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

impact of cyclic freezing on precipitation of silica in me sio2 h2o systems and geochemical implications for cryosoils and sediments abstract cyclic freezing of aqueous solutions may cause a significant precipitation of amorphous silica where about 90 mol of the primary dissolved silicic acid can be fixed in analogy to evaporation h 2 o molecules are separated from aqueous solutions by freezing and supersaturation with respect to amorphous silica can be reached the results show that higher proportions of silica precipitated at lower input si oh 4 concentration and at increasing numbers of freeze thaw cycles additionally dissolved metal ions me favour the formation of amorphous silica in the sequence of na 2 2 2 2 this may be mostly related to a decrease of amorphous silica solubility in the high concentrated aqueous solutions which remained after ice formation co precipitation yields me si ratios of the depositions between 1 and 4 re dissolution of the primary amorphous precipitates is incomplete accordingly such solids are suitable precursor materials for neoformation of silicates as secondary authigenic silicates like zeolites may be formed from such precursors it is suggested that their occurrence and exposure in cryosoils and sediments might be used as a proxy indicator of past climate cyclic freezing can significantly reduce the geochemical and ecological availability of silica on the other side the decrease of dissolved silica induced by freezing provokes an increase of undersaturation with respect to silicates in the re thawed soil or interstitial solutions which stimulates the dissolution of siliceous minerals and rocks

determination of the stable iron isotopic composition of sequentially leached iron phases in marine sediments abstract reactive iron oxyhydr oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved fe ii the adsorption of fe ii onto particle surfaces and the formation of authigenic fe minerals the partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally defined iron phases here we complement an existing sequential leaching method by developing a sample processing protocol for 56fe analysis which we subsequently use to study fe phase specific fractionation related to dissimilatory iron reduction in a modern marine sediment carbonate fe was extracted by acetate easily reducible oxides e g ferrihydrite and lepidocrocite by hydroxylamine hcl reducible oxides e g goethite and hematite by dithionite citrate and magnetite by ammonium oxalate subsequently the samples were repeatedly oxidized heated and purified via fe precipitation and column chromatography the method was applied to surface sediments collected from the north sea south of the island of helgoland the acetate soluble fraction targeting siderite and ankerite showed a pronounced downcore 56fe trend this iron pool was most depleted in 56fe close to the sediment water interface similar to trends observed for pore water fe ii we interpret this pool as surface reduced fe ii rather than siderite or ankerite that was open to electron and atom exchange with the oxide surface common extractions using 0 5 m hcl or na dithionite alone may not resolve such trends as they dissolve iron from isotopically distinct pools leading to a mixed signal na dithionite leaching alone for example targets the sum of reducible fe oxides that potentially differ in their isotopic fingerprint hence the development of a sequential extraction fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide range of environmental settings

tracing mechanisms controlling the release of dissolved silicon in forest soil solutions using si isotopes and ge si ratios the terrestrial biogenic si bsi pool in the soil plant system is ubiquitous and substantial likely impacting the land ocean transfer of dissolved si dsi here we consider the mechanisms controlling dsi in forest soil in a temperate granitic ecosystem that would differ from previous works mostly focused on tropical environments this study aims at tracing the source of dsi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions using stable si isotopes and ge si ratios relative to granitic bedrock clays minerals were enriched in si 28 and had high ge si ratios while bsi from phytoliths was also enriched in si 28 but had a low ge si ratio such a contrast is useful to infer the relative contribution of silicate weathering and bsi dissolution in the shallow soil on the release of dsi in forest floor leachate solutions the delta si 30 values in forest floor leachates 1 38 parts per thousand to 2 05 parts per thousand are the lightest ever found in natural waters and ge si ratios are higher in forest floor leachates relative to soil solution these results suggest dissolution of si 28 and ge enriched secondary clay minerals incorporated by bioturbation in organic rich horizons in combination with an isotopic fractionation releasing preferentially light si isotopes during this dissolution process ge si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals tree species influence si isotopic compositions and ge si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and to a lesser extent through differing si recycling c 2010 elsevier ltd all rights reserved

a review of the nature and origin of limestone microporosity abstract limestone microporosity is ubiquitous and extensively developed in most phanerozoic limestones from an economic perspective microporosity is important because it contributes substantially to the carbonate pore system which can host significant volumes of water and hydrocarbons therefore determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations reservoir characterization and fluid flow simulations from an academic standpoint microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments a fundamental process in carbonate diagenesis many types of micropores contribute to what has been referred to as microporosity but the vast majority is hosted among low magnesium calcite lmc microcrystals that are present in limestone matrix and allochems geochemical textural and mineralogical data from natural settings and laboratory experiments indicate that lmc microcrystals are diagenetic in origin more specifically these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high magnesium calcite hmc minerals and precipitation of lmc microcrystal cements the stabilization process is inferred to take place in the meteoric marine and burial diagenetic realms although it has not been directly observed carbon and oxygen isotopes as well as trace element data suggest that lmc microcrystals form during burial diagenesis in marine like fluids evidence suggests that porosity is not generated during this dissolution precipitation process but rather inherited from the precursor sediments the final arrangement of the micropores in a limestone however depends on the precise diagenetic pathway lmc microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size the three main textural classes granular framework fitted mosaic and clustered have been recognized across a wide range of ages depositional settings burial depths and precursor types and are characterized by distinct petrophysical properties such as porosity permeability and pore throat size observations from modern sediments also support the hypothesis that lmc microcrystals develop from aragonite and hmc dominated lime mud the origin of lime mud has been extensively studied but still highly debated of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the phanerozoic microporous limestones comprised of lmc microcrystals are equally abundant during times of aragonite seas and calcite seas which suggests that no special mineral precursor is required microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents unlike shallow marine limestones deep marine sediments start out as mostly lmc therefore mineralogical stabilization is not a significant process in chalk diagenesis

lacustrine chalky carbonates origin physical properties and diagenesis palaeogene of the madrid basin spain the palaeogene lacustrine chalky carbonates of the madrid basin are a peculiar type of very soft and friable carbonate facies with high porosity despite being covered by more than 800 m of sediment similar physical properties to those described in marine chalk reservoirs emphasize the interest in analysing and characterizing these carbonate facies within a lacustrine depositional system lithologically they are calcitic and or dolomitic poorly cemented carbonate muds with no significant amounts of skeletal debris clay minerals such as illite smectite and palygorskite are present between the carbonate crystals palygorskite is the most common covering the carbonate crystals and forming sheets between them these lacustrine chalky carbonates were formed in the basinal areas of the lake as the result of inorganic carbonate precipitation and or detrital sedimentation related to episodic reactivation of the adjacent fan systems their petrological geochemical and physical properties indicate that few textural and compositional modifications occurred during diagenesis their main physical properties are a very low dry bulk and grain density 1 6 2 2 and 2 62 g cm3 respectively and medium to high porosity 10 40 due to micropores 2 am 30 the convergence of lacustrine sedimentation dynamics rapid sedimentation the original mineralogy of these calcareous lacustrine muds relatively stable low magnesian calcite and dolomite the early formation of the palygorskite cement of these muds and the retention of mg enriched fluids in the pore system were decisive in the partial inhibition of calcite cementation compaction and recrystallization the chalky carbonates are also intercalated between impermeable littoral carbonate facies that impeded fluid flow through their pore systems

iron isotopes in the amazon river system weathering and transport signatures abstract trace metal clean plankton tows river samples aerosol leachates and porewater samples from the amazon river system were measured for their iron isotopic composition the overall isotopic range observed was 1 5 with fe isotopic variability observed between different types of tributaries dissolved fe and suspended load fe from two main channel sites were isotopically similar 0 1 to 0 3 in contrast the dissolved fe and suspended load fe from an organic rich tributary the negro river were isotopically distinct with the dissolved slightly isotopically heavy 0 3 and the suspended load isotopically light 0 9 the isotopically lightest sample from the amazon region was shelf porewater 1 3 in river water seawater mixing experiments the fe isotopic signal of dissolved fe of river water was modified by flocculation of isotopically heavy fe the variability of the fe isotopic composition of natural samples including biological and aqueous samples demonstrates that significant fractionation is associated with fe biogeochemistry in the environment

rayleigh fractionation of iron isotopes during pedogenesis along a climate sequence of hawaiian basalt abstract we measured iron isotopic composition of surface 10 20 cm and subsurface 50 70 cm basaltic soil horizons from the island of maui along a climate gradient mcg ranging from 2 2 to 4 2 m mean annual precipitation map all soil forming factors except climate were conserved the mcg has a documented decrease in fe with increasing rainfall that is highly correlated with decreasing mean annual eh values we found that increasing map from 2 8 to 4 2 m resulted in a surface plus subsurface average increase of 0 56 0 09 56fe with the subsurface consistently 0 33 0 06 56fe greater than the surface horizons based on loss of fe relative to nb rayleigh fractionation was observed with 103ln lost retained values of 0 37 0 03 and 0 34 0 04 for the surface and subsurface respectively equivalent 103ln lost retained values for the surface and subsurface soils suggests fe loss is driven by similar mechanisms throughout the soil profile our calculated fractionation factor is about 1 3 the magnitude of laboratory determined fractionation factors for fe reduction suggesting other processes organic complexation fe re precipitation modulate the net fe loss along the mcg these results offer field scale confirmation of laboratory experiments on model systems that show anoxic weathering reactions produce materials enriched in heavy fe isotopes

the influence of weathering processes on riverine magnesium isotopes in a basaltic terrain this study presents major trace element and mg isotope data for the dissolved load and suspended particulates of icelandic rivers draining dominantly basaltic catchments including both glacier fed and direct runoff rivers these samples provide the opportunity to understand the behaviour of mg isotopes during chemical weathering where variations due to lithology are not extant given the significant role of mg in the carbon cycle such variations may provide important information on the regulation of earth s climate hydrothermal waters groundwater precipitation glacial ice basalt glass olivine and representative soils have also been analysed the dissolved load shows a wide range of 26mg compositions compared to the parent basaltic glass 26mg 0 29 ranging from 0 96 to 0 64 while precipitation and hydrothermal waters possess 26mg values of 0 83 and 0 85 respectively with lower mg concentrations than the dissolved load biomass activity in vegetation and organic material in soils and rivers colloids appear to have little effect on the mg isotope compositions rather the data suggest that mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions in some samples seawater in the form of direct precipitation or glacial runoff also appears to be an important source of mg glacier fed rivers groundwaters and some direct runoff rivers with a high ph have higher 26mg than basalt which is most likely due to the incorporation of light mg isotopes in secondary minerals in contrast those direct runoff rivers which have a relatively low ph have low 26mg relative to basalt consistent with preferential incorporation of heavy mg isotopes into secondary phases although it is not possible to rule out some contribution from precipitation riverine suspended particulates are depleted in mobile elements and have 26mg compositions values both higher and lower than unweathered basalt in the glacier fed and direct runoff rivers where the 26mg of the dissolved phase is heavy due to the formation of secondary phases the suspended load is light because it contains more of those phases the opposite is true for the remainder of the direct runoff rivers which have low ph this could be due to dissolution of secondary minerals enriched in light mg which are unstable at low ph or the formation of new secondary phases

can fe isotope fractionations trace the pedogenetic mechanisms involved in podzolization abstract stable fe isotopes have shown the potential for tracing pedogenetic processes large isotopic fractionations were especially observed in podzols nevertheless a clear link between isotopic fractionation and elementary processes still needs to be established to spatially distinguish the mechanisms successively involved in pedogenesis we studied a podzolic chronosequence from vancouver island british columbia canada we analyzed depth variations in soil properties ph particle size fractions organic carbon si al ratio zr ti fe concentrations in different fe pools and fe isotopic compositions the si al ratio zr and ti demonstrated that the cox bay podzols developed from the same parental material satisfying the requirements of a chronosequence we showed that acidification to a ph of 4 7 was a prerequisite for the start of podzolization this first phase took place after 270 years furthermore we observed that once the required ph was reached 4 7 podzolization occurred rapidly over 50 years we found that fe isotope fractionation in the a e horizon was linked to mineral dissolution while in the podzolic b horizons this fractionation was clearly associated with the accumulation of fe organic complexes and poorly crystalline fe oxyhydroxides

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

silicon isotopes in allophane as a proxy for mineral formation in volcanic soils abstract weathering of basaltic ash in volcanic areas produces andosols rich in allophane and ferrihydrite since the rate of mineral formation is very useful in climate and geochemical modelling this study investigates si isotope compositions of allophane as a proxy for mineral formation allophane formed in contrasting conditions in five icelandic soil profiles displays silicon isotope signatures lighter than the basalt in less weathered soils 0 64 0 15 and heavier in more weathered organic rich soils 0 23 0 10 the fate of the dissolved si in those volcanic soils strongly depends on al availability in organic rich soils most of al is humus complexed and the results support that si precipitates as opaline silica by super saturation leaving an isotopically heavier dissolved si pool to form allophane with uncomplexed al this study highlights that si isotopes can be useful to record successive soil processes involved in mineral formation which is potentially useful in environmental paleo reconstruction

iron isotopic fractionation during continental weathering abstract the effect of continental weathering on the iron isotope compositions of natural materials is investigated unweathered igneous rocks pelagic clay and dust fall within the range 56 fe 0 0 3 rivers with large suspended loads also have 56 fe values near zero dilute streams have 56 fe values that trend towards lower 56 fe 1 suggesting that dissolved riverine iron is isotopically light relative to igneous rocks bulk soil and soil leaches display systematically different 56 fe profiles indicating that isotopically distinct fe pools are generated during pedogenesis nannofossil ooze which contains fe scavenged from the ocean water column has 56 fe 0 but is consistent with seawater dissolved fe having negative 56 fe it is inferred that continental weathering under modern oxidizing earth surface conditions preferentially releases dissolved fe with negative 56 fe which is transported in rivers to the ocean a preliminary analysis of the marine fe budget suggests that riverine fe has a substantial role in determining the 56 fe of both the modern and ancient oceans but other inputs particularly that from diagenesis of marine sediments may also be important since the chemical pathways of fe processing during weathering are dependent on oxidation state and biological activity fe isotopes may prove useful for detecting changes in these parameters in the geologic past

trace elements in a north sea drill core abstract chalk samples systematically taken along a drill core from one of the hydrocarbon producing fields of the north sea tyra field were analysed by a neutron activation technique involving measurement of radioisotopes with relatively short half lives elements determined include na al cl ca mn v sr dy mg ti ba and eu the major elements ca and al could be related to petrophysical parameters particularly al which shows a correlation with the silicification argillaceous content na and cl has a low content in the hydrocarbon bearing section of the drill core suggesting that the pore space is filled mainly by hydrocarbons a significant decrease of mn with depth probably suggests diagenesis of chalk prior to or with hydrocarbon emplacement investigations of drilling fluids and cuttings reveal a strong contamination of the latter mainly by ba chalk data from comparable onshore localities danish subbasin show a different geochemistry with generally less variation of elements with depth

inferences from the vertical distribution of fe isotopic compositions on pedogenetic processes in soils abstract the isotopic compositions of major elements in soils can help understand the mechanisms and processes that control the evolution of soils and the nature and dynamics of the soil constituents in this study we investigated the variations of the fe concentrations and isotopic compositions combined with classical soil parameters such as granulometry ph and c and n concentrations we selected three soils submitted to different hydrodynamic functioning along a toposequence a well drained cambisol and two hydromorphic soils an albeluvisol and a gleysol in the cambisol the isotopic variations were small indicating little redistribution of fe which we attributed to centimetric scale exchanges from the si bound to the weakly bound iron pools and insignificant subsurface fe export in contrast the hydromorphic soils showed an overall variation of 0 37 for 56 fe 56 fe 56 fe 54 fe sample 56 fe 54 fe irmm 014 1 1000 and an inverse correlation between the fe isotopic compositions and the oxide bound fe concentrations we suggest that in the uppermost horizon the mobilisation of oxide bound fe was due to the reducing conditions and predominantly involved the light fe isotopes similarly within the bt horizon of the albeluvisol the fluctuations of the water table level induced changes in the redox conditions and thus fe dissolution and transport of isotopically light fe the fe isotopic composition profile in the b c horizon of the gleysol is dominated by the signature of the parental material overall the variations of the underground water table combined with topography driven water flow were suggested to be the main mechanisms of fe translocation in these hydromorphic soils finally the comparison between fe isotope profiles in worldwide soils allows us to show that fe isotopic variations can help discriminate between various mechanisms and scales of fe transfer in soils and accordingly provide information on the evolution of soils when used in combination with pedological geochemical geographical and environmental characterisations

morphometry of micrite particles in cretaceous microporous limestones of the middle east influence on reservoir properties abstract microporosity may account for a significant part of the total porosity of cretaceous limestone reservoirs of the middle east in these microporous facies porosity is moderate to excellent up to 35 while permeability is poor to moderate up to 190 md micritic limestones also may form dense layers with very low porosity and permeability values micritic samples were collected from three fields of the habshan and mishrif formations to examine the spatial relationship with their porosity permeability and pore throat radius distributions two key parameters of the micritic particles are studied using scanning electron microscopy their morphology shape and inter crystal contacts and their crystallometry results reveal that micrite matrixes can be subdivided into three petrophysical classes class c strictly microporous limestones with coarse punctic to partially coalescent micrites is made up of coarse 2 m polyhedral to rounded micritic crystals it has good to excellent porosity 8 28 poor to moderate permeability 0 2 190 md and a mean pore threshold radius of more than 0 5 m the class c is usually observed in rudist shoal facies where relatively high hydrodynamic energy disfavoured deposition of the finer micritic crystals it also developed within meteoric leaching intervals below exposure surfaces class f strictly microporous limestones with fine punctic to partially coalescent micrites is composed of fine

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

copper and iron isotope fractionation during weathering and pedogenesis insights from saprolite profiles abstract iron and copper isotopes are useful tools to track redox transformation and biogeochemical cycling in natural environment to study the relationships of stable fe and cu isotopic variations with redox regime and biological processes during weathering and pedogenesis we carried out fe and cu isotope analyses for two sets of basalt weathering profiles south carolina usa and hainan island china which formed under different climatic conditions subtropical vs tropical unaltered parent rocks from both profiles have uniform 56 fe and 65 cu values close to the average of global basalts in the south carolina profile 56 fe values of saprolites vary from 0 01 to 0 92 in the lower reduced part and positively correlate with fe 3 fe r 2 0 90 whereas 65 cu values are almost constant by contrast 56 fe values are less variable and negatively correlate with fe 3 fe r 2 0 88 in the upper oxidized part where large 4 85 65 cu variation is observed with most samples enriched in heavy isotopes in the hainan profile formed by extreme weathering under oxidized condition 56 fe values vary little 0 05 0 14 whereas 65 cu values successively decrease from 0 32 to 0 12 with depth below 3 m and increase from 0 17 to 0 02 with depth above 3 m throughout the whole profile 65 cu positively correlate with cu concentration and negatively correlate with the content of total organic carbon toc overall the contrasting fe isotopic patterns under different redox conditions suggest redox states play the key controls on fe mobility and isotope fractionation the negative correlation between 56 fe and fe 3 fe in the oxidized part of the south carolina profile may reflect addition of isotopically light fe this is demonstrated by leaching experiments which show that fe mineral pools extracted by 0 5 n hcl representing poorly crystalline fe hydr oxides are enriched in light fe isotopes the systematic cu isotopic variation in the hainan profile reflects desorption and downward transport of isotopically heavy cu leaving the organically bound cu enriched in light isotope as supported by the negative correlation of 65 cu with toc r 2 0 88 the contrasting mostly positive vs negative cu isotopic signatures in the upper parts of these two profiles can be attributed to the different climatic conditions e g high rainfall at a tropical climate in hainan favors desorption and the development of organism whereas relatively dry climate in south carolina favors cu re precipitation from soil solutions and adsorption onto fe hydr oxides our results highlight the potential applications of fe and cu isotopes as great tracers of redox condition ancient climate and biological cycling during chemical weathering and pedogenic translocation

fe cycling in the shale hills critical zone observatory pennsylvania an analysis of biogeochemical weathering and fe isotope fractionation abstract during weathering fe in primary minerals is solubilized by ligands and or reduced by bacteria and released into soil porewaters such fe is then removed or reprecipitated in soils to understand these processes we analyzed fe chemistry and isotopic composition in regolith of the shale hills watershed a critical zone observatory in central pennsylvania overlying iron rich shale of the rose hill formation elemental concentrations were measured in soil from a well drained catena on a planar hillslope on the south side of the catchment based upon x ray diffraction and bulk elemental data loss of fe commences as clay begins to weather 15 cm below the depth of auger refusal more fe iii was present than fe ii in all soil samples from the ridge top to the valley floor both total and ferrous iron are depleted from the land surface of catena soils relative to the bedrock loss of ferrous fe is attributed mostly to abiotic or biotic oxidation loss of fe is most likely due to transport of micron sized particles that are not sampled by porous cup lysimeters but which are sampled in stream and ground waters the isotopic compositions 56 fe relative to irmm 014 of bulk fe and 0 5 n hcl extracted fe operationally designed to remove amorphous fe oxyhydr oxides range between 0 3 and 0 3 with 56 fe bulk extractable values between 0 2 and 0 4 throughout the soils along the catena 56 fe signatures of both bulk fe and hcl extracted fe become isotopically lighter as the extent of weathering proceeds the isotopic trends are attributed to one of two proposed mechanisms one mechanism involves fe fractionation during mobilization of fe from the parent material due to either fe reduction or ligand promoted dissolution the other mechanism involves fractionation during immobilization of fe oxyhydr oxides if the latter mechanism is true then shale which comprises one quarter of continental rocks could be an important source of isotopically heavy fe for rivers

pedogenesis and weathering rates of a histic andosol in iceland field and experimental soil solution study abstract it is important to study the rate determining processes of chemical weathering and soil formation in volcanic islands since a significant part of the carbon fixed by chemical weathering of silicates on earth is fixed at the surface of volcanic islands these soils are fertile and much of the river suspended matter delivered to the ocean stems from these islands this study determines the factors that drive the pedogenesis of a histic andosol in western iceland soil solutions were extracted from the profile in the field from undisturbed ex situ mesocosms and from repacked laboratory microcosms concentrations of measured and calculated inorganic species in the field and experimental soil solutions were used for thermodynamic and kinetic interpretation and to calculate the weathering rates the main primary rock constituent of the 205 cm thick soil profile was basaltic glass allophane content ranged from 2 to 22 and the soil carbon content ranged from 11 to 42 mean soil solution ph value ranged from 4 to 6 with the lowest value at 80 cm depth and highest between 150 to 205 cm the high solute concentrations in soil solutions in the beginning of the microcosm weathering experiment declined faster for anions than cations under field conditions inorganic anions were supplied by marine and anthropogenic rather than pedogenic sources and hence these anions were subsequently leached out during the experimental duration when there was a limited input of anions through experimental precipitation the factor which defined the rate at which each ion was depleted from the exchange complex of the soil decreased down the soil profile the release sequence at 50 cm depth was cl na so 4 f si the si and base cations experimental weathering rate when normalized to geographical surface area are similar to or lower than those measured from river catchments in southwestern iceland the dissolved al flux was much higher from the soil when compared to the river catchments field and experimental soil solutions were all highly undersaturated with respect to basaltic glass field and mesocosm samples were supersaturated with respect to secondary allophane and imogolite while samples from the microcosms were often undersaturated with respect to allophane and imogolite predicted dissolution rate was dictated by the soil solution a h 3 a al 3 activity ratio but slowed down by up to 20 and 30 by decreasing undersaturation in field and mesocosms respectively predicted dissolution rates according to the a h 3 a al 3 activity ratio increased up to factor of 7 30 and 37 by speciating al 3 with oxalate in field mesocosms and microcosms respectively speciation with oxalate which represents the maximum effect of the dissolved organic carbon doc on dissolution rates generally had more effect near the surface than at deep levels in the soil profile this study shows that at fixed temperature reactive surface area and composition of the volcanic glass in the soil the chemical weathering rates of andosols are dictated by 1 aeolian deposition rates and drainage which affect the saturation state and the a h 3 a al 3 activity ratio 2 the production of organic anions within the soil and 3 external supply of anions capable of complexing al 3

molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain the application of the molybdenum mo isotope system as a proxy for determining changes in the redox state of the oceans is predicated on the assumption that the composition of continental input can be characterised from crustal rock types and remains constant however it has recently been shown that the 98 95mo composition of global rivers varies between 0 15 and 2 4 and is therefore systematically heavier than the average composition of the continental crust 0 in order to understand the processes that control mo isotope fractionation during weathering this study presents 98 95mo and mo abundance data for rivers and estuarine samples from iceland that drain predominantly basaltic terrains resolvable differences are observed in the isotopic composition of the riverine mo sources ice 98 95moice 1 8 basaltic bedrock 98 95mobedrock 0 0 and hydrothermal waters 98 95mohydrothermal 3 4 systematic changes in the dissolved mo isotope composition are also observed within river catchments with 98 95mo values increasing from 0 in glacial rivers close to the source to 1 downstream consistent with mo isotopes being fractionated during weathering analysis of other riverine phases bedload colloids and iron precipitates demonstrates that these phases preferentially incorporate light mo isotopes and remain coupled to the dissolved load during riverine transportation a 98 95mo profile through the borgarfjor eth ur estuary exhibits a predominantly conservative mixing behaviour but suggests that the release of isotopically light mo from the particulate and or colloidal phases may occur in the low salinity part of the estuarine mixing zone 2010 elsevier b v

chemostratigraphy of upper cretaceous chalk in the danish subbasin the application of well cuttings to geochemical analyses was studied on the basis of the elemental distribution of mg sr mn and zn and the carbonate contents in upper cretaceous chalk sequences from six wells in the central part of the danish subbasin the cuttings were washed and dried at the well site with no other cleaning in the laboratory the analytical results from well cuttings were tested by comparison with data from corresponding cored intervals the results indicate that contamination by drilling mud is negligible in the material studied whereas caving occasionally may cause some veiling of the elemental distribution patterns the elemental data are therefore considered representative for carbonate sediments and the sequences studied a chemostratigraphic correlation of the sections is carried out on the basis of several chemostratigraphic markers each defined by one or more elements the chemostratigraphy hereby established for the central part of the danish subbasin is in accordance with the existing log stratigraphy and biostratigraphy in the area furthermore the geochemical profiles show remarkable similarity to corresponding profiles from the upper cretaceous in the north sea central graben the results emphasize the applicability of cuttings for geochemical analysis and of geochemistry as a useful tool for chemostratigraphy and basin analyses in the subsurface exploration for chalk as hydrocarbon reservoirs

fractionation of fe and cu isotopes in acid mine tailings modification and application of a sequential extraction method abstract sulfidic mine tailings have a high potential for contamination of the environment by triggering acid mine drainage hence it is crucial to understand metal mobilization processes and to develop monitoring tools metal isotope fingerprinting as a potential monitoring tool for metal sulfide oxidation processes was in the focus of this study by using stable isotope signatures of cu and fe for fe a six step sequential extraction method was applied in order to separate potential fe bearing minerals water soluble exchangeable fraction fe iii oxyhydr oxides fe oxides sulfides and organic compounds and residual silicates for cu this method was modified into a four step extraction method water soluble exchangeable fraction oxalate fraction bound to fe oxides and sulfides residual to verify accuracy and precision of the sequential extraction method for metal isotope analysis isotope fractionation during the extraction procedure was investigated employing minerals for which the mineral composition and the isotopic composition was known the developed procedure is suitable to separate target minerals with only a small loss in the elemental budget no significant isotope fractionation was observed during the extraction procedure application of this method on two sites of porphyry copper mine tailings in the atacama desert in chile chanaral bay revealed several implications about the mobilization of fe and cu in an environmental setting iron contents and fe isotope compositions are homogeneous with depth 0 61 cm 56fe 0 2 0 3 for the bulk and the fe hyr oxide fraction and only the deepest samples at 60 cm exhibited lower 56fe values 0 which are likely related to the occurrence of an alluvium at this depth the fe silicate fraction shows higher 56fe values 0 6 0 9 most likely because of preferential leaching of the light fe isotopes this consequently indicates a more pronounced fe isotope fractionation with depth as is expected from longer weathering the fe sulfide fraction is isotopically lighter compared to the fe hydr oxide fraction because during sulfide oxidation the heavy fe isotopes prefer the oxidized forms and oxidative precipitation results in an enriched fe isotopic signature for fe hydr oxides the cu isotope compositions of all bulk samples and individual fractions except the cu sulfides of one site ch1 exhibited a decrease of the 65cu values from the depth towards the surface which is potentially related to cu mobilization during capillary water rise in the arid climate a correlation of 65cu with ph indicates preferential adsorption of 65cu on fe oxy hydroxides at site ch1 which is evident by a change of 65cu from 0 5 to 0 7 in the water soluble fraction at another site ch12 where ph at depths was potentially not high enough for the formation of fe minerals that could adsorb cu only minor cu isotope fractionation was observed in the water soluble fraction the cu sulfide fraction at site ch1 exhibits higher 65cu values with an increase from the bottom 0 42 to the surface 0 92 which might be related to preferential leaching of the light isotopes e g by microorganisms

the groundwater resources and sustainable yield of cheju volcanic island korea hydrogeologic data of 455 water wells comprising geologic logs water qualities and aquifer test results are analyzed to determine hydrogeological characteristics water quality and sustainable yield of the groundwater resources of cheju volcanic island the groundwater of the island occurs in unconsolidated pyroclastic deposits and clinkers interbedded in highly jointed basaltic and andesitic rocks as high level basal and parabasal groundwater under unconfined conditions the total storage of groundwater is estimated at about 44 billion m3 the average transmissivity and specific yield of the aquifer are at about 0 34 m2 s 1 29300 m2 day 1 and 0 12 respectively the average annual precipitation is about 3 39 billion m3 of which 1 49 billion m3 equivalent to 44 0 of the total annual precipitation is recharged into aquifers with 0 638 billion m3 year 1 of runoff and 1 26 billion m3 year 1 of evapotranspiration based on a groundwater budget analysis the sustainable yield is estimated at about 0 62 billion m3 year 1 equivalent to 41 6 of annual recharge a low permeability marine sedimentary formation sehwari formation composed of loosely cemented sandy silt was recently found to be situated at 120 68 m below mean sea level if the said marine sediment is distributed as a basal formation of the freshwater zone of the island most of its groundwater will be of parabasal type so the marine sediment is one of the most important hydrogeological boundaries and groundwater occurrences in the area

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

diagenetic origins of the calcite microcrystals that host microporosity in limestone reservoirs abstract micron size pores are common in limestones and they host significant hydrocarbon reserves in conventional and unconventional plays worldwide previous studies have shown that the vast majority of micropores in limestones occur in the interstices between micron size calcite crystals that are present in nearly all depositional environments and burial depths throughout the phanerozoic in an effort to elucidate the genetic history of these calcite microcrystals a geochemical study was undertaken to investigate a global dataset collected from seven new subsurface microporous reservoirs as well as data from 21 previous studies bulk elemental analyses indicate that the vast majority of microcrystals are composed of low mg calcite average 14 mmol mol range 1 6 90 mg ca microcrystal sr mg ratios are consistent with abiotic calcite cements though overall mg ca and sr ca are lower than modern abiotic calcite cements reported in the literature higher sr ca observed in depositional chalks implies a biotic though probably not aragonitic precursor most calcite microcrystals have oxygen isotope compositions that are 0 to 4 more negative and carbon isotope compositions just slightly more negative 1 than contemporaneous abiotic marine calcite most stable oxygen and carbon isotopic data plot either along a burial trend anchored at its most positive end in the age equivalent marine calcite composition or parallel to that trend with an offset in 13c assuming that 13c offsets are due to marine carbon isotopic excursions the isotopic data are consistent with shallow burial diagenesis as the dominant diagenetic pathway for most calcite microcrystals for samples with information on porosity and depth more negative 18o ca 1 4 is associated with increased depth and reduced porosity implying that progressive overgrowth of calcite microcrystals is modifying bulk chemistry and rock properties of the reservoir during burial diagenesis a meteoric calcite line was evident only in two datasets this demonstrates that evidence of meteoric diagenesis can be preserved in the chemistry of calcite microcrystals but that its occurrence is rare in comparison with evidence for diagenesis during shallow burial geochemical data from calcite microcrystals are bulk samples i e measurements averaged over many many calcite microcrystals which themselves may have zoned chemistries as such microcrystal data require additional interpretation compared to samples from calcite macrocrystals where superposition puts samples into clear chronological order and produces diagenetic trends on for example 18o 13c plots bulk samples of calcite microcrystals should show mixing between phases on diagenetic trends with this model in mind the assembled database suggests that most samples are mixes of primary marine and burial calcite

phosphorus and iron cycling in deep saprolite luquillo mountains puerto rico abstract rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations in the luquillo mountains of puerto rico a 5 m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock we test the hypotheses that the organisms at depth are limited by the availability of two nutrients p and fe many tropical soils are p limited rather than n limited and dissolution of apatite is the dominant source of p we document patterns of apatite weathering and of bioavailable fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7 5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut iron isotopic compositions of 0 5 n hcl extracts of soil and saprolite range from about 56 fe 0 to 0 1 throughout the saprolite except at the surface and at 5 m depth where 56 fe 0 26 to 0 64 the enrichment of light isotopes in hcl extractable fe in the soil and at the saprolite bedrock interface is consistent with active fe cycling and consistent with the locations of high cell densities and fe ii oxidizing bacteria identified previously to evaluate the potential p limitation of fe cycling bacteria in the profile solid state concentrations of p were measured as a function of depth in the soil saprolite and weathering bedrock weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6 8 10 14 mol m 2 s 1 was calculated while surface communities depend on recycled nutrients and atmospheric inputs deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering while low available p may limit microbial activity within the middle saprolite fluxes of p from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite

iron speciation and isotope fractionation during silicate weathering and soil formation in an alpine glacier forefield chronosequence abstract the chemical weathering of primary fe bearing minerals such as biotite and chlorite is a key step of soil formation and an important nutrient source for the establishment of plant and microbial life the understanding of the relevant processes and the associated fe isotope fractionation is therefore of major importance for the further development of stable fe isotopes as a tracer of the biogeochemical fe cycle in terrestrial environments we investigated the fe mineral transformations and associated fe isotope fractionation in a soil chronosequence of the swiss alps covering 150 years of soil formation on granite for this purpose we combined for the first time stable fe isotope analyses with synchrotron based fe exafs spectroscopy which allowed us to interpret changes in fe isotopic composition of bulk soils size fractions and chemically separated fe pools over time in terms of weathering processes bulk soils and rocks exhibited constant isotopic compositions along the chronosequence whereas soil fe pools in grain size fractions spanned a range of 0 4 in 56 fe the clay fractions 56 fe for all soils the lightest isotopic composition was observed in a 1 m nh 2 oh hcl 25 acetic acid extract targeting poorly crystalline fe iii hydr oxides compared with easily leachable fe in primary phyllosilicates 0 5 m hcl extract and fe in residual silicates the combination of the fe isotope measurements with the speciation data obtained by fe exafs spectroscopy permitted to quantitatively relate the different isotope pools forming in the soils to the mineral weathering reactions which have taken place at the field site a kinetic isotope effect during the fe detachment from the phyllosilicates was identified as the dominant fractionation mechanism in young weathering environments controlling not only the light isotope signature of secondary fe iii hydr oxides but also significantly contributing to the isotope signature of plants the present study further revealed that this kinetic fractionation effect can persist over considerable reaction advance during chemical weathering in field systems and is not only an initial transient phenomenon

tidal effects on variations of fresh saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island jeju island korea summary we conducted various field studies at the seawater intrusion monitoring wells located in the eastern part of jeju island korea to observe the tidal effect on groundwater seawater flow in the coastal aquifer studies included monitoring the fluctuations of groundwater and tide levels electrical and temperature logging and 2 d heat pulse flowmeter tests according to time series analysis tidal effects on groundwater level reached up to 3 km inland from the coastline water level variation was more sensitive to tidal fluctuations near the coast and more related to rainfall toward inland areas temporal and spatial variations in the shape and location of the freshwater saltwater interface were analyzed using data from nine monitoring wells the results indicated that the interface toe is located at a distance of 6 8 km from the coastline and its location was related to geological layers present long term seasonal variations revealed no major changes in the interface minor variations were due to moving boundary conditions induced by tidal fluctuations using the two dimensional heat pulse flowmeter groundwater flow directions and velocities at four tidal stages were measured on three monitoring wells drilled into the multilayered aquifers this direct measurement enabled us to relate the differences of flow velocities and directions with geology and tidal fluctuations combining the results of ec logging and flowmeter tests we found a zone where freshwater and saltwater moved alternately in opposite directions as influenced by the tidal fluctuations integrating various physical logging and flowmeter data with water level fluctuations improved our understanding of the behavior of fresh and seawater flow in the coastal aquifers

the influence of weathering and soil organic matter on zn isotopes in soils zinc is an essential micronutrient that is ultimately released during mineral weathering in soils organic matter plays a key role in influencing zn partitioning and therefore on zn biogeochemical cycling soil organic matter is partitioned between carbon that is more readily available for decomposition by microorganisms and more stable carbon transiently preserved from decomposition the role of the stable pool of soil organic matter on zn biogeochemical cycling remains poorly understood the pool of stable carbon is controlled by combination with mineral constituents or is material that is intrinsically resistant to decomposition the zn stable isotopes are fractionated by interactions between zn and soil mineral and organic constituents this study reports the zn isotope composition of five icelandic soil profiles derived from the same parent basalt and characterized by contrasting degrees of weathering and organic matter content 66zn 0 10 0 05 to 0 35 0 02 the distribution of reactive mineral constituents available to form associations with soil organic matter and the amount of stable organic carbon throughout these soils the 66zn isotope variations are little influenced by mineral constituents but rather by soil organic matter content these data suggest that a combination of organic matter accumulation and zn loss by leaching is required to explain the observed decrease in zn concentration in soils and lighter soil 66zn with increasing organic carbon content these results suggest that the presence of stable organic carbon in soils provides a pool of light zn attributed to the zn isotope signature of organic matter partially preserved from decomposition crucially this stable organic carbon pool may also contribute to the formation of the light zn isotope sink reported in organic rich marine sediments a key output required to explain the oceanic mass balance of zn isotopes

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

extreme iron isotope fractionation between colloids and particles of boreal and temperate organic rich waters abstract significant and systematic variations of iron isotopic composition in surface water sample fractions obtained by frontal cascade filtration and ultrafiltration have been recorded in 1 subarctic organic rich boreal river and stream mire lake and soil solutions in northern taiga zone karelia nw russia and 2 temperate river and lake waters of the southern boreal zone central russia water samples were filtered in the field employing progressively decreasing pore size from 100 m to 1 kda followed by iron isotope analysis in all river samples there was a gradual increase of 57fe relative to irmm 14 with decreasing pore size from 0 4 0 1 at 100 m up to 4 2 0 1 at 10 kda fraction in the subarctic zone and from 0 024 0 2 at 100 m up to 1 2 0 2 at 10 kda in the temperate zone in the series of filtrates ultrafiltrates of subarctic and temperate streams and rivers the 57fe value decreases with increasing molar fe corg ratio therefore small size fe poor c rich colloids 1 10 kda and low molecular weight lmw fractions of oxygenated water exhibit strong enrichment in heavy isotope whereas high molecular weight fe rich colloids 100 kda 0 22 m and particles 1 100 m are isotopically lighter and closer to the continental crust fe isotope composition the relative enrichment of 1 10 kda ultrafiltrates in heavy isotopes suggests that low molecular weight ligands bind fe more strongly fe o c bonds than fe iii oxy hydr oxides fe o fe bonds in accord with quantum mechanics principles highly positive 57fe of the lmw fraction of labile and potentially bioavailable fe in small subarctic rivers may turn out to be a very important source of isotopically heavy fe in the arctic ocean the mechanisms involved in the production of this isotopically heavy fe may lead this tracer to become a new indicator of environmental changes occurring in the boreal zone

the low temperature geochemical cycle of iron from continental fluxes to marine sediment deposition suspended sediments from 34 major rivers geographically wide spread and 36 glacial meltwater streams have been examined for their variations in different operationally defined iron fractions fehr iron oxides soluble in dithionite fepr iron soluble in boiling hcl but not in dithionite and feu total iron less that soluble in boiling hcl river particulates show a close association between fehr and total iron fet reflecting the effects of chemical weathering which derive oxide iron from and retain it in close association with total iron consistent with this continental scale average fehr fet ratios vary with runoff ratios average river runoff per unit area average precipitation per unit area by contrast the diminished effects of chemical weathering produce no recognizable association of fehr with fet in glacial particulates and instead both fepr and feu are closely correlated with fet reflecting essentially pristine mineralogy a comparison of the globally averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in fehr fepr and fet but enriched in feu the river and glacial particulate data are combined with estimates of authigenic hydrothermal atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for fehr f e pr f e u and fet this budget suggests that the differences between riverine particulates and marine sediments can be explained by i preferentially removing fehr from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains salt marshes and estuaries and ii mixing the resulting riverine particulates with fehr depleted glacial particulates preliminary measurements of inner shore sediments are consistent with i above phanerozoic and modern normal marine sediments have similar iron speciation characteristics which implies the existence of a long term steady state for the iron cycle this steady state could be maintained by a glacioeustatic feedback where fehr enriched riverine particulates are either more effectively trapped when sealevel is high small ice masses diminished glacial erosion or are mixed with greater masses of fehr depleted glacial particulates when sealevel is low large ice masses enhanced glacial erosion further important controls on the steady state for fehr operate through the formation of euxinic sediments and ironstones which also provide sealevel dependent sinks for fehr enriched sediment

continuous measurements of riverine chemical constituents reveal the environmental effect of the 2014 2015 barðarbunga eruption in iceland abstract the 2014 2015 bar arbunga eruption caused one of the most intense and widespread volcanic derived air pollution events in centuries the volcanic plume was tracked all the way to central europe volcanic gas and aerosol dissolved in precipitation and accumulated as aerosol and snow on land in iceland here we report on the continuous monitoring of discharge and dissolved constituents of a direct runoff river located 85 km from the eruption site during the entire snowmelt period in 2015 osmotic samplers collected continuous water samples so short lived flood peaks would not be missed comparison between results from this 2015 study and data from 1998 to 2013 from the same river showed that average ph was 0 2 units lower in 2015 and the cl and so4 average concentration were 36 and 38 higher than prior to the eruption respectively integrated riverine fluxes of h and so4 were two to three times higher during summer 2015 than in the control years and the cl flux was 37 higher the measured excess h flux was however only a small fraction of the calculated excess h flux stemming from excess so4 and cl fluxes in 2015 which can be explained by ion exchange in the catchment soil water rock interaction consuming protons within the catchment and by elevated sea spray derived cl and so4 due to relatively high wind speed during the 2014 2015 winter in addition the small h increase in the river water can be the cause of the less than expected excess so4 riverine flux normalised to the total measured volcanogenic so2 and hcl emission fluxes 11 tg 0 1 tg which underscores the slow oxidation of so2 to so4 in relatively dry weather conditions and reduced sunlight during winter in iceland despite being sensitive to riverine ph and aluminium changes freshwater ecosystem was not threatened by the eruption since it did not cause large ph changes and the dissolved aluminium concentration was always low during the study period this study provided the means to quantify the integrated chemical fluxes and the impact of volcanic emissions on a riverine system in the vicinity of the eruption site and the implication of increased chemical fluxes of solutes of volcanic origin on the system

silicon isotopes record dissolution and re precipitation of pedogenic clay minerals in a podzolic soil chronosequence by providing the largest part of the reactive surface area of soils secondary minerals play a major role in terrestrial biogeochemical processes the understanding of the mechanisms governing neo trans formation of pedogenic clay minerals in soils is therefore of the utmost importance to learn how soils evolve and impact the chemistry of elements in terrestrial environments soil forming processes governing the evolution of secondary aluminosilicates in podzols are however still not fully understood the evolution of silicon si isotope signature in the clay fraction of a podzolic soil chronosequence can provide new insight into these processes enabling to trace the source of si in secondary aluminosilicates during podzol forming processes characterized by the mobilization transport and precipitation of carbon metals and si the si isotope compositions in the clay fraction comprised of primary and secondary minerals document an increasing light 28 si enrichment and depletion with soil age respectively in illuvial b horizons and eluvial e horizon the mass balance approach demonstrates that secondary minerals in the topsoil eluvial e horizons are isotopically heavier with 30 si values increasing from 0 39 to 0 64 in c a 200years while secondary minerals in the illuvial bhs horizon are isotopically lighter 30 si 2 31 compared to the original unweathered secondary minerals in bc horizon 30 si 1 40 the evolution of si isotope signatures is explained by the dissolution of pedogenic clay minerals in the topsoil which is a source of light 28 si for the re precipitation of new clay minerals in the subsoil this provides consistent evidence that in strong weathering environment such as encountered in podzols si released from secondary minerals is partially used to form tertiary clay minerals over very short time scales ca 300years our dataset demonstrates the usefulness to measure si isotope signatures in the clay fraction to discern clay mineral changes e g neoformation versus solid state transformation during soil evolution this offers new opportunity to better understand clay mineral genesis under environmental changes and the short term impact of the dissolution and re precipitation of pedogenic clay minerals on soil fertility soil carbon budget and elemental cycles in soil plant systems 2014 elsevier b v

iron and silicon isotope behaviour accompanying weathering in icelandic soils and the implications for iron export from peatlands abstract incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions which has implications for fe export from soils this study uses fe isotopes to assess the processes controlling fe export in a range of icelandic soils including peat soils derived from the same parent basalt where fe isotope variations principally reflect differences in weathering and drainage in poorly weathered well drained soils non peat soils the limited fe isotope fractionation in soil solutions relative to the bulk soil 57fesolution soil 0 11 0 12 is attributed to proton promoted mineral dissolution in the more weathered poorly drained soils peat soils the soil solutions are usually lighter than the bulk soil 57fesolution soil 0 41 0 32 which indicates that fe has been mobilised by reductive mineral dissolution and or ligand controlled dissolution the results highlight the presence of fe organic complexes in solution in anoxic conditions an additional constraint on soil weathering is provided by si isotopes the si isotope composition of the soil solutions relative to the soil 30sisolution soil 0 92 0 26 generally reflects the incorporation of light si isotopes in secondary aluminosilicates under anoxic conditions in peat soils the largest si isotope fractionation in soil solutions relative to the bulk soil is observed 30sisolution soil 1 63 0 40 and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation si supersaturation in solution with respect to amorphous silica is reached upon freezing when al availability to form aluminosilicates is limited by the affinity of al for metal organic complexes therefore the precipitation of amorphous silica in peat soils indirectly supports the formation of metal organic complexes in poorly drained soils these observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands fe export from soils as fe organic complexes will increase which in turn has implications for fe transport in rivers and ultimately the delivery of fe to the oceans

volcanic soils of iceland abstract the parent materials of all icelandic soils are volcanic in origin and most of the soils are classified as andosols iceland has the largest area of andosols in europe and may have 5 of all andosols in the world a new classification scheme based on the world reference base world reference base for soil resources world soil resources reports 84 fao rome is used to provide an overview of icelandic volcanic soils the data were acquired as part of a recently initiated effort to build a database for icelandic soils but are also partly based on data from the eu cost 622 action a literature review and unpublished icelandic research the main soil types in iceland are histosols histic andosols gleyic andosols brown andosols vitrisols and leptosols in the icelandic language a single term is used to refer to each soil type this system reflects the dominant influence of eolian deposition and drainage conditions on the formation of histosols and andosols the parent materials of icelandic soils are mixtures of tephra layers and eolian sediments consisting mostly of volcanic glass the steady flux of eolian sediments from unstable sandy deserts constantly recharges the surface with basaltic tephra materials the histosols 20 c have appreciable amounts of andic materials because of the eolian deposition the andosols histic gleyic and brown andosols show characteristic properties of soils dominated by andic materials with allophane contents of up to 30 ferrihydrite commonly accounts for 5 15 of the soil the contents of both allophane and ferrihydrite generally decrease with increasing organic c content of soil horizons as organic matter inhibits allophane formation and iron tends to become reduced and lost in poorly drained locations the vitrisols are soils of desert surfaces with low organic content but they usually meet criteria for andic materials with 30 glass and al 1 2fe o 0 4 cryoturbation is intense in iceland resulting in a range of geomorphic surface features the cryoturbation is related to both climatic factors and the andic soil properties including high water retention and rapid hydraulic conductivity

limited iron isotope variations in recent lateritic soils from nsimi cameroon implications for the global fe geochemical cycle laterites are oxidized fe rich soils covering one third of the continents and are drained by half of the continental waters they therefore represent a key component of the iron geochemical cycle at the earth s surface yet no iron isotope study has been conducted on recent laterites so far building on previous integrated morpho pedological studies of soils located in an equatorial rainforest southern cameroon we have undertaken a mineralogical elemental and isotopic study of iron in two lateritic profiles one is a borehole 3620 cm deep going to the parent granodioritic rock located at the top of a hill whereas the other is 675 cm deep and is located downhill mossbauer spectroscopy reveals that typically geq 96 of the soil s iron is held in nanocrystalline hematite and goethite iron isotope measurements performed by plasma source mass spectrometry show that fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size overall it is found that most samples display iron isotope signatures very close to the mean crustal value with a maximum range of 0 2 in delta 57 fe given that lateritic soils evolve over millions of years show large variations in fe concentrations and mineralogical abundances this range is surprisingly small when compared to other fe isotope studies of soils from different climatological contexts that can easily show delta 57 f e ranges in excess of 1 at the bulk sample scale the most likely explanation of this finding is that despite notable vertical and lateral fe mobility in the studied laterites from cameroon as computed using the open system mass fraction transport function tau fe w iron remained mostly in the oxidized form as shown by mossbauer pectroscopy this probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon this study therefore reveals that soils that remained as an open system for iron do not necessarily show large fe isotopic variations it is suggested that the fe drained from such lateritic soils should have delta 57 fe values within sim 0 1 of that of the continental crust this is in contrast with previous soil studies notably from sites located at higher latitudes that imply that these soils should release isotopically more variable fe to surface waters such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of fe in seawater

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

smectites and zeolites in ash from the 2010 summit eruption of eyjafjallajokull volcano iceland hydrothermal alteration minerals are often incorporated in volcanic ash from phreatic and phreatomagmatic activity here we assess the presence and abundance of such minerals in the ash materials produced during the april may 2010 initial phreatomagmatic phase i and subsequent magmatic phases ii and iii eruptions of eyjafjallajokull volcano iceland the results of x ray diffraction analyses reveal significant quantities of smectites up to 4 wt mainly as saponite and zeolites up to 7 wt in ash from phasei while a minor amount of smectites 0 5 wt is present in ash from the subsequent weak explosive activity phaseii both smectites and zeolites are absent in phaseiii ash this material was generated following abrupt rejuvenation of explosive activity in the absence of magma ice water interaction smectites and zeolites in phasei ash result primarily from scouring of altered volcanic rocks in the subsurface although some may derive also from water rock interaction within the summit ice cauldrons through which fragmented magma was injected we show that incorporation of smectites and zeolites in phasei ash can explain its anomalously high specific surface area further the presence of these minerals in ash may enhance its ability to act as ice nuclei as well as favour particle aggregation processes in the volcanic plume cloud finally the eyjafjallajokull eruption represents another case in which ash fallout acted as an exogenic source of 2 1 type clay minerals in volcanic soils

ekofisk first of the giant oil fields in western europe discovery of the giant ekofisk field in block 2 4 in the norwegian part of the north sea in 1969 was a major turning point in the exploration for petroleum in western europe since that time the north sea has proven to be one of the best areas for exploration anywhere in the world current production is 1 5 million barrels of oil per day and north sea proven reserves total 18 billion barrels with estimates of ultimate reserves as high as 40 billion barrels ekofisk is located in the central graben in the southern part of the norwegian sector of the north sea although several periods of tectonism have affected this area it has remained an intercratonic basin since devonian time the main elements of the tectonic fabric were established during the caledonian and hercynian orogenies and later remained as controlling features for facies and sediment distribution a stratigraphic history reveals petrography of the main chalk group environment of deposition and diagenetic history of the area a section outlines the preservation of porosity which has allowed the ekofisk and tor formations to retain an average porosity of 30 to 40 percent which would not have been possible under normal circumstances it is appropriate to state that the anomalously high porosity in ekofisk field is probably due to a combination of 1 overpressuring of the reservoir 2 magnesium rich pore fluids and 3 early introduction of hydrocarbons the six greater ekofisk fields now being developed were all located during the 1960s by reconnaissance seismic work the history of the geophysical exploration is outlined and a study has been made on source rock analysis and geopressures also outlined is the three phase greater ekofisk development program

variations of δ30si and ge si with weathering and biogenic input in tropical basaltic ash soils under monoculture in soils silicon released by mineral weathering can be retrieved from soil solution through clay formation si adsorption onto secondary oxides and plant uptake thereby impacting the si isotopic signature and ge si ratio of dissolved si dsi exported to rivers here we use these proxies to study the contribution of biogenic si bsi in a soil plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping delta si 30 and ge si ratios were determined in bulk soils 2 mm sand 50 2000 mu m silt 2 50 mu m amorphous si asi 2 50 mu m and clay 2 mu m fractions delta si 30 by mc icp ms nu plasma in medium resolution operating in dry plasma with mg doping delta si 30 vs nbs28 0 12 parts per thousand 2 sigma sd ge si computed after determination of ge and si concentrations by hr icp ms and icp aes respectively components of the asi fraction were quantified by microscopic counting phytoliths diatoms ashes compared to fresh ash delta si 30 0 38 parts per thousand ge si 2 21 mu mol mol 1 soil clay fractions 2 mu m were enriched in light si isotopes and ge with increasing weathering degree delta si 30 decreased from 1 19 to 2 37 parts per thousand and ge si increased from 4 10 to 5 25 mu mol mol 1 sand and silt fractions displayed delta si 30 values close to fresh ash 0 33 parts per thousand or higher due to saharian dust quartz deposition whose contribution was evaluated by isotopic mass balance calculation si isotopic signatures of bulk soils 2 mm were strongly governed by the relative proportions of primary and secondary minerals the bulk soil si isotopic budget could be closed indicating that all the phases involved were identified microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation delta si 30 and ge si values of clay fractions in poorly developed volcanic soils isotopically heavier and ge depleted in surface horizons support the occurrence of a dsi source from banana phytolith dissolution available for si sequestration in clay sized secondary minerals clay minerals formation and si adsorption onto fe oxide in the soil plant system delta si 30 and ge si are thus highly relevant to trace weathering and input of dsi from phytoliths in secondary minerals although not quantifying the net input of bsi to dsi c 2009 elsevier ltd all rights reserved

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

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

iron isotope fractionation during leaching of granite and basalt by hydrochloric and oxalic acids abstract transport of iron fe within hydrothermal and soil environments involves the transferral into aqueous solutions by leaching of complex polyminerallic rocks understanding the isotope fractionation mechanisms during this process is key for any application of the fe isotope system to biogeochemical studies here we reacted biotite granite and tholeiite basalt with 0 5 m hydrochloric acid and 5 mm oxalic acid solutions at ambient temperature solution aliquots were recovered over a seven day period and analysed for major and trace element concentrations and fe isotopic compositions in all experiments fe initially released into solution was isotopically lighter with 56fesolution rock as low as 1 80 in the granite hydrochloric acid system the oxalic acid experiments showed similar patterns but smaller fractionation in all experiments the 56fesolution rock reduced over time which would be in line with the formation of a leached layer as proposed before brantley s l liermann l j guynn r l anbar a icopini g a and barling j 2004 fe isotopic fractionation during mineral dissolution with and without bacteria geochim cosmochim acta 68 15 3189 3204 granite and basalts reacting with hydrochloric acid reached apparent steady state values of 0 60 0 15 and 0 40 0 20 respectively whilst experimental values with oxalic acid were 1 0 0 15 and 0 50 0 15 during the granite experiments alteration of biotite to chlorite followed by dissolution of chlorite were likely the dominant processes whilst in the basalt experiments dissolution of pigeonite was likely the principal source of fe variations in ph during the hydrochloric acid experiments were minimal remaining below 0 5 at all times in oxalic acid solutions the ph increased to over 4 leading likely to precipitation of secondary minerals and adsorption co precipitation of fe onto mineral surfaces these processes could contribute to the greater fractionation observed in the final stages of the oxalic acid experiments our results highlight the importance of mineralogy and fluid composition on the fe isotope systematics during weathering the fractionation processes identified for granites and basalts are in line with those inferred from field observations in soils sediments groundwater and hydrothermal deposits and from laboratory studies of single mineral leaching

magnesium retention on the soil exchange complex controlling mg isotope variations in soils soil solutions and vegetation in volcanic soils iceland understanding the biogeochemical cycle of magnesium mg is not only crucial for terrestrial ecology as this element is a key nutrient for plants but also for quantifying chemical weathering fluxes of mg and associated atmospheric co2 consumption requiring distinction of biotic from abiotic contributions to mg fluxes exported to the hydrosphere here mg isotope compositions are reported for parent basalt bulk soils clay fractions exchangeable mg seasonal soil solutions and vegetation for five types of volcanic soils in iceland in order to improve the understanding of sources and processes controlling mg supply to vegetation and export to the hydrosphere bulk soils 26mg 0 40 0 11 are isotopically similar to the parent basalt 26mg 0 31 whereas clay fractions 26mg 0 62 0 12 exchangeable mg 26mg 0 75 0 14 and soil solutions 26mg 0 89 0 16 are all isotopically lighter than the basalt these compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex successive adsorption desorption of heavy mg isotopes leads to the preferential loss of heavy mg from the soil profile leaving soils with light mg isotope compositions relative to the parent basalt additionally external contributions from sea spray and organic matter decomposition result in a mixture of mg sources on the soil exchange complex vegetation preferentially takes up heavy mg from the soil exchange complex 26mgplant exch 0 50 0 09 and changes in 26mg in vegetation reflect changes in bioavailable mg sources in soils this study highlights the major role of mg retention on the soil exchange complex amongst the factors controlling mg isotope variations in soils and soil solutions and demonstrates that mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of mg that is bioavailable for plants and is exported to the hydrosphere

the role of early lithification in development of chalky porosity in calcitic micrites upper cretaceous chalks egypt abstract petrological and geochemical examination of upper cretaceous chalks from the abu roash area and bahariya oasis egypt provide insights into factors which control porosity development in fine grained calcitic carbonates petrological studies indicate that primary skeletal fabrics of coccoliths and foraminifera in micrites of abu roash are well preserved in contrast primary skeletal fabrics of the bahariya oasis chalks are pervasively altered geochemical analysis of these chalks reveals a striking contrast in composition the high porosity chalks at bahariya oasis have depleted isotopic values 13 c 5 0 18 o 8 9 pdb and low concentrations of trace elements sr and na in contrast the low porosity chalks at abu roash are less depleted in terms of their isotopic values 13 c 1 0 18 o 4 0 and possess elevated concentrations of sr na fe and zn importantly fracture and collapse veins present only at abu roash are filled with calcite whose 18o value is coincident with that of altered micrites at bahariya oasis the lithic fragments cemented by this calcite however retain compositions identical to other lithified limestones at abu roash differences in fabric and chemistry of the pelagic marine micrites observed between abu roash and bahariya oasis reflect the degree of early lithification and compaction prior to exposure and meteoric diagenetic alteration associated with regional late cretaceous pre eocene subaerial unconformity

insights into iron sources and pathways in the amazon river provided by isotopic and spectroscopic studies abstract the present study investigated the weathering and transport mechanisms of fe in the amazon river a particular emphasis was placed on fe partitioning speciation and isotopic fractionation in the contrasting waters of the solimoes and negro rivers and their mixing zone at the beginning of the amazon river samples collected in the end member rivers and thirteen sites distributed throughout the mixing zone were processed through frontal vacuum filtration and tangential flow ultrafiltration to separate the different suspended solid fractions i e particulate p 0 45 m and p 0 22 m colloidal 0 22 m c 5 kda and truly dissolved elements td 57 fe values relative to irmm 14 while the particulate fractions yielded light isotopic compositions of 0 344 for p 0 22 m and 0 104 for p 0 45 m fractions the mineral particulate rich waters of the solimoes river had dissolved and colloidal fractions with light isotopic composition 0 532 and 0 176 respectively whereas the particulate fractions yielded 57 fe values close to those of the continental crust i e 0 029 for p 0 22 m and 0 028 for p 0 45 m ten kilometers downstream from the negro and solimoes junction the concentrations of colloidal and dissolved fe species deviate markedly from conservative mixing a maximum fe loss of 43 g l i e 50 of the dissolved and colloidal fe is observed 110 km downstream from the rivers junction the contrasting negro and solimoes rivers isotopic compositions along the pore sized water fractions is attributable to the biogeochemical processes involving different types of upland soils and parental materials for instance the isotopic composition of colloidal and dissolved fe from the negro river are consistent with fe oxidation and complexation mechanisms at the interface between waterlogged podzols and river networks as supported by strong organo fe complexes signals observed by epr conversely the particulate and colloidal fractions from the solimoes river have 57 fe consistent with strong mechanical erosion in the andean cordillera and upland soils as evidenced by high concentrations of fe 3 oxides sensu lato measured by epr the massive dissolved and colloidal fe removal is associated with the evolution of the physical and chemical composition of the waters i e ionic strength during mixing which influences organo fe 3 and fe 3 oxyhydroxides stability several models are discussed to explain fe non conservative behavior including dissociation of organo fe complexes and the subsequent formation of solid fe 3 oxyhydroxides and semiquinone free radicals as evidenced by epr spectra demonstrating that organo fe signals decrease as fe 3 oxyhydroxides and free radicals signals increase as in estuarine regions the mechanisms involving fe transfer and loss in the mixing zone has a negligible effect on the bulk water fe isotopic composition this result suggests that a tropical basin similar to the amazon river basin delivers to the ocean waters with an fe isotopic composition similar to that of the earth s continental crust

experimental constraints on kinetic and equilibrium silicon isotope fractionation during the formation of non biogenic chert deposits silicon isotopic compositions 30si of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments but interpretations are hampered by the lack of experimentally validated fractionation factors here we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch reactors at low temperature 10 35 c and near neutral ph 7 5 8 5 as analogue for non biogenic chert formation instantaneous fractionation factors derived from 30si values of the total dissolved sitd silica and mass balance computations with inst 30sippt 1000 30sitd 1000 decrease with progressive precipitation and reduced reaction rates this suggests that silica deposition in the batch reactors is kinetically dominated at the start of the experiments but approaches a metastable equilibrium after ca 400hours modelled kinetic fractionation factors range from 0 9965 at 10 c to 0 9976 at 20 c and 0 9993 at 35 c and ph8 5 whereas equilibrium isotope effects are smaller and range from 0 9995 at 10 c to 1 000 at 20 c and 1 0005 at 35 c our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated implying that the kinetic conditions of non biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks with particular relevance for those preserved in the archean chert record

the isotopic fingerprint of fe cycling in an equatorial soil plant water system the nsimi watershed south cameroon abstract following an initial study of a tropical lateritic hillside system showing little iron isotope fractionation despite a strong accumulation within the soil profile the present work investigates iron isotope signatures within the organic matter rich swamp system that represents 20 of the studied watershed surface from nsimi south cameroon this study considers the soil plant water continuum in order to better understand the fe elemental and isotopic transfer out of the ecosystem within the swamp system the iron isotope compositions of gleysol samples 57 fe irmm 14 0 6 are significantly heavier than both the continental crust baseline and the reference lateritic soils from the hillslope 57 fe irmm 14 0 1 this enrichment towards heavy isotopes is attributed to a preferential removal of light iron isotopes during soil forming processes pedogenic transformations i e gleyzation organic complexation of metal and leaching are responsible for the reductimorphic features observed in ferralitic horizons i e incomplete degradation of organic matter in surface and soil whitening favored by good draining conditions the organic carbon rich waters of the swamp system are prone to redox processes and strong metal chelation the dissolved iron i e fraction 57 fe signatures with a downstream enrichment in heavy isotopes from 0 511 0 266 to 1 076 0 240 the binding of iron fe iii with organic matter can explain the observed enrichment in heavy isotopes in the dissolved fraction on the contrary plant leaves are significantly enriched in light fe 57 fe of 0 665 0 035 and 1 119 0 080 relative to i the litter compartment 0 166 0 078 to 0 262 0 013 for 57 fe and ii the most superficial soils iron isotopic compositions in plants and litter vary as a function of both plant species and season hence the differences in fe isotopic compositions between the various studied compartments suggest that fe isotopes can be used i to study elemental transfers during soil pedogenesis in tropical environment and ii to better appraise and constrain iron biogeochemical cycle between surface horizon of soils surface waters and the vegetation

fractionation of fe isotopes by soil microbes and organic acids small natural variations in fe isotopes have been attributed to biological cycling however without understanding the mechanisms of fractionation it is impossible to interpret such variations here we show that the d 56 fe of fe dissolved from a silicate soil mineral by siderophore producing bacteria is as much as 0 8 lighter than bulk fe in the mineral a smaller isotopic shift is observed for fe released abiotically by two chelates and the magnitude of the shift increases with affinity of the ligand for fe consistent with a kinetic isotope effect during hydrolysis of fe at the mineral surface fe dissolved abiotically without chelates shows no isotopic shift the d 56 fe of the exchange fraction on soil grains is also lighter by 0 6 1 than fe from both hornblende and iron oxyhydroxides the kinetic isotope effect is therefore preserved in open systems such as soils when recorded in the rock record fe isotopic fractionation could document fe transport by organic molecules or by microbes where such entities were present in the geologic past

riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain this study presents u and li isotope and major and trace element data for the dissolved load suspended particulates and bedload for icelandic rivers draining predominantly basaltic catchments physical erosion rates range from 920 to 2084 t km2 yr with the higher values associated with glacier fed rivers chemical erosion rates range from 45 to 91 t km2 yr with lower rates being associated with glacier fed rivers uranium activity ratios 234u 238u are close to secular equilibrium in the suspended and bedloads but all dissolved load samples show values greater than unity ranging from 1 13 to 2 41 the highest 234u 238u values are found in glacier fed rivers and can be attributed to recoil effects as grinding by glaciers locally enhances rates of physical weathering activity ratios in glacial rivers decrease with distance from the glacial source due to input from non glacial tributaries which have high levels of dissolved uranium and lower activity ratios in contrast in non glacial rivers uranium activity ratios increase with distance downstream due to continued weathering in soils and of bedrock the 7li value of the suspended load is always lower than that of the bedload due to preferential retention of 6li in secondary minerals during weathering in turn the 7li value of the dissolved load is always greater than that of the bedload ranging from 17 0 to 43 7 7li for the dissolved load decreases with increasing levels of dissolved silicon and the saturation index of secondary minerals which suggests that 7li decreases with increasing chemical weathering there is no correlation between 7li values and uranium activity ratios for the dissolved load for non glacial rivers but for glacier fed rivers there is an increase in 7li with increasing 234u 238u suggesting that where physical comminution of mineral grains by glaciers is high chemical weathering is suppressed and vice versa

contrasting iron isotopic compositions in river suspended particulate matter the negro and the amazon annual river cycles abstract iron isotopic compositions fe irmm 14 56 of suspended particulate matter spm from two major rivers in the amazon basin the amazon river itself at the obidos station and the negro river at the serrinha station were investigated in the present study the main objective was to search for temporal variations during their annual river cycles fe irmm 14 56 values for the amazon river at obidos range between 0 00 and 0 15 indistinguishable from the average continental crust value in contrast the iron isotopic compositions of the negro river serrinha station spm vary between 0 34 and 0 82 whereas the dissolved matter is isotopically heavier in this river the lack of significant isotopic variations in the amazon river indicates that one individual spm subsurface sample is representative of the river during the whole annual river cycle in opposition to results obtained for the negro river the data suggest that in organic poor white water rivers such as the amazon iron isotopic signatures of the suspended fraction reflect a detrital crustal component with little isotopic fractionation on the other hand in the organic rich negro river which has tropical podzols as the main iron source the iron redox cycling at the water soil interface influences the iron isotopic composition

iron isotope fractionation in river colloidal matter temporal variations in the iron isotopic composition 56fe between 0 13 and 0 31 have been measured in the suspended fraction in a boreal river the major mechanism behind these variations is temporal mixing between two types of particles colloids fe oxyhydroxides and fe c colloids data in this study indicate that these two types of colloids have different fe isotope composition the fe c colloid has a negative 56fe value whereas the fe oxyhydroxide colloid is enriched in 56fe these two types of colloidal matter have different hydrogeochemical origin the fe c colloid reaches the river during storm events when the upper sections of the soil profile o and e horizons are flooded by a rising water table colloidal fe oxyhydroxides reach the river via inflow and subsequent oxidation of groundwater enriched in dissolved fe ii