using fractal crustal magnetization models in magnetic interpretation1 evidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three dimensional distribution of magnetization within the crust can be described as fractal this property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization specifically we address the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging when magnetic data are continued downwards the depth at which the power spectrum flattens out the white depth can be taken to be an estimate of the top of the source distribution this procedure assumes that individual sources are uncorrelated with each other taking into account the correlation of the magnetization using a fractal description leads to a reduction in this depth estimate gridding of randomly distributed magnetic measurements using kriging requires an estimate of the covariance of the data compared with the assumption of a white uncorrelated magnetization distribution using fractal covariances for kriging produces gridded estimates which more closely reflect the statistics of the underlying magnetization process and produce maps with a justifiable degree of smoothness

the paleoproterozoic snowball earth a climate disaster triggered by the evolution of oxygenic photosynthesis although biomarker trace element and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2 8 giga annum before present ga and perhaps as early as 3 7 ga a skeptical examination raises considerable doubt about the presence of oxygen producers at these times geological features suggestive of oxygen such as red beds lateritic paleosols and the return of sedimentary sulfate deposits after a 900 million year hiatus occur shortly before the 2 3 2 2 ga makganyene snowball earth global glaciation the massive deposition of mn which has a high redox potential practically requires the presence of environmental oxygen after the snowball new age constraints from the transvaal supergroup of south africa suggest that all three glaciations in the huronian supergroup of canada predate the snowball event a simple cyanobacterial growth model incorporating the range of c fe and p fluxes expected during a partial glaciation in an anoxic world with high fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on timescales as short as 1 million years as the geological evidence requiring oxygen does not appear during the pongola glaciation at 2 9 ga or during the huronian glaciations we argue that oxygenic cyanobacteria evolved and radiated shortly before the makganyene snowball

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

relation between soil age and silicate weathering rates determined from the chemical evolution of a glacial chronosequence long term weathering rates rlt were determined from base cation depletion in soil profiles developed on six granitic glacial moraines varying in age from 0 4 to 297 ka in the wind river mountains wyoming weathering rates were found to decrease with soil age t according to the power law equation rlt 15 t 0 71 where rlt is in meq m 2 yr 1 and t is in ka comparable data from the literature are consistent with this function and it is similar to power law equations describing changes in laboratory weathering rates with time on the basis of our study we predict that for a catchment unaffected by anthropogenic inputs and in a steady state with respect to the organic and cation exchange pools rlt should be higher than present day weathering rates derived from stream fluxes rpd by a factor of about 3 4

production preservation and biological processing of mass independent sulfur isotope fractionation in the archean surface environment mass independent fractionation of sulfur isotopes s mif in archean and paleoproterozoic rocks provides strong evidence for an anoxic atmosphere before 2 400 ma however the origin of this isotopic anomaly remains unclear as does the identity of the molecules that carried it from the atmosphere to earth s surface irrespective of the origin of s mif processes in the biogeochemical sulfur cycle modify the primary signal and strongly influence the s mif preserved and observed in the geological record here a detailed model of the marine sulfur cycle is used to propagate and distribute atmospherically derived s mif from its delivery to the ocean to its preservation in the sediment bulk pyrite in most sediments carries weak s mif because of microbial reduction of most sulfur compounds to form isotopically homogeneous sulfide locally differential incorporation of sulfur compounds into pyrite leads to preservation of s mif which is predicted to be most highly variable in nonmarine and shallow water settings the archean ocean is efficient in diluting primary atmospheric s mif in the marine pools of sulfate and elemental sulfur with inputs from so2 and h2s respectively preservation of s mif with the observed range of magnitudes requires the s mif production mechanism to be moderately fractionating 20 40 constraints from the marine sulfur cycle allow that either elemental sulfur or organosulfur compounds or both carried s mif to the surface with opposite sign to s mif in so2 and h2so4 optimal progress requires observations from nonmarine and shallow water environments and experimental constraints on the reaction of photoexcited so2 with atmospheric hydrocarbons

a bistable organic rich atmosphere on the neoarchaean earth it has been hypothesized that before widespread oxygenation about 2 45 billion years ago the earth s atmosphere contained an organic haze similar to that on titan however these theoretical predictions have not been substantiated by geological evidence here we use multiproxy geochemical analyses of sediments from the 2 65 2 5 billion year old ghaap group in south africa to reconstruct ocean and atmospheric chemistry during this time we find evidence for oxygen production in microbial mats and localized oxygenation of surface waters carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane consistent with the prediction of a hydrocarbon haze we use a photochemical model to corroborate our geochemical data our simulations predict transitions between two stable atmospheric states one with organic haze and the other haze free the transitions are presumably governed by variations in the amount of biological methane production during the archaean eon we find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the archaean before the rise of oxygen the atmosphere of the early earth may have consisted of an organic haze geochemical data and modelling suggest that from 2 65 to 2 5 gyr ago several transitions between hazy and haze free atmospheric conditions occurred potentially linked to variations in biogenic methane production

geodynamic controls on tertiary arc magmatism in ecuador constraints from u pb zircon geochronology of oligocene miocene intrusions and regional age distribution trends abstract we obtained u pb zircon ages of late tertiary intrusions in the northern andes to provide robust time calibration points for the intrusive geochronologic framework of ecuador which is mostly based on k ar data magmatism associated with intrusion emplacement ranges from 31 to 7 ma with ages mainly pooling in the late oligocene early miocene where both k ar and u pb data exist for a given intrusive system ages obtained by the different methods are usually concordant within 1 4 m y suggesting that k ar ages are accurate indicators for the timing of tertiary arc magmatism on a regional multi m y scale except for a single sample the investigated intrusions completely lack externally inherited zircons in agreement with dominantly zircon poor oceanic crustal basement domains spatio temporal distribution trends of tertiary arc magmatism inferred from screened u pb k ar and zircon fission track geochronologic data allow tracking of the progressive along arc broadening of a flat slab segment below southernmost ecuador in the mid to late miocene and of moderate slab shallowing in northern central ecuador in the late miocene these regional arc migration patterns correlate in time with the subduction of the buoyant inca plateau and the carnegie ridge seamount chain the temporal distribution of tertiary ecuadorian arc magmatism indicates a late oligocene early miocene arc magmatic flare up event comprising widespread ignimbrite eruption and batholith construction initiation of the flare up event coincides in time with accelerating less oblique farallon nazca south america plate convergence suggesting a positive feedback between convergence rates asthenospheric melt production mantle crust melt flux and upper crustal arc magmatic productivity in ecuador

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

exhumation and preservation of porphyry cu au deposits at oyu tolgoi south gobi region mongolia field relationships and u pb data zircon are used to constrain the timing of tectonomagmatic events that occurred shortly after the formation of late devonian 372 ma porphyry cu au deposits at oyu tolgoi mongolia in the two northernmost of the several discrete porphyry centers hugo dummett deposits upper devonian rocks positioned stratigraphically above copper gold mineralization preserve a record of uplift exhumation and subsequent burial shortly after mineralization strongly altered and mineralized quartz monzodiorite and volcanic rocks at hugo dummett are overlain by an unmineralized dacite sequence 369 2 ma the basal breccia and conglomerate of the dacite sequence contain mineralized porphyry clasts derived from erosion of the porphyry deposits two zircon grains recovered from one of these mineralized porphyry clasts yielded chemical abrasion thermal ionization mass spectrometry ca tims 206 pb 238 u dates of 374 0 1 9 and 371 4 1 6 ma which are consistent with previously published ages for mineralized intrusions this implies that oyu tolgoi or other nearby porphyry deposits were exhumed shortly after the main mineralization event weakly mineralized granodiorite dikes intruded the postmineralization dacite sequence as well as devonian sedimentary and volcaniclastic rocks that structurally overlie the dacite a weighted mean 206 pb 238 u age of 370 6 0 5 ma for this rock is based on the three youngest grains analyzed by the ca tims technique mswd 1 14 burial of the cu au deposits at oyu tolgoi soon after their exhumation was critical to their preservation exhumation and subsequent burial occurred at 371 ma during a short time frame within the overlapping u pb zircon ages for the quartz monzodiorite mineralized porphyry clast unmineralized dacite sequence and granodiorite unit

the mathematics of early diagenesis from worms to waves the changes that sediments undergo after deposition are collectively known as diagenesis diagenesis is not widely recognized as a source for mathematical ideas however the myriad processes responsible for these changes lead to a wide variety of mathematical models in fact most of the classical models and methods of applied mathematics emerge naturally from quantification of diagenesis for example small scale sediment mixing by bottom dwelling animals can be described by the diffusion equation the dissolution of biogenic opal in sediments leads to sets of coupled nonlinear ordinary differential equations and modeling organisms that eat at depth in the sediment and defecate at the surface suggests the one dimensional wave equation while the effect of waves on pore waters is governed by the two or three dimensional wave equation diagenetic modeling however is not restricted to classical methods diagenetic problems of concern to modern mathematics exist in abundance these include free boundary problems that predict the depth of biological mixing or the penetration of o2 into sediments algebraic differential equations that result from the fast reversible reactions that regulate ph in pore waters inverse calculations of input functions histories and the determination of the optimum choice in a hierarchy of possible diagenetic models this review highlights and explores these topics with the hope of encouraging further modeling and analysis of diagenetic phenomena

u rich archaean sea floor sediments from greenland indications of 3700 ma oxygenic photosynthesis abstract 3700 ma metamorphosed pelagic shale from west greenland contains up to 0 4 wt reduced carbon with 13c values down to 25 6 pdb peedee belemnite the isotopic signature and mode of occurrence suggest that the carbon derived from planktonic organisms the pb isotopic composition shows that the shale had high primary u th this indicates that organic debris produced a local reducing environment which precipitated u transported to the site of sedimentation by oxidized ocean water the existence of highly productive plankton that fractionated c isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 ma

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

timing of deposition and deformation of the moodies group barberton greenstone belt south africa very high resolution of archaean surface processes abstract thin and largely discontinuous volcanic tuff beds in the alluvial fluvial and marginal marine siliciclastic strata of the middle archaean moodies group barberton greenstone belt bgb contain rare to abundant zircon dating of magmatic zircon grains from five tuff beds and one cross cutting dike yield ages which span the entire spectrum of bgb history from ca 3550 ma to 3215 ma and show variable degrees of post depositional alteration until ca 2500 ma the youngest pristine zircon grains in all tuff samples analyzed by ca id tims chemical abrasion ion dissolution thermal ion mass spectrometry yield upper intercept and concordia ages between 3227 6 0 3 and 3216 0 3 ma and those grains analyzed by la sf icp ms laser ablation sector field inductively coupled plasma mass spectrometry and shrimp sensitive high resolution ion microprobe yield ages between 3228 9 and 3214 9 ma most of these ages are either identical within error or significantly older than a ca id tims age of 3223 4 0 3 ma obtained from zircon grains of a tuff horizon of the schoongezicht formation at the top of the underlying fig tree group we interpret this age to mark the time of volcanic eruption whereas the significantly older and overlapping ages of the stratigraphically higher moodies tuff zircon grains point either to a high degree of magmatic inheritance and or a detrital contribution from surrounding sedimentary rocks during or after ash tuff deposition the minimum age for deposition of moodies group sediments and deformation is provided by a la icp ms zircon u pb age of 3219 9 ma obtained from an undeformed felsic dike which crosscuts folded moodies strata this age overlaps with a precise ca id tims u pb zircon age of 3216 0 0 3 ma obtained from a tuff horizon of the upper moodies group overall the new age data collectively imply that the up to 3 7 km thick sandstone dominated moodies group was deposited and deformed within

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

the diffusive tortuosity of fine grained unlithified sediments abstract use of fick s first law to estimate solute fluxes is arguably the most common and important calculation related to the geochemistry and environmental chemistry of fine grained unlithified sediments in such porous media the diffusion coefficient in this law must be reduced by the square of the tortuosity to account for the longer diffusive paths experimental methods to estimate tortuosity are specialized or time consuming and not routinely done an analysis of available data shows that tortuosity is accurately correlated to readily measured porosity values through the simple relation 2 1 l n 2

low pco2 in the pore water not in the archean air arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 the solar luminosity during the archean 3 8 to 2 5 billion years ago was 20 25 lower than at present and was probably compensated at least in part by a stronger greenhouse effect1 2 3 rosing et al 4 estimate the archean partial pressure of carbon dioxide to have been about 10 3 bar on the basis of the simultaneous occurrence of magnetite fe3o4 and siderite feco3 in banded iron formations bifs a type of chemical sediment here we question a central assumption by rosing et al 4 that the mineralogy of bifs reflects near thermodynamic equilibrium with the atmosphere ocean system just as the presence of authigenic pyrite in modern sediments does not imply that the modern atmosphere is anoxic the mineralogy of bifs cannot be used to argue for a low archean atmosphere carbon dioxide is therefore still a viable greenhouse gas candidate with which to explain the warm archean climate

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

what caused the rise of atmospheric o2 abstract oxygenic photosynthesis appears to have evolved well before o 2 levels increased in the atmosphere at around 2 4 ga this has led to numerous suggestions as to what may have kept o 2 suppressed and then eventually allowed it to rise these suggestions include changes in the recycling of carbon and sulfur relative to water or hydrogen a switch from dominantly submarine to dominantly subaerial volcanism gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases a decline in deposition of banded iron formations a decline in nickel availability and various proposals to increase the efficiency of photosynthesis several of these different mechanisms could have contributed to the rise of o 2 although not all of them are equally effective to be considered successful any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates which shows relatively little change with time apart from transient but occasionally spectacular excursions the reasons for this constancy are explored here but are not fully resolved in the process of making these comparisons a self consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on earth like exoplanets

spatial coincidence and similar geochemistry of late triassic and eocene oligocene magmatism in the andes of northern chile evidence from the mmh porphyry type cu mo deposit chuquicamata district the mmh porphyry type copper molybdenum deposit in northern chile is the newest mine in the chuquicamata district one of largest copper concentrations on earth mineralized eocene oligocene porphyry intrusions are hosted by essentially barren triassic granodiorites despite a century of exploitation geologists still have problems in the mine distinguishing the triassic granodiorite from the most important ore carrying eocene porphyries in the district to resolve the problem internally consistent high quality geochemical analyses of the triassic and tertiary intrusives were carried out explaining the confusion they show that the rock units in question are nearly identical in composition and thus respond equally to hydrothermal alteration in detail the only difference in terms of chemical composition is that the main eocene oligocene porphyries carry relatively less fe and ni unexpectedly the mineralized eocene oligocene porphyries have consistently less u and th than other tertiary intrusions in the district a characteristic that may be valuable in exploration the supergiant copper molybdenum deposits in the central andes were formed within a narrow interval between 45 and 31 ma close to 7 of the 200 my duration of andean magmatism which resulted from subduction of oceanic lithosphere under south america since the jurassic although recent work has shown that subduction was active on the margin since paleozoic times pre andean pre jurassic gondwanan magmatism is often described as being very different having involved crustal melting and the generation of massive peraluminous rhyolites and granites this study shows that the indistinguishable late triassic and eocene oligocene intrusions occupy the same narrow ns geographic belt in northern chile if it is accepted that magma character may determine the potential to generate economic cu mo deposits then late triassic volcano plutonic centres in the same location in the south american margin could have contained valuable ore deposits although their preservation will depend on the level attained by pre mid jurassic erosion both late triassic and eocene oligocene magmatic events occurred during the waning stages of vigorous volcano plutonic cycles and both preceded apparent gaps in igneous activity rhaetian and post oligocene abrupt lateral shifts of the volcanic front and radical changes in the character of the magmas generated both late triassic and eocene oligocene intrusions were emplaced along the same narrow strip of crust it is probable that they both exploited the same deep crustal structures the eocene oligocene magmatic front was controlled by an orogen parallel shear system caused by oblique subduction it is possible that late triassic magmatism along the same belt had a similar setting the identified rhaetian gap in subduction and magmatism may have widespread implications

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

geographical variability of the first baroclinic rossby radius of deformation global 1 83 18 climatologies of the first baroclinic gravity wave phase speed c1 and the rossby radius of deformation l1 are computed from climatological average temperature and salinity profiles these new atlases are compared with previously published 5 83 58 coarse resolution maps of l1 for the northern hemisphere and the south atlantic and with a 1 83 18 fine resolution map of c1 for the tropical pacific it is concluded that the methods used in these earlier estimates yield values that are biased systematically low by 5 15 owing to seemingly minor computational errors geographical variations in the new high resolution maps of c1 and l1 are discussed in terms of a wkb approximation that elucidates the effects of earth rotation stratification and water depth on these quantities it is shown that the effects of temporal variations of the stratification can be neglected in the estimation of c1 and l1 at any particular location in the world ocean this is rationalized from consideration of the wkb approximation

iron in precambrian rocks implications for the global oxygen budget of the ancient earth abstract banded iron formations bif are prominent in sediments older than 2 ga however little is known about the absolute abundance of bif in archean and early proterozoic sediments and the source of the fe is still somewhat uncertain also unknown is the role that fe may have played in the maintenance of low oxygen pressures in the archean and early proterozoic atmosphere an analysis of the chemical composition of precambrian rocks provides some insight into the role of fe in precambrian geochemical cycles the fe content of igneous rocks is well correlated with their ti content plots of fe vs ti in precambrian sandstones and graywackes fall very close to the igneous rock trend plots of fe vs ti in precambrian shales also follow this trend but show a definite scatter toward an excess of fe phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter mn ti relations show a stronger indication of precambrian mn loss perhaps due to weathering under a less oxidizing early atmosphere these data show that fe was neither substantially added to nor significantly redistributed in archean and early proterozoic sediments enough hydrothermal fe was added to these sediments to increase the average fe content of shales by at most a factor of 2 this enrichment would probably not have greatly affected the near surface redox cycle or atmospheric oxygen levels continued redistribution of fe and mixing with weathered igneous rocks during the recycling of precambrian sediments account for the excellent correlation of fe with ti in phanerozoic shales and for the similarity between their fe ti ratio and that of igneous rocks

reconstructing earth s surface oxidation across the archean proterozoic transition the archean proterozoic transition is characterized by the widespread deposition of organic rich shale sedimentary iron formation glacial diamictite and marine carbonates recording profound carbon isotope anomalies but notably lacks bedded evaporites all deposits refl ect environmental changes in oceanic and atmospheric redox states in part associated with earth s earliest ice ages time series data for multiple sulfur isotopes from carbonateassociated sulfate as well as sulfi des in sediments of the transvaal supergroup south africa capture the concomitant buildup of sulfate in the ocean and the loss of atmospheric massindependent sulfur isotope fractionation in phase with sulfur is the earliest recorded positive carbon isotope anomaly convincingly linking these environmental perturbations to the great oxidation event ca 2 3 ga

preservation of reactive organic matter in marine sediments abstract it is generally assumed that the reactivity of organic matter and the amount preserved in sedimentary deposits necessarily increases with total sedimentation rate in some environments such as deltas where supply of unreactive terrigenous debris may vary independently of reactive organic matter input the amount of reactive organic material preserved can in fact theoretically correlate either directly or inversely with sedimentation rate the amount preserved can be shown quantitatively by transport reaction models to depend on 1 the relative importance of electron acceptor concentration in the overlying water 2 advection during sedimentation 3 dilution by sedimentation 4 solute diffusion 5 initial flux of organic matter and 6 the magnitudes of reaction rate constants both single reaction rate constant and multiple reaction rate constant models suggest that at steady state maximum preservation with respect to a given oxidant occurs when d s k w 2 where d s whole sediment diffusion coefficient of the electron acceptor k first order rate constant of the dominant organic fraction and w sedimentation rate this is the likely basis for the reported correlation between average reactivity k of carbon in a deposit percent carbon preservation and w 1 5 w 2 because of the variety of factors which determine these relationships such correlations are probably valid only within specific classes of depositional environments

the cycling and redox state of nitrogen in the archaean ocean organisms that produce oxygen through photosynthesis existed during the late archaean eon about 2 500 million years ago but controversial evidence suggests that they may have evolved several hundred million years earlier oxygen is expected to react with oceanic nitrogen altering its redox state the reaction leaves a signature in the isotopic composition of the nitrogen bound in organic matter here we present a record of the nitrogen isotopic composition of kerogen extracted from minimally altered shales from the campbellrand malmani platform in south africa between the palaeo archaean and about 2 670 million years ago the 15n values of the kerogen rose by about 2 we interpret this increase as an indication of the onset of coupled nitrification and denitrification or anammox reactions in the surface oceans which require the presence of free oxygen a second increase in nitrogen isotopic composition around 2 520 million years ago implies instability of the n cycle with loss of fixed n this evidence for available oxygen in the oceans occurs at least 200 million years before geochemical indications of the presence of significant levels of atmospheric oxygen we suggest that coupled nitrification and denitrification drove the loss of fixed inorganic nitrogen leading to nitrogen limitation and conclude that the low levels of biologically available nitrogen limited the growth of oxygen producing plankton delaying the accumulation of oxygen in the atmosphere the initial production of oxygen in early earth s oceans altered the redox chemistry and cycling of nitrogen a record of nitrogen isotopes from preserved organic matter indicates nitrogen cycling in the presence of free oxygen 2 67 billion years ago about 200 million years before the first geochemical evidence for atmospheric free oxygen

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

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

global climate perturbations during the permo triassic mass extinctions recorded by continental tetrapods from south africa several studies of the marine sedimentary record have documented the evolution of global climate during the permo triassic mass extinction by contrast the continental records have been less exploited due to the scarcity of continuous sections from the latest permian into the early triassic the south african karoo basin exposes one of the most continuous geological successions of this time interval thus offering the possibility to reconstruct climate variations in southern laurasia from the middle permian to middle triassic interval both air temperature and humidity variations were estimated using stable oxygen 18op and carbon 13cc isotope compositions of vertebrate apatite significant fluctuations in both 18op and 13cc values mimic those of marine records and suggest that stable isotope compositions recorded in vertebrate apatite reflect global climate evolution in terms of air temperature oxygen isotopes show an abrupt increase of about 8 c toward the end of the wuchiapingian this occurred during a slight cooling trend from the capitanian to the permo triassic boundary ptb at the end of the permian an intense and fast warming of 16 c occurred and kept increasing during the olenekian these thermal fluctuations may be related to the emeishan south china and siberian volcanic paroxysms that took place at the end of the capitanian and at the end of the permian respectively vertebrate apatite 13cc partly reflects the important fluctuations of the atmospheric 13c values the differences with marine curves being likely due to the evolution of local humidity both the oxygen and carbon isotope compositions indicate that the ptb was followed by a warm and arid phase that lasted 6 ma before temperatures decreased during the late anisian toward that of the end permian environmental fluctuations occurring around the ptb that affected both continental and marine realms with similar magnitude likely originated from volcanism and methane release

application of apatite u th he thermochronometry to the determination of the sense and amount of vertical fault displacement at the chuquicamata porphyry copper deposit chile chuquicamata is the world s largest porphyry copper deposit notwithstanding the fact that a portion of the orebody has been faulted off by postmineralization movement along the west fault in order to locate the missing portion of the orebody in the vertical dimension a study was designed to estimate the sense and amount of vertical displacement along this major structure by measuring the u th he and fission track ages of vertically distributed apatite samples from each of the crustal blocks fortuna and chuquicamata intrusive complexes bordering the fault apatite u th he ages range from 32 to 16 ma whereas apatite fission track ages range from 33 to 28 ma reflecting the lower closure temperature of the u th he thermochronology method approximately 75 degrees vs approximately 125 degrees c for cooling rates of approximately 10 degrees c m y the u th he ages decrease systematically with depth in both blocks however the age elevation curve for the western fortuna block is shifted vertically with respect to the eastern chuquicamata block indicating that the postmineralization denudation was significantly greater to the west the minimum vertical displacement along the west fault is estimated to be 600 or 100 m implying that the missing portion of the chuquicamata deposit should be located at a present day elevation of at least 3 600 m the new apatite ages combined with previous thermochronometric data rb sr u pb ar ar reveal rapid cooling rates approximately 100 degrees c m y for the chuquicamata deposit following emplacement at about 35 ma thereby indicating that the cu mineralization took place at a depth of less than 4 km

porewater ph and authigenic phases formed in the uppermost sediments of the santa barbara basin abstract in this paper porewater and solid phase analyses are used in combination with in situ o 2 and ph microelectrode measurements to characterize early diagenetic processes in the uppermost sediments of the santa barbara basin california rapid reduction of dissolved oxygen nitrate solid phase manganese and iron and dissolved sulfate is observed between sediment depths of 0 and 2 cm reductive solubilization of ferric iron phases releases fe 2 adsorbed phosphate and fluoride to the porewaters and contributes to a sharp increase in porewater ph between 2 and 4 cm sulfate reduction rates peak ph levels off and acid volatile sulfides and pyrite become the dominant forms of solid phase iron saturation state calculations which depend largely on ph indicate that the porewaters of the santa barbara basin become saturated with respect to carbonate fluorapatite and calcite within the first 0 25 mm of the sediment and are highly supersaturated by and below 2 cm in spite of this result porewater evidence of phosphate and fluoride removal into a solid phase is observed only in the first 5 cm of some cores whereas dissolved ca profiles suggest dispersed calcite precipitation throughout the sediment column this finding is interpreted as an indication of the nonsteady state nature of the surface reactions that may given sufficient nucleation sites and time lead to carbonate fluorapatite genesis in anoxic sediments finally microelectrode ph profiles from two other basins in the california borderlands are presented these demonstrate that the porewaters of the santa barbara basin are more alkaline than those of other basins this outcome is attributed to the lack of particle mixing and a unique interplay between fe liberation and fes precipitation reactions in the santa barbara basin

timing and formation of porphyry cu mo mineralization in the chuquicamata district northern chile new constraints from the toki cluster the recently discovered toki cluster which includes the toki quetena genoveva miranda and opache porphyry cu mo prospects is located 15 km south southwest of the chuquicamata radomiro tomic mines in northern chile these prospects occur in an area of 5 6 km and are completely covered with neogene alluvial deposits inferred resources for the cluster are estimated at about 20 mt of fine copper with toki and quetena contributing 88 of these resources mineralization in these deposits is associated with tonalite porphyries that intruded andesites and dacites of the collahuasi group and intrusions of the fortuna los picos granodioritic complex hypogene mineralization in the toki cluster consists mainly of chalcopyrite bornite with minor molybdenite with mineralization grading outward to a chalcopyrite pyrite zone and ultimately to a pyrite halo alteration is dominantly of the potassic type with k feldspar and hydrothermal biotite sericitic alteration is relatively restricted to late quartz pyrite veins d type veins previous k ar geochronology for the cluster yielded ages within a range of 34 to 40 ma four new re os ages for toki indicate that molybdenite mineralization occurred in a single pulse at 38 ma re os ages for three different molybdenite samples from quetena are within error of the toki mineralization ages these ages are concordant with a new zircon u pb age of 38 6 0 7 ma from the tonalite porphyry in quetena two re os ages for genoveva 38 1 0 2 and 38 0 0 2 ma are also within error of the toki and quetena molybdenite ages four re os molybdenite ages for opache range between 36 4 and 37 6 ma the miranda prospect is the youngest with an age of 36 ma four new re os ages for the chuquicamata deposit range between 33 and 32 ma whereas nine new 40ar 39ar ages of biotite muscovite and k feldspar range between 32 and 31 ma analyzed molybdenites have re and os concentrations that vary between 21 3 099 ppm and 8 1 231 ppb respectively the highest re and os concentrations are found in the toki prospect three new 40ar 39ar ages for the toki cluster are younger than the re os mineralization ages the age spectra for these three samples show evidence of excess argon and have similar inverse isochron ages of 35 ma that probably reflect a late hydrothermal phyllic event the new geochronological data presented here for the toki cluster indicate that molybdenite mineralization occurred within a very short period probably within 2 ma and synchronously at 38 ma in three mineralization centers toki quetena and genoveva furthermore mineralization at the toki cluster preceded the emplacement of the chuquicamata deposit 35 31 ma and indicates that porphyry cu mo mineralization occurred episodically over a period of several million years in the chuquicamata district

u pb geochronologic constraints on the crustal evolution of the long range inlier newfoundland the long range mountains of newfoundland expose the northeastern most basement inlier of the appalachian orogen u pb results for two samples of basement gneiss reveal crustal formation ages of 1466 10 ma western brook pond charnockite and 1530 8 ma cat arm road gneiss indicating an affinity with pinwarian magmatism 1510 1450 ma in labrador a third sample of basement gneiss from the cat arm road dam site yielded a minimum formation age of 1631 ma these basement gneisses were intruded by granite plutons during two periods of grenvillian magmatism group i at 1032 1022 ma and group ii at 993 985 ma group i intrusions include the 1032 0 1 5 ma lomond river granite and the 1022 0 2 0 ma lake michel igneous suite group ii intrusions include the 999 4 ma potato hill charnockite 993 7 ma horse chops granite 984 9 1 6 ma cloud river granite and may include the apsy granite the metamorphic history of the long range inlier is complex and three discrete proterozoic metamorphic events can

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

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

geology and geochemistry of paleosols developed on the hekpoort basalt pretoria group south africa the hekpoort paleosols comprise a regional paleoweathering horizon developed on 2 224 0 021 ga basaltic andesite lavas at the top of the hekpoort formation of the pretoria group transvaal supergroup south africa in five separate profiles from outcrops along road cuts near waterval onder and the daspoort tunnel and in three drill cores from the bank break area bb3 bb8 and bb14 the top of the paleosol is a sericite rich zone the sericite zone grades downward into a chlorite rich zone in core bb8 and in the road cut at the daspoort tunnel we sampled the underlying or parent basaltic andesite into which the chlorite zone grades we did not obtain samples of the parent material at waterval onder and in cores bb3 and bb14 but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core bb8 and at the daspoort tunnel the presence of apparent rip up clasts of the paleosol in the overlying ironstones of the strubenkop formation in the cores from bank break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at waterval onder point to a subaerial weathering origin very little if any al ti zr v or cr moved a discernible distance during weathering of any of the five profiles the vertical distribution of fe mg mn ni and co indicates that these elements were largely removed from the top of the soil during weathering the overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an fe 2 rich smectite the loss of fe from the top of the soil during weathering of the hekpoort paleosols indicates that atmospheric p o 2 was less than 8 x 10 4 atm about 2 22 ga fe 2 rich smectite should only precipitate during soil formation if atmospheric p co 2 is less than or equal to 2 x 10 2 atm rye kuo and holland 1995 ca and na were largely lost during weathering some na was apparently added to the sericite zone in cores bb3 bb8 and bb14 after weathering all five profiles are enriched in k and rb and most are enriched in ba the distribution of these elements indicates that they all were added during post weathering hydrothermal metasomatism rb sr analysis of the paleosol at the daspoort tunnel indicates that metasomatism last affected that profile 1 925 0 032 ga macfarlane and holland 1991

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

precambrian paleosols and atmospheric co2 levels abstract precambrian atmospheric pco2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols in addition to theoretical flaws with that model recalculation of pco2 levels using more recent thermodynamic data gives significantly lower values a new model based on the mass balance of weathering in paleosols gives consistent results from three separate 2 2 ga old paleosols the calculated pco2 value of 23 3 3 times present atmospheric levels is insufficient to overcome the faint young sun paradox suggesting that another greenhouse gas such as ch4 was present at elevated levels relative to the present atmosphere to account for the discrepancy applying this new approach to other precambrian paleosols a proterozoic pco2 curve is generated which indicates consistently high pco2 from 2 5 to 1 8 ga ago and a substantial drop in atmospheric pco2 at some time between 1 8 and 1 1 ga ago

two ages of porphyry intrusion resolved for the super giant chuquicamata copper deposit of northern chile by ela icp ms and shrimp zircon u pb ages measured in situ by excimer laser ablation inductively coupled plasma mass spectrometry ela icp ms and verified by sensitive high resolution ion microprobe shrimp on ore bearing felsic porphyries from the chuquicamata porphyry copper molybdenum deposit northern chile identify two discrete igneous events the volumetrically dominant east porphyry has an age of 34 6 0 2 ma whereas the bench and west porphyries yield ages of 33 3 0 3 ma and 33 5 0 2 ma respectively the age of the east porphyry is indistinguishable from a re os age for early molybdenite mineralization 35 ma and the oldest reported 40 ar 39 ar ages for hydrothermal alteration confirming a genetic link with mineralization previous geological studies and 40 ar 39 ar and re os geochronology identify two main hydrothermal events high temperature potassic alteration with chalcopyrite at 33 4 0 3 ma followed by lower temperature quartz sericite alteration with pyrite at 31 1 0 3 ma the ages of the west and bench porphyries match the ages for potassic alteration younger quartz sericite alteration may reflect an additional fourth intrusion concealed at depth the anomalously large size of chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic hydrothermal systems

early permian to late triassic batholiths of the chilean frontal cordillera 28 31 s shrimp u pb zircon ages and lu hf and o isotope systematics abstract one of the major geological units of the main andean range frontal cordillera of north central chile is a group of composite and heterochronous late paleozoic early mesozoic batholiths that extends for 500 km roughly ns along from 26 to 31 s ten new shrimp zircon crystallization ages together with 11 recently published u pb zircon ages by other authors indicate an episodic intrusion history which can be divided in 4 groups mississippian earliest carboniferous 330 326 ma cisuralian earliest permian 301 284 ma latest permian to middle triassic 264 242 ma and late triassic 225 215 ma volcanic rocks in the area span a similar time lu hf and o isotopic systematics in zircon grains from eight of the plutonic rocks indicate the magma source areas have contributed variable amounts of crustal and mantle components zircon 18 o values evolve from crustal values 7 in the earliest permian intrusives to mantle values in the latest permian to upper triassic including evidence for likely hydrothermal alteration of the source 4 zircon ehf values vary in a good linear correlation with the 18 o isotopes from 6 to 0 in rocks older than 270 ma increasing to 2 to 7 from lower to upper triassic 250 to 215 ma the petrogenetic constraints indicated by these values suggest that the influence of magma sources varied with time from predominantly crustal to mantle like in accord with the regional tectonic models the earliest permian rocks were generated in a subduction related magmatic arc which varied towards an extension related environment in the latest permian and triassic

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

uranium isotope fractionation suggests oxidative uranium mobilization at 2 50ga abstract geochemical data from late archean sedimentary rocks point to photosynthetic o 2 production and at least intermittent occurrences of locally mild oxidative weathering and surface ocean oxygenation oxygen oases prior to the early paleoproterozoic great oxidation event for example distinctive authigenic enrichments of mo and re in euxinic anoxic and sulfidic black shales are best explained by the oxidative mobilization of these metals from crustal sulfide minerals and their accumulation as oxyanions in seawater in contrast it is not clear if low u enrichments in the same shales reflect negligible oxidation of u from the upper crust or a very small oceanic u inventory that was derived from oxidative u mobilization here we report u isotope data for the 2 50 ga mt mcrae shale hamersley basin western australia which provides a more sensitive test for the presence or absence of authigenic u compared to u concentrations and enrichment factors normalized to average shale or upper crustal compositions that may not be representative of the local detrital composition we find instances where the u isotope composition in the upper mt mcrae shale 238 u 0 2 to 0 0 relative to standard srm950a is isotopically heavier than average upper crust 238 u 0 31 0 14 2sd based on granitoids and basalts the high 238 u values point to u isotope fractionation in the late archean marine environment and hence indicate the presence of a small amount of dissolved u in seawater and authigenic u in the mt mcrae shale volume dependent equilibrium isotope fractionation during the reduction of dissolved u vi to insoluble u iv like that observed in the modern black sea may explain the high 238 u signatures if u removal from bottom waters was not quantitative alternatively quantitative u removal would require that late archean seawater had high 238 u which could have arisen from the preferential sequestration of 235 u to fe oxyhydr oxide minerals elsewhere in the hamersley basin the supracrustal 238 u signatures are associated with some of the highest mo and re enrichments in the mt mcrae shale as well as distinctive mo s and n isotope signatures that are indicative of mild environmental oxygenation hence our findings suggest that small amounts of u were oxidatively mobilized from the upper crust at 2 50 ga unlike mo and re however u oxidation may not have occurred on land instead we hypothesize that oxidative u mobilization occurred by submarine weathering in an oxygen oasis

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

metamorphic textures and geochemistry of the cyprus type massive sulfide lenses at zurabad khoy iran abstract the upper cretaceous volcanogenic exhalative fe cu massive sulfide lenses at zurabad iran occur within spilitized basalts of the khoy ophiolite both the host rocks and the sulfide lenses are metamorphosed to greenschist facies and have been subjected to weak deformation causing textural changes in the ore the presence of preserved banded pyrite pyrrhotite chalcopyrite and sphalerite in a matrix rich in chlorite indicates that the ore was a primary facies of synvolcanic hydrothermal massive sulfide mineralization similar to those formed in modern seafloor environments and in cyprus type ophiolite deposits coarse grained pyrite porphyroblasts or crystalloblasts recrystallized pyrrhotite and chalcopyrite together with regional silicate sulfide foliation were produced by sub seafloor metamorphism followed by regional metamorphism and ophiolite obduction brittle deformation and shearing produced cataclastic textures in pyrite which were filled by chalcopyrite and pyrrhotite post obduction and or post metamorphic events produced late veinlets of pyrite chalcopyrite and gangue minerals supergene processes related to gossan formation converted pyrite pyrrhotite and chalcopyrite to marcasite and birdseye pyrite primary sphalerite contains an average of 14 mole fes which shows equilibration at temperatures below 240 c and a pressure less than 1 kilobar based on its equilibrated textures with other sulfides and silicates in the foliation pyrrhotite was a primary mineral in the zurabad volcanogenic massive sulfide lenses therefore pyrrhotite can be primary and with pyrite forms a buffer that constrains sulphur fugacity during regional metamorphism on the evidence of textures mineralogy and geochemistry pyrite is stable in the greenschist facies provided that the activity of sulphur remains high of prime importance in exploration of the massive sulfide lenses in the zurabad area has been the recognition of banded gossans and coarse pyrite porphyroblasts which may prove to be a successful exploration tool in the search for further massive sulfide mineralization in the district

mineralogical constraints on precambrian pco2 arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 rosing and others1 recently proposed a new model for the early earth s atmospheric composition they suggest on the basis of mineral assemblages in sedimentary rocks that atmospheric co2 and ch4 concentrations have been consistently overestimated this proposal is intriguing given that high concentrations of these gases are traditionally considered to be the solution to the faint young sun paradox of why the earth did not freeze over in our sun s youth when solar luminosity was greatly decreased rosing and others1 instead invoke a decreased planetary albedo during the earth s early history new insights into this problem which has long captivated those interested in the earth s early climate and biological history are welcome however we believe that the arguments presented by rosing and others1 for roughly modern values of the partial pressure of co2 are not robust

microbial fixation of methane carbon at 2 7 ga was an anaerobic mechanism possible 1 in earth s history the most extreme episode of 13c depletion in sedimentary organic matter occurred about 2 7 billion years ago this isotopic excursion is widely interpreted as resulting from increased burial of organic carbon derived from microbial fixation of 13c depleted methane modern microbial physiologies involve either oxygen or sulfate as electron acceptors in such processes until now the isotopic signal has been exclusively assigned to growth of aerobic methanotrophic bacteria and consequently interpreted as an indication for the presence of molecular oxygen in late archaean surface environments the alternative anaerobic pathway involving sulfate as the electron acceptor was dismissed because this process has been considered inefficient in producing biomass sufficient to account for significant fractions of 13c depleted carbon in some archaean rocks calculations based on recent findings at two modern sites now show however that anaerobic processes could account for the archaean signal

geodynamic evolution of the northern and central andes during early to middle mesozoic times a tethyan model the early to middle mesozoic sedimentary and magmatic history of the northern and central andes i e the nne trending colombian ecuadorian segment and the nw trending peruvian segment exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings the late triassic to late liassic period began with a widespread marine transgression from the latest triassic the colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in peru during this time no significant magmatic activity is recorded except in the emerging colombian area this period is interpreted as the result of rifting of a tethyan oceanic arm which separated the colombian and palaeo mexican margins from latest liassic an important calc alkaline magmatic arc developed along the emergent colombian segment further south the north peruvian shelf probably emerged but marine sedimentation continued in southern peru during middle and early late jurassic times the colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation along the peruvian segment a turbiditic trough emergent areas and continental basins were created and the scarcity of calc alkaline magmatism suggests that only very local subduction took place this period is regarded as one of the southeastward subduction beneath the colombian segment and of tethyan oceanic crust originating in the newly formed colombian oceanic arm this pattern would have induced a chiefly left lateral transform motion along the peruvian segment by kimmeridgian tithonian times the palaeogeographic framework had drastically changed along the colombian segment magmatic activity ceased and continental accretions occurred along dextral strike slip sutures in peru tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation in the coastal area arc related volcanism indicated that subduction took place beneath this segment this geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the tethyan ridges and replacement by pacific spreading centres inducing a roughly northeastward convergence direction

sediment residence times constrained by uranium series isotopes a critical appraisal of the comminution approach abstract quantifying the rates of landscape evolution in response to climate change is inhibited by the difficulty of dating the formation of continental detrital sediments we present uranium isotope data for cooper creek palaeochannel sediments from the lake eyre basin in semi arid south australia in order to attempt to determine the formation ages and hence residence times of the sediments to calculate the amount of recoil loss of 234u a key input parameter used in the comminution approach we use two suggested methods weighted geometric and surface area measurement with an incorporated fractal correction and typical assumed input parameter values found in the literature the calculated recoil loss factors and comminution ages are highly dependent on the method of recoil loss factor determination used and the chosen assumptions to appraise the ramifications of the assumptions inherent in the comminution age approach and determine individual and combined comminution age uncertainties associated to each variable monte carlo simulations were conducted for a synthetic sediment sample using a reasonable associated uncertainty for each input factor and including variations in the source rock and measured 234u 238u ratios the total combined uncertainty on comminution age in our simulation for both methods of recoil loss factor estimation can amount to 220 280 ka the modelling shows that small changes in assumed input values translate into large effects on absolute comminution age to improve the accuracy of the technique and provide meaningful absolute comminution ages much tighter constraints are required on the assumptions for input factors such as the fraction of recoil lost 234th and the initial 234u 238u ratio of the source material in order to be able to directly compare calculated comminution ages produced by different research groups the standardisation of pre treatment procedures recoil loss factor estimation and assumed input parameter values is required we suggest a set of input parameter values for such a purpose additional considerations for calculating comminution ages of sediments deposited within large semi arid drainage basins are discussed

climatic and vegetation control on sediment dynamics during the last glacial cycle as climate is changing rapidly there is an increasing need to understand how water and soil resources respond to climate change soil and sediment dynamics are sensitive to several external factors such as climate vegetation type and distribution human activity and tectonic activity however the relationship between erosion and changes in these factors is difficult to constrain with current available approaches here we show that uranium isotopes in sediments from river paleochannels can be used to reconstruct variations in the residence time of sediments in a catchment over the past 100 k y we find that sediment residence times increase by an order of magnitude during interglacials compared to glacial periods this is interpreted as a change in sediment stores in the landscape that are tapped by catchment erosion young upland soils during glacial periods reworking of old alluvial sediments during interglacials a direct correlation is found between the sediment residence time and climatic parameters sea surface temperature atmospheric carbon dioxide content and paleorainfall estimates suggesting that during a glacial cycle sediment dynamics closely follow variations in climate however this relationship is not simple because there is no correlation between sediment residence time and paleodischarge estimates because sediment residence time variations correlate with changes in vegetation inferred from pollen data it is hypothesized that the influence of climate on erosion over a glacial cycle may be indirect and operates via the influence of climate on the type of plant ecosystems within a catchment if verified elsewhere this conclusion would emphasize the important role of biology in the physical evolution of earth s surface here observed over a 100 k y time scale

relationship between copper tonnage of chilean base metal porphyry deposits and os isotope ratios re os isotopes in pyrite and chalcopyrite from early high temperature hypogene alteration assemblages in chilean porphyry copper deposits identify the source of os and by inference cu in these ore systems typical concentrations for os in both pyrite and chalcopyrite are between 7 and 30 ppt 10 12 g g and for re between 0 200 and 10 ppb 10 9 g g re os isochrons yield ages and initial 187 os 188 os ratios for the sulfides the isotopic data reflect the relative contributions of copper from the mantle and crust in chilean porphyry copper deposits seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in chile were studied the initial osmium ratios of the first stage of mineralization at each of the deposits range from 0 15 to 5 these values are more radiogenic than the present chondritic mantle 0 13 and indicate significant crustal contributions of os to the magmatic and or hydrothermal systems there is a strong correspondence between the total copper content and initial os isotopic ratios in base metal porphyry deposits the larger deposits have lower initial os ratios than the smaller less significant deposits this relationship implies that larger deposits acquire a greater proportion of os from the mantle the initial os ratio of samples in the central segment of porphyry copper deposits of northern chile also decreases with decreasing age of the deposit a plausible interpretation of the re os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources

fault zone development and strain partitioning in an extensional strike slip duplex a case study from the mesozoic atacama fault system northern chile abstract upper crustal strike slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system detailed field mapping of the mesozoic atacama fault system in the coastal cordillera of northern chile documents the progressive development of second and third order faults forming a duplex at a dilational jog between two overstepping master faults the sinistral strike slip nnw striking jorgillo and bolfin faults these are constituted by a meter wide core of foliated s c ultracataclasite and cataclasite flanked by a damage zone of protocataclasite splay faults and veins lateral separation of markers along master faults is on the order of a few kilometers second order nw striking oblique slip subsidiary fault zones do not show foliated ultracataclasite lateral sinistral separations are in the range of 10 to 200 m with a relatively minor normal dip slip component in turn third order east west striking normal faults exhibit centimetric displacement oblique slip sinistral normal fault zones located at the southern termination of the bolfin fault form a well developed imbricate fan structure they exhibit a relatively simple architecture of extensional and extensional shear fractures bound by low displacement shear fractures kinematic analysis of fault slip data from mesoscopic faults within the duplex area document that the nw striking and the ew striking faults accommodate transtension and extension respectively examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement we interpret that the duplex formed by progressive linkage of horsetail like structures at the southern tip of the bolfin fault that joined splay faults coming from the jorgillo and coloso faults the geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike slip fault zones in the upper crust

jurassic to palaeogene tectono magmatic evolution of northern chile and adjacent bolivia from detrital zircon u pb geochronology and heavy mineral provenance heavy mineral provenance data presented in this article reinforce evidence for significant late palaeogene deformation and relief formation along the western margin of the central andean plateau late eocene oligocene onset of molasse type sedimentation records initial range uplift strikingly different basement sources of sediments deposited to the east and west of the late palaeogene range indicate that initial relief development was governed by a bivergent thrust system significantly higher sediment accumulation rates to the east of the range compared to the west suggest that the generated relief acted already as an effective orographic barrier at that time higher precipitation and denudation along the eastern slope facilitated deeper erosion of the trust belt

oxidative sulfide dissolution on the early earth recent evidence suggests that the biological production of oxygen in earth s surface oceans may have preceded the initial accumulation of large amounts of oxygen in the atmosphere by 100 million years or more however the potential effects of early oxygen production on surface ocean chemistry have remained little explored and questions persist regarding the locus of oxidation of crustal material i e subaerial and or submarine settings here we revisit the notion of spatially restricted oxygen oases in the archean surface ocean by employing a simple steady state box model of the surface ocean in a coastal upwelling system using pyrite as an example we then explore the possibility that oxygenic photosynthesis in such a system could support the widespread oxidation of crustal sulfide minerals without concomitant accumulation of oxygen in the atmosphere we find that it is possible to establish strong air sea gas exchange disequilibrium with respect to o 2 however in marine settings there is an apparent timescale mismatch between the kinetics of oxidative dissolution and the rate at which sulfide minerals delivered physically to shallow marine sediments will be buried below the zone of oxygen penetration estimated dissolution timescales compare somewhat more favorably with typical timescales of soil development and physical weathering transport in the subaerial realm despite the much lower dissolved oxygen concentrations inferred at gas exchange equilibrium with atmospheric po 2 relative to those attainable in productive regions of the surface ocean however broad scale subaerial weathering of sulfides would likely have required either local o 2 production at the site of weathering or transient increase in atmospheric po 2 above the most plausible levels characteristic of a pervasively reducing atmosphere

theoretical constraints on oxygen and carbon dioxide concentrations in the precambrian atmosphere simple one dimensional climate models suggest that carbon dioxide concentrations during the archean must have been at least 100 1000 times the present level to keep the earth s surface temperature above freezing in the face of decreased solar luminosity such models provide only lower bounds on co2 so it is possible that co2 levels were substantially higher than this and that the archean climate was much warmer than today periods of extensive glaciation during the early and late proterozoic on the other hand indicate that the climate at these times was relatively cool to be consistent with climate models co2 partial pressures must have declined from approximately 0 03 to 0 3 bar around 2 5 ga ago to between 10 3 and 10 2 bar at 0 8 ga ago this steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data which implies that pco2 did not change appreciably during that time oxygen was essentially absent from the earth s atmosphere and oceans prior to the emergence of a photosynthetic source probably during the late archean during the early proterozoic the atmosphere and surface ocean were apparently oxidizing while the deep ocean remained reducing an upper limit of 6 x 10 3 bar for po2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean the establishment of oxidizing conditions in the deep ocean marked by the disappearance of banded iron formations approximately 1 7 ga ago permitted atmospheric oxygen to climb to its present level o2 concentrations may have remained substantially lower than today however until well into the phanerozoic

molybdenum isotope evidence for mild environmental oxygenation before the great oxidation event abstract we use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of o2 in the environment 50 ma before the great oxidation event goe in a high resolution profile from the 2 5 ga mt mcrae shale the molybdenum isotope compositions 98 95mo from samples throughout the sequence span a range from 0 99 to 1 86 all samples have heavier 98 95mo compared to average upper continental crust in addition the upper s1 and lower s2 black shale units within the mt mcrae shale exhibit systematic differences in average isotopic compositions and distinct patterns of 98 95mo variation heavier 98 95mo values occur in the s1 unit where 98 95mo correlates with mo enrichments in the s2 unit 98 95mo is not as heavy and is relatively invariant based on sedimentary fe proxies we infer that s1 sediments were deposited under euxinic conditions so that mo removal from bottom waters was likely quantitative thus 98 95mo in this interval likely records coeval seawater the lighter 98 95mo values in the s2 unit may indicate a less fractionated ocean mo inventory relative to the s1 unit however sedimentary fe proxies suggest that s2 sediments accumulated under a water column that was ferruginous rather than euxinic raising the possibility of non quantitative mo scavenging and hence an expressed 98 95mo fractionation relative to coeval seawater because any associated fractionations during this process would have favored the light isotope in sediments the lighter 98 95mo values in the s2 unit represent a lower limit on the value in contemporaneous seawater after evaluating a range of hypotheses we conclude that the isotopically heavy 98 95mo values seen throughout the mt mcrae shale likely reflect the effects of oxidative weathering and adsorption of mo to oxide mineral surfaces on land or in surface oceans the extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global mo isotope budget because of the small ocean mo inventory in the late archean documented by low concentrations of mo and low mo toc the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans however when juxtaposed against the record of 98 95mo through time our findings provide further evidence of the onset of environmental oxygenation before the goe

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

towards accurate numerical calibration of the late triassic high precision u pb geochronology constraints on the duration of the rhaetian numerical calibration of the late triassic stages is arguably the most controversial issue in mesozoic stratigraphy despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end triassic mass extinction here we report new chemical abrasion isotope dilution thermal ionization mass spectrometry zircon u pb dates for volcanic ash beds within the aramachay formation of the pucara group in northern peru that place precise constraints on the age of the norian rhaetian boundary nrb and the duration of the rhaetian the sampled ash bed bearing interval is located just above the last occurrence of the bivalve monotis subcircularis placing this stratigraphic sequence in the uppermost norian perhaps ranging into the earliest rhaetian zircon u pb dates of ash beds constrain the deposition age of this interval to be between 205 70 0 15 ma and 205 30 0 14 ma providing precise constraints on the age of the nrb combined with previously published zircon u pb dates for ash beds bracketing the triassic jurassic boundary we estimate a duration of 4 14 0 39 m y for the rhaetian this ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end triassic mass extinction

late archean euxinic conditions before the rise of atmospheric oxygen life on earth is thought to have coevolved with the chemistry of the oceans and atmosphere and the shift from an anoxic to an oxic world across the archean proterozoic boundary represents a fundamental step in this process in order to understand the relative influence of biological and geological factors on this transition we must constrain key variables in seawater chemistry before the great oxidation event ca 2500 ma we present a multielement c s fe mo biogeochemical study of ca 2662 ma shales from the hamersley province in western australia our data reveal a sustained episode of fe limited pyrite formation under an anoxic and sulfidic euxinic water column this is the oldest known occurrence of euxinia in earth9s history and challenges the paradigm of persistently fe rich archean oceans bulk trace metal chemistry and preservation of strong mass independent s isotope fractionations in sedimentary pyrites indicate that ocean euxinia was possible prior to oxidative weathering suggesting that sulfidic waters may have been common throughout the archean eon c s fe systematics suggest that oxygenic photosynthesis was the primary source of organic carbon in the basin and the absence of mo enrichments highlights a potential link between inefficient nitrogen fixation and the delayed arrival of the great oxidation event

soil development at snail s pace evidence from a 6 ma soil chronosequence on basalt in north queensland australia abstract in semi arid tropical australia basaltic lava flows with ages of 0 01 0 89 2 46 2 64 3 4 and 5 59 ma show a trend of progressive soil thickness increase of ca 0 30 m ma this rate is 10 to 1000 times slower than most soils on unconsolidated parent materials such as alluvium dune sand and glacial deposits reverse polarity magnetisation of pedogenic hematite in the lower b horizons of soils on the four oldest flows indicates acquisition of the remanence prior to the brunhes matuyama polarity transition at 0 78 ma preservation of such a reverse polarity implies that the remanence carrier pedogenic hematite has been unchanged by chemical weathering or physical disturbance by soil biota for at least 0 78 ma a conceptual model of soil development is presented which implies that prior to 0 78 ma soil weathering may have occurred to a greater depth because summer rainfall was significantly higher than present the extremely slow rate of soil formation implied from these basaltic soils in northern australia is a clear warning that agricultural use of such soils is unlikely to be sustainable against accelerated surface erosion

the evolution of the global selenium cycle secular trends in se isotopes and abundances abstract the earth s surface has undergone major transitions in its redox state over the past three billion years which have affected the mobility and distribution of many elements here we use se isotopic and abundance measurements of marine and non marine mudrocks to reconstruct the evolution of the biogeochemical se cycle from 3 2 gyr onwards the six stable isotopes of se are predominantly fractionated during redox reactions under suboxic conditions which makes se a potentially valuable new tool for identifying intermediate stages from an anoxic to a fully oxygenated world 82 78 se shows small fractionations of mostly less than 2 throughout earth s history and all are mass dependent within error in the archean especially after 2 7 gyr we find an isotopic enrichment in marine 0 37 0 27 relative to non marine samples 0 28 0 67 paired with increasing se abundances student t tests show that these trends are statistically significant although we cannot completely rule out the possibility of volcanic se addition these trends may indicate the onset of oxidative weathering on land followed by non quantitative reduction of se oxyanions during fluvial transport the paleoproterozoic great oxidation event goe is not reflected in the marine 82 78 se record however we find a major inflection in the secular 82 78 se trend during the neoproterozoic from a precambrian mean of 0 42 0 45 to a phanerozoic mean of 0 19 0 59 this drop probably reflects the oxygenation of the deep ocean at this time stabilizing se oxyanions throughout the water column since then reduction of se oxyanions has likely been restricted to anoxic basins and diagenetic environments in sediments in light of recent cr isotope data it is likely that oxidative weathering before the neoproterozoic produced se oxyanions in the intermediate redox state se iv whereas the fully oxidized species se vi became more abundant after the neoproterozoic rise of atmospheric oxygen

reported supergene sphalerite rims at the chuquicamata porphyry deposit northern chile revisited evidence for a hypogene origin previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in chuquicamata in a mineral assemblage including typically chalcocite digenite cu1 85s1 12 and u1 99s1 01 covellite cus and cu1 08s0 92 and sphalerite up to 1 2 wt fe microscopic observations on samples from a central and a southern section completed by scanning electronic microscope sem backscattering electron imaging and electron microprobe analyses suggest that all sphalerite in chuquicamata is hypogene a scenario backed by observations in each step that explains the formation of the peculiar sphalerite rims is the following 1 precipitation of chalcopyrite typical of the early and main hydrothermal stages 2 precipitation of sphalerite rimming chalcopyrite and in voids and in weakness sites during the late hydrothermal stage 3 partial or total replacement of chalcopyrite by chalcocite digenite during the late hydrothermal stage and 4 formation of lamellar covellite principally at the expense of chalcopyrite suggesting increasingly oxidizing and or acidic conditions this covellite may be linked to supergene processes and or to the late hydrothermal stage the findings of the present work have implications for the position of the lower limit of the supergene enrichment in the eastern southern part of the deposit as the sphalerite rims now interpreted as hypogene were the only potential supergene sulfide in the eastern vertical section of the southern section

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

sediment transport time measured with u series isotopes results from odp north atlantic drift site 984 abstract high precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition the approach is demonstrated on fine grained late pleistocene deep sea sediments from ocean drilling program site 984a on the bjorn drift in the north atlantic the sediments are siliciclastic with up to 30 carbonate and dated by 18o of benthic foraminifera nd and sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from iceland and a distal continental source during glacial periods an unexpected finding is that the 234u 238u ratios of the silicate portion of the sediment isolated by leaching with hydrochloric acid are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance the 234u depletions are inferred to be due to recoil loss of 234th and are used to calculate comminution ages of the sediment the time elapsed between the generation of the small 50 m sediment grains in the source areas by comminution of bedrock and the time of deposition on the seafloor transport times the difference between comminution ages and depositional ages vary from less than 10 ky to about 300 to 400 ky for the site 984a sediments long transport times may reflect prior storage in soils on continental shelves or elsewhere on the seafloor transport time may also be a measure of bottom current strength during the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from iceland that is rapidly transported to the site of deposition the comminution age approach could be used to date quaternary non marine sediments soils and atmospheric dust and may be enhanced by concomitant measurement of 226ra 230th 230th 234u and cosmogenic nuclides

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

late paleozoic early triassic magmatism on the western margin of gondwana collahuasi area northern chile abstract the basement in the altiplano high plateau of the andes of northern chile mostly consists of late paleozoic to early triassic felsic igneous rocks collahuasi group that were emplaced and extruded along the western margin of the gondwana supercontinent this igneous suite crops out in the collahuasi area and forms the backbone of most of the high andes from latitude 20 to 22 s rocks of the collahuasi group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main andes of chile the frontal cordillera of argentina choiyoi group or choiyoi granite rhyolite province and the eastern cordillera of peru thirteen new shrimp u pb zircon ages from the collahuasi area document a bimodal timing for magmatism with a dominant peak at about 300 ma and a less significant one at 244 ma copper mo porphyry mineralization is related to the younger igneous event initial hf isotopic ratios for the 300 ma zircons range from about 2 to 6 indicating that the magmas incorporated components with a significant crustal residence time the 244 ma magmas were derived from a less enriched source with the initial hf values ranging from 2 to 6 suggestive of a mixture with a more depleted component limited whole rock 144nd 143nd and 87sr 86sr isotopic ratios further support the likelihood that the collahuasi group magmatism incorporated significant older crustal components or at least a mixture of crustal sources with more and less evolved isotopic signatures

characterisation of triassic rifting in peru and implications for the early disassembly of western pangaea abstract triassic igneous and sedimentary rocks exposed within the basement of the andes were deposited in a series of rifts and may record the early disassembly of western pangaea these rocks are sporadically exposed along almost the entire length of western south america although reliable geochronological and isotopic data are sparse we combine new geochronological zircon u pb isotopic hf nd and geochemical data with stratigraphic observations to constrain the age and tectonic setting of the mitu rift of southern peru the peruvian mitu rift is compared with other triassic rifts in colombia and ecuador palanda rift 240 225 ma bolivia mitu rift triassic bolivia chile and argentina e g cuyo basin 246 230 ma and conclusions are reached regarding the relationship between triassic extension along the western margin of pangaea and the eventual formation of the proto caribbean and central atlantic oceans the mitu rift peru was active during 245 240 to 220 ma and was synchronous with rifting along the pacific margin of colombia and ecuador along the chilean margin and western argentina and probably rifting within bolivia rifting north of the huancabamba deflection was accompanied by subduction and led to seafloor spreading whereas rifting along the peruvian and chilean margins mainly occurred in the absence of subduction and terminated prior to the formation of extensive transitional crust extension within peru and chile probably occurred via a combination of transtension steepening and detachment of an arrested slab we propose that plate tectonic forces initiated the early break up of pangaea by attenuating its margins and enhancing mantle upwelling prolonged extension may have propagated along pre existing weak zones that extended into the continental interior captured melts derived from the upwelled mantle forming a lip e g central atlantic magmatic province became hot and weak and eventually lead to the formation of a juvenile ocean e g central atlantic

on the nature of methane gas hydrate dissociation during the toarcian and aptian oceanic anoxic events the magnitude and timing of a major rapid negative carbon isotope excursion recorded in marine and terrestrial matter through the early toarcian early jurassic and early aptian early cretaceous oceanic anoxic events oaes have been proposed to be the result of large methane gas hydrate dissociation events here we develop and evaluate a global carbon isotope mass balance approach for determining the responses of each component of the exogenic carbon cycle terrestrial biosphere atmosphere and ocean the approach includes a dynamic response of the terrestrial carbon cycle to methane related co2 increases and climatic warming our analyses support the idea that both the early toarcian and early aptian isotopic curves were indicative of large episodic methane releases 5000 and 3000 gt respectively promoting warm greenhouse conditions in the mesozoic these events are calculated to have increased the atmospheric co2 concentration by 900 and 600 ppmv respectively and land surface temperatures by 2 5 to 3 0 c however we show that much of the methane released from oceanic sediments is rapidly sequestered by terrestrial and marine components in the global carbon cycle and this effect strongly attenuated the potential for ancient methane gas hydrate dissociation events to act as major amplifiers in global warming an increase in oceanic carbon sequestration is consistent with the deposition of globally distributed black shales during these oaes our analyses point to the urgent need for high resolution marine and terrestrial carbon isotope records to better characterize the nature of the toarcian and aptian events and improve our interpretation of their consequences for the global carbon cycle

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

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

atmospheric sulfur rearrangement 2 7 billion years ago evidence for oxygenic photosynthesis mass independently fractionated sulfur isotopes mif s provide strong evidence for an anoxic atmosphere during the archean moreover the temporal evolution of mif s shows increasing magnitudes between 2 7 and 2 5 ga until the start of the great oxidation event g o e at around 2 4 ga the conclusion of a completely anoxic atmosphere up to the g o e is in contrast to recent studies on redox sensitive elements which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the g o e in order to investigate this apparent inconsistency we present multiple sulfur isotopes for 2 71 ga pyritic black shales derived from the kidd creek area ontario canada these samples display high positive 33s values up to 3 8 and the typical late archean slope in 36s 33s of 0 9 in contrast the time period before 3 2 2 73 ga is characterized by greatly attenuated mif s magnitudes and a slope in 36s 33s of 1 5 we attribute the increase in 33s magnitude as well as the contemporaneous change in the slope of 36s 33s to changes in the relative reaction rate of different mif s source reactions and changes in atmospheric sulfur exit channels both of these are dependent on atmospheric ch4 co2 and o2 mixing ratios we propose a distinct change in atmospheric composition at 2 7 ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed neoarchean mif s record our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2 7 billion years ago

benthic perspective on earth s oldest evidence for oxygenic photosynthesis the great oxidation event goe is currently viewed as a protracted process during which atmospheric oxygen increased above 10 5 times the present atmospheric level pal this threshold represents an estimated upper limit for sulfur isotope mass independent fractionation s mif an archean signature of atmospheric anoxia that begins to disappear from the rock record at 2 45 ga however an increasing number of papers have suggested that the timing for oxidative continental weathering and by conventional thinking the onset of atmospheric oxygenation was hundreds of million years earlier than previously thought despite the presence of s mif we suggest that this apparent discrepancy can be resolved by the earliest oxidative weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere such as biological soil crusts and freshwater microbial mats covering riverbed lacustrine and estuarine sediments we calculate that oxygenic photosynthesis in these millimeter thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant s mif signature as continental freeboard increased significantly between 3 0 and 2 5 ga the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass balance perspective these observations help reconcile evidence for pre goe oxidative weathering with the history of atmospheric chemistry and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the goe

uranium series isotopes in river materials insights into the timescales of erosion and sediment transport abstract the uranium series isotope signatures of the suspended and dissolved load of rivers have emerged as an important tool for understanding the processes of erosion and chemical weathering at the scale of a watershed these signatures are a function of both time and weathering induced fractionation between the different nuclides provided appropriate models can be developed they can be used to constrain the residence time of river sediment this chronometer is triggered as the bedrock starts weathering and the inferred timescale encompasses the residence time in the weathering profile storage in temporary sediment deposits e g floodplain and transport in the river this approach has been applied to various catchments over the past five years showing that river sediments can reside in a watershed for timescales ranging from a few hundreds of years iceland to several hundreds of thousands of years lowlands of the amazon various factors control how long sediment resides in the watershed the longest residence times are observed on stable cratons unaffected by glacial cycles or more generally climate variability and human disturbance shorter residence times are observed in active orogens andes or fast eroding recently glaciated catchments iceland in several cases the residence time of suspended sediments also corresponds to the time since the last major climate change the u series isotope composition of rivers can also be used to predict the river sediment yield assuming steady state erosion is reached by comparing this estimate with the modern sediment yield obtained by multi year sediment gauging it is clear that steady state is seldom reached this can be explained by climate variability and or human disturbance steady state is reached in those catchments where sediment transport is rapid iceland or where the region has been unaffected by climate change and or human disturbance u series are thus becoming an important tool to study the dynamics of erosion

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion

evidence for free oxygen in the neoarchean ocean based on coupled iron molybdenum isotope fractionation abstract most geochemical proxies and models of atmospheric evolution suggest that the amount of free o2 in earth s atmosphere stayed below 10 5 present atmospheric level pal until the great oxidation event goe that occurred between 2 2 and 2 4 ga at which time free o2 in the atmosphere increased to approximately 10 1 to 10 2 pal although photosynthetically produced o2 oases have been proposed for the photic zone of the oceans prior to the goe it has been difficult to constrain absolute o2 concentrations and fluxes in such paleoenvironments here we constrain free o2 levels in the photic zone of a late archean marine basin by the combined use of fe and mo isotope systematics of ca mg carbonates and shales from the 2 68 to 2 50 ga campbellrand malmani carbonate platform of the kaapvaal craton in south africa correlated fe and mo isotope compositions require a key role for fe oxide precipitation via oxidation of aqueous fe ii by photosynthetically derived o2 followed by sorption of aqueous mo to the newly formed fe oxides a dispersion reaction model illustrates the effects of fe oxide production and mo sorption to fe oxides and suggests that a few to a few tens of m free o2 was available in the photic zone of the late archean marine basin consistent with some previous estimates the coupling of fe and mo isotope systematics provides a unique view into the processes that occurred in the ancient shallow ocean after production of free o2 began but prior to oxygenation of the deep ocean or significant accumulation of free o2 in the atmosphere these results require oxygenic photosynthesis to have evolved by at least 2 7 ga and suggest that the neoarchean ocean may have had a different oxygenation history than that of the atmosphere the data also suggest that the extensive iron formation deposition that occurred during this time was unlikely to have been produced by anoxygenic photosynthetic fe ii oxidation finally these data indicate that the ocean had significant amounts of o2 at least 150 myr prior to previously proposed whiffs of o2 at the archean to proterozoic transition

new insights into archean sulfur cycle from mass independent sulfur isotope records from the hamersley basin australia we have measured multiple sulfur isotope ratios 34 s 33 s 32 s for sulfide sulfur in shale and carbonate lithofacies from the hamersley basin western australia the v 33 s values v 33 swn 33 s30 515un 34 s shift from 31 9 to 6 9x over a 22 m core section of the lower mount mcrae shale v2 5 ga likewise sulfide sulfur analyses of the jeerinah formation v2 7 ga yield v 33 s values of 30 1 to 8 1x over a 50 m section of core despite wide variations in v 33 s and n 34 s these two shale units yield a similar positive correlation between v 33 s and n 34 s in contrast pyrite sulfur analyses of the carawine dolomite v2 6 ga yield a broad range in n 34 s 3 2 to 16 2x but a relatively small variation and negative values in v 33 s 32 5 to 31 1x the stratigraphic distribution of n 33 s n 34 s and v 33 s in western australia allows us to speculate on the sulfur isotopic composition of archean sulfur reservoirs and to trace pathways in the archean sulfur cycle our data are explained by a combination of massindependent fractionation mif in the atmosphere and biological mass dependent fractionation in the ocean in the archean volcanic sulfur bearing gas species were photolysed by solar ultraviolet uv radiation in an oxygen free atmosphere resulting in mif of sulfur isotopes aerosols of s8 with v 33 ss 0 and sulfuric acid with v 33 s6 0 formed from the products of uv photolysis and carried mass independently fractionated sulfur into the hydrosphere the signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass dependent fractionation on mass independent atmospheric effects multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur metabolizing organisms a 2003 elsevier science b v all rights reserved