sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

a redox stratified ocean 3 2 billion years ago abstract before the great oxidation event goe 2 4 2 2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen producing microorganisms recently however it has been proposed that transient oxygenation of shallow seawater occurred between 2 8 and 3 0 billion years ago here we present a novel combination of stable fe and radiogenic u th pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3 2 ga based on analysis of the manzimnyama banded iron formation bif fig tree group south africa this unit is exceptional in that proximal shallow water and distal deep water facies are preserved when compared to the distal deep water facies the proximal samples show elevated u concentrations and moderately positive 56 fe values indicating vertical stratification in dissolved oxygen contents confirmation of oxidizing conditions using u abundances is robustly constrained using samples that have been closed to u and pb mobility using u th pb geochronology although redox sensitive elements have been commonly used in ancient rocks to infer redox conditions post depositional element mobility has been rarely tested and u th pb geochronology can constrain open or closed system behavior the u abundances and 56 fe values of the manzimnyama bif suggest the proximal shallow water samples record precipitation under stronger oxidizing conditions compared to the distal deeper water facies which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time this work therefore documents the oldest known preserved marine redox gradient in the rock record the relative enrichment of o 2 in the upper water column is likely due to the existence of oxygen producing microorganisms such as cyanobacteria these results provide a new approach for identifying free oxygen in earth s ancient oceans including confirming the age of redox proxies and indicate that cyanobacteria evolved prior to 3 2 ga

volcaniclastic rocks and volcaniclastic facies in the middle precambrian aphebian belcher group northwest territories canada the middle precambrian flaherty formation consists of a thick sequence of volcanic rocks of both effusive and explosive origin derived from a source area west of the belcher islands the effusive

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

precise u pb zircon ages for the molson dyke swarm and the fox river sill constraints for early proterozoic crustal evolution in northeastern manitoba canada precise u pb zircon ages have been obtained for samples from the molson dyke swarm and the fox river sill in ne manitoba canada the ages determined for the cross lake and cuthbert lake dykes are 1 883 7 1 5 1 7and 1 883 2 ma respectively and are in excellent agreement with the 1 882 9 1 4 1 5ma age obtained for the fox river sill these results support the contention that the emplacement of the fox river sill and the molson dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities the timing of early proterozoic mafic magmatism in the western superior province appears to be synchronous with igneous activity in other parts of the circum superior belt and in the trans hudson orogen to the west the emplacement of the molson dyke swarm at 1 883 ma indicates a 700 ma interval of quiescence between the final igneous activity that is recorded in the archean basement and dyke intrusion the presence of deformed equivalents of molson dykes in the thompson nickel belt indicates that the intense deformation in this belt occurred sometime after 1 883 ma

emergence of modern continental crust about 3 billion years ago the continental crust provides a record of earth s evolution analysis of the geochemical signature of continental crust formed since the hadean points to the initiation of plate tectonics about 3 billion years ago

palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the late archaean witwatersrand basin oxygen deficient atmosphere or hydrothermal alteration petrographic and sulphur isotope studies support the long held contention that rounded grains of pyrite in siliciclastic sequences of the late archaean witwatersrand supergroup originated as placer grains the grains areconcentrated at sites where detrital heavy minerals are abundant within quartz pebble conglomerates and quartzose sandstones depositional sites with abundant pyrite are 1 within the matrix of bar type clast supported conglomerates 2 on scoured or winnowed surfaces and 3 on stratification planes the grains are internally compact or porous with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source rocks during erosion and sediment transport a large range in textures reflects source rock lithologies with known varieties linked to sedimentary hosted diagenetic pyrite volcanic hosted massive sulphide deposits and hydrothermal pyrite laser ablation sulphur isotope analysis of pyrite reveals a broader range in 3 4 s values 5 3 to 6 7 than that of previously reported conventional bulk grain analyses 1 to 4 rounded pyrite from the steyn reef has significant variation in 3 4 s values 4 7 to 6 7 that establishes heterogeneous sulphur compositions with even adjacent grains having diverse isotopic signatures the heterogeneity supports a placer origin for rounded pyrite euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted 3 4 s values 0 5 to 2 5 are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration the placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere its distribution in three sequences witwatersrand supergroup ventersdorp contact reef and black reef and in other sequences not linked to witwatersrand type au u ore deposits implies deposition of redox sensitive detrital heavy minerals during the late archaean consequently rounded grains of detrital pyrite are strong indicators of an oxygen poor atmosphere while not confirming a placer origin for gold in witwatersrand au u ore deposits the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization

the 13c12c ratios of siderite and organic matter of a modern metalliferous hydrothermal sediment and their implications for banded iron formations abstract a hydrothermal ferruginous sediment precipitated by the activity of the microaerophilic chemolithotrophic bacterium galionella ferruginea has been sampled in a bay of nea kameni island santorini greece due to high fugacity of carbon dioxide precipitation of siderite also occurred the 13c value pdb was determined to be 6 5 in the siderite and 26 8 in the organic matter of mainly microbiological origin the enrichment of isotopically light carbon in carbonates of banded iron formations 13 c 10 has recently been discussed in literature the dilution of these marine carbonates by isotopically light carbon is suggested to have resulted from bacterial oxidation of reduced organic carbon during early diagenesis it is further suggested that fe iii in the newly precipitated sediment served as electron acceptor during oxidation in a hydrothermal model for the origin of banded iron formations this is consistent with the data presented here it is therefore postulated from the results presented that the reduced organic carbon in banded iron formations originates from microaerophilic chemolithotrophic bacteria

atmospheric oxygenation caused by a change in volcanic degassing pressure around two and a half billion years ago following the end of the archaean eon the atmosphere turned from anoxic to weakly oxic in what is known as the great oxidation event using a model of volcanic degassing gaillard et al demonstrate that a preceding period of continental crust formation may have been the trigger they propose that as continents emerged and volcanoes became increasingly subaerial rather than submarine magmatic volatiles were degassed at lower pressures leading to a progressive oxidation of the gases released this shift to a release of sulphur as sulphur dioxide rather than as hydrogen sulphide could then have fed marine sulphate reduction and the eventual oxygenation of earth s atmosphere the precambrian history of our planet is marked by two major events a pulse of continental crust formation at the end of the archaean eon and a weak oxygenation of the atmosphere the great oxidation event that followed at 2 45 billion years ago this oxygenation has been linked to the emergence of oxygenic cyanobacteria1 2 and to changes in the compositions of volcanic gases3 4 but not to the composition of erupting lavas geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3 5 billion years ago5 6 here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation using thermodynamic calculations simulating gas melt equilibria in erupting magmas we suggest that mostly submarine archaean volcanoes produced gases with so2 h2s 1 and low sulphur content emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late archaean then led to widespread subaerial volcanism which in turn yielded gases much richer in sulphur and dominated by so2 dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere

redox state of the archean atmosphere evidence from detrital heavy minerals in ca 3250 2750 ma sandstones from the pilbara craton australia the presence of detrital uraninite and pyrite in fluvial placers of the witwatersrand basin south africa has been used to infer low levels of atmospheric oxygen during the archean 2500 ma however recent studies advocate a hydrothermal origin for these minerals thereby limiting their value as constraints on the composition of the early atmosphere in contrast ca 3250 2750 ma fluvial siliciclastic sediments from the pilbara craton in australia have never undergone significant hydrothermal alteration and their heavy minerals are of unequivocal detrital origin these heavy minerals include the redox sensitive phases pyrite uraninite and gersdorffite along with more inert zircon rutile chromite and monazite locally siderite is a major constituent to 90 of the heavy mineral population with grains displaying evidence for several episodes of erosion rounding and subsequent authigenic overgrowth detrital siderite is very rare in post archean sandstones largely due to its instability in oxidizing environments however its frequent survival of prolonged transport in well mixed and therefore well aerated archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present moreover concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

petrological and stable isotope studies of carbonate and sulfide minerals from the gunflint formation ontario evidence for the origin of early proterozoic iron formation abstract the 1 9 ga gunflint formation in the thunder bay district of ontario principally comprises iron formation overlain by an iron poor limestone dolostone member the upper limestone member rocks are virtually unmetamorphosed retaining primary textures except locally around diabase sills the gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron formation and to study the changes from iron rich to iron poor sedimentation of the carbonate minerals in iron rich units siderite formed at or near the sediment water interface with ankerite and calcite forming later during diagenesis and during metamorphism around sills heaviest 13c values of siderite and of dolomite in the upper limestone member are consistently near 0 throughout the stratigraphic section indicating that marine bicarbonate was the source of carbon a spread of values between 0 and 7 for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis whereas 13c values more negative than 7 are the result of metamorphism the heaviest 18o values from unmetamorphosed rocks 5 3 pdb for siderite 6 1 for ankerite and 6 7 for calcite are considered to represent the original marine composition lighter values to 17 represent isotopic exchange reactions with pore fluids at higher temperatures and or isotopic exchange with 18o depleted meteoric water low s c ratios and a relatively narrow range of positive 34s values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean locally higher sulfide contents and a wider range of 34s values were the product of the introduction of sulfur after deposition it is proposed that the gunflint formation was deposited in a stratified water column with anoxic bottom water the depositional basin had restricted communication with the open ocean and was affected by distal volcanism hydrothermal activity associated with the volcanism provided a large source of dissolved iron and possibly silica which helped buffer o2 and sulphate in water to low levels low concentrations of sulphate limited the generation of h2s that would otherwise have restricted the solubility of iron during periods of increased hydrothermal activity the anoxic oxic water boundary moved upwards permitting transport of iron to the shallow shelf where it was precipitated as siderite iron hydroxides iron silicates or pyrite depending on physico chemical conditions the transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary

rise of atmospheric oxygen and the upside down archean mantle 1 the establishment of an oxygen rich atmosphere dramatically altered the evolution of life on earth most of the recent discussion of the topic has been focused on the timing of the event rather than on its mechanism here we draw upon recent developments in the understanding of earth s interior to propose that the rise of oxygen followed a geologically abrupt period of mantle overturn and or intense plume activity near the archean proterozoic transition 2470 2450 million years ago the magmatic event has already been linked to the widespread deposition of oxide facies banded iron formation and the rise of oxygen has been implicated as the trigger for iron deposition and earth s first major glaciation we argue that these events are all related to a change in redox state of volcanic gases brought about by deep seated late archean and earliest paleoproterozoic magmatism

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

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

metallogenesis of an early proterozoic foreland sequence eastern hudson bay canada on the e side of hudson bay the nw margin of the archaean superior structural province is overlain by an unmetamorphosed little deformed early proterozoic supracrustal sequence the sequence is divisible into three parts possible tectosomes each characterized by a distinct style of mineralization the arkosic alluvial richmond gulf group was shed eastward from a thermal uplift and has been preserved only in a later aulacogen the richmond gulf graben it contains an apatite cemented redbed secondly a widespread post aulacogen carbonate rich w facing miogeocline blanketed the region a pyritic zone developed at its basal contact is particularly thick and hosts disseminated copper mineralization where formed on the arkose in the graben a pyritic brecciated stromatolite rich horizon within the carbonate contains galena and sphalerite thirdly widespread iron formation and a local 60 cm thick manganese carbonate bed overlying w derived basalt flows flysch and arkosic molasse all of uncertain tectonic affinity terminate the sequence

the ongeluk basaltic andesite formation in griqualand west south africa submarine alteration in a 2222 ma proterozoic sea abstract the ongeluk lavas form part of the palaeoproterozoic transvaal griqualand west supracrustal sequence of the archaean kaapvaal craton of south africa they form a thick shallow marine volcanic sequence of pillow lava massive flows and hyaloclastite which together with their comagmatic subaerial hekpoort correlate once covered most of the craton in this study the magmatic composition and alteration features of ongeluk samples were distinguished using geochemistry and radiogenic isotopes the ongeluk hekpoort magma was a basaltic andesite derived from anomalous kaapvaal craton lithosphere with a long history of high u pb high rb sr and chur like sm nd ratios little evidence for magmatic fractionation was found although contamination of the lower lavas with cr rich material is evident in both hekpoort and ongeluk sequences the alteration of ongeluk lavas is characterised using diagrams which compare pillow core rim pairs hyaloclastite and massive lava samples with the estimated original magma composition alteration is ascribed to a shallow 87 sr 86 sr ratios decreased from magmatic 0 7071 towards seawater 0 7025 during submarine alteration the rb sr system also reveals a much later 1200 ma phase of ca and rb mobility in a few samples in contrast the submarine alteration affected pb isotope ratios so little that a 2222 13 ma isochron age is regarded as a refinement of earlier dates for the ongeluk hekpoort extrusion the ongeluk hyaloclastites acted as a sink for k and rb in the overlying sea while the mn and ca entering the sea could have contributed to the overlying kalahari manganese deposits

lithological and geochemical evidence of fe and mn pathways during deposition of lower proterozoic banded iron formation in the krivoy rog basin ukraine

supergene alteration since the upper cretaceous on alkaline igneous and metasomatic rocks of the pocos de caldas ring complex minas gerais brazil abstract this contribution presents field evidence mineralogical observations and geochemical data of different modes of lateritic weathering which affected the alkaline magmatic rocks and their metasomatic alteration products since the upper cretaceous field relations demonstrate that ferralitic bauxitic weathering is confined to well drained hill crest and upper slope settings in the pocos de caldas area bauxite normally rests directly on parent rock stagnant drainage and reducing conditions beneath tropical peat deposits along valley floors and in lower slope situations favour sialitic saprolitic weathering the additional importance of high porosity and permeability is demonstrated by co occurring parent rocks with strongly contrasting response on weathering attack in normal nepheline syenites and phonolites nepheline breaks down first and nearly instantly upon weathering this results in high porosity giving water and atmospheric gases easy access to the more resistant minerals thus in well drained morphological positions bauxite is generated if the same original magmatic parent rocks are shielded against pervasive weathering attack by pre lateritic metasomatic alteration no comparable early permeability arises a fine grained and dense metasomatic assemblage of kaolinite illite microcline and smectite combined with the annealing of fractures and joints is able to prevent bauxite generation and saprolite is formed instead this metasomatically induced saprolite is significantly richer in illite than normal saprolite bauxite formation results in a strong depletion of mg ca na k si ni co cu zn rb ba sr y and ree sialitic saprolitic weathering leads to a generally less pronounced depletion of essentially the same elements silicon and ni are more efficiently retained than in bauxite mobile elements are mn ca na k mg co zn rb ba sr and ree cerium can be re precipitated under oxidizing local conditions closely associated positive and negative ce anomalies are due to inhomogenously concentrated precipitates in small scale spatial alternation with zones of nearly total depletion a general correlation between the type of laterite saprolite or bauxite and ce retainment or depletion does not exist host phases for most of the immobile trace elements are goethite and related plasmas in bauxites and saprolites immobility is exhibited by v pb ga nb zr th and u relict zircon is too rare to explain a seemingly retarded hree loss in saprolites

the silicon and oxygen isotope compositions of precambrian cherts a record of oceanic paleo temperatures oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperatures during the precambrian during the past decade the advance of in situ analysis of si isotopes has enhanced the interest on cherts as a paleo environmental proxy the coupled o and si isotope composition variations show secular and correlated trends that have been interpreted as a progressive cooling of the ocean however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation in this case the secular oxygen and silicon isotope variation are considered as reflecting a mixing between seawater and hydrothermal sources a key point in this discussion deals with the origin of cherts sedimentary hydrothermal or chemically silicified therefore several petrographical and geochemical criteria are proposed to recognize the pristine sedimentary origin of a chert namely they are 1 the occurrence of microquartz 2 a 18o rich bulk oxygen isotopic composition 3 the occurrence of large 18o ranges at a micrometer scale 4 variation of trace element compositions coupled with 30si 5 the occurrence of large ranges of 30si in pure microquartz these criteria should be regarded as guides to the identification of pristine diagenetic cherts in order to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis this article will review the different interpretations about o and si isotope variation and propose a model of formation based on ancient and modern chert studies

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

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

the role of the global carbonate cycle in the regulation and evolution of the earth system we review one of the most ancient of all the global biogeochemical cycles and one which reflects the profound geochemical and biological changes that have occurred as the earth system has evolved through time that of calcium carbonate caco3 in particular we highlight a mid mesozoic revolution in the nature and location of carbonate deposition in the marine environment driven by the ecological success of calcareous plankton this drove the creation of a responsive deep sea sedimentary sink of caco3 the result is that biologically driven carbonate deposition provides a significant buffering of ocean chemistry and of atmospheric co2 in the modern system however the same calcifying organisms that under pin the deep sea carbonate sink are now threatened by the continued atmospheric release of fossil fuel co2 and increasing acidity of the surface ocean we are not yet in a position to predict what the impact on caco3 production will be or how the uptake of fossil fuel co2 by the ocean will be affected this uncertainty in the future trajectory of atmospheric co2 that comes from incomplete understanding of the marine carbonate cycle is cause for concern

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

precise determination of variations in the 131ce 142ce ratios of marine ferromanganese nodules abstract new and precise data on the 138 ce 142 ce ratio in marine manganese nodules obtained from the atlantic and pacific oceans fall within a narrow range of 0 6 to 1 1 in terms of ce one tenth of per mil difference from the standard the nodule ce isotopic data are used as a surrogate of surrounding seawater and the ce values ranging from 0 6 to 0 2 for the central pacific are clearly more negative than those 0 2 to 1 1 for the atlantic by coupling the ce with nd for 143nd 144nd it is inferred that the relative contribution of the continental derived to mantle derived sources is smaller in the pacific than in the atlantic if we assume that the oceanic 138 ce 142 ce and 143 nd 144 nd ratios are determined by mixing of two end members i e the continental crust and morb mid ocean ridge basalt the oceanic variation of ce is greater than that of nd relative to their difference in the two end members this strongly suggests that ce and nd are fractionated within the ocean because of the difference in their mean residence times 90 165 yr for ce and 1000 1500 yr for nd and the 138 ce 142 ce and 143 nd 144 nd ratios modified by scavenging and oceanic mixing are recorded in the manganese nodules

stratigraphic changes of ge si ree y and silicon isotopes as insights into the deposition of a mesoarchaean banded iron formation in order to determine the origin of silicon si in banded iron formation bif we have undertaken a multi tracer study combining ree y data ge si ratios and si isotopes d 30 si on stratigraphically resolved layers from a 2 95 ga bif from the pongola supergroup south africa si in both si rich and fe rich layers has a common origin represented by a seawater reservoir strongly influenced by continent derived freshwaters 10 and very limited o 0 1 high t hydrothermal fluids as indicated by eu anomalies and y ho ratios the coevolution of d 30 si signatures of si and fe rich

morphological adaptations of 3 22 ga old tufted microbial mats to archean coastal habitats moodies group barberton greenstone belt south africa microbial life was well established and widespread by the paleoarchean however the degree of evolutionary advancement such as microbial motility intra and inter species interactions phototropism or oxygenic photosynthesis by that time remains highly debated the 3 22 ga moodies group in the barberton greenstone belt bgb south africa are earth s oldest well preserved siliciclastic tidal deposits they exhibit a unique assemblage of microbial mats providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings the fossil mats are preserved as kerogenous laminations 0 5 1 mm thick that can be traced laterally for 15 km in a 1000 m thick succession of fine to coarse grained tidal sandstones and conglomerates we here present a detailed stratigraphic and depositional facies analysis documenting the association of the three principal mat morphotypes with specific environmental settings 1 planar type in coastal floodplain 2 wavy type in intertidal and 3 tufted type in upper inter to supratidal facies all mat types indicate a flourishing phototrophic biota moreover the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments

pervasive oxygenation along late archaean ocean margins the photosynthetic production of oxygen in the ocean is thought to have begun at least 2 7 billion years ago the geochemistry of marine sediments deposited 2 6 billion years ago suggests that ocean margins were oxygenated at least 100 million years before the first significant increase in atmospheric oxygen concentrations

volcanic stratigraphy geochronology and gold deposits of the archean hope bay greenstone belt nunavut canada the archean hope bay greenstone belt is located in the bathurst block of the northeastern part of the slave structural province a predominantly archean granite greenstone metasedimentary terrane the greenstone belt is dominated by mafic volcanic rocks with less common felsic volcanic and volcaniclastic products and subordinate ultramafic bodies and metasedimentary rocks three main au deposits have been defined in the greenstone belt with a cumulative resource of over 10 million ounces moz of au as of april 2007 the hope bay greenstone belt can be divided into a series of coherent panels of strata that tend to be fault bounded and collectively are used to construct a composite stratigraphic column of the belt constrained by u pb geochronology on felsic suites the oldest felsic suite the flake lake suite ca 2700 ma is a succession of felsic volcanic rocks that has a tholeiitic geochemical affinity interpreted as a product of rift related volcanism overlying this suite is a series of well constrained calc alkaline mainly felsic volcanic rocks which have ages of ca 2690 ma square lake suite ca 2686 ma windy felsic suite ca 2677 ma koignuk suite and ca 2662 ma clover lake suite all of which are interpreted as products of arc volcanism the transition from rift to arc related volcanism divides the greenstone belt into upper and lower volcanic cycles with the transition occurring at about ca 2690 ma the older cycle of volcanic rocks has a distinctive group of mafic rocks consisting of mafic pillowed flows which have an fe rich tholeiitic geochemical affinity which is not recognized in the younger volcanic cycle these fe rich tholeiitic mafic volcanic rocks commonly show a spatial relationship to au mineralization acting as host strata to the main au deposits in the greenstone belt the younger cycle ca 2690 ma of mafic volcanic rocks tends to have a tholeiitic affinity with some minor andesitic volcanic rocks near the top of the composite stratigraphic column detrital zircon work was carried out for all the main sedimentary successions recognized in the hope bay greenstone belt the detrital zircon results show that in each case the sediments were derived mainly from local sources as very few anomalously old zircons were recognized a conglomerate outcropping in the north part of the belt is dominated by locally derived material with detrital zircon work supporting this interpretation the three main au deposits at hope bay are all associated with the older cycle of volcanic rocks interpreted as products of rift related volcanism the structural setting of each deposit is different but each is related to d2 strain in structural and or stratigraphic settings which provided dilational environments to localize auriferous vein related mineralization or localized alteration and mineralization of favorable host lithologies the older cycle of tholeiitic felsic and fe rich mafic volcanic rocks are rift related volcanic products which are time equivalent to the kam group in the yellowknife supergroup overlying these rocks are calc alkaline felsic volcanic rocks and tholeiitic mafic volcanic rocks which represent the transition to calc alkaline arc related volcanism which correlates to the banting group of the yellowknife supergroup overlying the younger ca 2677 ma felsic volcanic rocks are a succession of conglomerates consisting mainly of locally derived clasts representing collapse of the arc into a fluvial dominated environment this sedimentary succession is possibly equivalent to the regionally extensive burwash basin of the yellowknife supergroup

the comparative behaviors of yttrium and lanthanides in the seawater of the north pacific yttrium has long been recognized as an ekalanthanide because of its chemical contiguity relative to the chemistries of rare earth elements which are in recent years intensively utilized for elucidation of metal scavenging processes in the ocean here we present the first detailed depth profile of y iii in seawater together with the lanthanides in the north pacific ocean the concentrations of y iii range 70 290 pmol kg and show the nutrient like profile best resembling that of ho iii amongst the other rare earth elements the results agree well with an expectation based on the similarity in their ionic radii and hence stability constants of complexation with carbonate ions yet the ho iii y iii ratios in seawater systematically increase with depth suggesting that y and ho are fractionated during scavenging by natural marine particulates this is likely to result from the different complexation behavior in that y iii is more weakly complexed than ho iii with soft organic ligands on the surface of particulate matter during scavenging in the surface water but once released into seawater in the deep sea y iii is complexed with carbonate ions equally or stronger than ho the pattern of deep water enrichment in the lanthanide series appears to be consistent with the recent observation of partitioning between suspended particles and seawater our precise measurements also indicated that pr and tb best resemble nd and dy respectively in their oceanic behavior whereas ho and tm are intermediate between their neighboring rare earth elements

sources of iron formations in the archean isua and malene supracrustals west greenland evidence from la ce and sm nd isotopic data and ree abundances abstract isotopic ratios of ce and nd and abundances of rare earth elements rees and ba are reported for the iron formations of the archean isua and malene supracrustals west greenland it is suggested that the sm nd system of the isua iron formations was disturbed by metamorphisms at 1680 ma oxide facies iron formation and 2740 ma carbonate facies iron formation however the malene iron formation samples show similar nd isotopic ratios and ree patterns indicating that they suffered little disturbance after deposition at 2800 ma initial ce and nd values estimated at 2800 ma for the malene iron formation are close to chur chondritic uniform reservoir of 3 samples two show negative ce and positive nd and the other gives chondritic ce and negative nd these initial values suggest that the sources for the malene iron formation had a chiefly chondritic or rather mantle like depleted nature without a notable long term negative ce anomaly similar ree features are indicated for the sources of the isua iron formation by the estimated initial ce and nd values at 3760 ma of these samples

the effectiveness of the paleoproterozoic biological pump a δ13c gradient from platform carbonates of the pethei group great slave lake supergroup nwt samples of carbonate cements collected along a depth transect of the pethei platform a 1 9 ga stromatolitic reef reveal a small 0 5 carbon isotope gradient between shallow and basinal facies the magnitude of this gradient would conventionally be interpreted as indicating low export of organic matter from ocean surface waters but steady state simulations using a two box model of the paleoproterozoic ocean suggest that the small carbon isotope gradient could instead be due to high partial pressures of carbon dioxide in the paleoproterozoic atmosphere which would increase the ocean9s dissolved inorganic carbon content and damp the effects of biological pumping if the paleoproterozoic atmosphere were indeed enriched in atmospheric co 2 in compensation for a less luminous sun these results indicate that the planktonic biota was a significant component of the precambrian global carbon and nutrient cycles affecting if not regulating as today the composition of both surface and deep ocean waters

quantifying the areal extent and dissolved oxygen concentrations of archean oxygen oases abstract several lines of evidence indicate that the advent of oxygenic photosynthesis preceded the oxygenation of the atmosphere perhaps by as much as 300 million years the fate of biogenic o2 prior to its appearance in the atmosphere remains speculative but recent work suggests that o2 was locally available within the surface ocean to support aerobic microbial ecosystems simple mass balance predicts that locally oxygenated environments oxygen oases could exist in areas of high productivity if the local rate of o2 production by oxygenic photosynthesis exceeded the combined rate of o2 loss by a number of processes e g exchange with the atmosphere transport within the ocean reaction with reduced aqueous species biological consumption the areal extent of these environments and the dissolved o2 concentrations that could have persisted in an otherwise anoxic ocean however are key uncertainties in our understanding of the spatiotemporal redox evolution of the early earth system we use an earth system model of intermediate complexity that has been modified to simulate a theoretical archean biosphere in order to explore redox heterogeneity in the late archean surface ocean we demonstrate that oxygen oases are an expected consequence of oxygenic photosynthesis beneath an essentially o2 devoid atmosphere and that oxygenated surface waters need not be restricted to shallow coastal environments or microbial mats within oxygen oases o2 concentrations locally approach 1 10 m for a large range of plausible archean conditions although o2 concentrations in the open ocean are exceedingly low biologically relevant dissolved o2 concentrations are widespread in our hypothetical surface ocean

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

was the deposition of large precambrian iron formations linked to major marine transgressions geochemical evidence suggests the iron and silica rich precipitates that compose large neoarchean and paleoproterozoic iron formations formed along a chemocline between iron rich deep water and iron poor surface water sedimento logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions we suggest this occurred because the chemocline could only impinge on continental shelves during times of sea level rise or highstand on the other hand the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgressions in others to explain this we propose that iron concentrations in the precambrian ocean were greatest at mid water depths in a zone on the order of 100 m thick radiometric age dates from the hamersley basin of western australia suggest major iron formation deposition had a periodicity of ca 20 to 125 m y close to second order cycles in phanerozoic sea level driven by long te

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

correlation of paleoproterozoic glaciations based on u pb zircon ages for tuff beds in the transvaal and huronian supergroups abstract the rise in atmospheric oxygen between 2 45 and 2 2 ga has been linked to the demise of a methane rich atmosphere and the onset of multiple glaciations culminating in a possible snowball earth the glacial deposits represent possible global marker horizons however their correlation is uncertain because key sedimentary successions are undated hampering paleoenvironmental reconstructions three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern africa with those in north america however it has also been suggested that there were four glaciations and that the youngest was a snowball earth event recorded only in south african successions in situ u pb zircon ages for tuffs in southern african and north american successions establish the existence of four glaciations between 2 45 and 2 22 ga geochronological and stratigraphic data demonstrate that the three oldest glaciations predate 2 31 ga and that the final glaciation which is only recognized in south africa occurred between 2 26 and 2 22 ga the new age calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations at 2 31 ga the first marine sulfate evaporites were deposited contemporaneously with 13 c enriched carbonates indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen the appearance of red beds and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels which culminated with the fourth glaciation between 2 26 and 2 22 ga after which the atmosphere remained irreversibly oxygenated

first discovery of ferruginous chert arenites in the early precambrian hamersley group of western australia the shaly upper part of the wittenoom dolomite which belongs to the 2 5 ga hamersley group of western australia locally contains thin layers to lenses of ferruginous chert arenite that are very similar to cherty arenites found in granular precambrian iron formations where they are best preserved the clasts in the wittenoom arenites consist of chert with finely disseminated inclusions of hematite or iron silicate in some layers the clasts are elongated and petrographically very similar to thinly laminated cherts in the surrounding layers suggesting that they are rip up clasts redeposited locally in other arenites however the chert clasts are peloidal to oolitic suggesting a shallower source and a longer distance of transportation both types were probably transported during short lived high energy events such as turbidity currents and or bottom return flow induced by storms restriction of the arenites to the northeastern part of the hamersley basin and other sedimentological data suggest that these currents were moving toward the south and west these occurrences of ferruginous chert arenite provide new evidence that the textural differences between large banded iron formations such as those of the hamersley group and large granular iron formations such as those of the lake superior region and nabberu basin primarily reflect differences in hydrodynamic conditions

origin of negative ce anomalies in barberton sedimentary rocks deduced from la ce and sm nd isotope systematics abstract the origin of sedimentary rocks with negative ce anomalies among several samples derived from the barberton greenstone belt 3250 ma south africa was investigated using la ce and sm nd isotopic systematics together with light rare earth element lree abundances negative ce anomalies were recognized in two sedimentary rocks a banded ferruginous chert and a shale their ce ce values are 0 40 and 0 81 respectively initial ce isotopic ratio calculated for the ferruginous chert 138 ce 142 ce 3250 ma 0 0225059 is far lower than chur value at 3250 ma and even the value at earth formation 138 ce 142 ce chur 4560 ma 0 0225409 the calculated initial ce isotopic ratio for the shale with a ce anomaly is also lower than that of other rocks without ce anomalies these results indicate that the negative ce anomalies were not inherited from an archean redox change but occurred following later episodes of alteration two possible formation models were examined to estimate a plausible occurrence age for the negative ce anomalies one is characterized by ce extraction and the other is by an addition of la nd and sm both models indicate that negative ce anomalies arose before 1100 ma this study suggests that la ce isotopic systematics provides a unique tool to constrain the occurrence age of negative ce anomalies observed in archean sedimentary rocks and gives some hints about the redox condition in the archean ocean and the evolution of the earth

banded iron formations through much of precambrian time similarities between proterozoic 1 8 2 5 gyr and archean 2 5 gyr banded iron formations are probably more significant than their differences the contrasts largely reflect differences in the tectonic settings of proterozoic and archean terrains archean banded iron formations are not as thick nor laterally as extensive as the major proterozoic iron formations nevertheless some archean iron formations have strike lengths of over 150 200 km and may have been quite extensive prior to the deformation that has affected most archean terrains stratigraphic sequences in which iron formations occur are highly variable and indicate that iron formations formed in many depositional environments sedimentary textures in the iron formations are dominated either by granules and oolites or laminations including microbanding reflecting differences in their physical conditions of deposition granular and oolitic textures are abundant in only three proterozoic depositional basins and most precambrian iron forma

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

microfossils from the neoarchean campbell group griqualand west sequence of the transvaal supergroup and their paleoenvironmental and evolutionary implications the oldest filament and colonial coccoid containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the neoarchean approximately 2600 ma old campbell group ghaap plateau dolomite lime acres member obtained at lime acres northern cape province south africa the assemblage is biologically diverse including entophysalidacean eoentophysalis sp probable chroococcacean unnamed colonial coccoids and oscillatoriacean cyanobacteria eomycetopsis cf filiformis and siphonophycus transvaalensis as well as filamentous fossil bacteria archaeotrichion sp filamentous possible microfossils unnamed hematitic filaments also occur the campbell group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the transvaal intracratonic sea although only moderately to poorly preserved they provide new evidence regarding the paleoenvironmental setting of the campbell group sediments extend the known time range of entophysalidacean cyanobacteria by more than 400 million years substantiate the antiquity and role in stromatolite formation of archean oscillatoriacean cyanobacteria and document the exceedingly slow hypobradytelic evolutionary rate characteristic of this early evolving prokaryotic lineage

depositional age of the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa and implications for low level re os geochronology and paleoproterozoic global correlations abstract we present new re os isotope data from the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa to further evaluate low level re os geochronology of organic rich sedimentary rocks and improve global correlations of paleoproterozoic sedimentary rocks the klipputs member contains re 0 3 0 8 ppb total os 34 177 ppt and common os 192 os 10 66 ppt abundances that overlap upper crustal abundances of 0 2 2 ppb 30 50 ppt and 12 21 ppt respectively five samples yield a model 1 re os date of 2479 20 ma 22 ma if the 187 re decay constant uncertainty is included 2 mean square of weighted deviates mswd 0 01 using the cr vi h 2 so 4 digestion protocol dissolution of the same sample powders using inverse aqua regia yields a model 3 re os date of 2862 190 ma 2 mswd 148 comparison of re os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital os whereas the cr vi h 2 so 4 medium did not the re os date of 2479 22 ma for the klipputs member is consistent with published geochronological ages from underlying and overlying stratigraphic units suggesting the cr vi h 2 so 4 protocol yields accurate re os depositional ages from ors containing very low levels of hydrogenous re and hydrogenous radiogenic os hence there is potential for widespread application of low level re os ors geochronology including sedimentary successions deposited before the great oxidation event the initial 187 os 188 os 0 12 0 01 from the klipputs member cr vi h 2 so 4 regression indicates that dissolved os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and or cosmic dust rather than oxidative weathering of the radiogenic upper crust in light of the re os depositional age for the klipputs member we propose that the stratigraphic units in the upper hamersley group above the brockman iron formation western australia are younger than the ghaap group diamictite from the doradale formation in the ghaap group is constrained to be older than 2479 22 ma and thus may not be correlative with glacial deposits assigned to the oldest paleoproterozoic glaciation

increased subaerial volcanism and the rise of atmospheric oxygen 2 5 billion years ago the hypothesis that the establishment of a permanently oxygenated atmosphere at the archaean proterozoic transition approximately 2 5 billion years ago occurred when oxygen producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older to sustain vanishingly low oxygen levels despite near modern rates of oxygen production from approximately 2 7 2 5 billion years ago thus requires that oxygen sinks must have been much larger than they are now here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the archaean era enhanced submarine volcanism was abruptly and permanently diminished during the archaean proterozoic transition observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the proterozoic submarine volcanoes are more reducing than subaerial volcanoes so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen

uranium in iron formations and the rise of atmospheric oxygen abstract the concept of the great oxidation event goe during which atmospheric oxygen rose precipitously and perhaps to near modern levels around 2 4 2 1 billion years ago ga has become entrenched in our views on secular atmospheric evolution multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the archean proterozoic boundary however it remains unclear precisely when this transition occurred due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures here we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium u concentrations in iron formations if just as if are important archives of the redox state of seawater concentrations of redox sensitive u in if are faithful proxies for oxidative continental weathering and associated delivery of dissolved u to seawater our dataset suggests that there was an increase in u redox cycling and transport at ca 2 47 ga just before the permanent loss of mass independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the goe further there is significant temporal variability in the if u record that we propose reflects dynamic precambrian redox conditions we provide additional support for earlier suggestions that the goe was a protracted event marked by vacillating oxygen levels

possible earthquake induced soft sediment faulting and remobilization in pennsylvanian alluvial strata southern new brunswick canada braided river sheetflood and playa lake sediments of the tynemouth creek formation lower pennsylvanian show superficial synsedimentary faulting and subsurface post depositional sediment remobilization a prominent palaeosol has been offset 90 cm on two faults movement on which clearly preceded deposition of the overlying beds seventeen to 25 m below this horizon large load structures upward branching siltstone intrusions and three types of sandstone dike are recognised intrusion occurred on several occasions at different depths of burial and was the result of rapid probably earthquake induced dewatering this interpretation is supported by the location of the study area only a few kilometres to the north of the cobequid chedabucto fault on which major strike slip movement occurred during the pennsylvanian

geochemistry of the krivoy rog banded iron formation ukraine and the impact of peak episodes of increased global magmatic activity on the trace element composition of precambrian seawater abstract pure superior type banded iron formation bif samples from the krivoy rog supergroup ukraine are excellent archives of ambient early precambrian seawater they show low concentrations of incompatible elements such as zr hf and th and shale normalized rare earths and yttrium rey sn patterns similar to those of modern seawater i e heavy rey sn enriched patterns with positive la sn gd sn and y sn anomalies lack of ce sn and presence of positive eu sn anomalies indicate rey contributions to anoxic ferruginous seawater from high temperature hydrothermal fluids the depositional age of the krivoy rog bif is ill defined but a late archean to paleoproterozoic age has been suggested based on u pb zircon ages for units stratigraphically above and below the bif we determined sm nd isotopic compositions of pure and impure samples from the krivoy rog bif which yield an errorchron with an apparent age of 2406 350 ma mswd 15 that falls within this broad age range all pure bif samples show chondrite normalized subscript cn rey patterns with strong positive eu cn anomalies that are typical for archean but rather rare and much less pronounced in proterozoic bifs associated schists also show archean rather than post archean style rey distributions the rey geochemistry of both chemical and epiclastic sediments therefore is more consistent with a late archean rather than a post archean depositional age of the krivoy rog supergroup initial nd values of impure bifs and of associated schist reveal variable contributions from ttgs less radiogenic in nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying new krivoy rog group the purest krivoy rog bif representing local krivoy rog seawater displays an nd 2 60 ga value of 2 3 this value is less radiogenic than impure krivoy rog bifs or other near contemporaneous neoarchean pure chemical sediments to preserve this specific local isotopic fingerprint in anoxic archean seawater the krivoy rog bif must have been deposited in an isolated sea basin with limited exchange with ferruginous deep waters of the open ocean a compilation of rey data for high purity precambrian bifs reveals that eu cn eu cn ratios of precambrian seawater follow a general global evolution curve that shows specific peaks which reflect times of increased high temperature hydrothermal rey input into seawater following declining eu cn eu cn ratios from the eoarchean to the mesoarchean the ratios suddenly rise at 2 7 ga and reach a maximum at 2 6 ga indicating an increased flux of high temperature hydrothermal rey to neoarchean seawater which supports the hypothesis that times of widespread bif deposition coincided with periods of intense submarine hydrothermal activity probably triggered by major mantle plume events this association is supported by a strong increase of the nd t values of pure seawater archives at 2 7 2 6 ga which reflects an increased flux of mantle nd into seawater these results suggest that eu rey systematics and potentially nd systematics are robust tools to indentify episodes of enhanced mantle plume activity

biologically recycled continental iron is a major component in banded iron formations banded iron formations bifs record a time of extensive fe deposition in the precambrian oceans but the sources and pathways for metals in bifs remain controversial here we present fe and nd isotope data that indicate two sources of fe for the large bif units deposited 2 5 billion y ago high end and 56fe signatures in some bif samples record a hydrothermal component but correlated decreases in end and 56fe values reflect contributions from a continental component the continental fe source is best explained by fe mobilization on the continental margin by microbial dissimilatory iron reduction dir and confirms for the first time to our knowledge a microbially driven fe shuttle for the largest bifs on earth detailed sampling at various scales shows that the proportions of hydrothermal and continental fe sources were invariant over periods of 100 103 y indicating that there was no seasonal control although fe sources varied on longer timescales of 105 106 y suggesting a control by marine basin circulation these results show that fe sources and pathways for bifs reflect the interplay between abiologic hydrothermal and biologic processes where the latter reflects dir that operated on a basin wide scale in the archean

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

the geology and geochemistry of the palaeoproterozoic makganyene diamictite abstract the palaeoproterozoic earth experienced a global glacial event at 2400 ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans the transvaal supergroup in the griqualand west basin south africa hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes the focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic makganyene formation in order to constrain its palaeoenvironmental settings the makganyene formation forms the base of the postmasburg group and has been regarded as resting on an erosive regional unconformity throughout the northern cape province systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal the makganyene formation is in fact conformable with underlying formations of the koegas subgroup in the deep southern prieska basin and rests on an unconformity only on the shallow ghaap platform to the north east the makganyene formation displays lateral facies changes that reflect the palaeogeography of the study area and the advance and retreat of ice sheets shelves geochemical investigations of glacial strata of the makganyene formation demonstrate that underlying banded iron formations of the transvaal supergroup acted as the main clastic source for the diamictite detritus geographic variations in bulk composition of the diamictites correlate well with field observations and show that sorting processes were controlled largely by the morphology of the palaeobasin carbon isotope results emphasize the transitional nature of the makganyene formation in terms of the environmental conditions that resulted in widespread global glaciation in the palaeoproterozoic on the basis of the above geological evidence it is proposed that the transvaal supergroup in the northern cape province represents a continuous depositional event that lasted approximately 250 ma and hence provides a unique opportunity for assessing the transitional changes experienced by the palaeoproterozoic earth

u pb zircon ages from athapuscow aulacogen east arm of great slave lake n w t canada athapuscow aulacogen is an early proterozoic intracratonic basin located in the east arm of great slave lake between the slave and northwest churchill provinces athapuscow aulacogen comprises three stratigraphic sequences the wilson island group the great slave supergroup and the et then group new u pb zircon ages provide constraints on the development of the aulacogen the blachford lake intrusive suite consists of an older alkaline phase hearne channel granite dated at 2175 7 ma and a younger peralkaline phase thor lake syenite dated at 2094 10 ma confirming the suggestion that the two phases may not be related a felsite from the wilson island group has an age of 1928 11 ma the wilson island group is intruded by epizonal granites butte island intrusive suite one of which has an age of 1895 8 ma the wilson island group and the butte island instrusive suite are entirely allochthonous with respect to the slave craton rocks of the great slave supergroup overlie mylonitized wilson isl

the antiquity of oxygenic photosynthesis evidence from stromatolites in sulphate deficient archaean lakes the tumbiana formation about 2700 million years old was largely deposited in ephemeral saline lakes as judged by the unusual evaporite paragenesis of carbonate and halite with no sulfate stromatolites of diverse morphology occur in the lacustrine sediments some with palimpsest fabrics after erect filaments these stromatolites were probably accreted by phototropic microbes that from their habitat in shallow isolated basins with negligible sulfate concentrations almost certainly metabolized by ozygenic photosynthesis

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

determination of the fe ii aq magnetite equilibrium iron isotope fractionation factor using the three isotope method and a multi direction approach to equilibrium abstract magnetite is ubiquitous in the earth s crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical fe cycling magnetite is also the most abundant fe oxide in banded iron formations bifs that have not been subjected to ore forming alteration magnetite is therefore an important target of stable fe isotope studies and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable fe isotope fractionation factors for magnetite relative to fluids and other minerals in this study we utilized the three isotope method 57fe 56fe 54fe to explore isotopic exchange via an enriched 57fe tracer and natural mass dependent fractionation using 56fe 54fe variations during reaction of aqueous fe ii fe ii aq with magnetite importantly we employed a multi direction approach to equilibrium by reacting four 57fe enriched fe ii solutions that had distinct 56fe 54fe ratios which identifies changes in the instantaneous fe isotope fractionation factor and hence identifies kinetic isotope effects we find that isotopic exchange can be described by two 56fe 54fe fractionations where an initial rapid exchange 66 isotopic mixing within 1 day involved a relatively small fe ii aq magnetite 56fe 54fe fractionation followed by slower exchange 25 isotopic mixing over 50 days that was associated with a larger fe ii aq magnetite 56fe 54fe fractionation this later fractionation is interpreted to approach isotopic equilibrium between fe ii aq and the total magnetite all four fe ii solutions extrapolate to the same final equilibrium 56fe 54fe fractionation for fe ii aq magnetite of 1 56 0 20 2 at 22 c additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous fe ii yield final 56fe 54fe fractionations that are identical to those of the exchange experiments our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for fe h 2 o 6 2 from rustad et al 2010 and stoichiometric magnetite from mineev et al 2007 and these relations may be combined with the experimental constraints to determine the temperature dependence of the fe ii aq magnetite fractionation factor 10 3 ln f e i i aq magnetite 0 145 0 002 10 6 t 2 0 10 0 02 where t is in k part of the reason for large discrepancies in calculated fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies given the significant effect of octahedral versus tetrahedral fe isotope fractionation that has been calculated when our results are applied to bif genesis our experimentally determined fe ii aq magnetite fractionation factor indicates that magnetite siderite mineral pairs in 2 5 ga bifs did not form in fe isotope equilibrium with each other or with ancient seawater iron oxides in such bifs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans indicating that such bif minerals may not be suitable proxies for ancient paleoenvironments

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

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

u pb ages from the nimish formation and montagnais glomeroporphyritic gabbro of the central new quebec orogen canada three u pb zircon dates were obtained from two igneous suites associated with cycle 2 sedimentation in the central new quebec orogen labrador trough in the dyke lake area of the western part of the orogen schefferville zone the nimish formation includes a polymictic conglomerate containing quartz syenite cobbles that crystallized at 1877 8 1 3 ma these cobbles are petrographically and geochemically linked to the nimish volcanics which are intercalated with the sokoman iron formation consequently the syenite date gives an approximate age for both nimish magmatism and ironstone deposition in the schefferville zone a date of ca 2 65 ga obtained from a nimish trachyte in the same area indicates that zircons in the sample are xenocrysts farther east at howse lake howse zone a montagnais plagioclase glomeroporphyritic gabbro sill crystallized at 1884 0 1 6 ma the howse lake sill which intrudes turbidites of the menihek formation is considered comagmatic with the basalts that cap the form

geochemistry and si o fe isotope constraints on the origin of banded iron formations of the yuanjiacun formation lvliang group shanxi china abstract banded iron formations bifs within the lvliang region of shanxi province china are hosted by sediments of the yuanjiacun formation part of the paleoproterozoic lvliang group these bifs are located in a zone where sedimentation changed from clastic to chemical deposition indicating that these are superior type bifs here we present new major trace and rare earth element ree data along with fe si and o isotope data for the bifs in the yuanjiacun within the fe deposits at yuanjiacun jianshan and hugushan when compared with post archean australian shale paas these bifs are dominated by iron oxides and quartz contain low concentrations of al 2 o 3 tio 2 trace elements and the ree and are light rare earth element lree depleted and heavy rare earth element hree enriched the bifs also display positive la y and eu anomalies high y ho ratios and contain 30 si depleted quartz with high 18 o values that are similar to quartz within siliceous units formed during hydrothermal activity these data indicate that the bifs within the yuanjiacun formation were precipitated from submarine hydrothermal fluids with only negligible detrital contribution none of the bif samples analyzed during this study have negative ce anomalies although a few have a positive ce anomaly that may indicate that the bifs within the yuanjiacun formation formed during the great oxidation event goe within a redox stratified ocean the positive ce anomalies associated with some of these bifs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed ho lree and ce 4 these deposits formed during reductive dissolution at the oxidation reduction transition zone or in deeper level reducing seawater the loss of ce lree and ho to seawater and the deposition of these elements with iron hydroxides caused the positive ce anomalies observed in some of the bif samples although the limited oxidizing ability of surface seawater at this time meant that y ho and lree hree ratios were not substantially modified unlike similar situations within stratified ocean water during the late paleoproterozoic magnetite and hematite within the bifs in the study area contain heavy fe isotopes 56 fe values of 0 24 1 27 resulting from the partial oxidation and precipitation of fe 2 to fe 3 in seawater in addition mass independent fractionation of sulfur isotopes within pyrite indicates that these bifs were deposited within an oxygen deficient ocean associated with a similarly oxygen deficient atmosphere even though the bifs within the yuanjiacun formation formed after initiation of the goe

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

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

a statistical analysis of the carbon isotope record from the archean to phanerozoic and implications for the rise of oxygen organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales permanent burial of organic carbon in the crust or mantle oxidizes the surface environment atmosphere ocean and biosphere by removing reduced carbon it has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout earth9s history thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen schidlowski 1988 catling and others 2001 holland 2002 holland 2009 kump and others 2009 rothman 2015 however the opposite conclusion has been drawn from the same carbon isotope record des marais and others 1992 bjerrum and canfield 2004 to test these opposing claims we compiled an updated carbon isotope database and applied both parametric and non parametric statistical models to the data to quantify trends and mean level changes in fractional organic carbon burial with associated uncertainties and confidence levels we first consider a conventional mass balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon for this model statistical analysis implies fractional organic burial has increased over earth history by a factor of 1 5 relative to organic burial at 3 6 ga with the 95 percent confidence interval ranging from factors of 1 2 to 2 0 an increase in organic burial by a factor of 1 2 cannot explain the rise of oxygen whereas an increase by a factor of 2 could conceivably explain the rise of oxygen there is however a highly significant and well constrained increase in organic burial from the proterozoic to the phanerozoic we also analyze changes in the difference between carbonate and organic carbon isotopic ratios over earth history there is a statistically significant increase in this difference from the early to late archean possibly caused by increased biological fractionation due to methanotrophic recycling this transition is consistent with the evolution of oxygenic photosynthesis at 2 8 ga or earlier finally we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes specifically ocean crust carbonatization and authigenic carbonates because the size of these fluxes has a large poorly constrained range our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all however it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time in which case conclusions from the conventional model would be valid

prolonged history of episodic fluid flow in giant hematite ore bodies evidence from in situ u pb geochronology of hydrothermal xenotime abstract absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the earth s crust yet many ore deposits remain undated because suitable mineral chronometers have not been found the origin of giant hematite ore deposits which are hosted in precambrian banded iron formations bifs remains contentious several models have been formulated based on different sources and timing for the mineralizing fluids supergene metamorphic syn orogenic late orogenic extensional collapse and syn extensional precise geochronology of the ore offers a means of discriminating between these models in this study two u pb chronometers xenotime and monazite have been identified in high grade hematite ore bodies from the mount tom price mine in the hamersley province northwestern australia both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite hematite pseudomorph after magnetite and microplaty hematite indicating that the xenotime and monazite precipitated during mineralization in situ u pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events our results suggest that ore genesis may have commenced as early as 2 15 ga with subsequent hydrothermal remobilization and or mineralization at 2 05 ga 1 84 ga 1 67 ga 1 59 ga 1 54 ga 1 48 ga and 0 85 ga the location of the ore bodies along ancient fault systems and the coincidence of at least some of the u pb phosphate dates with episodes of tectonothermal activity in the adjacent proterozoic capricorn orogen implies that fluids were channelled through major structures in the southern pilbara craton during discrete phases of tectonic compression and extension our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow in contrast to the aforementioned models our results imply that iron ore formation was probably a long lived multi stage process spanning more than one billion years

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

earth s earliest microbial mats in a siliciclastic marine environment 2 9 ga mozaan group south africa this study provides evidence for the existence of filamentous cyano bacteria forming sediment stabilizing mats in shelf environments at 2 9 ga the oldest known occurrence of microbial mats in siliciclastic rocks the mesoarchean mozaan group south africa features fine grained quartzites of an ancient shallow shelf environment these sandstones contain wrinkle structures which in thin section reveal filamentous textures forming carpet like microbial mat fabrics the textures resemble the trichomes of modern cyanobacteria chloroflexi or sulfur oxidizing proteobacteria mineralogical geochemical and isotopic analyses acre consistent with a biological origin of the filament like textures carbon filaments with biogenic isotopic signatures 1 3 c 24 2 0 5 are closely associated with hematite goethite and chert minerals which may derive from the former presence of oxygen within the microbial mats detrital quartz zircon and rutile in the mats could indicate baffling trapping and binding of the bacterial communities

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

constraining atmospheric oxygen and seawater sulfate concentrations during paleoproterozoic glaciation in situ sulfur three isotope microanalysis of pyrite from the turee creek group western australia abstract previous efforts to constrain the timing of paleoproterozoic atmospheric oxygenation have documented the disappearance of large mass independent sulfur isotope fractionation and an increase in mass dependent sulfur isotope fractionation associated with multiple glaciations at least one of these glacial events is preserved in diamictites of the 2 4 ga meteorite bore member of the kungarra formation turee creek group western australia outcrop exposures of this unit show the transition from the boolgeeda iron formation of the upper hamersley group into clastic glaciomarine sedimentary rocks of the turee creek group here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution as well as the occurrence of detrital pyrite in the meteorite bore member the 15 3 range of 33 s in one sample containing detrital pyrite 3 6 to 11 7 is larger than previously reported worldwide and there is evidence for mass independent sulfur isotope fractionation in authigenic pyrite throughout the section 33 s from 0 8 to 1 0 the 90 range in 34 s observed 45 5 to 46 4 strongly suggests microbial sulfate reduction under non sulfate limiting conditions indicating significant oxidative weathering of sulfides on the continents multiple generations of pyrite are preserved typically represented by primary cores with low 34 s 34 s 4 7 and enrichments in as ni and co the preservation of extremely sharp sulfur isotope gradients 30 p o 2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides yet remained low enough to permit the production and preservation of mass independent sulfur isotope fractionation

fluid flow in extensional environments numerical modelling with an application to hamersley iron ores the mechanical feasibility of focusing both surface and basinal derived fluids towards sites of iron ore genesis during proterozoic deformation in the hamersley province is tested here by computer simulation finite difference modelling of porous media flow during extensional deformation of a mountain range shows that surface fluids are drawn towards areas of failure and focus into the centre of the mountain the addition of permeable structures such as a normal fault provides focused fluid pathways in which mechanical and geological conditions are particularly conducive to both upward and downward flow upward flow from the base of the fault within the model overall is favoured by low permeability basement materials and supra hydrostatic pore pressures downward migration of fluids becomes more prominent as extension progresses and upward fluid flow from the base diminishes the introduction of sedimentary layering into the models allows lateral fluid flow such that sites of potential fluid mixing may then occur within permeable iron formation units close to the fault zone allowing parts of the stratigraphy to become more permeable as a function of high fluid flux simulates permeability enhancement by silica dissolution as a mechanism for iron ore genesis the involvement of both basinal and surficial fluids in the genesis of the ore deposits is supported by the mechanical models and in addition provides an explanation for a progression from relatively reduced to oxidised conditions at the mt tom price deposit and possibly other large deposits with time

microbiological processes in banded iron formation deposition banded iron formations have been studied for decades particularly regarding their potential as archives of the precambrian environment in spite of this effort the mechanism of their deposition and specifically the role that microbes played in the precipitation of banded iron formation minerals remains unresolved evidence of an anoxic earth with only localized oxic areas until the great oxidation event ca 2 45 to 2 32 ga makes the investigation of o2 independent mechanisms for banded iron formation deposition relevant recent studies have explored the long standing proposition that archean banded iron formations may have been formed and diagenetically modified by anaerobic microbial metabolisms these efforts encompass a wide array of approaches including isotope ecophysiological and phylogeny studies molecular and mineral marker analysis and sedimentological reconstructions herein the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post depositional alteration are described the main findings of recent years are summarized and compared here and suggestions are made regarding cross disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition

biological fe oxidation controlled deposition of banded iron formation in the ca 3770 ma isua supracrustal belt west greenland the redox balance of the archean atmosphere ocean system is among the most significant uncertainties in our understanding of the earliest history of earth s surface zone most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca 2 45 ga after the great oxidation event but there is less agreement on when o2 production began and how this may have been consumed by reduced species such as fe ii in the oceans the fe redox cycle through time has been traced using banded iron formations bifs and fe isotopes are increasingly used to constrain the conditions of earth s paleoenvironments including the pathways of formation of bifs iron isotope analyses of bifs from the 3 7 to 3 8 ga isua supracrustal belt isb obtained by micro sampling of magnetite rich layers and conventional analysis as well as by in situ femtosecond laser ablation fs la icp ms indicate a consistently narrow range of non zero 56fe values analysis of magnetite by fs la icp ms allows for precise and accurate micron scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry sims analyses magnetite 56fe values range from 0 4 to 1 1 among different bands but within individual layers magnetite grains are mostly homogeneous although these bifs have been metamorphosed to amphibolite facies the metamorphism can neither explain the range in fe isotope compositions across bands nor that between hand samples the isotopic compositions therefore reflect primary low temperature sedimentary values the positive 56fe values measured from the isb magnetites are best explained by deposition of fe iii oxides produced by partial oxidation of fe ii rich ocean water a dispersion reaction model which accounts for rates of hydrothermal fe ii aq input rates of oxidation and rates of fe oh 3 settling suggests exceptionally low o2 contents 0 001 of modern o2 contents in the photic zone such low levels suggest an anoxygenic pathway is more likely and the data can be well modeled by anoxygenic photosynthetic fe ii oxidation comparison of the fe isotope data from the isua bifs with those from the 2 5 ga bifs from the hamersley and transvaal basins australia and south africa respectively suggests a striking difference in fe sources and pathways the 2 5 ga magnetite facies bifs of australia and south africa have 56fe values that range from 1 2 to 1 2 over small scales and are on average close to 0 which is significantly lower than those reported here from the isua bifs the wide range in fe isotope compositions for the hamersley and transvaal bifs in concert with c and o isotope data have been interpreted to reflect bacterial dissimilatory fe iii reduction dir the absence of low 56fe values in the isua bifs as well as the lack of fine scale isotopic heterogeneity may indicate formation prior to widespread dir

detrital zircon ages from early proterozoic quartzites wisconsin support rapid weathering and deposition of mature quartz arenites abstract the ages of more than 200 detrital zircons from two proterozoic quartzites the baraboo and hamilton mounds quartzites from central wisconsin have been measured utilizing a laser ablation icpms technique both quartzites contain penokean 1 83 1 89 ga and archean components as well as a younger post penokean component the 69 detrital zircons from hamilton mounds contain major pre penokean age populations at 3 2 2 9 and 2 55 ga penokean ages at 1895 1853 and 1811 ma and a post penokean population at 1772 ma most detrital zircon ages are similar to previously measured ages of locally underlying gneisses and intrusive rocks of the marshfield terrane the 147 detrital zircons from two samples of the baraboo quartzite show a grossly similar age distribution with populations at 1754 and 1850 ma and 2 55 ga however the zircon populations in the baraboo quartzite differ from those of hamilton mounds in having a large population at 1920 ma a broad distribution of ages from 2 0 to 2 4 ga a