gravity anomalies and segmentation at the east coast usa continental margin summary the free air gravity edge effect anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust while crustal thinning is a major contributor recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it at some margins to be highly segmented along their strike in this paper we use a combined 3 d flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of atlantic type continental margins we focus on the east coast usa since a substantial amount of high quality seismic reflection and refraction gravity and magnetic data already exists for this margin by calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free air anomaly we have determined the best fit elastic thickness te structure of the margin we show that 0 10 5 a integrated strength of the lithosphere these results imply that weak regions abut strong ones at the east coast usa margin te generally decreases with increase in the amounts of crustal thinning and the flexed basement curvature k suggesting it is controlled at least in part by the mechanical structure of the pre rift lithosphere and yielding due to flexural loading however there is considerable scatter suggesting other factors such as along strike changes in crustal composition irrespective we show that an isostatic anomaly that takes into account the best fit te distribution is significantly reduced in spectral power compared with one which is computed assuming only airy compensation i e te 0 km this is not to imply that rifting and sediment loading completely accounts for the anomalies along strike and across strike the east coast usa margin to the contrary significant isostatic anomaly highs and lows persist especially in inner and middle shelf regions one of the most prominent is an arcuate 670 km long high with flanking lows offshore carolina that we attribute to magmatism during the initial stages of continental break up

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

a 56 million year rhythm in north american sedimentation during the phanerozoic abstract long term 10 myr fluctuations in climate sea level and sedimentation have been documented in the stratigraphic record but the lack of well constrained data series has made it difficult to rigorously evaluate cyclic periodic or quasi periodic changes at this scale here we utilize a new compilation of the coverage area of sedimentary rocks in north america to investigate the dominant modes orders of stratigraphic variability and to evaluate potential long period cyclic changes in sedimentation on the continent during the phanerozoic our analysis resolves two principal temporal modes of variability 1 a strongly sinusoidal mode with a periodicity of 56 myr 3 myr and 2 a longer term phanerozoic mode the m curve linked to the wilson cycle which is indistinguishable from a stochastic autoregressive process the newly identified 56 myr cycle in sedimentation delineates most of the cratonic sequences that have previously been identified qualitatively in north america but here we propose a quantitative redefinition that includes nine distinct units and two mega sequences the timing of the 56 myr beat in sedimentation is consistent with an orogenic oscillator source or an oscillatory dynamic in mantle convection and its tempo is statistically similar to a known rhythm in number of marine animal genera in the global fossil record thus the identification of a significant periodic signal in the sedimentary record of north america provides new evidence for an important tectonic and or mantle scale cyclic process that links both large scale biological evolution and physical environmental change

construction of the cambrian and ordovician seawater 87sr 86sr curve abstract the estimated path of seawater 87 sr 86 sr during the cambrian and ordovician is based on 172 analyses the sample suite is largely from locations in oklahoma new york tennessee and texas where rocks of this age are widely exposed and well studied the oldest reliable samples examined in this study are of middle cambrian age and yield an 87 sr 86 sr near 0 70888 sw 19 the estimated ratio rises to near 0 70915 sw 8 in the lower upper cambrian the highest value during the phanerozoic and probably for all geologic time the estimated path drifts lower to near 0 70903 sw 4 at the cambrian ordovician boundary this further declines to near 0 70875 sw 32 at the end of the ibexian and reaches a value near 0 70821 sw 86 at the whiterockian mohawkian boundary the decline in the seawater 87 sr 86 sr continues into later mohawkian time to flatten abruptly near 0 70782 sw 125 this ratio is essentially constant through the latest ordovician the curve rises near to a ordovician silurian boundary value poorly constrained from our data near 0 70784 sw 123 the early paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of sr continental sr dominated seawater from the beginning of the cambrian into late cambrian from that time as the cambrian transgression proceeded and radiogenic crystalline rocks were covered oceanic sr began to dominate sr from low ratio oceanic sources dominated through early mohawkian time an abrupt balance is struck between oceanic and continental sources to end the decline in the late ordovician and maintained until near the end of the ordovician the general curve shape follows the pattern suggested by proposed sealevel changes the rise and fall of seawater 87 sr 86 sr during the cambrian and ordovician characterized seawater variation during all the paleozoic

regolith residence time and the concept of surface age of the piedmont peneplain the common upland weathering profile on foliated metamorphic rocks on the appalachian piedmont between alabama and maryland includes a 50 cm 1 m thick clay rich soil b horizon over 10 20 m of saprolite the saprolite retains the volume fabric and structure of the underlying parent rock but has lost about one third of the mass of the parent rock where the saprolite is thickest beneath upland drainage divides those divides are of loow slope and relief the flatness of the piedmont drainage divides provides the appearance of a planation surface of accordant summits or peneplain i interpret this flatness to be the result of three active processes volume reduction during compaction of saprolite into soil loss of mass in dissolved solids draining the soil b horizon and loss of mass from the soil surface by erosion of clay size particles the cumulative effects of these upland processes are to convert saprolite into soil to limit soil thickness and residence time and to limit the age of the oldest residual soils on the surface this results in a continual lowering of the piedmont peneplain surface this interpretation is corroborated by evidence of topographic inversion of fluvial gravels since the miocene near the fall line study of baseflow dissolved solids draining piedmont basins indicates a minimum rate of saprolite production of about 4 m ma 1 this minimum rate of production near the fall line is less than the estimated rate of surface lowering and saprolite consumption during the late tertiary and quaternary periods of topographic inversion measurement of cosmogenic 10be inventories in two 15 20 mores of piedmont upland regolith indicate a minimum residence time of residual soil and saprolite of about 1 ma this indicates a possible maximum rate of saprolite production and erosion of about 20 m ma 1 using the best available evidence therefore the typical piedmont upland regolith has a residence time of between 1 and 5 ma residence times may not be the same everywhere and ages may vary systematically across the surface the geomorphic evidence of topographic inversion and similar regolith profiles in other humid temperate and tropical regions strongly suggests that production and removal of piedmont saprolite has been an ongoing quaternary process and that most of the piedmont regolith is significantly younger than 5 ma independent geophysical evidence suggests that the rate of uplift probably involving tilting of parts of the piedmont blue ridge and valley and ridge has averaged between 5 and 20 m ma 1 through the cenozoic

physical and chemical weathering rates and co2 consumption in a tropical lateritic environment the upper niger basin the chemical composition of niger river water measured bimonthly at bamako mali during the period 1990 1992 provides an estimate of present weathering rates in the upper niger basin the dominant weathering process is kaolinite formation monosiallitization however seasonal variations promote gibbsite formation in the rainy season september and smectite development in the dry season may the results show that lateritic profiles continue to develop even during very dry episodes the rate of profile development calculated as the difference between the chemical weathering rate at the base of the soil profile and mechanical erosion rate at the soil surface is about 1 3 to 3 7 m myr a comparison between 43 river basins of the world shows that for similar runoff the co2 flux consumed by silicate weathering is about two times lower in lateritic areas than in nonlateritic zones

using a structural approach to identify relationships between soil and erosion in a semi humid forested area south india biogeochemical and hydrological cycles are currently studied on a small experimental forested watershed 4 5 km 2 in the semi humid south india this paper presents one of the first data referring to the distribution and dynamics of a widespread red soil ferralsols and chromic luvisols and black soil vertisols and vertic intergrades cover and its possible relationship with the recent development of the erosion process the soil map was established from the observation of isolated soil profiles and toposequences and surveys of soil electromagnetic conductivity em31 geonics ltd lithology and vegetation the distribution of the different parts of the soil cover in relation to each other was used to establish the dynamics and chronological order of formation results indicate that both topography and lithology gneiss and amphibolite have influenced the distribution of the soils at the downslope the following parts of the soil covers were distinguished i red soil system ii black soil system iii bleached horizon at the top of the black soil and iv bleached sandy saprolite at the base of the black soil the red soil is currently transforming into black soil and the transformation front is moving upslope in the bottom part of the slope the chronology appears to be the following black soil bleached horizon at the top of the black soil streambed bleached horizon below the black soil it appears that the development of the drainage network is a recent process which was guided by the presence of thin black soil with a vertic horizon less than 2 in deep three distinctive types of erosional landforms have been identified 1 rotational slips type 1 2 a seepage erosion type 2 at the top of the black soil profile 3 a combination of earthflow and sliding in the non cohesive saprolite of the gneiss occurs at midslope type 3 types 1 and 2 erosion are mainly occurring downslope and are always located at the intersection between the streambed and the red soil black soil contact neutron probe monitoring along an area vulnerable to erosion types 1 and 2 indicates that rotational slips are caused by a temporary watertable at the base of the black soil and within the sandy bleached saprolite which behaves as a plane of weakness the watertable is induced by the ephemeral watercourse erosion type 2 is caused by seepage of a perched watertable which occurs after swelling and closing of the cracks of the vertic clay horizon and within a light textured and bleached horizon at the top of black soil type 3 erosion is not related to the red soil black soil system but is caused by the seasonal seepage of saturated throughflow in the sandy saprolite of the gneiss occurring at midslope c 2006 elsevier b v all rights reserved

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

surface reflectance of mars observed by crism mro 1 multi angle approach for retrieval of surface reflectance from crism observations mars reco 1 this article addresses the correction for aerosol effects in near simultaneous multi angle observations acquired by the compact reconnaissance imaging spectrometer for mars crism aboard the mars reconnaissance orbiter in the targeted mode crism senses the surface of mars using 11 viewing angles which allow it to provide unique information on the scattering properties of surface materials in order to retrieve these data however appropriate strategies must be used to compensate the signal sensed by crism for aerosol contribution this correction is particularly challenging as the photometric curve of these suspended particles is often correlated with the also anisotropic photometric curve of materials at the surface this article puts forward an innovative radiative transfer based method named multi angle approach for retrieval of surface reflectance from crism observations mars reco the proposed method retrieves photometric curves of surface materials in reflectance units after removing aerosol contribution mars reco represents a substantial improvement regarding previous techniques as it takes into consideration the anisotropy of the surface thus providing more realistic surface products furthermore mars reco is fast and provides error bars on the retrieved surface reflectance the validity and accuracy of mars reco is explored in a sensitivity analysis based on realistic synthetic data according to experiments mars reco provides accurate results up to 10 reflectance error under favorable acquisition conditions in the companion article photometric properties of martian materials are retrieved using mars reco and validated using in situ measurements acquired during the mars exploration rovers mission

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

macrostrat a platform for geological data integration and deep time earth crust research characterizing the lithology age and physical chemical properties of rocks and sediments in the earths upper crust is necessary to fully assess energy water and mineral resources and to address many fundamental questions in the earth sciences although a large number of geological maps regional geological syntheses and sample based measurements have been produced there is no openly available database that integrates rock record derived data while facilitating large scale quantitative characterization of the volume age and material properties of the upper crust here we describe macrostrat a relational geospatial database and supporting cyberinfrastructure that is designed to enable quantitative spatial and geochronological analyses of the entire assemblage of surface and subsurface sedimentary igneous and metamorphic rocks macrostrat now contains general comprehensive summaries of the age and properties of 33 903 lithologically and chronologically defined geological units distributed across 1 474 regions in north and south america the caribbean new zealand and the deep sea sample derived data including fossil occurrences in the paleobiology database more than 180 000 geochemical and outcrop derived measurements and more than 2 1 million bedrock geologic map units from over 170 map sources are linked to specific macrostrat units and or lithologies macrostrat has generated numerous quantitative results and its data infrastructure is being used in several independently developed mobile applications however it is necessary to expand geographic coverage and to refine age models and material properties to arrive at a more precise characterization of the upper crust globally

shield effect on continental weathering implication for climatic evolution of the earth at the geological timescale abstract a model sensitivity study is performed in order to explore the impact of the development of thick weathering profiles in the equatorial area on the geological evolution of climate long term climatic evolution is driven by the balance existing between solid earth degassing and continental silicate weathering the co 2 consumption through continental silicate weathering is here calculated as a function of climate based on the correlation existing with mean annual continental runoff for large drainage basins these correlations appear to be dependent on the climatic regime under mid latitude temperate climate the silicate weathering is 5 times more sensitive to continental runoff than in the warm humid equatorial belt these correlations emphasize the role of the thick weathering profiles existing in equatorial area shielding the bedrock from further weathering we use this set of relationships between runoff and silicate weathering as a function of the geoclimatic zone to explore the impact of this shield effect on the mesozoic evolution of climate using the geoclim model compared to previous work we suggest that atmospheric co 2 was generally underestimated in the past evidencing a complex interplay between the paleogeographic setting and the shield effect particularly calculated atmospheric co 2 during the early middle jurassic rises up to 1600 ppmv while it was limited to 700 ppmv when shield effect was neglected corresponding to a global warming of more than 4 c this study emphasizes the need for the building up of numerical models describing the growth of the weathering profiles that can be coupled to vegetation and climate models to investigate biogeochemical cycle and climate evolution even at the geological timescale

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

geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in central africa a simple geochemical balance of lateritization processes governing the development of several tens of meters of weathering profiles overlain by ferricretes is estimated on the basis of detailed mineralogical and geochemical data the lateritic weathering mantle of the haut mbomou area in central africa is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite a mottled clay layer a soft nodular layer a soft ferricrete and a ferricrete in which kaolinite gibbsite goethite and hematite occur in various quantities incongruent dissolution of kaolinite leads to the formation of gibbsite in the upper saprolite whereas the hematite does not clearly replace the kaolinite according to an epigene process in the upper ferruginous layers of the profiles instead that kaolinite is also transformed into gibbsite according to an incongruent dissolution under hydrated and reducing conditions induced by a relatively humid climatic pattern the respective relations of the silica iron and aluminum balances and the al substitution rate of the hematite on the one hand and of rhg rhg 100 hematite hematite goethite and the kaolinite on the other hand to the consumption or the release of protons h permit differentiation of aggrading ferruginization and degradation processes operating in the different lateritic weathering profiles the al substitution rate of the fe oxyhydroxides varies according to the nature of lateritization processes e g saprolitic weathering and aggrading ferruginization vs degradation the observations and results indicate that the ferruginization process of the weathering materials of parent rocks is not a simple ongoing process as often thought this suggests that the actual lateritic weathering mantle of the haut mbomou area may result from different stages of weathering and erosion during climatic changes

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

regolith mass balance inferred from combined mineralogical geochemical and geophysical studies mule hole gneissic watershed south india the aim of this study is to propose a method to assess the long term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics geochemistry and mineralogy we initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the karnataka plateau peninsular india in the transition sub humid zone of this climatic gradient we have studied the pristine forested small watershed of mule hole 4 3 km2 mainly developed on gneissic substratum mineralogical geochemical and geophysical investigations were carried out i in characteristic red soil profiles and ii in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots in addition 12 electrical resistivity tomography profiles ert with an investigation depth of 30 m were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles the location of the ert profiles is based on a previous electromagnetic survey with an investigation depth of about 6 m the soil cover thickness was inferred from the electromagnetic survey combined with a geological pedological survey taking into account the parent rock heterogeneity the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices loss on ignition weathering index of parker chemical index of alteration comparing these indices with electrical resistivity logs it has been found that a value of 400 ohm m delineates clearly the parent rocks and the weathered materials then the 12 inverted ert profiles were constrained with this value after verifying the uncertainty due to the inversion procedure synthetic models based on the field data were used for this purpose the estimated average regolith thickness at the watershed scale is 17 2 m including 15 2 m of saprolite and 2 m of soil cover finally using these estimations of the thicknesses the long term mass balance is calculated for the average gneiss derived saprolite and red soil in the saprolite the open system mass transport function indicates that all the major elements except ca are depleted the chlorite and biotite crystals the chief sources for mg 95 fe 84 mn 86 and k 57 biotite only are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18 the ca accumulation can be attributed to the precipitation of caco3 from the percolating solution due to the current and or the paleoclimatic conditions overall the most important losses occur for si mg and na with 286 106 mol ha 62 of the total mass loss 67 106 mol ha 15 of the total mass loss and 39 106 mol ha 9 of the total mass loss respectively al fe and k account for 7 4 and 3 of the total mass loss respectively in the red soil profiles the open system mass transport functions point out that all major elements except mn are depleted most of the oligoclase crystals have broken down with a loss of 90 the most important losses occur for si na and mg with 55 106 mol ha 47 of the total mass loss 22 106 mol ha 19 of the total mass loss and 16 106 mol ha 14 of the total mass loss respectively ca al k and fe account for 8 6 4 and 2 of the total mass loss respectively overall these findings confirm the immaturity of the saprolite at the watershed scale the soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric co2

neoproterozoic glacial origin of the great unconformity the great unconformity a profound gap in earth s stratigraphic record often evident below the base of the cambrian system has remained among the most enigmatic field observations in earth science for over a century while long associated directly or indirectly with the occurrence of the earliest complex animal fossils a conclusive explanation for the formation and global extent of the great unconformity has remained elusive here we show that the great unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late neoproterozoic crustal erosion and sediment subduction event of unprecedented scale these excursions the great unconformity preservational irregularities in the terrestrial bolide impact record and the first order pattern of phanerozoic sedimentation can together be explained by spatially heterogeneous neoproterozoic glacial erosion totaling a global average of 3 5 vertical kilometers along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard

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

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

behavior of sm and nd in a lateritic soil profile a study of lateritic soils and samples of ground and river waters was carried out in the nsimi zoetele a tropical watershed in the southern cameroon the nd isotopic compositions and concentrations of nd and sm were determined it was found that the nd isotopic composition of the river waters was much more radiogenic than the parent rocks and that the nd in the waters is not homogeneous but is carried by different dissolved and complexed components that are not isotopically homogenized the soil profile shows a regular increase in e nd going from the parent rock e nd 36 to e nd 18 near the top of the profile the nd transported in the river is thus not representative of the parent rock but reflects the results of differential weathering of constituent minerals and the redeposition of ree in phosphates and a significant contribution of radiogenic nd from dust the concentration of nd in the river water is far above that found in temperate climate rivers and thus this type of tropical river may play a dominant role in the marine nd and ree budget it is suggested that the correlation of ree with doc is related to doc fixing some dissolved ree but that the ree in solution is governed by other mechanisms no major shifts were found in sm nd however a regular progression from the parent rock through the lateritic profile was found the upper laterite profile shows large almost uniform depletions in all ree below tb and enrichment above complementary behavior was found in the lower part of the section the concentration of nd relative to the immobile elements zr and ti in the laterite is depleted by a factor of 10 th nd and sm are enriched in the lowest zone sampled and must reflect redeposition of ree from the upper part of the weathering section and is associated with phosphate formation it is concluded that the soil evolution involves both differential dissolution of primary phases from the parent rock significant to major input of ree from atmospheric dust from other regions and the formation of diagenetic phases particularly phosphates

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

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

covariation in macrostratigraphic and macroevolutionary patterns in the marine record of north america students of earth history have long recog nized the correlation between the quantity of preserved sedimentary rock and the diversity of life recorded as fossils but paleontologists have yet to determine whether this pattern refl ects a causal relationship or a unidirec tional sampling bias in fossil data imposed by preserved rock quantity distinguishing between these two alternatives has been com plicated by the fact that many of the basic patterns of paleontologic and lithologic co variation have yet to be quantifi ed rigorously here we present the fi rst analyses of the co variation between the macrostratigraphic and macroevolutionary histories of north america based on geographically and tem porally explicit co occurrences of rocks and fossils the analyses use independent quan titative summaries of the stratigraphic and fossil records by integrating the paleobiology database paleodb and macrostrat a mac rostratigraphy database for north america which allows a more direct comparison of the stratigraphic and biological histories of the continent than has heretofore been possible within the macrostrat database the rock re cord is divided into discrete packages of sedi ment that are bound by hiatuses resolvable at the stage level using per interval per pack age rates of sediment package initiation and truncation and genus fi rst and last appear ances herein regional origination and extinc tion we fia substantially stronger positive correlation between sediments and biology for extinction like parameters than we do for origination like parameters four of the largest coincident pulses of regional extinc tion and sediment truncation occur during the widely recognized end ordovician late permian end triassic and end cretaceous mass extinction intervals a further compari son of the global ranges of north american genera to north american macrostratigra phy indicates that the regional and global extinction of genera are more likely to oc cur in the same stage than are global and regional originations together these results suggest that our general understanding of biodiversity dynamics from the fossil record may not be strongly biased by the preserva tion of sediments and leaves open the pos sibility that certain large perturbations to the earth system are responsible for major changes of state in both the sedimentary and biological systems

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

ferricrete biochemical degradation on the rainforest savannas boundary of central african republic in southeastern central african republic the lateritic weathering mantles are capped by 2 to 5 m thick ferricretes which previously formed under a seasonal tropical climate the actual humid tropical climatic conditions result in the biophysical disaggregation of the ferricretes everywhere the forest develop that lead to a soil formation composed of ferricrete relicts and ferruginous nodules embedded in a soft bioturbated micro aggregated clay ferruginous matrix the potential effect of the biological activity e g termites on the soil and vegetation dynamics and therefore on the evolution of the previous consolidated ferricrete is discussed following its mechanical disaggregation by the forest tree roots the ferricrete underwent a chemical degradation under the combined effect of hydration and redox conditions which result from the biodegradation and oxidation of the organic matter the transformation of the ferricrete into a soft bioturbated micro aggregated clay ferruginous soil matrix implies the hematite dissolution and the kaolinite transformation into gibbsite which may characterize a late bauxitization secondary gibbsitization of kaolinite of previous ferricrete profiles linked to a late quaternary environmental climate and vegetation change however the geochemical and mineralogical patterns of the matrix not only reflect the geochemistry of the parental ferricrete but also depend on the physical transfers of quartz and heavy minerals from the lower horizons of the profile hence the use of geochemical indexes such as ti and or zr for mass balance calculations in the lateritic weathering profiles is precluded because these supposedly inert chemical elements are not simply accumulated by in situ chemical weathering process

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

chemical and isotopic changes in williston basin brines during long term oil production an example from the poplar dome montana brine samples were collected from 30 conventional oil wells producing mostly from the charles formation of the madison group in the east and northwest poplar oil fields on the fort peck indian reservation montana dissolved concentrations of major ions trace metals sr isotopes and stable isotopes oxygen and hydrogen were analyzed to compare with a brine contaminant that affected groundwater northeast of the town of poplar two groups of brine compositions designated group i and group ii are identified on the basis of chemistry and 87sr 86sr ratios the solute chemistry and sr isotopic composition of group i brines are consistent with long term residency in mississippian carbonate rocks and brines similar to these contaminated the groundwater group ii brines probably resided in clastic rocks younger than the mississippian limestones before moving into the poplar dome to replenish the long term fluid extraction from the charles formation collapse of strata at the crest of the poplar dome resulting from dissolution of charles salt in the early paleogene probably developed pathways for the ingress of group ii brines from overlying clastic aquifers into the charles reservoir such changes in brine chemistry associated with long term oil production may be a widespread phenomenon in the williston basin

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

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

contrasting patterns and connections of rock and biotic diversity in the marine and non marine fossil records of north america abstract macrostratigraphy uses packages of continuous sedimentation that are bound by hiatuses of non deposition erosion or alternations between environments lithologies to characterize spatiotemporal patterns of sedimentation previous work has linked the macrostratigraphy of marine shelf environments to many different macroevolutionary patterns in the marine animal fossil record here we use an improved macrostratigraphic database for north america combined with fossil occurrence data in the paleobiology database to show that macroevolutionary and macrostratigraphic patterns are correlated more strongly in marine environments than in non marine environments we also test the hypothesis that the temporal distribution of lithological diversity measured as evenness similar to ecological evenness based on the number of lithostratigraphic units with a given lithologic type within each time interval has a relationship to genus level taxonomic diversity in both the marine and non marine realms uneven sampling of lithologies and their corresponding depositional environments can bias our perception of taxonomic diversity by causing taxa from poorly sampled environments to be underrepresented in the fossil record our results show a negative correlation between lithologic evenness and marine vertebrate generic richness less even sampling of lithologies corresponds to a higher observed taxonomic richness and no correlation with lithologic evenness and non marine diversity measures this suggests a difference in the relationship between biologic and sedimentary processes at work in the marine and non marine realms and possibly differences in the magnitude of the bias imposed by the rock record on the underlying biologic patterns

seasonality change and the third millennium bp rainforest crisis in southern cameroon central africa the third millennium bp crisis of the central african rainforest is not sufficiently understood the low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event and its causal climatic parameters are debated we present a high resolution pollen profile from the swamp site nyabessan in the southern cameroonian rainforest covering the period 3100 2300 cal yr bp between 3100 and 2500 cal yr bp the climate was favourable for a regional evergreen forest with caesalpiniaceae and lophira and a local raphia swamp forest around 2500 2400 cal yr bp a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations the dominance of trema orientalis a pioneer well adapted to seasonal desiccation points to a much more accentuated seasonality after 2500 cal yr bp which seems to be linked to a southwards shift of the itcz during the northern hemisphere winter months we propose that the rain forest crisis between 2500 and 2200 cal bp created favourable conditions for farming and paved the way for a major expansion of bantu speaking populations

behaviour of li isotopes during continental weathering the bidar laterite profile india weathering of large basaltic provinces is a major sink of atmospheric carbon dioxide and is also an important part of the geochemical cycle of many other elements this study aims to improve our understanding of basaltic weathering by analysis of trace elements and li isotopes in a lateritic profile developed upon the deccan traps flood basalt at its base the profile consists of unaltered basalt that has a li concentration and 7li value of 5 ppm and 2 5 respectively which is upwards progressively altered to a highly weathered laterite iron rich residuum at the surface high concentrations of fe2o3 85 wt at 11 m depth indicate the presence of a palaeowatertable the transition metals the rare earth elements rees and u are also strongly enriched in this horizon whereas fluid mobile elements such as li and rb are depleted relative to the precursor basalt and the bulk 7li value is very low 5 5 a high proportion of leachable li is found in this horizon and this is enriched in 6li relative to the bulk we suggest that leaching extracts the labile weathering products and these have high 6li as 7li has been preferentially mobilised during basalt weathering throughout the rest of the profile li concentrations and li isotope values are best explained by two component mixing between the precursor basalt and aeolian material with high li and low 7li mass balance calculations show that at least 50 of the li present in the profile could have been supplied by a mineral aerosol that has a 7li value between 0 and 1 3 the depth integrated 7li value of the profile is 0 6 which suggests that 7li has been preferentially leached from the aerosol after its deposition the source of this aeolian material is most likely to be ancient continental crust it is possibly derived from archaen proterozoic terrain to the south and southeast of the deccan and or material from central asia similar to that which forms chinese loess and which is supplied to the north pacific basin