influence of organics and silica on fe ii oxidation rates and cell mineral aggregate formation by the green sulfur fe ii oxidizing bacterium chlorobium ferrooxidans kofox implications for fe ii oxidation in ancient oceans most studies on microbial phototrophic fe ii oxidation photoferrotrophy have focused on purple bacteria but recent evidence points to the importance of green sulfur bacteria gsb their recovery from modern ferruginous environments suggests that these photoferrotrophs can offer insights into how their ancient counterparts grew in archean oceans at the time of banded iron formation bif deposition it is unknown however how fe ii oxidation rates cell mineral aggregate formation and fe mineralogy vary under environmental conditions reminiscent of the geological past to address this we studied the fe ii oxidizer chlorobium ferrooxidans kofox a gsb living in co culture with the heterotrophic geospirillum strain kofum we investigated the mineralogy of fe iii metabolic products at low high light intensity and in the presence of dissolved silica and or fumarate silica and fumarate influenced the crystallinity and particle size of the produced fe iii minerals the presence of silica also enhanced fe ii oxidation rates especially at high light intensities potentially by lowering fe ii toxicity to the cells electron microscopic imaging showed no encrustation of either kofox or kofum cells with fe iii minerals though weak associations were observed suggesting co sedimentation of fe iii with at least some biomass via these aggregates which could support diagenetic fe iii reduction given that gsb are presumably one of the most ancient photosynthetic organisms and pre date cyanobacteria our findings on the one hand strengthen arguments for photoferrotrophic activity as a likely mechanism for bif deposition on a predominantly anoxic early earth but on the other hand also suggest that preservation of remnants of fe ii oxidizing gsb as microfossils in the rock record is unlikely

diversity of iron and silica precipitation by microbial mats in hydrothermal waters iceland implications for precambrian iron formations direct examination of microbial mats from icelandic hot springs with transmission electron microscopy and energy dispersive x ray spectroscopy revealed a consortium of bacterial cells in varying stages of mineralization differences in observed mineralogy largelyreflectdifferencesinthechemistryofthehydrothermalwaters silica richspheroids formedepicellularlyoncellwallsandsurroundingsheathsandcapsulesofmicroorganisms and insomecases intracellularlywhenpresumablythecell s hadlysed commonly these precipitateswereobservedcoalescingtoformamatrixofamorphoussilicathatcompletely encapsulated the cells and or replaced their cytoplasmic material however in other cells the precipitates were composed of amorphous granules made exclusively of iron and silica inapproximatelyequalproportions atonelocality thebacteriaformedseveralepicellular iron minerals ranging from iron mineralized capsules tofine grained spheroids of amorphous ferric hydroxide and acicular aggregates of goethite the complete encrustation of bacterial cells by silica iron or a combination of both may greatly enhance their preservation potential such that these mineralized microorganisms may conceivably represent future microfossils thus we may be witnessing contemporaneous biomineralization processes that are similar to those of the geologic past particularly with regard to the origin of some precambrian banded iron formations

microbe clay mineral interactions clays and clay minerals are common components in soils sediments and sedimentary rocks and they play an important role in many environmental processes iron is ubiquitous in clays and clay minerals and its oxidation state in part controls the physical and chemical properties of these fine grained minerals the structural ferric iron in clay minerals can be reduced either chemically or biologically biological reductants include mesophilic and thermophilic microorganisms from diverse environments such as soils sediments sedimentary rocks and hydrothermal hot springs multiple clay minerals have been used for microbial reduction studies including dioctahedral smectiteillite series palygorskite chlorite and their various mixtures in natural soils and sediments all of these clay minerals are reducible by microorganisms under various conditions with smectite nontronite being the most reducible and illite the least the rate and extent of bioreduction depends on many experimental factors such as the type of microorganisms and clay minerals solution chemistry and temperature despite significant efforts current understanding of the mechanisms of microbial reduction of ferric iron in clay minerals is still limited whereas some studies have presented evidence for a solid state reduction mechanism others argue that the clay mineral structure partially dissolves when the extent of reduction is high this inconsistency may be related to several experimental conditions and their specific effects are discussed in this paper whereas past experiments have been largely conducted in well controlled laboratory systems recent efforts have attempted to transfer knowledge to the field to improve our understanding of more complex soil systems for better agricultural practices biologically reduced clay minerals are also important agents in remediating inorganic and organic contaminants in soil and groundwater systems this paper reviews the most recent developments and suggests some directions for future research

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

method to determine appropriate source models of large earthquakes including tsunami earthquakes for tsunami early warning in central america large earthquakes such as the mw 7 7 1992 nicaragua earthquake have occurred off the pacific coasts of el salvador and nicaragua in central america and have generated distractive tsunamis along these coasts it is necessary to determine appropriate fault models before large tsunamis hit the coast in this study first fault parameters were estimated from the w phase inversion and then an appropriate fault model was determined from the fault parameters and scaling relationships with a depth dependent rigidity the method was tested for four large earthquakes the 1992 nicaragua tsunami earthquake mw7 7 the 2001 el salvador earthquake mw7 7 the 2004 el astillero earthquake mw7 0 and the 2012 el salvador nicaragua earthquake mw7 3 which occurred off el salvador and nicaragua in central america the tsunami numerical simulations were carried out from the determined fault models we found that the observed tsunami heights run up heights and inundation areas were reasonably well explained by the computed ones therefore our method for tsunami early warning purpose should work to estimate a fault model which reproduces tsunami heights near the coast of el salvador and nicaragua due to large earthquakes in the subduction zone

upb geochronologic constraints on deposition of the campbellrand subgroup transvaal supergroup south africa abstract a volcanic ash bed that crops out 40 50 m below the contact between the campbellrand subgroup and the kuruman iron formation transvaal supergroup south africa yields an u pb zircon age of 2521 3 ma this new age demonstrates that the campbellrand malmani carbonate platform was deposited during late archean time and is the oldest extensively preserved carbonate platform known in addition the age constrains deposition of the kuruman griquatown and penge iron formations to latest archean to earliest paleoproterozoic time although a previously reported pb pb age of 2557 49 ma for dolomites at the base of the campbellrand subgroup is broadly consistent with this u pb zircon age new pb isotopic data for carbonate cements and massive sulfides lie along the same pb pb array as the dolomites this relationship implies that the isochron is the result of post depositional resetting of the u pb system in dolomite by fluids responsible for sulfide mineralization and that the age which corresponds to its slope has no geological significance

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

influence of the intraseasonal variability on heat waves in subtropical south america the influence of the intraseasonal variability on heat wave development over subtropical south america during austral summer is analyzed the role of the south atlantic convergence zone sacz on this development is documented results show that intraseasonal variability can explain on average at least 32 of summer temperature variance moreover 73 of the heat waves in subtropical south america develop in association with an active sacz the analysis of pentad maps shows that warm conditions in the region under study develop in association with the strengthening of an anticyclonic anomaly which is discernible over the subtropical regions at least 15 days before temperature peak occurrence that circulation anomaly is embedded in a large scale rossby wave train extending along the south pacific ocean that is linked to convection anomalies at the equatorial western and central pacific ocean in addition the development of the anticyclonic circulation over subtropical south america appears to be strengthened by the subsidence conditions promoted by the active sacz which result in temperature rise in the subtropical region under relatively dry conditions on the other hand during the last 2 days of evolution sacz activity weakens and the progressive temperature rise in the region is dominated by warmer and moister air being anomalously advected from the north results confirm the important role that sacz activity on intraseasonal time scales has in inducing persistent circulation anomalies at the subtropical regions that can result in the development of persistent heat waves and very extreme daily temperature

dust to dust evidence for the formation of primary hematite dust in banded iron formations via oxidation of iron silicate nanoparticles abstract conventional models for the deposition of banded iron formations bifs envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide hydroxide particles in surface waters that settled to form laterally extensive layers of iron rich sediment a fundamental tenet of this model is that fine grained hematite so called dusty hematite in least altered bifs represents the dehydration product of original oxide hydroxide precipitates however this premise has never been proven we have investigated the origin of the earliest formed iron oxides in chert in well preserved bifs of the 2 63 2 45 billion year old hamersley group australia we find that laminated chert in bifs show progressive stages of in situ alteration from gray green chert containing iron silicate nanoparticles to red chert with abundant hematite dust analysis of textures by transmission electron microscopy of samples from the transition zone between gray green and red chert reveals that dusty hematite formed after partial dissolution of iron silicate nanoparticles by the precipitation of iron oxides in resulting cavities these observations suggest that hematite dust is not a relict of an original seawater precipitate but the end product of post depositional oxidation our observations are consistent with paleomagnetic results from the hamersley group which record two major phases of magnetic remanence carried by hematite that post date deposition by more than 200 million years our results may provide an alternative explanation for the origin of jasper in bifs deposited before the start of the great oxidation event about 2 4 billion years ago if correct it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early precambrian seawater furthermore our results suggest that the primary iron precipitate in bifs was iron silicate mud that was silicified at or just below the sediment water interface a hypothesis that requires neither dissolved oxygen nor photosynthetic life but was an inorganic chemical process reflecting anoxic oceans enriched in iron and silica

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

iron formation carbonates paleoceanographic proxy or recorder of microbial diagenesis the chemical and isotopic compositions of carbonates are commonly used as proxies for ancient seawater or paleoenvironments iron formation if carbonates have been used as evidence for an anoxic fe ii rich archean and paleoproterozoic ocean and high atmospheric co 2 contents it has been proposed however that microbial fe cycling dominates the chemical and isotopic compositions of if carbonates suggesting less direct applicability as an oceanic proxy here were use an isotope tracer that is not affected by biological processes or isotopic fractionation the radiogenic 87 rb87 sr system to test the applicability of if carbonates as a paleoenvironmental proxy we focus on the 2 5 ga campbellrand platform transvaal basin south africa that records a shift from ca mg carbonates to if carbonates during a marine transgression when coupled with previously determined fe c and o isotope compositions it becomes clear that the if carbonates studied here do not refl ect seawater compositions but instead record extensive microbial fe cycling in the soft sediment prior to lithifi cation these results question the use of if carbonates to infer seawater compositions and paleoenvironmental conditions including estimates for atmospheric co 2 contents

primary silica granules a new mode of paleoarchean sedimentation in the modern silica cycle dissolved silica is removed from seawater by the synthesis and sedimentation of silica biominerals with additional sinks as authigenic phyllosilicates and silica cements fundamental questions remain however about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in neoproterozoic time the abundance of siliceous sedimentary rocks in archean sequences mainly in the form of chert strongly indicates that abiotic silica precipitation played a significant role during archean time it was previously hypothesized that these cherts formed as primary marine precipitates but substantive evidence supporting a specific mode of sedimentation was not provided we present sedimentologic petrographic and geochemical evidence that some and perhaps many archean cherts were deposited predominately as primary silica grains here termed silica granules that precipitated within marine waters this mode of silica deposition appears to be unique to archean time and provides evidence that primary silica precipitation was an important process in archean oceans understanding this mechanism promises new insights into the archean silica cycle including chert petrogenesis microfossil preservation potential and archean alkalinity budgets and silicate weathering feedback processes

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

evidence of anoxic to oxic atmospheric change during 2 45 2 22 ga from lower and upper sub huronian paleosols canada paleoweathered zones in four underground drill holes in quirke ii mine and one drill hole by denison mines in elliot lake area ontario canada were studied for petrography mineralogy and whole rock chemistry these paleosols formed beneath 2 45 ga old lower huronian formations are characterized by upward depletion of fetotal and fe3 in the whole rock by upward depletion of fetotal fe mg ratios in chlorites and fine grained muscovite and by the presence of fe2 minerals e g pyrite pyrrhotite ilmenite in contrast the paleosols underlying upper huronian strata 2 22 ga old exhibit an upward increase in fe3 and fetotal and contain hematite and goethite uraninite and pyrite bearing paleoplacers occur in lower huronian strata whereas the upper huronian formations contain red beds and are devoid of detrital uraninite and pyrite these differences between the sub huronian paleoweathering profiles and mineralogical characteristics of the lower huronian strata overlying the paleosols support the previously postulated oxyatmoversion theory and restricts the time interval for its occurrence to between 2 45 and 2 22 ga assuming that few tens of millions of years were required for the deposition of lower huronian groups oxyatmoversion likely occurred at about 2 4 ga

sedimentology of the kuruman and griquatown iron formations transvaal supergroup griqualand west south africa abstract iron formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow water carbonate platform at the time of deposition of the campbellrand carbonate sequence a subsequent transgression resulted in the deposition of open shelf iron formation on top of the campbellrand carbonate platform progradational sedimentation coupled with shoaling followed and an iron formation sequence represented by the kuruman and griquatown iron formations was deposited this sequence consists from the base upwards of stacked open shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units toe of slope greenalite siderite rhythmites platform slope greenalite siderite rhythmites with grainflow bands platform edge sideritic orthochemical and allochemical iron formations epeiric sea orthochemical and allochemical sideritic hematitic and greenalitic iron formations supratidal disclutites and lacustrine banded greenalite lutite landwards the lacustrine felutites were followed by deltaic chloritic claystone siltstone and quartz wacke of the koegas subgroup autochthonous ferhythmites of the kuruman iron formation reach a maximum development in a basin near prieska whereas orthochemical and allochemical units are more abundant on the kaapvaal craton iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in eh and ph in the depository and may be related to biological activity chert mesobanding in the iron formations is essentially of an early diagenetic origin

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

precipitation of iron silicate nanoparticles in early precambrian oceans marks earth s first iron age the early ocean was characterized by anoxic iron rich ferruginous conditions before the rise of atmospheric oxygen 2 45 b y ago a proxy for ferruginous conditions in the ancient ocean is the deposition of banded iron formations bifs which are iron and silica rich chemical sediments whose constituents were largely derived from seawater although experiments simulating ancient ocean chemistry support the rapid growth of iron silicate phases the main iron precipitates are hypothesized to have been ferric oxyhydroxides the paradox between the prevailing reducing conditions and the deposition of oxidized iron phases is explained by biologically mediated oxidation in the water column new high resolution microscopy of bifs and shales throughout the 2 63 2 45 b y old hamersley group australia reveals the presence of vast quantities of nanometer sized iron silicate particles in laminated chert the nanoparticles are finely disseminated in early diagenetic chert and locally define sedimentary lamination indicating that they represent relicts of the original sediments by inference from experimental studies simulating the composition of the early precambrian ocean we suggest that the nanoparticles precipitated from anoxic seawater enriched in silica and dissolved iron and were silicified upon deposition the prevalence of iron silicate nanoparticles implies that they were pervasive background precipitates in ferruginous silica enriched oceans forming the primary sediments of bifs during periods of enhanced submarine mafic volcanism our results imply that silicate precipitation was a major sink of seawater iron and silica before the great oxidation event and because of the reactivity of nanoparticle surfaces may also have influenced the transport and geochemical cycling of trace metals and nutrients our hypothesis that the basic building blocks of bifs were predominantly iron silicate muds rather than iron oxides and or hydroxides may lead to new insights into seawater chemistry on the early earth and the role of biology in the deposition of bifs

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

nickel partitioning in biogenic and abiogenic ferrihydrite the influence of silica and implications for ancient environments fe iii oxyhydr oxides are ubiquitous in modern soils and sediments and their large surface area leads to scavenging of trace elements experimental trace element partitioning between fe iii oxyhydr oxides and aqueous solutions have been used to elucidate the geochemical composition of the precambrian oceans based on the trace element concentrations in precambrian banded iron formations bifs however previous partitioning experiments did not consider the potential influence of microbially derived organic material even though it is widely believed that bacterial phytoplankton was involved in fe ii oxidation and the deposition of bif primary minerals therefore the present study focuses on sorption of ni to and co precipitation of ni with both biogenic ferrihydrite precipitated by the freshwater photoferrotroph rhodobacter ferrooxidans sw2 and the marine photoferrotroph rhodovulum iodosum as well as chemically synthesized ferrihydrite we considered the influence of cellular organic material medium composition and the availability of dissolved silica our results show a preferential association of ni with ferrihydrite and not with the microbial cells or extracellular organic substances we found that the addition of silica 2 mm did not influence ni partitioning but led to the encrustation of some cells with ferrihydrite and amorphous silica the two to threefold lower ni fe ratio in biogenic as compared to abiogenic ferrihydrite is probably due to a competition between ni and organic matter for sorption sites on the mineral surface additionally the competition of ions present at high concentrations in marine medium for sorption sites led to decreased ni sorption or co precipitation based on our data we conclude that if the fe iii minerals deposited in bifs were at least to some extent biological then the ni concentrations in the early ocean would have been higher than previously suggested this study shows the importance of considering the presence of microbial biomass and seawater ions in paleomarine reconstructions

carbonate petrography kerogen distribution and carbon and oxygen isotope variations in an early proterozoic transition from limestone to iron formation deposition transvaal supergroup south africa the transition zone comprises campbellrand microbialaminated replacing cryptalgalaminate limestone and shale with minor dolomite conformably overlain by the kuruman iron formation of which the basal part is characterized by siderite rich microbanded iron formation with minor magnetite and some hematite containing units the iron formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones the sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees c and pressures of 2 kbars carbonate minerals in the different rock types are represented by primary micritic precipitates now recrystallized to microsparite early precompactional sparry cements and concretions deep burial limpid euhedral sparites and spar cements precipitated from metamorphic fluids in close contact with diabase sills paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them kerogen occurs as pigmentation in carbonate crystals as reworked organic detritus in clastic textured carbonate units and as segregations of kerogen pigment around late diagenetic carbonate crystals locally kerogen may also be replaced by carbonate spar carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies integration of sedimentary petrographic geochemical and isotopic results makes it possible to distinguish between depositional early diagenetic deep burial and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence major conclusions are that deep burial thermal decarboxylation led to 13c depletion in euhedral ferroan sparites and 13c enrichment in kerogen organic carbon metamorphic sparites are most depleted in 13c carbonates in oxide rich iron formations are more depleted in 13c than those in siderite rich iron formation whereas the kerogens in oxide banded iron formations bif are more enriched this implies that the siderite rich iron formations were not derived from oxide rich iron formation through reduction of ferric iron by organic matter organic matter oxidation by ferric iron did however decrease the abundance of kerogen in oxide rich iron formation and led to the formation of isotopically very light sparry carbonates siderite and calcmicrosparite both represent recrystallized primary micritic precipitates but differ in their 13c composition with the siderites depleted in 13c by 4 6 per mil on average relative to calcmicrosparite this means that the siderites were precipitated from water with dissolved inorganic carbon depleted in 13c by about 9 per mil relative to that from which the limestones precipitated this implies an ocean system stratified with regard to total carbonate with the deeper water from which siderite rich iron formation formed depleted in 13c iron formations were deposited in areas of very low organic matter supply depletion of 13c may therefore derive not from degradation of organic matter but from hydrothermal activity a conclusion which is supported by 18o composition of the carbonate minerals and trace element and rare earth element ree compositions of the iron formations

primary hematite in neoarchean to paleoproterozoic oceans banded iron formations bifs are ironand silica rich chemical sedimentary rocks formed throughout the archean and paleoproterozoic eras the presence of hematite fe2o3 and magnetite fe3o4 in bifs has led to the widespread assumption that fe ii oxidation must have occurred in the ancient oceans via either a biological or chemical mechanism however it is unclear whether the ferric iron now present in bif represents the original ferric oxyhydroxide e g ferrihydrite fe oh 3 precipitated in the water column or if it is the result of later stage circulation of oxidizing fl uids through the sediment pile in this study we conducted high resolution microscopic investigations on bif from the 2728 ma abitibi greenstone belt located in the superior province of the canadian shield and the 2460 ma kuruman iron formation in south africa to ascertain the timing and paragenesis of the hematite three types of hematite are identifi ed by high resolution electron microscopic characterization and selected area electron diffraction 1 3 5 nm ultrafi ne hematite particles in the iron oxide rich bands h1 2 submicrometer subhedral to euhedral hema tite crystals randomly distributed in the chert matrix of transitional zones between iron oxide and chert rich bands h2 and 3 needle like radial and fi brous hematite that replaced stilpnomelane or carbonates and is distributed along fractures or layer boundaries h3 we interpret the fi rst two types as primary minerals dehydrated from precursor ferric oxyhydroxides h1 remains ultrafi ne in size while h2 has undergone an ostwald coarsening process facilitated by internal fl uids produced during amorphous silica to quartz transformation h3 is a later stage mineral formed by external fl uid mediated replacement of iron silicates or carbonates these results indicate that a signifi cant fraction of the hematite in the bif originated from ferric oxyhydroxide precursors importantly this implies that photosynthetic fe ii oxidation by either a direct or indirect biological mechanism did exist in seawaters from which some bif material was deposited

biogenic fe iii minerals from formation to diagenesis and preservation in the rock record abstract fe metabolizing bacteria are intimately linked to the cycling of fe in modern environments and have likely been key players in the evolution of the earth s biogeosphere fe minerals have also been suggested as a key preservative of cell organic matter in sediments keeping otherwise labile phases conserved at least on time scales of 100 000 years the interpretation of a biological influence on the fe rock record is difficult without a deeper understanding of the mechanisms of biogenic fe iii and fe ii mineral formation the character of these minerals and their diagenesis over short and long time scales here we present the recent advances in the study of abiogenic and biogenic fe iii minerals in particular we focus on the role of fe ii oxidizing bacteria in the deposition of ancient banded iron formations bif we discuss this work within the framework of the main challenge separating biogenic from abiogenic processes over deep time we describe how efforts in isotope geochemistry biomarker research mineral analysis and biogeochemistry are helping to establish a window to the past finally we present some new approaches that help investigate the main processes leading to the formation and potential fate of fe organic matter aggregates

green rust formation controls nutrient availability in a ferruginous water column iron rich ferruginous conditions were a prevalent feature of the ocean throughout much of earth s history the nature of elemental cycling in such settings is poorly understood however thus hampering reconstruction of paleoenvironmental conditions during key periods in earth evolution this is particularly true regarding controls on nutrient bioavailability which is intimately linked to earth s oxygenation history elemental scavenging during precipitation of iron minerals exerts a major control on nutrient cycling in ferruginous basins and the predictable nature of removal processes provides a mechanism for reconstructing ancient ocean chemistry such reconstructions depend however on precise knowledge of the iron minerals formed in the water column here we combine mineralogical and geochemical analyses to demonstrate formation of the mixed valence iron mineral green rust in ferruginous lake matano indonesia carbonated green rust gr1 along with signifi cant amounts of magnetite forms below the chemocline via the reduction of ferrihydrite further we show that uptake of dissolved nickel a key micronutrient required for methanogenesis is signifi cantly enhanced during green rust formation suggesting a major control on methane production in ancient ferruginous settings

haematite pigmentation events and palaeomagnetic recording implications from the pilbara print stone western australia summary haematite pigment is a common constituent of sedimentary rocks but its contribution to the natural remanent magnetization of rocks is poorly understood here we describe magnetic properties of two distinct pigment types that produce a characteristic decorative print stone foundinthe 2 5gamount mcrae shaleformation hamersley province westernaustralia distinct magnetic remanence directions observed in the print stone can be correlated to each pigment type by comparison with the australian apparent polar wander path the remanence carried by uniformly distributed pigment can be dated to 15 25 ma while two age options the mesoproterozoic 1 5 ga or the middle carboniferous 320 310 ma are permissible for the remanence carried by the pigment responsible for the distinctive newsprint pattern magnetic properties and demagnetization characteristics of the different pigment types overlap significantly and thus are not predictive of the dominant remanence carrier magnetic characteristics of the uniformly distributed pigment vary significantly on short spatial scales strong local control on pigment formation raises the possibility that a primary remanent magnetization may survive locally in pockets within sedimentary red bed formations

formation of fine grained metal and silicate precipitates on a bacterial surface bacillus subtilis abstract the ability of the gram positive bacterium b subtilis to bind and nucleate precipitates from silicate anions has been studied over 24 weeks in the presence of fe and a1 at concentrations close to those levels in soils and at slightly acid 5 5 and basic 8 0 ph in all cases formation of silicate crystallites quasi crystalline precipitates on the bacterial surfaces was observed bacterially mediated minerals were more diverse in composition and morphology less crystalline smaller and sometimes more abundant than those that were abiotically formed fe pretreatment of the bacterial cells enhanced the binding of silicate at ph 8 0 walls which were not pretreated with fe bound silicate more favourably at acid values when heavy metals pb cd zn cr ni cu were added to the mixture at ph 4 5 silicate retention was greatly favoured giving greater retention of either si or metals than was seen in abiotic controls experiments with only heavy metals showed a high affinity of the bacterial walls for the metals even at low temperatures 4 c it is postulated that a cationic bridging mechanism is involved in the binding of silicate anions by bacterial cell walls

simulating precambrian banded iron formation diagenesis abstract post depositional diagenetic alteration makes the accurate interpretation of key precipitation processes in ancient sediments such as precambrian banded iron formations bifs difficult while microorganisms are proposed as key contributors to bif deposition the diagenetic transformation of precursor fe iii minerals associated with microbial biomass had not been experimentally tested we incubated mixtures of ferrihydrite proxy for biogenic ferric oxyhydroxide minerals and glucose proxy for microbial biomass in gold capsules at 1 2 kbar and 170 c both wet chemical analysis and mineralogical methods microscopy x ray diffraction and mossbauer spectroscopy were used to analyze the reaction products under these conditions ferrihydrite fe iii oh 3 transforms to hematite fe 2 iii o 3 magnetite fe ii fe 2 iii o 4 and siderite fe ii co 3 silica coated ferrihydrite prepared at conservative si fe ratios as predicted for the precambrian oceans and mixed with glucose yielded hematite and siderite whereas magnetite could not be identified microscopically our results show that electron transfer from organic carbon to fe iii minerals during temperature pressure diagenesis can drive the production of key bif minerals our results also demonstrate that the post depositional mineralogy of bif does not directly archive the oceanic or atmospheric conditions present on earth during their lithification as a consequence atmospheric composition regarding concentrations of methane and co 2 during the time of bif mineral deposition cannot be directly inferred from bif mineralogical data alone

o2 constraints from paleoproterozoic detrital pyrite and uraninite redox sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the archean and early paleoproterozoic atmosphere today these minerals are rapidly chemically weathered within short transport distances prior to the rise of oxygen low o2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes after the rise of oxygen redox sensitive detrital grains effectively vanish from the sedimentary record to get a better understanding of the timing of this transition we examined sandstones recorded in a scientifi c drill core from the south african 2 415 ga koegas subgroup a mixed siliciclastic and iron formation bearing unit deposited on the western deltaic margin of the kaapvaal craton in early paleoproterozoic time we observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section indicating the rise of oxygen is younger than 2 415 ga to better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain earth surface redox conditions we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental o2 concentrations even conservative model calculations for deltaic depositional systems with suffi cient transport distances approximately hundreds of kilometers show that redox sensitive detrital grains are remarkably sensitive to environmental o2 concentrations and they constrain the archean and early paleoproterozoic atmosphere to have 3 2 10 5 atm of molecular o2 these levels are lower than previously hypothesized for redox sensitive detrital grains but they are consistent with estimates made from other redox proxy data including the anomalous fractionation of sulfur isotopes the binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric o2 concentrations rose substantially at this time and were never again suffi ciently low 0 01 atm to enable survival and preservation of these grains in short transport systems

paleomagnetism of a neoarchean paleoproterozoic carbonate ramp and carbonate platform succession transvaal supergroup from surface outcrop and drill core griqualand west region south africa detailed paleomagnetic study across the archean paleoproterozoic boundary interval preserved in the ghaap group transvaal supergroup of the kaapvaal craton reveals a complex series of viscous and thermo chemical magnetic overprints despite this complex history a potential primary single polarity remanence direction was revealed at high temperature demagnetization steps in about 10 of orientated drill core samples only this component declination 132