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

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

u sr isotopic speedometer fluid flow and chemical weathering rates in aquifers abstract both chemical weathering rates and fluid flow are difficult to measure in natural systems however these parameters are critical for understanding the hydrochemical evolution of aquifers predicting the fate and transport of contaminants and for water resources water quality considerations 87 sr 86 sr and 234 u 238 u activity ratios are sensitive indicators of water rock interaction and thus provide a means of quantifying both flow and reactivity the 87 sr 86 sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate similarly the 234 u 238 u ratio of natural ground waters is a balance between the flow rate and the dissolution of solids and recoil loss of 234 u from the solids by coupling these two isotope systems it is possible to constrain both the long term ca 100 s to 1000 s of years flow rate and bulk dissolution rate along the flow path previous estimates of the ratio of the dissolution rate to the infiltration flux from sr isotopes 87 sr 86 sr are combined with a model for 234 u 238 u to constrain the infiltration flux and dissolution rate for a 70 m deep vadose zone core from hanford washington the coupled model for both 234 u 238 u ratios and the 87 sr 86 sr data suggests an infiltration flux of 5 2 mm yr and bulk silicate dissolution rates between 10 15 7 and 10 16 5 mol m 2 s the process of recoil enrichment while primarily responsible for the observed variation in 234 u 238 u of natural systems is difficult to quantify however the rate of this process in natural systems affects the interpretation of most u series data models for quantifying the recoil loss fraction based on geometric predictions surface area constraints and chemical methods are also presented the agreement between the chemical and theoretical methods such as direct measurement of 234 u 238 u of the small grain size fraction and geometric calculations for that size fraction is quite good

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

climate control on erosion distribution over the himalaya during the past 100 ka sediment samples from a 50 m long core representing 100 ka of deposition taken from the ganga plain on the campus of the indian institute of technology kanpur were analyzed for sr and nd isotope compositions both 87 sr 86 sr and nd vary significantly with depth in the core 0 72701 0 76708 and 14 4 to 16 6 respectively within the range for silicate rocks of the higher and the lesser himalaya the variations in the isotope compositions reflect variations in the mixing proportion of sediments from the higher and lesser himalaya the two major sediment sources to the ganga the opposite trends in 87 sr 86 sr and nd depth profiles further confirm this hypothesis the isotope profiles exhibit two major excursions ca 20 ka and ca 70 ka ago coinciding with periods of precipitation minima and larger glacial cover these excursions are the result of a decrease in the proportion of sediment from the higher himalaya due to a decrease in monsoon precipitation and an increase in glacial cover that are in turn caused by lower solar insolation this study highlights the significant influence of climate on erosion in the himalaya

microbiotic soil crusts in the sahel of western niger and their influence on soil porosity and water dynamics abstract microbiotic soil crusts are common features of the surface of fallow land in western niger we investigated the interaction between these microbial covers and the porosity and water dynamics of soils at the surface of a sahelian landscape the soil pore system was examined by microscopic observations and mercury porosimetry the soil water retention capacity was measured using a richard pressure membrane apparatus runoff measurements were performed in situ at a 1 m2 scale under natural rainfall samples with dense cyanobacterial cover had microbially originated pores ranging from 0 04 and 50 m in size these samples also showed higher total soil porosity and retained two to four times more water than samples with thin microbial cover these properties are closely related to the hydrophobic nature of cyanobacterial components final runoff values obtained on densely covered surfaces were significantly higher compared to those measured on surfaces with thin microbial cover due to the geometry of the microbially originated pore system and its functioning

detecting induced in situ erodibility of a dust producing playa in australia using a bi directional soil spectral reflectance model abstract two sites on a playa claypan in south western queensland australia were chosen to represent the variability in soil surface conditions known to control wind erosion and dust production in the region the material was very fine and comprised of large amounts and different types of salts high electrical conductivity a propensity for slaking and the presence of cyanobacteria the surfaces of 4 plots at each site were modified using in situ rainfall simulation and wind tunnel abrasion the changes in the surface conditions were recorded using digital images and multi angular spectral measurements of reflectance and inverted against a soil bi directional spectral reflectance model optimised values of the model parameters produced the single scattering albedo ssa and a description of the scattering behaviour of the soil surfaces that included an estimate of microscopic roughness the model parameters removed the effect of the measurement conditions illumination and viewing geometry on the spectral reflectance abrasion of the untreated surfaces demonstrated that the variability in the antecedent surface conditions played an important role in their response rainfall drying and subsequent abrasion appeared to reset the soil surface conditions and controlled the spatial and temporal variation in the soil surface conditions redundancy analyses conducted separately for ssa of selected wavebands model parameter values and treatments elucidated the subtle and interactive effects of the treatments and surface responses the results showed that high intensity rainfall and abrasion was associated with wavebands in the visible region and reduced roughness low intensity rainfall was associated with wavebands in the near infrared and short wave infrared regions and increased roughness the analysis was interpreted as the result of eluviation due to rainsplash impact which caused size segregation and the preferential translocation of iron oxides to the surface and the creation of a thick surface seal and caused a thin crust which enabled the movement of salts and or mineral clays to the surface because of the strong evaporation gradient a poor relationship between aerodynamic resistance and microscopic roughness suggested that features of soil surface erodibility created by natural environmental processes such as rainsplash and wind erosion required a correction for macroscopic roughness the values of the model parameters appeared to be insensitive to the directional nature of the abrasion process the findings are significant for the development of this work towards an airborne or satellite based assessment of soil surface erodibility and erosion

considerations for u series dating of sediments insights from the flinders ranges south australia abstract uranium isotope ratios have been determined for the fine grained detrital fraction of pleistocene wilkawillina valley fill sediments four local proterozoic bedrock samples and fine grained aeolian material from a sand dune deposit of the flinders ranges south australia the aim was to quantify the comminution age i e the time elapsed since physical weathering of the bedrock and residence time of the valley fill sediments and to place tighter constraints on input parameters for the comminution age calculation despite using two independent approaches for determination of the recoil lost fraction of 234 u from the sediment weighted geometric and surface area estimates samples fail to produce realistic comminution ages and hence residence times the issues involved in the ability to determine sediment comminution ages are discussed the 234 u 238 u activity ratio of the local bedrock is not in secular equilibrium despite the bedrock being much older than 1 ma i e the timeframe for 234 u and 238 u to reach secular equilibrium in a closed system using the average flinders ranges bedrock 234 u 238 u ratio instead of an assumed 234 u 238 u activity ratio of unity for the source would significantly reduce calculated residence times this result warrants concern for future studies using the comminution approach for which a secular equilibrium source 234 u 238 u activity ratio is assumed significant input of aeolian material may modify the measured 234 u 238 u activity ratios such input may be more tightly constrained in future studies using rare earth element and radiogenic isotopic data future comminution studies would benefit from further consideration of the importance of 1 leaching lost 234 u from source rock and bulk sediment samples 2 wind deposition of fine grained material and 3 the appropriateness and robustness of sample pre treatment procedures

sr and nd isotopes in river sediments from the ganga basin sediment provenance and spatial variability in physical erosion 1 sr and nd concentrations and isotope compositions in sediment of the ganga river from gangotri to rajmahal and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the ganga subbasins sr and nd in silicates range from 37 to 138 and from 10 to 36 g g with 87sr 86sr and nd of 0 7474 0 8428 and 25 5 to 15 5 respectively the results suggest that 65 of ganga mainstream sediments are derived from the higher himalayan crystallines highlighting intense physical erosion in this region the 87sr 86sr values of sediments in the gangetic plain show nearly identical trends during two seasons with a sharp and significant decrease at barauni downstream of gandak confluence this brings out the major impact of the sediment contribution from the gandak to the ganga mainstream model calculation suggests that about half of the ganga sediment at rajmahal is sourced from the gandak the erosion rates in the himalayan subbasins of the ganga range between 0 5 and 6 mm a where a is years with the gandak having the highest erosion rate high relief and intense precipitation over the headwater basins of the gandak appear to drive the rapid and focused erosion of this basin the results of this study and those in literature suggest that the eastern syntaxis brahmaputra the western syntaxis indus and the gandak have much higher physical erosion rates than the other himalayan basins focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

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

higher erosion rates in the himalaya geochemical constraints on riverine fluxes the modern erosion rate of continental scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains these two parameters are very difficult to measure directly in continental scale basins in this paper we examine the chemical composition of the suspended load bedload and dissolved load of the ganga and brahmaputra rivers and compare them with the average composition of himalayan source rocks a mass balance equation of erosion fluxes shows that a si rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock it corresponds to bedload sediment and flood plain deposits which are enriched in quartz by mineral sorting during transport the combined budget of si al and fe in the river system allows us to estimate this si rich flux by this method the total himalayan erosion is estimated to be twice the measured flux of suspended load the comparison between the brahmaputra and the ganga shows that the eastern himalaya has a higher erosion rate 2 9 mm yr than the western himalaya 2 1 mm yr this is likely the result of the higher runoff in the brahmaputra basin the intensity of the monsoon acts as a first order control of the erosion rate in the range

potential effects of alpha recoil on uranium series dating of calcrete abstract evaluation of paleosol ages in the vicinity of yucca mountain nevada at the time the site of a proposed high level nuclear waste repository is important for fault displacement hazard assessment uranium series isotope data were obtained for surface and subsurface calcrete samples from trenches and boreholes in midway valley nevada adjacent to yucca mountain 230 th u ages of 33 surface samples range from 1 3 to 423 thousand years ka and the back calculated 234 u 238 u initial activity ratios ar are relatively constant with a mean value of 1 54 0 15 1 which is consistent with the closed system behavior subsurface calcrete samples are too old to be dated by the 230 th u method u pb data for post pedogenic botryoidal opal from a subsurface calcrete sample show that these subsurface calcrete samples are older than 1 65 million years ma old enough to have attained secular equilibrium had their u th systems remained closed however subsurface calcrete samples show u series disequilibrium indicating open system behavior of 238 u daughter isotopes in contrast with the surface calcrete where open system behavior is not evident data for 21 subsurface calcrete samples yielded calculable 234 u 238 u model ages ranging from 130 to 1875 ka assuming an initial ar of 1 54 0 15 the mean value calculated for the surface calcrete samples a simple model describing continuous recoil loss predicts that the 234 u 238 u and 230 th 238 u ars reach steady state values 2 ma after calcrete formation potential effects of open system behavior on 230 th u ages and initial 234 u 238 u ars for younger surface calcrete were estimated using data for old subsurface calcrete samples with the 234 u loss and assuming that the total time of water rock interaction is the only difference between these soils the difference between the conventional closed system and open system ages may exceed errors of the calculated conventional ages for samples older than 250 ka but is negligible for younger soils

the strontium isotopic budget of himalayan rivers in nepal and bangladesh abstract himalayan rivers have very unusual sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic we present sr o and c isotopic data from river and rain water bedload and bedrock samples for the western and central nepal himalaya and bangladesh including the monsoon season central himalayan rivers receive sr from several sources carbonate and clastic tethyan sediments high himalayan crystalline hhc gneisses and granitoids with minor marbles carbonates and metasediments of the lesser himalaya lh and miocene recent foreland basin sediment from the siwaliks group and the modern flood plain in the tethyan himalaya rivers have dissolved sr 6 mol l and 87sr 86sr 0 717 with a large contribution from moderately radiogenic carbonate rivers draining hhc gneisses are very dilute with sr 0 2 mol l and 87sr 86sr 0 74 lesser himalayan streams also have low sr 0 4 mol l and are highly radiogenic 87sr 86sr 0 78 highly radiogenic carbonates of the lh do not contribute significantly to the sr budget because they are sparse and have very low sr in large rivers exiting the himalaya sr systematics can be modeled as a mixture between tethyan rivers where slightly radiogenic carbonates mean 87sr 86sr 0 715 are the main source of sr and lesser himalaya waters where extremely radiogenic silicates 0 8 are the main source of sr hhc waters are less important because of their low sr rivers draining the siwaliks foreland basin sediments have sr 4 mol l and 87sr 86sr 0 725 weathering of silicates in the siwaliks and the flood plain results in a probably significant radiogenic 0 72 0 74 input to the ganges and brahmaputra g b but quantification of this flux is limited by uncertainties in the hydrologic budget the g b in bangladesh show strong seasonal variability with low sr and high 87sr 86sr during the monsoon sr in the brahmaputra ranges from 0 9 mol l and 0 722 in march to 0 3 mol l and 0 741 in august we estimate the seasonally weighted flux from the g b to be 6 5 108 mol yr with 87sr 86sr 0 7295

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

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

microbiotic crusts in the high equatorial andes and their influence on paramo soils although microbiotic crusts are known from many lowland deserts they have apparently never been reported from alpine areas this paper describes microbiotic crusts from an equatorial high andean location paramo de piedras blancas in venezuela the main objectives are to describe the basic characteristics of the crusts to examine the properties of the soils on which they occur and to discuss their possible geomorphic significance for paramo soils properties of crusts and of the soils beneath them were compared with those of adjacent bare soils at three sites between 4360 and 4550 m altitude measurements on unconfined soil compressive strength and infiltration rates were taken in the field crust specimens were also gathered and later examined and sectioned under a microscope to ascertain micromorphological details laboratory data included soil texture organic matter content color ph water storage at field capacity and stability of soil aggregates crusts formed discontinuous garland like patches parallel to contours or covered small round soil buds produced by frost sorting microbiotic crusts contained many plants including an unclassified hepatic genus marsupella dumort a moss grimmia longirostris hook and several lichen species crusts consisted of a thin organic layer 5 35 mm overlying mineral soil and showed three parallel bands with diffuse boundaries the outer layer had green reddish brown live plant shoots and stems with few fine mineral grains the middle layer had anatomically recognizable dark brown plant material and a greater content of mineral grains the light colored basal layer had little organic matter mixed with coarse mineral particles it also contained many small orthovughs vesicles and some larger arched chambers caused mainly by frost activity soils with a microbiotic crust were darker than bare soils organic matter content in crust areas was two to three times higher than in adjacent bare soil ph dropped 0 3 units below crusts soils with crust had finer texture than bare ground the content of fines in crusts was 1 5 times higher than in bare soils but gravel in these was four times higher than below cryptogams indices of particle concentration showed that grains 0 5 1 mm were more abundant in crusts finer soils in crusts may result from a combination of a prior random texture variation b cryptogam colonization of fine soil patches sorted by frost c interception and capture of aeolian dust by cryptogams especially by mosses and d reduction of fine particle erosion by crusts as coarse grains are lost mean particle size within crusts drops geomorphic effects of crusts were also significant a infiltration rates were 70 greater in crust than in noncrust soils 67 0 vs 40 4 mm min b crust soils stored more water at field capacity 48 7 88 4 than bare areas 20 5 42 c crust soils had a higher resistance to raindrop erosion than bare areas 47 5 drops 0 1 g vs 10 2 drops 0 1 g undisturbed crust specimens were even more effective in resisting drop impact surviving up to 1100 drops 0 1 g d cryptogams increased soil compressibility more than 200 from 54 g cm2 in bare soils to 118 1 g cm2 in crust areas all these geomorphic changes should result in lower rates of soil erosion in paramo areas with dense crusts

runoff generation fostered by physical and biological crusts in semi arid sandy soils abstract biological soil crusts occur extensively in semi arid regions in the western part of niger they are associated with various types of physical soil crusts in fallows and in the tiger bush ecosystem a landscape with a typical banded pattern consisting of densely vegetated bands of small trees and shrubs alternating with bare soil bands despite their widespread occurrence not much research has been done regarding their role on runoff generation in the sahelian environment the present study aims at i highlighting the relative contribution of biological soil crusts in infiltration and runoff on crusted soil in respect with other soil factors ii assessing the impact of underlying physical crusts associated with a wide range of microbial cover and iii gathering the data required for an evaluation of the potential of microbial cover in water redistribution in an arid environment and for modelling purposes the study was based on runoff measurements performed in situ at the level of ten plots 1 m 2 each under two simulated rainfalls those experiments were coupled with the visual characterization and image analysis of soil surface conditions as well as micromorphological examinations using a light microscope both on the field and at the laboratory visual estimation of crust cover was found consistent with the estimation based on the image analysis biological soil crusts capped the summit of pre existing physical soil crusts i e structural crusts formed in situ by the impact of water drop and erosion crusts resulting from water or wind erosion of structural crusts the percentage of microbial cover in the 1 m 2 plots range from 39 to 80 55 in average on structural crusts compared to 4 to 29 17 in average on erosion crusts runoff coefficient varied with both the level of microbial cover and the type of physical soil crust higher runoff coefficients were found where biological soil crusts had developed on erosion crusts as compared to structural crusts runoff coefficients recorded from erosion crusts increased with the level of microbial cover while negative correlations were found on structural crusts they show that the clogging effect of microbial cover for the plasmic layer of erosion crusts is higher than that of the sand layer of structural crusts separating biological soil crusts according to the types of the underlying physical crusts explain therefore the contradictory findings related to their role on infiltration and runoff generation those data will help to evaluate the potential role of biological soil crusts in the modelling of water redistribution in the arid environment of the western part of niger

10be derived himalayan denudation rates and sediment budgets in the ganga basin abstract the himalayas represent the archetype of mountain building due to active continental collision and are considered in many studies as the locus of intense interactions between climate denudation and tectonics estimates of modern denudation rates across the entire range remain however relatively sparse in this study in situ produced cosmogenic 10 be concentrations were measured in detritic quartz in order to determine basin scale denudation rates for the central part of the himalayan range river sand was sampled over several years in the main trans himalayan rivers from the himalayan front to the ganga outlet in bangladesh the calculated 10 be denudation rates of the trans himalayan river basins range from 0 5 to 2 4 mm yr 1 average 1 3 mm yr 1 and vary by up to a factor of 3 between sampling years these denudation rates strongly contrast with the 0 007 mm yr 1 denudation rate of southern tributary basins draining the indian craton this work also shows that in the ganga basin no systematic evolution of average 10 be concentrations is observed during floodplain transfer implying that distal samples can be used to estimate the integrated denudation rate of the whole central himalayan range samples from the ganga in bangladesh display remarkably low variability in 10 be concentration implying an average himalayan denudation rate of 1 0 1 1 mm yr 1 however within the floodplain several samples suggest a recent perturbation of sediment transport dynamics with a recent increase in the relative sediment contribution from southern tributaries the himalayan sediment flux deduced from the 10 be denudation rate of the range is 610 230 mt yr 1 this flux is consistent within uncertainty with sediment fluxes derived from sediment gauging the similarity of the two flux estimates suggests that himalayan erosion fluxes have remained stable over the last centuries even if the large uncertainties associated with each method hamper more precise assessments

recycling of amazon floodplain sediment quantified by cosmogenic 26al and 10be the modern amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1 3 m y in floodplains by reworking of floodplain deposits this sediment is being incorporated into amazonian streams we conclude this from using the ratio of in situ produced cosmogenic 26 al to 10 be in detrital sediment from rivers draining the sediment producing andean guyana and brazilian shield areas as well as the largest amazonian lowland rivers samples from the modern amazon river yield 26 al 10 be ratios between 3 8 and 5 5 and thus represent mixing between a non buried sediment end member and a formerly buried sediment end member the non buried end member is sourced in the andes with a ratio of 6 5 reflecting recent erosion of hillslopes exposed to cosmic rays and the buried end member is potentially represented by deeply stored floodplain sediment as old as miocene age with a ratio of 3 that was partially shielded from cosmic rays this simple picture of binary mixing is complicated by grain size effects coarser grained sediment from non andean settings lowlands cratonic shields contains slightly more formerly buried material whereas finer grained lowland sediment mostly records non buried 26 al 10 be ratios and nuclide concentrations that are indicative of an andean source our assessment shows an incorporation of formerly buried old sediment of central amazonian floodplains to the fresh andean derived sediment mainly carried in the amazon trunk stream we interpret our results to mean that the amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current holocene channel

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

uranium series disequilibrium in river sediments and waters the significance of anomalous activity ratios abstract samples of river water suspended and bottom sediments from the fenland catchment system draining into the wash u k have been analysed for the longer lived u th series radionuclides 238u 234u 230th and 232th the aim of this work was to assess the effects of open system phase transfer on the u series disequilibria observed in the river waters and sediments and to investigate the controls exerted on this phase transfer by the hydrological regime two important aspects of the u series radionuclide mobilisation in this modern river system have been observed 1 u series radionuclides are in a continuous state of flux between river sediments and river water and 2 u sorption on sediment particles is seasonably variable sequential extraction of whole sediments revealed that the u series isotopic characteristics of the ion exchangeable organic bound carbonate amorphous fe mn oxyhydroxides and crystalline fe mn oxides are controlled to a significant degree by the acquisition of u isotopes from the aqueous phase during summer when surface run off and river discharges are generally low the u isotope acquisition by sediment particles results in anomalously high234u 238u activity ratios 1 this is of importance for studies which rely upon the isotopic and elemental separation of u series radionuclides in the weathering system i e mass balance weathering rate and sediment yield assessments

evaluating the removal of non detrital matter from soils and sediment using uranium isotopes this study was supported by an arc discovery project grant dp1093708 and arc future fellowship ft0990447 to ad and a university of wollongong postgraduate scholarship award to anm

u series disequilibria in minerals from gandak river sediments himalaya abstract the impact of mineralogical sorting on u th ra variations observed for the ganga sediments in the himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the u series isotopic system in himalayan minerals for this purpose u th and ra concentrations along with 238u 234u 230th 226ra radioactive disequilibria were measured in several pure mineral fractions i e monazite zircon titanite ilmenite rutile garnet magnetite quartz biotite and muscovite separated from one bank sediment of the gandak river ganga basin the data obtained through a double digestion protocol in high pressure and high temperature vessels confirm that u and th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in u and th namely monazite titanite and epidote as well as major minerals with lower u and th concentrations such as quartz more importantly the data clearly indicate that similarly to whole rock samples the u th ra systematics of individual minerals have likely recorded complex u th ra fractionations in particular the 226ra enrichment observed in the biotite and clay fractions likely results from the affinity of ra for phyllosilicate minerals while the 230th and to a lesser extent the 234u enrichment observed for the other minerals might result from a nuclide adsorption process on the fe oxide coatings present on the grains or directly on the mineral surface the linear trend outlined by the minerals phases in the 226ra 230th vs 230th 234u diagram suggests that 230th is probably mobilized from zircons the only fraction presenting 230th 234u

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

geochemical and isotopic sr u monitoring of soil solutions from the strengbach catchment vosges mountains france evidence for recent weathering evolution abstract this study presents the results of a geochemical and isotope u and sr study of soil solutions from two experimental sites in the strengbach catchment one under spruce trees and the other under beech trees the soil solution data combined with the data from soil and vegetation samples indicate that the dissolved u filtered to 0 45 m in the soil solutions is primarily of lithogenic origin implying that the colloidal u is secondarily associated with organic matter or organo metallic complexes present in the soil solutions the uranium activity ratios and sr isotope ratios of soil solutions are used to decipher the nature of the minerals that control the lithogenic contribution to the soil solutions a comparison of soil solution data collected over the period 2004 2006 with those collected over the period 1993 2010 indicates a decrease in the ca and k exported by the soil solutions between 1998 and 2004 at the spruce site and a significantly smaller decrease at the beech site in addition the sr isotope ratios of the soil solutions vary significantly between these two periods at the spruce site but not or much less at the beech site such observations suggest that the forest ecosystem at the spruce site is in a transient state associated with recent changes in weathering reactions the data also suggest that the comparison of u and sr isotopes in soil solutions and plant samples could be a relevant method for determining whether a forest ecosystem is in a steady or transient state

the relationship between riverine u series disequilibria and erosion rates in a basaltic terrain u series isotopes have been measured in the dissolved phase suspended load and bedload of the main rivers draining basaltic catchments in iceland for the dissolved phase 234u 238u and 238u 230th range between 1 08 and 2 2 and 7 4 and 516 respectively for the suspended load and bedload 234u 238u and 238u 230th range from 0 97 to 1 09 and from 0 93 to 1 05 respectively chemical erosion rates calculated from dissolved major elements range between 13 and 333 t km 2 yr 1 physical erosion rates have also been estimated from existing data and range between 21 and 4864 t km2 yr with an average of 519 t km 2 yr 1 u series disequilibria indicate that weathering in iceland operates at close to steady state conditions a model of continuous weathering indicates a maximum weathering timescale of 10 kyr with an average rate of uranium release into water of 1 6 10 4 yr 1 which is significant when compared to granitic terrains located at similar latitudes and to tropical basaltic terrains all river waters display 234u 238u greater than secular equilibrium consistent with the effects of alpha recoil the same dissolved phase 234u 238u exhibit a negative trend with physical erosion rates explained by the dominant effect of close to congruent chemical weathering of hyaloclastites in the younger basaltic terrains therefore chemical erosion rate and mineral weathering susceptibility play a major role in determining 234u 238u disequilibria in basaltic river waters comparison of global data for river basins in which weathering was recently strongly limited indicates a negative correlation between silicate weathering rates estimated with major elements and the age of weathering estimated with u series disequilibria this strongly suggests a key role of time and soil thickness on the chemical erosion of silicates

a rouse based method to integrate the chemical composition of river sediments application to the ganga basin 1 the ganga river is one of the main conveyors of sediments produced by himalayan erosion determining the flux of elements transported through the system is essential to understand the dynamics of the basin this is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions using acoustic doppler current profiler adcp acquisitions with sediment depth profile sampling of the ganga in bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments the model shows that ca 390 30 mt of sediments are transported on average each year through the ganga at haring bridge bangladesh modeled average sediment grain size parameters d 50 and d 84 are 27 4 and 123 9 mm respectively grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation the integrated sediment flux is characterized by low al si and fe si ratios that are close to those inferred for the himalayan crust this implies that only limited sequestration occurs in the gangetic floodplain the stored sediment flux is estimated to c a 10 of the initial himalayan sediment flux by geochemical mass balance the associated globally averaged sedimentation rates in the floodplain are found to be ca 0 08 mm yr and yield average himalayan erosion rate of ca 0 9 mm yr this study stresses the need to carefully address the average composition of river sediments before solving large scale geochemical budgets citation lupker m c france lanord j lave j bouchez v galy f metivier j gaillardet b lartiges and j l mugnier 2011 a rouse based method to integrate the chemical composition of river sediments application to the ganga basin

morphology and microstructure of microbiotic soil crusts on a tiger bush sequence niger sahel abstract microbiotic crusts resulting from the colonization of soil surfaces by communities of micro organisms dominated by cyanobacteria eucaryotic algae or lichens are widespread in arid and semi arid regions in the tiger bush of niger sahel microbiotic crusts occur in bare bands between densely vegetated bands the tiger bush soils are sandy loams with ph 4 7 6 5 three different microbiotic crusts are recognised forming strips parallel to the vegetation banding they are all formed mainly by filamentous cyanobacteria dominated by schizothrix other important genera are scytonema lyngbya microcoleus nostoc and phormidium the filamentous micro organisms trap sand particles and finer particles stick on the filament surfaces these effects enhance soil cohesion and resistance to erosion soil porosity is also increased by the microbial cover with additional pores delineated by filaments on the surfaces of crusts and porous organic bodies derived from microbial remains at depth the microbiotic crusts are essential components of the tiger bush ecosystem water leaving the lower sides of vegetated bands results in plant decay and on the upslope sides vegetation grows because of increased water supply microbial covers help resist erosion and increase water and nutrient retention thus providing substrates for the expansion of plants from adjacent vegetated bands

comparative behavior of sr nd and hf isotopic systems during fluid related deformation at middle crust levels abstract we have carried out a comparative rb sr sm nd and lu hf isotopic study of a progressively deformed hercynian leucogranite from the french massif central belonging to the la marche ductile shear zone in order to investigate the respective perturbation of these geochronometers with fluid induced deformation the one meter wide outcrop presents a strongly deformed and mylonitized zone at the center and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone ten samples have been carefully collected every 10 cm north and south away from the strongest deformed mylonitic zone they have been analyzed for a complete major trace element data set oxygen isotopes rb sr sm nd and lu hf isotopic systematics we show that most of major and trace elements except sio2 alkaline elements k2o rb and some metal transition elements cu are progressively depleted with increasing deformation this depletion includes ree y but also hfs elements ti hf zr nb which are commonly considered as immobile elements during upper level processes variations in elemental ratios with deformation e g decrease in lree mree hree nd hf th sr increase in rb sr u th and constant sr nd lead to propose the following order of element mobility u th sr nd hf hree we conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing f rich hydrothermal fluids associated with deformation a temperature of the fluid of 470 480 c can be deduced from the 18o equilibrium between quartz muscovite pairs elemental fractionation induces perturbation of the rb sr geochronometer the well defined 87rb 86sr 87sr 86sr correlation gives an apparent age of 294 19 ma slightly younger than the 323 4 ma age of leucogranites in this area this apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer and exhumation of the ductile crust shortly after the leucogranite emplacement sm nd and lu hf isochron type diagrams do not define any correlation because of the low fractionated sm nd and lu hf ratios isotopic data demonstrate that only the lu hf geochronometer system is not affected by fluid circulation and gives reliable tdm age 1 29 0 03 ga and ehf signatures by contrast the sm nd geochronometer system gives erroneous old tdm ages of 2 84 4 ga there is no positive end ehf correlation because of decreasing end values with deformation at constant ehf values however end ehf values remain in the broad end ehf terrestrial array which strongly indicates that fluid induced fractionation can contribute to the width of the terrestrial array the strong ehf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the french massif central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust

mobility of u series nuclides during basalt weathering an example from the deccan traps india abstract u series disequilibria have been measured by tims and mc icp ms in dissolved phases and suspended sediments of the main basaltic rivers from the deccan trap region india for dissolved phases 234u 238u 230th 238u and 226ra 238u range between 1 11 and 1 28 between 0 03 10 2 and 1 35 10 2 and between 0 02 and 0 16 respectively for suspended particles 234u 238u 230th 238u and 226ra 238u range from 0 95 to 1 1 from 1 23 to 1 59 and from 0 1 to 0 47 respectively 230th 238u and 234u 238u systems show direct links with indices of weathering the data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady state because present day physical erosion rates are higher than expected 100 400 mm ka versus 40 mm ka for a steady state process for the narmada and the tapti basins models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40 90 ka in contrast with basins located in northern latitudes erosion in the deccan basins does not appear to have been significantly disturbed during the last glaciation 234u 238u disequilibria in the dissolved phase of large rivers world wide are also explored and two key factors are highlighted in this study the dissolution of carbonates releasing dissolved u at secular equilibrium and the silicate weathering rate estimated at the scale of the watershed in particular a positive correlation is found between dissolved 234u 238u and basaltic weathering rates for the deccan that could reflect a key role of physical erosion since for silicate monolithological basins chemical erosion rates are primarily controlled by physical erosion rates for basins with mixed lithology dissolved organic matter also plays a key role in the preferential release of 234u from silicate minerals 234u 238u ratios in rivers therefore reflect a mixture between u released by carbonate dissolution and 234u preferentially released from silicate minerals located in high physical erosion and peat areas

adsorption of rees on kaolinite and halloysite a link to the ree distribution on clays in the weathering crust of granite abstract the clay minerals kaolinite and halloysite are dominant hosts for rare earth elements rees in the weathered crust elution deposited ree deposits however the accumulation of rees on kaolinite and halloysite has not been systematically compared in this study the adsorption of complete ree series on kaolinite and halloysite was investigated under different ph levels and ionic strengths which was linked to the enrichment and fractionation of rees within the clay fraction of a typical weathered crust elution deposited ree deposit at low ionic strength the adsorption of rees on kaolinite and halloysite increased with increase in ph with some noticeable fluctuation observed on halloysite at high ph all the rees were adsorbed to a similar extent without apparent fractionation except for the slight enrichment of heavy rees hrees at high ph at high ionic strength ree adsorption exhibited a linear increase with the increase in ph particularly at high ph with hrees being preferentially adsorbed compared to halloysite kaolinite possessed a higher specific surface area ssa normalized adsorption capacity towards rees these observed adsorption characteristics could possibly explain the distribution of rees in the clay fraction along the ree deposit the decrease of ion exchangeable ree content with depth was significant for kaolinite while the ree fractionation was ascribed to the selective adsorption of hrees on both kaolinite and halloysite the enrichment and fractionation mechanism of rees on kaolinite and halloysite were also discussed in terms of the surface chemistry and morphology of the clay minerals and the variations of chemical properties across the ree group

u th ra variations in himalayan river sediments gandak river india weathering fractionation and or grain size sorting abstract understanding the origin of u th ra variations in the ganga river sediments is a prerequisite for correctly using u series nuclides to constrain the sediment transport times in himalayan rivers for this purpose u th and ra concentrations along with 238 u 234 u 230 th 226 ra radioactive disequilibria were analyzed in bank bedload and suspended sediments from the gandak river one of the main tributaries of the ganga river the data confirm that u and th budgets of the himalayan sediments are significantly influenced by minor resistant minerals such as zircon garnet and ti bearing minerals the dissolution of which required the use of a high pressure acid digestion process most importantly the results indicate that the variations in 238 u 232 th and 230 th 232 th activity ratios and 238 u 234 u 230 th 226 ra disequilibria in sediments along the river alluvial plain mainly reflect modifications in the mineralogical and grain size compositions rather than the degree of weathering during transport the 238 u 232 th and 230 th 232 th activity ratios in the bank and bed sediments are related to variations in the minor primary minerals strongly enriched in u and th i e zircon ree bearing minerals and ti bearing minerals whereas the activity ratios in the suspended load are related to variations in the proportions of clay fe oxyhydroxides and the silt sand fraction which contains u and th bearing minor minerals the data also indicate that 238 u 234 u 230 th 226 ra disequilibria are strongly influenced by secondary mineral phases the 230 th budget is likely mainly controlled by fe oxyhydroxides and the 226 ra budget is likely mainly controlled by clay minerals therefore the variations in the 238 u 234 u 230 th 232 th system in the sediments of the gandak river cannot simply be interpreted as the result of fractionation due to chemical transformation of the bulk sediment during its transport within the alluvial plain and or the result of radioactive decay consequently they cannot be used to infer long sediment transport times within the gandak plain 10 100 ka as previously proposed such analytical and interpretative artifacts are certainly not specific to the present study on the gandak basin these issues will certainly be encountered anytime this technique is applied to alluvial systems in which the u and th budgets of the sediments are influenced by heavy minerals that can be sorted during the transport of sediments within the plain

properties and spatial distribution of microbiotic crusts in the negev desert israel abstract playing a cardinal role in surface stabilization and in carbon and nitrogen fixation microbiotic crusts play a crucial role in arid regions where they may serve as useful biomarkers for wind power and wetness duration this is especially the case on relatively unstable and infertile sand dunes in the negev desert where high correlations between the crust chlorophyll content and the daytime wetness duration were found yet only scarce data are available as to the possible link between the chlorophyll content and other physical color thickness strength crack density surface roughness and infiltrability and biological protein carbohydrate organic matter and species composition factors which determine in turn the crust type and its effect upon geomorphological and ecological processes no data are available on crust type distribution these were the aims of the current research when a cluster analysis was performed five types of microbiotic crusts were defined four of which were cyanobacterial a d and one moss dominated crust e the crusts differed in their physical and biological properties they showed an increase in chlorophyll content protein carbohydrates and organic matter from a to e with concomitant increase in species diversity thickness roughness and strength but with some variables crack density and infiltrability showing a reversed trend at the moss dominated crust the increase in the biomass components of the crust and the gradual change of the physical properties are explained by the improved physical conditions primarily wetness duration which facilitates longer hours of photosynthetic activity and consequently the introduction of additional more mesic species such as green algae lichens and mosses extended wetness duration was found to shift the crust type from cyanobacterial to moss dominated crust the spatial distribution of the crusts as verified by crust mapping coincided with the daylight surface duration which in turn was controlled by topography aspect angle and slope position it implies that whereas initial physical conditions dictates species composition and thus crust type the crust type in turn is responsible for characterizing the physical properties of the surface which may largely affect ecological and geomorphological processes

effects of microbiotic crusts under cropland in temperate environments on soil erodibility during concentrated flow several studies illustrate the wind and water erosion reducing potential of semi permanent microbiotic soil crusts in arid and semi arid desert environments in contrast little is hitherto known on these biological crusts on cropland soils in temperate environments where they are annually destroyed by tillage and quickly regenerate thereafter this study attempts to fill the research gap through a a field survey assessing the occurrence of biological soil crusts on loess derived soils in central belgium in space and time and b laboratory flume 2 m long experiments simulating concentrated runoff on undisturbed topsoil samples 0 4 0 1 m2 quantifying the microbiotic crust effect on soil erosion rates three stages of microbiotic crust development on cropland soils are distinguished 1 development of a non biological surface seal by raindrop impact 2 colonization of the soil by algae and gradual development of a continuous algal mat and 3 establishment of a well developed microbiotic crust with moss plants as the dominant life form as the silt loam soils in the study area seal quickly after tillage microbiotic soil crusts are more or less present during a large part of the year under maize sugar beet and wheat representing the main cropland area on average the early successional algae dominated crusts of stage 2 reduce soil detachment rates by 37 whereas the well developed moss mat of stage 3 causes an average reduction of 79 relative soil detachment rates of soil surfaces with microbiotic crusts compared with bare sealed soil surfaces are shown to decrease exponentially with increasing microbiotic cover b 0 024 for moss dominated and b 0 006 for algae dominated crusts in addition to ground surface cover by vegetation and crop residues microbiotic crust occurrence can therefore not be neglected when modelling small scale spatial and temporal variations in soil loss by concentrated flow erosion on cropland soils in temperate environments copyright 2007 john wiley sons ltd

trace element and nd isotopic evidence for sediment sources in the mid proterozoic vindhyan basin central india abstract ancient sediments reflect the compositions of old eroded continental crustal material and preserve geochemical signatures of paleo tectonic and volcanic events geochemical study of such sediments provides insight into tectonic events that shaped the face of the earth nd isotopes are best suited for such studies because rare earth elements ree specifically sm and nd are not fractionated during post depositional processes such as erosion transportation and diagenesis of organic poor sediments and hence reflect the source composition of these sediments sedimentation in the proterozoic vindhyan basin of central india with a 4 km thick succession of unmetamorphosed and mostly undeformed sediments lasted over 1000 ma from 1630 ma up to the precambrian cambrian boundary these sediments provide an ideal opportunity to study the evolution of ancient continental crust as well as the tectonic evolution of peninsular india the occurrence of porcellanites in the lower part of the basin indicates active volcanism during deposition of the vindhyan sediments these tuffs show large ion lithophile element lile enrichment as well as depletions in nb and ta in primitive mantle normalized plots characteristic of an arc volcanic source in combined trace element ratio plots these volcanic rocks indicate mixing between mantle derived melts and the continental crust and overlap with the composition of global arcs their initial nd at 1630 ma ranging from 5 7 to 0 9 is similar to those observed in andean type continental margin arcs trace element concentration patterns as well as nd isotopic compositions of the clastic and non clastic sediments of the vindhyan basin from one of the type localities of the vindhyan stratigraphy in the son valley are vastly different from those of the bundelkhand granite which is the basement for these sediments indicating that these granitic rocks were not a major source of sediments for the vindhyan basin our nd isotopic study clearly indicates that sedimentation in the vindhyan basin which lasted over 1 ga was not continuous the boundaries between the semri and kaimur groups as well as the rewa and bhander groups are characterized by abrupt changes in the nd isotopic compositions indicating changes in provenance with sediments in the kaimur and bhander groups being derived from more juvenile sources compared to the semri and rewa groups respectively this inference is also supported by changes in t dm and f sm nd for the sediments across these boundaries the strikingly different trace element patterns of the rewa group sediments compared to the underlying kaimur sediments indicates that sediment sources also changed across the kaimur rewa boundary multiple changes in provenance in the vindhyan sediments indicate that this basin was tectonically active during the entire span of sedimentation while there is some uncertainty in determining the exact sources of sediments for the vindhyan basin we argue on the basis of our geochemical in particular nd isotopic ratios data and available paleocurrent data that majority of the sediments were derived from a now extinct andean type arc source in the south that resulted from a southerly dipping subduction prior to the collision of the bhandara and bundelkhand cratons according to this reconstruction the vindhyan basin was initiated as a foreland basin after the collision of these two cratons

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

soils of a dune coastal salt marsh system in relation to groundwater level micro topography and vegetation under a semiarid mediterranean climate in se spain abstract twenty one soil profiles were described analysed and classified in a salt marsh dune system in se spain sites for sampling were selected according to micro topographical positions and vegetation types the pedons showed an a c type profile most of the profiles at summit positions had a microbiotical crust in the upper centimetres formed by fungal mycelia that contribute to agglutination of sand particles in two soils a buried ab horizon could be identified soils at inter dune depressions showed redoximorphic features with diffuse boundaries therefore indicating the actual activity of the oxidation reduction processes the presence of remains of posidonia oceanica buried in many of the profiles indicates both a progressive regression of the shoreline and the accretion of the dune system sand grains were the most abundant size fraction generally higher than 65 salinity strongly increased at the depressions with cl so42 na and mg2 being the most abundant ions the microtopographical position at the dune had more influence on the vegetation cover and the soil type than did the distance to the shoreline most of the profiles at summit sites were colonised by trees pinus halepensis and juniperus phoenicea subsp turbinata shrubs pistacea lentiscus rhamnus lycioides thyme brushwoods crucianella maritima teucrium dunense or grasses ammophila arenaria and they were classified as arenosols or torripsammens the slopes of the depressions were dominated by schoenus nigricans living on gleysols or halaquepts and aquisalids or limonium cossonianum living on solonchaks or aquisalids juncus maritimus occupied footslope or bottom positions of the depressions growing on solonchaks or aquisalids and gleysols or halaquepts the succulent halophytes arthrocnemum macrostachyum and sarcocornia fruticosa were typical species at bottom positions living on solonchaks and fluvisols or aquisalids in two of the profiles at interdune depressions a petrocalcic horizon appeared

cyanobacterial diversity and toxicity of biocrusts from the caspian lowland loess deposits north iran abstract biocrusts and adjacent sediments were collected from the so called loess plateau of northern iran and the loess deposits on the foothills of alborz mountains the mineralogical and granulometric analyses characterized sediments as quartz rich clayey or sandy silts cyanobacterial diversity colony morphology and grain stabilization were studied by light and scanning electron microscopy which showed glue like layers of cyanobacterial eps and dense networks of filamentous cyanobacteria immobilizing finer and bigger grains respectively amorphous tuft and globular cyanobacterial colony structures were also detected 15 cyanobacterial genera were identified in the biocrust samples including microcoleus vaginatus as the common species in all samples aphanocapsa aphanothece calothrix chroococcus chroococcidiopsis cyanosarcina hassallia homeothrix nostoc oculatella schizothrix scytonema tolypothrix and trichocoleus two biocrust samples showed traces of toxicity in a protein phosphatase inhibition assay targeting microcystins and similarly acting compounds the artemia salina assay revealed an elevated toxic response in one sample

sediment residence times constrained by uranium series isotopes a critical appraisal of the comminution approach abstract quantifying the rates of landscape evolution in response to climate change is inhibited by the difficulty of dating the formation of continental detrital sediments we present uranium isotope data for cooper creek palaeochannel sediments from the lake eyre basin in semi arid south australia in order to attempt to determine the formation ages and hence residence times of the sediments to calculate the amount of recoil loss of 234u a key input parameter used in the comminution approach we use two suggested methods weighted geometric and surface area measurement with an incorporated fractal correction and typical assumed input parameter values found in the literature the calculated recoil loss factors and comminution ages are highly dependent on the method of recoil loss factor determination used and the chosen assumptions to appraise the ramifications of the assumptions inherent in the comminution age approach and determine individual and combined comminution age uncertainties associated to each variable monte carlo simulations were conducted for a synthetic sediment sample using a reasonable associated uncertainty for each input factor and including variations in the source rock and measured 234u 238u ratios the total combined uncertainty on comminution age in our simulation for both methods of recoil loss factor estimation can amount to 220 280 ka the modelling shows that small changes in assumed input values translate into large effects on absolute comminution age to improve the accuracy of the technique and provide meaningful absolute comminution ages much tighter constraints are required on the assumptions for input factors such as the fraction of recoil lost 234th and the initial 234u 238u ratio of the source material in order to be able to directly compare calculated comminution ages produced by different research groups the standardisation of pre treatment procedures recoil loss factor estimation and assumed input parameter values is required we suggest a set of input parameter values for such a purpose additional considerations for calculating comminution ages of sediments deposited within large semi arid drainage basins are discussed

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

climatic and vegetation control on sediment dynamics during the last glacial cycle as climate is changing rapidly there is an increasing need to understand how water and soil resources respond to climate change soil and sediment dynamics are sensitive to several external factors such as climate vegetation type and distribution human activity and tectonic activity however the relationship between erosion and changes in these factors is difficult to constrain with current available approaches here we show that uranium isotopes in sediments from river paleochannels can be used to reconstruct variations in the residence time of sediments in a catchment over the past 100 k y we find that sediment residence times increase by an order of magnitude during interglacials compared to glacial periods this is interpreted as a change in sediment stores in the landscape that are tapped by catchment erosion young upland soils during glacial periods reworking of old alluvial sediments during interglacials a direct correlation is found between the sediment residence time and climatic parameters sea surface temperature atmospheric carbon dioxide content and paleorainfall estimates suggesting that during a glacial cycle sediment dynamics closely follow variations in climate however this relationship is not simple because there is no correlation between sediment residence time and paleodischarge estimates because sediment residence time variations correlate with changes in vegetation inferred from pollen data it is hypothesized that the influence of climate on erosion over a glacial cycle may be indirect and operates via the influence of climate on the type of plant ecosystems within a catchment if verified elsewhere this conclusion would emphasize the important role of biology in the physical evolution of earth s surface here observed over a 100 k y time scale

biological soil crust development and its topsoil properties in the process of dune stabilization inner mongolia china this study was undertaken at horqin sand land inner mongolia northern china field samples of biological soil crusts bscs and underlying topsoil 0 5 cm under bsc were taken in areas of different dune stabilization stages and their physicochemical properties were analyzed including particle size distribution bulk density organic matter nitrogen phosphorus electrical conductivity ec ph and caco3 content the results revealed that semi mobile dunes semi fixed dunes and fixed dunes had developed a physical crust algae crust and moss crust respectively the thickness hardness water content fine fraction and nutrient contents of bscs were gradually increasing along the dune stabilization gradient meanwhile bsc establishment and development enhanced the bulk density silt and clay content and nutrients of the topsoil under it in an increasing tend from semi mobile dune to fixed dune organic matter concentrations and other nutrients in the 0 5 cm topsoil layer under bscs were significantly higher compared to unconsolidated soil control moreover there were strong significant positive correlations between topsoil and bscs organic matter total nitrogen available nitrogen available phosphorus caco3 and 0 05 mm particle content suggesting that bscs have an influence on some of the properties of the underlying topsoil

the evolution of the vindhyan basin vis a vis the narmada son lineament central india from deep seismic soundings abstract the evolution of the vindhyan basin has been studied from deep seismic soundings block tectonics appear to be active throughout the geological history in this region the crust is divided into four major blocks by deep faults at narsinghgarh katangi and jabalpur the crustal block between narsinghgarh and katangi forms a graben in the crystalline basement giving rise to the great vindhyan basin in which precambrian vindhyan sediments of about 5 5 km thickness are deposited the crustal block between katangi and jabalpur is a horst block which represents the narmada son lineament a crustal depth section obtained from wide angle reflections shows a large number of reflector segments up to the mono boundary indicating crustal heterogeneity the boundary faults at the northern and the southern ends of the vindhyan basin extend up to the moho the crustal thickness varies from 39 5 to 45 km

the microstructure of microbiotic crust and its influence on wind erosion for a sandy soil surface in the gurbantunggut desert of northwestern china abstract microscopic examination of microbiotic soil crust shows that the profiles of soil with a dense cyanobacterial cover had two different layers a surface thin layer composed of aeolian born materials and an organic layer formed by filamentous cyanobacteria associated with sand particles the results indicate that microbiotic cover is an important determinant of sand fixation in the gurbantunggut desert northern part of xinjiang northwestern china microscopic examination of microbiotic crusts in this study revealed an intricate network of filamentous cyanobacteria and exopolysaccharides which binds and entraps sand grains and conglutinate fine particles with each other resistance to wind erosion paralleled the different disturbance levels on microbiotic soil crust sandy soil surface disturbances resulted in greatly decreased soil resistance to wind erosion maximum wind tunnel velocity in this test 25 m s 1 did not lead to any wind erosion on the surface of undisturbed microbiotic soil crust i e 100 covered by microbiotic soil crust as for different disturbance levels the highest threshold friction velocity was seen in the sand surface with 10 disturbance of microbiotic crust the surface microbiotic soil crusts have great effects on wind erosion rates wind erosion rates for sandy soil with 0 crust cover was about 46 21 17 times the soil with 90 crust cover at wind velocities of 18 22 25 m s 1 respectively this study confirms that the planners and managers of nature reserves in this area should understand the important ecological roles of microbiotic crust in desert ecosystems the reduction of trampling on the soil will eventually result in the re establishment of biological crusts and their associated organisms and ultimately lead to lower levels of wind erosion additionally strategies should be developed to manage livestock and oil exploration in order to avoid concentrated zones of impact

erosion timescales derived from u decay series measurements in rivers the relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades the principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates in this study 234u 238u 230th 238u and 226ra 238u were measured by tims and by mc icp ms in the dissolved and suspended loads of rivers from the mackenzie basin northwest territories canada the data show a complementary nature between 234u 238u 230th 238u in the dissolved and suspended loads while 226ra is characterized by a more complex behavior modeling of fractionation of u series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed 9 28 10 ka as well as the rates of release of u series nuclides the results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the mackenzie river

introduction to u series geochemistry during the last century the earth sciences underwent two major revolutions in understanding the first was the recognition of the great antiquity of the earth and the second was the development of plate tectonic theory these leaps in knowledge moved geology from its largely descriptive origins and established the modern quantitative earth sciences for any science and particularly for the earth sciences time scales are of central importance until recently however the study of time scales in the earth sciences was largely restricted to the unraveling of the ancient history of our planet for several decades earth scientists have used a variety of isotope chronometers to unravel the long term evolution of the planet a fuller understanding of the physical and chemical processes driving this evolution often remained elusive because such processes occur on time scales 1 105 years which are simply not resolvable by most conventional chronometers the u series isotopes however do provide tools with sufficient time resolution to study these earth processes during the last decade the earth sciences have become increasingly focused on fundamental processes and u series geochemistry has witnessed a renaissance with widespread application in disciplines as diverse as modern oceanography and igneous petrology the uranium and thorium decay series contain radioactive isotopes of many elements in particular u th pa ra and rn the varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments while the varied half lives of the nuclides allows investigation of processes occurring on time scales from days to 105 years u series measurements have therefore revolutionized the earth sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the earth the application of u series geochemistry to the earth sciences was thoroughly summarized in 1982

assessing the effect of sequential extraction on the uranium series isotopic composition of a basaltic weathering profile abstract soil sustainability implies maintaining the balance between soil erosion and production while it is known how to assess soil erosion only recently we have been able to estimate rates of soil and saprolite namely regolith production using uranium series isotopes this method assesses the time elapsed since rock forming minerals start fractionating the u series isotopes in this study we assess a sample pre treatment protocol that has the potential to improve the method used to estimate regolith production rates we propose that removal or partial removal of secondary phases precipitated from solution during pedogenesis solution derived phases and organic matter from regolith may improve the accuracy of this method this is tested using sequential extraction followed by etching as sample pre treatment here we assess their effect on the u series isotopic composition of regolith and infer whether they minimize the presence of solution derived phases and organic matter we applied sequential extraction and etching to a basaltic weathering profile bedrock saprolite and soil and compared the u series isotopic composition before and after treatment we also measured major elements concentrations and assessed mineralogy the bedrock was in secular equilibrium and sequential extraction resulted in unchanged 234u 238u activity ratios while increased 230th 238u in contrast etching resulted in increased 234u 238u and 230th 238u activity ratios which is attributed to the removal of primary minerals relative to the untreated bedrock the untreated saprolite showed no changes in u and th concentrations and activity ratios we hypothesise that during the conversion of bedrock into saprolite u and th budgets are unaffected moreover major element and mineralogical analyses suggest that during this process rock forming minerals are converted into secondary phases clays we hypothesise that during this conversion the u series isotopes are not fractionated therefore the removal of these secondary phases is not necessary relative to the saprolite the soil showed gains of u and th 234u 238u 1 and 230th 238u 1 to minimize the presence of solution derived phases and organic matter in basaltic weathering profiles we suggest that only soil samples should undergo sequential extraction because only these are significantly affected by solution derived phases and organic matter additionally our experiments show the existence of fractionation processes that are often overlooked in u series isotopic studies i e implantation of 234u and 230th recoiled from u rich mineral i e glass into adjacent u poor phases e g pyroxene and feldspar

uranium series comminution ages of continental sediments case study of a pleistocene alluvial fan obtaining quantitative information about the timescales associated with sediment transport storage and deposition in continental settings is important but challenging the uranium series comminution age method potentially provides a universal approach for direct dating of quaternary detrital sediments and can also provide estimates of the sediment transport and storage timescales the word comminution means to reduce to powder reflecting the start of the comminution age clock as reduction of lithic parent material below a critical grain size threshold of 50 mu m to test the comminution age method as a means to date continental sediments we applied the method to drill core samples of the glacially derived kings river fan alluvial deposits in central california sediments from the 45 m core have independently estimated depositional ages of up to 800 ka based on paleomagnetism and correlations to nearby dated sediments we characterized sequentially leached core samples both bulk sediment and grain size separates for u nd and sr isotopes grain size surface texture and mineralogy in accordance with the comminution age model where 234u is partially lost from small sediment grains due to alpha recoil we found that 234u 238u activity ratios generally decrease with age depth and specific surface area with depletions of up to 9percent relativemore to radioactive equilibrium the resulting calculated comminution ages are reasonable although they do not exactly match age estimates from previous studies and also depend on assumptions about 234u loss rates the results indicate that the method may be a significant addition to the sparse set of available tools for dating detrital continental sediments following further refinement improving the accuracy of the method requires more advanced models or measurements for both the recoil loss factor fa and weathering effects we discuss several independent methods for obtaining fa on individual samples that may be useful for future studies less

predominant floodplain over mountain weathering of himalayan sediments ganga basin we present an extensive river sediment dataset covering the ganga basin from the himalayan front downstream to the ganga mainstream in bangladesh these sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column mineral sorting is the first order control on the chemical composition of river sediments taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain by comparing sediments sampled at the himalayan front with sediments from the ganga mainstream in bangladesh it is possible to budget weathering in the floodplain assuming a steady state weathering regime in the floodplain the weathering of himalayan sediments in the gangetic floodplain releases ca 189 92 10 9 and 69 22 10 9 mol yr of carbonate bound ca and mg to the dissolved load respectively silicate weathering releases 53 18 10 9 and 42 13 10 9 mol yr of na and k while the release of silicate mg and ca is substantially lower between ca 0 and 20 10 9 mol yr additionally we show that sediment hydration h2o is a sensitive tracer of silicate weathering that can be used in continental detrital environments such as the ganga basin both h2o content and the d h isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions by comparing the chemical composition of river sediments across the floodplain with the composition of the eroded himalayan source rocks we suggest that the floodplain is the dominant location of silicate weathering for na k and h2o overall this work emphasizes the role of the gangetic floodplain in weathering himalayan sediments it also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account 2012 elsevier ltd all rights reserved

microbial geomorphology a neglected link between life and landscape abstract whilst recognition is increasing that life and landscapes are intimately related as evidenced by growing research into ecosystem engineering biogeomorphology and allied topics the microbial contribution to such interactions has been relatively neglected a revolution in environmental microbiology based on molecular techniques is now driving a reconsideration of the role of microbial processes in geomorphology at all scales recent research illustrates the hitherto unknown microbial diversity present in many extreme geomorphic environments such as hyperarid deserts subglacial lakes hot springs and much richer microbial life than previously suspected within the soils and sediments that blanket most other landscapes such microbial communities have been found to play important geomorphic roles across a wide range of environments notably in weathering precipitation of minerals and protecting surfaces from erosion these geomorphic roles can also be conceptualised as examples of ecosystem engineering and can pave the way for further plant geomorphology and zoogeomorphology processes three key aspects which emerge from a review of microbial influences on earth surface processes are a that microbes play roles on a continuum from full control to passive involvement b that complex and widespread communities of microorganisms are involved and c that microbial activity usually affects several earth surface processes at once examples of the contribution of microbial life to geomorphology over long medium and short timescales suggest that microorganisms play key geomorphological roles in two major situations on the cusp between stable states and in extreme environments where higher plant and animal life is limited and many abiotic processes are also constrained the dominant link between microbial life and geomorphology appears to take on different forms depending on the timescale under consideration with a stabilising microbial role apparent over short timescales being replaced by a denudational role over longer timescales further research involving microbiologists and geomorphologists is now needed to address three main questions i e under what conditions are microbial and geomorphological processes most closely linked what scale issues surround links between microbes and geomorphology and how do microbiological processes underpin broader biogeomorphological interactions

role of a cyanobacterial cover on structural stability of sandy soils in the sahelian part of western niger microbiotic soil crusts mostly formed by cyanobacteria are widespread on the surface of fallow land in western niger they lie adjacent to completely bare soils we have investigated the structural stability of these soils by testing aggregate breakdown under fast wetting slow wetting and mechanical breakdown the tests were effected on aggregates ranging from 3 to 5 mm in size these experiments were completed by micromorphological examinations under light and scanning electron microscope for all tests the fragment size distribution and the mean weight diameter mwd revealed the great stability of aggregates from soils with a cyanobacterial cover mwd 1 82 to 3 10 mm compared to those from soils devoid of cyanobacterial cover mwd 0 25 to 1 26 mm fast and slow wetting of microbiotic soil crust material induced a lesser disaggregation of aggregates compared to mechanical breakdown on the contrary fast wetting and mechanical breakdown of aggregates from soils devoid of cyanobacterial cover induced a greater disaggregation than slow wetting microscopic examination of microbiotic crusts revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions which binds and entraps mineral particles on the soil surface organo mineral aggregates ranging from 0 10 to 0 12 mm were observed below the superficial crusts filaments and residual organic matter bind tightly soil particles thus giving a compact structure the great stability of aggregates of soil with cyanobacterial cover is likely related to the binding and gluing effect of cyanobacteria and derived organic matter this is consistent with the positive correlation between mwd values and organic carbon content the present results thereby confirm the resistance to erosion of soil with microbial cover as indicated by field measurements

u series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains the case of the himalayan rivers 238u 234u 230th radioactive disequilibria were analyzed in suspended sediments collected at different depths from the ganges river and one of its main tributaries the narayani gandak river results associated with bedload sediment data suggest that uranium series u series disequilibria in river sediments of the ganges basin vary with grain size and sampling location the range of observed u series disequilibria is explained by a mixing model between a coarse grained sediment end member represented by bedload and bank sediments and a fine grained end member that both originate from himalaya but undergo different transfer histories within the plain the coarse grained sediment end member transits slowly i e several 100 s ky in the plain whereas the fine grained sediment end member is transferred much faster 20 25 ky as indicated by the absence of significant variations in th isotope composition of the fine grained sediment end members these results show that u series isotopes can be used to quantify the various transfer times of river sediments of different sizes and infer that there can be an order of magnitude of difference or more between the transfer time of suspended and bedload sediments this underlines that a good knowledge of the proportion of suspended vs bedload sediments transported in the river is required to accurately assess how fast erosion products are transferred in a catchment and how fast a catchment is likely to respond to external forcing factors

uranium series isotopes in colloids and suspended sediments timescale for sediment production and transport in the murray darling river system abstract we have measured 238u 234u 230th radioactive disequilibria in five size fractions of colloids and suspended sediments for four rivers from the murray darling river basin se australia continuous compositional trends of composition are observed for 234u 238u and 230th 238u activity ratios over the range of size fractions studied they are explained by the variable contribution of two components detrital alumino silicates and organic matter using relationships between activity ratios and the loss on ignition it is possible to estimate the u series isotopic composition of both detrital and organic end members which provide the best estimate for the composition of the true detrital phase i e controlled only by the removal of elements through weathering and the true dissolved phase i e colloid free respectively when performing a single filtration at 0 2 m it appears that size fractions and

link between climate and catchment erosion in the himalaya during the late quaternary abstract few studies using geochemistry of sediments from plains and delta of the himalayan river system have reported contrasting results on the shift in erosion of different lithotectonic units across the strike of himalayan orogen in response to changes in the intensity of indian summer monsoon ism and glacial cover during the late quaternary here we present records of the 143nd 144nd end and 87sr 86sr ratios and 13c of sediment organic matter 13csom in bulk sediment samples from two 45 m long alluvial cores collected from a paleo sutlej channel in sirhind punjab at an upstream connection of a large river ghaggar hakra with present day sutlej river in nw india the isotopic variations are compared with the available proxy records that document changes in the ism and himalayan glacial extent significant covariation in down core profiles of end 14 1 to 22 2 and 87sr 86sr 0 72251 0 79456 reveals changing contributions of sediment derived from the higher and lesser himalaya end member sources in the catchment higher end and lower 87sr 86sr during early marine isotope stage mis 1 and late mis3 reflects increased contribution from the higher himalaya as a result of receding glacial cover as well as intense ism that penetrates into the interior of the higher himalaya during warm periods the 13csom varies from 28 5 to 21 3 and shows a 5 7 increase from colder mis2 to warmer early mis1 implying an increase in c4 plant abundance during the same period in response to intensification of the ism in the post last glacial maximum period these variations in sediment provenance and vegetation pattern thus highlight the pronounced influence of climate on distribution of erosion and vegetation types in the nw himalaya

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

sediment transport time measured with u series isotopes results from odp north atlantic drift site 984 abstract high precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition the approach is demonstrated on fine grained late pleistocene deep sea sediments from ocean drilling program site 984a on the bjorn drift in the north atlantic the sediments are siliciclastic with up to 30 carbonate and dated by 18o of benthic foraminifera nd and sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from iceland and a distal continental source during glacial periods an unexpected finding is that the 234u 238u ratios of the silicate portion of the sediment isolated by leaching with hydrochloric acid are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance the 234u depletions are inferred to be due to recoil loss of 234th and are used to calculate comminution ages of the sediment the time elapsed between the generation of the small 50 m sediment grains in the source areas by comminution of bedrock and the time of deposition on the seafloor transport times the difference between comminution ages and depositional ages vary from less than 10 ky to about 300 to 400 ky for the site 984a sediments long transport times may reflect prior storage in soils on continental shelves or elsewhere on the seafloor transport time may also be a measure of bottom current strength during the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from iceland that is rapidly transported to the site of deposition the comminution age approach could be used to date quaternary non marine sediments soils and atmospheric dust and may be enhanced by concomitant measurement of 226ra 230th 230th 234u and cosmogenic nuclides

importance of atmospheric inputs and fe oxides in controlling soil uranium budgets and behavior along a hawaiian chronosequence abstract a long term budget of uranium calculated for a chronosequence of hawaiian soils demonstrates the importance of mineral aerosol deposition as a geochemical pathway a series of sequential extractions on the same soils shows a strong association between fe oxides and uranium especially the amorphous and poorly crystalline fe oxides present in intermediate age soils 20 and 150 ka despite intense leaching conditions soil u contents were found to increase between 20 fold and 60 fold with age along the chronosequence from 9 to 17 g cm 2 at 0 3 ka sites to 344 543 g cm 2 at the 4100 ka site an open system transport function comparing u to an index least mobile element nb was used to calculate u gains or losses from the soils relative to the input from parent material weathering results show small losses or small gains 25 of u occurred in soils 150 ka and younger while gains of up to 531 u occurred in soils older than 150 ka highlighting the importance of mineral aerosol dust deposition at the older sites in the 4100 ka soil the ratio of dust derived u inputs to parent material derived u inputs is approximately 6 sequential extractions show that between 0 1 and 1 2 of the total u in the soils was extracted with ammonium acetate indicating that very little u exists as exchangeable cations a second extraction employing citrate dithionate and hydroxylamine hydrochloride was performed to extract u associated with fe oxides average percent occurrences for u in the fe oxide extraction for each profile are 7 at 0 3 ka 53 at 20 ka 40 at 150 ka and 17 at 4100 ka illustrating an increase with age to 20 ka followed by a subsequent decline this parallels the trend observed in amorphous fe oxide content suggesting that adsorption on amorphous fe oxides is the dominant control on u in the intermediate age soils

the provenance and crustal residence ages of british sediments in relation to palaeogeographic reconstructions 24 smnd isotope analyses of fine grained phanerozoic and modern clastic sediments from britain and quebec are presented in combination with published data they have been used to calculate crustal residence ages tcr and to assess the provenance of the british sedimentary mass sediments now preserved on either side of the suture formed by the closure of the iapetus ocean ca 400 ma ago were derived from isotopically distinct source regions sediments north of the suture are characterised by1 7 tcr 2 8 ga whereas those to the south exhibit a smaller range and an average value of 1 6 ga the northern and southern source regions were most probably laurentia and gondwana respectively it seems likely that a third source perhaps baltica provided the lower palaeozoic southern uplands succession sediments deposited in southern britain after the closure of iapetus were derived mainly from the recycling of older sediments the tectonic rearrangements which occurred during the phanerozoic are not reflected in the smnd isotopic structure of the southern british sedimentary mass suggesting that the caledonian and hercynian orogenies and even the grenville orogeny involved minimal accretion of new mantle derived material into the british and adjacent continental crust smnd analyses of fine grained clastic sediments provide a powerful sedimentological tool for elucidating palaeogeography clastic source areas sediment recycling and maturity and some aspects of sediment transport

impacts of soil crusts on soil physicochemical characteristics in different rainfall zones of the arid and semi arid desert regions of northern china soil crusts commonly form in arid and semi arid desert regions once shifting sand dunes stabilize the crusts affect soil properties such as texture and fertility however the role of crusts depends on the species composition their physiological activity and their stage of evolution and varies with precipitation and season in this study the soil properties of physical and biological crusts in different rainfall zones of the arid and semi arid desert regions of northern china were analyzed more developed crusts had finer soil texture the impact on chemical properties was greater in areas with higher precipitation contents of soil organic carbon soc total nutrients available n and exchangeable ions did not vary among crust types in a hyperarid region soc total n and p and available n increased with crust development at 150 400 mm rainfall soil crusts increased available p during the initial stage of development crust development increased available k at 300 400 mm precipitation but not at 250 mm precipitation caco3 changes were related to windblown dust and soil moisture in the crust electrical conductivity and total salinity varied in response to variation of soil nutrients and aeolian salt accumulation the ph did not vary greatly between crust types and sites crust development was accompanied by soil alkalization ca2 was more abundant than other cations crust development did not influence cl but increased hco3 and na during crust development so4 2 and k contents decreased and mg2 increased in regions with 250 mm precipitation the opposite changes occurred at 300 400 mm precipitation

wind transport of sand surfaces crusted with photoautotrophic microorganisms abstract wind tunnel experiments show that while surfaces inoculated with photoautotrophic organisms are stable in winds up to 19 m s 1 only filamentous cyanobacteria especially nostoc commune afford considerable protection against abrasion from windborne particles crusts formed by the polysaccharide secreting alga chlamydomonas acidophila break down easily under impact the entrainment of large low density aggregates possibly contributing to higher transport rates than would otherwise be expected for untreated sediment surface disintegration encompasses a suite of complex scale dependent processes which vary with the structure thickness and strength of the crust independent measurements of crust strength based on flexure testing are in good agreement with wind tunnel evaluation of crust stability the accommodation of a relatively large amount of deformation without rupture is an important property of fibrous photoautotrophic mats perhaps outweighing consideration of their peak strength

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

a new raba chromatographic separation and its application to ra mass spectrometric measurement in volcanic rocks we have adapted the recent chromatographic method described by e p horwitz and coworkers for sr separation to perform an efficient ba ra separation for measuring ra isotopes in volcanic rocks by mass spectrometry extraction chromatography is used with a solution of a crown ether 4 4 5 bis tert butylcyclohexano 18 crown 6 in 1 octanol adsorbed on an inert substrate as an extractant the elchrom company markets this resin under the name sr spec we show that this technique gives a very efficient ba ra separation which compares favourably with previous chromatographic methods the chemical and mass spectrometric procedure is described in detail and is applied to ra measurements in volcanic rocks

sediment residence times in catchments draining to the gulf of carpentaria northern australia inferred by uranium comminution dating abstract uranium u isotopes are useful for constraining the timescales of weathering and erosion processes the 234u 238u activity ratio parentheses denote activity ratio of fine grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to

carbon sinks in small sahelian lakes as an unexpected effect of land use changes since the 1960s saga gorou and dallol bosso sw niger for several decades global change has resulted in an increase in runoff in the sahelian belt provoking major changes in the quality and quantity of sediments transported by drainage networks one of the astonishing consequences is the establishment of numerous permanent lakes the origins of particulate organic matter om preserved within lacustrine sediments of three lakes were investigated by coupling optical observations palynofacies and bulk geochemistry rock eval 6 pyrolysis an initial estimate of particulate organic carbon oc stored in these lacustrine sediments was assessed soil organic matter som was sampled from the surface 0 10 cm of various land use and land cover areas and was characterized and compared with sedimentary organic matter our results reveal that lake tankalawal is subjected to high autochthonous organic sedimentation toc ranges between 3 0 and 10 0 wt while lakes bangou kirey and bi are characterized by weak sedimentation of non indigenous om originating from the soil erosion and shore vegetation toc 3 0 wt in sediments the effects of early diagenesis on the om induce not only a loss of labile and aquatic om but also a significant loss of terrestrial om which is supposed to be more resistant than its aquatic counterpart both the preservation of om in top sediments and the relative preservation of terrestrial om impact the oc storage in lakes indeed oc storage in lacustrine sediments lake bangou kirey was two to seven times higher than the oc storage in sahelian soils where greater contributions of terrestrial om to sedimentary om are associated with lower oc storage in lacustrine sediments for lakes bangou kirey and bi oc accumulation rates were also assessed due to the identification of a sedimentary limit corresponding to the establishment of permanent lakes dated earlier 1960s high oc fluxes were estimated and ranged between 104 and 213 g oc m 2 yr 1 compared with other oc accumulation rates for various african lakes these high values are similar to those calculated for reservoirs and are related to anthropogenic pressure soil textures favoring erosion and proper physical and chemical conditions for om preservation in sediments accordingly in these sahelian environments that are generally viewed as non efficient in storing oc we claim that global change could promote a new oc sink if other similar studies reinforce our assertion then regional c budgets should be revisited