waters exhibit low concentrations of major and trace elements and doc elements are carried in these solutions in a true dissolved form except al and rare earth elements rees by contrast the higher abundances of al fe and trace elements in coloured waters are controlled by the colloidal fraction thermodynamic equilibrium calculations show that clear waters are in equilibrium with kaolinite and iron oxi hydroxide which are major minerals in the weathered soil for coloured waters the aqueous speciation of ca mg cu fe al la and th was calculated taking into account the complexes with humic acids speciation calculations for cu fe al la th show a strong complexation with humic acids in good agreement with the results of the filtration experiments by contrast although filtration experiments show a strong control of major cations by organic matter for example 75 for ca speciation calculations reveal that their complexes with humic ligands do not exceed a few percent of total dissolved elements this discrepancy is explained as an artefact induced by the organic colloids and occurring during the filtration procedure finally both filtration experiments and speciation calculations show that organic matter plays an important role in natural doc rich waters organic acids increase significantly the dissolution rates of silicates and oxi hydroxides and thus the amounts of solutes and of complexed elements leaving the catchment

solute generation and co2 consumption during silicate weathering under subtropical humid climate northern okinawa island japan abstract the chemical characteristics of freshwaters draining the silicate rocks in the northern part of okinawa island were studied to understand solute generation processes and to determine rates of chemical weathering and co2 consumption it was observed that the water chemistry is highly influenced by marine aerosols contributing more than 60 of total solute significant positive correlations observed for chloride versus dissolved silica and chloride versus bicarbonate suggest a strong influence of evapotranspiration on the seasonality of solute concentration it was also found that chemical weathering has been highly advanced in which the dominant kaolinite minerals are being gibbsitized carbonic acid was found to be the major chemical weathering agent releasing greater than 80 of weathering derived dissolved cations and silica while the remaining portion was attributed to weathering by sulfuric acid generated via oxidation of pyrite contained in the rocks the flux of basic cations weathering derived silica and co2 consumption were relatively high due to favourable climatic condition topography and high rate of mechanical erosion silicate weathering rates for basic cations were estimated to be 6 7 9 7 ton km 2 y 1 carbon dioxide consumed by silicate weathering was 334 471 kmol km 2 y 1 which was slightly higher than that consumed by carbonate weathering in general divalent cations mg and ca and bicarbonate alkalinity derived from carbonate dissolution were higher than those from silicate weathering as a consequence the evolution of chemical species in the freshwaters of northern area of okinawa island to a large extent could be explained by mixing of two components characterized by waters with na and cl as predominant species and waters enriched with ca2 and hco3

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

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

elemental release rates from dissolving basalt and granite with and without organic ligands bacteria fungi lichen and plants all produce organic acids which can strongly affect weathering by increasing the solubility and mobility of elements leaching by organic acids may therefore produce trace element signatures which could record the presence of life in the rock record from early earth to elucidate this effect long term column experiments were performed with powdered granite and basalt with and without 0 01 m citrate at ph 6 for 45 weeks both granite 8 44 10 13 3 39 10 13 and basalt 2 94 10 14 6 47 10 14 dissolution rates mol ca mg m 2 s 1 respectively were enhanced in the presence of citrate relative to the organic free controls granite 3 17 10 14 4 4 10 15 and basalt 1 01 10 14 1 04 10 14 enhanced release of individual elements in the presence of citrate was strongly correlated with the stability constant of the citrate element complex elements which might be useful as biosignatures are those elements that showed a strong enrichment in the presence of citrate zr sc and mn basalt v and zn granite and y la ce th ti al p pb ni and fe both basalt and granite release of these elements from the rock material in the columns is consistent with dissolution of apatite fe sulfides fe oxides augite in basalt and apatite sphene hornblende in granite similar groups of elements have been reported to be enriched in organic rich rivers suggesting leaching of strongly complexed elements could be useful as biosignatures and may have left mineralogical traces on early earth

dissolution of radiation damaged zircon in lateritic soils zircon crystals from lateritic soils at nsimi cameroon were investigated using electron microprobe analysis empa transmission electron microscopy tem scanning electron microscopy sem and raman spectroscopy to determine the extent of radiation damage from alpha decay events the soils belong to a small watershed developed on granitic rocks of the congo craton 2 9 ga interactions with fluids are evidenced by significant cao up to 1 5 wt al2o3 up to 2 9 wt and fe2o3 up to 2 9 wt concentrations in uo2 rich regions 0 05 to 1 wt of the zircon regional heating up to 500 c related to the pan african orogeny about 0 6 ga ago has lead to the recrystallization of the radiation damaged grains and the formation of a nanoporous microstructure the correlation observed between the presence of dissolution features and the actual damage state of zircon shows that zircon dissolution occurs under tropical weathering conditions and with preferential dissolution of the highly radiation damaged regions congruent dissolution of zircon and the limited mobility of zr are supported by the absence of zirconium oxide precipitates in the fractures of weathered grains of zircon

present weathering rates in a humid tropical watershed nsimi south cameroon the study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks common for the temperate zone has not yet been widely applied to the tropics especially humid areas this paper presents an updated database for a six year period for the small experimental watershed of the mengong brook in the humid tropics nsimi south cameroon this watershed is developed on precambrian granitoids north congo shield and consists of two convexo concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter mineralogical and geochemical investigations were carried out in the protolith the saprolite the hillside lateritic soils and the swamp hydromorphic soils biomass chemical analyses were done for the representative species of the swamp vegetation the groundwater was analysed from the parent rock saprolite weathering front to the upper fringe in the hillside and swamp system the chemistry of the wet atmospheric and throughfall deposits and the mengong waters was monitored in the nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter these organic colloids contribute significantly to the mobilization and transfer of fe al zr ti and th in the uppermost first meter of the swamp regolith when the organic colloid content is low i e in the hillside groundwater th and zr concentrations are extremely low 3 pmol l icp ms detection limits strongly insoluble secondary thorianite tho 2 and primary zircon zrsio 4 crystals control their mobilization respectively this finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed biogenic recycling significantly influences the low cl input fluxes sodium is a good tracer of chemical weathering in the watershed the sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate even though low 2 8 mm kyr the chemical weathering rate predominates over the mechanical weathering rate 1 9 mm kyr compared to the rio icacos watershed the most studied tropical site the chemical weathering fluxes of silica and sodium in the mengong are 16 and 40 times lower respectively this is not only related to the protective role of the regolith thick in both cases but also to differences in the hydrological functioning this is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level

estimation of weathering rates and co2 drawdown based on solute load significance of granulites and gneisses dominated weathering in the kaveri river basin southern india abstract the solute load of the kaveri river south india and its tributaries draining diverse precambrian terrains during pre monsoon and monsoon periods was determined using average annual flow total drainage area and atmospheric input corrected major ion concentrations of these rivers chemical weathering rates annual fluxes of different ionic species to the ocean and co2 consumption rates were estimated bicarbonate is the most dominant ion 27 79 of anion budget in all the river samples collected during monsoon period followed by ca2 whereas in case of pre monsoon water samples na is the most dominant ion in meq l two approaches were adopted to estimate silicate and carbonate weathering rates in the drainage basin at musuri silicate weathering rate swr is 9 44 0 29 tons km2 a and carbonate weathering rate cwr is 1 46 0 16 tons km2 a more than 90 of the total ionic budget is derived from weathering of silicates in the kaveri basin co2 consumption rate in the basin for silicate weathering f co 2 sil is 3 83 0 12 105 mol km2 a upper limit which is comparable with the himalayan rivers at upper reaches for carbonate weathering f co 2 carb co2 consumption rate is 0 15 0 03 105 mol km2 a in the kaveri basin the lower limit of co2 consumption rate corrected for h2so4 during silicate and carbonate weathering is f co 2 sil is 3 24 1005 mol km2 a and f co 2 carb 0 13 105 mol km2 a respectively co2 sequestered due to silicate weathering in the kaveri basin is 25 41 0 82 109 mol a which represents 0 21 0 01 of global co2 drawdown this may be due to tropical climatic condition high rainfall during both sw and ne monsoon and predominance of silicate rocks in the kaveri basin

the isotopic fingerprint of fe cycling in an equatorial soil plant water system the nsimi watershed south cameroon abstract following an initial study of a tropical lateritic hillside system showing little iron isotope fractionation despite a strong accumulation within the soil profile the present work investigates iron isotope signatures within the organic matter rich swamp system that represents 20 of the studied watershed surface from nsimi south cameroon this study considers the soil plant water continuum in order to better understand the fe elemental and isotopic transfer out of the ecosystem within the swamp system the iron isotope compositions of gleysol samples 57 fe irmm 14 0 6 are significantly heavier than both the continental crust baseline and the reference lateritic soils from the hillslope 57 fe irmm 14 0 1 this enrichment towards heavy isotopes is attributed to a preferential removal of light iron isotopes during soil forming processes pedogenic transformations i e gleyzation organic complexation of metal and leaching are responsible for the reductimorphic features observed in ferralitic horizons i e incomplete degradation of organic matter in surface and soil whitening favored by good draining conditions the organic carbon rich waters of the swamp system are prone to redox processes and strong metal chelation the dissolved iron i e fraction 57 fe signatures with a downstream enrichment in heavy isotopes from 0 511 0 266 to 1 076 0 240 the binding of iron fe iii with organic matter can explain the observed enrichment in heavy isotopes in the dissolved fraction on the contrary plant leaves are significantly enriched in light fe 57 fe of 0 665 0 035 and 1 119 0 080 relative to i the litter compartment 0 166 0 078 to 0 262 0 013 for 57 fe and ii the most superficial soils iron isotopic compositions in plants and litter vary as a function of both plant species and season hence the differences in fe isotopic compositions between the various studied compartments suggest that fe isotopes can be used i to study elemental transfers during soil pedogenesis in tropical environment and ii to better appraise and constrain iron biogeochemical cycle between surface horizon of soils surface waters and the vegetation

temperature dependence of silicate weathering in nature how strong a negative feedback on long term accumulation of atmospheric co2 and global greenhouse warming estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent arrhenius activation energy of 18 4 kcal mol 77 0 kj mol much higher than most laboratory values this finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought this result provides a stronger negative feedback on long term greenhouse warming than has been assumed in most models of global carbon cycling the present estimate was determined from the ratio of feldspar weathering rates determined by geochemical mass balance in the southern blue ridge mountains of north carolina united states temperature a function of elevation is the only factor that differs between the two catchments parent rock type aspect hillslope hydrology and vegetation type and successional stage are the same in both

zircon an immobile index in soils interpretation of any element s mobility in soils needs rigorous means of calculating losses or gains in relation to parent rock the first mass balance calculations were based on isoelement changes major elements during conversion of fresh rocks to weathered materials harrisson 1933 leneuf 1959 isovolumetric mass balance calculations were applied for the first t ime in 1955 to determine mass transfers in saprolite millot and bonifas 1955 since that time numerous works have been carried out on mass balance calculations in determining transfers during weathering brimhall and dietrich 1986 recently equations used to calculate volumetric changes and net mass gain or loss of mobile elements have been based on zirconium mostly from the mineral zircon the aim of the paper is to question the axiomatic immobility of zircon in soils and then to assess the limit of mass balance calculations in surficial weathering systems

on the iron isotope homogeneity level of the continental crust abstract assessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer in recent studies we concluded that most igneous rocks from the continental crust were homogeneous to 71 wt sio2 that we concluded were significantly heavier than less silicic rocks from the crust on the basis of fe isotopic analyses of seven bulk rock samples from different geological settings beard and johnson beard b l johnson c m 2006 comment on heavy iron isotope composition of granites determined by high resolution mc icp ms by f poitrasson and r freydier chemical geology volume 222 pages 132 147 chem geol this issue challenge this finding on the basis of previously published data it is shown here that this controversy stems from the confusion between whole rock and mineral iron isotope analyses since this isotopic tracer like many others is more heterogeneous at the mineral scale than the bulk rock scale thus minerals cannot be used in place of bulk rock fe isotope compositions the controversy also arises from different levels of analytical uncertainties if only the best fe isotope probes of planetary reservoirs bulk rocks are considered then the conclusion that igneous rocks with sio2 71 wt are significantly heavier than the mean 57fe of the continental crust poitrasson f freydier r 2005 heavy iron isotope composition of granites determined by high resolution mc icp ms chem geol 222 132 147 remains valid nonetheless given that granites represent only a minor fraction of the continental crust this finding does not change the bulk igneous fe isotope composition estimate previously defined for the earth

redistribution and mobilization of titanium zirconium and thorium in an intensely weathered lateritic profile in western australia abstract the mobility of titanium zirconium and thorium elements commonly considered insoluble during supergene weathering is still not well understood especially in intensely weathered regolith thus an intensely weathered lateritic profile jg developed on meta granitoids in jarrahdale western australia was investigated the mobility of ti zr and th has been assessed at both mineral assemblage and profile scales and the mode of occurrence has been investigated through the combined use of geochemical data from bulk regolith particle size fractions and sequential extractions with in situ data determined by electron probe micro analyzer and synchrotron x ray powder diffraction neoformed poorly crystalline phases containing trace to minor amounts of zr ce and th unassociated with silicates or phosphates were identified on the walls of al fe rich pores in the ferruginous duricrust this implies that some mobilization and redistribution of zr and th occurs within a sample scale breakdown of primary thorite and rare earth element rich fluorocarbonates is thought to be the source for zr and th in the neoformed phases rather than zircon thus the mineral hosts of zr ti and th in the parent rock and their relative susceptibility to weathering are the fundamental controls on subsequent mobility during initial weathering trace amounts of th in secondary phases such as rhabdophane and florencite show translocation of th at the mineral scale while strong partitioning of th into gravel rather than matrix reflects redistribution of th at the profile scale the absence of primary sphene from the regolith and the presence of partially dissolved ilmenite and rutile grains in the ferruginous mottled zone suggest mobilization and translocation of ti at a mineral assemblage scale furthermore the fluctuation of ti zr in the ferruginous zone is in contrast to the consistency of zr hf throughout the profile in general within the range of parent meta granitoid this suggests that ti and zr fractionate from each other and partition between gravel and matrix during extreme weathering and advanced lateritization this study demonstrates that ti zr and th are mobile at a variety of scales an important consideration that is often overlooked when calculating element mass flux in intensely weathered regolith