water chemistry of the southern tibetan plateau an assessment of the yarlung tsangpo river basin the yarlung tsangpo is the largest river draining the southern tibetan plateau along the himalayan ranges in this paper the ions and elements in the water of the yarlung tsangpo were studied in general the water of the yarlung tsangpo had a fairly high buffering capacity ph 8 8 and oxidation environment orp 190 mv total dissolved solids averaged approximately 157 mg l 1 which is higher than that of the global mean level under the dominant rock weathering due to the abundant carbonates around the river basin the ionic chemistry of the yarlung tsangpo was mainly composed of ca2 and hco3 in addition with a large amount water discharged into the river groundwater e g numerous chloride rich hot springs is another potentially important source of ions that affect the water chemistry in the yarlung tsangpo basin although watercourses on the tibetan plateau can generally be considered pristine concentrations of some toxic elements i e cd and pb were found to be higher than the who guideline for drinking water due to increasing influence from anthropogenic activities e g tourism mining operations and municipal wastewater discharge which constitute a risk to human livelihoods in both local and surrounding regions

recent developments in dating ancient crustal fluid flow several isotopic techniques have recently been developed or substantially improved allowing the accurate determination of the timing of fluid related processes these include k ar rb sr and sm nd dating of diagenetic minerals in clastic rocks u pb dating of carbonates sm nd dating of fluorite and uraninite re os and rb sr dating of sulfides 40ar 39ar and rb sr dating of fluid inclusions and u pb dating of ilmenite magnetite sphene and rutile while many of these techniques require further evaluation some have already resulted in new constraints on a variety of crustal fluid flow models

sources of metals in the porgera gold deposit papua new guinea evidence from alteration isotope and noble metal geochemistry the porgera gold deposit is spatially and temporally associated with the late miocene mafic alkalic epizonal porgera intrusive complex pic located in the highlands of papua new guinea png the highlands region marks the site of a tertiary age continent island arc collision zone located on the northeastern edge of the australasian craton the pic was emplaced within continental crust near the lagaip fault zone which represents an oligocene suture between the craton and volcano sedimentary rocks of the sepik terrane magmatism at porgera probably occurred in response to the late miocene elimination of an oceanic microplate and subsequent early pliocene collision between the craton margin and an arc system located on the bismarck sea plate gold mineralization occurred within 1 ma of the time of magmatism metasomatism accompanying early disseminated au mineralization in igneous host rocks resulted in additions of k rb mn s and co2 and depletions of fe mg ca na ba and sr rare earth and high field strength elements remained largely immobile pervasive development of illite k feldspar quartz carbonate alteration assemblages suggests alteration by mildly acidic 200 to 350 c fluids at high water rock ratios strontium and lead isotopic compositions of minerals from early base metal sulphide veins associated with k metasomatism and later quartz roscoelite veins carrying abundant free gold and tellurides are remarkably uniform e g 87sr86sr 0 70745 0 00044 n 10 207pb204pb 15 603 0 004 n 15 these compositions fall between those of unaltered igneous and sedimentary host rocks and specifically sedimentary rocks from the jurassic om formation which underlies the deposit igneous rocks 87sr86sr 0 7035 207pb204pb 15 560 om formation 87sr86sr t 0 7153 207pb204pb 15 636 it is therefore suggested that the hydrothermal fluids acquired their sr and pb isotopic signatures by interaction with or direct derivation from a plutonic root of the pic and host sedimentary rocks of the om formation it is likely that au was also derived from one or both of these two sources concentrations of au in unaltered igneous and sedimentary rocks from porgera 10 ppb au do not indicate that either lithology represents a significantly enriched protore although au and platinum group element pge abundances in the igneous rocks suggest a mild primary magmatic enrichment of au relative to the pge average au pt pd mantlenormalized 14 0 6 5 n 8 evidence that the porgera magmas were rich in volatiles permits speculation that au may have been concentrated in a magmatic fluid phase but alternative possibilities such as derivation of au by hydrothermal leaching of solidified igneous materials or sedimentary rocks cannot be excluded at this time

origins of nd sr pb isotopic variations in single scheelite grains from archaean gold deposits western australia accessory gangue scheelite cawo4 from the archaean mt charlotte lode au deposit can be divided into two types with different rare earth element ree signatures in some scheelite grains specific ree signatures are reflected by different cathodoluminescence colours which can be used to map their often complex oscillatory intergrowths domains with specific ree contents from two grains were sampled for sm nd rb sr and pb isotopic analyses using a micro drilling technique type i scheelite is strongly enriched in middle ree mree and eu anomalies are either absent or slightly positive four fragments collected from type i regions of two crystals have initial 87sr 86sr and end values ranging from 0 70141 to 0 70163 and 2 5 to 3 5 respectively and pb isotope ratios reflecting the composition of greenstone sequence this may indicate that nd and pb have their source either locally or regionally in the greenstones basic greenstone lithologies have 87sr 86sr 0 7015 and the radiogenic sr signatures indicate that part of the sr originated from felsic lithologies located either within or beneath the host greenstone pile alternatively the sr signature may have evolved from preferential leaching of a rb rich mineral during hydrothermal alteration of the greenstone the ree patterns of type ii scheelite are either flat or mree depleted and have strong positive eu anomalies three fragments collected from type ii regions of the same two crystals have initial 87sr 86sr ratios and end values between 0 70130 and 0 70146 and 1 1 to 2 6 respectively and pb isotope signatures that are once again similar to that of the greenstone this implies that 87sr 86sr ratios in type ii fluids were closer to those of the host dolerite 0 7008 0 7013 due to more extensive fluid interaction with the dolerite a positive correlation between na and ree suggests that ree3 are accommodated by the coupled substitution ree3 na 2 ca2 into both type i and type ii scheelite this is consistent with a fractional crystallisation model to explain the change in ree patterns from type i to type ii but not with a model involving different coupled substitutions and fluids from different origins we propose that the complex ree and isotopic signatures of scheelite at mt charlotte are related to small m to medium km scale processes involving mixing between fresh batches of hydrothermal fluid with fluids that had already been involved in extensive wall rock alteration the very high end values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high sm nd ratios however tiny 20 m grains of secondary hydroxyl bastnasite were found within micro fractures of one scheelite grain containing an extremely high end signature the hydroxyl bastnasite probably formed during recent ree redistribution within the scheelite as a result of meteoric fluid circulation the scale of this cryptic low temperature alteration is sufficient to explain the anomalously high endi values observed in scheelite from western australia

small domes on venus probable analogs of icelandic lava shields on the basis of observed shapes and volumetric estimates we interpret small dome like features on radar images of venus to be analogs of icelandic lava shield volcanoes using morphometric data for venusian domes in aubele and slyuta in press as well as our own measurements of representative dome volumes and areas from tethus regio we demonstrate that the characteristic aspect ratios and flank slopes of these features are consistent with a subclass of low icelandic lava shield volcanoes lils lils are slightly convex in cross section with typical flank slopes of 3 plausible lava shield production rates for the venusian plains suggest formation of 53 million shields over the past 0 25 ga the cumulative global volume of lava that would be associated with this predicted number of lava shields is only a factor of 3 4 times that of a single oceanic composite shield volcano such as mauna loa the global volume of all venusian lava shields in the 0 5 20 km size range would only contribute a meter of resurfacing over geologically significant time scales thus venusian analogs to lils may represent the most abundant landform on the globally dominant plains of venus but would be insignificant with regard to the global volume of lava extruded as in iceland associated lavas from fissure eruptions probably dominate plains volcanism and should be evident on the higher resolution magellan radar images

hydrochemical water evolution in the aral sea basin part i unconfined groundwater of the amu darya delta interactions with surface waters summary the aral sea which has been affected by lake level lowering of approximately 25 m and a salinity increase from 10 to 100 g l since 1963 represents along with the amu dary delta a dynamic hydrological system under an arid climate regime the system receives river water inflow at high seasonal and inter annual variability from remote alpine source areas in the amu darya delta there is a distinct salinity contrast between the low salinity river water 1 g l and the salinity of the unconfined gw gw unconf 10 95 g l the gw unconf levels are predominantly controlled by the seepage of the river water inflow and gw discharge into the shrinking aral sea in june 2009 and august 2009 we sampled water from various sources including surface waters gw unconf lake water and soil leachates for chemical analyses evaporative enrichment precipitation dissolution of gypsum and precipitation of calcite drive the gw unconf to an nacl so 4 water type presenting a positive correlation between na and so 4 we model the hydrochemical evolution of the gw unconf in a box model which considers the capillary rise of near surface gw the precipitation of minerals in the unsaturated horizon and the seasonal re flushing of adhesive residual brines and soluble salts the model documents a rapid increase in salinity over a few annual cycles furthermore the model simulations demonstrate the importance of the aeolian redistribution of soluble salts on the hydrochemical gw evolution in a lab experiment halite hexahydrite and starkeyite are precipitated during the late stages of evaporative enrichment from a representative local brine processes specific to different water compartments plausibly explain the variations of selected element ratios for example the precipitation of low sr calcite in irrigation canals and natural river branches of the delta lowers ca sr the dissolution of gypsum in soils ca sr mole ratio 150 and the possible precipitation of srso 4 associated with sr depletion in adhesive residual brines increases ca sr in seepage and re increases ca sr in the unconfined gw aral sea water which receives high ca sr surface and groundwater inflow developed due to continued precipitation of high ca sr calcite the almost lowest ca sr ratio 25 over time we observed spatial variations in the gw unconf composition i ammonium levels increase strongly due to interaction with lake sediments rich in organic matter and ii distinct increases in levels of nitrate u mo and se locally reflect oxygenation when gw levels decrease the amu darya delta acts as a sink for boron uptake via terrestrial vegetation and a source for bromide release by degradation of organically bound br our results concerning the hydrochemical evolution of the gw unconf and additional data from the aral sea constrain the parameter gw discharge in water budget models of the lake and improve the basis for palaeoclimatic interpretations of sediment records from the aral sea

strontium isotopic compositions and potassium and rubidium contents of formation waters in sedimentary basins clues to the origin of the solutes abstract strontium isotopic compositions of formation waters in sedimentary basins relate primarily to sr from dissolution replacement of alkali feldspar minerals dissolution recrystallization precipitation of marine carbonate and sulfate minerals and illitization formation waters in sedimentary basins are often depleted in k but enriched in rb relative to evaporated seawater and have k rb ratios commonly between 200 and 2000 the k and rb characteristics of formation waters suggest that deep burial conversion of smectite to illite has a major influence on the evolutionary history of these solutes

short term impact of de vegetation on organic inorganic interactions and cation transfers in uppermost soil horizons abstract the soil solutions of a natural soil covered by grass and of the same soil after grass removal were collected and analyzed after each rainfall and percolation through the two soils during a fifteen month experiment to identify and quantify the water mineral plant interactions during this period of time plastic tubes containing calcite and smectite particles were inserted into the two soils some powder of both being recovered after three and fifteen months water leached and the leachates analyzed the contents of various elements including the rare earth elements were measured in the soil solutions and the water leachates to identify the extent of the interactions and evaluate the contribution of the plants the immediate impact of de vegetation was recorded in the soil solutions that exported systematically more si and ca and often more mg from de vegetated soil the supplement of exported si was probably released by altering microorganisms as a response to de vegetation whereas that of mg resulted probably from dissolution of oxy hydroxides and that of ca from partial dissolution of the calcite reference mineral the removed elements were about 50 times higher in the soil solutions than in the plain rainwater before percolation through the soils the contents of the dissolved organic carbon and ammonia of the soil solutions from de vegetated soil were about one third higher than in those of the vegetated soil on the basis of the elemental contents of the soil solutions that percolated through the two soils removal of plants did not significantly affect adsorption of the nutrient k on the inserted smectite and calcite during the initial three months however less k was adsorbed on smectite and calcite of the de vegetated soil than on the same minerals of the same soil covered with grass during the following twelve months indicating in turn that more k was removed from de vegetated soil within a one year time

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

nd sr pb isotopic constraints on metal and fluid sources in w sb au mineralization at woxi and liaojiaping western hunan china this paper presents nd sr pb isotope data on scheelite inclusion fluids and residues of gangue quartz and sulfides from the w sb au ore deposits at woxi and liaojiaping ljp in the xuefeng uplift belt xub western hunan china sm and nd concentrations in scheelite from woxi are much lower than in scheelite from ljp and can be distinguished by their high 147sm 144nd ratios of 1 25 from the much lower ratios around 0 26 in scheelites from the ljp end values compared to values at 200 ma which is the average timing of granitoid emplacement during the indosinian yanshanian periods in the xub are around 10 for the ljp and compare well with the range of 5 to 11 defined by the granitoids whereas they are around 27 for scheelite from woxi this might indicate that rees in the mineralizing fluids at ljp originated from granitoids that are concentrated along the southern border of the xub whereas in the case of woxi the original fluids might have been masked by rees released during intense high temperature wall rock alteration of unexposed precambrian basement rocks at depth sr isotopes of scheelite from these two deposits show similar relations to host nearby rocks in that 87sr 86sr t 200 ma ratios of 0 721 for ljp scheelite agree with values ranging between 0 718 0 726 for granitoids whereas these ratios are much higher i e 0 745 for scheelite from woxi and correspond to the 87sr 86sr t 200 ma ratio range of 0 743 0 749 for precambrian host slates crushing experiments to release inclusion fluids from gangue quartz and sulfides deposited during later stages of ore deposition in both deposits failed to provide accurate and geologically meaningful two point fluid residue tie lines in rb sr isochron diagrams however sr released from fluid inclusions generally reveals lower initial 87sr 86sr ratios than the respective residues and shows affinities to 87sr 86sr t 200 ma values of indosinian yanshanian granitoids both at woxi and ljp pb stepwise leaching of scheelite and sulfides did not result in sufficient spreads in pb isotope diagrams and therefore no information regarding exact mineralization ages in the two deposits could be deduced overall ore pb isotopes reveal upper crustal signatures and are compatible with pb isotope signatures of the predominant precambrian slates in the woxi area steep trajectories through late stage quartz sulfide mineralization in pb isotope diagrams may hint at mixing scenarios involving pb from the host rocks and a component with lower 207pb 204pb and 208pb 204pb ratios relative to 206pb 204pb ratios which cannot be linked to any known reservoir in the xub mining district sr isotopes of four out of seven residual sulfide samples from woxi plot along a paleomixing line at an age of 199 8 ma supporting a mixing scenario for the fluids indicated by the pb isotopes and pointing to a possible genetic relationship with the emplacement of indosinian yanshanian granitoids the budgets of rees rb sr and pb in the original fluids were severely affected by contamination of these elements apparently during intense wall rock alteration but after sealing of the major pathways the mineralizing fluids tend to have better preserved their original signatures these attest a genetic relationship between the metallogeny in the xub w sb au province and the emplacement of indosinian yanshanian granitoids during mesozoic intracontinental tectonic uplift and thrusting

sm nd direct dating of the collins bay hydrothermal uranium deposit saskatchewan primary pitchblende mineralization from the collins bay unconformity type uranium deposit in northern saskatchewan has been dated using the sm nd method the mineral isochron age of 1281 80 ma is in good agreement with other geologic and geochronological data and demonstrates the potential of the sm nd technique for the direct dating of hydrothermal mineral deposits 23 references 5 figures 7 tables

rare earth element distributions in uraninites implications for ore genesis abstract analysis of the rare earth element ree distributions in uraninites from a variety of hydrothermal mineral deposits indicates that wide variations are to be anticipated although crystal structure must exert a fundamental influence upon ree substitution in uraninite the present study suggests that other factors in particular hydrothermal fluid chemistry may be of equal importance total ree ree contents range from 15 000 ppm in uraninites from ultrametamorphic deposits to eu eu lree hree 0 08 1 10 they are interpreted to result from variations in the oxygen fugacity and the nature of the anionic complexes in the hydrothermal fluids responsible for ore transport additionally in uraniferous quartz pebble conglomerates the ree patterns of the detrital uraninites provide direct evidence regarding the provenance of these sedimentary u ores

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

the magellan venus radar mapping mission the nasa magellan venus radar mapper spacecraft was launched into an interplanetary transfer trajectory to venus on may 4 1989 and will be placed into orbit around venus on august 10 1990 the orbiter carries a 12 cm wavelength multimode radar system in the synthetic aperture mode it is capable of imaging most of the venus surface at a resolution of better than 300 m approaching 120 m over more than half the planet in the altimeter mode it will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter where the surface relief is not too extreme in the radiometer mode the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 k at a resolution of 2 k tracking of the orbiter s coherent radio telemetry transmitter will permit observations of small accelerations related to gravitational inhomogeneities in the planet s interior objectives of the mission include deducing the geological history of the surface and the geophysical state of the interior specific attention focuses on the origin present distribution and activity of four geological processes that modify the surface 1 volcanic and tectonic 2 impact 3 erosional depositional and chemical and 4 isostatic and convective data products resulting from the mission will be made available to members of the scientific community through nasa s planetary data system

the apure river geochemistry of major and selected trace elements in an orinoco river tributary coming from the andes venezuela the analysis of physicochemical variables and selected dissolved elements was performed on the apure river waters for 15 months the variables ph alkalinity dissolved o2 conductivity and na ca mg and cd concentrations showed maximum values during low water whereas k si fe al mn zn cu cr and dissolved organic carbon doc showed maximum concentrations during rising and high water five important factors were found to control the amount and temporal variability of the dissolved elements lithology hydrology vegetation floodplain processes redox conditions and organic complexation weathering of silicates carbonates and evaporites in the andes provides most of the proportion of na ca mg and hco3 to waters the temporal variability of these ions is controlled by a dilution process although si can be taken up by the biomass si and k can be leached from the floodplain by weathering of clays microbial decay of the submerged plants in the floodplain during the inundation periods provides doc and k to river waters and changes the redox conditions in water the changing redox conditions control the solubility of mn zn and fe dissolved mn is a function of ph dependent redox process whereas zn solubility is controlled by scavenging of zn during the oxidation of mn2 to mno2 positive relationships between al fe cu cr and doc suggest that these elements are complexed by organic colloids generated in the floodplain moreover the binding capacity of fe with doc increases under reducing conditions although cd seems to be provided by weathering in the andes several processes can affect the mobility of cd during transport copyright 2010 john wiley sons ltd

samarium neodymium isotope systematics of hydrothermal calcites from the xikuangshan antimony deposit hunan china the potential of calcite as a geochronometer abstract the xikuangshan antimony mine located at central hunan china is the largest antimony deposit in the world hydrothermal calcites from the xikuangshan mine display relatively large variations in ree abundance and sm nd ratios and are characterized by mree and hree enriched and lree depleted patterns sm nd isotopic compositions of syn sulfide calcites fall on well defined lines in the isochron plot calcites from early and late stage mineralization form two distinct isochrons corresponding to 155 5 1 1 and 124 1 3 7 ma respectively the isochron ages suggest that sb mineralization took place during the late jurassic early cretaceous period in the xikuangshan district consistent with previous geological observations and with independent isotope data samarium neodymium dating of calcites shows potential for determining ages of hydrothermal mineralization

evaluation of rammelsbergite nias2 as a novel mineral for 187re 187os dating and implications for unconformity related u deposits abstract rammelsbergite nias2 is often present in ore deposits worldwide in association with other arsenides sulphides and sulfarsenides this work demonstrates the value of the application of the re os isotopic system to the dating of rammelsbergite using the example of the cigar lake uranium deposit athabasca basin saskatchewan canada our results show that rammelsbergite concentrates significant and variable amounts of re when crystallizing up to 1 6 g g 1 while incorporating minimal common os such characteristics make this mineral an ideal target for re os isotopic dating by both isochron and model age methods remarkable agreement of the re os age of the rammelsbergite with the u pb age of the uranium oxide grains found inside one of the massive rammelsbergite veins confirms the applicability of the method the selected rammelsbergite samples from the cigar lake uranium deposit yield a 187re 187os isochron age of 1239 33 20 ma these results challenge a long standing petrogenetic model which postulates contemporaneous u oxide and ni co arsenide deposition in unconformity related uranium deposits the new re os age of rammelsbergite veins indicates that arsenide crystallization postdated the deposition of the primary uranium oxides at 1461 1341 ma implying that uranium oxides and nickel arsenides as well as other sulphides and sulfarsenides were not precipitated contemporaneously during a single hydrothermal event as previously proposed the 187re 187os data suggest that crystallization of rammelsbergite and co genetic sulphides arsenides and sulfarsenides was linked to the ca 1270 ma mackenzie magmatic event marked by the emplacement of mafic dikes throughout the canadian shield including in the area of the athabasca basin this study therefore suggests a new hypothesis for the origin of nickel cobalt sulfur and arsenic in unconformity related u deposits of the athabasca basin more broadly the successful application of the 187re 187os rammelsbergite chronometer in this geological context suggests that its application in other ore deposit environments worldwide is likely to provide similar insights in this respect the 187re 187os chronometer applied to rammelsbergite and potentially to other arsenides and sulfarsenides represents a valuable complement to molybdenite dating because it can be used in systems that lack molybdenite

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

strontium isotopes trace biological activity in the critical zone along a climate and vegetation gradient abstract weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock the geogenic nutrient pathway however the prominence of this link is dependent on the efficiency of nutrient recycling from plant litter the organic nutrient cycle isotope ratios of strontium sr an element that behaves similarly to ca in ecosystems confer two types of information radiogenic sr isotopes inform as to the sources of sr and the degree of weathering while stable sr isotopes constrain partitioning between compartments of the critical zone bedrock water secondary solids and plants to date however neither the reactions nor the mass balance between compartments that fractionate sr isotopes nor the fractionation factors involved are well understood here we present geochemical budgets of sr using radiogenic and stable sr isotopes and ca sr ratios at four sites along a substantial climate and primary production gradient in the coastal mountains of chile we found that sr release through weathering is isotopically congruent and released sr is not strongly isotopically fractionated either during secondary mineral formation or transfer into the exchangeable pool despite this the 88sr 86sr ratio of bio available sr which should reflect the ratio of dissolved sr is higher than that of rock and regolith we propose that this offset is caused by plants while 88sr 86sr in plant organs at the four study sites systematically increased from roots towards their leaves whole plant sr isotope compositions indicate preferential uptake of light sr into plants with a fractionation of up to 0 3 relative to the bio available pool despite this strong biological fractionation 88sr 86sr ratios in bio available sr do not covary with biomass production across our study sites because with greater plant growth sr is recycled more times after release by weathering an isotope neutral process rather the loss of sr from the ecosystem in solid organic material sets the isotope ratio of dissolved or bio available sr organic solids thus appear to constitute a significant export path of elements released during weathering with the removal of solid plant debris reducing the recycling factor of sr and possibly that of other mineral nutrients too

high k rb ratios in stream waters exploring plant litter decay ground water and lithology as potential controlling mechanisms the mechanisms causing the k rb ratio in stream waters to attain considerably higher ratios c 1000 than in the major source rocks such as granites and basalts c 275 and 250 and in the upper continental crust c 260 are explored because k is a nutrient and rb is not the k rb ratio in biological materials is typically higher than in soils and rocks it is therefore possible as recently suggested in the scientific literature that a considerable portion of the k carried by rivers has circulated via biota and been leached into water bodies during litter decay in this study we can provide detailed information on the development of the k rb ratio during long term decay of litter from typical plants in the boreal zone this data show that it is indeed possible for decaying litter to deliver degradation products strongly enriched in k over rb however after revisiting data collected in regional hydrogeochemical studies over europe and ground water data collected at two contrasting sites in the boreal region we suggest that lithology could also be an important controlling factor over phanerozoic sedimentary and or limestone rich rocks in particular the k rb ratio is generally high perhaps mostly due to extensive retention of rb released by the weathering of source minerals we thus argue that geochemistry together with biological and climatological factors is responsible for the high k rb ratios in streams and rivers worldwide

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

plant decay as a major control of river dissolved potassium a first estimate abstract living plants which acquire significant amounts of k primarily from weathered silicate materials may be considered as an intermediate storage for part of k that is ultimately of the origin of silicate mineral weathering part of this plant k is transferred to streams and rivers as dead vegetation on the ground surface and within the soil beneath the surface becomes decomposed and leached by surface and ground waters with the remaining k being recycled to new growth of plants the plant k transferred to rivers was never included in any previous budget model that assessed the sources and pathways of k in global river a popularly current view claims that nearly 87 of the total k in global river water comes from weathered silicate source relative to 5 from evaporite deposits 1 from atmospheric salt cycling and 7 from fertilizers in fact some fraction of this silicate material derived k in global river should belong to plant source k derived from leaching of dead vegetation on the surface and in the soils by surface and ground waters the importance of decay of vegetation on the surface and in soils as being a source of k in global river was assessed by using a mass balance equation that expresses the k rb weight ratio of global river in terms of fractional contributions of k from various sources to global river water and the k rb ratios of the contributing sources considering a wide range of values for the k rb ratio of each source of k in global river water calculations have shown that of the proposed 87 silicate derived k in global river water nearly 70 to 30 can be attributed to leaching of dead vegetation on the surface and in soils by surface and ground waters whereas the remaining 17 to 57 can be assigned to leaching of the weathered silicate materials by surface and ground waters a more realistic estimate can be based on taking into account the average crustal silicate k rb ratio of about 250 300 by this measure the k derived from leaching of decayed plant by surface and ground waters is at least as important as the k derived from leaching of weathered silicate materials by surface and ground waters 43 44 each and may be even more important than the k derived from leaching of the weathered silicate materials 70 72 for the plant source k in contrast to 15 17 for the silicate source k as a control of the dissolved k in global river water we have also estimated that nearly 97 98 of k released from dead vegetation on the surface and in the soils is cycled back each year to newly grown plants

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

metallogenic significance of trace element and u pb isotope data for uraninite rich mineral separates from the labrador central mineral belt thirteen concentrates of uraniferous material were prepared from uranium occurrences in the central mineral belt of labrador host rocks to these occurrences include granitoid rocks of the archean basement ca 2000 ma metasedimentary rocks of the lower aillik group and 1860 ma felsic volcanic rocks of the upper aillik group common lead corrected pb isotope data from inductively coupled plasma mass spectrometry analyses define 207pb 206pb ages ranging from 1805 to 1697 ma for all but one sample with a mean age of 1752 27 ma 1 ages calculated for individual samples are similar to those derived by previous workers using standard analytical techniques eleven of these samples define linear trends that intersect the u pb concordia at 1741 23 ma and a tera wasserburg curve at 1740 21 ma respectively these data suggest that the occurrences are epigenetic with respect to host rocks and possibly related to a common metallogenic event therefore resolving a long standing controversy about the timin

from evaporated seawater to uranium mineralizing brines isotopic and trace element study of quartz dolomite veins in the athabasca system abstract stable isotope o h c radiogenic isotope sr nd and trace element analyses have been applied to quartz dolomite veins and their uranium u bearing fluid inclusions associated with proterozoic unconformity related uo 2 uraninite ores in the athabasca basin canada in order to trace the evolution of pristine evaporated seawater towards u mineralizing brines during their migration through sediments and basement rocks fluid inclusion data show that quartz and dolomite have precipitated from brines of comparable chemistry excepted for relatively small amounts of co 2 found in dolomite hosted fluid inclusions however 18 o values of quartz veins 18 o 11 to 18 and dolomite veins 18 o 13 to 24 clearly indicate isotopic disequilibrium between quartz and dolomite hence it is inferred that this isotopic disequilibrium primarily reflects a decrease in temperature between the quartz stage 180 c and the dolomite stage 120 c the 13 c values of co 2 dissolved in dolomite hosted fluid inclusions 13 c 30 to 4 and the 13 c values of dolomite 13 c 23 5 to 3 5 indicate that the co 2 dissolved in the mineralizing brines originated from brine graphite interactions in the basement the resulting slight increase in the fluid partial pressure of co 2 p co 2 may have triggered dolomite precipitation instead of quartz 18 o values of quartz veins and previously published 18 o values of the main alteration minerals around the u ores illite chlorite and tourmaline show that quartz and alteration minerals were isotopically equilibrated with the same fluid at 180 c the ree concentrations in dolomite produce paas normalized patterns that show some similarities with that of uo 2 and are clearly distinct from that of the other main ree bearing minerals in these environments monazite zircon and aluminum phosphate sulfate aps minerals the radiogenic isotope compositions of dolomite 87 sr 86 sr i 0 7053 to 0 7161 and e nd t 8 8 to 20 3 differ from one deposit to another reflecting both heterogeneity in the basement geology and variable preservation of the original composition of brines the previously published 87 sr 86 sr i and e nd t values of uo 2 compare with the most evolved dolomites i e dolomites precipitated from brines that exchanged the most with the basement this reinforces a close genetic link between dolomites and uo 2 deposition and implies that uo 2 deposition occurred in a cooling system during the transition from quartz to dolomite formation the 18 o and d values of the mineralizing brines 18 o 1 to 8 and d 150 to 50 are considerably shifted from that of their theoretical original values acquired during evaporation of seawater 18 o 3 and d 40 the positive 18 o shift is explained by protracted fluid rock interaction within the basin and basement rocks the negative d shift is attributed to incomplete mixing between the u mineralizing brines and low d water this low d water was likely produced during the abiogenic synthesis of bitumen by fisher tropsch like reactions involving co 2 derived from brine graphite interaction in the basement and radiolytic h 2 the resulting low d brines have been equilibrated with alteration minerals this may explain why some alteration minerals yield anomalously low d values whose significance has long been debated

the impact of biotic abiotic interfaces in mineral nutrient cycling a study of soils of the santa cruz chronosequence california abstract biotic abiotic interactions between soil mineral nutrients and annual grassland vegetation are characterized for five soils in a marine terrace chronosequence near santa cruz california a mediterranean climate with wet winters and dry summers controls the annual cycle of plant growth and litter decomposition resulting in net above ground productivities of 280 600 g m 2 yr 1 the biotic abiotic a b interface separates seasonally reversible nutrient gradients reflecting biological cycling in the shallower soils from downward chemical weathering gradients in the deeper soils the a b interface is pedologically defined by argillic clay horizons centered at soil depths of about one meter which intensify with soil age below these horizons elevated solute na ca mg ca and sr ca ratios reflect plagioclase and smectite weathering along pore water flow paths above the a b interface lower cation ratios denote temporal variability due to seasonal plant nutrient uptake and litter leaching potassium and ca exhibit no seasonal variability beneath the a b interface indicating closed nutrient cycling within the root zone whereas mg variability below the a b interface denotes downward leakage resulting from higher inputs of marine aerosols and lower plant nutrient requirements the fraction of a mineral nutrient annually cycled through the plants compared to that lost from pore water discharge is defined their respective fluxes fj plants qj plants qj plants qj discharge with average values for k and ca fk plants 0 99 fca plants 0 93 much higher than for mg and na fmg plants 0 64 fna plants 0 28 the discrimination against rb and sr by plants is described by fractionation factors ksr ca 0 86 krb k 0 83 which are used in rayleigh fractionation mixing calculations to fit seasonal patterns in solute k and ca cycling krb k and k 24 mg 22 mg values derived from isotope data in the literature fall within fractionation envelopes bounded by inputs from rainfall and mineral weathering ksr ca and k 44 ca 40 ca fractionation factors fall outside these envelopes indicating that ca nutrient cycling is closed to these external inputs small net positive k and ca fluxes 6 14 mol m 2 yr 1 based on annual mass balances indicate that the soils are accumulating mineral nutrients probably as a result of long term environmental disequilibrium

terrestrial ecosystems and the global biogeochemical silica cycle 1 most research on the global si cycle has focused nearly exclusively on weathering or the oceanic si cycle and has not explored the complexity of the terrestrial biogeochemical cycle the global biogeochemical si cycle is of great interest because of its impact on global co2 concentrations through the combined processes of weathering of silicate minerals and transfer of co2 from the atmosphere to the lithosphere a sizable pool of si is contained as accumulations of amorphous silica or biogenic silica bsi in living tissues of growing plants known as phytoliths and after decomposition of organic material as remains in the soil the annual fixation of phytolith silica ranges from 60 200 tmol yr 1 and rivals that fixed in the oceanic biogeochemical cycle 240 tmol yr 1 internal recycling of the phytolith pool is intense with riverine fluxes of dissolved silicate to the oceans buffered by the terrestrial biogeochemical si cycle challenging the ability of weathering models to predict rates of weathering and consequently changes in global climate consideration must be given to the influence of the terrestrial bsi pool on variations in the global biogeochemical si cycle over geologic time and the influence man has had on modifying both the terrestrial and aquatic biogeochemical cycles