relationships between heavy metals distribution and organic matter cycling in mangrove sediments conception bay new caledonia mangroves of new caledonia act as a buffer between a lagoon of more than 20 000 km2 and the island which suffers intense processes of erosion resulting from urbanization and natural resources exploitation this preliminary study aims at determining how heavy metals are distributed in mangrove sediments and pore waters in relationship with their organic content to reach our goal a series of 50 cm deep cores were collected in the mangrove of conception bay the various coring sites are representative of live forests avicennia marina rhizophora stylosa dead forest and intertidal unvegetated area the ranges of concentrations in sediments were the following mol g 1 cu 0 08 to 0 51 co 0 01 to 0 38 ni 0 03 to 3 55 cr 0 36 to 3 11 zn 0 68 to 2 36 mn 1 13 to 5 0 and fe 22 64 to 721 69 heavy metals distribution within sediments and pore water appear to result from diagenetic processes linked to om decomposition beneath the dead avicennia forest and in the unvegetated sediments the higher the organic content the higher the metal concentration in the solid phase beneath living mangrove stands despite a higher organic content as well as higher sulphur content heavy metals concentrations were not higher than those measured in the other sampling sites beneath these forests redox conditions were mainly controlled by the length of waterlogging and the activity of root system we suggest that because of the specificity of the avicennia root system and its position in the intertidal zone heavy metals are more bioavailable and potentially more mobile than beneath rhizophora stand

ni speciation in a new caledonian lateritic regolith a quantitative x ray absorption spectroscopy investigation abstract changes in ni speciation in a 64 m vertical profile of a new caledonian saprolitic lateritic regolith developed over ultramafic rocks under tropical weathering conditions were investigated by exafs spectroscopy quantitative analysis of the exafs spectra by linear combination least squares fitting lc lsf using a large set of model compound spectra showed that ni hosted in primary silicate minerals olivine and serpentine in the bedrock is incorporated in secondary phyllosilicates serpentine and fe oxides goethite in the saprolite unit and mainly in goethite in the laterite unit a significant concentration of ni up to 30 of total ni is also hosted by mn oxides in the transition laterite i e the lowest part of the laterite unit which contains large amounts of mn oxides however the amount of ni associated with mn oxides does not exceed 20 of the total ni in the overlying laterite unit this sequence of ni species from bedrock to laterite yields information about the behavior of ni during tropical weathering of ultramafic rocks the different ni distributions in phyllosilicates in the bedrock randomly distributed and in the saprolite unit clustered indicate two generations of ni bearing phyllosilicates the first which formed at higher temperature is related to serpentinization of oceanic crust whereas the second one which formed at lower temperature is associated with post obduction weathering of ultramafic rocks in addition the observed decrease in the proportion of ni hosted by mn oxides from the transition laterite to the upper lateritic horizons indicates dissolution of mn oxides during the last stages of differentiation of the lateritic regolith i e lateritization finally the ubiquitous occurrence of ni bearing goethite emphasizes the major role of this phase in ni speciation at the different weathering stages and suggests that goethite represents the major host for ni in the final tropical weathering stages of new caledonian ultramafic rocks

comment on geomorphological paleontological and 87sr 86sr isotope analyses of early pleistocene paleoshorelines to define the uplift of central apennines italy

formation processes of framboidal pyrite abstract pyrite framboid formation may be the result of four consecutive processes 1 nucleation and growth of initial iron monosulfide microcrystals 2 reaction of the microcrystals to greigite fe 3 s 4 3 aggregation of uniformly sized greigite microcrystals i e framboid growth and 4 replacement of greigite framboids by pyrite the uniform morphology uniform size range and ordering of the microcrystals in individual framboids as well as the range of observed framboid structures from irregular aggregates to densely packed spherical aggregates and polyframboids are consequences of these processes using dlvo theory derjaguin landau verwey and overbeek we have evaluated the stability of colloidal iron monosulfide suspensions with ionic strengths typical of marine and lacustrine waters in addition to van der waals attractive and double layer repulsive forces a term is included to account for the ferrimagnetic properties of greigite numerical models predict that magnetically saturated greigite particles 0 1 m in diameter will rapidly aggregate in either marine or fresh water the aggregation model is in agreement with the sequence of greigite formation followed by pyrite framboid formation established in a previous experimental study sweeney and kaplan 1973 and is consistent with the occurrence of framboids composed of other magnetic minerals e g greigite magnetite and magnesioferrite based on the temperature dependent magnetic properties of greigite and aging experiments in hydrothermal solutions this mechanism for framboid formation via precursor greigite could operate to temperatures of 200 c consistent with the occasional occurrence of pyrite framboids in the paragenesis of metalliferous ore deposits

the partitioning of transitional metals fe mn ni cr in mangrove sediments downstream of a ferralitized ultramafic watershed new caledonia abstract in new caledonia one third of the island is composed of ultramafic rocks and lateritic soils enriched in fe ni and cr open cast mining occurs all around the island and processes of erosion and sedimentation which occur naturally along the coastline are strongly amplified by mining activities due to their position at the interface between land and sea mangroves receive extensive amounts of particles emanating from rivers through estuaries the purpose of this study is to understand the distribution and partitioning of some transitional metals fe mn ni cr in sediments and pore waters in a mangrove swamp which is situated downstream of a catchment characterized by lateritic soils that were exploited a century ago quantitative analyses on bulk and after selective extraction were carried out on cores collected along a transect within the intertidal zone i e beneath a rhizophora stylosa stand an avicennia marina stand and within a salt flat mean metal concentrations were mol g 1 fe 1997 ni 44 2 cr 31 9 mn 8 8 thus ni cr and fe concentrations in this study are substantially higher than mangrove world average in addition ni concentrations are 10 to 100 times higher than in other new caledonian mangrove developing downstream of a catchment not composed of ultramafic rocks the studied mangrove is characterized by gradients of water and organic contents with depth and along the intertidal zone which induced different redox conditions and thus different metals partitioning transitional metals are deposited in the mangrove mainly as oxides and or oxy hydroxides that are subsequently dissolved by bacteria for the decomposition of organic matter and which leads to a strong increase of metals in the dissolved phase then dissolved metals were precipitated with organic and sulfide compounds to conclude organic diagenesis in mangrove sediments leads to the transfer of transitional metals from oxide form to organic and sulfide forms

contrasting isotopic signatures between anthropogenic and geogenic zn and evidence for post depositional fractionation processes in smelter impacted soils from northern france abstract zinc isotopes have been studied along two smelter impacted soil profiles sampled near one of the largest pb and zn processing plants in europe located in northern france about 50 km south of lille mean 66zn values along these two soil profiles range from 0 22 0 17 2 to 0 34 0 17 2 at the lowest horizons and from 0 38 0 45 2 to 0 76 0 14 2 near the surface the 66zn values in the lowest horizons of the soils are interpreted as being representative of the local geochemical background mean value 0 31 0 38 whereas heavier 66zn values near the surface of the two soils are related to anthropogenic zn this anthropogenic zn occurs in the form of franklinite znfe2o4 bearing slag grains originating from processing wastes at the smelter site and exhibiting 66zn values of 0 81 0 20 2 the presence of franklinite is indicated by exafs analysis of the topsoil samples from both soil profiles as well as by micro xanes analysis of the surface horizon of a third smelter impacted soil from a distant site these results indicate that naturally occurring zn and smelter derived zn exhibit significantly different 66zn values which suggests that zinc isotopes can be used to distinguish between geogenic and anthropogenic sources of zn in smelter impacted soils in addition to a possible influence of additional past sources of light zn likely zn sulfides and zn sulfates directly emitted by the smelter the light 66zn values in the surface horizons compared to smelter derived slag materials are interpreted as resulting mainly from fractionation processes associated with biotic and or abiotic pedological processes zn bearing mineral precipitation zn complexation by organic matter and plant uptake of zn this conclusion emphasizes the need for additional zn isotopic studies before being able to use zn isotopes to trace sources and pathways of this element in surface environments

xas evidence for ni sequestration by siderite in a lateritic ni deposit from new caledonia mineralogical and spectroscopic analyses were conducted on a lateritic ni deposit from southern new caledonia results show that ni is incorporated in siderite feco3 found between 37 and 40 m depth in the laterite and saprolite units of the regolith sem edxs analyses of siderite rich samples indicate that a significant amount of nickel can be hosted by this crystalline phase 0 8 wt nio ni and fe k edge extended x ray absorption fine structure exafs spectroscopic analyses of the siderite rich samples from the regolith as well as comparison with synthetic ni bearing and ni free siderites demonstrate isomorphous substitution of ni2 for fe2 in the siderite structure linear combination fitting lcf of the ni k edge exafs data reveals that this ni bearing siderite species accounts for more than 90 of the total ni pool 1 wt nio in the siderite rich horizons of the regolith in addition lcf analysis of the exafs spectra indicates that goethite and serpentine are the major ni hosts in the upper horizons laterite and lower horizons saprolite of the regolith respectively formation of siderite an uncommon mineral species in such oxidized environments is attributed to the development of swampy conditions in organic rich lateritic materials that accumulated at the bottom of dolines these results thus show the importance of siderite as a host for nickel in lateritic ni deposits that have been affected by late hydromorphic and reducing conditions

synchrotron based speciation of chromium in an oxisol from new caledonia importance of secondary fe oxyhydroxides in new caledonia the weathering of ultamafic rocks under a tropical climate has led to the residual accumulation of trace elements in lateritic soils widely dominated by fe oxyhydroxides the speciation of trace elements such as cr ni and co in these oxisols remains a major subject of interest regarding mining and environmental issues we have assessed the speciation of chromium in the upper part of an oxisol by combining bulk and spatially resolved chemical analyses epma and sem eds with synchrotron based spectroscopic data exafs and xanes epma indicates that the main hosts for chromium in the bedrock sample are the silicates forsterite enstatite and lizardite hosting of chromium in these easily weatherable mineral species could lead to a significant loss of this element upon weathering however total analyses of major elements indicate only a slight depletion of cr together with an immobility of fe and al and drastic losses of si and mg after the weathering of the bedrock such a low mobility of chromium is likely related to its significant incorporation in goethite and hematite formed after the weathering of fe 2 bearing primary silicates this efficiency of secondary fe oxyhydroxides at immobilizing chromium is demonstrated by quantitative analysis of exafs data that indicates that these mineral species host between 67 and 75 wt of total cr compared to the 18 to 22 wt of total cr hosted by chromite in addition sem observation and sem eds analyses performed on the oxisol samples also show some evidence for chemical weathering of chromite chromite could then represent a past and or present source of chromium upon extended tropical weathering of the studied oxisol rather than a stable host these results emphasize the importance of secondary fe oxyhydroxides compared to cr spinels on chromium hosting in oxisols developed upon tropical weathering of ultramafic rocks although the trapping mechanism of chromium mainly corresponds to incorporation within the structural network of goethite and hematite sorption reactions at the surface of these mineral species could also be involved in such a process in addition considering their potential oxidative reactivity that can generate cr 6 or enhance the chemical weathering of chromite the occurrence of mn oxides could significantly modify the behavior of chromium upon weathering these considerations indicate that further studies are needed to assess the actual potential of chromium release from oxisols developed upon weathering of ultramafic rocks under a tropical climate

legacy impacts of acid sulfate soil runoff on mangrove sediments reactive iron accumulation altered sulfur cycling and trace metal enrichment abstract the off site impacts of iron rich acidic runoff from drained acid sulfate soils ass on the geochemistry of estuarine sediments are relatively poorly constrained in this study we examine the geochemistry of intertidal zone mangrove sediments and porewaters from an estuarine tributary that received 15 20 years of acute ass drainage compared to adjacent control sites mixing of acidic iron rich ass drainage waters with circumneutral ph estuarine waters drove precipitation of fe iii floc and has led to significant localized accumulation of reactive fe species fer 1 m hcl and citrate buffered dithionite extracts up to 4200 mol g 1 as well as significantly elevated porewater fe2 1000 m in ass impacted sediments x ray diffraction indicates that goethite lepidocrocite and ferrihydrite are important fe iii minerals in surficial sediments at ass impacted sites the abundant fer is shifting the equilibrium of reduced inorganic sulfur ris mineral formation pathways towards accumulation of s 0 and acid volatile sulfide avs species and is buffering porewater s ii to low concentrations in contrast ris speciation at the control sites is dominated by pyrite scr and porewater s ii is present some porewater metals i e al mn cu cr are significantly elevated at sites proximal to the source of ass drainage compared to control sites abundant fer also provides a substrate for sorption and accumulation of metals and correspondingly there is significant relative enrichment of reactive metal fractions 1 m hcl extractable in sediments at the ass impacted sites with al mn u cu cr ni this study demonstrates that ass drainage which causes accumulation of reactive fe in mangrove environments can substantially influence sediment biogeochemistry by altering reduced inorganic sulfur cycling and enhancing the availability of some trace metals

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

redox stratification and heavy metal partitioning in avicennia dominated mangrove sediments a geochemical model abstract mangrove forest sediments can provide a sink for trace metals because the mangroves create a baffle that promotes the accumulation of fine grained organic matter rich sediment which is usually sulphidic due to the presence of sulphate reducing bacteria direct adsorption complexing with organic matter and the formation of insoluble sulphides all contribute to the trapping of metals the concentration and chemical speciation of the metals are influenced by the distribution of geochemically distinct horizons within the sediment in horizons with a ph 7 and an eh 100 mv oxidation horizons most metals are present as exchangeable or oxide bound species in most cores two oxidation and two reduction horizons can be recognised but dark mottles of low eh 100 mv sediment can be found around mangrove roots and burrows in the reduction horizons the depth to each horizon differs between cores and can change in response to seasonal shifts in the position of the water table a model is presented that accounts for the development of the oxidation and reduction horizons within the avicennia dominated mangrove forest sediment and describes the major controls on metal cycling within the sediment

biomineralization of iron phosphates in the water column of lake pavin massif central france the availabilities of iron and phosphorus have considerably impacted biological productivity in past and present natural aquatic environments and therefore have been key regulators of climate changes over geological time scales microbial organisms are known to play important roles in reactions that drive fe and p cycling at redox interfaces in earth s surface environments here we study the depth variations of fe and p speciation in lake pavin massif central france a deep bottom depth 92 m and permanently stratified lake with anoxic and ferruginous conditions in the water column below 60 m depth we particularly focus on the potential roles of microbes on fe and p transformations and traces left by these processes in the sedimenting particular matter bulk chemical analyses powder x ray diffraction and x ray absorption spectroscopy xas at the fe k edge were performed to characterize the mineralogy and fe oxidation state of solid particles at different depths in the water column and in sediments deposited at the bottom of the lake fe is mainly hosted by fe iii oxyhydr oxides and phyllosilicates in the shallower oxygenated water column of the lake 25 m the amount of fe in suspended matter increases with depth and an additional amorphous fe ii fe iii phosphate phase is detected close to the chemocline at 56 m depth while vivianite an fe ii phosphate with a formula of fe ii 3 po4 2 8 h2o becomes dominant in the deeper anoxic water 67 m and 86 m depths fe oxyhydr oxides are preserved down to these depths in the water column suggesting that fe reduction has little impact on the particulate fe budget over the monimolimnion these fe oxyhydr oxides undergo reductive dissolution at the surface of the sediments where vivianite is the main fe bearing phase these results are confirmed by imaging at the micrometer and nano scales using scanning electron microscopy sem transmission electron microscopy tem and synchrotron based scanning transmission x ray microscopy stxm as well as high spatial and energy resolution xanes spectra at the fe l2 3 edges and the c k edge moreover polyphosphate accumulating microorganisms were observed by microscopy and chemical imaging at all depths except in the deeper part of the lake 86 m and some fe was associated with these polyphosphate inclusions below 56 m finally the fe phosphates encrust microbial bodies suggesting that microbial activities might be involved in the formation of these phases based on comparisons with laboratory experiments we propose that fe oxidation in contrast to fe reduction may play a role in the precipitation of fe phosphates in the water column of lake pavin polyphosphate accumulating microorganisms could also be involved in fe phosphate formation in the lake for instance by increasing dissolved phosphate concentrations in the monimolimnion

exafs analysis of iron cycling in mangrove sediments downstream a lateritized ultramafic watershed vavouto bay new caledonia mangrove forests are the dominant intertidal ecosystem of tropical coastlines in new caledonia mangroves act as a buffer zone between massive fe lateritic deposits and a lagoon partly registered by unesco as a world heritage site the new caledonian mangroves are characterized by a botanical gradient composed of three main vegetal stands i e rhizophora spp avicennia marina and salt flat which relies on the duration of tidal immersion that imposes gradients of pore water salinity oxygenation and organic content in the sediment in the present study we have determined the distribution and speciation of fe in mangrove sediments along this botanical gradient by using x ray absorption spectroscopy xas at the fe k edge both xanes and exafs results show that iron speciation strongly follows the redox boundaries marking the intertidal and depth zonations fe bearing minerals eroded from lateritic outcrops mainly goethite a feooh and phyllosilicates serpentine and talc are the major fe hosts in the upward horizons these mineral species progressively disappear with increasing depth where pyrite fes2 forms in the hydromorphic rhizophora and avicennia zones sulfate reduction is not observed in the drier salt flat zone in addition to these reduction processes intense re oxidation of aqueous fe ii and pyrite leads to the formation of poorly ordered ferrihydrite lepidocrocite c feooh and likely goethite in the upper sediments beneath avicennia and rhizophora stands the relative proportion of the newly formed poorly ordered ferrihydrite and lepidocrocite is found to be higher in the rhizophora mangrove stand which is the closest to the shore tidal fluctuations may thus be a major cause for continuous fe reduction oxidation cycles in the vegetated mangrove stands which could significantly affect the iron mass balance in mangrove systems

geochemistry of iron and manganese in soils and sediments of a mangrove system island of pai matos cananeia sp brazil abstract five zones along a transect of 180 m were selected for study on the island of pai matos sao paulo brazil four of the zones are colonised by vascular plants spartina sp laguncularia lg avicennia av and rhizophora rh and were denominated soils and the other zone which lacks vegetation was denominated sediment sd the geochemical conditions differed significantly in soils and sediment and also at different depths the soils were oxic eh 350 mv or suboxic eh 350 100 mv at the surface and anoxic eh

sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils abstract we examined the solubility mineralogy and geochemical transformations of sedimentary fe in waterways associated with coastal lowland acid sulfate soils class the waterways contained acidic ph 3 26 3 54 fe iii rich 27 138 m surface water with low molar cl so 4 ratios 0 086 5 73 the surficial benthic sediments had high concentrations of oxalate extractable fe iii due to schwertmannite precipitation kinetically favoured by 28 30 of aqueous surface water fe being present as the fe iii so 4 species subsurface sediments contained abundant pore water hco 3 6 20 mm and were reducing eh ii concentrations were high 2 mm and were constrained by precipitation dissolution of siderite the near neutral reducing conditions also promoted so 4 reduction and the formation of acid volatile sulfide avs the results show for the first time for class associated waterways that sedimentary avs consisted mainly of disordered mackinawite in the presence of abundant pore water fe ii precipitation dissolution of disordered mackinawite maintained very low i e ii concentrations such low concentrations of s ii caused slow rates for conversion of disordered mackinawite to pyrite thereby resulting in relatively low concentrations of pyrite 1 as fe compared to disordered mackinawite up to 590 mol g 1 as fe this study shows that interactions between schwertmannite goethite siderite disordered mackinawite and pyrite control the geochemical behaviour of sedimentary fe in class associated waterways

ni cycling in mangrove sediments from new caledonia abstract covering more than 70 of tropical and subtropical coastlines mangrove intertidal forests are well known to accumulate potentially toxic trace metals in their sediments and thus are generally considered to play a protective role in marine and lagoon ecosystems however the chemical forms of these trace metals in mangrove sediments are still not well known even though their molecular level speciation controls their long term behavior here we report the vertical and lateral changes in the chemical forms of nickel which accumulates massively in mangrove sediments downstream from lateritized ultramafic deposits from new caledonia where one of nature s largest accumulations of nickel occurs to accomplish this we used ni k edge extended x ray absorption fine structure exafs spectroscopy data in combination with microscale chemical analyses using scanning electron microscopy coupled with energy dispersive x ray spectroscopy sem edxs after principal component and target transform analyses pca tt the exafs data of the mangrove sediments were reliably least squares fitted by linear combination of 3 components chosen from a large model compound spectral database including synthetic and natural ni bearing sulfides clay minerals oxyhydroxides and organic complexes our results show that in the inland salt flat ni is hosted in minerals inherited from the eroded lateritic materials i e ni poor serpentine 44 58 ni rich talc 20 31 and ni goethite 18 24 in contrast in the hydromorphic sediments beneath the vegetated avicennia and rhizophora stands a large fraction of ni is partly redistributed into a neoformed smectite pool 20 69 of ni montmorillonite and ni speciation significantly changes with depth in the sediment indeed ni rich talc 25 56 and ni goethite 15 23 disappear below 15 cm depth in the sediment and are replaced by ni sorbed pyrite 23 52 in redox active intermediate depth layers and by pyrite 34 55 in the deepest sediment layers ni incorporation in pyrite is especially observed beneath an inland avicennia stand where anoxic conditions are dominant in contrast beneath a rhizophora stand closer to the ocean where the redox cycle is intensified due to the tide cycle partial re oxidation of ni bearing pyrites favors nickel mobility as confirmed by ni mass balance estimates and by higher ni concentration in the pore waters these findings have important environmental implications for better evaluating the protective role of mangroves against trace metal dispersion into marine ecosystems they may also help in predicting the response of mangrove ecosystems to increasing anthropogenic pressure on coastal areas

preserving the palaeoenvironmental record in drylands bioturbation and its significance for luminescence derived chronologies luminescence osl dating has revolutionised the understanding of late pleistocene dryland activity however one of the key assumptions for this sort of palaeoenvironmental work is that sedimentary sequences have been preserved intact enabling their use as proxy indicators of past changes this relies on stabilisation or burial soon after deposition and a mechanism to prevent any subsequent re mobilisation as well as a dating technique osl especially at the single grain level can be used to gain an insight into post depositional processes that may distort or invalidate the palaeoenvironmental record of geological sediment sequences this paper explores the possible impact of bioturbation the movement of sediment by flora and fauna on luminescence derived chronologies from quaternary sedimentary deposits in texas and florida usa which have both independent radiocarbon chronologies and archaeological evidence these sites clearly illustrate the ability of bioturbation to rejuvenate ancient weathered sandy bedrock and or to alter depositional stratigraphies through the processes of exhumation and sub surface mixing of sediment the use of multiple osl replicate measurements is advocated as a strategy for checking for bioturbated sediment where significant osl heterogeneity is found caution should be taken with the derived osl ages and further measurements at the single grain level are recommended observations from the linear dunes of the kalahari show them to have no bedding structure and to have osl heterogeneity similar to that shown from the bioturbated texan and florida sites the kalahari linear dunes could have therefore undergone hitherto undetected post depositional sediment disturbance which would have implications for the established osl chronology for the region

sulfur biogeochemical cycling and novel fe s mineralization pathways in a tidally re flooded wetland abstract sulfur biogeochemical cycling and associated fe s mineralization processes exert a major influence over acidity dynamics electron flow and contaminant mobility in wetlands benthic sediments and groundwater systems while s biogeochemical cycling has been studied intensively in many environmental settings relatively little direct information exists on s cycling in formerly drained wetlands that have been remediated via tidal re flooding this study focuses on a tidal wetland that was drained in the 1970s causing severe soil and water acidification and subsequently remediated by controlled re flooding in 2002 we examine so 4 2 reduction rates and fe s mineralization at the tidal fringe 7 years after the commencement of re flooding the initial drainage of the wetland examined here caused in situ pyrite fes2 oxidation resulting in the drained soil layers being highly acidic and rich in so 4 2 bearing fe iii minerals including jarosite kfe3 so4 2 oh 6 tidal re flooding has neutralized much of the previous acidity with the pore water ph now mostly spanning ph 5 7 the fastest rates of in situ so 4 2 reduction up to 300 nmol cm 3 day 1 occur within the inter tidal zone in the near surface soil layers to 60 cm below ground surface the so 4 2 reduction rates correlate with pore water dissolved organic c concentrations thereby suggesting that electron donor supply was the predominant rate determining factor elemental s was a major short term product of so 4 2 reduction comprising up to 69 of reduced inorganic s in the near surface soil layers this enrichment in elemental s can be partly attributed to interactions between biogenic h2s and jarosite a process that also contributed to enrichment in pore water fe2 up to 55 mm and so 4 2 up to 50 mm the iron sulfide thiospinel greigite fe3s4 was abundant in near surface soil layers within the inter to sub tidal zone where tidal water level fluctuations created oscillatory redox conditions there was evidence for relatively rapid pyrite re formation within the re flooded soil layers however the results indicate that pyrite re formation has occurred mainly in the lower formerly drained soil layers whereas the accumulation of elemental s and greigite has been confined towards the soil surface the discovery that pyrite formation was spatially decoupled from that of elemental s and greigite challenges the concept that greigite is an essential precursor required for sedimentary pyrite formation in fact the results suggest that greigite and pyrite may represent distinct end points of divergent fe s mineralization pathways overall this study highlights novel aspects of fe s mineralization within tidal wetlands that have been drained and re flooded in contrast to normal undisturbed tidal wetlands as such the long term biogeochemical trajectory of drained and acidified wetlands that are remediated by tidal re flooding cannot be predicted from the well studied behaviour of normal tidal wetlands

iron mineralogy across the oxycline of a lignite mine lake iron rich pelagic aggregates of microbial origin named iron snow are formed in the water column of some acidic lignite mine lakes we investigated the evolution of fe mineralogy across the oxycline of the lusatian lake 77 germany at two sampling sites differing by their ph and mixing profiles the central basin cb of this lake shows a dimictic water regime with a non permanent anoxic deep layer and a homogeneous acidic ph all over the water column ph 3 in contrast the northern basin nb is meromictic with a permanently anoxic bottom layer and a ph increase from ph 3 in the mixolimnion superficial part of the lake to ph 5 5 in the monimolimnion anoxic bottom layer fe minerals above and below the oxycline were identified using x ray absorption spectroscopy xas at the fe k edge and further characterized down to the atomic scale by high resolution transmission electron microscopy hrtem and scanning transmission electron microscopy stem coupled to energy dispersive x ray spectroscopy edxs we explored local fe redox state and c speciation using scanning transmission x ray microscopy stxm at the fe l2 3 edges and c k edge schwertmannite fe8o8 oh 8 2x so4 x identified as the sole fe mineral in cb was polycrystalline consisting in the aggregation of nanodomains of 2 3 nm each one exhibiting the crystal structure of schwertmannite in contrast schwertmannite was partly 40 converted to aluminous ferrihydrite when reaching the oxycline in nb this mineralogical transformation was most probably due to a combination of abiotic and microbial anaerobic processes promoting ph increase and release of fe ii e g via heterotrophic fe iii reduction that induce the catalytic hydrolysis of schwertmannite to ferrihydrite mineral products were stabilized in the monimolimnion by the adsorption of aluminum silicate and organic matter noteworthy local fe redox state heterogeneities were observed with a few areas enriched in fe ii as evidenced by stxm analyses at the fe l2 3 edges these local redox heterogeneities could arise from microbial activity e g fe iii and or sulfate reduction all these results provide an in depth mineralogical overview of iron phases forming in lake 77 as a basis for future investigations of microbial vs abiotic parameters controlling their stability and transformation

manganese and iron geochemistry in sediments underlying the redox stratified fayetteville green lake abstract manganese and iron are redox sensitive elements that yield clues about biogeochemistry and redox conditions both in modern environments and in the geologic past here we investigated mn and fe bearing minerals preserved in basin sediments underlying fayetteville green lake a redox stratified lake that serves as a geochemical analogue for paleoproterozoic oceans synchrotron source microprobe techniques xrf xanes and xrd and bulk geochemical analyses were used to examine the microscale distribution and speciation of mn fe and s as a function of depth in the top 48 cm of anoxic lake sediments manganese was primarily associated with calcite grains as a manganese rich carbonate that precipitated in the chemocline of the water column and settled through the euxinic basin to collect in lake sediments iron was preserved in framboidal iron sulfides that precipitated in euxinic bottom waters and underwent transformation to pyrite and marcasite in the sediments previous studies attribute the formation of manganese rich carbonates to the diagenetic alteration of manganese oxides deposited in basins underlying oxygenated water our study challenges this paradigm by providing evidence that mn bearing carbonates form in the water column and accumulate in sediments below anoxic waters consequently manganoan carbonates preserved in the rock record do not necessarily denote the presence of oxygenated bottom waters in ocean basins