reduction of oh contamination in quantification of water contents using nanosims imaging abstract quantification of water content is relevant in various topics in geology and planetary sciences nanosims has capabilities for high spatial resolution imaging and offers opportunities to accurately quantify water contents at fine scale on small surface areas the main concern using ion microprobe techniques is to estimate and minimize contribution of water contamination from residual gas in the sample chamber sticking onto the surface of the sample here we tackle a set of sputtering analytical parameters and we evaluate their relative influence on the oh si ratio we demonstrate that a high erosion rate reached using a primary beam intensity of 25 pa is sufficient to lower this oh contamination for basaltic glass this leads us to describe a procedure to correct for oh contamination and thus determine accurate values of oh si ratio in order to quantify water contents in silicate materials using nanosims imaging

nitrogen isotope evidence for anoxic deep marine environments from the mesoarchean mosquito creek formation australia abstract current evidence for oxygenated environments in the mesoarchean is limited to the shallowest marine and fluvio lacustrine settings it is not until the neoarchean that signs of oxygenated surface waters above outer shelf and basinal depositional environments become evident in order to further explore the mesoarchean redox landscape for signs of basinward surface water oxygenation we present nitrogen and carbon isotope ratios from the turbiditic mosquito creek formation of the nullagine group 2 9 ga the 15n and 13corg values are invariant around 1 8 and 32 respectively throughout a 70 m section of drill core suggesting an ecosystem dominated by nitrogen fixers anaerobic nitrogen cycling and co2 fixation by the calvin cycle when compared with other archean isotopic data these results i provide further evidence that the mosquito creek formation was deposited in a marine basin and ii contain 15n values that highlight the prevalence of nitrogen fixation by mo nitrogenase and the dearth of aerobic nitrogen metabolisms in the mesoarchean

aqueous alteration in the northwest africa 817 nwa 817 martian meteorite samples of a new martian meteorite of the nakhlite family nwa 817 contain traces of an iron rich alteration product textural arguments indicate that this alteration product has been formed on the parent body of the meteorite mars the chemical composition and structural data x ray diffraction transmission electron microscopy and vibrational spectroscopy show that the alteration mineral is a hydrous phase from the smectite family major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive eu anomaly olivine and possibly apatite hydrogen isotope data display negative deltad values ranging from 60 to 280parts per thousand in olivine and pyroxenes and from 140 to 181parts per thousand in the alteration phase the values of deltad for the alteration product show a small scatter with a mean value of 170 14parts per thousand these values are lower than those previously obtained on other martian meteorites which give mainly positive deltad values these positive values have been interpreted as resulting from the interaction of the martian meteorites with water from the martian atmosphere ruling out the effect of terrestrial alteration it is suggested that alteration in the nwa 817 meteorite was likely produced on mars by the circulation of an aqueous fluid originating from a chemical reservoir such as the martian mantle which has not equilibrated with a fractionated martian atmosphere c 2002 elsevier science b v all rights reserved

a compositional classification scheme for meteoritic chondrules a taxonomic scheme is proposed for the main component of the primitive chondrite meteorites the chondrules the scheme provides insight into the variety of chondrules originally produced in the primordial solar nebula and the effects on them of secondary processes on meteorite parent bodies it may also contribute to understanding the origin of compositional diversity in the chondrites

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

introduction initial investigations of a neoarchean shelf margin basin transition transvaal supergroup south africa abstract the late archean to earliest paleoproterozoic transvaal supergroup preserves both thick platform carbonates and basinal successions rich in iron formation two new drill cores sponsored by the agouron institute document the platform basin transition thus facilitating the reconstruction of both late archean basin development and biogeochemical processes at play just before the great oxidation event interpreted in light of previous outcrop and subsurface studies the new cores provide an unusually detailed framework of time and environment for sedimentological palemagnetic and geochemical analyses the papers in this special issue constitute the first fruits of research on the agouron cores but undoubtedly not the last

forward and back diffusion through argillaceous formations the exchange of solutes between aquifers and lower permeability argillaceous formations is of considerable interest for solute and contaminant fate and transport we present a synthesis of analytical solutions for solute diffusion between aquifers and single aquitard systems validated in well controlled experiments and applied to several datasets from laboratory and field scale problems with diffusion time and length scales ranging from 10 2 108 years and 10 2 102 m one dimensional diffusion models were applied using the method of images to consider the general cases of a finite aquitard bounded by two aquifers at the top and bottom or a semi infinite aquitard bounded by an aquifer the simpler semi infinite equations are appropriate for all domains with dimensionless relative diffusion length zd 0 7 at dimensionless length scales above this threshold application of semi infinite equations to aquitards of finite thickness leads to increasing errors and solutions based on the method of images are required measured resident solute concentration profiles in aquitards and flux averaged solute concentrations in surrounding aquifers were accurately modeled by appropriately accounting for generalized dynamic aquifer aquitard boundary conditions including concentration gradient reversals dimensionless diffusion length scales were used to illustrate the transferability of these relatively simple models to physical systems with dimensions that spanned 10 orders of magnitude the results of this study offer guidance on the application of a simplified analytical approach to environmentally important layered problems with one or two diffusion interfaces

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

evolution of primary producers and productivity across the ediacaran cambrian transition abstract nearly all extant animal phyla first appeared during the ediacaran cambrian transition a revolution in primary producer ecology and marine productivity has been proposed as a bottom up ecological driver for this rapid diversification of metazoans however the control s driving the evolution of primary producers and primary productivity around the ediacaran cambrian transition and their potential relationship to the cambrian explosion are not fully understood in this study we measured the nitrogen content and the nitrogen isotopic composition of kerogen in the piyuancun and hetang formations using samples collected from the chunye 1 well on the lower yangtze block in western zhejiang our isotope results suggest that nitrate was the dominant n source fueling primary productivity during deposition of the entire piyuancun formation and lower hetang formation with only a minor contribution from n 2 fixation quantitative reconstructions of paleoproductivity based on organic carbon and pyrite content and sedimentation rate indicate that productivity either stayed the same or increased during the transition from the piyuancun formation to the overlying hetang formation thus fixed n availability was not the ultimate factor limiting primary productivity during the ediacaran cambrian transition combining our data with published nitrogen isotope records from the early middle ediacaran to cambrian stage 4 suggests that no 3 was the dominant n species fueling phytoplanktonic productivity during the broad cambrian explosion interval from its earliest roots in the ediacaran through the main episode in cambrian stage 3 eukaryotes lack the ability to fix nitrogen and preferentially assimilate nitrate the no 3 dominated euphotic zone from the early middle ediacaran to cambrian stage 3 may have promoted the radiation of relatively large eukaryote phytoplankton which may have been an important driver of early animal diversification

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

nitrogen isotope evidence for stepwise oxygenation of the ocean during the great oxidation event abstract the earth s oxygenation represents one of the most important environmental drivers of life s evolution with the first rise known as the great oxidation event goe corresponding to unpreceded accumulation of atmospheric o2 changes in the flux of marine nutrients and possibly global glaciations however the detailed evolution of the goe is still debated as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation here we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from two drill cores t2 and t3 in the early paleoproterozoic turee creek group western australia to track the redox evolution of marine conditions during the goe t2 core samples of the kungarra formation which consists of clastic sedimentary rocks overlaid by the glaciogenic meteorite bore member were deposited 2 31 ga ago t3 core intercepts from bottom to top quartzite of the koolbye formation and shales and stromatolitic carbonates of the kazput formation which were deposited around 2 25 ga samples from t2 show minor variations of 13corg avg 34 5 1 7 n 30 with no significant difference between siliciclastic and glaciogenic sedimentary rocks in contrast t3 samples display an increase in 13corg from 32 0 to 24 8 n 54 from shales to carbonates in both t2 and t3 cores 13corg values are inversely correlated with al2o3 suggesting a strong petrological control on 13corg values inferred here as resulting from variable contributions of detrital organic matter bulk n contents are low from 13 5 to 56 7 ppm and 15 7 to 53 4 ppm in t2 and t3 samples respectively the 15n values show a bimodal distribution with one mode at 2 6 in t2 and another at 8 8 in t3 independent from lithological variations this 15n values shift between t2 and t3 is interpreted as reflecting a change from dominating n2 fixers to no3 assimilating organisms this implies an increase of no3 availability and thus of o2 concentration during the time interval separating the deposition of t2 and t3 sediments dissolved no3 and o2 concentrations of the turee creek marine basin are estimated from two models using n isotope data the dissolved no3 concentration has an upper limit ranging from 1 91 to 3 04 m about one order of magnitude below the average value of modern oceans the lower limit for dissolved oxygen concentration ranges from 1 8 to 4 4 m which is two orders of magnitude lower than modern oceans together with previous studies the present data place quantitative constraints on the redox changes associated with the great oxidation event and illustrate a stepwise increase of no3 bioavailability between 2 31 to 2 25 ga in relation with increasing o2 level

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

nitrogen isotope and trace element composition characteristics of the lower cambrian niutitang formation shale in the upper middle yangtze region south china abstract the early cambrian is a key interval in the global development of biological evolution in which occurred many important environmental events and organic rich sedimentary layers were deposited which formed important hydrocarbon source rocks the lower cambrian niutitang formation is one of the most important marine sedimentary layers in the yangtze region south china and it records abundant important geological information that can be used for the reconstruction of the lower cambrian paleo climates and paleo environments nitrogen isotope compositions record and reflect variations in the original sedimentary environment and the redox conditions of marine ecosystems trace elements are also reliable indicators of sedimentary environments in this study we collected samples from the lower cambrian niutitang formation shale in the upper middle yangtze region of south china and measured the nitrogen isotope values of bulk sediments 15nbulk as well as their organic carbon isotope values 13corg total organic carbon toc contents total nitrogen tn contents and trace element concentrations the 15nbulk values of the niutitang shale range from 0 6 to 2 8 they show a certain positive excursion from bottom to top in the longitudinal direction and they exhibit a significant correlation with the longitudinal variations in trace elements these trace element characteristics suggest that the lower section of the niutitang formation existed in a strongly reduced sedimentary environment and was located in a hydrothermally active layer during the deposition of the lower section of black shale the nitrogen cycle was dominated by anaerobic nitrogen fixation thus its nitrogen isotope values are relatively lighter in contrast during the deposition of the upper section of the niutitang formation the redox conditions of water column were unstable the oxygen contents increased and oxygenated reactions were intermittent or localized thus producing relatively strong isotopic fractionation and causing the 15nbulk values in the upper section of black shale to exhibit a positive excursion we conclude that the sedimentary environment is the key factor affecting nitrogen isotopes and that the redox conditions of the water column exhibit a significant correlation with the nitrogen isotope values

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

nitrogen isotope dynamics of the cariaco basin venezuela 1 we report 15n 14n measurements of water column nitrate and ammonium sinking particles and sediments from the cariaco basin an anoxic marine basin off the coast of venezuela water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic anoxic interface 275 m being free of a large denitrification signal the 15n of shallow thermocline nitrate is 3 5 significantly lower than the mean deep ocean nitrate 15n of 5 this may be due to the nitrification of newly fixed n whether it occurs within the basin or in open atlantic waters that flow into the cariaco over the sill the 15n 14n of the sinking flux in the deepest trap 1250 m is similar to that of thermocline nitrate as expected given the complete consumption of nitrate in the surface layer moreover the 15n 14n of the seafloor sediment is similar to that of the sinking flux as is common in environments of high export production low o2 and good organic matter preservation thus the modern cariaco basin records the 15n 14n of the thermocline nitrate which in turn may record the input of newly fixed n to the upper ocean be it local or more regional in origin

an expanded shale δ98mo record permits recurrent shallow marine oxygenation during the neoarchean abstract multiple attempts have been made using the ancient shale record to track the molybdenum isotope composition 98mo of seawater during the final two hundred million years of the archean eon 2 7 to 2 5 billion years ago or ga this seawater 98mo value is important because it should have scaled with levels of ocean oxygenation during the runup to the great oxidation event goe unfortunately however it is difficult to tell if the majority of the existing late archean shale 98mo record tracks an ancient seawater value here we further attempt to track pre goe seawater 98mo using an expanded and well characterized shale sample set from western australia jeerinah wittenoom and mt sylvia formations and south africa nauga and klein naute formations most importantly and in contrast to most previous mo isotope studies of similarly aged shales local redox conditions for our shales have been independently constrained using the iron fe speciation proxy and bottom water mo contents at the time of deposition have been qualitatively estimated using mo toc ratios local redox conditions and bottom water mo availability are important parameters because transfer of the seawater 98mo to sediments today is shown to be dependent on these conditions according to our updated sedimentary 98mo record seawater 98mo commonly exceeded 1 0 between 2 69 ga and 2 50 ga in order to drive such a heavy seawater 98mo there must have been a marine sink with a strong preference for lighter mass mo isotopes frequently present over this timeframe the operation of some anaerobic processes in late archean marine settings could theoretically explain the heavier seawater 98mo such processes are known to promote the preferential retention of lighter mass mo isotopes in marine sediments today e g interactions between mo and organic matter or the formation of thio complexes alternatively or in addition adsorption of lighter mass mo isotopes to fe and mn oxide minerals formed in oxygenated marine environments can explain the heavier seawater 98mo a compilation of previous work suggests that oxygenated shallow marine environments were fairly common during the late archean and thus mo adsorption to fe mn oxides formed in these settings probably played an important role in driving heavy seawater 98mo over this timeframe

transient surface ocean oxygenation recorded in the 2 66 ga jeerinah formation australia many paleoredox proxies indicate low level and dynamic incipient oxygenation of earth s surface environments during the neoarchean 2 8 2 5 ga before the great oxidation event goe at 2 4 ga the mode tempo and scale of these redox changes are poorly understood because data from various locations and ages suggest both protracted and transient oxygenation here we present bulk rock and kerogen bound nitrogen isotope ratios as well as bulk rock selenium abundances and isotope ratios from drill cores sampled at high stratigraphic resolution through the jeerinah formation 2 66 ga fortescue group western australia to test for changes in the redox state of the surface environment we find that both shallow and deep depositional facies in the jeerinah formation display episodes of positive primary 15n values ranging from 4 to 6 recording aerobic nitrogen cycling that requires free o2 in the upper water column moderate selenium enrichments up to 5 4 ppm in the near shore core may indicate coincident oxidative weathering of sulfide minerals on land although not to the extent seen in the younger mt mcrae shale that records a well documented whiff of atmospheric oxygen at 2 5 ga unlike the mt mcrae shale jeerinah selenium isotopes do not show a significant excursion concurrent with the positive 15n values our data are thus most parsimoniously interpreted as evidence for transient surface ocean oxygenation lasting less than 50 my extending over hundreds of kilometers and occurring well before the goe the nitrogen isotope data clearly record nitrification and denitrification providing the oldest firm evidence for these microbial metabolisms

petrology of feo poor porphyritic pyroxene chondrules in the semarkona chondrite abstract the mineralogy and petrology of feo poor porphyritic pyroxene and olivine rich chondrules in the semarkona ll3 0 chondrite are described in detail in an extension of the textural classification scheme these chondrules are designated types iab and ib in type iab chondrules the proportion of olivine phenocrysts is between 20 80 and in type ib chondrules olivine constitutes

early diagenetic quartz formation at a deep iron oxidation front in the eastern equatorial pacific a modern analogue for banded iron chert formations the mechanisms of early diagenetic quartz formation under low temperature conditions are still poorly understood in this study we investigated lithified cherts consisting of microcrystalline quartz recovered near the base of a 420 m thick miocene holocene sequence of nannofossil and diatom ooze at a drill site in the eastern equatorial pacific ocean drilling program site 1226 precipitation seems still ongoing based on a sharp depletion in dissolved silica at the depth of the cherts also palaeo temperatures reconstructed from 18o values in the cherts are in the range of adjacent porewater temperatures opal a dissolution appears to control silica concentration throughout the sequence while the solution remains oversaturated with respect to quartz however at the depth of the sharp depletion in dissolved silica quartz is still saturated while the more soluble silica phases are strongly undersaturated hence precipitation of quartz was initiated by an auxiliary process a process previously observed to assist in the nucleation of quartz is the adsorption of silica on freshly precipitated iron oxides indeed a deep iron oxidation front is present at 400 m below seafloor which is caused by upward diffusing nitrate from an oxic seawater aquifer in the underlying oceanic crust sequential iron extraction showed a higher content of the adsorbed iron hydroxide fraction in the chert than in the adjacent nannofossil and diatom ooze x ray absorption near edge structure xanes spectroscopy revealed that iron in the cherts predominantly occurs in illite and amorphous iron oxide whereas iron in the nannofossil and diatom ooze occurs mainly in smectite mossbauer spectroscopy also indicated the presence of illite that is to 97 oxidized two possible mechanisms may be operative during early diagenetic chert formation at iron oxidation fronts 1 silica precipitation is catalysed by adsorption to freshly precipitated iron oxide surfaces and 2 porewater silica concentration is locally decreased below opal a and opal ct saturation allowing for precipitation of the thermodynamically more stable phase quartz this mechanism of chert formation at the iron oxidation front in suboxic zones may explain why early diagenetic microcrystalline chert only occurs sporadically in modern marine sediments it may also serve as a modern analogue for the deposition of much more abundant banded iron chert formations at the time of the great oxidation event around 2 4 ga bp which was probably the largest iron oxidation front in earth s history

interstellar hydroxyl in meteoritic chondrules implications for the origin of water in the inner solar system abstract ion probe determinations of water concentrations and d h ratios have been performed on different phases pyroxene olivine and mesostasis of chondrules from the bishunpur and semarkona ll3 chondrites mean water concentrations average 1290 ppm in olivine 500 to 2100 ppm 2400 ppm in pyroxene 400 to 9800 ppm and up to 3950 ppm in the mesostasis 1000 to 16500 ppm in one chondrule an iron oxide phase exhibits a mean water concentration of 1 70 wt 1 21 to 2 15 wt hydrogen isotopic measurements reveal the presence of two water sources with high and low d h ratios pyroxenes range from 74 to 479 10 6 with a mean value of 186 10 6 n 54 the mesostasis from 124 to 203 10 6 with a mean value of 152 10 6 n 8 olivines from 63 to 209 10 6 with a mean value of 130 10 6 n 13 the iron oxide phase from 157 to 360 10 6 with a mean value of 219 10 6 n 7 hydroxyls in pyroxenes have been identified as submicroscopic poorly crystallized amphibole lamellae through high resolution transmission electron microscopy considering the broad range of d h ratios in pyroxene chondrules alteration in a unique meteoritic parent body seems to be excluded an early hydroxylation of the pyroxene occurring during chondrule formation due to the presence of water bearing minerals among chondrule precursors seems the only possible interpretation of the data accordingly the distribution of d h ratios in chondrules reflects that of their precursors such a distribution is quite similar to that found in matrix minerals as recorded by water concentration and d h ratio the mesostasis has been subjected to hydrothermal alteration after the incorporation of the chondrules into the meteoritic parent body this alteration took place with water having a mean d h ratio reflecting the mixing of the d depleted and d rich sources the d rich component is interpreted as a preserved interstellar source while the low d h ratio reflects a water component that underwent isotopic exchange with protosolar molecular hydrogen

the linkage of nitrogen isotopic composition and depositional environment of black mudstones in the upper triassic yanchang formation ordos basin northern china abstract in this study we report new analyses of stable nitrogen isotopic compositions 15n of the black mudstones from the chang 7 member of the upper triassic yanchang formation in the ordos basin china to examine the possible linkage of such isotopic compositions with the depositional environment these mudstones were deposited mainly under suboxic bottom water column conditions the core descriptions and trace element concentrations differ between samples obtained from the chang 73 and chang 71 2 members situated at the bottom and top of the chang 7 member respectively such that the depositional condition of the former implies a more reduced water regime the bulk rock nitrogen isotope 15nbulk values of the samples obtained from the chang 73 member are higher than those of the chang 71 2 member no significant differences were identified in the organic carbon isotopes 13corg total organic carbon toc or total nitrogen tn values between the two units the results of 15nbulk cross plotting with the toc tn and carbon to nitrogen atomic ratio c n show an absence of significant modification of the primary nitrogen isotopic compositions from post depositional processes given the depositional history of these units we suggest that the sedimentary redox environment can have a significant influence on 15nbulk denitrification which occurs mainly in suboxic conditions plays an important role in such an environment resulting in 15n enrichment during the original deposition thus owing to its sensitivity to sedimentary redox environments the 15nbulk index can be an effective tool for evaluating depositional redox conditions particularly for further subdivision of suboxic environments

a test of the nitrogen limitation hypothesis for retarded eukaryote radiation nitrogen isotopes across a mesoproterozoic basinal profile abstract nitrogen limitation caused by trace metal scarcity under euxinic ocean conditions has been proposed as an explanation for the delayed radiation of eukaryotes until at least the late mesoproterozoic however evidence for how the nitrogen cycle was operating during the middle precambrian is so far rare more specifically it is unknown which steps in the biogeochemical nitrogen cycle e g nitrogen fixation nitrification or ammonification were rate limiting and thus controlling microbial community structures the mesoproterozoic belt supergroup in western montana hosts a variety of facies ranging from shallow to deep water and thus offers the opportunity to address this issue bulk 15 n values show a clear trend from 1 in the deepest part of the basin to 5 along basin margins which suggests that coastal areas were sufficiently oxygenated for aerobic nitrogen cycling the total fractionation of carbon isotopes between carbonate and organic carbon 13 c increases from 20 to 32 in the same direction possibly indicating an ecological response to redox stratification and nitrogen speciation evidence from the belt supergroup is thus consistent with the idea that nutrient availability may have restricted early eukaryotic organisms to a narrow range of habitats which thus prevented a global rise to ecological dominance until concentrations of fixed nitrogen increased in the global open ocean

origin of two distinct multiple sulfur isotope compositions of pyrite in the 2 5ga klein naute formation griqualand west basin south africa abstract quadruple sulfur isotope ratios 32s 33s 34s 36s were measured by a high precision sf6 method for early diagenetic pyrite in the 2 5 ga upper prieska facies of the griqualand west basin from the agouron drill core gkp01 measured 33s 33s 0 515 34s values range between 2 3 and 8 5 with no apparent meter scale stratigraphic trend for 70 m section it was found that early diagenetic pyrites occur in two texturally and isotopically distinct populations such that bulk rock isotope data would reflect mixing of these two sulfur pools nodular and layered pyrite yield mostly negative down to 2 3 and relatively constant 33s values over a few tens of cm scale disseminated pyrites in contrast yield positive 0 0 8 5 33s values that are heterogeneous in sub cm scales the former pyrite sulfur was likely derived from seawater sulfate through microbial sulfate reduction and the latter preferentially sampled aqueous h2s s0 reservoirs in late archean oceans relatively constant 33s values for layered pyrite suggest archean sulfate reservoirs had residence times of a few million years most data yield a well defined 36s 33s ratios of 0 9 suggesting predominance of a single s mif source reaction four analyses of a two cm length sample 306 5 m yield 36s 33s slope of 1 6 this slope is consistent with the relationship expected for a symmetry breaking isotope effect during the s s2 reaction but deviations from the main archean array i e 36s 33s 0 9 may also result from various combinations of so photolysis and so2 oxidation high spatial resolution high precision sf6 measurements of archean pyrite provide significant new insights into the origin of archean sulfur mass independent fractionation s mif and the late archean sulfur cycle

contaminant transport in layered porous media 2 applications in a companion paper analytical solutions describing the fate of a solute in a layered aquifer system are given in the study presented here these closed form solutions are utilized in solution of several problems of relevance to layered aquifer analysis initially the example problems discussed here are primarily chosen for the purpose of verification of some of these analytical solutions these applications are later extended to other aquifer aquitard configurations to demonstrate the range of problems that may be analyzed using these procedures thus in this sense the problems discussed here cover only a sample of cases which may be studied utilizing the solutions developed earlier in the examples discussed here the importance of advective transport in the vertical direction is emphasized for contaminant transport analysis in a layered system

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

aqueous alteration of carbonaceous chondrites evidence for preaccretionary alteration a review in most groups of carbonaceous chondrites minerals occur that are formed due to aqueous alteration in the nebula and or within meteorite parent bodies for determining the evolution of materials in the early solar system it is of significant importance to clearly identify evidence for either nebular or planetary aqueous alteration therefore results from the study of chondrites have fundamental implications for ideas concerning nebular dynamics gas solid interactions in the nebula and accretionary processes considering the topic of this review it is important to define nebular and parent body processes the solar nebula activity should include condensation processes of high and low temperature components as well as processes of chemical fractionation and grain size sorting mixing of solids and gas and interactions between early formed solids with the remaining gas but it should exclude processes that occurred in small uncompacted protoplanetary objects that may have been totally destroyed again before accretion of the final meteorite parent bodies therefore the term preaccretionary instead of nebula is used in this paper to include all these processes that may have occurred in small precursor planetesimals currently there is no doubt that parent body aqueous alteration is a fundamental process in the evolution of several groups of carbonaceous chondrites however due to textural and mineralogical observations and chemical analyses strong arguments have been found also indicating preaccretionary aqueous alteration of distinct components in carbonaceous chondrites in this paper evidence for preaccretionary aqueous alteration in carbonaceous chondrites and their components taken from previous studies is discussed in detail the strongest evidence for preaccretionary alteration comes from studies of cm chondrites the survival of highly unequilibrated mineral assemblages in accretionary rims and sharp contacts between chondrule glass and surrounding phyllosilicates are only two important arguments for preaccretionary alteration features among many others discussed in the paper similar observations were also made in cr ch and related chondrites due to the small abundance of water bearing phases in co and cv chondrites the origin of phyllosilicates in these groups is less clear a preaccretionary origin of hydrous phases in ca al rich inclusions cais has also been suggested by several scientists in ci chondrites no strong indications for such processes have been found mainly due to heavy brecciation and severe parent body alteration

spatial and temporal trends in precambrian nitrogen cycling a mesoproterozoic offshore nitrate minimum abstract fixed nitrogen is an essential nutrient for eukaryotes as n 2 fixation and assimilation of nitrate are catalyzed by metalloenzymes it has been hypothesized that in mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses restricting eukaryotes to near shore environments and limiting their evolutionary innovation to date this hypothesis has only been tested in the belt supergroup 1 4 ga with results that support an onshore offshore nitrate gradient as a potential control on eukaryote ecology here we present bulk nitrogen and organic carbon isotopic data from non isochronous cross basinal facies across the bangemall 1 5 ga and the roper 1 4 1 5 ga basins to better understand the extent and variability of onshore offshore nitrogen isotope gradients in the mesoproterozoic both basins show an average 1 2 enrichment in 15 n bulk from deep to shallow facies with a maximum range from 1 offshore to 7 5 onshore unlike the belt basin the bangemall and roper basins show some offshore 15 n bulk values that are enriched beyond the isotopic range associated with biological n 2 fixation alone this suggests a mixture of aerobic and anaerobic metabolisms offshore in shallow waters where 15 n bulk enrichment peaks an aerobic nitrogen cycle was evidently operating even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the bangemall and roper basins our data are consistent with a lateral gradient in nitrate availability within the photic zone with higher concentrations in near shore environments than offshore the variability in 15 n bulk values in each depositional environment and the consistently low 15 n bulk values from mesoproterozoic units compared to the paleoproterozoic and neoproterozoic suggest that nitrate concentrations in the global ocean were likely low this trend is now seen in all three mesoproterozoic basins so far examined and contrasts with the paleoproterozoic and neoproterozoic where nearly all 15 n bulk data plot above the n 2 fixation window thus we propose that the mesoproterozoic ocean was characterized by a nitrate minimum with the lowest concentrations in offshore environments this inference is consistent with a mesoproterozoic o 2 decline following a temporary paleoproterozoic o 2 peak and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near shore environments delaying their rise to ecological dominance until the neoproterozoic era

testing models of pre goe environmental oxidation a paleoproterozoic marine signal in platform dolomites of the tongwane formation south africa abstract carbonate facies of the upper tongwane formation preserve a largely overlooked record of pre goe paleoproterozoic seawater this inventory has survived despite a complex paragenetic history involving both diagenetic and contact metamorphic processes bif mineral assemblages are dominated by a medium grade grunerite overprint and the formation of prograde and retrograde riebeckite the massive dolomite member a platform top carbonate which caps the succession is characterized by marine rey sn patterns lacking significant negative ce sn anomalies 13 c carb values of between 1 83 and 2 95 vpdb that are slightly elevated above paleoproterozoic normal marine values systematically decreasing 13 c carb values measured in monotonously recrystallized talc bearing slope carbonates reflect interaction with devolatilization fluids during contact metamorphism caused by the intrusion of the bushveld igneous complex slope carbonates offer no insight into marine paleoredox conditions thus despite contact metamorphism paleoproterozoic marine signals are retained in platform top dolomites however they do not support claims for widespread oxygen accumulation in shallow depositional environments of the transvaal basin on the eve of the goe

tectonic and geochronological constraints on late archaean and palaeoproterozoic stratigraphic correlation within and between the kaapvaal and pilbara cratons recent radiometric dating of carbonates and banded iron formations on the kaapvaal craton southern africa has suggested that late archaean and palaeoproterozoic carbonate sedimentation was diachronous across the craton we present new shrimp u pb zircon ages of 2583 5 ma and 2588 7 ma for two samples of a tuff bed 320 km apart in the upper oak tree formation transvaal supergroup these ages in conjunction with published data support previously established sequence stratigraphic correlations that show the temporal equivalence of late archaean carbonates in the region a review of the sequence stratigraphy and geochronology of the mount bruce megasequence set on the pilbara craton western australia and the newly named highveld megasequence set on the kaapvaal craton indicates a remarkably similar geohistory both cratons are characterized by a divergent megacycle hamersley range and vaal megasequences followed by a convergent megacycle chichester range and drakensberg megasequences our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time c 2590 ma which permits more meaningful geotectonic correlations between them

isotopic evidence for the origin of an acid sulphate alteration styrian basin austria the gossendorf volcanic body is the only one in the styrian basin that shows extensive hydrothermal alteration k ar dating of primary volcanic biotite and alteration products alunite suggests that the emplacement of the volcanic body and hydrothermal alteration took place synchronously 15 ma ago the stable isotope compositions of the alteration products such as opal barite pyrite and alunite combined with crystallographia investigations indicate temperatures between 150 and 200 c for the formation of the alteration zones the calculated stable isotopic compositions of the parent fluid responsible for the alteration show an exogene marine component which interacted with the host rock sulphur isotopic compositions of sulphur sulphides and sulphates indicate disequilibrium and progressive oxidation this fact combined with the mineral zonation of the alteration zone reflects not only change in the ph but also change in the fo2 of the ascending fluids

solutions approximating solute transport in a leaky aquifer receiving wastewater injection a mathematical model amenable to analytical solution techniques is developed for the investigation of contaminant transport from an injection well into a leaky aquifer system which comprises a pumped and an unpumped aquifer connected to each other by an aquitard a steady state groundwater flow field is assumed where the injected fluids move horizontally in the pumped aquifer and vertically in the aquitard the unpumped aquifer is assumed to remain in hydrostatic condition due to its large transmissivity descriptions of groundwater velocities are based on appropriate leaky aquifer well hydraulics in general the modeling approach neglecting longitudinal dispersion in the pumped aquifer and lateral dispersion in the aquitard does not introduce significant error to the modelling results as shown by the good agreement between numerical solutions including these two transport mechanisms and the solutions of the proposed model within the context of the assumed hydrogeological conditions the proposed model appears to be a valid approximation to the problem of interest type curves for concentration evaluated at the well bore and the tope of the aquitard are given which can be employed to estimate whether the unpumped aquifer would be polluted as the consequence of the injection neglecting the leakage effectsmore in a leaky aquifer can cause significant error in studying the transport problem less

an analytical solution of two dimensional reactive solute transport in an aquifer aquitard system 1 this article presents an analytical study of two dimensional reactive solute transport in an aquifer aquitard system advection longitudinal and vertical dispersion first order irreversible decay and linear sorption are included for aquifer transport advection vertical diffusion first order irreversible decay and linear sorption are considered for aquitard transport this study solves transport equations in the aquifer and aquitards simultaneously and maintains continuities of concentration and mass flux at the aquifer aquitard interfaces the transport equations are first solved in laplace domain and the solutions are then inverted numerically to yield solutions in the real time domain the zhan solution concentration profiles in the aquifer and the aquitards are obtained and mass transported to the aquitards is also calculated closed form steady state solutions of concentration and mass have been obtained the zhan solution is compared with solutions derived with the averaged volumetric sink source approach the aa solution the aa solution substantially overestimates the concentration and the mass in the upper aquitard when an upward velocity exists in the aquitard the penetrating depths of the solute in the aquitard are similar for the aa solution and the zhan solution

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

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

geological nitrogen cycle and atmospheric n2 over phanerozoic time using the average nitrogen carbon ratio n c of sedimentary organic matter coal and volcanic metamorphic gases existing models of the carbon cycle have been used to calculate the fluxes of nitrogen to and from the atmosphere via the weathering burial and metamorphic volcanic decomposition of organic matter results indicate no appreciable change less than one percent of that present in the mass of atmospheric n 2 over phanerozoic time this is in sharp contrast to atmospheric o 2 which shows notable variations variation of the mass of o 2 but not that of n 2 must have led to variation in total atmospheric pressure with time results are not altered by considering n c ratios ten times higher than the average for sedimentary rocks nor by considering nitrogen release from silicate weathering the residence time of n 2 in the atmosphere relative to exchange with and storage in crustal rocks is estimated to be about one billion years

late archean molecular fossils from the transvaal supergroup record the antiquity of microbial diversity and aerobiosis abstract cores recovered during the agouron griqualand drilling project contain over 2500 m of well preserved late archean transvaal supergroup sediments dating from ca 2 67 to 2 46 ga bitumen extracts of these strata were obtained using clean drilling sampling and analysis protocols that avoided overprinting syngenetic molecular fossil signatures with contaminant hydrocarbons comparisons of biomarker contents in stratigraphically correlated intervals from diverse lithofacies in two boreholes separated by 24 km as well as across a 2 gyr unconformity provide compelling support for their syngenetic nature the suite of molecular fossils identified in the late archean bitumens includes hopanes attributable to bacteria potentially including cyanobacteria and methanotrophs and steranes of eukaryotic origin this molecular fossil record supports an origin in the archean eon of the three domains of cellular life as well as of oxygenic photosynthesis and the anabolic use of o2

differential metamorphic effects on nitrogen isotopes in kerogen extracts and bulk rocks abstract the last decade has seen a steady rise in the number of publications on nitrogen isotopes in sedimentary rocks which have become an established tool for investigating the evolution of life and environmental conditions nitrogen is contained in sedimentary rocks in two different phases bound to kerogen or substituted in potassic minerals mostly k bearing phyllosilicates and feldspars isotopic measurements and interpretations typically focus either on kerogen extracts alone or on bulk rocks that include both phases the community is split about which sample type more accurately captures the original composition of biomass to address this question we combined nitrogen isotopes and carbon to nitrogen ratios with carbon to hydrogen ratios which act as an independent proxy for metamorphic alteration our results reveal that metamorphism drives kerogen bound nitrogen isotopically lighter while silicate bound nitrogen becomes heavier for rocks up to greenschist facies the isotopic effect of this internal partitioning up to 3 4 is larger than the isotopic effect of metamorphic nitrogen loss from the system up to 1 2 the opposite may be true for higher metamorphic grades we conclude that for low grade sedimentary rocks with more than 60 of their total nitrogen residing in the silicate phase the primary isotopic composition of the biomass is best approximated by bulk rock measurements whereas for high grade rocks the kerogen extract may be the more accurate proxy the isotopic difference between nitrogen phases can serve as a rough indicator of the degree of metamorphic alteration

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

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

modulation of oxygen production in archaean oceans by episodes of fe ii toxicity oxygen accumulated in the surface waters of the earth s oceans 1 and atmosphere 2 several hundred million years before the great oxidation event between 2 4 and 2 3 billion years ago 3 before the great oxidation event periods of enhanced submarine volcanism associated with mantle plume events 4 supplied fe ii to sea water these periods generally coincide with the disappearance of indicators of the presence of molecular oxygen in archaean sedimentary records 5 the presence of fe ii in the water column can lead to oxidative stressinsomeorganismsasaresultofreactionsbetweenfe ii and oxygen that produce reactive oxygen species 6 here we test the hypothesis that the upwelling of fe ii rich anoxic water into the photic zone during the late archaean subjected oxygenic phototrophic bacteria to fe ii toxicity in laboratory experiments we found that supplying fe ii to the anoxic growth medium housing a common species of planktonic cyanobacteriadecreasedboththeeci encyofoxygenicphotosynthesis and their growth rates we suggest that this occurs because of increasing intracellular concentrations of reactive oxygen species we use geochemical modelling to show that fe ii toxicity in conditions found in the late archaean photic zone could have substantially inhibited water column oxygen production thus decreasing fluxes of oxygen to the atmosphere we therefore propose that the timing of atmospheric oxygenationwascontrolledbythetimingofsubmarine plumetype volcanism with fe ii toxicity as the modulating factor possible biological explanations for the slow tempo of atmospheric oxidation invoke limitations on the efficiency of early oxygenic photosynthesis such as the toxicity of oxygen to the organisms who first produced it 7 or that a paucity of key nutrients limited growth of oxygenic phototrophs in late archaean sea water 8 iron is a key limiting nutrient for oxygenic photosynthesis in modern oxic oceans and enhanced fe fluxes to the surface ocean are generally thought to have increased primary productivity over geologic time 9 coastal environments in the late archaean experienced enhanced fluxes of fe in its reduced soluble form fe ii from upwelling sea water saturated with fe ii supplied from hydrothermal leaching of oceanic crust 10 11 in the absence of land plants in the late archaean a reasonable assumption is that most oxygen was produced or built up locally in coastal marine environments that experienced such upwelling 5 12 and oxygenic phototrophs would have experienced these fe ii fluxes nearly half of earth s primary productivity and hence its oxygen production is today produced in the oceans 13 and planktonic marine cyanobacteria such as synechococcus and prochlorococcus account for up to 25 of this number 14 as previous work documented significant oxidative stress when planktonic cyanobacteria were

the microstructure of semarkona and bishunpur abstract the ordinary chondrites semarkona and bishunpur appear to have experienced in situ hydrous alteration here we report the results of the first detailed tem examination of them which supports this conclusion in semarkona all but the most magnesian mafic minerals have been destroyed and primary sulphides most likely troilite have been partially altered to phyllosilicates and ni rich pyrrhotite in bishunpur alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix during hydrous alteration of both meteorites the temperature probably did not exceed 260 c the equilibrium composition of the gas present during the alteration of semarkona was not solar but was dominated by h2o the formation of smectite does not require the presence of liquid water however evidence for the redistribution of elements such as ca zn and si over tens of microns or more implies that there was at least some transient grain boundary fluid present the alteration in semarkona and bishunpur is far from complete thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the cm and ci carbonaceous chondrites superficially the mineralogy mineral chemistry and bulk chemistry of semarkona matrix rims resemble those of some chondritic interplanetary dust particles cidps the ordinary chondrite parent bodies may therefore be a source of some cidps

micron scale d h heterogeneity in chondrite matrices a signature of the pristine solar system water abstract organic matter and hydrous silicates are intimately mixed in the matrix of chondrites and in situ determination of their individual d h ratios is therefore challenging nevertheless the d h ratio of each pure component in this mixture should yield a comprehensible signature of the origin and evolution of water and organic matter in our solar system we measured hydrogen isotope ratios of organic and hydrous silicates in the matrices of two carbonaceous chondrites orgueil ci1 and renazzo cr2 and one unequilibrated ordinary chondrite semarkona ll3 0 a novel protocol was adopted involving nanosims imaging of h isotopes of monoatomatic h and molecular oh secondary ions collected at the same location this allowed the most enriched component with respect to d to be identified in the mixture using this protocol we found that in carbonaceous chondrites the isotopically homogeneous hydrous silicates are mixed with d rich organic matter the opposite was observed in semarkona hydrous silicates in semarkona display highly heterogeneous d h ratios ranging from 150 to 1800 10 6 d smow 40 to 10 600 organic matter in semarkona does not show such large isotopic variations this suggests limited isotopic exchange between the two phases during aqueous alteration our study greatly expands the range of water isotopic values measured so far in solar system objects this d rich water reservoir was sampled by the ll ordinary chondrite parent body and an estimate 9 of its relative contribution to the d h ratio of water in oort cloud family comets is proposed

nitrogen isotope evidence for water mass denitrification during the early toarcian jurassic oceanic anoxic event bulk sedimentary nitrogen isotope 15ntot data have been generated from lower jurassic black carbon rich shales in the british isles and northern italy deposited during the early toarcian oceanic anoxic event a pronounced positive 15ntot excursion through the exaratum subzone of the falciferum zone defined by characteristic ammonites in the british isles broadly correlates with a relative maximum in weight percent total organic carbon and in some sections with a negative 13corg excursion upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of 15ntot and parallels may be drawn with quaternary sediments of the arabian sea gulf of california and northwest mexican margin the development of early toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity approximately coincident phenomena include the following a relative climatic optimum realignment of major oceanic current systems and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

the bulk valence state of fe and the origin of water in chondrites abstract there is abundant petrologic evidence for the oxidation of fe during the aqueous alteration of chondrites and water must have been the oxidant for this process the h2 lost from the chondrite parent bodies as a result of fe oxidation would have been isotopically very light enriching any residual water in d the extents of the d enrichments will have depended on the fractions of water consumed and the temperatures during fe oxidation here we have estimated the likely ranges of water consumed by fe oxidation in the ci cm cr and ll parent bodies as well as the likely range of changes in water h isotopic compositions this would have produced we first used fe xanes to determine the fe valences of bulk meteorite powders in orgueil ci1 a number of cms and crs that experienced varying degrees of alteration and semarkona ll3 00 the total ranges of bulk fe valences we obtained were orgueil 2 77 cms 2 40 2 63 crs 1 46 2 54 and semarkona 2 10 combining previous estimates of the present water oh contents of our samples with the present bulk fe valences and an estimated range of initial bulk fe valences we estimate the likely ranges of fractional water losses to have been orgueil 15 26 semarkona 73 83 cms 23 48 and crs 39 62 the associated maximum and minimum changes in the h isotopic compositions of the remaining water were estimated assuming the equilibrium h2 h2o isotopic fractionation factor rayleigh fractionation of the h2 and oxidation temperatures of 0 200 c using previous estimates of the water h isotopic compositions in the chondrites the ranges of estimated d values for the initial chondritic waters are orgueil 672 to 422 cms 676 to 493 crs 527 to 56 and semarkona 527 to 154 the ci cm cr and ordinary chondrites all accreted water with similar h isotopic compositions that were distinct from the compositions of comets or saturn s moon enceladus thus the carbonaceous chondrites are unlikely to have come from comets or from bodies that were scattered into the asteroid belt from comet forming regions by orbital migration of the giant planets if the carbonaceous chondrites did form in the outer solar system as some models predict it was probably not beyond 7 au however based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner solar system

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

classification metamorphic history and pre metamorphic composition of chondrules abstract sears et al 1992 and dehart et al 1992 have proposed that the existing scheme for chondrule classification which is based on modal mineralogy texture and mineral composition should be replaced by a new scheme that is based primarily on the compositions of olivine phenocrysts and mesostases but also on their cathodoluminescence we have used published and unpublished compositional data for chondrules in ordinary and carbonaceous chondrites to compare the two classification schemes we find that the new scheme does not preserve the identity of the existing chondrule groups even in the least metamorphosed chondrites because of difficulties in assigning chondrules to the proposed groups and the lack of olivine grains or mesostasis areas of sufficient size 30 of chondrules in some type 3 ordinary chondrites cannot be classified in the new scheme using compositional data for type 4 6 chondrites the existing scheme provides more information about the history of individual chondrules than the new scheme we conclude that textural and modal criteria should be retained for the classification of chondrules

paired carbonate organic carbon and nitrogen isotope variations in lower mississippian strata of the southern great basin western united states abstract the early mississippian k o kinderhookian osagean carbon isotope 13c excursion or tice mid tournaisian carbon isotope excursion is one of the most prominent positive 13c excursions of the phanerozoic recent studies raise uncertainties about the representative shape single vs double spikes and magnitude of this 13c excursion 3 to 6 in south china 5 5 in europe and 7 in north america and the 3 unidirectional increase in nitrogen isotopes across the 13c excursion which is unanticipated considering the amount of organic carbon burial required to form the 13c excursion and the resultant oxygen increase and global cooling to test if stratigraphic completeness and spatial isotope variations caused such uncertainties we have conducted paired carbonate carbon 13ccarb organic carbon 13corg and nitrogen 15n isotope analyses across the k o interval in two well exposed sections of the southern great basin western united states the two sections represent proximal shallow water and distal deep water depositional settings of a west dipping carbonate ramp in the distal ramp section where no exposure surface is present both 13ccarb and 13corg show double spikes with peak 13ccarb values up to 7 and a negative shift down to 4 between the peaks in the proximal shallower water section where two karstic disconformities are observed 13corg shows similar double spikes but 13ccarb displays only a single peak with the highest value of 5 5 the missing 13ccarb spike is likely caused by diagenetic alteration below a karstic disconformity that lowered 13ccarb but not 13corg values resulting in smaller magnitude of the 13ccarb excursion these features suggest that the 7 magnitude and double spikes are more representative of the k o 13c excursion in the southern great basin the smaller magnitude of the k o 13ccarb excursion in some sections of the great basin and the tice in other sections globally may have overprinted with local environmental diagenetic signal or resulted from stratigraphic hiatus truncation which needs to be clarified in future research the 15n across the k o 13c excursion in the distal ramp section is decoupled from 13c with the majority of 15n values around 4 1 that do not show any obvious temporal trend in contrast 15n values in the shallow water section is coupled with the k o 13c excursion with a 3 positive shift from 4 to 7 at the rising limb of the 13c excursion and a negative shift from 7 to 1 2 at the falling limb of the 13c excursion the 15n trend from the distal ramp section is in some extent comparable with that documented from a section in south china while the coupled 13c 15n pattern in the proximal section seems better explain the potential redox change across a prominent 13c excursion considering the sensitivity of 15n to redox conditions of depositional environments a more comprehensive 15n study in a broader paleogeographic context is required to better understand the interactions between carbon and nitrogen cycles across the k o interval a critical transition from the mid paleozoic greenhouse clime to late paleozoic ice age lpia

contaminant transport in layered porous media 1 general solution analysis of transport of contaminants in a layered aquifer system still poses significant challenges although considerable progress has been made in the literature toward the understanding of this problem in this study an analytical approach is chosen as the mode of analysis and a closed form solution is developed for a quasi three dimensional advective diffusion equation defined for a layered aquifer the layered aquifer is idealized as an aquifer aquitard system in which the solution domain is considered to be finite and two dimensional in the aquifer region and infinite and one dimensional in the adjacent aquitards the analytical solution outlined emphasizes the advection of contaminant in the longitudinal direction in the aquifer and also in the transverse vertical direction in the aquiferaquitard system the analytical solution also includes the two dimensional diffusion of the contaminant in the aquifer one dimensional diffusion in the aquitards and first order reaction and retardation terms in both aquifer and aquitards

nitrogen and organic carbon isotope stratigraphy of the yangtze platform during the ediacaran cambrian transition in south china abstract n and corg isotope results are presented from six sections along a west east transect in the south china basin scb covering both shallow and deeper domains in order to investigate biogeochemical cycling stratigraphic correlation and isotope systematics over the crucial ediacaran cambrian transition 15n bulk values range between 3 and 7 while 13corg values range between 21 and 39 similar isotopic trends have been identified for both these proxies across the basin although hiatuses differences in depositional setting and syn depositional bacterial fermentation may have caused some inconsistencies a trend towards negative n isotope values can be recognized above the pc c boundary in both shallow and deeper basin realms across the yangtze platform this negative 15n excursion is probably a response to photic zone anoxia and intense nitrogen fixation assimilation by diazotrophic cyanobacteria and green purple sulfur bacteria gsb and psb the xiaotan section and a composite section from the yangtze gorges area show meaningful similarities in their nitrogen isotope trends interpreted as chemocline fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform using carbon isotope stratigraphy we correlate boundary strata across the platform to test the wider significance of nitrogen isotopic variations increased bioturbation and food chain complexity across the ediacaran cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early cambrian reflecting the beginning of a new more biologically controlled era

formation pathways of proto kerogens in holocene sediments of the upwelling influenced cariaco trench venezuela abstract proto kerogens were isolated by extraction and hf hc1 treatment from core samples of holocene sediments of the cariaco trench with interpolated ages of 900 2850 and 6000 years and examined via a combination of microscopic spectroscopic and pyrolytic methods it appears that these proto kerogens were chiefly formed from phytoplanktonic components via the degradation recondensation pathway the natural sulfurisation pathway only afforded a minor contribution in spite of the conditions prevailing in the water column and sediments that correspond to those generally considered as especially favourable for the formation of sulfurised organic matter proto kerogen formation via sulfurisation i e the endpoint of the continuum leading to insoluble high molecular weight structures cross linked by sulfur and resistant to acid hydrolysis is therefore a rather slow process under these conditions however the contribution of sulfurised moieties to the total proto kerogen substantially increased with depth due to continuous sulfurisation in the time depth interval whereas formation through degradation recondensation is almost complete for the 900 years old sample onwards proto kerogen formation via carbohydrate sulfurisation is faster than lipid sulfurisation and only sulfurised carbohydrates were detected in the shallowest sample in contrast sulfurised lipids occur in the other two proto kerogens moreover their contribution relative to sulfurised carbohydrates increases with depth probably due to the higher resistance of lipids to mineralisation compared to carbohydrates

an osmium based method for assessing the source of dissolved rhenium and molybdenum to archean seawater abstract authigenic enrichments of re and mo in 2 7 2 5 ga organic rich mudrocks orm indicate that dissolved re and mo were present in late archean seawater however it can be difficult to determine if the dissolved re and mo were sourced from oxidative crustal weathering or magmatic hydrothermal extraterrestrial mhe inputs comparison of re mo and 192 os non radiogenic os isotope concentrations and initial 187 os 188 os isotope compositions from re os isochron regressions of orm can shed insight on the source of re and mo to archean seawater seawater 187 os 188 os reflects the relative contributions of radiogenic crustal versus unradiogenic mhe os inputs to seawater during the phanerozoic transient increases in magmatic hydrothermal activity marked by unradiogenic 187 os 188 os excursions and higher 192 os concentrations in orm are accompanied in some cases by higher re concentrations these unradiogenic os sources could thus have played a greater role in the archean marine re and possibly mo budget given that the riverine re and mo flux at that time was likely smaller than the phanerozoic flux hence a positive correlation between re mo and 192 os concentrations in archean orm that yields unradiogenic initial 187 os 188 os may reflect a mhe origin for dissolved re and mo in the overlying paleo seawater local oxidative weathering in the presence of photosynthetic o 2 of mafic ultramafic rocks and crustal sulfide minerals with low re os ratios is an alternative explanation but the two scenarios cannot be readily distinguished on the basis of initial 187 os 188 os by contrast an initial 187 os 188 os from archean orm that is statistically higher than the mhe baseline of 0 10 0 11 points to oxidative mobilization of re os and mo from crustal sulfide minerals rhenium and mo concentrations that are negatively correlated with 192 os concentrations in archean orm yielding unradiogenic initial 187 os 188 os suggests that some re and mo were derived from oxidative weathering such negative correlations may reflect an increased marine os sink adsorption to oxide minerals during a mild increase in local environmental o 2 levels these concepts may help trace the onset of photosynthetic o 2 production and accumulation along archean ocean margins

spatio temporal evolution of ocean redox and nitrogen cycling in the early cambrian yangtze ocean abstract the early cambrian was a critical interval for the earth system during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans a variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early cambrian ocean these include the development of a well oxygenated deep ocean at the base of cambrian stage 3 commencing at 521 ma or alternatively persistent and widespread anoxic ferruginous conditions throughout the early cambrian ocean here we present redox sensitive trace element rste fe speciation and n and c isotope 15nsed and 13corg data for samples from a section zhongnancun of the early cambrian niutitang formation which was deposited on the outer shelf of the yangtze block south china the fe speciation and rste data provide evidence of a transition from euxinic through ferruginous to oxic conditions during deposition of the niutitang formation the combination of these new data with existing data from the inner shelf to basin environment implies regional redox stratification across the yangtze block during cambrian stages 2 and 3 with oxic shallow waters above ferruginous deep waters and spatial variability in the degree of mid depth euxinia oxygenation of deeper waters may have occurred by early cambrian stage 4 514 ma a compilation of 15n values from multiple early cambrian sections of the yangtze block indicate that n2 fixation dominated the nitrogen cycle during late cambrian stage 2 low 15n values

isotopic evidence for massive oxidation of organic matter following the great oxidation event the stable isotope record of marine carbon indicates that the proterozoic eon began and ended with extreme fluctuations in the carbon cycle in both the paleoproterozoic 2500 to 1600 million years ago ma and neoproterozoic 1000 to 542 ma extended intervals of anomalously high carbon isotope ratios 13c indicate high rates of organic matter burial and release of oxygen to the atmosphere in the neoproterozoic the high 13c interval was punctuated by abrupt swings to low 13c indicating massive oxidation of organic matter we report a paleoproterozoic negative 13c excursion that is similar in magnitude and apparent duration to the neoproterozoic anomaly this shunga francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen rich atmosphere

chondrules and other components in c o and e chondrites similarities in their properties and origins we describe three types of chondrules that are common to h3 ll3 cm2 co3 and cv3 chondrites low and high feo porphyritic olivine chondrules contain olivine with fa0 3 8 and fa5 50 respectively and can easily be distinguished petrographically poikilitic pyroxene chondrules have 1 20 vol olivine fa0 2 8 enclosed by low ca pyroxene fs0 5 7 and also occur in e chondrites these three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups metallic fe ni grains containing 0 1 1 cr si and p are present in low feo olivine chondrules from all type 2 and least metamorphosed type 3 chondrites metal compositions reflect reduction during chondrule formation in the nebula not nebular condensation opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy and probably have related origins we conclude that chondrules in all known chondrite groups share similar nebular origins

marine biogeochemical cycling during the early cambrian constrained by a nitrogen and organic carbon isotope study of the xiaotan section south china the precambrian cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet when changes in chemical and physical conditions together with key biological innovations helped to trigger a biodiversity explosion the yangtze platform south china mainly characterized by continuous and unaltered successions of this age is ideally suited to high resolution palaeomarine investigations of this event in this study 15n and 13corg records from xiaotan section yunnan sw china were investigated in order to provide insight into variations in primary productivity ecological developments and marine environment the ediacaran early cambrian xiaotan section is characterized by relatively high sedimentation rates and variable lithologies carbonates cherts phosphorites sandstones and siltstones that alternate through nearly 600 meters thickness organic carbon isotope values vary between 36 and 21 tightly following and helping to complete published 13ccarb trends the base of the cambrian is characterized in this section by a significant drop from 25 to 35 mirroring the 13ccarb fall observed at this and other sections worldwide higher in the section in the dahai member values increase to 20 again demonstrating communication between doc and dic pools during the early cambrian on 105 106 year time scales from the base of the overlying shiyantou formation 13corg values remain around 30 until the end of the section testifying to an interval of more modest change in dic 13c that would be consistent with biostratigraphic correlations nitrogen isotope values vary independently from carbon isotope trends exhibiting several major 15n cycles with values fluctuating between 9 and 1 robust trends in 15n within the dahai member testify to changes in the ratio between nitrogen fixation and denitrification possibly reflecting fluctuations in the water column chemocline as 13c values reached their global acme up section nitrogen isotope values show dampened cyclicity with values remaining between 0 and 5 indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles the overall variability in 15n closely resembles that of recent marine sediments while the observed cyclicity is defined by several samples in all cases these observations argue for good preservation of original isotopic signatures which bodes well for future n isotope studies of the precambrian cambrian boundary

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

hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites we present new hydrogen isotope data for separated matrix hydrated chondrules and other hydrated coarse silicate fragments from nine carbonaceous chondrites these data were generated using a micro analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids we also re evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites hydrogen isotope compositions of matrix and whole rock samples of cm chondrites are correlated with oxygen isotope indices major and minor element abundances and abundance and isotope ratios of other highly volatile elements these correlations include a monotonic decrease in d with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements including c n and ar between extremes of 0 least altered most volatile rich and 200 most altered least volatile rich in plots involving only abundances and or isotope ratios of highly volatile elements ci chondrites fall on the high d volatile rich end of the trends defined by cm chondrites i e ci chondrites resemble the least altered cm chondrites in these respects these trends suggest the protoliths of the cm chondrites i e before aqueous alteration contained an assemblage of volatiles having many things in common with those in the ci chondrites if so then the volatile element inventory of the ci chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized ci meteorites differences in volatile element chemistry between the ci and average cm chondrites can be attributed to aqueous alteration of the latter previous models of carbonaceous chondrite aqueous alteration have suggested 1 the protoliths of the cm chondrites are volatile poor objects like the co or cv chondrites and 2 the ci chondrites are more altered products of the same process producing the cm chondrites both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile element geochemistry of these rocks we present a model for aqueous alteration of the cm chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites our model requires among other things that the water infiltrating cm chondrites had a d value of 158 consistent with initial accretion of cm parent bodies at 4 au

water content of mantle garnets garnet megacrysts from colorado plateau diatremes green knobs garnet ridge and the wesselton kimberlite south africa commonly contain a structural hydrous component the colorado plateau samples range from 0 0 to 0 26 wt h 2 o and the wesselton samples contain from 0 01 to 0 07 concentrations were measured using p 2 o 5 cell coulometry h 2 gas manometry and thermogravimetry these were used to calibrate infrared integrated absorbance in the 3 m region which is a more sensitive measure of total o h content than the other analytical methods infrared absorbance patterns were also used to differentiate structural hydrous component from water contained in alteration and included phases the structure of the hydrous component in these garnets appears to be the classic h 4 o 4 4 sio 4 4 profiles at 100 m intervals across these samples show flat concentration profiles or slightly increasing concentration toward the center a large range of water content among samples appears to represent real differences in water fugacity at the point where the garnets equilibrated garnets in eclogite nodules from south africa and the solomon islands were also studied but were either anhydrous or too badly altered to determine the content of structurally bound water the high concentration of hydrous component in the colorado plateau samples is consistent with other indicators of high volatile content in that region of the mantle the water content of mantle garnets may prove to be an accurate indicator of mantle water fugacities

ree bearing sulfide in bishunpur ll3 1 a highly unequilibrated ordinary chondrite abstract this is a report of the first unambiguous discovery of rare earth elements ree in iron sulfide fes in an ordinary chondrite the content and the pattern of ree as well as the reduced state of the chondrules with the ree containing sulfide suggest that the sulfide inside the bishunpur ll3 1 chondrules is of igneous origin formed during crystallization of the chondrule melts the similarities in the content and the pattern of ree between chondrule sulfide and matrix sulfide in bishunpur indicate a genetic relationship the presence of igneous sulfide inside chondrules provides constraints on the ambient nebular temperature and the duration of chondrule formation the results of this study allow on the basis of ree content and pattern discrimination of igneous sulfide from condensate and secondary sulfide in chondrites and other interplanetary materials

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

analytical and approximate solutions to radial dispersion from an injection well to a geological unit with simultaneous diffusion into adjacent strata analytical and approximate solutions are developed for radial dispersion in aquifers with simultaneous diffusion into adjacent strata contaminants from the injection well are transported within the main aquifer by advection and mechanical dispersion assuming a steady state and radially diverging ground water flow field the leakage of contaminants from the main aquifer to the aquitards is accounted for by molecular diffusion a mathematical model consisting of two coupled differential equations is proposed to investigate concentration distributions in the main aquifer as well as in the adjacent aquitards by making use of laplace transforms analytical solutions valid for small time periods are obtained without difficulty to complement these analytical solutions concentration distributions for intermediate and large time intervals are determined approximately by numerically inverting the appropriate transformed solution in the laplace domain with the stehfest algorithm excellent agreement exists between analytical solutions and approximate solutions for small time intervals supporting the validity of approximate solutions for intermediate and large time periods it is shown that the diffusive leakage may significantly delay contaminant movement within the main aquifer at relatively large time periods the solutions can be applied to study radial dispersion in granular aquifers bounded by relatively low permeability aquitards or in planar fractures contained in porous formations

the distribution of hydroxyl in garnets from the subcontinental mantle of southern africa 166 garnets of dominantly mantle origin were analyzed for oh content by infrared ir spectroscopy ir spectra in the 3400 3700 cm 1 region display consistent absorption patterns attributable to oh structurally bound within the garnet crystal occasionally contaminated by low intensity oh absorptions from microscopic inclusions the principal structural oh absorption occurs near 3570 cm 1 with the appearance of additional absorptions near 3512 cm 1 and 3650 cm 1 dependent on garnet composition or paragenesis samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern africa oh abundances using the best currently available calibration range from less than 1 up to 135 ppm h 2o and increase in the general order as follows on craton eclogites group 1 basaltic kimberlite alnoite alkali basalt the oh contents of common lithospheric granets from coarse peridotites including several phlogopite bearing samples are typically less than 20 ppm h 2o for tectonic settings of kimberlites both on and off the archaean kaapvaal craton ti rich garnets from deformed peridotites are richer in oh supporting previous suggestions of association of these xenoliths with putative megacryst magmas subcalcic cr rich xenocrysts diamond inclusion garnets and garnets from diamondiferous eclogites have very low oh contents similar to eclogites and depleted peridotites without macroscopic diamonds the oh content of southern african peridotite and eclogite garnets are significantly lower on average than those previously examined from the colorado plateau diatremes while details of emplacement related h mobility in garnets remain to be established our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle although garnets do not appear to constitute a large reservoir of mantle hydrogen the large stability range of oh bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer

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

extraterrestrial water in micrometeorites and cosmic spherules from antarctica an ion microprobe study the d h ratios and water contents were measured by ion microprobe analysis in 52 individual antarctic micrometeorites amms and 10 antarctic cosmic spherules acss containing nuggets of iron hydroxide cops phase in amms d values vary from 366 to 249 and water contents lie between 0 4 3 7 wt the cops nuggets in cosmic spherules have high water contents 2 to 8 wt and exhibit d values from 144 to 167 which is indicative of an extraterrestrial origin of their constituent water the silicate portion of acss also contain extraterrestrial h equivalent to 0 l to 1 2 wt water deuterium exchange experiments were performed with isotopically spiked water these experiments demonstrate that water in mineral phases of amms and acss is indigenous and does not result from contamination during residence in antarctic ice the frequency distribution of d h ratios in amms allows us to further narrow the relationship between amms and carbonaceous chondrites to cm and ci chondrites but contrasts with that of stratospheric interplanetary dust particles idps of similar sizes from 10 to 50 m the relatively narrow range of d h ratios measured in amms as well as in acss which are more resistant and thus less susceptible to collection biases suggests that d rich idp like particles are very rare in our amms collections this indicates that these d rich grains might constitute a minor fraction of the micrometeorite flux in the interplanetary medium and that possible collection biases in antarctica would not be responsible for their strong depletion in the amms collections

nitric acid from volcanoes abstract atmospheric cycling of nitric acid and other nitrogen bearing compounds is an important biogeochemical process with significant implications for ecosystems and human health volcanoes are rarely considered as part of the global nitrogen cycle but here we show that they release a previously unconsidered flux of hno3 vapour to the atmosphere we report the first measurements of nitric acid vapour in the persistent plumes from four volcanoes masaya nicaragua etna italy and villarrica and lascar chile mean near source volcanic plume concentrations of hno3 range from 1 8 to 5 6 mol m 3 an enrichment of one to two orders of magnitude over background 0 1 1 5 mol m 3 using mean molar hno3 so2 ratios of 0 01 0 02 0 05 and 0 07 for villarrica masaya etna and lascar respectively combined with so2 flux measurements we calculate gaseous hno3 fluxes from each of these volcanic systems and extend this to estimate the global flux from high temperature non explosive volcanism to be 0 02 0 06 tg n yr 1 while comparatively small on the global scale this flux could have important implications for regional fixed n budgets the precise mechanism for the emission of this hno3 remains unclear but we suggest that thermal nitrogen fixation followed by rapid oxidation of the product no is most likely in explosive ash rich plumes no may result from or at least be supplemented by production from volcanic lightning rather than thermal n fixation we have calculated no production via this route to be of the order of 0 02 tg n yr 1