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

absolute accumulation and isotope fractionation of si and fe during dolomite weathering and terra rossa formation abstract the behavior of silicon si and iron fe and their isotope fractionation during carbonate weathering and terra rossa formation are unclear here we focus on a 4 05 m thick terra rossa profile developed on dolomite in the yunnan guizhou plateau china we measured the mineral and geochemical composition magnetic susceptibility and si and fe isotopic composition of the terra rossa ferromanganese concretions fmcs and the dolomite and its insoluble residues the results indicate that there is absolute accumulation of si and fe during dolomite weathering and terra rossa formation due to the extraneous input of dissolved si and fe this means that the insoluble residues of dolomite are not the only source for terra rossa formation in addition we suggest that the high frequency dependent magnetic susceptibility fd and rubification of the terra rossa are the result of the oxidation of extraneous dissolved fe and the neoformation of finely dispersed fe oxides overall the terra rossa is characterized by the enrichment of light si and fe isotopes compared with insoluble dolomite residues and bulk dolomite the extraneous input of dissolved fe is characterized by a light isotopic signature whereas the si isotopic signature of the extraneous source is unclear due to modification by late desilicification si isotope fractionation in the terra rossa is mainly controlled by chemical weathering and associated mobilization and redistribution of si the 30 si values of terra rossa decrease with increasing weathering intensity while the re precipitation of mobilized si leads to the significant enrichment of isotopically heavy si in the basal terra rossa the 30 si values of terra rossa are not significantly affected by the adsorption of the fe and al oxyhydr oxides by contrast the major process that fractionates fe isotopes in the dolomite terra rossa weathering system is unclear in the terra rossa profile the 56 fe values of the fmcs are significantly lower than those of the surrounding terra rossa matrix these low 56 fe signatures of the fmcs reflect the preferential translocation of the light fe isotope while the fmcs may be able to record the low 56 fe characteristics of pore water

geology and geochemistry of the yuanjiacun banded iron formation in shanxi province china constraints on the genesisthe genesis of the yuanjiacun bify men et al

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

micrometer scale chemical and isotopic criteria o and si on the origin and history of precambrian cherts implications for paleo temperature reconstructions abstract oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the precambrian these reconstructions have been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography criterion 1 chert is composed mostly of microquartz on the bulk oxygen isotopic composition criterion 2 bulk 18 o has to be close enough to the maximum 18 o value previously measured in other cherts of the same age and on the presence of a large 18 o range at the micrometer scale criterion 3 18 o range of 10 at 2 m however these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts we report in situ si and o isotope compositions and trace element concentrations in seven chert samples ranging from 1 88 to 3 5 ga in age correlations between 30 si and al 2 o 3 and k 2 o tio 2 reveal that microquartz is of three different origins i e diagenetic hydrothermal or silicification moreover chert samples composed mostly of diagenetic microquartz show a large range of 30 si at the micrometer scale 1 7 4 5 consistent with the large range of 18 o previously found in the gunflint diagenetic cherts we propose two further quantitative criteria to assess the origin state of preservation and diagenetic history of cherts criterion 4 uses trace element concentrations coupled with 30 si to ascribe the origin of cherts among three possible end members diagenetic hydrothermal and silicified criterion 5 is the presence of a large range of 30 si in pure diagenetic microquartz in the seven samples analyzed in this study only one from the gunflint iron formation at 1 88 ga passes all the criteria assessed here and can be used for paleo temperature reconstructions the criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis

coordinated spectral and xrd analyses of magnesite nontronite forsterite mixtures and implications for carbonates on mars 1 mineral detection on mars largely relies on laboratory data of minerals and mineral mixtures the objective of this study is to provide reflectance spectra in the visible near infrared vnir and mid ir regions x ray diffraction xrd data and mossbauer spectra of a suite of carbonate phyllosilicate and olivine mixtures in order to facilitate identification and characterization of these minerals on mars remote sensing observations indicate that combinations of these minerals are present in ancient rocks on mars around the isidis basin and in gusev crater magnesite nontronite and forsterite size fractions 125 m were selected for this study results of the vnir reflectance analyses illustrate the complexity of vnir spectra of mixtures analyses of the nir band depths near 2 3 2 5 3 4 and 4 m showed clear trends with carbonate abundance although the data are not linear mixtures of magnesite and nontronite exhibited a band near 2 3 m much closer to that observed for nontronite than that for magnesite vnir analyses of the mixtures indicated that a small amount of forsterite in any of the mixtures contributed a large increase in the broad 1 m band and hence the red slope characteristic of fe2 bearing minerals mid ir mixture spectra were dominated by magnesite and forsterite and nontronite was much more difficult to detect by mid ir spectra in the mixtures this could be related to why phyllosilicates are detected in many locations on mars using data collected by the compact reconnaissance imaging spectrometer for mars but not detected using data collected by the thermal emission spectrometer mossbauer spectroscopy is well suited for analyses of fe2 and fe3 bearing minerals and modeling of the peak areas gave well correlated trends for nontronite and forsterite abundances where abundant fe was present xrd full pattern fitting analyses were performed on the magnesite forsterite series giving results within 6 wt of the actual values with a mean difference between actual and calculated values of 2 4 wt this study provides important laboratory data for characterizing the spectral and xrd properties of mineral mixtures that will facilitate mineral identification on mars carbonates in particular have been primarily observed at low abundances and in small outcrops and they are frequently found mixed with other minerals through analyses of mineral mixtures using multiple data sets this study seeks to provide ground truthing that will enable better coordination of carbonate detections in the dust and rocks of mars

germanium silicon fractionation in a tropical granitic weathering environment abstract germanium silicon ge si ratios were determined on quartz diorite bedrock saprolite soil primary and secondary minerals phytolith soil and saprolite pore waters and spring water and stream waters in an effort to understand ge si fractionation during weathering of quartz diorite in the rio icacos watershed puerto rico the ge si ratio of the bedrock is 2 mol mol with individual primary mineral phases ranging between 0 5 and 7 mol mol the ratios in the bulk saprolite are higher 3 mol mol than values measured in the bedrock the major saprolite secondary mineral kaolinite has ge si ratios ranging between 4 8 and 6 1 mol mol the high ge si ratios in the saprolite are consistent with preferential incorporation of ge during the precipitation of kaolinite bulk shallow soils have lower ratios 1 1 1 6 mol mol primarily due to the residual accumulation of ge poor quartz ge si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths spring water and baseflow stream waters have the lowest ge si ratios 0 27 0 47 mol mol reflecting deep initial weathering reactions resulting in the precipitation of ge enriched kaolinite at the saprolite bedrock interface mass balance calculations on saprolite require significant loss of si and al even within 1 m above the saprolite bedrock interface higher pore water ge si ratios 1 2 mol mol are consistent with partial dissolution of this ge enriched kaolinite pore water ge si ratios increase up through the saprolite and into the overlying soil but never reach the high values predicted by mass balance perhaps reflecting the influence of phytolith recycling in the shallow soil

expert system based mineral mapping in northern death valley california nevada using the airborne visible infrared imaging spectrometer aviris abstract integrated analysis of imaging spectrometer data and field spectral measurements were used in conjunction with conventional geologic field mapping to characterize bedrock and surficial geology at the northern end of death valley california and nevada a knowledge based expert system was used to automatically produce image maps showing the principal surface mineralogy from airborne visible infrared imaging spectrometer aviris data linear spectral unmixing of the aviris data allowed further determination of relative mineral abundances and identification of mineral assemblages and mixtures the imaging spectrometer data show the spatial distribution of spectrally distinct minerals occurring both as primary rockforming minerals and as alteration and weathering products field spectral measurements were used to verify the mineral maps and field mapping was used to extend the remote sensing results geographically referenced image maps produced from these data form new base maps from which to develop improved understanding of the prosesses of deposition and erosion affecting the present land surface

weathering dust and biocycling effects on soil silicon isotope ratios abstract silicon isotope ratios 30si of bulk mineral materials in soil integrate effects from both silicon sources and processing here we report 30si values from a climate gradient of hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy the results demonstrate informative relationships between the mass fraction of soil si depletion and 30si in upper 1 m deep soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved si in basalt watershed rivers and driven by preferential 28si removal from the dissolved phase during precipitation in contrast after correcting for the influence of dust secondary mineral si depletion and 30si values in shallow

micrometer silicon isotope diagnostics of soils by uv femtosecond laser ablation this study presents the first si isotope data of the principle si pools in soils determined by a uv femtosecond laser ablation system coupled to a multicollector inductively coupled plasma mass spectrometer mc icp ms this method reveals accurate and precise si isotope data on bulk materials and at high spatial resolution on the mineral scale the following si pools have been investigated a the si source to soils on all major silicate minerals on thin sections from bedrock fragments in the soil profiles b bulk soils particle size 2 mm after fusion to glass beads with an iridium strip heater or pressed into powder pellets c separated clay fractions as pressed powder pellets and e separated phytoliths as pressed powder pellets multiple analyses of three rock standards bhvo 2 agv 1 and rgm 1 as fused glass beads and as pressed powder pellets reveal 30si values within the expected range of igneous rocks the mpi ding reference glass kl2 g exhibits the same si isotope composition after remelting by an iridium strip heater showing that this technique does not alter the isotope composition of the glass we used this approach to investigated two immature cambisols developed on sandstone and paragneiss in the black forest germany respectively bulk soils show a largely uniform si isotope signature for different horizons and locations which is close to those of primary quartz and feldspar with 30si values around 0 4 soil clay formation is associated with limited si mobility which preserves initial si isotope signatures of parental minerals an exception is the organic horizon of the paragneiss catchment where intense weathering leads to a high mobility of si and significant negative isotope signatures as low as to 1 00 in bulk soils biogenic opal in the form of phytoliths exhibits negative si isotope signatures of about 0 4 these results demonstrate that uv femtosecond laser ablation mc icp ms provides a tool to characterize the si isotope signature of the principle si pools left behind after weathering and si transport have altered soils these results can now serve as a fingerprint of the residual solids that can be used to explain the isotope composition of dissolved si in soil solutions and river water which is mostly enriched in the heavy isotopes

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

hydrothermal formation of clay carbonate alteration assemblages in the nili fossae region of mars abstract the compact reconnaissance imaging spectrometer for mars crism has returned observations of the nili fossae region indicating the presence of mg carbonate in small

terrestrial analogs for interpretation of infrared spectra from the martian surface and subsurface sulfate nitrate carbonate and phyllosilicate bearing atacama desert soils 1 hyperarid 6 5 m was not suited for salt and phyllosilicate detection because of overlapping spectra from primary silicates e g feldspar water and carbon dioxide the visible near infrared 0 35 2 5 m region was suited for detecting carbonate nitrate gypsum water of hydration and phyllosilicate hydroxyls without interference from primary silicates however gypsum water of hydration features can obscure phyllosilicate hydroxyl carbonate and nitrate features if gypsum levels are high overtone combination absorption features in the midinfrared were determined to be the best indicators of sulfate 4 48 4 70 m nitrate 4 12 m and carbonate 3 98 m because interferences from overlapping primary silicate and water features are not present in this region interferences from co2 and thermal emission effects in the overtone combination region are possible but may be minimized by corrective techniques infrared analysis of atacama desert soils can provide insight into the spectral search of sulfate nitrate carbonate and phyllosilicate containing soils on mars

identification of spectrally similar materials using the usgs tetracorder algorithm the calcite epidote chlorite problem a scheme to discriminate and identify materials having overlapping spectral absorption features has been developed and tested based on the u s geological survey usgs tetracorder system the scheme has been applied to remotely sensed imaging spectroscopy data acquired by the airborne visible and infrared imaging spectrometer aviris instrument this approach was used to identify the minerals calcite epidote and chlorite in the upper animas river watershed colorado the study was motivated by the need to characterize the distribution of calcite in the watershed and assess its acid neutralizing potential with regard to acidic mine drainage identification of these three minerals is difficult because their diagnostic spectral features are all centered at 2 3 m and have similar shapes and widths previous studies overestimated calcite abundance as a result of these spectral overlaps the use of a reference library containing synthetic mixtures of the three minerals in varying proportions was found to simplify the task of identifying these minerals when used in conjunction with a rule based expert system some inaccuracies in the mineral distribution maps remain however due to the influence of a fourth spectral component sericite which exhibits spectral absorption features at 2 2 and 2 4 m that overlap the 2 3 m absorption features of the other three minerals whereas the endmember minerals calcite epidote chlorite and sericite can be identified by the method presented here discrepancies occur in areas where all four occur together as intimate mixtures it is expected that future work will be able to reduce these discrepancies by including reference mixtures containing sericite

the application of aster remote sensing data to porphyry copper and epithermal gold deposits abstract this paper reviews the performance characteristics of the advanced spaceborne thermal emission and reflection radiometer aster remote sensor the standard data products and applications of the most recently developed image processing methods applied to aster data as a tool for mapping hydrothermal alteration mineral zones associated with porphyry copper and epithermal gold mineralization and related host rock lithology hydrothermal alteration zones associated with porphyry copper deposit such as phyllic argillic and propylitic mineral assemblages can be discriminated from one another by virtue of their spectral absorption features which are detectable by aster swir spectral bands the identification of the phyllic zone is important in the initial stages of porphyry copper exploration as an indicator of high economic potential for copper mineralization two new crosstalk corrected aster swir reflectance products including ast 07xt and refl1b are more reliable than previous aster data products for regional mineral mapping without use of additional spectral data from the site for calibration four types of algorithms were used to extract spectral information of aster data 1 band ratio indices and logical operator based methods 2 principal components and enhancement based methods such as principal component analysis pca and minimum noise fraction mnf 3 shape fitting based algorithms such as spectral angle mapper sam matched filtering mf and mixture tuned matched filtering mtmf and 4 partial unmixing methods such as linear spectral unmixing lsu and constrained energy minimization cem this review emphasizes that the logical operator algorithms can be best suited for hydrothermal alteration mineral mapping including phyllic and argillic zones associated with porphyry copper mineralization in a regional scale shape fitting based and partial unmixing algorithms are robust and reliable for detecting particular mineral and mineral assemblages in hydrothermal alteration zones in a district scale consequently the integration of the results derived from the logical operator shape fitting based and partial unmixing algorithms can produce comprehensive and accurate information for the reconnaissance stages of copper and gold exploration at both regional and district scales all of the methods and applications reviewed in this paper demonstrate the utility of aster data for exploration of the porphyry copper and epithermal gold deposits around the world

imaging spectroscopy earth and planetary remote sensing with the usgs tetracorder and expert systems 1 imaging spectroscopy is a tool that can be used to spectrally identify and spatially map materials based on their specific chemical bonds spectroscopic analysis requires significantly more sophistication than has been employed in conventional broadband remote sensing analysis we describe a new system that is effective at material identification and mapping a set of algorithms within an expert system decision making framework that we call tetracorder the expertise in the system has been derived from scientific knowledge of spectral identification the expert system rules are implemented in a decision tree where multiple algorithms are applied to spectral analysis additional expert rules and algorithms can be applied based on initial results and more decisions are made until spectral analysis is complete because certain spectral features are indicative of specific chemical bonds in materials the system can accurately identify and map those materials in this paper we describe the framework of the decision making process used for spectral identification describe specific spectral feature analysis algorithms and give examples of what analyses and types of maps are possible with imaging spectroscopy data we also present the expert system rules that describe which diagnostic spectral features are used in the decision making process for a set of spectra of minerals and other common materials we demonstrate the applications of tetracorder to identify and map surface minerals to detect sources of acid rock drainage and to map vegetation species ice melting snow water and water pollution all with one set of expert system rules mineral mapping can aid in geologic mapping and fault detection and can provide a better understanding of weathering mineralization hydrothermal alteration and other geologic processes environmental site assessment such as mapping source areas of acid mine drainage has resulted in the acceleration of site cleanup saving millions of dollars and years in cleanup time imaging spectroscopy data and tetracorder analysis can be used to study both terrestrial and planetary science problems imaging spectroscopy can be used to probe planetary systems including their atmospheres oceans and land surfaces

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

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

mapping hydrothermal alteration zone through aster data in gadag schist belt of western dharwar craton of karnataka india alteration zones are developed in a wide range of geological milieus such as volcano plutonic rocks hydrothermal alteration zones are considered an important guide for mineral exploration and can act as a potential zone of mineralization the minerals in alteration zones have very diagnostic absorption features in visible near infrared vnir and shortwave infrared swir regions of the electromagnetic spectrum the advanced space borne thermal emission and reflection radiometer aster data have the potential to identify mineral assemblages in different hydrothermal alteration zones because of its 14 spectral bands which cover a wide spectral range in the present article fcc band ratio and spca techniques were applied to map the hydrothermal alteration zones in gadag schist belt gsb of western dharwar craton wdc of karnataka fcc has highlighted lithology alteration zones and given a fair idea about the lithological control and alteration occurrences in the area band ratio technique for muscovite kaolinite chlorite iron oxide gossan and silica alteration have yielded promising results spca analysis successfully mapped hydrothermal alteration and delineated phyllic argillic and propylitic zones in the area these results were verified through the existing geological map and by geological fieldwork in the area and these results are satisfactory the total area altered by different alteration process was also calculated in the gsb iron alteration zone phyllic argillic propylitic and silica alteration zones were effectively mapped with the help of aster data

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

revised crism spectral parameters and summary products based on the currently detected mineral diversity on mars the investigation of hyperspectral data from the mars reconnaissance orbiter compact reconnaissance imaging spectrometer for mars crism and the observatoire pour la mineralogie l eau les glaces et l activitie omega on mars express has revealed an increasingly diverse suite of minerals present on the martian surface a revised set of 60 spectral parameters derived from corrected spectral reflectance at key wavelengths in crism targeted observations and designed to capture the known diversity of surface mineralogy on mars is presented here as summary products some of the summary products have strong heritage to omega spectral parameter calculations this paper also presents newly derived parameters that highlight locations with more recently discovered spectral signatures type locations for the diversity of currently identified mineral spectral signatures have been compiled into a library presented in this work our analysis indicates that the revised set of summary products captures the known spectral diversity of the surface and successfully highlights and differentiates between locations with differing spectral signatures the revised spectral parameter calculations and related products provide a useful tool for scientific interpretation and for future mission landing site selection and operations

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

interpretation of the environmental change of dongting lake middle reach of yangtze river china by 210pb measurement and satellite image analysis abstract the present study examined the recent environmental history of the dongting lake located in the middle yangtze river region central china the sediment was recovered by 22 vibrocores in different lake sectors primarily characterizing subaqueous delta and bay subfacies high sedimentation rates revealed by 210pb measurement in different subfacies of the lake ranged from 0 77 to 2 33 cm year these rates equate to deltaic sedimentation associated with mobile channels resulting from yangtze flood events satellite images confirm that the dongting lake does not contain the biggest yangtze floods moreover comparative bathymetric surveys verify that the dongting lake has lost almost two thirds of its total area in the past century due primarily to siltation by yangtze sediment in the late 19th century a major flood induced channel avulsion resulted in the coupling of the four tributary subbasin to the dongting lake causing a massive increase in the sediment supply into the lake since then hydrological data demonstrate that about 83 of the lake s annual sediment influx are derived from the yangtze in addition pressures from a growing population and associated human activities such as reclamation embanking aquaculture slope modification and deforestation are compounding the loss of the lake surface area and associated storage capacity

human impacts on the changjiang yangtze river basin china with special reference to the impacts on the dry season water discharges into the sea abstract the annual mean discharge from the upper changjiang yangtze basin has shown a significant decreasing trend since the end of the 19th century since the 1970s the monthly mean discharge to the sea has also shown a dramatic decrease during dry seasons this paper examines the human impacts on the major hydrological processes in the changjiang river basin with a special focus on their influence on the discharge from the drainage basin to the sea during the dry season climatic warming has been obvious since the 1960s in the headwater area resulting a continuous retreat of glaciers while the increased evaporation is responsible for the dropping of lake water levels and decrease in water area such a trend continuing into the coming decades will significantly change the seasonal hydrological processes especially the dry season discharges from the upper basin the decreasing vegetation cover and the increasing reservoir volume capacity also impacted on the water discharge over the past decades although in different ways the possible impacts of the three gorges dam on the monthly variation of water discharge to the sea are discussed with special emphasis in the middle basin discussions are focused on the effect of decreasing lake area of increasing reservoir capacity and of irrigated agriculture on the temporal changes of water discharge since the 1950s the human impacts on water discharge from the lower basin to the sea are mostly attributed to water transfer to both tributary and neighboring drainage basins by a large number of electric pumping stations and sluices the total water transferring capacity is more than 5000 m3 s along the lower river studies indicate that in a dry season the water discharge to the sea is greatly reduced and results in strong saltwater intrusion in the estuary

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

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

the silicon isotope composition of the upper continental crust the upper continental crust ucc is the major source of silicon si to the oceans and yet its isotopic composition is not well constrained in an effort to investigate the degree of heterogeneity and provide a robust estimate for the average si isotopic composition of the ucc a representative selection of well characterised continentally derived clastic sediments have been analysed using high precision mc icpms analyses of loess samples define a narrow range of si isotopic compositions 30si 0 28 to 0 15 this is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples the average loess 30si is 0 22 0 07 2 s d identical to average granite and felsic igneous compositions therefore minor chemical weathering does not resolvably affect bulk rock 30si and loess is a good proxy for the si isotopic composition of unweathered crystalline continental crust the si isotopic compositions of shales display much more variability 30si 0 82 to 0 00 shale si isotope compositions do not correlate well with canonical proxies for chemical weathering such as cia values but do correlate negatively with insoluble element concentrations and al si ratios this implies that more intensive or prolonged chemical weathering of a sedimentary source with attendant desilicification is required before resolvable negative si isotopic fractionation occurs shale 30si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine derived silica in such samples using the data gathered in this study combined with already published granite si isotope analyses a weighted average composition of 30si 0 25 0 16 2 s d for the ucc has been calculated

human impact on floods and flood disasters on the yangtze river abstract in the middle reaches of the yangtze river the floods have become more and more frequent and the water level rises higher than before the damages are becoming ever more serious this is primarily a consequence of human activity in the river basin three aspects deserve particular attention first destruction of vegetation has led to soil erosion in the upper reaches in the past 30 years the forest cover has been reduced to half while the area exposed to severe erosion doubled in size in the long run this can be expected to increase flooding second land reclamation and siltation has reduced lake sizes this has resulted in decrease of the flood storage capacity third the construction of levees has caused flood levels to rise due to restricted flood discharge capacity establishment of the great jinjiang levee caused silting up of the riverbed and valley in the mid reaches of yangtze consequently the discharge capacity decreased to 60 000 68 000 m 3 s which is sufficient only for ordinary floods this article concludes that the deteriorating flood situation is the result of inappropriate human intervention in the natural environment it is suggested that the appropriate strategy should change from keeping the flood away to giving the flood way related tactics and strategies under consideration are briefly summarized

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

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

geochemistry of rare earth elements in the mainstream of the yangtze river china abstract water suspended matter and sediment samples were taken from 8 locations along the yangtze river in 1992 the concentration and speciation exchangeable bound to carbonates bound to fe mn oxides bound to organic matter and residual forms of rare earth elements la ce nd sm eu tb yb and lu were determined by instrumental neutron activation analysis inaa the contents of the soluble fraction of rees in the river are low and rees mainly reside in particulate form in the particles the chondrite normalized distribution patterns show significant lree enrichment and eu depletion while normalized to shales both sediments and suspended matter samples show relative lree enrichment and hree depletion rees are relatively enriched in fine grained fractions of the sediments the speciation characteristics of rees in the sediments and suspended matter are very similar the amount of the five forms follows the order residual bound to organic matter bound to fe mn oxides bound to carbonates exchangeable about 65 to 85 of rees in the particles exist in the residual form and the exchangeable form is very low high proportions of residual rees reveal that rees in sediments and suspended matter are controlled by their abundances in the earth s crust carbonate fe mn oxide and organic fractions of rees in sediments account for 2 4 6 9 5 2 11 1 and 7 3 14 0 of the total contents respectively they are similar to those in the suspended matter this shows that carbonates fe mn oxides and organic matter play important roles during the particle water interaction processes by normalization to shales the 3 forms of rees follow convex shapes according to atomic number with middle ree sm eu and tb enrichment while light ree and heavy ree are depleted

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

biogenic silica in wetlands and their relationship with soil and groundwater biogeochemistry in the southeastern of buenos aires province argentina although phytoliths constitute part of the wetland suspended load there are few studies focused on the quantification of them in the biogenic silica bsi pool so the aim of this paper is both to determine bsi content diatoms and phytoliths and its relationship with dissolved silica in surface waters and the influence of soil and groundwater si biogeochemistry in los padres wetland buenos aires province argentina in the basin of the los padres wetland dissolved silica dsi concentration is near 840 232 mol l and 211 83 275 92 mol l in groundwaters and surface waters respectively bsi represents an 5 6 22 1 of the total suspension material and 8 34 of the total mineralogical components of the wetland bottom sediments dsi and bsi vary seasonally with highest bsi content diatoms specifically during the spring summer in correlation to the lowest dsi concentration dsi 660 917 5 mol l and phytolith 3 35 5 84 concentrations in the inflow stream are higher than in the wetland and its outflow stream 19 1 113 mol l 0 45 3 2 respectively probably due to the high phytolith content in soils the high silica concentration in the soil solution and the groundwater inflow diatom content 5 16 8 in the wetland and its outflow stream is higher than in the inflow stream 0 45 1 97 controlling dsi in this system the understanding of the groundwater surface water interaction in an area is a significant element for determining the different components and the role that they play on the local biogeochemical cycle of si

yangtze river of china historical analysis of discharge variability and sediment flux hydrological records covering a 100 year period from the upper middle and lower yangtze river were collected to examine the temporal and spatial distribution of discharge and sediment load in the drainage basin the yangtze discharge as expected increases from the upper drainage basin downstream only an estimated 50 of the discharge is derived from the upper yangtze with the rest being derived from the numerous tributaries of the middle and lower course however the distribution of sediment load along the yangtze is the reverse of that observed for discharge with most of the sediment being derived from the upper basin a dramatic reduction in sediment load by 0 8 108 tons year occurs in the middle yangtze because of a marked decrease in slope and the change to a meandering pattern from the upper yangtze rock sections considerable siltation also occurs in the middle yangtze drainage basin as the river cuts through a large interior dongting lake system sediment load in the lower yangtze while significantly less than that of the upper river is somewhat higher than the middle yangtze because of additional load contributed by adjacent tributaries a strong correlation exists between the discharge and sediment load along the yangtze drainage basin during the dry season as lower flows carry lower sediment concentration during the wet season a strong correlation is also present in the upper yangtze owing to the high flow velocity that suspends sand on the bed however a negative to poor correlation occurs in the middle and lower yangtze because the flow velocity in these reaches is unable to keep sand in suspension transporting only fine grained particles downstream hydrological data are treated for 30 years 1950 1980 when numerous dams were constructed in the upper yangtze drainage basin at yichang and hankou hydrological stations records revealed a decreasing trend in annual sediment load along with slightly reduced annual discharge at the same stations this can be interpreted as the result of water diversion primarily for agriculture sediment load at datong further downstream is quite stable and not influenced by slightly reduced discharge furthermore sediment concentration at the three hydrological stations increased which can be attributed to sediment loss in association with intensifying human activity especially in the upper drainage basin such as deforestation and construction of numerous dams mean monthly sediment load of these 30 years pulses about 2 months behind discharge implying dam released sediment transport along the entire river basin during the high water stage

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

soluble sulfate in the martian soil at the phoenix landing site 1 sulfur has been detected by x ray spectroscopy in martian soils at the viking pathfinder opportunity and spirit landing sites sulfates have been identified by omega and crism in valles marineris and by the spectrometers on the mer rovers at meridiani and gusev the ubiquitous presence of sulfur has been interpreted as a widely distributed sulfate mineralogy one goal of the wet chemistry laboratory wcl on nasa s phoenix mars lander was to determine soluble sulfate in the martian soil we report here the first in situ measurement of soluble sulfate equivalent to 1 3 0 5 wt as so 4 in the soil the results and models reveal so 4 predominately as mgso 4 with some caso 4 if the soil had been wet in the past epsomite and gypsum would be formed from evaporation the wcl derived salt composition indicates that if the soil at the phoenix site were to form an aqueous solution by natural means the water activity for a dilution of greater than 0 015 g h 2 o g soil would be in the habitable range of known terrestrial halophilic microbes

geochemistry and si o fe isotope constraints on the origin of banded iron formations of the yuanjiacun formation lvliang group shanxi china abstract banded iron formations bifs within the lvliang region of shanxi province china are hosted by sediments of the yuanjiacun formation part of the paleoproterozoic lvliang group these bifs are located in a zone where sedimentation changed from clastic to chemical deposition indicating that these are superior type bifs here we present new major trace and rare earth element ree data along with fe si and o isotope data for the bifs in the yuanjiacun within the fe deposits at yuanjiacun jianshan and hugushan when compared with post archean australian shale paas these bifs are dominated by iron oxides and quartz contain low concentrations of al 2 o 3 tio 2 trace elements and the ree and are light rare earth element lree depleted and heavy rare earth element hree enriched the bifs also display positive la y and eu anomalies high y ho ratios and contain 30 si depleted quartz with high 18 o values that are similar to quartz within siliceous units formed during hydrothermal activity these data indicate that the bifs within the yuanjiacun formation were precipitated from submarine hydrothermal fluids with only negligible detrital contribution none of the bif samples analyzed during this study have negative ce anomalies although a few have a positive ce anomaly that may indicate that the bifs within the yuanjiacun formation formed during the great oxidation event goe within a redox stratified ocean the positive ce anomalies associated with some of these bifs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed ho lree and ce 4 these deposits formed during reductive dissolution at the oxidation reduction transition zone or in deeper level reducing seawater the loss of ce lree and ho to seawater and the deposition of these elements with iron hydroxides caused the positive ce anomalies observed in some of the bif samples although the limited oxidizing ability of surface seawater at this time meant that y ho and lree hree ratios were not substantially modified unlike similar situations within stratified ocean water during the late paleoproterozoic magnetite and hematite within the bifs in the study area contain heavy fe isotopes 56 fe values of 0 24 1 27 resulting from the partial oxidation and precipitation of fe 2 to fe 3 in seawater in addition mass independent fractionation of sulfur isotopes within pyrite indicates that these bifs were deposited within an oxygen deficient ocean associated with a similarly oxygen deficient atmosphere even though the bifs within the yuanjiacun formation formed after initiation of the goe

modeling novel stable isotope ratios in the weathering zone when rock is converted to weathering products the involved processes can be fingerprinted using the stable isotope ratios of metals for example li mg ca fe sr and metalloids b si here we construct a framework for interpreting these novel stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context the approach is applicable to any novel stable isotope system and is based on a simple steady state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays and uptake of nutrients by plants the model results show that the isotope composition of a given element in the weathering zone compartments depends on 1 the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material consisting of secondary precipitates and organic matter 2 the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants a relationship is established between isotope ratios isotope fractionation factors and indexes for chemical weathering such as chemical depletion fractions cdf and elemental mass transfer coefficients derived from simple elemental concentration measurements from this relationship isotope fractionation factors can be calibrated from chemical and isotope data measured on field material furthermore we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock water and sediment this calculation can be applied to samples from soils from rivers and from the sedimentary record and does not require knowing the isotope fractionation factors involved in the reactions finally we apply the model to the oceanic li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the cenozoic

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

the use of reflectance spectroscopy and the prehnite spectral index to target gold mineralization at the bonfim polymetallic skarn deposit serido mobile belt borborema province brazil abstract the serido mobile belt hosts numerous mineralized skarns in the borborema province northeastern brazil the bonfim w mo au bi te skarn occurs within a neoproterozoic metasedimentary sequence serido group within marble and schist lenses or at the contacts between both an asd fieldspec 3 high resolution spectrometer was used to characterize the mineralogy of a representative drill core interval 181 m from the bonfim deposit in order to generate the spectral characterization to define the gold mineralization skarns are characterized by the presence of diopside actinolite tremolite clinozoisite zoisite muscovite and prehnite prehnite and epidote constitute the main components of the calc silicate assemblage associated with late stage gold mineralization the diagnostic absorption feature of prehnite at 1478 nm oh can be used to identify its presence and the prehnite index proposed in this case study a reflectance ratio of 1465 nm 1478 nm allows highlighting the prehnite enriched sections along the core geochemical data show a positive correlation between gold and the presence of a prehnite absorption at 1478 nm the results suggest that the prehnite spectral signature can be used as a reliable and efficient rapid indicator for gold skarns perhaps under geologically similar circumstances to those at bonfim

hydrothermal alteration and short wavelength infrared swir characteristics of the tongshankou porphyry skarn cu mo deposit yangtze craton eastern china abstract the early cretaceous tongshankou cu mo deposit is located in the eastern yangtze craton and comprises both porphyry and skarn mineralization the porphyry ore is hosted in a granodiorite porphyry with the skarn mineralization found along the contacts with the carbonate host rocks alteration in the porphyry mineralization can be divided into three stages potassic alteration phyllic alteration and a carbonate stage with phyllic alteration associated with the main porphyry mineralization no propylitic alteration is present at tongshankou the skarn type alteration comprises five stages early skarn stage late skarn stage oxide stage quartz sulfide stage and late vein stage short wave length infrared swir analysis identified 15 hydrothermal minerals in the tongshankou deposit including illite dickite halloysite phengite talc muscovite saponite gypsum chlorite prehnite montmorillonite serpentine phlogopite actinolite kaolinite with montmorillonite illite and chlorite being the most common swir parameters and the electron microprobe results of chlorites show that chlorites close to the mineralization center tend to be iron rich and have high feoh absorption position pos 2250 2251 nm consequently chlorite may be a useful indicator mineral for mineralization in the tongshankou deposit pos 2250 is not correlated to temperature as calculated using a chlorite geothermometer but does correlate with the iron contents of the chlorites our results show that the ore forming fluids are fe rich in the proximity to the mineralization but mg rich towards the distal area unlike in other porphyry deposits the white mica show no consistent variation in the tongshankou deposit and cannot be used as vectors towards mineralization the fe mg al poor carbonate host rocks of the tongshankou deposit restrict the development of a propylitic alteration zone the fe mg al host rocks also prevent tschermak exchange in white mica siiv mg fe vi alivalvi making white mica pos 2200 and illite crystallinity ic randomly distributed our results show that swir spectroscopy of chlorite may be an applicable exploration tool in skarn related hydrothermal systems

geochemical stratigraphy sedimentology and mo isotope systematics of the ca 2 58 2 50 ga old transvaal supergroup carbonate platform south africa abstract the neoarchean paleoproterozoic transvaal supergroup in south africa contains the well preserved stromatolitic campbellrand malmani carbonate platform which was deposited in shallow seawater shortly before the 2 40 2 32 ga great oxidation event goe this platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for isotope geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the goe mo isotopes in sedimentary archives are widely used as a proxy for redox changes in modern and ancient environments and recent evidence suggests that oxy molybdate moo 4 2 is directly transferred from ocean water to inorganic carbonates with negligible fractionation thus reflecting oceanic mo isotope signatures in this study we analyzed major and trace element compositions as well as mo isotopic compositions of carbonate and mudstone samples from the kmf 5 drill core geochemical indicators such as fe and mn concentrations and fe to mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform heavy 30 si values of silicified carbonates between 0 53 and 2 35 point to si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint this assessment is supported by the frequent observation of rip up structures of silica chert layers within the entire sedimentary succession the 98 mo values of whole rock samples throughout the malmani campbellrand platform range between 0 82 and 1 40 similar to values reported for deeper slope carbonates from the griqualand west area but variations are independent from lithology or depositional water depth these large variations in 98 mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere ocean system at that time in agreement with earlier mo isotopic studies on campbellrand carbonates and shales a similar range in 98 mo values for carbonates between 0 40 and 0 87 however was also found on the hand specimen scale indicating the danger of a sample bias on the mo isotopic stratigraphy of this carbonate platform results of previously unpublished adsorption experiments of mo on caco 3 clearly indicate that the mo inventory of malmani campbellrand carbonates was not only influenced by primary adsorption from seawater but to a much larger degree by secondary processes during early diagenesis which also affected the mo isotopic composition of the samples on a local scale our results indicate that mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through earth s history nevertheless we interpret our heavy mo isotopic signatures from carbonates as a minimum value for neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic mo reservoir at that time

characterization of aster spectral bands for mapping of alteration zones of volcanogenic massive sulphide deposits abstract in this study the oxidized and hydrothermally altered zones including propylitic argillic and phyllic zones associated with volcanogenic massive sulphide deposits in the sohar shinas region of the sultanate of oman are studied based on absorption characters of spectral bands of advanced spaceborne thermal emission and reflection radiometer aster in this work we have used selected imaging processing methods including decorrelation stretching band ratios linear spectral unmixing lsu and mixture tuned matched filtering mtmf as well as petrological and geochemical characteristics of the alteration zones to map and understand the distribution of these alteration zones results indicate that 1 the decorrelation of aster spectral bands 3 6 and 8 discriminate extrusive basalts and the host lithology of altered zones 2 the application of aster band ratios 5 3 1 2 oxidized zone 4 6 5 argillic zone and 5 7 6 phyllic zone delineate alteration zones spatially associated with mineralized zones 3 the utilization of aster indices for oh bearing altered minerals e g kaolin and alunite indices confirmed the presence of alteration minerals in the mineralized zone 4 alteration end members derived using the spectral angle mapper sam method in combination with linear spectral unmixing lsu can be used to map the distribution of the weathering and alteration zones 5 short wavelength infrared swir measurements on field samples were used to confirm spectral measurements from aster data and 6 microscopic study and xrd analysis of field samples confirmed the presence of hematite and goethite in the oxidized zone chlorite epidote and calcite in the propylitic zone kaolin and chlorite in the argillic zone sericite and illite in the phyllic zone the major minerals responsible to absorptions of aster bands 3 8 5 and 6 respectively this study demonstrated the sensor capability of aster and the potential use of the image processing methods and documented the spectral absorptions values of the altered minerals and the field and mineralogical characters of alteration zones of the volcanogenic massive sulphide deposits to map similar deposits elsewhere

silicon isotopes in 3 8 ga west greenland rocks as clues to the eoarchaean supracrustal si cycle we report mc icp ms si isotopic measurements 29si recalculated as 30si on micro subsamples at 500 m scale from several major rock types of the 3 8 ga old isua greenstone belt igb southern west greenland and surrounding eoarchaean terrains with a large overall range of variations 2 80 30si 0 68 they demonstrate strong involvement of surface fluids enriched with dissolved si the resistance of si isotopes to metamorphic resettings and metasomatic overprints is also established metabasaltic pillows and metasediments display similar 30si enriched signatures suggesting that emergent surfaces of the eoarchaean protocrust were composed of slightly weathered hydrothermally altered mafic ultramafic bodies isua magnetite quartz banded iron formation bif is strongly depleted in 30si relative to all coeval rocks this depletion supports rayleigh controlled precipitation from seafloor vented hydrothermal fluids in contrast banded quartz pyroxene rocks from akilia island some 150 km southwest of the igb which some authors have identified as bif related yield quartz with si isotopic composition 30si 0 36 similar to metamorphic derived quartz 30si 0 50 this supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers