the seafloor as the ultimate sediment trap using sediment properties to constrain benthic pelagic exchange processes at the goban spur the benthic diagenetic model omexdia has been used to reproduce observed benthic pore water and solid phase profiles obtained during the omex study in the goban spur area n e atlantic and to dynamically model benthic profiles at site omex iii 3660 m depth with the sediment trap organic flux as external forcing the results of the dynamic modelling show that the organic flux as determined from the lowermost sediment trap 400 metres above the bottom at omex iii is insufficient to explain the organic carbon and pore water profiles the best fitting was obtained by maintaining the seasonal pattern as observed in the traps while multiplying the absolute values of the flux by a factor of 1 85 the inverse modelling of diagenetic processes resulted in estimates of total mineralisation rate and of degradability of the organic matter at the different stations these diagenetic model based estimates are used to constrain the patterns of lateral and vertical transports of organic matter using the observed degradability as a function of depth we show that the observed organic matter fluxes at the different depths are consistent with a model where at all stations along the gradient the same vertical export flux occurs at 200 m and where organic matter sinks with a constant sinking rate of around 130 m d 1 if sinking rates were higher in the order of 200 m d 1 the observations could be consistent with an off slope gradient in export production of approximately a factor of 1 5 between the shallowest and deepest sites the derived high degradability of the arriving organic matter and the consistency of the mass fluxes at the different stations exclude the possibility of a massive deposition on the margin of organic matter produced on the shelf or shelf break however other hypotheses to explain the patterns found in the sediment trap data of both omex and other continental margin study sites also suffer from different inconsistencies further close examination of the flow patterns at the margin will be needed to examine the question

tectonic systems and the deposition of iron formation abstract the iron formation group of chemically precipitated cherty iron rich sediments ranges in age from early precambrian to recent and is composed of a variety of lithological facies iron formations are classified as lake superior and algoma types to emphasize the contrasting conditions in the spectrum of sedimentary tectonic environments in which they formed thick stratigraphic units of lake superior type iron formation which form the most extensive iron ranges of the world are part of sedimentary tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions the algoma type iron formations were deposited with volcanic rocks and greywacke in various tectonic volcanic systems formed under more dynamic tectonic conditions but are otherwise comparable to present day spreading ridges on the ocean floor the thick sequences of lake superior type iron formation deposited between 2 7 and 2 0 ga form part of major sedimentary tectonic systems that extended along the margins of continents direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments

a revised global estimate of dissolved iron fluxes from marine sediments literature data on benthic dissolved iron dfe fluxes mol m 2 d 1 bottom water oxygen concentrations o2bw m and sedimentary carbon oxidation rates cox mmol m 2 d 1 from water depths ranging from 80 to 3700 m were assembled the data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic dfe fluxes inline image where 170 mol m 2 d 1 is the maximum flux for sediments at steady state located away from river mouths this simple function unifies previous observations that cox and o2bw are important controls on dfe fluxes upscaling predicts a global dfe flux from continental margin sediments of 109 55 gmol yr 1 of which 72 gmol yr 1 is contributed by the shelf 2000 m of 41 21 gmol yr 1 is unsupported by empirical data previous estimates of benthic dfe fluxes derived using global iron models are far lower approximately 10 30 gmol yr 1 this can be attributed to i inadequate treatment of the role of oxygen on benthic dfe fluxes and ii improper consideration of continental shelf processes due to coarse spatial resolution globally averaged dfe concentrations in surface waters simulated with the intermediate complexity university of victoria earth system climate model were a factor of 2 higher with the new function we conclude that i the dfe flux from marginal sediments has been underestimated in the marine iron cycle and ii iron scavenging in the water column is more intense than currently presumed

geochemical characteristics of gold bearing boninites and banded iron formations from shimoga greenstone belt india implications for gold genesis and hydrothermal processes in diverse tectonic settings abstract this paper is the first documentation of boninites from the kudrekonda formation of shimoga greenstone belt sgb in western dharwar craton wdc india and the occurrence of native gold in the associated quartz veins we also investigate the gold mineralization hosted in the banded iron formations bifs of ganajur karajgi and palavanahalli areas of the sbg the kudrekonda metavolcanic rocks are composed of clinopyroxene orthopyroxene and plagioclase with secondary amphibole the geochemical attributes of these rocks based on mgo tio 2 ni cr and co contents al 2 o 3 tio 2 gd yb n and la sm n ratios classify them as boninites and their relative lile hfse abundance suggests magma generation in an intraoceanic arc setting the gd yb and dy yb ratios suggest parent magma origin through partial melting of a spinel lherzolite mantle source geochemical signatures of the boninites suggest high temperature shallow level melting of refractory mantle wedge metasomatized by fluids derived through dehydration of oceanic slab during subduction initiation quartz veins present in the kudrekonda boninites contain gold grains showing affinity towards quartz carbonate vein type greenstone hosted occurrences epma data record 88 5 90 3 wt au content with 7 7 8 4 wt fe 0 05 2 1 wt s and negligible ag the au is hosted in fe oxides bulk chemical analyses show 4 1 684 4 ppb au and 139 4 2186 4 ppb ag in the studied samples the gold mineralization of kudrekonda boninites is syngenetic mesothermal convergent margin orogenic type decarbonation desulphidization and metamorphism of subducted oceanic lithosphere and devolatilization of metasomatized enriched mantle wedge in intraoceanic arc setting are the viable processes that contributed to gold genesis in the kudrekonda boninites devolatilization during prograde metamorphism hydrothermal fluid generation syntectonic gold remobilization and migration of gold enriched fluids through shear zones account for au transportation and precipitation in quartz veins emplaced within the boninites the bifs of ganajur karajgi and palavanahalli areas of shimoga greenstone belt dominantly comprise alternating si rich layers of quartz and fe rich hematite and magnetite layers geochemical characteristics of the bif samples including their depleted ree positive eu anomalies negative to negligible ce anomalies and superchondritic y ho ratios suggest that the bifs were i derived by chemical sedimentation from paleo ocean enriched in dissolved iron and silica with marked contributions from hydrothermal fluids of volcanic origin and ii deposited in an off shelf marine environment proximal to a mid oceanic spreading centre where hydrothermal activity induced release of eu and y enriched hydrothermal solutions under low oxygen fugacity conditions native gold occurs in banded strata bound disseminated sulphide lenses compositionally characterized as pyrite arsenopyrite chalcopyrite and galena mineral chemical analyses reveal 73 100 au content with variable concentrations of ag 0 02 15 2 wt geochemical compositions of bifs are characterized by pronounced enrichment in au 706 15 831 ppb and ag 630 2435 ppb gold mineralization in shimoga bifs is epigenetic epithermal type akin to homestake type iron formation hosted vein and disseminated lode gold deposit gold genesis in these bifs is attributed to migration of oxygen depleted reducing sulphidic auriferous fluids of hydrothermal origin to the site of bif deposition that served as chemical traps for the auriferous fluids the kudrekonda boninites represent accretion and preservation of a slice of older oceanic crust onto a relatively younger active continental margin through arc continent collision during neoarchean time

low oxygen and high inorganic carbon on the vancouver island shelf 1 recently independent concerns about declining oxygen and ph conditions in the coastal ocean have emerged in coastal upwelling regions hypoxia can be driven by onshore advection of oxygen depleted offshore waters as well as by local biological consumption triggered by high productivity as both mechanisms can also decrease ph and carbonate saturation states coupled studies of oxygen and carbon are imperative a quasi two dimensional model coupling carbon oxygen and nitrogen was developed for the summer wind driven upwelling region off southern vancouver island using the regional ocean modeling system the physical model is coupled to an ecosystem module that tracks 11 state variables and allows nonfixed c n ratios for detritus and dissolved organic matter given uncertainties in sediment parameterizations in biophysical models three sediment models are compared and discussed results demonstrate that sediment associated processes play a dominant role in consuming oxygen from and releasing inorganic carbon to the bottom waters over the shelf this study also examines the unique characteristics of the southern vancouver island shelf two key features distinguish this region from other shelves in the california current system and protect inner shelf waters from severe hypoxia and corrosive i e undersaturated in aragonite conditions first the near shore vancouver island coastal current provides a source of oxygen and nutrients and forms a barrier that prevents upwelled waters depleted in oxygen and rich in carbon from penetrating the inner shelf second the greater width of the shelf dilutes these upwelled offshore waters and reduces their penetration onto the shallower shelf region

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

auriferous chert banded iron formation and related volcanogenic hydrothermal alteration atik lake manitoba small scale alteration pipes and stratiform alteration in archean glomeroporphyritic tholeiitic basalts at atik lake manitoba stratigraphically underlie silicate oxide banded iron formation bif and auriferous sulfide bearing chert the auriferous chert is locally interbedded with graphitic argillite indicating euxinic conditions during deposition cordierite gedrite rocks formed by recrystallization of alteration assemblages during the lower amphibolite facies metamorphism t 550 c p 2 5 kbar al2o3 tio2 zr and nb which were relatively immobile during alteration have been used to monitor igneous differentiation and alteration volcanogenic hydrothermal alteration resulted in depletion of ca si mg na and sr in the altered basalt and the addition of k fe rb and ba this was accompanied by mass and volume losses of up to 25 the mineralizing fluid was reducing and somewhat acidic rare earth element ree profiles of bif and graphitic argillite normalized to archean shale are less

a knowledge based reactive transport approach for the simulation of biogeochemical dynamics in earth systems 1 a knowledge based reactive transport model kb rtm for simulation of coupled transport and biogeochemical transformations in surface and subsurface flow environments is presented http www geo uu nl kbrtm the scalable web distributed knowledge base kb which combines information technology it an automatic code generator and database management facilitates the automated construction of complex reaction networks from comprehensive information stored at the level of biogeochemical processes the reaction centric approach of the kb rtm system offers full flexibility in the choice of model components and biogeochemical reactions the procedure coupling the reaction networks to a generalized transport module into rtms is also presented the workings of our kb rtm simulation environment are illustrated by means of two examples of redox and acid base chemistry in a typical shelf sediment and an aquifer contaminated by landfill plumes

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

the variability of benthic fluxes and sedimentary remineralization rates in response to seasonally variable organic carbon rain rates in the deep sea a modeling study

geochemistry of femn formations of the archaean sandur schist belt india mixing of clastic and chemical processes at a shallow shelf abstract the late archaean greenstone belts of karnataka are unique in having manganese formations with mineable ores an excellent example of these mn bearing greenstone belts is the sandur schist belt where fe mn formations are confined to the basin shelf clearly separated from the deeper water manganese free bif that accumulated at the basin margin and flanking the marine basin sedimentary textures and structures including stromatolites show that fe mn formations have been deposited on a shallow shelf within the photic zone and above wave base along with quartz fe mn minerals found in arenites argillites and carbonates include ripidolite mg chlorite sericite mn siderite kutnahorite dolomite ferroan dolomite psilomelane pyrolusite cryptomelane haematite and magnetite the manganese and iron contents of these rocks range up to 25 and 46 respectively al 2 o 3 tio 2 mgo cao mno 2 fe 2 o 3 t abundances and ratios show a wide variation ni cr co zr v sc rb sr u th ree lree hree la ce and eu anomalies and their binary relationships indicate that wherever the terrigenous component has increased the concentration of elements of felsic parentage such as zr and hf has gone up elevated concentrations of ni cr co and sc are contributed by chlorite and other components characteristic of basic volcanic debris the carbonates where clastic input is minimal are extremely depleted in ree and other trace elements it is proposed that feo mno and sio 2 probably were added to archaean ocean water at oceanic ridge vents the compositional data suggest that the fe mn formations of the sandur schist belt were generated by chemical and clastic sedimentary processes on a shallow shelf that led into a deeper basin open on one side in this environment chemical and clastic sediments and organic matter were mixed and buried during transgression chemical precipitation took place at the sediment water interface whereas at the time of regression these chemical sediments were buried by clastic sediments redox potential differences between the watermasses on the shallower and deeper shelf respectively fractionated fe and mn the fe mn formations were deposited within the photic zone and above wave base and in a setting where the water column was oxygenated but organic material accumulated by contrast bif was deposited below both wave base and photic zone in a relatively oxygen deficient setting where formation of stable mn oxides was not possible fe mn formation and bif provide an important constraint on greenstone tectonics a shorter distance between archaean oceanic ridge and shelf environments namely the smaller plates model appears to explain several features of fe mn formations bifs and greenstone belts

caco3 dissolution in california continental margin sediments the influence of organic matter remineralization in situ benthic flux chamber and oxygen microelectrode and shipboard porewater results have been used to quantify sea floor dissolution of caco3 on the continental rise adjacent to central california usa the porewater distributions and benthic fluxes of o2 no3 ta ca2 13c and tic are interpreted using a numerical simulation of organic matter remineralization and caco3 dissolution in marine sediments the processes considered in the simulation include organic matter oxidation by o2 no3 and so42 caco3 dissolution and precipitation hs and nh4 oxidation and sediment mixing and sediment accumulation calculated benthic fluxes of o2 no3 ta tic 13c and ca2 porewater concentrations of o2 no3 and nh4 and sediment distributions of organic carbon caco3 excess 210pb and 14c agree well with the measurements benthic fluxes of alkalinity and inferred caco3 dissolution rates cannot be explained on the basis of dissolution driven solely by bottom water undersaturation if the influence of metabolically produced co2 is included benthic fluxes are fully reconciled however this is in agreement with benthic chamber ca2 and 13c results that independently imply substantial caco3 dissolution in these sediments the above observations are in contrast to those reported by jahnke et al 1994 for the west african continental rise and the western equatorial pacific where 1 g diagenetic models predict dissolution fluxes larger than observed with benthic flux chambers we conclude that the extent of metabolic caco3 dissolution may vary regionally numerous factors such as the depth of metabolic co2 production and caco3 dissolution kinetics are known or predicted to influence metabolic dissolution among the factors that should be considered in reconciling these observations are l the extent to which sulfate reduction and reoxidation reactions may influence acid base properties in surface sediments and 2 the total sedimentary caco3 content of the sites that may influence porewater acidity through surface exchange reactions at the mineral surface

dynamics of fossil fuel co2 neutralization by marine caco3 a detailed model of the ocean circulation and carbon cycle was coupled to a mechanistic model of caco3 diagenesis in deep sea sediments to simulate the millennium scale response of the oceans to future fossil fuel co2 emissions to the atmosphere and deep sea simulations of deep sea injection of co2 show that caco3 dissolution is sensitive to passage of high co2 waters through the atlantic ocean but caco3 dissolution has a negligible impact on atmospheric pco2 or the atmospheric stabilization co2 emission in the coming centuries the ultimate fate of the fossil fuel co2 will be to react with caco3 on the seafloor and on land an initial caco3 dissolution spike reverses the net sedimentation rate in the ocean until it is attenuated by an enhanced vertical gradient of alkalinity after about 1000 years the magnitude of the initial spike is sensitive to assumptions about the kinetics for caco3 dissolution but subsequent behavior appears to be less model dependent neutralization by seafloor caco3 occurs on a timescale of 5 6 kyr and is limited to at most 60 70 of the fossil fuel release even if the fossil fuel release is smaller than the seafloor erodible inventory of caco3 additional neutralization by terrestrial caco3 restores a balance between caco3 weathering and seafloor accumulation on a timescale of 8 5 kyr while the deficit of seafloor caco3 the lysocline is replenished with an e folding timescale of approximately 18 kyr the final equilibrium with caco3 leaves 7 8 of the fossil fuel co2 remaining in the atmosphere to be neutralized by the silicate rock cycle on a time frame of hundreds of thousands of years

the role of the benthic biota in sedimentary metabolism and sediment water exchange processes in the goban spur area ne atlantic we provide an overview of the role of biological processes in the benthic boundary layer bbl and in sediments on the cycling of particulate organic material in the goban spur area northeast atlantic the benthic fauna sediment and bbl characteristics were studied along a transect ranging from 208 to 4460 m water depth in different seasons over 3 years near bottom flow velocities are high at the upper part of the slope 1000 1500 m and high numbers of filter feeding taxa are found there such that organic carbon normally passing this area during high flow conditions is probably trapped accumulated and or remineralised by the fauna overall metabolism in shelf and upper slope sediments is dominated by the macrofauna more than half of the organic matter flux is respired by macrofauna with a lower contribution of metazoan meiofauna 4 and anoxic and suboxic bacterial mineralisation 21 the remainder 23 being channelled through nanobiota and oxic bacteria by its feeding activity and movement the macrofauna intensely reworks the sediments on the shelf and upper slope mixing intensity of bulk sediment and of organic matter are of comparable magnitude the benthos of the lower slope and abyssal depth is dominated by the microbiota both in terms of total biomass 90 and carbon respiration about 80 the macrofauna 16 meiofauna 4 and megafauna 0 5 only marginally contribute to total carbon respiration at depths below 1400 m because large animals have a lower share in total metabolism mixing of organic matter within the sediments is reduced by a factor of 5 whereas mixing of bulk sediment is one to two orders of magnitude lower than on the shelf the food quality of organic matter in the sediments in the shallowest part of the goban spur transect is significantly higher than in sediments in the deeper parts the residence time of mineralisable carbon is about 120 d on the shelf and compares well with the residence time of the biota in the deepest station the mean residence time of mineralisable carbon is more than 3000 d an order of magnitude higher than that of biotic biomass

archean banded iron formations of india abstract archean banded iron formations bifs are important lithologies for understanding the early earth processes greenstone belts in the four archean cratonic nucleii of peninsular india namely singhbhum bastar dharwar and bundelkhand host bif units that can be important candidates for case studies the bifs in these greenstone successions represent both algoma and superior type associations the oldest record comes from the paleoarchean southern eastern and western iron group of the singhbhum craton and the sargur supracrustals of the dharwar craton mesoarchean and neoarchean greenstone belts in the dharwar and bundelkhand cratons record a number of cycles of bif deposition in a spectrum of depositional settings from manganiferous arenite carbonate shelf associations to deep water volcanics and graywacke associations the bifs from the neoarchean bailadila group in the bastar craton were deposited in siliciclastic shelf association compositional variations in major trace and ree patterns have been evaluated from published data high grade iron ores are mainly mined from the bifs and locally mn ores occur in association as well microbial carbonates associated with some of the paleoarchean as well as neoarchean iron manganese formations suggest early life activities the lithological association of the archean iron formations have been explained in terms of both active margin and passive margin depositional settings bifs in association with manganiferous formations as old as paleoarchean time provide an opportunity to probe into the redox stratification of oceanic hydrosphere and in turn the oxygenation of early atmosphere

recycling of organic matter in antarctic sediments a transect through the polar front in the southern ocean indian sector sediments from the polar front zone were sampled in the indian sector of the antarctic ocean as part of the french jgofs expedition antares 1 the first porewater distributions of o2 and no3 and organic carbon data in the solid phase in this part of the ocean were used to model the recycling of organic matter in sediments the data are described by a model containing two types of degradable organic matter with distinct reactivities we estimate that the reactivity of the most labile organic carbon is very close to that of fresh organic matter with an average c n ratio of 7 we estimate that particulate organic carbon fluxes deposited at the sediment water interface range between 0 2 and 0 8 mol c m l y l with two peaks near the polar front and the subantarctic front the flux 01 organic carbon deposited at the sediment water interface is unusually high and represents 10 20 of estimated primary production from these findings we conclude that production in the pelagic zone of this region is strongly linked to deposition and recycling in the sediment

the influence of organic matter diagenesis on caco3 dissolution at the deep sea floor abstract in situ benthic flux chamber measurements were performed at seafloor locations in the atlantic and pacific oceans where the bottomwaters were supersaturated approximately saturated and undersaturated with respect to calcite there was no evidence of significant dissolution at or above the saturation horizon below the saturation horizon however significant benthic fluxes of titration alkalinity and calcium out of the sediments were observed indicating appreciable dissolution where observed the rate of dissolution is consistent with a rate expression of reaction order 4 5 and a rate constant in the range of 0 05 to 0 5 per day a numerical simulation was constructed to represent organic matter and caco3 diagenesis at the seafloor at the supersaturated and near saturation sites the model over predicts the influence of metabolically produced co2 on the extent of caco3 dissolution it is possible that these results can be reconciled by a better understanding and parameterization of organic matter mineralization and porewater transport processes in surface sediments the primary factors to be considered include 1 the oxidation state of the carbon being oxidized 2 the vertical distribution of oxidation rates including the role of doc in sediment remineralization 3 the influence of nondiffusive transport of near surface porewaters and 4 the influence of episodic depositional events on organic and inorganic carbon cycling

changes in glaciers in hidden valley mukut himal nepal himalayas from 1974 to 1994 glaciological investigations were carried out in 1994 on the glaciers in hidden valley mukut himal nepal himalayas in order to make a comparison with observations made in 1974 most of the glaciers were found to have retreated by 30 60 m in terminus elevation over the 20 years between the two studies rikha samba glacier the longest glacier in the valleys has retreated by about 200 m the areal average of the amount of surface lowering and the volume loss of the glacier was estimated to be 12 6 m ice equivalent and 13 of the total mass respectively the annual mass balance of 0 55 m a 1 water equivalent was obtained as an average for 20 years which is one of the largest negative values amongst small glaciers of the world

rare earth element geochemistry of some archean iron formations north of lake superior ontario rare earth element ree compositions of iron formation from two archean terrains in western ontario have been determined in order to assess the possible influence of hydrothermal activity on the ree patterns of chemical sediments one terrain is characterized by sulfide facies iron formation in association with volcanic flows and volcaniclastic sediments whereas the other is dominated by oxide facies iron formation intercalated within submarine fan clastic sediments mineral separates of chert magnetite and pyrite from the iron formations have low ree concentrations 20 30 ppm and display moderate to strong positive eu anomalies relative to archean shale the positive anomalies and lack of negative ce anomalies indicate that archean sea water from which iron formation locally precipitated was reduced although to varying degrees the ree patterns of mineral separates from a given locality are almost identical but the patterns for various localities differ in detail a number of iron formation

the calcite lysocline as a constraint on glacial interglacial low latitude production changes we investigate the response of the calcite lysocline to changes in the export production of the low latitude surface ocean the combined equatorial tropical and subtropical regions we employ different caco3 throughput schemes in a time dependent ocean carbon cycle model to separate the caco3 production lysocline balance from the other components of the model response and to estimate the effect of dissolution driven by organic matter decomposition in surface sediments with this model we carry out three experiments based on previous hypotheses for the cause of glacial interglacial atmospheric co2 variations 1 a simple increase in low latitude production 2 a decrease in its caco3 organic carbon rain ratio and 3 an increase in the depth of organic carbon regeneration none of these changes when taken alone can lower atmospheric co2 to glacial levels without violating observations of the glacial lysocline depth if a low latitude production increase were accompanied by a decrease in the caco3 organic carbon rain ratio then these simultaneous changes could lower atmospheric co2 to observed glacial levels without causing a significant change in the ocean average depth of the lysocline however even with these simultaneous changes the required increase in production is 50 or more of the modern rate of production

archaean microbiota from the donimalai formation dharwar supergroup india abstract syngenetic microbial trichomes are recorded from a black chert band interlayered with the banded iron formation belonging to the 2 6 ga old sandur schist belt of the dharwar sequence karnataka india the iron formation has interbeds of mafic pillow lava and tuffs although the fossil microbial community consists of highly biodegraded mats some well preserved structurally mineralized trichomes are also present electron microprobe analysis shows high percentage of sio 2 and p 2 o 5 in the filaments and the presence of carbon along the margins of some filaments rare earth element and isotopic data indicate a higher oxygen concentration than hitherto considered for archaean seawater in the present context there is presumptive evidence to suggest that this oxygen was produced by the photosynthetic activity of microbial biocoenoses

recent expansions of glacial lakes in the bhutan himalayas current conditions and secular changes in ice contact lakes in the bhutan china border region were investigated using satellite imagery more than 50 moraine dammed ice contact or ice proximal lakes were identified of which 14 were evidently growing in debris covered areas of valley glaciers lake growth is characterised by the initial appearance of supraglacial lakes and subsequent upstream expansion by a coalesced lake in small debris free or partially debris covered glaciers lake growth begins from a single lake lake growth has continued over a period ranging from 20 to 70 years with a growth rate of 70 m year in length and 0 04 km2 year in area these values observed at the northern side of the main himalayan range are smaller than those observed at the southern side the initial appearance year estimated from the expansion rate of most of the lakes in the southern and northern sides ranges from the 1950s to the 1970s and before the 1950s respectively

factors controlling long term phosphorus efflux from lake sediments exploratory reactive transport modeling abstract we perform for the first time a global sensitivity analysis on a generic diagenetic reaction transport model rtm to elucidate the effects of environmental thermodynamic and kinetic factors on the magnitudes of phosphorus release fluxes from aquatic sediments under a range of conditions typically found in lake sediments our model includes the processes that describe redox sensitive phosphorus releases the so called classical model and processes by which phosphorus mobilization is affected by porewater sulfate on decadal and longer time scales sediment phosphorus effluxes are primarily determined by sedimentation flux of reactive organic matter sedimentation flux of iron oxyhydroxides concentrations of dissolved oxygen and sulfate at the sediment water interface and the rate at which phosphate is immobilized in reduced sediment we show that the effects of these factors on phosphorus effluxes are interdependent and discuss the mechanisms of such interactions the dominant pathways by which dissolved sulfate increases phosphorus efflux in iron rich hypoxic sediments are discussed in detail in contrast to short term phosphorus releases such as described by the classical model long term phosphorus retention is controlled by phosphorus removal to deep reduced sediment rather than processes at the sediment water interface hence the results of short term laboratory or in situ studies such as sediment incubations experiments cannot be unequivocally extended to long time scales our results provide explanations to the reports that lake restoration measures such as restricting phosphorus inputs to a lake or oxygenating the lake s hypolimneon or both in the long term often fail to decrease sediment phosphorus effluxes the re deposition of sediment substances after their release into the water column the feedback often overlooked in sediment rtms can critically affect the magnitude and dynamics of phosphorus efflux from sediments depending on sediment history the same set of external boundary conditions can generate diagenetic regimes with either high or low phosphorus effluxes bistability

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

monitoring and prediction of shrinkage of a small glacier in the nepal himalaya in 1978 the first detailed glaciological studies were carried out on the small glacier ax010 in a glacier inventory of dudh kosi region in the nepal himalaya its length area and highest and lowest elevations in 1978 were 1 7 km 0 57 km2 5360 and 4950 m a s l respectively two resurveys of its terminus position and or surface elevations were carried out in 1989 and 1991 the glacier retreated by about 30 and 28 m during the periods 1978 89 and 1989 91 respectively in 1995 annual monitoring of this glacier by means of ground survey stake method and topographical mapping began in order to obtain its mass balance surface flow velocity and extent and link them with climatic conditions the results obtained in 1995 are summarized as follows 1 during the period 1991 95 the glacier retreated by 12 m 2 associated with the ice thickness loss in the lower part of the glacier the horizontal surface velocities along the center line in 1995 june october showed a remarkable decrease on the glacier tongue to about 50 of those in 1978 shrinkage of the glacier in the near future is predicted from a simple model calculation for the case that climatic conditions remain unchanged from 1995 the results show that the present shrinkage should continue and accelerate

archean seawater composition and depositional environment geochemical and isotopic signatures from the stromatolitic carbonates of dharwar craton india abstract the well preserved stromatolitic carbonates from neoarchean sandur chitradurga and shimoga greenstone belts of dharwar craton were studied to constrain the depositional conditions geochemical isotopic characteristics and the archean seawater temperatures these rocks display limited diversity in their morphological features such as columnar occasionally branched pseudo columnar domal crinkly laminated and oval elliptical shapes the stromatolites are associated with fe mn formations along with arenites argillites limestones and carbonaceous shales these stromatolites are cherty dolomites having variable sio2 mgo and cao contents the stromatolitic carbonates of the three greenstone belts possess low total rare earth elements plus ytterium rey along with positive la eu gd y anomalies and y ho ratios reflecting their deposition in marine conditions with variable hydrothermal and terrigenous inputs these stromatolites display predominantly negative 13c values ranging from 1 61 to 0 01 vpdb of sandur 2 53 to 1 35 vpdb of chitradurga and positive 13c of shimoga varying from 0 30 to 1 29 vpdb combined with the negative 18o values 20 95 to 7 72 vpdb our studies indicate fluctuating oxic to anoxic acidic to alkaline environment responsible for the deposition of these stromatolites with locally varying archean ocean temperatures from 25 to 75 c these conditions favoured the deposition of cherty dolomitic stromatolites in a shallow marine environment ranging from supra to subtidal zones with the minor influence of hydrothermal activity which is evident for the existence of archean smaller plate tectonics

modeling the calcite lysocline a numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea model results are compared with lysocline data from 13 regions in the atlantic pacific and indian oceans the model lysocline shape is sensitive to the calcite dissolution rate constant the calcite organic carbon and refractory material rain rates and the rates of oxic versus anoxic organic carbon degradation in the sediment the model is able to reproduce the observed lysocline within the constraints of the sediment trap and calcite accumulation data using a calcite dissolution rate constant of 30 100 d 1 a molar ratio of organic carbon to calcite rain rates of 0 5 1 0 and an initial caco3 fraction of 90 excluding organic carbon this rate constant is consistent with microelectrode results presented by archer et al 1989 the model predicts that 30 50 of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration consistent with previous modeling studies the range in the best fit value of the organic inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material rather than from scatter in the data lysocline data from the western equatorial atlantic are anomalous to the rest of the data this anomaly may be explained by high rates of refractory material sedimentation from the amazon river

distinctive features of late palaeozoic massive sulphide deposits in south china abstract more than 20 sediment hosted massive sulphide deposits occur in late palaeozoic basins in south china these deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their cu pb zn au and ag reserves the deposits and their host strata were commonly intruded by mesozoic granitoids remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein skarn and porphyry type orebodies in the same region or within a single deposit the ore containing basins occur in different tectonic settings the lower yangtze basin occurs on a passive continental margin where the deposits are high in cu and au with minor pb and zn and recoverable ag co and mo the ores have a lower concentration of radiogenic lead and 34s values close to zero fluid inclusions are highly saline and na rich fluids and metals of the lower yangtze region are interpreted to have been derived essentially from deep sources including the precambrian basement by contrast basins of the nanling region formed in an intracontinental setting developed on a folded caledonian basement these deposits are higher in pb zn sn and w as well as cu with recoverable ag sb hg u bi tl and mo the ores are characterised by a higher concentration of radiogenic lead and a wide variation of 34s composition fluid inclusions have lower salinities and higher k na ratios fluids are considered to have been sourced substantially from seawater by convection metals for the nanling deposits were essentially derived from the caledonian basement by leaching the contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore forming basins

geology age and origin of supracrustal rocks at akilia west greenland new mapping geochronology and geochemistry of the supracrustal enclave and associated orthogneisses at akilia west greenland support a 3825 6 ma age for the sequence and a chemical sedimentary origin for the controversial fe rich quartz pyroxene gneisses lithologies of the enclave comprise laterally continuous mappable units of mafic amphibolite ultramafic rocks and two fe rich quartz pyroxene units a minor anthophyllite garnet rock with chemical characteristics suggesting a sedimentary protolith was identified the earliest foliation parallels lithologic contacts this foliation was isoclinally folded about a steep hinge surface and then tightly refolded about a steep ns hinge plane the latest folding produced limbs exhibiting contrasting strains the full deformation history is shared by all lithologies of the enclave two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low strain limb the supracrustal anthophyllite garnet rock experienced zircon growth at 3600 and 2700 ma in common with amphibolites these ages are interpreted to reflect metamorphism at granulite and amphibolite facies respectively consistent with independent regional evidence the crosscutting orthogneiss sheets preserve a complex history of zircon growth but only earliest cores indicate th u exchange equilibrium with the bulk rock at magmatic conditions weighted mean 207 pb 206 pb ages of cores of 3730 7 ma and 3825 6 ma date the crystallization ages of the sheets indicating that the akilia supracrustal rocks were deposited 3819 ma quartz pyroxene rocks on akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon isotope evidence for life at the time of their formation an alternative proposal is an origin by metasomatic alteration our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body metamorphic zircons give a minimum 207 pb 206 pb age of 3589 13 ma regardless of origin the two models of formation of quartz pyroxene rocks were evaluated using field petrologic and geochemical tests the distribution of the units the absence of field or petrologic indicators of metasomatism and geochemistry oxygen and nontraditional stable isotopes immobile elements and ree together support a sedimentary but not a metasomatic origin because their protolith was sedimentary the quartz pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age despite strong deformation and polymetamorphism the akilia supracrustal enclave contains information about terrestrial surficial processes at 3819 ma

some geochemical indicators for discrimination between diagenetic and hydrothermal metalliferous sediments abstract two types of metalliferous sediments are known hydrothermal associated with tectonic faults and diagenetic enriched in precipitates from seawater they are similar in their appearance and chemical composition only the first type is an indicator for hydrothermal discharge therefore in exploration criteria for differentiating between the two are needed four possible indicators have been investigated and are discussed a differences in p y correlation b differences in cr zr correlation c different distribution patterns of ree and d different as contents the as content is easy to determine and seems to be at the present time the most suitable criterion for determining the genesis of the metalliferous sediments

on the coupling of benthic and pelagic biogeochemical models abstract mutual interaction of water column and sediment processes is either neglected or only crudely approximated in many biogeochemical models we have reviewed the approaches to couple benthic and pelagic biogeochemical models it is concluded that they can be classified into a hierarchical set consisting of five levels differing in the amount of detail given to the sediment processes the most complex approach level 4 fully couples water column processes to a vertically resolved biogeochemical sediment model first simplification is achieved by using a vertically integrated dynamic sediment model level 3 next is a reflective type of boundary level 2 where particulate material arriving at the sediment surface is instantaneously transformed into dissolved components then comes a set of models in which either the bottom water concentration of dissolved substances or the sediment water exchange is imposed level 1 finally in some biogeochemical models the bottom is plainly ignored level 0 we have tested these various approaches in a coupled physical pelagic benthic biogeochemical model for oxygen nitrogen and carbon cycling in continental shelf areas we discuss the various model approaches with respect to their impact on the pelagic system and point out some of the inconsistencies hidden in certain formulations we conclude that lower boundary types in which sediment fluxes or concentrations are imposed level 1 are especially badly designed because they fail to assure conservation of mass finally we suggest as best choice a level 3 approach in which the evolution of sedimentary particulate matter is part of the solution and where the bottom fluxes of dissolved constituents are parameterised based on mass budget considerations these simplified formulations represent the best balance between computational demand and attained accuracy

rare earth elements in the early archaean isua iron formation west greenland abstract the rare earth element ree patterns in the 3 8 ga old isua iron formation are generally flat resembling those of some primitive basalts samples with positive negative or no europium anomaly were found it is shown that diagenesis and metamorphism did not significantly change the ree patterns the presence or absence of europium anomalies in iron formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the archaean and precambrian the ree contents cannot be used to distinguish algoma type from superior type iron formations there appears to be a striking similarity between the archaean submarine exhalations and modern submarine hydrothermal systems it is considered likely that archaean and early precambrian seawater had a chondritic ree pattern with a slight enrichment of light ree

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

petrology and geochemistry of the boolgeeda iron formation hamersley basin western australia abstract the boolgeeda iron formation and overlying turee creek group hamersley basin western australia represent a conformable succession of sediment deposited between 2 45 and 2 22 ga this interval of geologic history is of significant interest because it spans the great oxidation event goe when oxygen first accumulated in earth s atmosphere here we present geochemical and petrographic data from the uppermost 45 m of the boolgeeda iron formation and an additional 30 m of the overlying kungarra formation mudstones as sampled from the turee creek drilling project 1 drill core tcdp1 this core captures the termination of bif deposition in the hamersley basin and coincides with a global decline in bif deposition in the paleoproterozoic we provide a continuous high resolution chemostratigraphic dataset of major and trace element concentrations as well as fe speciation data to assess the relationship between the rise of atmospheric oxygen and the subsequent decline in bif deposition we also highlight the interplay between local and global controls on the preservation of redox signatures including the rise and fall of local base level input of weathered continental material influx of reduced sulfur species associated with the continental weathering and global increases in atmospheric oxygen we interpret boolgeeda deposition to have taken place under shallow water oxic conditions overlying anoxic ferruginous deep water intermittent periods of oxidative weathering led to influxes of redox sensitive trace elements from land as continental sulfide minerals were weathered this also led to the temporary disappearance of mif s but o2 remained below the threshold capable of completely eliminating s isotope signatures associated with the mif s recycling

the temporal significance of transitions from melting to calving termini at glaciers in the central southern alps of new zealand termini of valley glaciers in the central southern alps are undergoing a three phase sequence of retreat that began in the 1950s an initial phase of melting and slow downwasting under thickening supraglacial debris mantles is associated with stationary or slowly retreating termini this is followed by a transitional phase of disruption of insulating debris mantles allowing the rapid growth of shallow thermokarst lakes the shape and location of which vary according to glacier gradient and marginal topography a third phase of rapid calving retreat of ice cliffs into deepening proglacial lakes develops from the transitional phase at no stage are terminal responses simply related to climate present transitions to calving retreat are related to the morphology of glacier termini particularly to large outwash heads landform evidence indicates that similar calving phases do not appear to have occurred since late pleistocene deglaciation so that present changes to glacier termini are of great significance

new constraints on the paleoarchean meteorite bombardment of the earth geochemistry and re os isotope signatures of spherule layers in the barb5 icdp drill core from the barberton greenstone belt south africa abstract archean spherule layers resulting from impacts by large extraterrestrial objects to date represent the only remnants of the early meteorite asteroid and comet bombardment of the earth only few archean impact debris layers have been documented all of them embedded in the 3 23 3 47 billion year old successions of the barberton greenstone belt bgb in south africa and the pilbara craton in western australia some of them might be correlated with each other given the scarcity of archean spherule deposits four spherule layer intersections from the recently recovered barb5 drill core from the central barberton greenstone belt analyzed in this study provide an opportunity to gain new insight into the early terrestrial impact bombardment despite being hydrothermally overprinted siderophile element abundance signatures of spherule rich samples from the barb5 drill core at least in part retained a meteoritic fingerprint the impact hypothesis for the generation of the barb5 spherule layers is supported by correlations between the abundances of moderately cr co ni and highly siderophile re os ir pt ru and pd elements whose peak concentrations and interelement ratios are within the range of those for chondrites rhenium osmium isotope evidence further support the impact hypothesis collectively this study provides evidence for extraterrestrial admixtures ranging between 40 and up to 100 to three of the four analyzed barb5 spherule layers and a scenario for their genesis involving i impact of a chondritic bolide into a sedimentary target ii varying admixtures of meteoritic components to target materials iii spherule formation via condensation in an impact vapor plume iv transportation of the spherules and sedimentation under submarine conditions followed by v moderate post impact remobilization of transition metals and highly siderophile elements

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

sediment redeposition in archean iron formation examples from the beardmore geraldton greenstone belt ontario abstract archean iron formation i f typically occurs over 5 15 m intervals within predominantly arenitic sedimentary rocks in the beardmore geraldton greenstone belt the i f consists primarily of dark laminated magnetite chemical component with common intercalations of siltstone sandstone clastic component which range in thickness from sub millimeters to tens of centimeters sedimentary structures in the latter beds which include graded bedding parallel lamination and load casts indicate deposition of clastic strata together with some magnetitic material by turbidity currents within intervals of alternating magnetite siltstone and sandstone beds the clastic component may coarsen and thicken upward over a scale of meters such sequences eventually pass vertically into a 100 clastic succession these features suggest progradation of submarine fan clastic sediment lobes with a minor magnetite component into an environment where essentially iron rich sediment the magnetite precursor was otherwise being deposited as background chemical rainout variations in coarseness and thickness of clastic beds in different sections within the i f are interpreted as reflecting proximal to distal locations relative to channel feed systems on the fan on the outcrop scale the background clastic free chemical sediment appears as blackish laminated magnetite thin sections however reveal that it actually consists of regular sub millimeter alternations between magnetite and chert these magnetite chert microlaminations are analogous to those of the hamersley basin in western australia trendall and blockley 1970 and similarly may reflect annual varvings

petrography and geochemical behaviour of trace element ree and precious metal signatures of sulphidic banded iron formations from the chikkasiddavanahalli area chitradurga schist belt india abstract the 1 2 km long and 250 m wide chikkasiddavanahalli c s halli hill range consists of mixed sulphidic oxide banded iron formations bifs and fe rich phyllites carbonaceous which overlie carbonated schistose and massive meta volcanics in stratigraphic succession the lithologies represent the ingaldhal formation and are an integral part of the chitradurga schist belt in the western dharwar craton the general strike at c s halli varies from n s to 340 with vertical to steep dips towards east and west the sulphides oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist and amphibolites facies metamorphism the bifs show mesobands of recrystallised cherts and iron sulphides such as pyrite arsenopyrite and silicates such as subordinate grunerite hornblende chlorite muscovite actinolite and minor carbonates such as ankerite calcite and magnesian siderite chemical data indicate depletion in ti mn co cu cr and ni in these iron formations most chondrite normalized ree patterns of the iron formation show moderate lree and hree enrichment coupled with strong positive eu anomaly the mineralized portions exhibit characteristic negative ce and eu anomalies eu eu 0 21 to 3 00 the total ree abundance varies correlates well with the iron contents of the bifs and similar to those exhibited by hydrothermal plumes chown e h dah e n mueller w g 2000 the relation between iron formation and low temperature alteration in a archean volcanic environment precambrian research 101 263 275 trace and ree data suggest that primary mantle derived hydrothermal solutions were added to the pore fluids of sediments of the chitradurga basin and supplied chemical constituents such as feo sio 2 and ree oxidation of feo to fe 2 o 3 was caused by the photosynthesis of primitive stromatolite building cyanobacteria geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation quartz calcite vein activity and sulphidation in the mixed sulphide oxide facies bif and associated iron phyllites in the c s halli area western dharwar craton india

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

geology geochemistry and genesis of bif of kushtagi schist belt archaean dharwar craton india the banded iron formation bif of the kushtagi schist belt dharwar craton is interbedded with metavolcanics the oxide fades cherty al2o3 2 bifs show large scale variations in their major and trace elements abundance cherty banded iron formation cbif is depleted in al2o3 tio2 zr hf and other trace elements like cr ni co rb sr v y and ree in comparison to shaley banded iron formation sbif depleted ree positive eu anomalies and the flat to hree enriched pattern of cbif indicate that fe and sio2 for these bifs were added to ambient ocean water by hydrothermal solutions at the amor vent sites it is inferred that the higher amount of hydrothermal fluid flux with a higher exit temperature provided enormous quantities of iron and silica fine grained sedimentation in the basin gave rise to the observed variability in the composition of bif during transgression a wave base was raised up consequently deposition of cbif became possible whereas during the regressive stage these chemical sediments were buried by and or mixed with the terrigenous sediments resulting in deposition of sbif and interbedded shales volcaniclastic activity within the basin appears to have contributed significantly to the composition of some sbif and shales the hydrothermal exhalative hypothesis combined with the archaean miniplate model explains most of the chemical features of the bifs of greenstone belts

carbon nutrient and trace metal cycling in sandy sediments a comparison of high energy beaches and backbarrier tidal flats abstract in order to evaluate the importance of coastal sandy sediments and their contribution to carbon nutrient and metal cycling we investigated two beach sites on spiekeroog island southern north sea germany and a tidal flat margin located in spiekeroog s backbarrier area we also analyzed seawater and fresh groundwater on spiekeroog island to better define endmember concentrations which influence our study sites intertidal sandy flats and beaches are characterized by pore water advection seawater enters the sediment during flood and pore water drains out during ebb and at low tide this pore water circulation leads to continuous supply of fresh organic substrate to the sediments remineralization products of microbial degradation processes i e nutrients and dissolved trace metals from the reduction of particulate metal oxides are enriched in the pore water compared to open seawater concentrations the spatial distribution of dissolved organic carbon doc nutrients po43 no3 no2 nh4 si oh 4 and total alkalinity trace metals dissolved fe and mn as well as sulfate suggests that the exposed beach sites are subject to relatively fast pore water advection which leads to organic matter and oxygen replenishment frequent pore water exchange further leads to comparatively low nutrient concentrations sulfate reduction does not appear to play a major role during organic matter degradation high nitrate concentrations indicate that redox conditions are oxic within the duneward freshwater influenced section while ammonification denitrification manganese and iron reduction seem to prevail in the ammonium dominated seawater circulation zone in contrast the sheltered tidal flat margin site exhibits a different sedimentology coarser beach sands versus finer tidal flat sands and nutrients dissolved manganese and doc accumulate in the pore water ammonium is the dominant pore water nitrogen species and intense sulfate reduction leads to the formation of sulfide which precipitates dissolved iron as iron sulfide these findings are due to slower advective pore water exchange in the tidal flat sediments this study illustrates how different energy regimes affect biogeochemical cycling in intertidal permeable sediments

iron formation in neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts abstract neoarchean deltaic deposits present in superior province canada contain an excellent record of shallow water siliciclastic and iron formation if deposition the deltas were fed by braided fluvial systems which commonly resulted in the development of small active lobes that repeatedly cycled through progradation and abandonment although a thick progradational deltaic assemblage was also investigated magnetite rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore proximal fluvial channels and distributary mouth bars of the braid delta deposits magnetite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces in ripples to barforms thicker accumulations of if up to 73 m form portions of transgressive systems tracts where chemical sedimentation dominated offshore equivalents of these successions do not contain chemical sediments sedimentologic data imply that if deposition in this well mixed nearshore water mass was not compatible with slow rates of constant iron compound precipitation in the world ocean combined with siliciclastic sediment starvation the model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore stimulating photosynthetically induced oxidation of fe2 under these conditions elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts

understanding the causes and consequences of past marine carbon cycling variability through models abstract on geological time scales the production and degree of recycling of biogenic carbon in the marine realm and ultimately its removal to sediments exerts a dominant control on atmospheric co 2 and hence variability in climate this is a highly complex system involving a myriad of inter connected biological chemical and physical processes for this reason alone linking observations often highly abstracted in the form of proxies to the primary processes involved and ultimately to explanatory hypotheses for specific geological events and transitions is challenging the past few decades have seen a progressive improvement in theoretical and process based understanding of the various components that make up the marine carbon cycle and hand in hand with this the development of numerical model representations of the complete system models have also been designed and or adapted with paleoclimate questions in mind and applied to quantitatively explore the role of the marine carbon cycle in both perturbations and long term geologic evolutionary trends in global climate and possible feedbacks between them however we must ask whether paleoclimate models incorporate sufficiently appropriate representations of the dynamics and sensitivities of the marine carbon cycle and indeed whether in the geological context we really know what these dynamics are here we provide a comprehensive overview of how marine carbon cycling and the biological carbon pump is treated in available paleoclimate models with the aim of critically evaluating their ability to help interpret past marine carbon cycle and climate dynamics to this end we first provide an overview of commonly used paleoclimate models and some of their associated paleo applications drawing from a wide range of global carbon cycle box models and earth system models of intermediate complexity emics secondly we review and evaluate the three dominant processes involved in the cycling of organic and inorganic carbon in the marine system and how they are represented in models namely biological productivity at the ocean surface remineralisation dissolution of particulate carbon within the water column and the benthic pelagic coupling at the seafloor we generate and employ illustrative examples using the model genie to show how different parameterisations of water column and sediment processes can lead to significantly different model projections our compilation reveals the prevalence of static parameterisations of marine carbon cycling among existing paleoclimate models which are commonly empirically derived from present day observations although such approaches tend to represent carbon transfer in the modern ocean well they are potentially compromised in their ability to reflect the true degree of freedom and strength of feedbacks with respect to past climate events particularly those characterised by environmental boundary conditions that differ fundamentally from today finally we discuss the importance of using models of different complexities and how questions of model uncertainty may start to be addressed

geochemical cycles in an ocean general circulation model preindustrial tracer distributions a state of the art report is given of the hamburg model of the oceanic carbon cycle the model advects geochemical tracers important to the carbon cycle by the currents of a general circulation model the geochemical cycling is driven by a michaelis menten type production kinetics the model is an extension of the bacastow and maier reimer 1990 model it is based on a more realistic current field and includes a mechanism of lysocline sediment interaction principal variables are co2 alkalinity phosphate oxygen and silicate the carbon variables are defined for 12c 13c and 14c separately in addition to these carbon isotopes 39a and 18o of dissolved oxygen are predicted the model predicts realistic global patterns of tracer distribution in the equatorial eastern pacific however the structures are exaggerated due to a strong upwelling which is a common feature of coarse resolution models of the general circulation of the ocean

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

microelectrode studies of organic carbon degradation and calcite dissolution at a california continental rise site fine scale porewater profiles of resistivity o2 ph and pco2 were measured in situ with microelectrodes at a site on the california continental rise they are reported here with more traditional measurements of shipboard porewater no3 alkalinity and tco2 and sediment organic c and caco3 profiles a numerical model encompassing diffusion advection and multiple reaction terms is applied to these data to characterize organic carbon degradation and caco3 dissolution processes near the sediment water interface a benthic o2 flux of 58 mol cm 2 yr 1 over the top 1 mm sediment is estimated based on one measured sub mm depth scale o2 profile the numerical model indicates that this high o2 consumption rate is related to a very labile fraction of organic c which has a degradation rate constant of approximately 4 5 10 8 s 1 1 4 yr 1 and which is not mixed downward by biological mixing seasonality in the benthic o2 consumption rate of this site may be attributed to variations in the input rate of this highly labile organic c degradation rate constants of the bulk organic c by oxygen reduction and by nitrate reduction are estimated to be about the same 1 10 9 s 1 the organic c rain rate responsible for the bulk organic c is estimated to be 53 mol cm 2 yr 1 this estimate exceeds the average rain rate determined by sediment trap collections at this site saturation state profiles calculated from the in situ ph and pco2 data and from modelling the in situ o2 and ph data both indicate that the porewater is undersaturated with respect to calcite to a depth of at least 8 cm the dissolution rate constant of calcite in this caco3 poor station is determined to be 5 10 day 1 assuming the reaction order is 4 5 which is close to the minimum estimates for environments richer in calcite a uniform calcite dissolution rate constant therefore may be adequate for representing carbonate dissolution in global scale models of ocean chemistry

simulation of early diagenetic processes in continental slope sediments off southwest africa the computer model cotam tested abstract pore water of marine sediments recovered from two stations of the continental slope off southwest africa were investigated we present computer simulations of in situ and laboratory concentration profiles of oxygen as well as laboratory concentration profiles of nitrate calcium ph and alkalinity the simulations were carried out with help of a numerical model cotam to describe the transport and the reaction of dissolved species in sediments cotam is based on an operator splitting approach comprising the independent calculation of transport and chemical reaction the consumption rates of oxygen and nitrate were determined by optimal fits to the measured pore water profiles of these species it could be shown that measured concentrations of nitrate in pore water correspond to a decomposition of organic matter with c n ratios between 3 and 3 7 however artificially increased subsurface nitrate concentrations due to core recovery cannot be excluded but our results show much greater deviations from expected concentrations assuming redfield stoichiometry than previously reported from comparative studies i e martin and sayles 1996 oxygen consumption in situ was shown to be distinctively lower than measured in multicorer cores after recovery simulations with varying denitrification rates indicate reduced diffusive nitrate release into the bottom water by up to 50 compared to shipboard results effects of nitrification and denitrification on pore water ph carbonate alkalinity and calcium concentrations were simulated by recalculating concentrations of these species with regard to calcite equilibrium for these calculations we used the standard software phreeqe as a subroutine of cotam calcium and carbonate alkalinity increase due to solid phase calcite dissolution ph was calculated to be an order of magnitude lower within the zone of oxygen depletion than indicated by shipboard results this difference is mainly related to non equilibrium conditions during ph measurements

extensive nitrogen loss from permeable sediments off north west africa the upwelling area off north west africa is characterized by high export production high nitrate and low oxygen concentration in bottom waters the underlying sediment consists of sands that cover most of the continental shelf due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen n loss via denitrification as reported for anthropogenically eutrophied regions however n loss from sands underlying naturally eutrophied waters is not well studied and in particular n loss from the north west african shelf is poorly constrained during two research cruises in april may 2010 2011 sediment was sampled along the north west african shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15n labeled nitrate areal n loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles areal n loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability the derived empirical relation between benthic n loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53 000 km2 using detailed sediment maps denitrification from this region amounts to 995 kt yr 1 average 3 6 mmol m 2 d 1 which is 4 times higher than previous estimates based on diffusive pore water transport sandy sediments cover 50 60 of the continental shelf and thus may contribute significantly to the global benthic n loss

effects of progressive oxygen depletion on sediment diagenesis and fluxes a model for the lower st lawrence river estuary we applied a diagenetic model to examine the effects of a decadal timescale progressive decrease in bottomwater oxygen concentration on sediment geochemistry and fluxes across the sediment water interface the model was calibrated using geochemical data acquired over the last 30 yr in the lower st lawrence river estuary canada where the bottom water oxygen concentration has been decreasing at an average rate of 1 mmol l21 yr21 over the past 70 yr benthic fluxes calculated from the model are comparable to fluxes derived from shipboard sediment incubations by propagating the model forward in time and allowing the oxygen concentration to decrease further at 1 mmol l21 yr21 we predict changes in the distributions and fluxes of iron manganese phosphorus nitrogen and sulfur as the oxygen concentration decreases the concentrations and distributions of reactive phases in the sediment change at an accelerating pace fluxes of reduced substances out of the sediment increase reactive iron and manganese oxide phases become depleted and the sediment becomes progressively enriched in iron sulfides a notable exception is the efflux of phosphate which remains invariant at the study site these changes are likely to become measurable within the next 20 yr model results are sensitive to the effects of decreasing oxygen concentrations on the benthos the response of which we represent by two different approximations of the bioturbation and bioirrigation formulations a gradual decrease and a thresholdtype decrease the principal obstacle to reliable predictions of how sediments will respond is the lack of knowledge about the response of bioturbation and bioirrigation to decreasing oxygen in bottom waters

trace rare earth element and oxygen isotopic systematics for the genesis of banded iron formations evidence from kushtagi schist belt archaean dharwar craton india abstract the ree distribution pattern shapes free 5 92 and 27 87 ppm in chert and hematite respectively positive eu anomalies eu eu 3 25 and 2 1 and la n yb n ratios 0 75 and 0 87 of the chert and hematite layers of the oxide facies bifs of kushtagi schist belt indicate that these constituents were derived from hydrothermal solutions generated and emplaced at an archaean ocean ridge however their oxygen isotopic composition have been modified by mixing with the ambient ocean water and diagenetic reactions 18o values 13 9 17 4 c and 8 6 to 3 3 in chert and hematite respectively yield 150 250 c temperature in the diagenetic and lowest greenschist facies field the mixing model of hydrothermal solutions with ocean water of 1 1000 ratio will not significantly change the oxygen isotope ratio of the ambient ocean water therefore while the trace element data have retained hydrothermal signatures oxygen isotopic signatures are imprinted by diagenetic processes

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise

the role of a transcrustal shear zone in orogenic gold mineralization at the ajjanahalli mine dharwar craton south india the ajjanahalli gold mine is spatially associated with a late archean craton scale shear zone in the eastern chitradurga greenstone belt of the dharwar craton india gold mineralization is hosted by an 100 m wide antiform in a banded iron formation original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite stilpnomelane minnesotaite sericite ankerite arsenopyrite pyrite pyrrhotite and gold at ca 300 to 350 c elements enriched in the banded iron formation include ca mg c s au as bi cu sb zn pb se ag and te whereas in the wall rocks as cu zn bi ag and au are only slightly enriched strontium correlates with cao mgo co2 and as which indicates cogenetic formation of arsenopyrite and mg ca carbonates the greater extent of alteration in the fe rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers the ore fluids as interpreted from their isotopic composition 18o 6 5 8 5 initial 87sr 86sr 0 7068 0 7078 formed by metamorphic devolatilization of deeper levels of the chitradurga greenstone belt arsenopyrite chalcopyrite and pyrrhotite have 34s values within a narrow range between 2 1 and 2 7 per mil consistent with a sulfur source in chitradurga greenstone belt lithologies based on spatial and temporal relationships between mineralization local structure development and sinistral strike slip deformation in the shear zone at the eastern contact of the chitradurga greenstone belt we suggest that the ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first order structure the ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics based on this model of fluid flow exploration should focus on similar low strain areas or potentially connected higher order splays of the first order shear zone

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered