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

silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

evaluation of groundwater quality in northern saudi arabia using multivariate analysis and stochastic statistics groundwater resources available in the paleozoic sedimentary formations form the most prolific source of water supply in the kingdom of saudi arabia in the absence of surface water supplies the available groundwater reserves in these aquifers have been used extensively to fulfill the agricultural and domestic water demands the main purpose of this paper was to evaluate the groundwater quality in the northern western part of saudi arabia using the classical water facies diagram piper and durov plot the more advanced methods of principal component analysis pca for determining the factors controlling the groundwater chemistry and stochastic geostatistics to understand the spatial distribution of different major elements in the groundwater of the study area were employed 40 groundwater samples from bore wells tapping the saq aquifer were collected and analyzed for the major ion chemistry and nitrates groundwater occurs within detrital sandstones of phanerozoic era in saq formations piper plot revealed that most of the water samples belonged to the mixed class ca mg cl so4 type and few samples falling within the ca mg hco3 type na cl type and ca mg so4 type of groundwater facies durov plot pointed towards simple dissolution silicate weathering ion exchange and reverse ion exchange as the main processes governing the chemistry of the area pca resulted in the extraction of two pc accounting for 78 11 of the data variability the extracted pcs pointed towards geogenic factors governing the groundwater quality stochastic geostatistics involved prediction maps of ordinary kriging and indicator kriging which provided detailed indications of spatial distribution of the chemical elements

silicon isotopes in granulite xenoliths insights into isotopic fractionation during igneous processes and the composition of the deep continental crust the silicon si cycle is of great current interest but the isotopic composition of the continental crust has not been determined magmatic differentiation generates liquids with heavier si and the lower crust thought to be dominated by cumulates and restites is predicted to have a light isotopic composition this is borne out by the composition of many types of granite which appear to have relative light si for their silica content here we report the si isotopic compositions of two granulite facies xenolith suites from the chudleigh and mcbride volcanic provinces australia providing new constraints on deep crustal processes and the average composition of the deep continental crust the xenoliths display a range of isotopic compositions 30si 0 43 to 0 15 comparable to that measured previously for igneous rocks the isotopic compositions of the mcbride xenoliths reflect assimilation and fractional crystallisation afc and or partial melting processes silicon and o isotopes are correlated in the mcbride suite and can be explained by afc of various evolved parent melts in contrast the chudleigh xenoliths have si isotope compositions predominantly controlled by the specific mineralogy of individual cumulates using the xenolith data and a number of weighting methods the si isotope compositions of the lower and middle crust are calculated to be 30si 0 29 0 04 95 s e and 0 23 0 04 95 s e respectively these values are almost identical to the composition of the bulk silicate earth implying minimal isotope fractionation associated with continent formation and no light lower crustal reservoir

silicon and chromium stable isotopic systematics during basalt weathering and lateritisation a comparison of variably weathered basalt profiles in the deccan traps india global biomass production is fundamentally affected by the hydrological cycling of elements at the earth s surface continental weathering processes are the major source for most bio essential elements in marine environments and therefore affect primary productivity in addition critical zone biomass depends on energy and chemical exchange reactions in weathering profiles the latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like ph and eh which regulate mineral break down rates and dictate the mobility and mass flux of elements two deccan traps basalt weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on si and cr and their isotopic systematics the quaternary chhindwara profile has progressed to a moderate degree of alteration saprolite while the paleogene bidar example displays an extremely altered laterite the chhindwara saprolite profile shows a near uniform cr and si concentration and isotopic composition whereas the bidar laterite profile is characterised by an intense loss of si a large enrichment of cr within the most altered uppermost levels and a wide range of cr stable isotope ratios 0 85 to 0 36 parts per thousand delta cr 53 52 a co variation between si and cr isotopes as well as their co variation with iron content provides empirical evidence that iron redistribution within the profile has a large effect on cr mobility and si isotopic fractionation therefore it is concluded that iron oxides exert a primary control over the isotopic composition of both cr and si in pore waters of laterites since laterite formation is promoted by tropical climates the results of this study provide new evidence to suggest that the hydrological cr and si fluxes originating from continental weathering have changed in accordance with large scale deep time climate variation and continental plate configuration an increased flux of si and greater magnitude of cr mobility and isotopic fractionation are possibly amplified under co2 rich greenhouse episodes and or when large landmasses were tectonically arranged at near equatorial latitudes

spatial and temporal variations of dissolved silicon isotope compositions in a large dammed river system abstract the dissolved silicon isotope composition 30si of river water serves as a key parameter in tracing the terrestrial silicon cycle however the lack of long term observations of rivers often makes it difficult to constrain the isotopic features of riverine dissolved silica dsi well in this study samples were collected during four basin scale surveys and monthly monitoring at the mouth of changjiang over the course of two years during the full operation period of the three gorges dam tgd the largest hydropower project in the world and the dsi concentration and 30si were measured the dsi concentration and 30si ranged from 91 3 m to 124 7 m and 1 00 to 1 86 for the main stream of changjiang and from 58 4 m to 194 0 m and 0 83 to 2 27 for the tributaries lakes respectively the mid lower tributaries lakes possessed mean dsi concentrations comparable to those of the upstream tributaries but higher 30si 30si increased by 0 7 in the main stream accompanied by a decrease in dsi concentration and elemental ratio of dissolved aluminum to si al si downstream from the tgd when the mid lower tributaries lakes were in the dry period suggesting co influence of enhanced clay formation productivity of freshwater diatoms as well as influx of 30si enriched tributaries lakes in controlling the evolution of 30si in the mainstream in contrast little longitudinal variation was observed when the mid lower stream was in the flooding period and 30si discharged by changjiang to the estuarine region was mainly controlled by the mixing between the main stream and the mid lower tributaries lakes groundwater could be relevant source of dsi in the main stream of changjiang as indicated by water mass balance calculation while its impact on 30si in the main stream was insignificant based on the long term monitoring at the river mouth the dsi flux weighted 30si value of changjiang was re estimated to be 1 4 which represented the best estimation of the riverine 30si discharged into the coastal waters by changjiang at this stage the temporal variation in 30si observed at the river mouth was affected by the annual flow cycles of the upper and mid lower streams of changjiang superimposed by seasonal aquatic diatom productivity combination with the historical dsi concentrations in the changjiang end member suggested that the three gorges reservoir does not play a significant role in direct retaining dsi at this stage while the severe sediment trapping within the reservoir may cause potential alteration in the silicate weathering and freshwater diatoms productivity in the mid lower reaches

mechanisms controlling the silicon isotopic compositions of river waters it has been proposed that silicon si isotopes are fractionated during weathering and biological activity leading to heavy dissolved riverine compositions in this study the first seasonal variations of stable isotope compositions of dissolved riverine si are reported and compared with concomitant changes in water chemistry four different rivers in switzerland were sampled between march 2004 and july 2005 the unique high resolution multi collector icp ms nu1700 has been used to provide simultaneous interference free measurements of 28si 29si and 30si abundances with an average limiting precision of 0 04 on 30si this precision facilitates the clarification of small temporal variations in isotope composition the average of all the data for the 40 samples is 30si 0 84 0 19 1 sd despite significant differences in catchment lithologies biomass climate total dissolved solids and weathering fluxes the averaged isotopic composition of dissolved si in each river is strikingly similar with means of 0 70 0 12 for the birs 0 95 0 22 for the saane 0 93 0 12 for the ticino and 0 79 0 19 for the verzasca however the 30si undergoes seasonal variations of up to 0 6 comparisons between 30si and physico chemical parameters such as the concentration of dissolved si and other cations the discharge of the rivers and the resulting weathering fluxes permits an understanding of the processes that control the si budget and the fate of dissolved si within these rivers the main mechanism controlling the si isotope composition of the mountainous verzasca river appears to be a two component mixing between the seepage of soil ground waters with heavier si produced by clay formation and superficial runoff associated with lighter si during high discharge events a biologically mediated fractionation can be excluded in this particular river system the other rivers display increasing complexity with increases in the proportion of forested and cultivated landscapes as well as carbonate rocks in the catchment in these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of si contributed by biological processes as opposed to abiotic weathering the presence of seasonal variations in si isotope composition in mountainous rivers provides evidence that extreme changes in climate affect the overall composition of dissolved si delivered to the oceans the oceanic si isotope composition is very sensitive to even small changes in the riverine si isotope composition and this parameter appears to be more critical than plausible changes in the si flux therefore concurrent changes in weathering style may need to be considered when using the si isotopic compositions of diatoms sponges and radiolaria as paleoproductivity proxies

temporal isotopic variations of dissolved silicon in a pristine boreal river abstract it has previously been concluded that the stable si isotopes are fractionated during geochemical and biogeochemical elemental transfers such as weathering and precipitation of clays and biogenic si which has opened up the possibility of using si as a tracer in natural terrestrial ecosystems furthermore quantification of the biogenic impact on the biogeochemical si cycle has attracted significant scientific interest since biological control has been suggested previous observations of seasonal variations in the dissolved si isotopic pattern further imply that high frequency riverine sampling during main hydrological events might provide important information about natural processes governing the fluvial biogeochemical si cycle therefore temporal variations in the isotopic composition of riverine dissolved si were investigated for the kalix river northern sweden the largest pristine river system in europe based on high frequency sampling during a period of 25 weeks from early april to early october 2006 temporal variations spanning 0 4 for 29si and 0 8 for 30si of dissolved si in the kalix river were observed during the period suggesting that the riverine dissolved si input to the oceans cannot be considered to have a constant si isotopic composition on a short time scale the results implicate biogeochemical si cycling via formation and dissolution of biogenic silica as processes significantly affecting the dissolved si transport in boreal systems during april to early october the si budget in the river system appeared to be controlled by relative si enrichment during high discharge events and relative si depletions in the subarctic mountainous and lake dominated areas the si enrichments and depletions were accompanied by decreasing and increasing riverine 29si and 30si respectively these isotope variations can be explained by release of plant derived silica depleted in heavier si isotopes during the spring snowmelt further increased volumetric contribution from the headwater and losses of dissolved si due to biogenic silica formation by diatoms in the subarctic lakes at a later period are expected to be responsible for the preferential losses of lighter isotopes as further verified by land cover analysis

suspended sediment and dissolved load budgets of two amazonian rivers from the guiana shield maroni river at langa tabiki and oyapock river at saut maripa french guiana this study presents the data collected within the framework of an observatory of research in environment on the amazonian basin the ore hybam it relates to the dissolved and solid loads of the two main rivers of french guiana the maroni and oyapock rivers running on the guiana shield and draining respectively 64 230 and 24 630 km 2 the low coefficient of variation of the average annual flows of the two rivers indicates an inter annual hydrological stability probably related to the immediate vicinity of the atlantic ocean the sedimentary load is mainly composed of quartz and kaolinite the total suspended solid tss concentrations are among the world s lowest values they range from 1 to 130 mg l 1 during the hydrological cycle with averages of 22 and 12 mg l 1 for the maroni and oyapock rivers respectively the seasonal variability of these values is significantly higher than that of hydrologic flows but without simple relationship with the discharge water chemical composition of the two rivers indicates a very weak mineralization very similar to that found in the amazonian rivers running on the brazilian and guianese shields and in the congo river and its tributaries in the central african shield seasonal variations are observed in both basins they correspond to higher concentrations during low water stage from october to february and to more diluted water during the flood from april to july a signature enriched in cl is present at the saut maripa station on the oyapock river indicating a more marked influence of the trade winds in this basin the computation of atmospheric contributions to ions budget indicated a weak contribution for ca2 and mg2 which originates mainly from water rock interactions in both stations while more than half of na is derived from atmospheric inputs

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

transformation of silicon in a sandy beach ecosystem insights from stable silicon isotopes from fresh and saline groundwaters abstract dissolved silicon isotope compositions 30si have been analysed for the first time in groundwaters of beach sediments which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater the fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736 m but homogeneous 30si of 1 0 0 15 by contrast the seawater is strongly depleted in dissolved silica with concentrations of 3 m and consequently characterised by high 30si of 3 0 the beach groundwaters are variably enriched in dissolved silica compared to seawater 23 192 m and concentrations increase with depth at all sampling sites the corresponding 30si values are highly variable 0 3 to 2 2 and decrease with depth at each site all groundwater 30si values are lower than seawater and most values are lower than dissolved 30si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments in contrast to open north sea sediments diatom dissolution or formation of authigenic silica in beach sediments is very low ca 5 mol si g 1 silica discharge from the beach to the coastal ocean is estimated as approximately 210 mol si yr 1 per meter shoreline considering the extent of coastline this is at least for the study area a significant amount of the total si budget and amounts to ca 1 of river and 3 5 of backbarrier tidal flat area si input

geomorphic controls on andean denudation rates abstract to predict erosion rates throughout the andes we conducted a multiple regression analysis of the sediment discharge from 47 drainage basins in the bolivian andes and various topographic climatologic and geologic parameters these mountainous basins are typically large 17 81 000 km2 documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm mean 11 000 end document km2 often have decades of measurement data on daily water and sediment discharge and display an extraordinary range of denudation 0 01 6 9 mm yr

tracing silicon cycling in the okavango delta a sub tropical flood pulse wetland using silicon isotopes chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction potentially confounding estimates of silicate weathering rates silicon isotopes d30si may be a useful tool to investigate these processes here we present 82 d30si measurements from surface waters pore waters biogenic silica bsi clays sand and vegetation from the okavango delta botswana a freshwater sub tropical flood pulse wetland hydrologically the okavango is dominated by evapotranspiration water losses to the atmosphere it receives an annual pulse of water that inundates seasonal floodplains while river baseflow is sufficient to maintain a permanent floodplain d30si in dissolved silica dsi in surface waters along a 300 km transect at near peak flood show a limited range 0 36 1 19 imply ing the delta is well buffered by a balance of processes adding and removing dsi from the surface water a key control on dsi concentrations is the uptake production of bsi and recycling of si by aquatic vegetation although the net isotopic effect is necessarily small since all bsi re dissolves on short timescales in the sediments bsi d30si n 30 ranges from less

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

silicon isotope variations accompanying basalt weathering in iceland abstract this study presents the si isotope compositions of the dissolved phase of twenty rivers from across iceland the high resolution nu plasma 1700 mc icp ms was used to provide interference free 30 si 28 si ratios with an average limiting precision of 0 04 30 si the si isotope composition ranges from 0 08 to 1 46 30 si averaging 0 63 0 38 1 sd this is only slightly lighter than the recently reported mean of 0 84 0 19 for high precision measurements of swiss rivers but is significantly lighter than compositions reported for some other rivers the si isotope composition varies slightly with the amount of glacial cover which provides evidence that the continental si input to the oceans may be climatically controlled and thus likely to vary over glacial interglacial cycles after correction for the dissolution of trace amounts of calcite in one region a correlation is found between 30 si and ca si this is consistent with the measured basaltic composition and indicates isotopic fractionation during si removal from the dissolved phase by calculating the fraction of si that is removed from the dissolved phase into secondary phases it is possible to use the riverine si flux si denudation rate to calculate the initial dissolution rate of si within these catchments from this study we estimate that the total amount of si being dissolved by primary weathering of iceland is 3 million tons per yr however only 50 of this si is released into the rivers and contributes to denudation using the si isotope composition and the si fraction removed it is possible to model the si isotope fractionation as either a rayleigh type fractionation or a steady state system both yield similar fractionation factors of about 1 5 30 si implying only one underlying fractionation process that is controlled by external parameters such as runoff furthermore rivers that follow a steady state fractionation model are associated with higher si dissolution rates and lower 30 si values indicating an inverse relationship between weathering rates and dissolved riverine si isotope composition

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

major and trace elements of river borne material the congo basin the congo river basin is the second largest drainage basin in the world after the amazon the materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the african continent this paper presents the chemical composition of the different phases carried in the congo rivers and is followed by a companion paper devoted to the modelling of major and trace elements the congo river between bangui and brazzaville as well as its main tributaries including a few organic rich rivers also called black rivers were sampled during the 1989 high water stage the three main phases suspended load dissolved load and bedload were analysed for twenty five major and trace elements concentrations normalized to the upper continental crust show that in each river suspended sediments and dissolved load are chemical complements for the most soluble elements ca na sr k ba rb and u while these elements are enriched in the dissolved loads they are considerably depleted in the corresponding suspended sediments this is consistent with their high mobility during weathering another type of complementarity is observed for zr and hf between suspended sediments and bedload related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads compared to other rivers absolute dissolved concentrations of ca na sr k ba rb and u are remarkably low surprisingly high dissolved concentrations are found in the congo waters for other trace elements e g rees especially in the black rivers on a world scale these concentrations are among the highest measured in rivers and are shown to be ph dependent for a number of dissolved trace elements the dissolved loads are systematically normalized to the suspended loads for each river in order to remove the variations of the element abundances owing to source rock variations normalized diagrams for rees are presented and extended to the other elements they strongly support the argument that the apparent higher solubility of trace elements in the congo waters is due to the presence in the dissolved load of a colloidal phase as a result of 0 2 m filtration an important result is that these colloids are strongly depleted in fe and al with respect to the other elements finally the comparison of the dissolved suspended and sandy transport fluxes of each element in the congo basin rivers shows that although the proportions of for example the rees in the dissolved loads of the majority of the congo basin rivers is close to 10 of the total transport flux up to 80 of the rees are transported by the so called dissolved load in the black rivers

chemical weathering and atmospheric soil co2 uptake in the andean and foreland amazon basins abstract this study is a geochemical investigation of the andean and foreland basins of the amazon river at high spatial and time resolution carried out within the framework of the hybam research program hydro geodynamics of the amazon basin monthly sampling was carried out at 27 gauging stations located in the upper tributaries of the amazon basin from north to south the napo maranon ucayali madre de dios beni and mamore rivers the aim of this paper is to estimate the present day chemical weathering rate cwr as well as the flux of co 2 consumption from total and silicate weathering in the andes and foreland amazon basins and to discuss their distribution as a function of geomorphic and structural parameters based on the forward method the napo and other ecuadorian basins present high silicate weathering rates in comparison with the other basins we confirm that the maranon and ucayali rivers control the amazon hydrochemistry due to the presence of salt rocks and carbonates in these basins the madre de dios beni and mamore basins do not contribute much to the amazon dissolved load this north to south cwr gradient can be explained by the combination of decreasing weatherable lithology surface and decreasing runoff rates from the north to the south the foreland part of the basins or mountain lowland transition accounts for nearly the same proportion of the amazon silicate chemical weathering and carbonate chemical weathering fluxes as the andean part this result demonstrates the importance of the sediment accumulation areas in the amazon basin weathering budget and can be explained by the occurrence of a higher temperature the deposition of fresh sediments from andean erosion and a higher sediment residence time than in the upper part of the basin with a total co 2 consumption rate of 744 10 3 moles km year 1 and a silicate co 2 consumption rate of 300 10 3 moles km year 1 the upper amazon river andes foreland part is the most intense part of the amazon basin in terms of atmospheric co 2 consumption by weathering processes it is an important co 2 sink by weathering processes but accounts for only somewhat more than half of the co 2 consumption by silicate weathering of the amazon basin this result points out the importance of the lowland part of the basin in the inorganic c silicate budget the upper amazon accounts for 2 4 of the world s silicate co 2 consumption which is the same proportion as for the southern and southern east himalaya and tibetan plateau

from oxisols to spodosols and histosols evolution of the soil mantles in the rio negro basin amazonia abstract the upper rio negro basin under a constantly humid equatorial climate is a low altitude peneplain of more than 165 000 km 2 prevailing soils are oxisols and spodosols and their distribution is usually related to the lithology of the parent materials spodosols being generally associated with sandy deposits after exploratory surveys in this extensive region six major soil geomorphic units have been identified in which selected toposequences have been studied by means of micromorphological chemical and mineralogical analyses detailed field analysis of the horizonation of the soil mantle has been carried out in three sequences consisting of oxisols ultisols and spodosols results show that sharp transitions within distances of less than one hundred meters separate the oxisols from the spodosols however the arrangement of the horizons in the soil mantle and similarities in micromorphological features chemical composition and mineral components between the adjacent horizons are evidences that genetic relationships link contiguous profiles the authors propose an alternative explanation for this soil distribution based on the lateral transformation of oxisols and ultisols into spodosols and on the lateral evolution of the giant spodosols into histosols peat and waterlogged ultisols interpretative models of landscape evolution as consequence of soil evolution are thus proposed

ge and si isotope signatures in rivers a quantitative multi proxy approach abstract solutes derived from the dissolution of silicate minerals play a key role in earth s climate via the carbon and other biogeochemical cycles silicon si is a unique constituent of silicate minerals and a biologically important nutrient so tracing its behavior in near surface environments may provide important insights into weathering processes however si released by weathering is variably incorporated into secondary mineral phases and biota obscuring signals derived from primary weathering processes due to chemical similarities germanium ge may help better understand the si cycle and its relationship to chemical weathering with this aim we report new measurements of the concentration and isotopic composition of ge for both the dissolved and particulate phases of a variety of global rivers these measurements are combined with analyses of concentration and isotopic ratio of si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems with this dataset we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers i e ge si 74 ge and 30 si this multi proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean dissolved 74 ge composition of rivers studied thus far range from 0 9 to 5 5 with a discharge weighted global average of 2 6 0 5 the ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks which is consistent with mass balance expectations the multi proxy modeling suggests that among the watersheds studied here the isotopic fractionation of si during secondary mineral phase precipitation 30 si s e c ranges from 2 7 to 0 2 which removes between 19 79 of the initial dissolved si while between 12 54 is incorporated by biota for ge modeling indicates that 79 98 of the dissolved load is incorporated into secondary mineral phases with a 74 ge s e c ranging from 4 9 to 0 3 the fractionation induced by biological uptake is calculated to range from 2 6 to 1 3 for 30 si b i o and 0 7 0 7 for 74 ge b i o in addition to improving our understanding of the coupled ge and si cycles our study provides a framework for using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

δ30si systematics in a granitic saprolite puerto rico granite weathering and clay mineral formation impart distinct and interpretable stable si isotope 30 si signatures to their solid and aqueous products within a saprolite clay minerals have 30 si values 2 0 more negative than their parent mineral and the 30 si signature of the bulk solid is determined by the ratio of primary to secondary minerals mineral specific weathering reactions predominate at different depths driving changes in differing 30 si pore water values at the bedrock saprolite interface dissolution of plagioclase and hornblende creates 30 si pore water signatures more positive than granite by up to 1 2 these reactions are the main contributor of si to stream water and determine its 30 si value throughout the saprolite biotite weathering releases si to pore waters but kaolinite overgrowth formation modulates its contribution to pore water si the influence of biotite on 30 si pore water is greatest near the bedrock where biotite derived si mixes with bulk pore water prior to kaolinite formation higher in the saprolite biotite grains have become more isolated by kaolinite overgrowth which consumes biotite derived si that would otherwise influence 30 si pore water because of this isolation which shifts the dominant source of pore water si from biotite to quartz 30 si pore water values are more negative than granite by up to 1 3 near the top of the saprolite

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

silicon isotope study on rice plants from the zhejiang province china silicon isotope compositions of 16 rice plants from the zhejiang province were studied and potential applications of silicon isotope study to agriculture and biogeochemistry were discussed it is found that the sio2 contents in ignited rice plant samples increased generally from roots through sl stem and leaves to husks large and systematic silicon isotope fractionation was observed among different organs of individual rice plants their 30si values show a generally increasing trend from roots and sl through husks to grains which can be explained by kinetic isotope fractionation in a rayleigh process a 30si value of 6 1 was obtained from two samples of rice grains which is much greater than the highest value 3 4 observed in terrestrial samples before it is inferred that monosilicic acid is the major silicon bearing component taken up by rice roots passive uptake of silicon is important for rice and evapotranspiration may be the major mechanism for the transportation and precipitation of silicon in rice plants it is suggested that silicon isotope study has a potential application to agriculture and biogeochemistry

the riverine silicon isotope composition of the amazon basin we present here the first large scale study of riverine silicon isotope signatures in the amazon basin the amazon and five of its main tributaries were studied at different seasons of the annual hydrological cycle the 30si signature of the dissolved silicon dsi exported to the estuary weighted for dsi flux for the period considered is estimated at 0 92 a river cross section shows the homogeneity of the amazon river regarding dsi concentration and isotope ratio the biogenic silica bsi concentration measured in surface water from all rivers is generally small compared to the dsi reservoir but large variations exist between rivers very low isotope signatures were measured in the upper rio negro 30si 0 05 0 06 which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation the si isotope fractionation in the andean tributaries and the amazon main stem can be explained by clay formation and follow either a rayleigh or a batch equilibrium fractionation model our results also suggest that the formation of 2 1 clays induces a fractionation factor similar to that of kaolinite formation

stable silicon isotopes of groundwater feldspars and clay coatings in the navajo sandstone aquifer black mesa arizona usa abstract we present some of the first analyses of the stable isotopic composition of dissolved silicon si in groundwater the groundwater samples were from the navajo sandstone aquifer at black mesa arizona usa and the si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed silicon isotope compositions were measured using high resolution multi collector inductively coupled mass spectrometry hr mc icp ms nu1700 nuplasma hr the quartz dominated bulk rock and feldspar separates have similar 30 si of 0 09 0 04 and 0 15 0 04 95 sem respectively and clay separates are isotopically lighter by up to 0 4 compared to the feldspars from isotopic mass balance considerations co existing aqueous fluids should have 30 si values heavier than the primary silicates positive 30 si values were only found in the shallow aquifer where si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals however 30 si decreases along the flow path from 0 56 to 1 42 representing the most negative dissolved si isotope composition so far found for natural waters we speculate that the enrichment in 28 si is due to dissolution of partly secondary clay minerals and low temperature silcretes in the navajo sandstone the discovery of the large range and systematic shifts of 30 si values along a groundwater flow path illustrates the potential utility of stable si isotopes for deciphering the si cycling in sedimentary basins tracing fluid flow and evaluating global si cycle

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

silicon isotopic fractionation in marine sponges a new model for understanding silicon isotopic variations in sponges abstract the silicon si isotope 30 si composition of deep sea sponges from near antarctica subantarctic waters tasmania seamounts and subtropical waters north of new zealand vary widely between 0 87 and 3 40 vs nbs28 depth profiles show that sponge 30 si compositions trend to lower values with increasing depth this is exemplified by sponges from the tasmania seamounts where 30 si varies from 0 87 to 3 13 over a depth range from 100 m to 1200 m these changes in 30 si of sponges are inconsistent with a rayleigh type isotope fractionation model requiring constant 30 si fractionation between sponge and seawater we conclude that overall si isotope fractionation 30 si 30 si sponge 30 si seawater is influenced by seawater si concentration with more fractionated lower isotope values being associated with sponges collected from waters high in si we invoke and fit a model whereby the 30 si fractionation varies as a function si influx and efflux using this model it appears that 30 si fractionation during transport into the sponge is constant at 1 34 the model also shows asymptotic behaviour with 30 si trending towards a maximum of 6 02 at very high si concentrations these results suggest that the 30 si composition of fossil spicules may be useful for reconstruction paleo si concentrations during the past

possible mechanisms for glacial earthquakes the large glacial earthquakes reported on by ekstrom et al 2003 2006 and tsai and ekstrom 2007 have previously been evaluated in terms of their seismic characteristics in this paper we attempt to take constraints such as known glacial ice properties outlet glacier size calving style and meltwater variability to construct a self consistent physical model of the glacial earthquake process since many glaciological parameters are poorly constrained we parameterize a number of important processes and estimate a wide range of possible values for some properties the range of model outputs is thus fairly large but it is still difficult to match observational constraints under most conditions we find that only a small class of models is able to satisfy the major observational constraints these models are characterized by 1 lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or 2 by nonequilibrium calving such as having large icebergs capsize into the glacier front although observational constraints cannot definitively rule out any of the proposed classes of mechanisms the calving model has much stronger support fortunately the various models make different predictions regarding observables that can potentially be measured in the near future

silicon isotope compositions of dissolved silicon and suspended matter in the yangtze river china abstract silicon isotope compositions of main channel samples of the yangtze river were systematically investigated along with their chemical compositions the concentration of suspended matter in the yangtze river tends to decrease from the upper reaches to the lower reaches corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow the silica contents of suspended matter vary from 52 1 to 56 9 and their 30si values vary from 0 to 0 7 both similar to those of shales from the upper to lower reaches the silica contents of suspended matter tend to increase whilst their 30si values tend to decrease both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter the concentrations of dissolved silicon vary from 97 to 121 mol l and their 30si values vary over a wide range from 0 7 to 3 4 from the upper to lower reaches dissolved silica concentrations tend to decrease and their 30si values tend to increase these trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches the major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass in wetlands and rice in paddy fields in drainage areas of the middle and lower reaches there is no correlation between 30si of dissolved silicon and that of suspended matter the 30sidiss spm values vary over a wide range of 1 0 3 7 indicating that 1 they are out of isotopic equilibrium 2 dissolved silicon and the associated suspended matter do not belong to one physico chemical system and 3 isotopic exchange rate between them is very slow the 30si value of dissolved silicon output from the yangtze river to the east sea is estimated to be 3 0 much higher than the values reported for the amazon and congo rivers this increases the 30si range of dissolved silicon in the world s rivers from 0 4 1 2 to 0 4 3 4

silicate weathering in the ganges alluvial plain the ganges is one of the world s largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny weathering and global climate change three regions can be recognised in the ganges basin with the himalayan orogeny to the north and the plateaus of peninsular india to the south together delimiting the ganges alluvial plain despite constituting approximately 80 of the basin weathering processes in the peninsula and alluvial plain have received little attention here we present an analysis of 51 water samples along a transect of the alluvial plain including all major tributaries we focus on the geochemistry of silicon and its isotopes area normalised dissolved si yields are approximately twice as high in rivers of himalaya origin than the plain and peninsular tributaries 82 51 and 32 kmol sio2 km 2 yr 1 respectively such dissolved si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the dsi mobilised during the initial weathering process is retained in secondary clay minerals however the silicon isotopic composition of dissolved si expressed as delta si 30 varies from 0 8 parts per thousand in the ganges mainstem at the himalaya front to 3 0 parts per thousand in alluvial plain streams and appears to be controlled by weathering congruency i e by the degree of incorporation of si into secondary phases the higher delta si 30 values therefore reflect decreasing weathering congruency in the lowland river catchments this is exploited to quantify the degree of removal using a rayleigh isotope mass balance model and consequently derive initial silica mobilisation rates of 200 150 and 107 kmol sio2 km 2 yr 1 for the himalaya peninsular india and the alluvial plain respectively because the non himalayan regions dominate the catchment area the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment 41 and 34 respectively c 2015 the authors published by elsevier b v less

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

desilication in archean weathering processes traced by silicon isotopes and ge si ratios abstract the influence of continental weathering on oceanic seawater compositions in the precambrian is poorly understood here we combine si isotopes and ge si ratios of a mesoarchean paleosol 2 95 ga and overlying shales as proxies for weathering processes and si mass transfer at the early earth s surface results show that as with modern soils neoformation of secondary clay minerals in the paleosol was associated with the fractionation of si isotopes and ge si ratios in response to chemical weathering and soil desilication furthermore the loss of fe bearing minerals most likely fe bearing smectites produced additional controls on si and ge mobility through the coupled dissolution and re precipitation of clay minerals under reductive conditions opposite fractionation behaviors are observed neoformed secondary clay minerals acted as a 28 si and ge sink whereas the leaching of fe bearing minerals released 28 si and ge into soil solutions an attempt to estimate 30 si and ge si signatures of released dissolved elements into soil solution provided 30 si and ge si signatures of about 0 1 and 1 4 mol mol respectively furthermore shales deposited shortly after paleosol formation display 30 si and ge si compositions that may be explained as mixtures of the recognized paleosol components the shale record suggests that weathering induced desilication and leaching of fe bearing minerals to a lesser extent might have been widely effective during the mesoarchean for the transfer of si from the continents to the hydrosphere comparison of si isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication relative to changes in hydrothermal and continent derived solute inputs to ocean and ocean cooling

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

organic complexation and translocation of ferric iron in podzols of the negro river watershed separation of secondary fe species from al species abstract the development of podzols in lateritic landscapes of the upper amazon basin contributes to the exportation of organic carbon and associated metals in the black waters of the negro river watershed we have investigated the distribution of feiii in the clay size fraction of eight organic rich horizons of waterlogged plateau podzols to unravel the weathering conditions and mechanisms that control its transfer to the rivers the speciation and amount of feiii stored in residual mineral phases of laterites or bound to organic compounds of weakly and well expressed podzols were determined by electron paramagnetic resonance spectroscopy combined with chemical analyses reducing conditions restrict the production of organo fe complexes in the subsoil b horizons of waterlogged podzols and most of the fe2 released from the dissolution of fe oxides is exported to the rivers via the perched groundwater however significant amounts of diluted feiii bound to organic ligands feiiiom and nano fe oxides are produced at the margin of the depression in the topsoil a horizons of weakly expressed podzols due to shorter periods of anoxia the downward translocation of organically bound metals from topsoil a to subsoil b horizons of podzols occurs in shorter distances for fe than it does for al this separation of secondary fe species from al species is attributed to the physical fractionation of their organic carriers in texture contrasted b horizons of podzols as well as to the effect of ph on metal speciation in soil solutions and metal binding onto soil organic ligands mostly for al this leads us to consider the topsoil a horizons of weakly expressed podzols as well as the subsoil bh horizon of better expressed ones as the main sources for the transfer of feiiiom to the rivers the concentration of feiiiom rises from soil sources to river colloids suggesting drastic biogeochemical changes in more oxygenated black waters of the negro river watershed the contribution of soil organic matter to the transfer of fe to rivers is likely at the origin of the peculiar fe isotope pattern recently recognized in podzolic environments

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

the effects of weathering variability and anthropogenic pressures upon silicon cycling in an intertropical watershed tana river kenya abstract we present the first study of the si isotopic composition of dry season river waters from the tana river kenya data encompasses the entire river basin with samples collected from headwaters to the estuary thereby capturing a salinity gradient in the headwaters the isotopic signature is affected by climate as a result of its control on soil drainage and weathering the 30 si signatures in the basin range from 0 69 up to 2 23 signatures are clearly affected by dams an increase in 30 si ratios of 0 54 and a decrease in the dissolved si dsi concentration by 41 were observed downstream of the masinga dam the largest of a succession of 5 hydroelectric dams this reduction in si load is most likely due to increased diatom productivity as the corresponding change in 30 si signature is consistent with the known fractionation by these organisms the 30 si composition of waters entering the estuary is ca 2 and dsi concentrations are 349 m in the estuary the dsi concentrations decrease linearly following the salinity while the 30 si ratio remains stable indicating the absence of processes affecting the dsi pool for the studied range of salinity

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

constraining modern day silicon cycling in lake baikal constraining the continental silicon cycle is a key requirement in attempts to understand both nutrient fluxes to the ocean and linkages between silicon and carbon cycling over different time scales silicon isotope data of dissolved silica 30sidsi are presented here from lake baikal and its catchment in central siberia as well as being the world s oldest and voluminous lake lake baikal lies within the seventh largest drainage basin in the world and exports significant amounts of freshwater into the arctic ocean data from river waters accounting for 92 of annual river inflow to the lake suggest no seasonal alteration or anthropogenic impact on river 30sidsi composition the absence of a change in 30sidsi within the selenga delta through which 62 of riverine flow passes suggests a net balance between biogenic uptake and dissolution in this system a key feature of this study is the use of 30sidsi to examine seasonal and spatial variations in dsi utilization and export across the lake using an open system model against deepwater 30sidsi values from the lake we estimate that 20 24 of dsi entering lake baikal is exported into the sediment record while highlighting the impact that lakes may have upon the sequestration of continental dsi mixed layer 30sidsi values from 2003 and 2013 show significant spatial variability in the magnitude of spring bloom nutrient utilization with lower rates in the north relative to south basin

clay mineral composition of river sediments in the amazon basin abstract clay minerals are important in evaluating the maturity of suspended sediments weathering intensity and source area however there are processes that can change the mineral assemblage such as river transportation deposition remobilization and tributary inputs in terms of water discharge and sediment yield the amazon is one of the largest rivers in the world most of the suspended sediments come from the andes crossing the lowlands before reaching the ocean this study measures the spatial distribution of clay mineral assemblages over the entire amazon basin the results obtained show the main features of the amazon river main stem and larger tributaries from their sources to their confluence clay mineral composition highlights the evolution of the madeira and maranon solimoes river which start in the andes with high illite chlorite content downstream smectite contents increase moreover all shield tributaries show high kaolinite content the lower amazon river is characterized by relative high smectite content different from the andean sources the clay mineral results show that suspended sediments of the amazon river have three main sources 1 the andes mountains 2 the amazon shields and 3 the piedmont basins especially the pastaza alluvial megafan and the fitzcarrald arch basin lateral bank erosion plays also a significant role by the introduction of more mature sediments into the river enriched in smectite

geomorphic regime modulates hydrologic control of chemical weathering in the andes amazon abstract the interplay between the hydrologic processes that supply store and route water in catchment systems and the chemical weathering reactions that add and remove solutes acts as an important control on chemical weathering fluxes in this study we use paired measurements of solute chemistry and runoff in four nested catchments that span the transition from the andes mountains to the amazonian foreland floodplain in peru in order to investigate the links between hydrology and weathering processes and to determine how these links change across a geomorphic gradient all of the sites show variation in elemental concentrations and ratios with runoff consistent with hydrologically driven changes in lithologic sources the extent of secondary mineral precipitation and potentially fluid flow paths in the andean sites solute concentrations are relatively constant despite large changes in runoff this is in direct contrast to the foreland floodplain site where solute concentrations are diluted as runoff increases in the andes amazon the concentration runoff behavior is correlated with the mean catchment slope angle which suggests that erosional processes by modulating the timescales over which weathering reactions occur within the critical zone can be an underlying control on solute production and therefore on chemical weathering due to the co variation between the geomorphic and hydrologic controls on chemical weathering weathering fluxes in andean sites are more sensitive to seasonal changes in runoff than in the foreland floodplain site

analysis of glacial earthquakes in 2003 ekstrom et al reported on the detection of a new class of earthquakes that occur in glaciated regions with the vast majority being in greenland the events have a characteristic radiation pattern and lack the high frequency content typical of tectonic earthquakes it was proposed that the events correspond to large and sudden sliding motion of glaciers here we present an analysis of all 184 such events detected in greenland between 1993 and 2005 fitting the teleseismic long period surface waves to a landslide model of the source we obtain improved locations timing force amplitudes and force directions after relocation the events cluster into seven regions all of which correspond to regions of very high ice flow and most of which are named outlet glaciers these regions are daugaard jensen glacier kangerdlugssuaq glacier helheim glacier the southeast greenland glaciers the northwest greenland glaciers rinks isbrae and jakobshavn isbrae event amplitudes range from 0 1 to 2 0 10 14 kg m force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s each region has a different temporal distribution of events all glaciers are more productive in the summer but have their peak activity in different months over the study period kangerdlugssuaq has had a constant number of events each year whereas jakobshavn had most events in 1998 1999 and the number of events in helheim and the northwest greenland glaciers has increased substantially between 1993 and 2005 the size distribution of events in kangerdlugssuaq is peaked above the detection threshold suggesting that glacial earthquakes have a characteristic size

the dynamics of si cycling during weathering in two small catchments in the black forest germany traced by si isotopes abstract silicon stable isotopes have emerged as a powerful proxy to investigate weathering because si uptake from solution by secondary minerals or by the vegetation causes significant shifts in the isotope composition in this study we determined the si isotope compositions of the principle si pools within two small catchments located on sandstone and paragneiss respectively in the temperate black forest germany at both settings clay formation is dominated by mineral transformation preserving largely the signature of parental minerals with 30si values of around 0 7 bulk soils rich in primary minerals are similar to bulk parental material with 30si values close to 0 4 topsoils are partly different because organic matter degradation has promoted intense weathering leading to 30si values as low as 1 0 water samples expose highly dynamic weathering processes in the soil zone 1 after spring snowmelt increased release of doc and high water fluxes trigger clay mineral dissolution which leads to 30si values down to 0 7 and 2 in course of the summer si uptake by the vegetation and secondary mineral formation drives dissolved si to typical positive 30si values up to 1 1 groundwater with 30si values of around 0 4 records steady processes in bedrock reflecting plagioclase weathering together with kaolinite precipitation an isotope mass balance approach reveals incongruent weathering conditions where denudation of si is largely driven by physical erosion erosion of phytoliths contributes 3 to 21 to the total si export flux which is in the same order as the dissolved si flux these results elucidate the si dynamics during weathering on catchments underlain of sedimentary origin prevailing on the earth surface and provide therefore valuable information to interpret the isotope signature of large river systems

riverine li isotope fractionation in the amazon river basin controlled by the weathering regimes we report li isotope composition 7li 7li of river borne dissolved and solid material in the largest river system on earth the amazon river basin to characterize li isotope fractionation arl a continental scale the 7li 7li in the dissolved load 1 2 to 32 is fractionated toward heavy values compared to the inferred bedrock 1 to 5 and the suspended sediments 6 8 to 0 5 as a result of the preferential incorporation of 66li into secondary minerals during weathering despite having very contrasted weathering and erosion regimes both andean headwaters and lowland rivers share similar ranges of dissolved 7li 7li 1 2 to 18 correlations between dissolved 7li 7li and li na and li mg ratios suggest that the proportion of li incorporated in secondary minerals during weathering act as the main control on the 7lidiss 7lidiss across the entire amazon basin a batch steady state fractionation model for andean and lowland rivers satisfactorily reproduces these variations with a fractionation factor between weathering products and dissolved load sec dis sec dis of 0 983 two types of supply limited weathering regimes can be identified for the lowlands clearwaters with dominant incorporation of li in secondary minerals and black waters e g rio negro where dissolution of secondary minerals enhanced by organic matter produces low 7li 7li apart from the black waters the 7li 7li of andean and lowland rivers is negatively correlated to the denudation rates with the lowest 7li 7li corresponding to the rivers having the highest denudation rates in contrast the main tributaries draining both the andes and the lowlands have higher 7li 7li compared to other rivers we propose that part of the dissolved li derived from weathering in the andes is re incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved 7li 7li along the course of these rivers unlike other rivers the dissolved 7li 7li in the main tributaries is best described by a rayleigh fractionation model with a fractionation factor sec dis sec dis of 0 991 altogether the control imposed by residence time in the weathering zone and floodplain processes results in i a non linear correlation between dissolved 7li 7li and the weathering intensity defined as w d and ii a positive relationship between the dissolved li flux and the denudation rate these results have important implications for the understanding of past ocean 7li 7li and its use as a paleo weathering proxy

stable silicon isotope signatures of marine pore waters biogenic opal dissolution versus authigenic clay mineral formation dissolved silicon isotope compositions have been analysed for the first time in pore waters 30sipw of three short sediment cores from the peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments the 30sipw varies between 1 1 and 1 9 with the highest values occurring in the uppermost part close to the sediment water interface these values are of the same order or higher than the 30si of the biogenic opal extracted from the same sediments 0 3 to 1 2 and of the overlying bottom waters 1 1 to 1 5 together with dissolved silicic acid concentrations well below biogenic opal saturation our collective observations are consistent with the formation of authigenic alumino silicates from the dissolving biogenic opal using a numerical transport reaction model we find that approximately 24 of the dissolving biogenic opal is re precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 mol si cm 2 yr 1 the fractionation factor between the precipitates and the pore waters is estimated at 2 0 dissolved and solid cation concentrations further indicate that off peru where biogenic opal concentrations in the sediments are high the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino silicate phases

greenland rumbles louder as glaciers accelerate flow rates of many large glaciers in greenland and antarctica have accelerated recently greenland earthquakes produced from glacier motion and calving have also increased dramatically

response of glacier basal motion to transient water storage the speed of a glacier is affected most by sudden jumps in the water supply to the glacier but it goes back to previous levels if high water inputs are sustained because the glacier s plumbing system adjusts

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

ge si ratios indicating hydrothermal and sulfide weathering input to rivers of the eastern tibetan plateau and mt baekdu abstract concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals formation and weathering of secondary clay minerals hydrothermal input and biological cycling formation and dissolution of opal phytoliths and growth of diatoms we applied the ge si ratio to assess the different sources of dissolved si in rivers hailing from the eastern tibetan plateau the salween mekong chang jiang yangtze hong red and huang he yellow and from mt baekdu the duman elevated riverine ge si ratios were observed in arid regions with high geothermal activity in the salween chang jiang and mt baekdu streams in the huang he and hong river basins geothermal influence was not as pronounced but weathering of sulfide and coal bearing minerals may be responsible for the high ge si ratios in rivers where inputs from hydrothermal and sulfide weathering are minimal our data mostly fall in the weathering limited regime of high riverine si concentrations and low ge si ratios

silicon isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe abstract silicon si stable isotopes have the potential to become a useful weathering proxy given that light si isotopes are preferentially incorporated into secondary clay minerals here we investigate how si depletion in soils and associated clay mineralogy influence the si isotope fractionation associated with clay mineral formation we report 30 si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from guadeloupe that were formed under contrasting climatic conditions strongly desilicated soils containing kaolinite that formed in wet areas high precipitation are compared with less desilicated soils containing smectite formed in drier conditions low precipitation clay fractions are isotopically lighter than the parent andesite 30 si 0 23 and increasingly lighter with si depletion in soils which supports the view that the si isotope composition in secondary clay fractions is controlled by the degree of soil desilication it is shown that the si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for si rich secondary clay minerals such as smectite and larger for si poor secondary clay minerals such as kaolinite this study provides new insights to better define si isotopes as a proxy for environmental conditions for clay neoformation

deep ocean nutrients during the last glacial maximum deduced from sponge silicon isotopic compositions the relative importance of biological and physical processes within the southern ocean for the storage of carbon and atmospheric pco2 on glacial interglacial timescales remains uncertain understanding the impact of surface biological production on carbon export in the past relies on the reconstruction of the nutrient supply from upwelling deep waters in particular the upwelling of silicic acid si oh 4 is tightly coupled to carbon export in the southern ocean via diatom productivity here we address how changes in deep water si oh 4 concentrations can be reconstructed using the silicon isotopic composition of deep sea sponges we report 30si of modern deep sea sponge spicules and show that they reflect seawater si oh 4 concentration the fractionation factor of sponge 30si compared to seawater 30si shows a positive relationship with si oh 4 which may be a growth rate effect application of this proxy in two down core records from the scotia sea reveals that si oh 4 concentrations in the deep southern ocean during the last glacial maximum lgm were no different than today our result does not support a coupling of carbon and nutrient build up in an isolated deep ocean reservoir during the lgm our data combined with records of stable isotopes from diatoms are only consistent with enhanced lgm southern ocean nutrient utilization if there was also a concurrent reduction in diatom silicification or a shift from siliceous to organic walled phytoplankton

the silicon isotopic composition of fine grained river sediments and its relation to climate and lithology abstract the 30si stable isotopic composition of silicon in soils and fine grained sediments can provide insights into weathering processes on continents with important implications on the si budget of modern and past oceans to further constrain the factors controlling the distribution of si isotopes in sediments we have analysed a large number n 50 of separate size fractions of sediments and suspended particulate materials collected near the mouth of rivers worldwide this includes some of the world s largest rivers e g amazon congo mackenzie mississippi murray darling nile yangtze and rivers from the case study areas of the congo river basin and northern ireland silt size fractions exhibit a mean si isotopic composition 30si 0 21 0 19 2 s d similar to that previously inferred for the upper continental crust in contrast clay size fractions display a much larger range of 30si values from 0 11 to 2 16 which yield a global 30siclay of 0 57 0 60 2 s d representative of the mean composition of the average weathered continental crust overall these new data show that the si isotopic signature transported by river clays is controlled by the degree of chemical weathering as inferred from strong relationships with al si ratios at a global scale the clay bound si isotopic composition of the world s largest river systems demonstrates a link with climate defining a general correlation with mean annual temperature mat in corresponding drainage basins while the distribution of si isotopes in river sediments also appears to be influenced by the tectonic setting lithological effects and sediment recycling from former sedimentary cycles our results pave the way for their use as paleo weathering and paleo climate proxies in the sedimentary record

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

turbulent mixing in the amazon river the isotopic memory of confluences rivers continuously discharge dissolved material to the oceans dissolved compounds partially result from water rock interactions which produce a large range of water chemical and isotopic compositions these waters are collected by rivers that are commonly assumed to be well mixed with regard to their different tributaries as a result of turbulent dispersion in this paper we test this hypothesis on the solimoes river at manacapuru the largest tributary of the amazon river by analyzing the sodium concentration and strontium isotopic composition of river water on a transverse section at different depths high precision measurements reveal lateral heterogeneities this reflects poor mixing between two main river masses that have distinct chemical and isotopic signatures a hundred kilometers downstream from their confluence the solimoes mainstream and the purus river using sodium concentration data the transverse dispersion coefficient is estimated for the studied solimoes reach the earth s largest river on which such an estimate now exists and is found to be 1 8 0 2 m2 s comparison with previously reported data highlights the potential role of bed morphology and islands in the efficiency of lateral mixing in large rivers we finally demonstrate that the characteristic length of lateral mixing downstream from confluences in large rivers is at least of several tens of kilometers

seasonality and increasing frequency of greenland glacial earthquakes some glaciers and ice streams periodically lurch forward with sufficient force to generate emissions of elastic waves that are recorded on seismometers worldwide such glacial earthquakes on greenland show a strong seasonality as well as a doubling of their rate of occurrence over the past 5 years these temporal patterns suggest a link to the hydrological cycle and are indicative of a dynamic glacial response to changing climate conditions

a silicon depleted north atlantic since the palaeogene evidence from sponge and radiolarian silicon isotopes despite being one of earth s major geochemical cycles the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica dsi concentration through geologic time remain poorly constrained silicon isotope ratios expressed as 30si in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling in particular the 30si of siliceous sponge spicules has been demonstrated to be a useful proxy for past dsi concentrations we analysed 30si in radiolarian tests and sponge spicules from the blake nose palaeoceanographic transect odp leg 171b spanning the palaeocene eocene ca 60 30 ma our 30si results range from 0 32 to 1 67 and 0 48 to 0 63 for the radiolarian and sponge records respectively using an established relationship between ambient dissolved si dsi concentrations and the magnitude of silicon isotope fractionation in siliceous sponges we demonstrate that the western north atlantic was dsi deplete during the palaeocene eocene throughout the water column a conclusion that is robust to a range of assumptions and uncertainties these data can constitute constraints on reconstructions of past ocean circulation previous work has suggested ocean dsi concentrations were higher than modern ocean concentrations prior to the cenozoic and has posited a drawdown during the early palaeogene due to the evolutionary expansion of diatoms our results challenge such an interpretation we suggest here that if such a global decrease in oceanic dsi concentrations occurred it must predate 60 ma

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

silicon isotope fractionation by marine sponges and the reconstruction of the silicon isotope composition of ancient deep water the silicon isotope composition d30si of biogenic opal provides a view of the silica cycle at times in the past reconstructions require the knowledge of silicon isotope fractionation during opal biomineralization the d30si of specimens of hexactinellid sponges and demosponges growing in the modern ocean ranged from 1 2 to 3 7 n 6 corresponding to the production of opal that has a d30si value 3 8 0 8 more negative than seawater silicic acid and a fractionation factor a of 0 9964 this is three times the fractionation observed during opal formation by marine diatoms and terrestrial plants and is the largest fractionation of silicon isotopes observed for any natural process on earth the d30si values of sponge spicules across the eocene oligocene boundary at ocean drilling program site 689 on maud rise range from 1 1 to 3 0 overlapping the range observed for sponges growing in modern seawater

the silicon isotope composition of granites because weathering results in preferential incorporation of the lighter si isotopes into clay minerals the si in magmas derived from meta sedimentary lithologies enriched in such phases should also be isotopically light relative to melts derived from igneous sources with the advent of new high precision techniques it should therefore be possible to resolve such differences hence si isotopes have to potential to distinguish between and characterise different magmatic source regions to explore this here we report the si isotopic compositions of a suite of phanerozoic i s and a type granites and mineral separates as well as samples of associated country rock the whole rock data fall between 30si 0 40 and 0 11 0 04 95 s e this range is more limited than that defined by previous low precision measurements of granites but is significantly broader than that displayed by more recent high precision measurements of high si extrusive igneous rocks as predicted s type granites are on average isotopically lighter than i and a type samples and negative correlations with alumina saturation index asi values and initial 87sr 86sr provide evidence that si isotope variations in granites can be explained by variable incorporation of an enriched sedimentary component however the relatively small isotopic perturbations and apparent degree of overlap between granite affinities suggest that si isotopes are not as sensitive to sedimentary input relative to more established proxies such as for example o isotopes the average si isotope value of all granites analysed in this study is 30si 0 23 0 15 2 s d which is entirely consistent with a recently published estimate for the continental crust based on an andesitic bulk composition and the well defined relationship between si isotopes and magmatic fractionation