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

large iron isotope fractionation at the oxic anoxic boundary in lake nyos abstract the degassing of volcanic lake nyos cameroon provides the opportunity to study the strong isotopic variation of dissolved fe ii in a well constrained redox cycle and to identify the governing processes by reaction transport modeling two depth profiles sampled in the lake in march 2004 and 2005 reveal an increase in iron concentrations and 57 fe from around 1 mg l 1 and 1 88 at 55 m depth up to 344 mg l 1 and 0 83 at the bottom of the lake respectively a steep increase in 57 fe was observed across the oxic anoxic boundary as many biological and geochemical processes are known to fractionate fe isotopes we used a calibrated reaction transport model to disentangle the processes governing the fe cycle the model combines the isotopic signatures of dissolved fe ii and settling fe iii particles with the concentration profiles and settling fluxes of the fe particles in the lake we show that the strong shift in 57 fe is caused by isotopic fractionation via dissimilatory fe reduction across the oxic anoxic boundary of lake nyos the shift towards more positive values below the oxic anoxic interface could be attributed to vertical mixing of a heavier component from the bottom of the lake

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 effect of early diagenesis on the fe isotope compositions of porewaters and authigenic minerals in continental margin sediments abstract iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the california continental margin where isotope compositions range from 56fe 3 0 to 0 4 at one site near monterey canyon off central california organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction dir and bacterial sulfate reduction whereas at our other site in the santa barbara basin dir appears to be comparatively small and production of sulfides fes and pyrite was extensive the largest range in fe isotope compositions is observed for fe ii aq in porewaters which generally have the lowest 56fe values minimum 3 0 near the sediment surface and increase with burial depth 56fe values for fes inferred from hcl extractions vary between 0 4 and 0 4 but pyrite is similar at both stations where an average 56fe value of 0 8 0 2 was measured we interpret variations in dissolved fe isotope compositions to be best explained by open system behavior that involves extensive recycling of feflux this study is the first to examine fe isotope variations in modern marine sediments and the results show that fe isotopes in the various reactive fe pools undergo isotopic fractionation during early diagenesis importantly processes dominated by sulfide formation produce high 56fe values for porewaters whereas the opposite occurs when fe iii oxides are present and dir is a major pathway of organic carbon respiration because shelf pore fluids may carry a negative 56fe signature it is possible that the fe isotope composition of ocean water reflects a significant contribution of shelf derived iron to the open ocean such a signature would be an important means for tracing iron sources to the ocean and water mass circulation

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

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

plant induced weathering of a basaltic rock experimental evidence abstract the active role of higher plants in the weathering of silicate minerals and rocks is still a question for debate the present work aimed at providing experimental evidence of the important role of a range of crop plants in such processes in order to quantitatively assess the possible effect of these diverse plant species on the weathering of a basaltic rock two laboratory experiments were carried out at room temperature these compared the amounts of elements released from basalt when leached with a dilute salt solution in the presence or absence of crop plants grown for up to 36 days for si ca mg and na plants resulted in an increase in the release rate by a factor ranging from 1 to 5 in most cases ca and na seemed to be preferentially released relative to other elements suggesting that plagioclase dissolved faster than the other constituents of the studied basalt negligible amounts of fe were released in the absence of plants as a consequence of the neutral ph and atmospheric po2 that were maintained in the leaching solution however the amounts of fe released from basalt in the presence of plants were up to 100 to 500 fold larger than in the absence of plants for banana and maize the kinetics of dissolution of basalt in the absence of plants showed a constantly decreasing release rate over the whole duration of the experiment 36 days no steady state value was reached both in the absence and presence of banana plants however in the latter case the rates remained at a high initial level over a longer period of time up to 15 days before starting to decrease for fe the maximum rate of release was reached beyond 4 days and this rate remained high up to 22 days of growth of banana the possible mechanisms responsible for this enhanced release of elements from basalt in the presence of plants are discussed although these mechanisms need to be elucidated the present results clearly show that higher plants can considerably affect the kinetics of dissolution of basalt rock therefore they need to be taken into account when assessing the biogeochemical cycles of elements that are major nutrients for plants such as ca mg and k but also micronutrients such as fe and nonessential elements such as si and na

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

geometry and grain size sorting of ripples on low energy sandy beaches field observations and model predictions there are very few field measurements of nearshore bedforms and grain size distribution on low energy microtidal beaches that experience low amplitude long period waves field observations are needed to determine grain size distribution over nearshore bedforms which may be important for understanding the mechanisms responsible for ripple development and migration additional nearshore field observations of ripple geometry are needed to test predictive models of ripple geometry ripple height length and sediment composition were measured in the nearshore of several low energy beaches with concurrent measurements of incident waves the distribution of sediment sizes over individual ripples was investigated and the performance of several models of ripple geometry prediction was tested both spatially and temporally sediment samples were collected from the crest and trough of 164 ripples the sand sized sediment was separated from the small amount generally 2 mm that was present within the sand sized fraction the ripple crests were found to be significantly coarser better sorted and more positively skewed than the troughs overall the troughs were finer than the crests but contained a greater proportion of the small fraction of sediment larger than 2 mm the field model of nielsen 1981 and the model of wiberg harris 1994 were found to be the most accurate models for predicting the wavelength of parallel ripples in the nearshore of the low energy microtidal environments surveyed the wiberg harris 1994 model was also the most accurate model for predicting ripple height temporal changes in ripple wavelength appear to be dependent on the morphological history of the bed

breaking probabilities for dominant surface waves on water of finite constant depth this paper extends our previous study of the breaking probability of dominant deep water gravity surface waves into the finite water depth environment it reports a unified behavior of the mean breaking statistics once the effects of finite water depth are taken into account the shallow water wave data that form the basis of this study were acquired at a field experiment site at lake george new south wales australia the breaking events were detected through visual observation of videotaped records of the wave field in combination with acoustic signatures of the breaking waves from a collocated hydrophone following banner et al 2000 we argue that when constant finite depth bottom influence is operative nonlinear hydrodynamical effects associated with energy and momentum fluxes within deforming wave groups remain the primary determinant of breaking onset this underpins our proposed finite depth water parameterization for the environmental dependence of dominant wave breaking probability given by the average number of breakers passing a fixed point per dominant wave period the additional influence of bottom interaction with the wind drift current shear and wind forcing are also included in our finite constant depth formulation this is a natural extension of our recently proposed deep water dependence and reduces to it as the significant wave height becomes much smaller than the water depth in common with the deep water case we propose that there exists a threshold of the significant peak steepness below which negligible dominant wave breaking occurs the available data show encouraging agreement with our proposed dependence with a correlation coefficient approaching 0 9

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

fe isotope variations in the modern and ancient earth and other planetary bodies iron the fourth most abundant element in the earth s crust has four naturally occurring stable isotopes 54fe 5 84 56fe 91 76 57fe 2 12 and 58fe 0 28 and the natural mass dependent isotope variations of fe in the rock record span a range of 4 per mil in 56fe 54fe ratios fig 1 the field of fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how fe is cycled in different environments iron typically occurs as either reduced ferrous fe in oxygen poor environments or as oxidized ferric iron in oxygen rich environments notably only the reduced species is soluble in oxygenated aqueous solutions unless the ph is low in the archean and early proterozoic the earth may have been relatively oxygen poor e g kasting et al 1979 grandstaff 1980 holland 1994 suggesting that there may have been significant quantities of fe 0 9 millimolar dissolved in the oceans as fe ii aq e g ewers 1983 sumner 1997 the extensive iron formations of archean to early proterozoic age may have been deposited from such fe ii rich oceans e g beukes and klein 1992 in the modern oxic oceans however fe contents are exceedingly low 1 nanomolar in the open oceans e g martin and gordon 1988 bruland et al 1991 martin 1992 johnson et al 1997 and it is now recognized that marine productivity is fe limited in parts of the open oceans e g martin and fitzwater 1988 martin et al 1989 1994 the differences in the behavior of fe with redox state and the significant isotope fractionations 1 or more in 56fe 54fe that are associated with redox conditions suggests that fe isotope studies will be extremely useful for tracing

refractory element mobility in volcanic soils refractory trace element concentrations in strongly weathered hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent rock values due to extensive mass loss of more soluble major elements during pedogenesis nb and ta exhibit virtually no mobility soil nb ta ratios are within the range of fresh bedrock even when soil nb concentrations are residually enriched by a factor of 10 in contrast al zr and hf are depleted relative to nb in surface soil horizons but are enriched at depth clearly indicating mobility of these elements variations in th nb ratios in soil profiles indicate significant th mobility within the soil column however mass balance calculations require that accretion of th enriched asian dust has resulted in a net increase in th in some soils soils developed on a 150 ka rainfall gradient show that the mobility and loss of zr increase with mean annual precipitation

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

germanium sequestration by soil targeting the roles of secondary clays and fe oxyhydroxides abstract germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes sequestration of ge by clay minerals is thought to be the dominant mechanism responsible for ge si fractionation during weathering in some environments ge may be sequestered by iron oxyhydroxides we examine ge behavior in soils along a well characterized climate gradient on the hawaiian island of maui to clarify the relationship between fe geochemistry and soil ge si ratios all of the sites are developed on basaltic lavas and tephras approximately 400 ky old are highly si depleted and have ge si ratios higher than parent material sites experiencing less than 3300 mm rain yr host oxidized highly weathered basaltic soils in which fe is present as secondary fe oxyhydroxide minerals in contrast the wetter sites 3340 mm yr show extensive fe reduction mobilization and loss we show that ge sequestration is independent of fe redox behavior along the gradient and thus precipitation of fe oxyhydroxides is not a major factor contributing to ge si fractionation during weathering instead elevated soil ge si ratios reflect partitioning of ge into secondary clay minerals plus additional retention of ge by refractory non silicates

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

ca cycling and isotopic fluxes in forested ecosystems in hawaii 1 biogeochemical processes fractionate ca isotopes in plants and soils along a 4 million year developmental sequence in the hawaiian islands we observed that plants preferentially take up 40ca relative to 44ca and that biological fractionation and changes in the relative contributions from volcanic and marine sources produce a significant increase in 44ca in soil exchangeable pools our results imply moderate fluxes enriched in 44ca from strongly nutrient depleted old soils in contrast with high 40ca fluxes in young and little weathered environments in addition biological fractionation controls divergent geochemical pathways of ca and sr in the plant soil system while ca depletes progressively with increasing soil age sr ca ratios increase systematically sr isotope ratios provide a valuable tracer for provenance studies of alkaline earth elements in forested ecosystems but its usefulness is limited when deciphering biogeochemical processes involved in the terrestrial ca cycle ca isotopes in combination with sr ca ratios reveal more complex processes involved in the biogeochemistry of ca and sr

ferrihydrite and allophane in four andepts from hawaii and implications for their classification abstract the mineralogy and composition of the waimea kukaiau akaka and hilo soils on the island of hawaii were studied using methods which included moessbauer spectroscopy and chemical dissolution the hilo soil which experiences 3000 mm year rainfall and the akaka soil 3800 mm year rainfall both contain large amounts about 25 of iron oxides of short range order together with other iron oxides gibbsite and allophane the iron oxides of short range order have characteristics similar to those of the reactive mineral ferrihydrite which is known to occur in a wide range of soils these characteristics however in two respects x ray diffraction and acid oxalate solubility do not rule out the presence or admixture of the much less known feroxyhite another closely related iron oxide of short range order most samples from these two soils dry irreversibly to give hard aggregates a property probably related to the presence of the iron oxides of shortrange order both the hilo and the akaka soils are classified as hydrandepts in the current usda soil taxonomy in the new proposed order of andisols these two soils would become hydrudands it is desirable to recognise the presence of short range order iron oxides in these soils the kukaiau soil currently classified as a dystrandept which occurs in a lower rainfall zone 1500 mm year contains about 25 allophane and lesser amounts of ferrihydrite other iron oxides and gibbsite the waimea soil occurs in the lowest rainfall zone 1000 mm year and the dominant mineral is halloysite but some allophane ferrihydrite and hematite are also present this soil is currently classified as a eutrandept under the andisol proposal the kukaiau and waimea soils would be a hapludand and a haplustand respectively quartz mica and other layer silicates were present in all four of the soils suggesting that there have been additions of eolian material

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

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

from a black to a gray box a mass balance interpretation of pedogenesis we utilize chemical elements as tracers in a mass balance analysis that provides functional relationships among soil chemical composition bulk density and volume change in relation to parent material these analytical functions are based on the principle of conservation of mass and include a term quantifying mass flux into out of the soil and between horizons we apply the technique to the oldest member of a chronosequence developed on marine terraces in northern california it is an alfisol that evolved from beach sand to its present state nearly one third of its weight is composed of secondary clay minerals in about 240 ky aside from large increases in organic carbon desilication is the dominant factor in soil evolution 29 50 83 g cm 2 si was leached from the beach sand during pedogenesis the rate of desilication is roughly 2 1 t km 2 yr 1 0 21 g cm 2 ky 1 an order of magnitude slower than that implied by the si denudation rate calculated for the mattole river watershed weathering of primary minerals and synthesis of secondary minerals is relatively well advanced suggesting that the rate of desilication may be declining the local beach is composed of quartz and sodic plagioclase with smaller amounts of chlorite mica and kaolinite the soil has substantially different mineralogy sand is dominated by quartz and clay is dominated by kaolinite halloysite chloritic intergrades and gibbsite bases are also leached though the total mass was much less 57 2 55 g cm 2 of na in the beach sand was lost as plagioclase weathered by focusing on elemental and mineralogical gains and losses we emphasize the essential connection between the pedologic environment and the external hydrochemical environment

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

the impact of climate on the biogeochemical functioning of volcanic soils abstract rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering secondary mineral synthesis and soil chemical properties along an arid to humid climosequence on kohala mountain hawaii we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall the lavas are influenced by a strong rain shadow with mean annual precipitation map averaging 160 mm near the coast and rising to 3000 mm near the summit a temperature decline from 24 to 15 c with increasing elevation is matched by lower potential evapotranspiration et a water balance model monthly precipitation minus monthly et defines three broad climate zones along the sampling transect an arid zone with moisture deficit in every month an intermediate zone with moisture deficit during low rainfall summer months and moisture surplus during high rainfall winter months and a humid zone with moisture surplus during every month the annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index the index reaches 1 total filling of the pore space on an annual basis at about 1400 mm map index values 1 imply intense leaching conditions because of pore water replacement in these 170 ka soils leaching losses of soluble base cations and si are nearly complete at index values 1 whereas only 60 of al has been lost at index values 1400 mm soil buffering capacity has been totally exhausted leading to low ph and low ecec the nonlinear decline in ecec is irreversible under natural conditions base cation depleted soils will remain so even if the climate shifts to drier conditions in contrast a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals elements are lost to leaching and surface chemistry is modified there is a time dimension implied in climate gradient studies soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system the details of this transfer depend on present and past climate and the age and erosional stability of landscapes

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

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

kinetic iron stable isotope fractionation between iron ii and iii complexes in solution abstract iron stable isotope fractionation between tris 2 2 bipyridine iron ii fe ii bipy 3 2 and iron iii chloride complexes has been measured using plasma source mass spectrometry the experimental protocol involves complexing fe ii ion with 2 2 bipyridine in a fe ii fe iii chloride solution and then separating the fe ii and fe iii solution species in 6 m hcl on an anion exchange resin large isotopic variations of 57 fe and 56 fe are experimentally measured in the two separated solution fractions with isotopic fractionations increasing from fe ii fe iii 25 to 174 units for 57 fe 54 fe and 17 to 117 units for 56 fe 54 fe the increase in fractionations correlates with a decrease in the mole fraction of fe ii in the solution fe fe ii fe ii fe iii that results from the dissociation and breakdown of fe ii bipy 3 2 complex in 6 m hcl solution the data variations are mainly ascribed to a kinetic fractionation occurring during this dissociation reaction mass balance calculations assuming that a rayleigh law describes the overall isotopic trends suggest a kinetic fractionation of ca 1 010 100 units the magnitude of this fractionation is attributed to the rupturing of the strong covalent bonds between 2 2 bipyridine and fe ii ion the experimental data confirm that the coordination chemistry of iron exhibits a profound control on its isotopic behaviour and that kinetic fractionations may play an important role in its isotope geochemistry as was also found in the pioneering experimental studies of the sulphur isotopic system in solution

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