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

evidence in pre 2 2 ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota the loss of fe from some pre 2 2 ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2 2 ga i have examined the behavior of fe in both pre and post 2 2 ga paleosols from depth profiles of fe 3 ti fe 2 ti and fe ti ratios and fe 3 ti vs fe 2 ti plots this new approach reveals a previously unrecognized history of paleosols essentially all paleosols regardless of age retain some characteristics of soils formed under an oxic atmosphere such as increased fe 3 ti ratios from their parental rocks the minimum oxygen pressure p o 2 for the 3 0 2 2 ga atmosphere is calculated to be about 1 5 of the present atmospheric level which is the same as that for the post 1 9 ga atmosphere the loss of fe common in paleosol sections of all ages was not due to a reducing atmosphere but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere this reductive dissolution of ferric hydroxides occurred either 1 after soil formation by hydrothermal fluids or 2 during and or after soil formation by organic acids generated from the decay of terrestrial organic matter terrestrial biomass on the early continents may have been more extensive than previously recognized

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

molybdenum isotopes in late archean carbonate rocks implications for early earth oxygenation abstract we present carbonate mo isotope data of the 2 64 2 5 ga old ghaap group transvaal supergroup south africa to contribute to the debate on early earth oxygenation mo fractionation in carbonates has just recently been introduced as a potential new proxy for ocean redox chemistry and it was suggested that under favorable conditions non skeletal carbonates closely monitor the ambient seawater 98 95 mo composition the record presented here is characterized by three phases of moderately to strongly fractionated 98 95 mo with values of up to 1 6 interrupted by two shifts towards detrital levels this near continuous presence of heavy 98 95 mo suggests a quasi permanent availability of free atmospheric o 2 because fractionation relative to the continental background signature occurs exclusively when mo is present as dissolved molybdate ions moo 4 2 we compare our carbonate data to a previously published black shale mo record on samples from the same drill core the general interpretation of both records is identical a rise in o 2 preceded the commonly accepted transition to a consistently oxygenated atmosphere by 150 ma however in the ghaap group carbonates fractionated mo is found at a time when the coeval black shales still indicate detrital sedimentation a combination of detritus admixture and or redox controlled mo isotope fractionation during shale formation in suboxic to anoxic waters is suggested as cause for a period characterized by a persistent offset between lighter shale and heavier carbonate 98 95 mo the apparent offset can then be attributed to the different geochemical behavior of the two archives with respect to the local depositional conditions redox vs non redox dependent such changes in the sedimentary environment might also explain why the carbonates do not reproduce rapid fluctuations found throughout the shale record but show a continuous signature with scarce outliers thus by means of the combined archives the conspicuous 98 95 mo scatter found in the shale record could be attributed to variable depositional conditions during its formation rather than global oxic anoxic fluctuations simultaneously the carbonates provide a better approximation of the long term 98 95 mo seawater evolution this study introduces the mo isotopic composition of carbonates as a promising new tool for paleo redox studies in particular when combined with black shales

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

molybdenum isotopic composition of modern and carboniferous carbonates we investigate the redox sensitive isotope system of molybdenum mo in marine carbonates to evaluate their potential as archive of the mo isotopic composition of coeval seawater we present mo isotope data delta 98 95 mo of modern skeletal and non skeletal carbonates as well as a variety of precipitates from the mid and late carboniferous the external reproducibility is determined by repeated analyses of two commercially available carbonate standards the resulting uncertainty of the low concentration samples is 0 1 parts per thousand 2 sigma analysis of modern ooid sands from the bahamas shows a consistently heavy mo isotopic composition delta 98 95 mo between 2 and 2 2 parts per thousand approaching modern mean seawater values delta 98 95 mo 2 3 parts per thousand 0 1 parts per thousand 2 sigma this suggests that isotope fractionation during mo uptake into non skeletal carbonate precipitates is small in contrast modern skeletal carbonates show variable isotopic compositions 0 1 to 2 2 parts per thousand which suggests a biologically controlled fractionation process the varying mo signatures found in carboniferous cement phases point to a strong response to local changes in fluid composition from which they precipitated overall we recognized three important factors to cause an offset relative to ocean water mo derived from skeletal components input of detrital mo and admixture of light hydroxide derived mo via diagenetic fluids all of these factors cause a lighter mo isotopic composition relative to seawater due to the apparent small isotope fractionation during mo uptake into non skeletal carbonates their delta 98 95 mo closely reflects the ambient fluid composition from these results we conclude that carbonates represent a promising new tool to characterize the water mass mo isotopic composition of marine paleo environments

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

the rise of atmospheric oxygen when did atmospheric oxygen rise to near present concentrations and what led to this rise as kasting explains in his perspective recent results have provided stronger than ever evidence that the switch from an anoxic to an oxic atmosphere occurred around 2300 millions years ago but researchers remain divided over the mechanism catling et al resurrect a previously proposed mechanism involving hydrogen escape to space with a couple of new twists whether their mechanism is viable depends on the oxidation state of the continental and oceanic crust at this crucial time of earth9s evolution

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

molybdenum in the northeast pacific ocean1 the concentration of dissolved molybdenum has been determined in several profiles from the northeast pacific ocean the salinity normalized concentration is essentially constant in all open ocean samples 107 nm 1 2 5 nm the dissolved distribution is not significantly affected by biological cycling

atypical mo isotope signatures in eastern mediterranean sediments suboxic sapropel s1 sediments of the eastern mediterranean carry an untypically light 98 95mo signal being even lighter than oxic mn enriched sediments and mn crusts nodules which were previously thought to carry the lightest mo isotope values the evaluation and comparison of oxic s1 diagenetically overprinted oxic s1 and suboxic s1 sediments indicates that this light isotope signal is primarily an oxic signal that has been overprinted by secondary diagenetic processes occurring after sediment burial such secondary processes bear the potential of additional mo isotope fractionation in particular in non steady state diagenetic environments that involve the discontinuous re location of the redox boundary

high precision 40 ar 39 ar dating of quaternary basalts from auckland volcanic field new zealand with implications for eruption rates and paleomagnetic correlations abstract the auckland volcanic field avf which last erupted ca 550 years ago is a late quaternary monogenetic basaltic volcanic field ca 500 km 2 in the northern north island of new zealand prior to this study only 12 out of the 53 identified eruptive centres of the avf had been reliably dated careful sample preparation and 40 ar 39 ar analysis has increased the number of well dated centres in the avf to 35 the high precision of the results is attributed to selection of fresh non vesicular non glassy samples from lava flow interiors sample selection was coupled with separation techniques that targeted only the groundmass of samples with 10 m wide coupled with ten increment furnace step heating of large quantities up to 200 mg of material the overall avf age data indicate an onset at 193 2 2 8 ka an apparent six eruption flare up from 30 to 34 ka and a 10 kyr hiatus between the latest and second to latest eruptions such non uniformity shows that averaging the number of eruptions over the life span of the avf to yield a mean eruption rate is overly simplistic together with large variations in eruption volumes and the large sizes and unusual chemistry within the latest eruptions rangitoto 1 and rangitoto 2 our results illuminate a complex episodic eruption history in particular the rate of volcanism in avf has increased since 60 ka suggesting that the field is still in its infancy multiple centres with unusual paleomagnetic inclination and declination orientations are confirmed to fit into a number of geomagnetic excursions with five identified in the mono lake two within the laschamp one within the post blake or blake and two possibly within the hilina pali

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

computerized tomographic analysis of displacement trajectories and three dimensional fold geometry above oblique thrust ramps computerized tomographic analysis of physical models of fold thrust structures permits nondestructive systematic analysis of variations in displacement trajectories and variations in three dimensional fold geometry as a function of thrust fault geometry the authors studied models in which sand layers with embedded transport parallel marker lines were shortened incrementally up to a maximum of 24 over rigid footwall fault templates these templates simulated a right stepping oblique ramp connecting two ramps perpendicular to the shortening direction in response to movement over the oblique ramp templates overturned folds formed and marker line deflections indicative of out of plane strains developed marker lines within layers originally truncated by fault templates were deflected leftward away from the right stepping oblique ramp during deformation whereas marker lines within sand layers that originally extended across the upper flat of the fault template were deflected in a more radial pattern differences in marker line deflections imply that displacement trajectories within natural thrust sheets may vary as a function of overburden and structural position furthermore their tomographic studies graphically demonstrate that out of plane strain and differential rotation of layers within thrust sheets occur in the vicinity of oblique ramps

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

sedimentology of the kuruman and griquatown iron formations transvaal supergroup griqualand west south africa abstract iron formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow water carbonate platform at the time of deposition of the campbellrand carbonate sequence a subsequent transgression resulted in the deposition of open shelf iron formation on top of the campbellrand carbonate platform progradational sedimentation coupled with shoaling followed and an iron formation sequence represented by the kuruman and griquatown iron formations was deposited this sequence consists from the base upwards of stacked open shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units toe of slope greenalite siderite rhythmites platform slope greenalite siderite rhythmites with grainflow bands platform edge sideritic orthochemical and allochemical iron formations epeiric sea orthochemical and allochemical sideritic hematitic and greenalitic iron formations supratidal disclutites and lacustrine banded greenalite lutite landwards the lacustrine felutites were followed by deltaic chloritic claystone siltstone and quartz wacke of the koegas subgroup autochthonous ferhythmites of the kuruman iron formation reach a maximum development in a basin near prieska whereas orthochemical and allochemical units are more abundant on the kaapvaal craton iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in eh and ph in the depository and may be related to biological activity chert mesobanding in the iron formations is essentially of an early diagenetic origin

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

authigenic molybdenum isotope signatures in marine sediments we present new mo isotope data from the mexican continen tal margin that in conjunction with previous data allow us to propose a mechanistic description of the mo isotope system in ma rine sediments we hypothesize that there are unique environmen tally dependent mo isotope signatures recorded in marine sedi ments that reflect the mechanisms responsible for authigenic mo accumulation open ocean anoxic sites defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water exhibit a 98 95 mo isotope signature of 1 6 we believe this val ue reflects mo sulfide formation via diagenetic processes within sediments quantitative formation of mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value 2 3 as typified by samples from the highly sulfidic black sea in contrast to these reducing settings manganese oxide rich sediments have measured mo isotope values that are more negative relative to seawater than any other sediment samples analyzed to date similar to fe mn crusts 0 7 most measured mo isotope compositions of marine sediments from open ocean settings appear to reflect a mix ture of lithogenic mo 0 0 and the mo signature of a specific authigenic mo accumulation mechanism we therefore suggest that mo isotopes may record unique signatures that reflect the domi nant chemical mechanism for mo sequestration into sediments

sulfidity controls molybdenum isotope fractionation into euxinic sediments evidence from the modern black sea molybdenum mo isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry it is therefore fundamental to understand the processes controlling mo partitioning into modern marine environments this study identifies the availability of dissolved sulfide as the dominant control on overall mo removal from the water column in euxinic systems mo isotopic composition of surface sediments from different localities of the black sea demonstrates complete fixation of mo only below 400 m water depth above a critical concentration of 11 mol l 1 aqueous hydrogen sulfide in the bottom water the mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2 3 in contrast significant mo isotope fractionation into less euxinic sediments is evident at shallower depths in the black sea as well as in temporarily euxinic deeps of the baltic sea consistent with the observed lower maximum sulfide concentrations in the respective water columns therefore mo isotope signatures in the modern black sea constrain the processes responsible for global mo removal from the ocean by euxinic sediments furthermore models of past ocean anoxia reconstruction have to consider that the seawater mo isotopic composition is not per se archived in euxinic sediments

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

pge re os and mo isotope systematics in archean and early proterozoic sedimentary systems as proxies for redox conditions of the early earth abstract re os data and pge concentrations as well as mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the barberton greenstone belt south africa fig tree and moodies groups 3 25 3 15 ga the belingwe greenstone belt zimbabwe manjeri formation ca 2 7 ga and shales from the witwatersrand ventersdorp and transvaal supergroups south africa ranging from 2 95 to 2 2 ga moderately oxidizing conditions are required to mobilize re and mo in the environment mo fractionation only occurs in solution and these parameters thus have potential use as paleoredox proxies for the early earth pge re abundance patterns of barberton greenstone belt sediments are uniform and very similar in shape to those of komatiites this indicates 1 that the pge came from a source of predominantly ultramafic composition and 2 that pge were transported and deposited essentially in particulate form sediments from the younger belingwe greenstone belt show more fractionated pge re patterns and have re os ratios 10 to 100 higher than those of barberton sediments their pge abundance patterns and re os ratios are intermediate between those of the mid archean shales and neoproterozoic to recent black shales they reflect scavenging of re from solution in the sedimentary environment 98 95 mo values of black shales of all ages correlate with their concentrations the barberton greenstone belt samples have 1 3 ppm mo similar to a granitoid basaltic source this mo has 98 95 mo between 1 9 and 2 4 relative to present day mean ocean water molybdenum momo and is thus not isotopically fractionated relative to such a source similar to the pge this indicates transport in solid form sediments from the belingwe greenstone belt show in part enhanced mo concentrations up to 6 ppm and mo isotope fractionation 98 95 mo up to 1 4 relative to momo the combined pge re and mo data show mainly reducing conditions in the mid archean and suggest that by 2 7 ga the atmosphere and oceans had become more oxidizing substantially younger samples from the transvaal supergroup to ca 2 2 ga surprisingly have mainly low mo concentrations around 1 ppm and show no significant mo isotope fractionation relative to the continental source among possible explanations for this are a return to reducing atmospheric conditions after 2 7 ga reservoir effects or mo removal by sulfide precipitation following sulfate reduction in early proterozoic oceans

rhenium in the black sea comparison with molybdenum and uranium rhenium concentrations were measured in the black sea to determine the behavior of this redox sensitive element in an anoxic basin under reducing conditions re is removed from solution leading to its depletion from black sea deep waters compared with uranium water column re and mo profiles show evidence of removal from mid depths suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than u black sea surface waters are enriched in re reflecting re in rivers draining into the northern black sea that are enhanced up to 80 fold over other large world rivers this enrichment is most likely anthropogenic and may be due to coal burning in the region a simple two box model is used to demonstrate that re concentrations in the black sea are not in steady state with respect to current riverine inputs and authigenic fluxes riverine inputs of u and mo are not as significant perhaps reflecting the fractionation of re from the other elements during a high temperature combustion process re2o7 is volatile above 270 c

an overview of the geology of the transvaal supergroup dolomites south africa in the neoarchaean intracratonic basin of the kaapvaal craton between approximately 2640 ma and 2516 ma two successive stromatolitic carbonate platforms developed deposition started with the schmidtsdrif subgroup which is probably oldest in the southwestern part of the basin and which contains stromatolitic carbonates siliciclastic sediments and minor lava flows subsequently the nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the transvaal supergroup epeiric sea around 2550 ma ago this transgression led to the development of a carbonate platform in the areas of the preserved transvaal and griqualand west basins which persisted for 30 50 ma during this time shales were deposited over the nauga formation carbonates in the southwestern portion of the epeiric sea a subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical iron rich silica precipitates of the banded iron formations bif over the entire basin carbonate precipitation in the archaean was largely due to chemical and lesser biogenic processes with stromatolites and ocean water composition playing an important role the stromatolitic carbonates in the preserved griqualand west and transvaal basins are subdivided into several formations based on the depositional facies reflected by stromatolite morphology and on intraformational unconformities interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins thorough dolomitisation of most formations took place at different post depositional stages but mainly during early diagenesis partial silicification was the result of diagenetic and weathering processes karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater such periods occurred locally at the time of carbonate and bif deposition main karstification however probably took place during an erosional period between approximately 2430 ma and 2320 ma

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

the carbonates of the transvaal and griqualand west sequences of the kaapvaal craton with special reference to the lime acres limestone deposit the campbell group carbonates were deposited in a basin transgressing from southwest to the east and to northeast at the southwestern margin of the kaapvaal craton in griqualand west mainly tidal flat carbonates and shallow marine deposits are found to be followed by subtidal carbonates marking the slow transgression north of the griquatown fault where the center of the griqualand west sub basin developed continuous subsidence matched by stromatolitic carbonate growth led to accumulation of the thickest carbonate pile 2000 m there the largest limestone deposits of south africa are located the deposit at lime acres and mining operations of the ppc lime ltd are described and discussed in respect to diagenetic processes that permitted limestone preservation

detrital pyrite in witwatersrand gold reefs x ray diffraction evidence and implications for atmospheric evolution pyrite is easily oxidized and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed compact rounded pyrite in the witwatersrand gold reefs of south africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite x ray precession photography reveals that petrographically featureless as poor grains are untwinned single crystals of high diffraction quality this new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits and excludes an origin by replacement of a pre existing detrital phase further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated as rich growth bands the geographically extensive witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and thus the late archean atmosphere of earth was likely essentially anoxic