chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks abstract chemical weathering indices are commonly used for characterizing weathering profiles by incorporating bulk major element oxide chemistry into a single metric for each sample generally on homogeneous parent rocks weathering indices change systematically with depth however the weathering of heterogeneous metamorphic rocks confounds the relationship between weathering index and depth in this paper we evaluate previously defined chemical weathering indices for their suitability in characterizing weathering profiles developed on felsic heterogeneous metasedimentary bedrock from coweeta hydrologic laboratory north carolina usa among the weathering indices evaluated here the weathering index of parker wip is the most appropriate for application to weathering profiles on heterogeneous and homogeneous parent rock because the wip includes only the highly mobile alkali and alkaline earth elements in its formulation it yields values that differ greatly from those of the parent rock in addition the wip allows for aluminum mobility unlike other weathering indices these characteristics combine to make the weathering index of parker the most applicable index for studying the weathering of heterogeneous metasedimentary rocks however the wip should be applied judiciously as alkali and alkaline earth metals may be readily depleted during weathering in addition to reflecting weathering the chemical index of alteration cia chemical index of weathering ciw plagioclase index of alteration pia and vogt s residual index v may be sensitive to subtle geochemical changes such as hydrothermal alteration along a fault and or alteration at the water table weathering indices that include iron in their calculations were not included in this study because they do not distinguish between ferric and ferrous iron the oxidation state and distribution of iron in a weathering profile is influenced by the proportion of oxidation states in the pre weathered metamorphic rock and interaction with groundwater during weathering

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

geochemistry of the kola river northwestern russia the kola river in the northern part of the kola peninsula northwestern russia flows into the barents sea via the kola bay the river is a unique place for reproduction of salmon and an important source of drinking water for more than 500 000 people in murmansk and the surrounding municipalities to evaluate the environmental status of the kola river water sampling of the dissolved 0 22 m phases was performed at 12 sites along the kola river and its tributaries during 2001 and 2002 major ca k mg na s si hco3 and cl and trace al as ba cd co cr cu fe mn mo ni pb sr ti and zn elements total and particulate organic c toc and poc n and p were analysed comparison with the boreal pristine kalix river northern sweden shows that except for na cl al cu and ni which exceed the concentrations in the kalix river by as much as 2 3 times the levels of other major and trace elements are close to or even below the levels in the kalix river however the results also demonstrate that pollutants from the three major sources 1 the cu ni smelter in monchegorsk 2 the open pit fe mine and ore concentration plant in olenegorsk and 3 the varlamov the medveziy and the zemlanoy creeks draining the area of the large agricultural enterprises in the lower part of the watershed have a major influence on the water quality of the kola river

a giant late archean lake system the meentheena member tumbiana formation fortescue group western australia abstract the 2720 ma old meentheena member of the tumbiana formation fortescue group is a 30 50 m thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of 680 km across the pilbara craton western australia limestones of the member consist of flat pebble intraformational conglomerate oolite grainstone calcisiltite and calcilutite siliciclastics consist of shale siltstone and sandstone and many of these are tuffaceous sedimentary structures include abundant symmetrical ripples ripple cross stratification desiccation cracks teepee structures and planar to wavy lamination the member is superficially similar in appearance wherever it crops out however the correlation of individual beds and units over distances greater than 15 km is problematic there are no regional marker beds radiometric ages from tuffaceous units young to the west by a few million years four lithofacies are defined a flat pebble conglomerate b ripple cross laminated oolite grainstone c planar to wavy laminated calcilutite calcisiltite and d shale siltstone stromatolites are found in all three of the limestone facies evaporites are rare and include halite pseudomorphs the four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession associations may not have all four lithofacies but do reflect the same deepening upward pattern the uppermost portions of shale siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure measured sections contain as many as 25 of these parasequences indicating fluctuating water levels limestone lithology and facies commonly change abruptly laterally at the meter and greater scale and vertically centimeter to meter scale the sedimentary structures differences and abrupt changes in lithofacies patterns the inability to correlate confidently between regions and the differences in age from east to west suggest a complex diachronous depositional system of fluctuating water levels in separate bodies of water the limestone is dominated by sedimentary structures indicative of shallow water within wave base however it is devoid of evidence of any tidal influence possibilities for depositional environments include a tideless sea or a lacustrine system 87sr 86sr values from limestone differ from known archean marine carbonates ree patterns are decidedly non marine as well a lacustrine system is favored for the meentheena member based on sedimentary structures the abruptness of lithologic and facies changes the facies association lateral gradation into fluvial deposits the inability to correlate many units and beds for more than a few to several kilometers geochemical data stromatolites and comparisons with younger lacustrine basins as well as marine successions there is no single line of evidence that unequivocally establishes a lacustrine origin however multiple lines of evidence are collectively consistent with a lacustrine origin the meentheena member would therefore represent the oldest known carbonate rich lacustrine system which we call the meentheena lakes system

geochemistry of precambrian carbonates 3 shelf seas and non marine environments of the archean a comprehensive whole rock study of mineralogical chemical and isotopic attributes of archean carbonates suggests that their lithologies and facies have been controlled by tectonic setting in the first two papers of this series we have shown that the dominant lithology of sedimentary carbonates in greenstone belt settings is limestone fe rich dolostone of ankeritic composition in older examples in this paper we suggest that the archean shelf sequences are mostly dolostone and the contemporaneous lacustrine playa lakes are characterized by limestone facies the present study is of the shelf environments of the archean represented by the pongola supergroup of south africa and the hamersley group wittenoom and carawine dolomite of australia the lacustrine playa examples have been sampled from the ventersdorp supergroup of south africa and the fortescue group of australia geological trace element and oxygen isotope considerations of the shelf carbonates suggest that their original mineralogy may have been aragonite and that the pongola dolostones probably represent a direct dolomitization product of this precursor in contrast the stabilization of the hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone prior to dolomitization subsequently both sequences but the pongola supergroup much more so were subjected to pervasive thermal events which overprinted their chemical and isotopic signatures consideration of the best preserved samples yields 0 9 2 1 and 6 l pdb for the 13c and 18o respectively for the initial archean signature if compared to phanerozoic carbonates the archean 13c values are comparable but the oxygen isotope measurements again support the primary nature of 18o depletion in ancient marine sediments regardless of its ultimate cause the 87sr86sr of 2 5 ga old sea water was not more than 0 7033 if this value reflects the contemporaneous sea water it would indicate the commencement of a significant 87sr input from the growing and aging continents it is however entirely possible that the slight enrichment in radiogenic 87sr relative to the contemporaneous mantle represents only an unresolved artefact of post depositional alteration if so the 87sr86sr of the 2 5 ga old sea water may still have been within a mantle range the lacustrine limestones retain a strong facies control in their trace element attributes they are also 4 depleted in 13c compared to marine carbonates and strongly depleted in 18o 19 l pdb these light 18o values are somewhat enigmatic unless the lakes were of the high altitude latitude variety the 87sr86sr of 0 711 for the ventersdorp lakes suggests that their drainage basin incorporated not only volcanic rocks but also aged continental crust archean carbonates of all lithologies and facies are enriched in mn and fe compared to their phanerozoic counterparts

plio pleistocene lacustrine stromatolites from lake turkana kenya morphology stratigraphy and stable isotopes abell p i awramik s m osborne r h and tomellini s 1982 plio pleistocene lacustrine stromatolites from lake turkana kenya morphology stratigraphy and stable isotopes sediment geol 32 1 26 a sequence of fossil stromatolites from lake turkana in kenya was examined for 51so and 513c content these stromatolites ranging in age from holocene 10 000 yrs b p to middle pliocene 3 m y showed a variety of growth forms from oncolitic columnar layered to bulbous heads the stromatolites used in our study contain filamentous blue green algae of one morphological type and rare coccoids thus the stromatolites are considered biogenic the stable isotope ratios for oxygen and carbon indicate changing climatic conditions ranging from a cool wet climate prior to ca 1 9 m y to much drier warmer conditions around 1 4 m y followed in turn by a somewhat cooler and wetter climate at the end of the pleistocene

isotopic evolution of the biogeochemical carbon cycle during the proterozoic eon abstract time related changes in the proterozoic sedimentary isotopic record of organic carbon and carbonates are consistent with the following 1 an increase in the oxidation state of the environment 2 a transition towards microbial populations dominated by producers and utilizers of o 2 and so 4 2 and 3 a relatively minor increase in organic 13 c during postdepositional thermal alteration 13 c increased 3 as h c declined from 0 8 to 0 1 the progressive oxidation of the environment was episodic for example between 2 2 and 1 9 ga crustal and atmospheric reservoirs of o 2 fe 3 and so 4 2 increased relatively rapidly and this increase was too large to be attributed solely to the declining rate of oxidant consumption by volcanic and hydrothermal fluxes of reduced species additional fundamental changes seem to be required in the tectonic style of organic sedimentation and preservation episodic changes in both the biogeochemical c cycle and atmospheric o 2 levels during the proterozoic eon likely influenced the evolution of eukarya

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

higher erosion rates in the himalaya geochemical constraints on riverine fluxes the modern erosion rate of continental scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains these two parameters are very difficult to measure directly in continental scale basins in this paper we examine the chemical composition of the suspended load bedload and dissolved load of the ganga and brahmaputra rivers and compare them with the average composition of himalayan source rocks a mass balance equation of erosion fluxes shows that a si rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock it corresponds to bedload sediment and flood plain deposits which are enriched in quartz by mineral sorting during transport the combined budget of si al and fe in the river system allows us to estimate this si rich flux by this method the total himalayan erosion is estimated to be twice the measured flux of suspended load the comparison between the brahmaputra and the ganga shows that the eastern himalaya has a higher erosion rate 2 9 mm yr than the western himalaya 2 1 mm yr this is likely the result of the higher runoff in the brahmaputra basin the intensity of the monsoon acts as a first order control of the erosion rate in the range

evidence for microbial life in synsedimentary cavities from 2 75 ga terrestrial environments fluviolacustrine sediments of the 2 75 ga hardey formation fortescue group australia preserve pendant columnar structures with stromatolitic lamination within synsedimentary cavities the millimeter sized finger like columns strongly resemble microbialites from modern basaltic caves and indicate the likely presence of microbial biofilms the ancient microbial columns are preserved by chert and locally occur as reworked clasts indicating a near depositional age for the structures sulfur isotopic analysis of pyrite in the columns and adjacent carbonaceous matrix yields 34scdt cdt canyon diablo troilite values between 8 5 and 19 showing significant fractionation characteristic of biological cycling of sulfur organic matter in cavity ceilings and shale matrix has 13cpdb pdb peedee belemnite values between 55 and 43 suggesting the presence of methanotrophs our results suggest that 2 75 ga terrestrial environments supported a microbial ecosystem including microbes that inhabited synsedimentary hollows extending the fossil record of coelobionts by 1 5 b y subsurface cavities represent a new habitable microenvironment for early life on earth and an analogue for ancient life on mars

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

biogenic versus metamorphic origins of diverse microtubes in 2 7 gyr old volcanic ashes multi scale investigations abstract titanite bearing microtubes in metabasalts have been interpreted as evidence for early microbial life thriving in the oceanic crust up to 3 5 ga ago based on similarities with microtubular pitting occurring in modern volcanic glass an alternative abiogenic origin as ambient inclusions trails aits has been debated but textural evidence such as the absence of diagnostic terminal inclusions was used to discard this possibility here we describe a variety of microtubes occurring in pyroclastic tuff of the 2 72 ga tumbiana formation western australia some of which resemble titanite filled microtubes reported in metamorphosed archean pillow basalts to assess the origin of the tumbiana microtubes we investigated their structure and chemistry and their root zone down to the sub micrometer scale using a combination of raman microspectroscopy scanning electron microscopy sem focused ion beam fib milling and scanning transmission electron microscopy stem coupled with energy dispersive x ray spectrometry edxs we report four types of microtubes type 1 comprises calcite microtubes type 2 comprises aits showing terminal pyrite and trails with negative crystal shapes type 3 comprises titanite microtubes in chlorite displaying morphologies similar to those reported in 3 5 ga old archean lavas at the sub micrometer scale they display a dendritic structure depending on their localization type 3 microtubes have been divided in three main sub types i type 3a extends in pores filled by chloritized cement ii type 3b occurs in pyroclasts and iii type 3c overgrows and thus post dates type 2 aits type 4 comprises titanite microtubes confined to quartz and calcite rims types 3 and 4 microtubes are terminated by sub micrometric carbonaceous and or mineral inclusions altogether these features suggest that the tumbiana titanite microtubes have formed as dendritic aits associated with displacement of mineral and or organic inclusions in pyroclasts and in surrounding void filling cements during recrystallization they are therefore unlikely to represent relics of the activity of early microbial life rather abiotic features formed in the presence of organic matter during burial metamorphism in that sense they may represent some sort of indirect evidence for life

remarkably preserved prokaryote and eukaryote microfossils within 1ga old lake phosphates of the torridon group nw scotland abstract the 1000 ma torridon group of northwest scotland are here shown to contain a rich diversity of organic walled microfossils preserved in exceptional detail within sedimentary phosphate the phosphatic nodules and bands in which they occur are autochthonous as are the organic fossils they contain the associated sedimentology of this phosphate is shown to be consistent with a lacustrine setting informal taxonomic treatment of the microfossils allows statistical assessment of the relative abundances of different morphotypes across a range of lacustrine environments exceptional preservation of pristine organic walled eukaryotes and prokaryotes together with a taphonomic spectrum in both physicochemical and biological degradational conditions allows a picture to be built of the earliest lacustrine ecological communities

geochemistry of large river suspended sediments silicate weathering or recycling tracer this study focuses on the major and trace element composition of suspended sediments transported by the world s largest rivers its main purpose is to answer the following question is the degree of weathering of modern river borne particles consistent with the estimated river dissolved loads derived from silicate weathering in agreement with the well known mobility of elements during weathering of continental rocks we confirm that river sediments are systematically depleted in na k ba with respect to the upper continental crust for each of these mobile elements a systematics of weathering indexes of river borne solids is attempted a global consistency is found between all these indexes important variations in weathering intensities exist a clear dependence of weathering intensities with climate is observed for the rivers draining mostly lowlands however no global correlation exists between weathering intensities and climatic or relief parameters because the trend observed for lowlands is obscured by rivers draining orogenic zones an inverse correlation between weathering intensities and suspended sediment concentrations is observed showing that the regions having the highest rates of physical denudation produce the least weathered sediments finally chemical and physical weathering are compared through the use of a simple steady state model we show that the weathering intensities of large river suspended sediments can only be reconciled with the silicate derived dissolved load of rivers by admitting that most of the continental rocks submitted to weathering in large river basins have already suffered previous weathering cycles a simple graphical method is proposed for calculating the proportion of sedimentary recycling in large river basins finally even if orogenic zones produce weakly weathered sediments we emphasize the fact that silicate chemical weathering rates and hence co2 consumption rates by silicate weathering are greatly enhanced in mountains simply because the sediment yields in orogenic drainage basins are higher hence the parameters that control chemical weathering rates would be those that control physical denudation rates

a 3 800 million year isotopic record of life from carbon in sedimentary rocks an increased ratio of 12c to 13c an indicator of the principal carbon fixing reaction of photosynthesis is found in sedimentary organic matter dating back to almost four thousand million years ago a sign of prolific microbial life not long after the earth s formation partial biological control of the terrestrial carbon cycle must have been established very early and was in full operation when the oldest sediments were formed

spatial structure in the thermospheric horizontal wind above poker flat alaska during solar minimum a new all sky imaging wavelength scanning fabry perot spectrometer was used to record high resolution r 200 000 spectra of the 630 nm thermospheric optical emission above poker flat alaska these spectra were used to derive spatially resolved maps of the horizontal wind vector at approximately 250 km altitude we describe the procedure used to infer vector winds from line of sight doppler shifts along with its limitations we present the time evolution of the vector wind fields obtained from this method for 6 nights of observation five of the 6 nights contained periods when we inferred the existence of significant curvature divergence or shear in the thermospheric wind across our instrument s 1000 km diameter field of view the sixth night exhibited little spatial structure and is shown for comparison we compare these results with a generic solar minimum winter time run of the national center for atmospheric research s thermosphere ionosphere and electrodynamics general circulation model while agreement was good at the start and end of the night considerable differences were found in the late evening and midnight sectors some possible origins for these discrepancies are proposed in particular we suggest that the f region horizontal wind may be deflected by upwelling vertical winds which are in turn driven by e region heating in the auroral zone we note that both the instrument used and our high latitude implementation of the analysis procedure are new experimental techniques thus the data presented here should be regarded as preliminary and if possible be validated by comparison with results from other techniques

antarctic microbial mats a modern analog for archean lacustrine oxygen oases the evolution of oxygenic photosynthesis was the most important geochemical event in earth history causing the great oxidation event goe 2 4 b y ago however evidence is mixed as to whether o2 production occurred locally as much as 2 8 b y ago creating o2 oases or initiated just prior to the goe the biogeochemical dynamics of possible o2 oases have been poorly constrained due to the absence of modern analogs however cyanobacteria in microbial mats in a perennially anoxic region of lake fryxell antarctica create a 1 2 mm o2 containing layer in the upper mat during summer providing the first known modern analog for formation of benthic o2 oases in lake fryxell benthic cyanobacteria are present below the oxycline in the lake mat photosynthesis rates were slow due to low photon flux rate 1 2 mol m 2 s 1 under thick ice cover but photosynthetic o2 production was sufficient to sustain up to 50 mol o2 l 1 sandwiched between anoxic overlying water and anoxic sediments we hypothesize that archean cyanobacteria could have similarly created o2 oases in benthic mats prior to the goe analogous mats may have been at least partly responsible for geological evidence of oxidative weathering prior to the goe and habitats such as lake fryxell provide natural laboratories where the impact of benthic o2 oases on biogeochemical signatures can be investigated

continental rifting and the archean proterozoic transition in the western australian shield the norseman wiluna belt dominated by lowermost sequences of deformed and metamorphosed basalts and komatiites and the fortescue group with successions of gently dipping little metamorphosed basalts and rare komatiites are typical examples of archean style greenstones and proterozoic style platform sequences respectively isotopic evidence however indicates that they are broadly coeval and both archean ca 2 7 0 05 ga and geologic evidence suggests that both relate to continental rifting their contrasting volcanism and subsequent deformation are inferred to result from their positions relative to rift axes and the state of the basement the norseman wiluna belt representing a failed rift in stretched relatively hot crust and the fortescue group representing a continental basalt succession on thick relatively cold crust the fortescue group immediately follows a major period of crustal reworking and stabilization whereas eruption of the greenstones of the norseman wiluna belt was closely followed by such an event thus the diachronous change in tectonic style from greenstones to platforms the archean proterozoic transition in this case appears to relate to a finite period of stabilization of continental crust rather than to a fundamental change in tectonic style or process with time

microbially induced sedimentary structures in archean sandstones a new window into early life abstract until now the most valuable information on the early life on the archean earth derived from bacterial fossils and stromatolites preserved in precipitated lithologies such as chert or carbonates also shales contain complex biomarker molecules and specific isotopes constitute an important evidence for biogeneicity in contrast because of their low potential of fossil preservation sandstones have been less investigated but recent studies revealed a variety of microbially induced sedimentary structures miss that differ greatly from any other fossils or sedimentary structures wrinkle structures multidirected ripple marks biolaminites and other macrostructures indicate the former presence of photoautotrophic microbial mats in shallow marine to tidal paleoenvironments the miss form by the mechanical interaction of microbial mats with physical sediment dynamics that is the erosion and deposition by water agitation the structures occur not only in archean tidal flats but in equivalent settings throughout earth history until today miss are not identified alone by their macroscopic morphologies in thin sections the structures display the carpet like fabrics of intertwined filaments of the ancient mat constructing microorganisms geochemical analyses of the filaments proof their composition of iron minerals associated with organic carbon in conclusion microbial mats colonize sandy tidal settings at least for 3 2 ga years therefore archean sandstones constitute an important archive for the exploration of early life

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

early non marine life evaluating the biogenicity of mesoproterozoic fluvial lacustrine stromatolites abstract the record of life on land or in non marine environments during the precambrian is sparse limiting our ability to understand life outside of marine settings before the advent of animals stromatolites from such environments are known but demonstrating stromatolite biogenicity remains difficult as stromatolite growth can be controlled by a spectrum of biologic chemical and biologically mediated processes stromatolites from the mesoproterozoic 1 09 ga copper harbor conglomerate an alluvial fan fluvial and lacustrine deposit offer an interesting test for the presence and nature of microbial life in shallow mesoproterozoic non marine settings stromatolites from a siltstone facies are interpreted as biogenic as they contain detrital rich laminae likely indicative of trapping and binding by microbes and fenestral fabrics suggestive of desiccation or lift off structures in mats via the presence of gas perhaps o2 from photosynthesis or other gases from mat decay the stromatolites formed as microbial mats grew over a mudflat or sandflat with carbonate filled desiccation cracks on an erosive topography and thus provide evidence for life in a very shallow predominantly desiccated environment stromatolites from a conglomerate facies are microdigitate and record both isopachous laminae with radial fibrous calcite fans and botryoids typically considered abiogenic in origin as well as wavy conical laminations likely indicative of the former presence of microbial mats the conglomerate facies stromatolites are interpreted to have formed in a flooded braidplain setting with restricted circulation this study supports the suggestion that microbial communities were abundant in non marine environments in the midcontinent rift during the mesoproterozoic it also highlights how variable environmental factors can influence stromatolite growth even in similar depositional settings and with a consistent microbial presence

examining archean methanotrophy the carbon isotope ratios preserved in sedimentary rocks can be used to fingerprint ancient metabolisms organic carbon in late archean samples stands out from that of other intervals with unusually low 13 c values 45 to 60 it was hypothesized that these light compositions record ecosystem wide methane cycling and methanotrophy either of the aerobic or anaerobic variety to test this idea we studied the petrography and carbon and oxygen isotope systematics of well known and spectacular occurrences of shallow water stromatolites from the 2 72 ga tumbiana formation of western australia we examined the carbonate cements and kerogen produced within the stromatolites because methanotrophy is expected to leave an isotopic fingerprint in these carbon reservoirs mathematical modeling of archean carbonate chemistry further reveals that methanotrophy should still have a discernible signature preserved in the isotopic record somewhat diminished from those observed in phanerozoic sedimentary basins due to higher dissolved inorganic carbon concentrations these stromatolites contain kerogen with 13 c org values of 50 by microsampling different regions and textures within the stromatolites we determined that the isotopic compositions of the authigenic calcite cements show a low degree of variation and are nearly identical to values estimated for seawater at this time the lack of low and variable 13 c carb values implies that methanotrophy does not explain the low 13 c org seen in the coeval kerogen these observations do not support a methanotrophy hypothesis but instead hint that the late archean may constitute an interval wherein autotrophs employed markedly different biochemical processes of energy conservation and carbon fixation

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

nitrogen isotope evidence for alkaline lakes on late archean continents abstract nitrogen isotope ratios in ancient sedimentary rocks are generally interpreted as a proxy for metabolic nitrogen pathways and the redox state of the water column fractionation processes occurring under anoxic alkaline conditions during the dissociation of nh 4 to h and volatile nh3 are frequently overlooked although this mechanism imparts large isotopic fractionations here we propose that nh3 volatilization is largely responsible for 15n values of up to 50 at high c n ratios in the late archean tumbiana formation this sequence of sedimentary rocks represents a system of lakes that formed on subaerial flood basalts and were partly filled by basaltic volcanic ash aqueous alteration of volcanic glass followed by evaporative concentration of ions should have led to the development of high alkalinity with a ph of 9 or higher as in modern analogues in this sedimentologically unusual setting nitrogen isotope ratios thus provide indirect evidence for the oldest alkaline lake system in the rock record these very heavy lacustrine 15n values contrast markedly with those of archean marine sedimentary rocks making a precambrian soda ocean unlikely today alkaline lakes are among the most productive ecosystems on earth some nutrients in particular molybdenum are more soluble at high ph and certain prebiotic reactions would likely have been favored under alkaline conditions in similar settings earlier in earth s history hence alkaline lakes in the archean could have been significant for the origin and early evolution of life

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

late archean initiation of manganese metallogenesis its significance and environmental controls abstract manganese metallogenesis was initiated in a modest scale during the late archean time selectively in india and brazil sedimentary deposits occur in both greenstone belts and high grade granulite terranes in shallow water shelf regimes only and are often intimately associated with stromatolites the changes in the rates of continental crustal growth mantle heat flux and outgassing along with the evolution of tectonic regimes sedimentary environments the state of the biosphere and finally the compositions of interacting atmosphere hydrosphere system from the early to the late archean time have been traced as interdependent parameters and interpreted to explain the first appearance of sedimentary manganese deposits promoted by possibly localized oxygenated environments compared to the spectacular manganese event represented by the giant paleoproterozoic kalahari deposit south africa which is interpreted to have been formed as a consequence of the rapid increase in atmospheric oxygen level ca 2 5 2 4 ga ago the modest initiation of the late archean manganese depositions was triggered by localized photosynthetic oxygen supply forming basin margin oxygen oases as a prologue for the succeeding stratified ocean system the dissolved manganese in the basin water that was oxidized to form shelf type shallow water deposits is suspected by analogy with contemporary banded iron formations bifs but not proved to be of hydrothermal ancestry the initiation of concentrated manganese sedimentation during the late archean by itself provides an evidence for an increase in the oxygen content in the contemporary atmosphere hydrosphere system compared to that in the earlier period supporting the geochemical indications from paleoweathering profiles

geochemistry of dissolved and suspended loads of the seine river france anthropogenic impact carbonate and silicate weathering this study focuses on the chemistry of the seine river system one of the major rivers in europe and constitutes the first geochemical investigation of both suspended and dissolved loads of this river the seine river drains a typical mesozoic cenozoic sedimentary basin the paris basin constituted of limestones mixed or interbedded with terrigenous sediments derived from the paleoreliefs bordering the mesozoic and cenozoic seas in the context of quantifying the global influence of carbonate and silicate weathering on atmospheric co2 consumption the seine river offers the possibility of examining weathering rates in a flat sedimentary environment under temperate climatic conditions one of the major problems associated with the seine river as with many temperate rivers is pollution we propose in this paper 2 approaches in order to correct the dissolved load of the seine river for anthropogenic inputs and to calculate weathering rates of carbonates and silicates the first uses the dissolved load of rivers and tries to allocate the different solutes to different sources a mixing model based on elemental ratios is established and solved by an inversion technique the second approach consists in using the suspended load geochemistry under steady state conditions we show that the geochemistry of suspended sediments makes it possible to estimate the amount of solutes released during the chemical weathering of silicates and thus to calculate weathering rates of silicates the total dissolved load of the seine river at paris can be decomposed into 2 of solutes derived from natural atmospheric sources 7 derived from anthropogenic atmospheric sources 6 derived from agriculture 3 derived from communal inputs and 82 of solutes derived from rock weathering during high floods the contribution of atmospheric and agriculture inputs predominates the weathering rate of carbonates is estimated to be 48 t km2 yr 25 mm 1000 yr only 10 of carbonates are transported in a solid form the rest being transported in solution co2 consumption by carbonate weathering approaches 400 103 mol km2 yr in the seine river at paris about 2 3 mg l of dissolved cations are found to originate from the chemical weathering of silicates by taking dissolved silica into accounts the total dissolved load derived from silicate weathering is about 6 7 mg l this value is minimal because biological uptake of silica probably occur in the seine river the chemical weathering rate of aluminosilicates is estimated to be 2 t km2 yr the ratio of physical over chemical weathering of silicates range between 1 and 3 and the total chemical and physical erosion rates of sedimentary silicates are about 2 3 mm kyr the co2 consumption by silicate weathering 15 24 103 mol km2 yr and is independent of dissolved silica concentration silicate consumption is thus 20 times less than carbonate consumption in the paris basin compared to the neighboring granitic areas the sedimentary region drained by the seine river has 2 to 3 times lower co2 consumption rates we attribute this difference to the cation depleted nature of the seine basin aluminosilicates which are of sedimentary origin at a world scale the chemical denudation rates found for the seine basin are very low and comparable to those given for tropical lowland rivers draining silicates such as the rivers of the congo and amazon basins in spite of huge climatic differences we attribute this similarity to the low mechanical denudation that characterizes these two types of regions

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

composition and syngeneity of molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup pilbara craton western australia shales of very low metamorphic grade from the 2 78 to 2 45 billion year old ga mount bruce supergroup pilbara craton western australia were analyzed for solvent extractable hydrocarbons samples were collected from ten drill cores and two mines in a sampling area centered in the hamersley basin near wittenoom and ranging 200 km to the southeast 100 km to the southwest and 70 km to the northwest almost all analyzed kerogenous sedimentary rocks yielded solvent extractable organic matter concentrations of total saturated hydrocarbons were commonly in the range of 1 to 20 ppm g g rock but reached maximum values of 1000 ppm the abundance of aromatic hydrocarbons was 1 to 30 ppm analysis of the extracts by gas chromatography mass spectrometry gc ms and gc ms metastable reaction monitoring mrm revealed the presence of n alkanes mid and end branched monomethylalkanes cyclohexylalkanes acyclic isopre noids diamondoids tri to pentacyclic terpanes steranes aromatic steroids and polyaromatic hydrocarbons neither plant biomarkers nor hydrocarbon distributions indicative of phanerozoic contamination were de tected the host kerogens of the hydrocarbons were depleted in 13 c by 2 to 21 relative to n alkanes a pattern typical of although more extreme than other precambrian samples acyclic isoprenoids showed carbon isotopic depletion relative to n alkanes and concentrations of 2 methylhopanes were relatively high features rarely observed in the phanerozoic but characteristic of many other precambrian bitumens molecular parameters including sterane and hopane ratios at their apparent thermal maxima condensate like alkane profiles high mono and triaromatic steroid maturity parameters high methyladamantane and methyldiaman tane indices and high methylphenanthrene maturity ratios indicate thermal maturities in the wet gas gener ation zone additionally extracts from shales associated with iron ore deposits at tom price and newman have unusual polyaromatic hydrocarbon patterns indicative of pyrolytic dealkylation the saturated hydrocarbons and biomarkers in bitumens from the fortescue and hamersley groups are characterized as probably syngenetic with their archean host rock based on their typical precambrian molecular and isotopic composition extreme maturities that appear consistent with the thermal history of the host sediments the absence of biomarkers diagnostic of phanerozoic age the absence of younger petroleum source rocks in the basin and the wide geographic distribution of the samples aromatic hydrocarbons detected in shales associated with iron ore deposits at mt tom price and mt whaleback are characterized as clearly archean based on their hypermature composition and covalent bonding to kerogen copyright 2003 elsevier ltd

ubiquitous occurrence of basaltic derived paleosols in the late archean fortescue group western australia abstract the 2 76 ga old mount roe basalt paleosols mr 1 and mr 2 recognized near the base of the 2 76 2 69 ga fortescue group in western australia represent some of the oldest definite examples of archean paleoweathering profiles the loss of fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the late archean atmosphere respectively however the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post weathering alteration over primary soil profiles here we report new exposures of the mr 1 paleosol as well as a number of new basalt derived paleosol occurrences distributed in the mount roe basalt and the 2 73 ga old kylena formations of the fortescue group we show that all these paleosols including mr 1 and mr 2 were strongly affected by post weathering reductive alteration nevertheless we discovered early lithologic units locally preserved within the hydrothermally altered paleosols which feature distinct chemical and mineralogic compositions these include i 13c depleted carbonaceous rich diaspore pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and ii green hard core material mostly composed of fe sericite associated with authigenic sphene and containing early relics of sulfate bearing iron rich smectite or berthierine we argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage while berthierine and sphene formed during diagenesis through the circulation of reductive fluids because the depletion of iron observed in the fortescue paleosols is not solely due to pedogenesis but also to post weathering alteration particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer recognition of the regional scale syn depositional alteration of the fortescue subaerial basalts over the north pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the pilbara craton during continental break up and rifting such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and or lacustrine systems favoring the development of microbial life in shallow waters

stromatolite assemblages of the late precambrian in china abstract the late precambrian as used in this paper is represented by the sections in the jixian chihsien and yangtze gorges these rocks correspond to the middle late proterozoic of some authors and range in age from c 1900 to 600 ma the succession can be subdivided into four systems the changcheng jixian qingbaikou and sinian systems all of which yield abundant stromatolites based on the latest studies we suggest a new sixfold classification scheme supertype type subtype family genus and species and refer the stromatolites to 13 families and 50 genera including some material published already in china the stromatolites of the late precambrian of china may be represented by elements from yanshan southern liaodong peninsula and kaiyang of guizhou etc they are subdivided into eight stromatolite assemblages and correlated throughout most of china

lacustrine stromatolites from the kyzylgir formation pliocene altai mountains ussr abstract the pliocene lacustrine kyzylgir formation crops out in the altai mountain region of the ussr this formation contains a wide variety of stromatolite morphologies ranging from stratiform structures to columns composing bioherms and biostromes large stromatolitic fields composed of bioherms biostromes and a variety of individual stromatolites are also found such morphological diversity on several constructional levels is unique among known lacustrine stromatolites

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

constraints on ocean carbonate chemistry and pco2 in the archaean and palaeoproterozoic one of the great problems in the history of earth s climate is how to reconcile evidence for liquid water and habitable climates on early earth with the faint young sun predicted from stellar evolution models possible solutions include a wide range of atmospheric and oceanic chemistries with large uncertainties in boundary conditions for the evolution and diversification of life and the role of the global carbon cycle in maintaining habitable climates increased atmospheric co 2 is a common component of many solutions but its connection to the carbon chemistry of the ocean remains unknown here we present calcium isotope data spanning the period from 2 7 to 1 9 billion years ago from evaporitic sedimentary carbonates that can test this relationship these data from the tumbiana formation the campbellrand platform and the pethei group exhibit limited variability such limited variability occurs in marine environments with a high ratio of calcium to carbonate alkalinity we are therefore able to rule out soda ocean conditions during this period of earth history we further interpret this and existing data to provide empirical constraints for carbonate chemistry of the ancient oceans and for the role of co 2 in compensating for the faint young sun

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

chemically oscillating reactions during the diagenetic oxidation of organic matter and in the formation of granules in late palaeoproterozoic chert from lake superior abstract filamentous and coccoidal microfossils have been reported since the 1950 s from a range of granular cherts from the late palaeoproterozoic southwestern superior craton canada united states however the chemical and mineral compositions of granules the presence of microfossils in granules and the common presence of granules in intercolumnar space of stromatolitic chert are poorly documented and explained furthermore the depositional model for the origin of granules in wave agitated waters does not entirely explain their mineral diversity nor their characteristic morphologies and patterns we report on the crystallinity of organic matter mineral diversity and compositions of microfossils in granules from three different kinds of late palaeoproterozoic cherts namely phosphatic organic and haematitic stromatolitic organic rich chert from the gunflint fm contains granules with euhedral carbonate and equidistant concentric laminations of organic matter akin to fractal patterns from the belousov zhabotinsky b z chemically oscillating reaction these granules also contain authigenic anatase ferric ferrous silicates and fe oxides filamentous and coccoidal microfossils similar to those of the gunflint occur in chert from the biwabik formation and share morphology and co occur with mn siderite and apatite granules in phosphatic chert in the michigamme formation often contain filamentous and coccoidal microfossils composed of organic matter sericite and apatite bulk carbonate associated with these michigamme granular phosphatic chert beds has systematically negative 13ccarb values around 3 1 0 9 1 and 18ocarb smow between 20 8 and 30 7 which suggest some contribution from the diagenetic oxidation of organic matter notably residual carboxylic acid is detectable in c xanes spectra of organic matter from granular phosphatic chert which is a residual reactant of b z type reactions along with previously reported observations of pyritised microfossils from the gunflint formation these distinct mineralogies indicate variable modes of preservation for the products of chemically oscillating reactions that likely relate to the availability of different oxidants in the diagenetic environment we conclude that the late palaeoproterozoic shallow marine environments of the lake superior area were populated by morphologically similar micro organisms and that the diagenetic oxidation of organic matter through chemically oscillating reactions contributed to the formation of spheroidal rosettes granules and concretions during a late palaeoproterozoic great putrefaction event granules in chert that contain organic matter or microfossils thus provide a reliable petrographic context to search for a record of putrefaction of microbial life on the early earth and on other ancient planetary surfaces

geochemistry of trace elements in surface waters of the arno river basin northern tuscany italy trace element geochemistry of the arno river and its main tributaries was investigated on the basis of two sampling campaigns carried out in november 1996 and june 1997 by analyzing filtered and unfiltered water samples fe and al are found in solution mainly as colloidal particles of size lower than 0 45 m in june lower flow rate fe and al are enriched in the filtered waters from the main river and this feature was interpreted in terms of higher water temperature promoting the formation of smaller particles thus reducing their aggregation properties iron and al show perfectly synchronous downstream profiles along the arno river correlate quite well each to other and display abrupt concentration increases near to florence where the lithology of the catchment changes from siliciclastic dominated to clay sand lacustrine marine dominated the same behaviour is shown by most of the other trace elements in the river thus supporting a general lithological control trace elements in the final part of the arno river are influenced by flocculation processes in addition to mixing adsorption phenomena on oxy hydroxides are denoted by good elemental correlations with fe and al sporadic anomalous concentration values possibly related to anthropogenic contributions may prevent such correlations referring to the quality of waters for potable use and fish life toxic elements are below the acceptable limits of current european regulations with few exceptions for hg exceeding guideline values multivariate analysis groups trace elements according to geochemical affinities and natural or anthropogenic sources thus distinguishing contaminated from uncontaminated samples the results achieved in this work will help regional and national authorities for compliance with the eu water policy especially in assessing the water quality at the river basin scale and its vulnerability to human activities

chemical and physical weathering in the min jiang a headwater tributary of the yangtze river abstract the min jiang is a major headwater tributary of the yangtze chang jiang its source is in the undeveloped eastern tibetan plateau but it flows through the heavily populated sichuan four rivers basin on its way to join the yangtze main channel the dissolved major element composition was determined in the tibetan headwaters at the rising stage and at four hydrologic stations in a monthly time series the min jiang is alkaline and has high dissolved load compared to other world rivers carbonate weathering dominates especially in the source region of the min jiang main channel and silicate weathering and evaporite dissolution gain some importance in the dadu he tributary the si na k ratios of the dissolved load and the clay mineralogy of the bed material suggest that silicate weathering in the watershed is superficial i e soluble cations are being preferentially leached many major elements show lower concentrations at high summer discharge but there is also additional input such that the concentration discharge relationship is not purely a result of dilution the discharge weighted total dissolved solid tds flux based on data from the three upstream monitoring stations is 10 10 6 tons year 6 of the yangtze at mouth and the chemical denudation rate as measured by tds yield is 115 tons km 2 year for the min jiang higher than the average yangtze basin 85 tons km 2 year the common extrapolation using annual average discharge and spot sampling of dissolved and suspended material during rising or falling stage can reasonably estimate chemical fluxes to 20 but seriously underestimate the physical flux sampling the peak flash flow is critical for the suspended particulate material concentrations of so 4 cl and ca are significantly elevated as the river debouches onto the populated sichuan basin

stratigraphy and sedimentary petrology of an archean volcanic sedimentary succession at mt goldsworthy in the pilbara block western australia implications of evaporite nahcolite and barite deposition abstract a approximately 3 3 ga sedimentary succession at mt goldsworthy in the northeastern pilbara block australia contains beds composed of silicified pseudomorphs of nahcolite nahco3 or barite baso4 this sedimentary succession correlates with the corboy formation and consists of lower middle and upper sedimentary units which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks the lower and middle units are predominantly siliciclastic whereas the upper unit is characterized by ferruginous cherts and banded iron formations with minor proportions of intercalated mature sandstone the succession defines an overall upward fining and deepening trend the detrital materials were derived from older greenstone successions volcaniclastics and reworked precipitative beds silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit the formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from na hco3 brines in a closed or semi closed evaporitic basin bladed barite deposits occur mainly in the middle unit which was deposited in a sub aerial to shallow marine environment barite may have precipitated as a result of mixing of so42 rich seawater and ba2 rich hydrothermal fluids the mt goldsworthy sedimentary succession records an archean shallow to sub aerial sedimentary environment that probably developed in a continental margin setting

the effect of weathering in the buyukmelen river basin on the geochemistry of suspended and bed sediments and the hyrogeochemical characteristics of river water duzce turkey abstract the buyukmelen river is expected to be a water source that can supply the drinking water needs of istanbul until 2040 the drinking and utility water needs of istanbul are to transport water from the buyukmelen river to istanbul via pipeline and pump it into the alibeykoy and omerli reservoirs when their water levels drop the buyukmelen river is located in the province of duzce in 170 km east of istanbul and its water basin is approximately 2250 km 2 the buyukmelen river flows muddy in the rainy season and into the black sea the chemical compositions of natural waters alter due to interaction with geological formations physical and chemical weathering of various rocks and the effects of mining and agricultural production a research was conducted at the buyukmelen river basin to determine the effects of hydrological processes therefore the samples of rocks soil stream water suspended and bed sediment were collected from the buyukmelen river basin geochemical and water chemistry analyses of samples were performed at als chemex laboratories canada the bed sediments contain quartz calcite plagioclase amphibole and clay minerals the clay minerals in the suspended and bed sediment samples are kaolinite smectite and illite the water samples collected from the aksu kucukmelen asarsu and ugursuyu streams and the buyukmelen river in winter december 2005 and summer june 2006 periods are rich in ca and hco 3 ions the ions most abundant in rainfall sample are hco 3 and na the ion compositions of surface waters have increased due to the weathering of limestone agglomerate and volcanic sandstones light acidic rainfall semi arid black sea climate in the buyukmelen basin the suspended sediment amount of the buyukmelen river in the rainy season december 2005 is 174 mg l according to the water contamination regulation of turkey the buyukmelen rivers belongs to quality class 4 based on al ion and to quality class 3 based on fe ion in winter period and to quality class 2 based on mn concentration in summer period chemical index of alteration cia indices observed in the suspended and bed sediments average of 55 suggest that their source area underwent moderate degrees of chemical weathering processes according to upper continental crust ucc values the suspended sediment was rich in elements such as fe 2 o 3 cao mgo mno tio 2 p 2 o 5 v cr co cu zn as cd sb hg and pb the element concentrations of the suspended sediments were related to size fractionation mainly of clay content the mentioned enrichment was contributed by agglomerate basalt volcanic sandstone and graywacke from rocks in the study area source of ions such as al fe mn ba cr co cu ni ti and hg and major in the buyukmelen river is interaction with rocks such as the agglomerate basalt andesite volcanic sandstone and graywacke as suggested by singh et al 2005 before weathering of some rocks in the buyukmelen river basin it was determined that they were graywacke and literanite based on the geochemistry of the suspended and bed sediments

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

aspects of proterozoic stromatolite biostratigraphy in western australia abstract stromatolites are abundant at many horizons in the proterozoic of western australia recent advances in knowledge of proterozoic stratigraphy of the state have provided a more detailed framework for interpreting the stromatolite data than has been available previously in the 1 7 ga earaheedy group of the nabberu basin a characteristic stromatolite assemblage occurs and within the basin a biostratigraphic succession can be recognized the assemblage contains several new forms which belong to new groups the need to erect new groups for these early proterozoic stromatolites is in agreement with recent studies in canada northern europe and south africa and suggests that the problem of younger or late proterozoic stromatolite groups in early proterozoic rocks mentioned by previous workers is a result of a lack of rigour in defining taxa examination of type material is necessary to determine how closely the earaheedy forms resemble those described from these other regions in western australia some stromatolite forms have a restricted vertical range and similar taxa occur in beds of approximately the same age in widely separated areas e g kimberley group and earaheedy group scorpion group and limbunya birrindudu mcarthur mt rigg and mt albert groups and bungle bungle dolomite tolmer and bullita groups moora and bangemall groups kai ki beds louisa downs mount house and albert edward groups stromatolite diversity shows a decline in the number of taxa at about 1 1 ga in the bangemall group more data are required to determine whether this decline is universal or specific to the bangemall group this study indicates that a stromatolite biostratigraphy for western australia is feasible and is consistent with data from other parts of australia thus emphasis on correlation should be placed on the stromatolite form rather than the group and intercontinental correlations should be attempted only when local biostratigraphic schemes have been firmly established

methanotrophs regulated atmospheric sulfur isotope anomalies during the mesoarchean tumbiana formation western australia abstract the 2 73 billion year old tumbiana formation of the fortescue group in the pilbara craton western australia preserves one of the best records of the fortescue negative carbon isotope excursion with organic 13 c values as low as 60 the prevailing environmental conditions at this biological excursion are not well understood here we present an integrated carbon 12 c 13 c and multiple sulfur 32 s 33 s 34 s 36 s isotopes study of a pristine diamond drill core through the tumbiana formation collected at meentheena by the pilbara drilling project our results show centimeter to meter scale heterogeneities in sulfur and carbon contents and isotope compositions organic 13 c values vary at the meter scale and display a bimodal distribution with peaks at about 44 and 34 this bimodal distribution is interpreted to reflect variations in the contribution of methanotrophic metabolism to the biomass the 34 s v cdt values of sedimentary pyrite vary from 5 73 to 2 75 sulfur isotopes sulfur contents and s c ratios are variable suggesting a complex local biotic sulfur cycling sedimentary pyrite preserves mass independent fractionation of sulfur mif s with 33 s v cdt ranging between 0 25 and 1 64 average 0 47 corresponding to the characteristic dampened mesoarchean anomalies positive correlation between sulfide mif s and a 13 c depletion of organic matter argues for a causal link between methanotrophs activity and the preservation of mif s anomalies in the rock record

lacustrine facies dependence of highly 13c depleted organic matter during the global age of methanotrophy abstract highly 13c depleted organic matter reported from neoarchean formations worldwide has led to the concept of a global age of methanotrophy gam in the neoarchean a temporal peak in the gam is suggested by values as low as 61 that are reported from rocks deposited at 2 7 ga here we analyse previously reported values report new field observations and isotope data and re evaluate the depositional settings of several units of this age we find a statistically significant lowering of 13corg values in units of neoarchean age compared to values reported from other precambrian intervals both older and younger confirming the existence of the gam however we also report a correlation between very low 13corg values and lacustrine units deposited during the neoarchean we hypothesize methanogenesis may have been promoted in some neoarchean lakes due to local deficiencies of oxidants specifically fe3 and so4 relative to the archean oceans lower availability of these oxidants could have limited higher energy yield metabolisms such as sulfate and iron reduction and provided an ecological niche for methanogens ultimately resulting in the local burial of biomass highly depleted in 13c we conclude that the exceptionally low 13corg values reported from formations deposited at 2 7 ga could represent the prevalence of closed basin depositional environments preserved in the limited outcrop available rather than a peak in the global age of methanotrophy at this time

neoarchean 2 7 ga lacustrine stromatolite deposits in the hartbeesfontein basin ventersdorp supergroup south africa implications for oxygen oases abstract the hartbeesfontein basin contains the most extensive deposits of archean lacustrine stromatolites on the kaapvaal craton with stromatolitic facies occurring over 100 km 2 in beds up to 7 m thick stromatolitic dolostones and cherts both preserve evidence of microbial processes dolomitic stromatolites have grumelous microspar textures between organic rich laminae suggestive of carbonate precipitation within microbial mats stromatolitic laminae within chert preserve detrital material beyond the angle of repose indicating the trapping and binding of grains by microbial mats stromatolitic cherts also preserve fenestral textures and filamentous microfossils many fenestrae have rounded shapes surrounded by filamentous laminae and appear to have formed in situ within stromatolite fabrics before lithification fenestrae within stromatolitic chert resemble hourglass associated fenestrae noted from recent silica stromatolites from yellowstone national park and are interpreted to originate from gas bubbles forming within stromatolite building mats the preservation of delicate structures in hartbeesfontein stromatolitic chert e g filamentous microfossils and gas related fenestrae implies rapid lithification of microbial mats while the mm to cm scale of fenestrae indicate equally rapid rates of microbial gas production textural and mineralogical evidence associated with gas related fenestrae support the presence of oxygenic photosynthesis which in turn strengthens previous hypotheses on archean lakes as potential oxygen oases before the great oxidation event

evidence for 2 0 ga continental microbial mats in a paleodesert setting abstract early evolved microbial communities characterized the initial biological invasion of precambrian continental landscapes in modern arid settings microbial mats and biological soil crusts are well developed and stabilize sediment the paleoproterozoic makgabeng formation in south africa is one of the oldest and best preserved dryland systems on earth six types of microbial mat related structures are now recognized within these depositional systems this paper presents three newly discovered structures that include tufted microbial mat biological soil crusts and gas escape features in addition to three previously documented structures that include roll up features sand cracks and wrinkled features these discoveries demonstrate that microbial communities were well established and inhabited diverse continental settings by 2 0 ga approximately 200 million years after the onset of the great oxidation event

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

geochronology of the karadere basement nw turkey and implications for the geological evolution of the istanbul zone earlier geological work in the istanbul zone western pontide tectonic belt has revealed the presence of extensive basement outcrops exposed underneath palaeozoic and mesozoic to tertiary cover sequences the basement of suspected neoproterozoic age plays an important role in understanding the crustal accretion process in nw turkey we report the first results of a detailed pb pb and u pb zircon study complemented by nd sr whole rock and mineral data from basement rocks exposed in the karadere valley safranbolu area five samples were selected for this study comprising three metagranitoids and two metasediments zircon geochronology indicates that the metagranitoids were formed during late proterozoic pan african magmatic events between 590 and 560 ma the rocks are of tonalitic and granitic composition and have low nb y ratios and ti contents consistent with those of arc rocks a continental arc setting is supported by their sr and nd isotope data that indicate a contribution of a mantle source as well as crustal assimilation during magma genesis the metasediments can clearly be distinguished from the metagranitoids by their higher 87sr 86sr ratios and lower end values at 580 ma which supports the suggestion that the arc was underlain by mature continental crust zircons from the metasediments yield a range of pb pb ages between 1 860 and 710 ma thirty per cent of them fall between 890 and 710 ma possibly suggesting a derivation from gondwana afro arabian regions a sm nd garnet whole rock analysis obtained on a metagranite gives an age of 559 8 ma which either reflects pre metamorphic magmatic growth of garnet in a felsic melt or a syntectonic high temperature metamorphic event uplift and cooling of the basement is further constrained by rb sr biotite ages of 548 545 ma these lower cambrian mineral ages demonstrate that the istanbul zone was not thermally reactivated during the hercynian cimmerian or alpine orogeny in contrast to its neighbouring tectonic zones confirming its role as a suspect terrane in the modern western pontide tectonic belt

implications of in situ calcification for photosynthesis in a 3 3 ga old microbial biofilm from the barberton greenstone belt south africa timing the appearance of photosynthetic microorganisms is crucial to understanding the evolution of life on earth the ability of the biosphere to use sunlight as a source of energy photoautotrophy would have been essential for increasing biomass and for increasing the biogeochemical capacity of all prokaryotes across the range of redox reactions that support life typical proxies for photosynthesis in the rock record include features such as a mat like laminated morphology stratiform domical conical often associated with bulk geochemical signatures such as calcification and a fractionated carbon isotope signature however to date in situ calcification related to photosynthesis has not been demonstrated in the oldest known microbial mats we here use in situ nanometer scale techniques to investigate the structural and compositional architecture in a 3 3 billion year ga old microbial biofilm from the barberton greenstone belt thus documenting in situ calcification that was most likely related to anoxygenic photosynthesis the josefsdal chert microbial biofilm jcmb formed in a littoral photic environment it is characterised by a distinct vertical structural and compositional organisation the lower part is calcified in situ by aragonite progressing upwards into uncalcified kerogen characterised by up to 1 sulphur followed by an upper layer that contains intact filaments at the surface crystallites of pseudomorphed pyrite are also associated with the biofilm suggesting calcification related to the activity of heterotrophic sulphur reducing bacteria in this anoxygenic nutrient limited environment the carbon required by the sulphur reducing bacteria could only have been produced by photoautotrophy we conclude that the josfsdal chert microbial biofilm was formed by a consortium of anoxygenic microorganisms including photosynthesisers and sulphur reducing bacteria

identification of an archean marine oxygen oasis abstract the early earth was essentially anoxic a number of indicators suggest the presence of oxygenic photosynthesis 2700 3000 million years ma ago but direct evidence for molecular oxygen o2 in seawater has remained elusive here we report rare earth element ree analyses of 2800 million year old shallow marine limestones and deep water iron rich sediments at steep rock lake canada these show that the seawater from which extensive shallow water limestones precipitated was oxygenated whereas the adjacent deeper waters where iron rich sediments formed were not we propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species fe ii to insoluble fe iii oxyhydroxide and estimate that at least 10 25 m oxygen concentration in seawater was required to accomplish this at steep rock this agrees with the hypothesis that an ample supply of dissolved fe ii in archean oceans would have hindered limestone formation there is no direct evidence for the oxygen source at steep rock but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria our findings support the view that during the archean significant oxygen levels first developed in protected nutrient rich shallow marine habitats they indicate that these environments were spatially restricted transient and promoted limestone precipitation if archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron rich seas then early oxygen oases are less elusive than has been assumed

the weathering of basalt and relative mobilities of the major elements at belbex france abstract a basalt and its weathered profile have been analysed for major elements graphical examination shows al fe and ti to have been essentially immobile determining mobility sequences for this deposit is complicated by the long pedogenetic history of the area three different sequences are presented two for different stages of weathering integrated over time and one that reflects the weathering regime of the present day

earliest life on earth evidence from the barberton greenstone belt south africa abstract studies investigating the structure and diversity of earth s record of life older than 3 2 ga are restricted to two locations worldwide in which sedimentary rocks have escaped regional high grade metamorphism and penetrative deformation the pilbara craton of western australia and the barberton greenstone belt in the kaapvaal craton of south africa this paper provides a south african perspective on the evidence of paleoarchean life a record that is often overlooked in the literature it aims to summarize and critically review previously reported claims of early life in the bgb gives an overview of the latest findings and provides an outlook on potential future discoveries the 15 km thick volcanic sedimentary succession making up the barberton supergroup was deposited between 3 55 to ca 3 20 ga and can be subdivided in three stratigraphic units that provide a unique window into a diverse and widespread paleoarchean microbial ecosystem landscape putative biosignatures occur almost throughout the entire bgb stratigraphy and range from carbonaceous cherts containing filamentous spheroidal and lenticular microstructures traces of hydrothermal biofilms photosynthetic microbial mats remnants of pseudocolumnar stromatolites and large organic walled spheroidal microfossils of currently unknown affinity the bgb also contains one of the world s oldest known record of tufted microbial mats which extensively colonized tidally influenced siliciclastic shorelines and were most likely formed by filamentous photosynthesizers other mat associated biosignatures include silicified gas bubbles domes and lenses that likely formed due to metabolic activity or the decay of buried organic matter some of these subsurface voids beneath the cohesive mats were inhabited by the earliest known forms of cavity dwelling microbial communities that were probably dominated by chemotrophic or photosynthetic microbes recently discovered terrestrial microbial mats once thriving in a fluvial dominated setting represent the oldest macroscopically visible fossil traces of life on land which is also supported by the occurrence of nearby paleosols that carry signals of biogenic sulfur fractionation the wealth of preserved microbial biosignatures from marine fluvial hydrothermal and possibly planktonic settings combined with the high spatial and temporal resolution of the barberton greenstone belt deposits is truly exceptional consequently the bgb deserves an equal level of attention and protection for future generations like its australian counterpart

implications for neoarchaean ocean chemistry from primary carbonate mineralogy of the campbellrand malmani platform south africa the precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform building process during deposition of the 2560 2520 ma campbellrand malmani carbonate platform south africa aragonite fans and fibrous coatings are common in unrestricted shallow subtidal to intertidal facies they are also present in restricted facies but are absent from deep subtidal facies decimetre thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies the proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0 to 65 depending on the facies subtidal facies commonly contain 20 35 in situ precipitated carbonate demonstrating that neoarchaean sea water was supersaturated with respect to aragonite carbonate crystal growth rates were rapid compared with sediment influx rates and the dynamics of carbonate precipitation were different from those in younger carbonate platforms the abundance of aragonite pseudomorphs suggests that sea water ph was neutral to alkaline whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans

how earth s atmosphere evolved to an oxic state a status report the evolution of the earth s atmosphere is essentially the story of atmospheric oxygen virtually every realm of the earth sciences biology geology geochemistry oceanography and atmospheric science is needed to piece together an understanding of the history of oxygen over the past decade new data from these fields has shown that there were two significant increases in atmospheric o2 levels at around 2 4 2 3 and 0 8 0 6 billion years ago respectively throughout earth history oceanic sulfate concentrations appear to have increased in accord with greater o2 levels while levels of methane a strong greenhouse gas may have inversely mirrored o2 both oxic transitions occurred in eras characterized by bsnowball earthq events and significant disturbances in the carbon cycle perhaps associated with increases in o2 and losses of methane to understand what controlled the oxygenation of the atmosphere it is necessary to determine how o2 is consumed on geologic time scales through reaction with reductants released from the earth s crust and mantle there was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere and a plausible explanation is that excess reductants scavenged photosynthetic o2 from the early atmosphere however a quantitative understanding of how and why o2 became abundant on our reducing planet is still lacking thus the study of the early atmosphere remains a frontier field with much to be discovered d 2005 elsevier b v all rights reserved

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

evidence for stabilization of the pilbara block australia the stabilization of continental crust to form cratons represents one of the major features of earth history1 major structural geochemical2 3 and to a lesser extent sedimentological changes4 associated with the development of stable crustal blocks are recorded mainly around 2 500 myr ago1 this change however appears to have been globally diachronous and cratonic sequences as old as 3 100 myr are recorded5 i now present the results of a recent regional stratigraphical and sedimentological study on the 2770 myr old lower fortescue group6 7 previously classified lithologically as proterozoic8 but following the recent recommendation of a 2 500 myr archaean proterozoic chronological boundary9 is referred to as archaean in the pilhara block10 western australia a major stabilization of the underlying granitoid greenstone terrain appears to have pre dated fortescue basin formation this event which available geochronology suggests was relatively rapid marked a major changing point in the tectonic evolution of the pilhara block

water quality and hydrogeochemical characteristics of the river buyukmelen duzce turkey the river buyukmelen is located in the province of duzce in northwest turkey and its water basin is approximately 470 km2 the aksu kucukmelen and ugursuyu streams flow into the river buyukmelen it flows into the black sea with an output of 44 m3 s 1 the geological succession in the basin comprises limestone and dolomitic limestone of the yilanli formation sandstone clayey limestone and marls of the akveren formation clastics and volcano clastics of the caycuma formation and cover units comprised of river alluvium lacutrine sediments and beach sands the river buyukmelen is expected to be a water source that can supply the drinking water needs of istanbul until 2040 therefore it is imperative that its water quality be preserved the samples of rock soil stream water suspended bed and stream sediments and beach sand were collected from the buyukmelen river basin they were examined using mineralogical and geochemical methods the chemical constituents most commonly found in the stream waters are na mg2 so2 4 cl and hco3 in the guz stream water ca2 in the abaza stream water and k in the kuplu stream water the concentrations of na k ca2 mg2 so2 4 hco 3 cl as pb ni mn cr zn fe and u in the kuplu and guz stream waters were much higher than the world average values the dilaver gubi tepekoy maden celik and abaza streams interact with sedimentary rocks and the kuplu and guz streams interact with volcanic rocks the amount of suspended sediment in the river buyukmelen in december 2002 was 120 mg l 1 the suspended and bed sediments in the muddy stream waters are formed of quartz calcite plagioclase clay kaolinite illite and smectite muscovite and amphibole minerals as co cd cr pb ni zn and u have all accumulated in the buyukmelen river bed sediments the muddy feature of the waters is related to the petrographic features of the rocks in the basin and their mineralogical compositions as most of the sandstones and volcanic rocks basalt tuffite and agglomerate are decomposed to a clay rich composition at the surface thus the suspended sediment in stream waters increases by physical weathering of the rocks and water rock interaction owing to the growing population and industrialization water demand is increasing the plan is to bring water from the river buyukmelen to istanbul s drinking water reservoirs according to the water pollution regulations the river buyukmelen belongs to quality class 1 based on hg cd pb as cu cr zn mn se ba na cl and so2 4 and to quality class 3 based on fe concentration the concentration of fe in the river buyukmelen exceeds the limit values permitted by the world health organization and the turkish standard because water from the river buyukmelen will be used as drinking water it will have an adverse effect on water quality and humans if not treated in advance in addition the inclusion of mn and zn in the elmali drinking water reservoir of istanbul and fe in the river buyukmelen water indicates natural inorganic contamination mn zn and fe contents in the waters are related to geological origin moreover the river buyukmelen flow is very muddy in the rainy seasons and it is inevitable that this will pose problems during the purification process copyright 2005 john wiley sons ltd

weathering of the ganga alluvial plain northern india implications from fluvial geochemistry of the gomati river abstract in the northern part of the indian sub continent the ganga alluvial plain gap feeds its weathering products to the ganga brahmaputra river system one of the world s largest fluvial systems the authors present a geochemical study of the gap weathering products transported by the gomati river the ganga river tributary to understand weathering processes of an alluvial plain in a humid sub tropical climate a total of 28 sediment samples were collected during the monsoon season and were analysed by x ray fluorescence spectrometry for 25 major and trace elements bulk chemistry of the channel flood and suspended sediments mostly consists 90 80 and 75 respectively of three elements al si and fe major element concentrations normalised with respect to upper continental crust ucc show strong depletion of highly mobile elements na ca and enrichment of immobile elements ti si silica enrichment in the sand fraction is probably caused by chemical weathering of feldspar mineral sorting during fluvial transportation acts as the single important factor that controls the geochemistry of these weathering products and also strongly influences major and trace element distribution in the individual sediment samples trace element ba cr cu nb ni pb v and zn concentrations were strongly correlated with major element si al fe mn and k concentrations indicating that the abundance of trace elements is controlled by the same processes that control the major element distribution in these sediments the gap weathering products were geochemically distinguished as arkose to litharenite in rock classification chemical mobility normalised with respect to tio 2 in ucc indicates that si na zr ba and sr mainly derived from feldspar muscovite and biotite are lost during weathering iron and zn remained immobile during weathering and were strongly adsorbed by phyllosilicates and concentrated in fine grained sediment fractions the chemical index of alteration indicates that the gap has experienced chemical weathering of incipient to moderate intensity the gap weathering products also demonstrated a progressive incomplete alteration in the alluvial sequence made up of the himalayan derived sediments a model has been proposed to better understand weathering processes and products of the gap in temporary storage of 50 ka in a humid sub tropical climate

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

microbially influenced formation of 2 724 million year old stromatolites nanoscale evidence suggests that the tumbiana formation stromatolites in australia were influenced by microbial activity in the stromatolites clusters of organic globules are closely associated with 2 724 million year old aragonite crystals