iron isotope variations in holocene sediments of the gotland deep baltic sea holocene sediments from the gotland deep basin in the baltic sea were investigated for their fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron the sediments display variations in 56fe differences in the 56fe 54fe ratio relative to the irmm 14 standard from 0 27 0 09 to 0 21 0 08 samples deposited in a mainly limnic environment with oxygenated bottom water have a mean 56fe of 0 08 0 13 which is identical to the mean fe isotopic composition of igneous rocks and oxic marine sediments in contrast sediments that formed in brackish water under periodically euxinic conditions display significantly lighter fe isotope signatures with a mean 56fe of 0 14 0 19 negative correlations of the 56fe values with the fe al ratio and s content of the samples suggest that the isotopically light fe in the periodically euxinic samples is associated with reactive fe enrichments and sulfides this is supported by analyses of pyrite separates from this unit that have a mean fe isotopic composition of 1 06 0 20 for 56fe the supply of additional fe with a light fe isotopic signature can be explained with the shelf to basin fe shuttle model according to the fe shuttle model oxides and benthic ferrous fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins the data furthermore suggest that the euxinic water has a negative dissolved 56fe value of about 1 4 to 0 9 if negative fe isotopic signatures are characteristic for euxinic sediment formation widespread euxinia in the past might have shifted the fe isotopic composition of dissolved fe in the ocean towards more positive 56fe values

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

tectono metallogenetic evolution of the fe cu deposit of dominga northern chile the dominga district in northern chile 2082 mt at 23 3 fe 0 07 cu shows a spatial and genetic affinity among distinctive structural elements and fe cu rich paragenetic mineral assemblages deep seated ne to e striking structural elements form a right lateral duplex like structural system early structural system ess that cuts a regionally extensive alteration stage i zone the ees system served as a locus and as path for the emplacement of biotite magnetite alteration mineralization stage iia as veins and fe bearing layers following altered volcano sedimentary strata nw striking actinolite magnetite hydrothermal breccias coeval with and part of the ess include apatite stage iib crystallized at 127 15 ma u pb 2 the ess was also the locus of subsequent alteration mineralization represented by k feldspar epidote and albite stage iiia and fe cu rich vermiculite anhydrite chalcopyrite stage iiib mineral associations shallowly developed nne striking left lateral structural elements defining the el tofo structural system etss probably part of the atacama fault system clearly crosscut the ess minerals associated with alteration mineralization stage iiib also occur as veins and as part of hydrothermal breccias of the etss marking the transition from the ess to etss molybdenite associated with alteration mineralization stage iiib yielded a re os age of 127 1 0 7 ma 2 both the ess and etss were cut by left lateral nw to e striking shallowly developed structural elements intermediate structural system iss on which a hematite calcite assemblage stage iv occurs mostly as infill material of veins and fault veins the iss is cut by n striking left lateral and shallowly developed structural elements late structural system lss showing no evidence of alteration mineralization estimated strain and stress fields indicate an overall nw trending shortening compression and ne trending stretching tension strike slip regime probably due to oblique subduction during the mesozoic however the orientations of the stress and strain fields calculated for each structural system suggest a back and forth rotation pattern during transition from one structural system to the other as they change between transtension and transpression and between alteration mineralization stages

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

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

tectonic denudation and topographic development in the spanish sierra nevada 1 the denudation history of the rapidly uplifting western part of the spanish sierra nevada was assessed using apatite fission track aft ages and 10be analyses of bedrock and fluvial sediments major contrasts in the denudation history are recorded within the 27 km2 rio torrente catchment upland areas are characterized by low relief low slope angles and locally the preservation of shallow marine sediments which have experienced 2 km of denudation since circa 4 ma the minimum denudation rates of 0 4 mm yr 1 derived from aft also contrast with the slow medium term 104 years erosion rates 0 044 0 015 mm yr 1 estimated from 10be measurements at high elevations the local medium long term contrasts in denudation rates within the high sierra nevada indicate that much of the unroofing occurs by tectonic denudation on flat lying detachments in lower elevation parts of the catchment rapid river incision coupled to rock uplift has produced 1 6 km of relief implying that the rivers and adjacent hillslopes close to the edge of the orogen are sensitive to normal fault driven changes in base level however these changes are not transmitted into the low relief slowly eroding upland areas thus the core of the mountain range continues to increase in elevation until the limits of crustal strength are reached and denudation is initiated along planes of structural weakness we propose that this form of tectonic denudation provides an effective limit to relief in young orogens

fe isotopic composition of the quaternary red clay in subtropical southeast china redoxic fe mobility and its paleoenvironmental implications abstract to probe fe isotopic signatures of the quaternary red clay qrc in subtropical southeastern china two qrc profiles located in southern anhui province southeast china were studied the whole profiles were generally in a narrow range in 56 fe of 0 040 0 061 0 117 0 048 the yellow brown earth ybe and uniform red clay urc of the upper profiles were similar in 56 fe to the xiashu loess along the yangtze river southeast china and the loess paleosols and tertiary red clay in the chinese loess plateaus northwest china as well as the baseline of terrestrial igneous rocks the slight variation of 56 fe from the ybe to the urc suggests that the chemical weathering and rubification on the aerobic surface did not fractionate fe isotopes significantly the reticulate red clay rrc underlying the ybe and urc fluctuated in 56 fe to some extent the white veins separated from the lower rrc were depleted of fe oxides and heavier in fe isotopes and the red veins on the contrary were isotopically lighter in fe 56 fe of the white veins 0 345 0 053 on average was significantly higher than the bulk samples 0 033 0 044 on average and the red veins 0 021 0 046 on average this suggests that the reticulate textures of the rrc were formed by the reductive removal re precipitation of fe due to the cyclical changes of redoxic conditions fe isotopic composition of the qrc was dominantly controlled by the redoxic mobility of fe rather than the degree of weathering 56 fe of the profiles was not correlated with the paleoclimatic events and thus cannot be used as a paleoclimatic proxy the fractionation of fe isotopes in the lower rrc however indicated the occurrence of over humid paleoclimate with alternations between drought and wetness in southern china during the pleistocene

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

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

iron isotopes in the early marine diagenetic iron cycle measurements of fe oxyhydroxides reactive fe iii in two shallow sediment profiles from reducing and oxic environments on the arabian sea margin demonstrate fe isotope fractionation during early marine diagenesis reactive fe iii has 56 fe values between 0 77 and 0 19 values are lowest at the top of the sediment profile and considerably lower than bulk sediment fe 56 fe 0 preferential reduction and dissolution of light fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive fe iii residual initially the isotopic composition of reactive fe iii evolves down core according to the fractionation factor typical of microbial dissimilatory fe reduction deeper in the profile 56 fe values remain unchanged despite further fe reduction here another process with a different fractionation factor becomes dominant probably reduction by dissolved sulfide the 56 fe of the residual reactive fe iii suggests that 25 of the initially present reactive fe iii was reduced by microbial fe reduction when fe is diagenetically recycled between reducing sediments at depth and an oxic top layer the process depending on recycling efficiency may result in the accumulation of light fe in the top layer while the complementary heavier residual is buried fe diffusing from the seafloor back into the ocean should reflect the low 56 fe diagenetic source of dissolved fe

iron isotopes in an archean ocean analogue abstract iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the proterozoic and phanerozoic this increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time we tested these contrasting scenarios through a detailed study of anoxic and ferruginous lac pavin france which can serve as a modern analogue of the archean ocean a depth profile in the water column of lac pavin shows a remarkable increase in dissolved fe concentration 0 1 1200 m and 56fe values 2 14 to 0 31 across the oxic anoxic boundary to the lake bottom the largest fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron leaving the residual fe enriched in light isotopes the analysis of four sediment cores collected along a lateral profile one in the oxic layer one at the redox boundary one in the anoxic zone and one at the bottom of the lake indicates that bulk sediments porewaters and reactive fe mostly have 56fe values near 0 0 0 2 similar to detrital iron in contrast pyrite 56fe values in sub chemocline cores 60 65 and 92 m are highly variable and show significant deviations from the detrital iron isotope composition 56fepyrite between 1 51 and 0 09 average 0 93 importantly the pyrite 56fe values mirror the 56fe of dissolved iron at the redox boundary where near quantitative sulfate and sulfide drawdown occurs suggesting limited iron isotope fractionation during iron sulfide formation this finding has important implications for the archean environment specifically this work suggests that in a ferruginous system most of the fe isotope variability observed in sedimentary pyrites can be tied to water column cycling foremost to the oxidation of dissolved ferrous iron this supports previous suggestions that enhanced iron isotope variability in the archean may record a unique stage in earth s history where partial ferrous iron oxidation in upwelling water masses was a common process probably linked to oxygenic or anoxygenic photosynthesis

subsurface processes at the lucky strike hydrothermal field mid atlantic ridge evidence from sulfur selenium and iron isotopes at lucky strike near the azores triple junction the seafloor setting of the hydrothermal field in a caldera system with abundant low permeability layers of cemented breccia provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution coupled analyses of s isotopes performed in conjunction with se and fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems these data provide a tool for resolving the different abiotic and potential biotic near surface hydrothermal reactions the 34s between 1 5 and 4 6 and se values between 213 and 1640 ppm of chalcopyrite suggest a high temperature end member hydrothermal fluid with a dual source of sulfur sulfur that was leached from basaltic rocks and sulfur derived from the reduction of seawater sulfate in contrast pyrite and marcasite generally have lower 34s within the range of magmatic values 0 1 and are characterized by low concentrations of se 50 ppm for 82se 76se ratios the 82se values range from basaltic values of near 1 5 to 7 the large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between se leached from igneous rock and se derived from seawater we interpret the se isotope signature to be a result of leaching and mixing of a fractionated se source located beneath hydrothermal chimneys in the hydrothermal fluid at lucky strike we consider two sources for s and se 1 the end member hydrothermal fluid with basaltic se isotopic values 1 5 and typical s isotope hydrothermal values of 1 5 2 a fractionated source hosted in subsurface environment with negative 34s values probably from bacterial reduction of seawater sulfate and negative 82se values possibly derived from inorganic reduction of se oxyanions fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by s isotopes fe isotope systematic reveals that se rich high temperature samples have 57fe values close to basaltic values 0 whereas se depleted samples precipitated at medium to low temperature are systematically lighter 57fe values between 1 to 3 an important implication of our finding is that light fe isotope composition down to 3 2 may be explained entirely by abiotic fractionation in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions

secular variation of iron isotopes in north atlantic deep water a high precision iron isotope time series for a ferromanganese crust demonstrates that the iron isotope composition in north atlantic deep water has changed substantially over the past 6 million years and that iron isotope variations in the crust are closely correlated to those of lead isotopes the close correlation between the two isotope series indicates that the observed iron isotope variations predominantly reflect those of iron input from terrigenous sources and provides no evidence for biologically induced mass fractionation within north atlantic deep water

focal mechanisms for sub crustal earthquakes in the gulf of cadiz from a dense obs deployment 1 an eleven month deployment of 25 ocean bottom seismometers provides an unprecedented opportunity to study low magnitude local earthquakes in the complex transpressive plate boundary setting of the gulf of cadiz known for the 1755 lisbon earthquake and tsunami 36 relocated earthquakes ml 2 2 to 4 8 concentrate at 40 60 km depth near the base of the seismogenic layer in 140 ma old oceanic mantle lithosphere and roughly align along two perpendicular nne ssw and wnw ese striking structures first motion focal mechanisms indicate compressive stress for the cluster close to the northern horseshoe fault termination which trends perpendicular to plate convergence focal mechanisms for the second cluster near the southern termination of the horseshoe fault indicate a strike slip regime providing evidence for present day activity of a dextral shear zone proposed to represent the eurasia africa plate contact we hypothesize that regional tectonics is characterized by slip partitioning

high resolution moho topography map beneath iberia and northern morocco from receiver function analysis abstract crustal thickness maps at regional scales are typically compiled using estimations inferred from different geophysical datasets providing a variable coverage of the investigated area consequently spurious effects related to changes in data resolution or artifacts in grid interpolation may affect significant zones of those maps the topoiberia iberarray broad band seismic network covering the iberian peninsula and northern morocco with stations distributed on a regular 60 60 km grid provides a unique opportunity to avoid such technical problems and to obtain a crustal thickness map derived from a same method sampling evenly all the region data from more than 340 stations has been gathered and analyzed using the p to s conversion phases at the moho discontinuity receiver functions the crustal thickness has been inferred applying the classical h stacking technique though in regions of complex crustal structure we have preferred to estimate the thickness directly from the arrival time of the converted phase at some sites the topography of the moho discontinuity is strongly correlated with tectonic processes the investigated area extending from the sahara platform to the bay of biscay has a great geodynamic diversity including north to south crustal imbrication in the pyrenean and cantabrian range a large and relatively undisturbed variscan massif in the center of iberia and areas of complex and still not completely understood geodynamics in the alboran crust domain and the atlas range the crustal thickness map reflects this diversity showing variations reaching 30 km between the thickest and thinnest zones of continental crust the final map has an overall similarity with previous estimations of the crustal thickness using independent data as those coming from more sparse deep seismic sounding profiles but provides further constraints at regional scale

a critical look at iron paleoredox proxies new insights from modern euxinic marine basins enrichments in reactive iron occur under euxinic marine conditions that is where dissolved sulfide is present in the water column these enrichments result primarily from the export of remobilized iron from the oxic shelf which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments strongly elevated ratios of highly reactive iron to total iron fehr fet and total iron to aluminum fet al and high degrees of pyritization dop are each products of this enrichment process these paleoredox proxies are among the most faithful recorders of ancient euxinia contrary to previous arguments iron enrichment is decoupled from biogenic sediment inputs but it does appear to be a uniquely euxinic phenomenon in other words we can rule out a major contribution from preferential physical transport of fehr rich detrital sediment to the deep basin which could also operate under oxic conditions furthermore enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins elevated fet al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net fehr addition these conclusions are supported by observations in the modern black sea orca basin and effingham inlet fet al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration however this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism resulting in only intermediate dop values for euxinic sediments and fet al ratios that mimic the oxic shelf such dilution effects are well expressed in black sea basinal turbidites and rapidly accumulating muds on euxinic basin margins under conditions of persistent euxinia varying extents of fehr enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation the magnitude of enrichment is a function of the source shelf to sink ocean basin areal ratio suggesting that iron proxies can also record ocean scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia for the first time manganese data are interpreted in light of the redox shuttle model as for the iron data the black sea orca basin and effingham inlet show enrichments in total manganese in the deep euxinic basin suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin the black sea data reveal iron and manganese enrichment across the broad deep euxinic basin suggesting efficient lateral transport and deep water mixing tied to the physical properties of the water column

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

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

modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans the increase in atmospheric oxygen ca 2 4 ga had a significant impact on the geochemical cycling of fe the history of environmental oxygenation may be recorded in the fe isotope composition of archean and proterozoic sediments but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations here we present fe isotope data and iron aluminum ratios from the black sea oxic shelf and euxinic basin the isotope data demonstrate that shelf fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin we propose that there is net transport of isotopically light fe from sediments of the shelf to those of the distal anoxic basin consistent with enrichments in reactive fe seen in the deep basin the low 56 fe benthic fe flux is generated during the coupling of microbial fe iii reduction or sulfidization with fe 2 aq oxidation low 56 fe values reported previously from late archean sedimentary pyrites may be an isotopic fingerprint of analogous fe redox cycling in the late archean oceans this interpretation implies shallow water fe redox recycling in the late archean we predict that the light isotopic compositions of the late archean will prove to be distinct from those of the early archean before fe redox cycling became an important process and we infer that this difference may be related to the presence of oxygen in the surface ocean

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

the biogeochemical cycle of iron and associated elements in lake kinneret abstract iron biogeochemical cycling and distribution between particulate reactive colloidal dissolved oxine labile fe ii fractions were studied in the seasonally stratified mesotrophic lake kinneret this article presents various aspects of the fe budget in the lake and relates them to the chemical reactivity of various physicochemical forms of fe the budget of fe in lake kinneret is dominated by fluvial fe load rather than by internal recycling of fe from the sediment as shown by the fact that 75 to 94 of the variance in fe concentrations in the lake can be explained by the fluctuation in the water discharge of the jordan river iron associated with phytoplankton accounts for 9 16 of the bulk particulate fe in the lake however within patches of the dominate algae the dinoflagellate peridinium gatunense algal fe accounts for more than 70 of the lake s particulate fe the algal fe is predominantly intracellular and the fe chlorophyll a ratios were within the range of published values for dinoflagellates and cyanobacteria iron associated with particles larger than 0 025 m 20 300 nm accounts for 80 95 of total fe in the epilimnion of lake kinneret throughout the year in contrast this fraction of fe is dominant in the hypolimnion only during the period of lake mixing iron concentration of different size fractions 0 025 m in the surface water covaried throughout the research period these covariations suggest dynamic transformations of fe between different size fractions either due to partial dissolution precipitation or desorption adsorption oxine labile fe concentrations the fe fraction considered chemically labile and available for phytoplankton ranged from 15 to 75 nm in wintertime the oxine labile fe accounts for only 10 20 of the total fe while in other seasons most of the fe is oxine labile oxine labile fe concentrations always exceed the 0 025 m filtered fe implying that some of the larger particles contain oxine labile fe and therefore are reactive the fraction of reactive particles in lake kinneret 10 80 is high relative to that of the marine environments and can most likely be attributed to its fluvial source which contains a significant proportion of reactive fe oxide and hydrous oxide particles the annual variability in the epilimnetic concentrations of other trace elements and nutrients such as al mn cd zn pb and p were studied and grouped according to their resemblance with river water discharge redox processes or biological uptake and scavenging distribution patterns of al pb and cd resembled that of fe suggesting that similar processes control the concentrations of these metals namely significant contribution from the watershed high affinity to particulate matter and little control of biota on their fate in the lake other elements such as zn and p are more affected by biological uptake while mn is more affected by redox cycling within the lake

iron isotope fractionation during biogeochemical cycle information from suspended particulate matter spm in aha lake and its tributaries guizhou china abstract iron isotope compositions of suspended particulate matters spm collected from the aha lake an artificial lake in the karst area of yun gui plateau and its tributaries in summer and winter were investigated for our understanding of the behavior of fe isotopes during iron biogeochemical cycling in lake 56fe values of spm display statistically negative shift relative to irmm 014 samples from the lake display a range from 1 36 to 0 10 in summer and from 0 30 to 0 07 in winter while river samples vary from 0 88 to 0 07 in summer and from 0 35 to 0 03 in winter the average iron isotope composition of aerosol samples is 0 10 which is very similar to that of igneous rocks 0 09 the spm in most rivers and water column showed seasonal variation in 56fe value the 56fe values of spm in summer were lower than in winter the seasonal variation in 56fe value of the riverine spm should be ascribed to the change in source of particulate fe and geochemical process in the watershed more particulate fe was leached from soil and produced by weathering of pyrite widely distributed in coal containing strata it is suggested that both allochthonous inputs and the redox iron cycling control the variations of 56fe values for spm in lake during summer stratification an fe cycle named ferrous wheel is established near the redox boundary where the upwardly diffusing fe ii is oxidized and the reactive fe oxides formed will continuously sink back into the reduction zone to complete the cycle the 56fe values for spm reach the minima 0 88 for db station and 1 36 for ljk station just near the redox boundary as a result of the fe cycling where a rough 45 to 76 of fe in these particles was produced by the repetitive cycle due to random transportation and diffusion 56fe values of the particles near the redox zone distributed into approximately a gaussian shape the good negative correlation existed between 56fe values and fe al ratios for db station suggesting that they together can be used as good indicators of the redox driven fe transformations

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

iron isotopes in acid mine waters and iron rich solids from the tinto odiel basin iberian pyrite belt southwest spain the isotopic composition of fe was determined in water fe oxides and sulfides from the tinto and odiel basins south west spain as a consequence of sulfide oxidation in mine tailings both rivers are acidic 1 45 ph 3 85 and display high concentrations of dissolved fe up to 420 mmol l 1 and sulphates up to 1190 mmol l 1 the 56fe of pyrite rich samples from the rio tinto and from the tharsis mine ranged from 0 56 0 08 to 0 25 0 1 56fe values for fe oxides precipitates that currently form in the riverbed varied from 1 98 0 10 to 1 57 0 08 comparatively narrower ranges of values 0 18 0 08 and 0 21 0 14 were observed in their fossil analogues from the pliocene pleistocene and in samples from the gossan the oxidized layer that formed through exposure to oxygen of the massive sulfide deposits 0 36 0 12 to 0 82 0 07 in water 56fe values ranged from 1 76 0 10 to 0 43 0 05 at the source of the tinto river fractionation between aqueous fe iii and pyrite from the tailings was less than would be expected from a simple pyrite oxidation process similarly the isotopic composition of gossan oxides and that of pyrite was different from what would be expected from pyrite oxidation in rivers the precipitation of fe oxides mainly jarosite and schwertmannite and lesser amounts of goethite from water containing mainly more than 99 fe iii with concentrations up to 372 mmol l 1 causes variable fractionation between the solid and the aqueous phase 0 98 56fesolid water 2 25 the significant magnitude of the positive fractionation factor observed in several fe iii dominated water may be related to the precipitation of fe iii sulphates containing phases

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