continuously increasing δ98mo values in neoarchean black shales and iron formations from the hamersley basin abstract we present mo c and o isotope data from black shales carbonate and oxide facies iron formations from the hamersley group western australia that range in age from 2 6 to 2 5 billion years the data show a continuous increase from near crustal 98 mo values of around 0 50 for the oldest marra mamba and wittenoom formations towards higher values of up to 1 51 for the youngest sample of the brockman iron formation thereby the trend in increasing 98 mo values is portrayed by both carbonate facies iron formations and black shales considering the positive correlation between mo concentration and total organic carbon we argue that this uniformity is best explained by molybdate adsorption onto organic matter in carbonate iron formations and scavenging of thiomolybdate onto sulfurized organic matter in black shales a temporal increase in the seawater 98 mo over the period 2 6 2 5 ga is observed assuming an overall low mo isotope fractionation during both mo removal processes oxide facies iron formations show lowest mo concentrations lowest total organic carbon and slightly lower 98 mo compared to nearly contemporaneous black shales this may indicate that in iron formation settings with very low organic matter burial rates the preferential adsorption of light mo isotopes onto fe oxyhydr oxides becomes more relevant a similar mo isotope pattern was previously found in contemporaneous black shales and carbonates of the griqualand west basin south africa the consistent and concomitant increase in 98 mo after 2 54 billion years ago suggests a more homogenous distribution of seawater molybdate with uniform isotopic composition in various depositional settings within the hamersley basin and the griqualand west basin the modeling of the oceanic mo inventory in relation to the mo in and outflux suggests that the long term build up of an isotopically heavy seawater mo reservoir requires a sedimentary sink for isotopically light mo the search for this sink i e adsorption onto mn oxides in well oxygenated surface oceans and or subaerial environments or incomplete thiomolybdate formation in weakly sulfidic settings remains debated but its relevance becomes more important closer to the great oxidation event and is probably related to already weakly oxidizing conditions even prior to the 2 5 ga whiff of oxygen

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

earth s early o2 cycle suppressed by primitive continents a decrease in mafic continental crust coincides with the rise of o2 in the earth s surface environments about 3 billion years ago according to an analysis of sediment chemistry reduced rates of serpentinization of mafic material which produces chemicals that react with o2 could explain the link

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

a redox stratified ocean 3 2 billion years ago abstract before the great oxidation event goe 2 4 2 2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen producing microorganisms recently however it has been proposed that transient oxygenation of shallow seawater occurred between 2 8 and 3 0 billion years ago here we present a novel combination of stable fe and radiogenic u th pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3 2 ga based on analysis of the manzimnyama banded iron formation bif fig tree group south africa this unit is exceptional in that proximal shallow water and distal deep water facies are preserved when compared to the distal deep water facies the proximal samples show elevated u concentrations and moderately positive 56 fe values indicating vertical stratification in dissolved oxygen contents confirmation of oxidizing conditions using u abundances is robustly constrained using samples that have been closed to u and pb mobility using u th pb geochronology although redox sensitive elements have been commonly used in ancient rocks to infer redox conditions post depositional element mobility has been rarely tested and u th pb geochronology can constrain open or closed system behavior the u abundances and 56 fe values of the manzimnyama bif suggest the proximal shallow water samples record precipitation under stronger oxidizing conditions compared to the distal deeper water facies which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time this work therefore documents the oldest known preserved marine redox gradient in the rock record the relative enrichment of o 2 in the upper water column is likely due to the existence of oxygen producing microorganisms such as cyanobacteria these results provide a new approach for identifying free oxygen in earth s ancient oceans including confirming the age of redox proxies and indicate that cyanobacteria evolved prior to 3 2 ga

two step deoxygenation at the end of the paleoproterozoic lomagundi event abstract the ca 2 1 ga francevillian group of gabon was deposited in the aftermath of the great oxidation event goe and records the lomagundi event le which is the most pronounced and long lived carbon isotope excursion in the geologic record moreover the sedimentary succession contains putative evidence for the earliest appearance of macro eukaryotes an emerging paradigm is that the end of the le was accompanied by a deoxygenation event that preceded the apparent stability of environmental and redox conditions as well as the carbon cycle characteristic of the mesoproterozoic however the processes that led to deoxygenation some 300 to 200 ma after the beginning of the goe are not well understood here we present a multi proxy stable isotope 34 s 33 s 36 s 98 mo 13 c org 13 c carb and 18 o carb study of the francevillian group we suggest that sedimentation of the lower part of the francevillian group took place during the le in oxygenated shallow waters with elevated sulfate concentrations two episodes of anoxic water shoaling during deposition of the upper francevillian group correspond with broader marine deoxygenation and a contraction of the seawater sulfate reservoir this shoaling of anoxic conditions may be linked to intense submarine hydrothermal and volcanic activity that led to sedimentary manganese deposits we propose that increased concentrations of aqueous hydrothermally sourced reductants drove oxygen consumption during the first deoxygenation event and established a sulfidic oxygen minimum zone at the margin of the shallow shelf carbonates with positive 13 c carb values characteristic of the le precipitated during this first stage of deoxygenation the second deoxygenation separated from the previous event by a period of well oxygenated conditions was marked by a stronger contraction of the seawater sulfate reservoir and coincided with the end of the le during this time widespread euxinic conditions were established in shallow above storm wave base marine environments the presence of a shallow water redoxcline points to a generally low oxygen atmosphere ocean system further the negative co variation between 34 s and 13 c values in sediments of the francevillian group and other sedimentary successions of similar age worldwide suggests that the inferred two step deoxygenation corresponding to the end of the le reflects global rather than local events that likely occurred between 2 1 and 2 05 ga ago

sulfur and carbon isotopic evidence for metabolic pathway evolution and a four stepped earth system progression across the archean and paleoproterozoic abstract the earth s mantle has provided a ready redox gradient of sulfur compounds so2 h2s since the stabilization of the crust and formation of the ocean over 4 billion years ago and life has evolved a multitude of metabolic pathways to take advantage of this gradient these transitions are recorded in the sulfur and carbon isotope signals preserved in the rock record in the genomic records of extant microorganisms and in the changing mantle and crust structure composition and cycling here we have assembled approximately 20 000 sulfur 34s 33s 36s and carbon 13c isotope data points from scientific publications spanning over five decades of geochemical analyses on rocks deposited from 4 0 to 1 5 ga we place these data in the context of molecular clock and tectonic and surface redox indicators to identify overarching trends and integrate them into a holistic narrative on the transition of the earth s surface towards more oxidizing conditions the greatest extreme in 34s values of sulfide minerals 45 5 to 54 9 and sulfate minerals 13 6 to 46 6 as well as 13c values in carbonate minerals 16 8 to 29 6 occurred in the period following the great oxidation event goe while the greatest extremes in organic carbon 13c values 60 9 to 2 4 and sulfide and sulfate mineral 33s and 36s values 4 0 to 14 3 and 12 3 to 3 2 respectively occurred prior to the goe from our observations we divide transitions in earth s history into four periods period 1 4 00 to 2 80 ga during which geochemical cycles were initialized period 2 2 80 to 2 45 ga during which s and c isotope systems exhibit changes as conditions build up to the goe period 3 2 45 to 2 00 ga encompassing the goe and period 4 after 2 00 ga after which s and c isotopic systems remained relatively constant marking a time of earth system geochemical quiescence using these periods we link changes in s and c isotopes to molecular clock work to aid in interpreting emerging metabolic functions throughout earth s history while underscoring the need for better proxies for robust evolutionary analyses specifically results indicate 1 an early development of sulfide oxidation and dissimilatory sulfite reduction followed by disproportionation and then sulfate reduction to sulfite resulting in a fully biologically mediated sulfur cycle by 3 25 ga 2 support for the acetyl coenzyme a pathway as the most likely earliest form of biologically mediated carbon fixation following methanogenesis 3 an increasingly redox stratified ocean in the neoarchean with largely oxic surface water and euxinic bottom water during the first half of the paleoproterozoic and 4 that secular changes in earth system crustal cycling dynamics and continent formation likely played a key role in driving the timing of the goe finally based on geochemical data we suggest that the paleoproterozoic be divided into a new era of the eoproterozoic from 2 45 to 2 00 ga and the paleoproterozoic from 2 00 to 1 60 ga

subglacial meltwater supported aerobic marine habitats during snowball earth the earth s most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the cryogenian period 720 to 635 ma aerobic eukaryotes must have survived the snowball earth glaciations requiring the persistence of oxygenated marine habitats yet evidence for these environments is lacking we examine iron formations within globally distributed cryogenian glacial successions to reconstruct the redox state of the synglacial oceans iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice covered oceans but increasing oxidation with proximity to the ice shelf grounding line we propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans creating glaciomarine oxygen oases the confluence of oxygen rich meltwater and iron rich seawater may have provided sufficient energy to sustain chemosynthetic communities these processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations

a comparison of bulk versus laser ablation trace element analyses in banded iron formations insights into the mechanisms leading to compositional variability abstract banded iron formations bif represent one of the few chemical sedimentary archives available for exploring the redox state of the precambrian atmosphere ocean system and the marine availability of biologically critical trace nutrients through time temporal records compiled for a number of trace elements show a relatively wide spread in concentrations spanning several orders of magnitude within a single deposit it has been suggested that this spread is present in both bulk rock digestion and laser ablation data however the comparability of data from the two methods has not been thoroughly evaluated here we provide a statistical assessment of how comparable laser ablation data of individual bif hematite and magnetite grains are to one another as well as how closely averages of laser ablation data approximate bulk rock values to do so we use laser ablation high resolution inductively coupled plasma mass spectrometry la hr icp ms data from 14 bif slabs and corresponding powders that span four different formations we focus in particular on bif from the 3 75 ga nuvvuagittuq supracrustal belt quebec canada and multiple bif deposits of the 2 6 2 45 ga hamersley group north western australia representing deposits separated by over a billion years and of differing metamorphic grade the scaling between selected transition metals is also evaluated in light of expectations for possible delivery mechanisms of trace elements from seawater to bif precursor sediments the adsorption to hydrous ferric oxides biological vectors and detrital influence our findings show that average trace element concentrations between bulk versus laser ablation data typically vary within an order of magnitude furthermore reproducibility between the two methods appears to be independent of formation age metamorphic grade or depositional setting these findings underscore the overall fidelity of the bif record at capturing seawater compositional signatures at various scales we demonstrate that evaluations of ancient seawater chemistry using the bif record are best served by a combined approach utilizing both bulk rock and laser ablation data as bulk methods offer the most conservative proxy data for ancient seawater composition but may mask significant enrichments or depletions that are detectable at the grain scale by laser ablation

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

trace element and stable c o and radiogenic sr isotope geochemistry of stromatolitic carbonate rocks of the mesoarchaean pongola supergroup implications for seawater composition abstract the mesoarchaean pongola supergroup hosts one of the oldest well preserved stromatolitic carbonate successions major and trace element geochemistry along with c o and sr isotopic analyses of three stratigraphic sections of stromatolitic carbonate rocks were undertaken in order to infer the depositional environment and to unravel the chemical characteristics of contemporaneous seawater the carbonate rocks consist mainly of dolomite and to a lesser extent ankerite their variable sr concentrations of 95 to 450 mg kg are interpreted as resulting from early to late dolomitization of metastable carbonates aragonite and or high mg calcite facies control and calcitization during meteoric diagenesis 18o values range from 19 6 to 12 0 vpdb and reflect recrystallization of carbonates caused by high water rock interactions during diagenesis and metamorphism a 13c value of 2 5 vpdb was measured in carbonate samples least affected by alteration and we suggest that it approximates the composition of dissolved inorganic carbon in coeval seawater the samples display variable 87sr 86sr isotopic ratios from as low as 0 708 to 0 777 these ratios are considerably higher than those expected in archaean open ocean water masses buffered by hydrothermal interaction with the mantle we interpret the highly radiogenic sr isotopic ratios in some carbonate samples to reflect isotopic exchange with radiogenic sr from siliciclastic volcaniclastic components in the samples and fluids from clay rich layers intercalated with the carbonates the least radiogenic sr isotopic ratio may approximate the mixed isotopic composition of seawater in the epicontinental basin and of riverine freshwater draining the kaapvaal craton the dolomitic carbonate samples show shale normalized rare earth element and yttrium distribution patterns that resemble those of modern ocean water with positive lasn gdsn and ysn anomalies superchondritic y ho ratios a small negative cesn anomaly and depleted light rare earth elements relative to the heavy rare earth elements these anomalies are not as pronounced as expected for an open marine setting which together with the sr isotopic results support a more restricted setting with riverine input and fluctuations in marine ingressions a convex upward shale normalized rare earth element and yttrium pattern is probably a result of several factors including organic matter complexation and element fractionation during estuarine mixing the small negative cesn anomaly may suggest small amounts of free oxygen in shallow waters due to local oxygenic microbial activity

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

co2 drawdown and cooling at the onset of the great oxidation event recorded in 2 45 ga paleoweathering crust abstract the archean atmosphere is thought to have been devoid of oxygen but instead containing high concentrations of greenhouse gases such as co2 and possibly ch4 that were required to keep the earth surface environments warm enough for maintenance of liquid water under faint young sun conditions earlier studies have suggested that the co2 concentrations must have been as high as over 1000 times present atmospheric levels pal to overcome the effects of weaker solar luminosity however more recent studies of precambrian paleosols imply existence of at least tenfold lower but still high pco2 levels the appearance of minute amounts of atmospheric o2 at the transition of archean and proterozoic is well documented however the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated different paleoclimate proxies of the 2 45 ga kuksha paleoweathering crust suggest its formation in a cool temperate climate paleoatmospheric pco2 estimates by geochemical and isotopic mass balance models suggest remarkably low paleoatmospheric co2 levels with the best guess estimates of atmospheric co2 between 1 and 10 pal we speculate that the kuksha paleoweathering crust either predates or partly overlaps with the onset of huronian glaciation and corresponds to a time interval of low co2 level during the first recorded large glaciation in earth history

mineralogical and isotopic characterization of graphite deposits in the western part of the north qaidam orogen and east kunlun orogen northeast tibetan plateau china abstract medium to high grade metamorphic rocks comprising numerous graphite deposits are widely distributed in the periphery of the qaidam basin northern tibetan plateau china we present textural xrd and raman spectroscopy characteristics of graphite 13c results of graphite and cogenetic calcite and 18o results of calcite in three graphite occurrences located at the periphery of qaidam basin graphite displays hexagonal platy flaky irregular polygonal ovate morphologies with occasional spherulitic textures graphite morphologies combined with xrd and raman spectroscopy indicate that graphite has relatively high crystallinity raman microspectroscopy of graphite shows that peak metamorphic temperatures vary between 690 and 801 c graphite 13cv pdb values range between 26 3 and 21 6 indicating organic carbon as the major carbon source for graphite calcite 13cv pdb and calcite 18ov smow values range between 10 8 and 3 9 and between 6 6 and 29 5 respectively calcite 13cv pdb results show that organic carbon was mixed with inorganic carbon which resulted in an increase of 13cv pdb calcite 18ov smow values indicate that the graphite deposits experienced late stage water rock interaction the positive and negative anomalies of calcite 13cv pdb indicate that organic matter oxidation and recycling locally occurred during the final stage of the great oxidation event the wide distribution of regional metamorphic organogenic graphite deposits provides evidence for large scale paleoproterozoic biological activity in the periphery of the qaidam basin

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

iron isotope composition of some archean and proterozoic iron formations abstract fe isotopes can provide new insight into redox dependent biogeochemical processes precambrian iron formations if are deserving targets for fe isotope studies because they are composed predominantly of authigenic fe phases and record a period of unprecedented iron deposition in earth s history we present fe isotope data for bulk samples from 24 archean and proterozoic if and eight phanerozoic fe oxide rich deposits these data reveal that many archean and early paleoproterozoic iron formations were a sink for isotopically heavy fe in contrast to later proterozoic and phanerozoic fe oxide rich rocks the positive 56 fe values in if are best explained by delivery of particulate ferric oxides formed in the water column to the sediment water interface because if are a net sink for isotopically heavy fe there must be a corresponding pool of isotopically light fe in the sedimentary record earlier work suggested that archean pyritic black shales were an important part of this light sink before 2 35 billion years ago ga it is therefore likely that the persistently and anomalously low 56 fe values in shales are linked with the deposition of isotopically heavy fe in if in the deeper parts of basins if deposition produced a residual isotopically light dissolved fe pool that was captured by pyritic fe in shales local dissimilatory fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced fe isotope heterogeneity in marine sediments and an iron shuttle may have transported isotopically light fe from shelf sediments to the basin nevertheless water column processing of hydrothermally delivered fe likely had the strongest influence on the bulk iron isotope composition of archean and paleoproterozoic iron formations and other marine sediments

evidence for oxygenic photosynthesis half a billion years before the great oxidation event the early earth was characterized by the absence of oxygen in the ocean atmosphere system in contrast to the well oxygenated conditions that prevail today atmospheric concentrations first rose to appreciable levels during the great oxidation event roughly 2 5 2 3 gyr ago the evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise but it has proved difficult to constrain the timing of this evolutionary innovation1 2 the oxidation of manganese in the water column requires substantial free oxygen concentrations and thus any indication that mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing mn oxides are not commonly preserved in ancient rocks but there is a large fractionation of molybdenum isotopes associated with the sorption of mo onto the mn oxides that would be retained here we report mo isotopes from rocks of the sinqeni formation pongola supergroup south africa these rocks formed no less than 2 95 gyr ago3 in a nearshore setting the mo isotopic signature is consistent with interaction with mn oxides we therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

initiation of modern style plate tectonics recorded in mesoarchean marine chemical sediments abstract the chemistry of the oceans in part reflects a balance between inputs from the continents and mantle traditionally it has been thought that archean ocean chemistry was dominated by mantle sources but recent work has suggested that continental weathering during the archean provided a much higher flux to the oceans than previously recognized here we present new rb sr and sm nd isotope compositions on carbonate dolomite and limestone from the 2 94 ga red lake and 2 80 ga steep rock groups in the superior province canada to assess the potential impact continental weathering had on ocean chemistry during the mesoarchean a time when initiation of modern style plate tectonics has been proposed to have occurred the low rb contents of all carbonate samples suggest that clastic contamination does not affect the sr isotope compositions using o and sr isotope modeling we identified unaltered samples and estimate a 87sr 86sr ratio of 0 70173 for seawater at 2 94 ga and 0 70182 at 2 80 ga strontium isotope compositions from both red lake and steep rock indicate that seawater was significantly more radiogenic than contemporaneous mantle and suggests that weathering of evolved continental crust was an important input to seawater continental weathering likely affected seawater chemistry through uplift of continental lithosphere during the initiation of modern style plate tectonics at 3 2 ga a model that is contrary to those that suggest the archean continents were small in extent and largely submerged initiation of modern style plate tectonics and associated continental weathering had an important effect on the biosphere including increased nutrient delivery as well as creation of ecological niches that allowed development of the first biologically produced shallow marine redox gradients

morphological adaptations of 3 22 ga old tufted microbial mats to archean coastal habitats moodies group barberton greenstone belt south africa microbial life was well established and widespread by the paleoarchean however the degree of evolutionary advancement such as microbial motility intra and inter species interactions phototropism or oxygenic photosynthesis by that time remains highly debated the 3 22 ga moodies group in the barberton greenstone belt bgb south africa are earth s oldest well preserved siliciclastic tidal deposits they exhibit a unique assemblage of microbial mats providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings the fossil mats are preserved as kerogenous laminations 0 5 1 mm thick that can be traced laterally for 15 km in a 1000 m thick succession of fine to coarse grained tidal sandstones and conglomerates we here present a detailed stratigraphic and depositional facies analysis documenting the association of the three principal mat morphotypes with specific environmental settings 1 planar type in coastal floodplain 2 wavy type in intertidal and 3 tufted type in upper inter to supratidal facies all mat types indicate a flourishing phototrophic biota moreover the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments

iron isotope fractionation during pyrite formation in a sulfidic precambrian ocean analogue the chemical response of the precambrian oceans to rising atmospheric o2 levels remains controversial the iron isotope signature of sedimentary pyrite is widely used to trace the microbial and redox states of the ocean yet the iron isotope fractionation accompanying pyrite formation in nature is difficult to constrain due to the complexity of the pyrite formation process difficulties in translating the iron isotope systematics of experimental studies to natural settings and insufficient iron isotope datasets for natural euxinic i e anoxic and sulfidic marine basins where pyrite formation occurs herein we demonstrate that a large permil level shift in the isotope composition of dissolved iron occurs in the black sea euxinic water column during syngenetic pyrite formation specifically iron removal to syngenetic pyrite gives rise to an iron isotope fractionation factor between fe ii and fes2 of 2 75 permil the largest yet reported for reactions under natural conditions that do not involve iron redox chemistry these iron isotope systematics offer the potential to generate permil level shifts in the sedimentary pyrite iron isotope record due to partial drawdown of the oceanic iron inventory the implication is that the iron stable isotope signatures of sedimentary pyrites may record fundamental regime shifts between pyrite formation under sulfur limited conditions and pyrite formation under iron limited conditions to this end the iron isotope signatures of sedimentary pyrite may best represent the extent of euxinia in the past global ocean rather than its oxygenation state on this basis the reinterpreted sedimentary pyrite fe isotope record suggests a fundamental shift towards more sulfidic oceanic conditions coincident with the great oxidation event around 2 3 billion years ago importantly this does not require the chemical state of the ocean to shift from mainly de oxygenated to predominantly oxygenated in parallel with the permanent rise in atmospheric oxygen contrary to other interpretations based on iron isotope systematics

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

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

fraction specific controls on the trace element distribution in iron formations implications for trace metal stable isotope proxies iron formations ifs are important geochemical repositories that provide constraints on atmospheric and ocean chemistry prior to and during the onset of the great oxidation event trace metal abundances and their mo cr u isotopic ratios have been widely used for investigating ocean redox processes through the archean and paleoproterozoic mineralogically ifs consist of three main fe bearing fractions 1 fe ca mg mn carbonates 2 magnetite and or hematite and 3 fe silicates these fractions are typically fine grained on a sub m scale and their co occurrence in varying amounts means that bulk rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on if mineralogy here we adapt an existing sequential extraction scheme for fe phases poulton and canfield 2005 to the high fe content in if and the specific three fraction mineralogy we optimized the scheme for magnetite dominated archean ifs using samples from the hematite poor asbestos hills subgroup if transvaal supergroup south africa previously commonly used hydroxylamine hcl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these if samples the acetate leach was tested at variable temperatures reaction times and under different atmospheres in order to ensure that all micro crystalline fe carbonates were effectively dissolved resulting in an optimum extraction for 48 h at 50 c under anoxic conditions the dissolution of magnetite by nh4 oxalate was also tested resulting in an optimum extraction for 24 h under an ambient atmosphere finally a hf hclo4 hno3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in if accuracy of the extraction technique was generally excellent as verified using 1 elemental recoveries 2 comparison of major and trace element distributions against mineralogy and 3 comparison to results from microanalytical techniques this study focuses on the distribution of three frequently used geochemical proxies in if u mo and cr molybdenum abundances in the kuruman and griquatown if are low and show an apparent correlation with mineralogical variability as determined by the sequential extraction this suggests that changes in bulk rock mineralogy rather than redox chemistry might significantly affect mo stable isotopes for cr a minor bulk rock stratigraphic increase can be related to the oxide and silicate fraction however a positive relationship with zr indicates that this was also controlled by detrital or volcanic ash input uranium is predominantly bound to the silicate fraction and shows clear correlations with zr and sc implying detrital reworking under anoxic conditions the discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry

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

origin of microbial biomineralization and magnetotaxis during the archean microbes that synthesize minerals a process known as microbial biomineralization contributed substantially to the evolution of current planetary environments through numerous important geochemical processes despite its geological significance the origin and evolution of microbial biomineralization remain poorly understood through combined metagenomic and phylogenetic analyses of deep branching magnetotactic bacteria from the nitrospirae phylum and using a bayesian molecular clock dating method we show here that the gene cluster responsible for biomineralization of magnetosomes and the arrangement of magnetosome chain s within cells both originated before or near the archean divergence between the nitrospirae and proteobacteria this phylogenetic divergence occurred well before the great oxygenation event magnetotaxis likely evolved due to environmental pressures conferring an evolutionary advantage to navigation via the geomagnetic field earth s dynamo must therefore have been sufficiently strong to sustain microbial magnetotaxis in the archean suggesting that magnetotaxis coevolved with the geodynamo over geological time

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

geochemical constraints on the genesis of the ekou banded iron formation shanxi province north china the ekou banded iron formation bif in the wutai area is hosted within the late archean baizhiyan formation the mineral assemblage is used to identify oxide and silicate facies the oxide facies is composed of magnetite and quartz and the silicate facies is characterized by the presence of silicate minerals a geochemical analysis shows that the major elemental compositions are dominantly sio2 and fe2o3t with very little al2o3 and tio2 and minor abundances of incompatible elements and transition elements these results indicate that negligible terrigenous materials were involved in the bif deposition the rare earth elements rees normalized by post archean australian shale exhibit the characteristics of light ree lree depletion heavy ree hree enrichment and positive la y and eu anomalies the y ho ratios are superchondritic these results indicate that the material that formed the ekou bif originated from the mixing of seawater and submarine hydrothermal fluids a ce anomaly deficiency and heavy fe isotope enrichment indicate that the ekou bif formed in an anoxic marine environment the 30sinbs 28 values for quartz in the ekou bif are similar to those of other bifs with distribution ranges of modern sinters and black smokers the 18ov smow distribution is similar to that of hydrothermal sedimentary siliceous rocks these results suggest that the formation of the ekou bif was closely connected to submarine volcanic exhalation activity

desilication in archean weathering processes traced by silicon isotopes and ge si ratios abstract the influence of continental weathering on oceanic seawater compositions in the precambrian is poorly understood here we combine si isotopes and ge si ratios of a mesoarchean paleosol 2 95 ga and overlying shales as proxies for weathering processes and si mass transfer at the early earth s surface results show that as with modern soils neoformation of secondary clay minerals in the paleosol was associated with the fractionation of si isotopes and ge si ratios in response to chemical weathering and soil desilication furthermore the loss of fe bearing minerals most likely fe bearing smectites produced additional controls on si and ge mobility through the coupled dissolution and re precipitation of clay minerals under reductive conditions opposite fractionation behaviors are observed neoformed secondary clay minerals acted as a 28 si and ge sink whereas the leaching of fe bearing minerals released 28 si and ge into soil solutions an attempt to estimate 30 si and ge si signatures of released dissolved elements into soil solution provided 30 si and ge si signatures of about 0 1 and 1 4 mol mol respectively furthermore shales deposited shortly after paleosol formation display 30 si and ge si compositions that may be explained as mixtures of the recognized paleosol components the shale record suggests that weathering induced desilication and leaching of fe bearing minerals to a lesser extent might have been widely effective during the mesoarchean for the transfer of si from the continents to the hydrosphere comparison of si isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication relative to changes in hydrothermal and continent derived solute inputs to ocean and ocean cooling

behaviors of trace elements in neoarchean and paleoproterozoic paleosols implications for atmospheric oxygen evolution and continental oxidative weathering abstract the behaviors of redox sensitive and or bio essential trace elements in neoarchean and paleoproterozoic paleosols ancient weathering profiles were investigated to better understand atmospheric oxygen evolution the loss or retention of individual trace elements in the paleosols can show how continental oxidative weathering and thus atmospheric oxygen evolution took place against age mainly due to their various redox potentials the v cr ni cu zn and mo concentrations of two paleoproterozoic paleosols were measured by inductively coupled plasma optical emission spectrometry and mass spectrometry and those as well as co w and u concentrations of nine neoarchean and paleoproterozoic paleosols were obtained from the literature the trace element behaviors were constrained by their degrees of loss or retention in the paleosols we applied two methods to the estimation i retention fraction of element m a mass ratio of element m of paleosol to parent rock using immobile elements such as ti and ii element element in particular si element correlations at different profile depths of a paleosol there are two distinct groups in trace element behavior in the neoarchean and paleoproterozoic paleosols co ni zn and w were lost from weathering profiles until the early paleoproterozoic but retained in the middle and late paleoproterozoic while v cr cu mo and u were retained in the profiles until the early paleoproterozoic or slightly later but lost from the profiles in the middle and late paleoproterozoic more precisely the timings of such loss and retention were different between trace elements during the paleoproterozoic the characteristics of these changes from retention to loss or from loss to retention indicate that the changes occurred and lasted throughout the paleoproterozoic the trace element behaviors accordingly suggest that continental weathering became oxidative progressively with age during almost the whole paleoproterozoic and thus atmospheric oxygen increased similarly with age in the same time period but not suddenly in a relatively short term the loss and retention of the trace elements may infer how they were released from the continent to the ocean against age in the neoarchean and paleoproterozoic

early earth cyanobacteria at work oxygen producing photosynthesis must have evolved before the pervasive oxidation of the atmosphere around 2 4 billion years ago but how long before is unclear geochemical analyses of ancient sedimentary rocks now suggest that cyanobacteria generated oxygen at least 3 billion years ago

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

evidence for episodic oxygenation in a weakly redox buffered deep mid proterozoic ocean abstract over the last two decades popular opinion about prevailing conditions in the mid proterozoic deep ocean has evolved from fully oxygenated to globally euxinic sulfidic to a more heterogeneous stratified water column with localized pockets of euxinia existing in predominantly iron rich ferruginous deep waters the animikie basin in the lake superior region has been essential in shaping our view of marine redox evolution over this time period in this study we present a multi proxy paleoredox investigation of previously unanalyzed strata of the late paleoproterozoic animikie basin using drill cores through the 1 85 ga stambaugh formation paint river group in the iron river crystal falls district of the upper peninsula of michigan usa based on previous tectonic reconstructions and analysis of sedimentary regimes the iron river crystal falls section captures strata from among the deepest water facies of the animikie basin in contrast to previous work on sedimentary rocks in this basin we find evidence from iron speciation trace metal and mo isotope data for episodes of at least local deep water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions while trace metal enrichments and iron speciation data suggest predominantly anoxic conditions the occurrence of mn rich intervals up to 12 3 wt mno containing abundant mn fe carbonate and a wide range of mo isotope data with extremely negative values 98 95mo 1 0 to 1 1 record the shuttling of mn oxides from surface waters through oxic or suboxic waters to the sediment water interface we propose that such conditions are analogous to those of locally restricted modern and holocene basins in the baltic sea which receive episodic inflow of oxygenated water producing similar geochemical signatures to those observed for the animikie basin we argue that the mid proterozoic was characterized by a lack of a strong redox buffer low sulfide ferrous iron and oxygen contents and thus was vulnerable to dramatic and at least local redox shifts including briefly oxygenated bottom waters a refined view of the mid proterozoic ocean is emerging one that was still predominantly anoxic but marked by regional heterogeneities and short term redox variability that may in part reflect a transitional state between prevailingly anoxic archean and predominantly oxic phanerozoic oceans

the molybdenum isotopic compositions of i s and a type granitic suites this study reports mo isotopic compositions for fifty two palaeozoic granitic rocks with contrasting source affinities a i and s type from the lachlan fold belt lfb and the new england batholith neb both in se australia and three compositionally zoned plutons loch doon criffell and fleet located in the southern uplands of scotland the results show relatively large variations in 98mo for igneous rocks ranging from 1 73 to 0 59 with significant overlaps between different types no relationships between 98mo and 18o or asi alumina saturation index are observed indicating that mo isotopes do not clearly distinguish igneous vs sedimentary source types instead effects of igneous processes source mixing regional geology as well as hydrothermal activity control the mo isotope compositions in these granites it is found that mo is mainly accommodated in biotite and to a lesser extent in hornblende hornblende and fe3 rich minerals may preferentially incorporate light isotopes as reflected by negative correlations between 98mo and k rb and fe2o3 there is a positive correlation between initial 87sr 86sr and 98mo in i type granitic rocks reflecting the admixing of material from isotopically distinct sources granitic rocks from scotland and australia display strikingly similar curvilinear trends in 98mo vs initial 87sr 86sr despite the differing regional geology localized hydrothermal effects can result in low 98mo in granite as seen in three samples from loch doon and criffell which have anomalously light 98mo of 1 based on this study an estimate of 98mo 0 14 0 07 95 s e for the phanerozoic upper crust is proposed this is slightly heavier than basalts indicating an isotopically light lower crust and or a systematic change to the crust resulting from subduction of isotopically light dehydrated slab and or pelagic sediment over time

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

a statistical analysis of the carbon isotope record from the archean to phanerozoic and implications for the rise of oxygen organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales permanent burial of organic carbon in the crust or mantle oxidizes the surface environment atmosphere ocean and biosphere by removing reduced carbon it has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout earth9s history thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen schidlowski 1988 catling and others 2001 holland 2002 holland 2009 kump and others 2009 rothman 2015 however the opposite conclusion has been drawn from the same carbon isotope record des marais and others 1992 bjerrum and canfield 2004 to test these opposing claims we compiled an updated carbon isotope database and applied both parametric and non parametric statistical models to the data to quantify trends and mean level changes in fractional organic carbon burial with associated uncertainties and confidence levels we first consider a conventional mass balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon for this model statistical analysis implies fractional organic burial has increased over earth history by a factor of 1 5 relative to organic burial at 3 6 ga with the 95 percent confidence interval ranging from factors of 1 2 to 2 0 an increase in organic burial by a factor of 1 2 cannot explain the rise of oxygen whereas an increase by a factor of 2 could conceivably explain the rise of oxygen there is however a highly significant and well constrained increase in organic burial from the proterozoic to the phanerozoic we also analyze changes in the difference between carbonate and organic carbon isotopic ratios over earth history there is a statistically significant increase in this difference from the early to late archean possibly caused by increased biological fractionation due to methanotrophic recycling this transition is consistent with the evolution of oxygenic photosynthesis at 2 8 ga or earlier finally we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes specifically ocean crust carbonatization and authigenic carbonates because the size of these fluxes has a large poorly constrained range our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all however it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time in which case conclusions from the conventional model would be valid

the formation of magnetite in the early archean oceans abstract banded iron formations bifs are iron and silica rich chemical sedimentary rocks that were deposited throughout much of the precambrian the biological oxidation of dissolved fe ii led to the precipitation of a ferric oxyhydroxide phase such as ferrihydrite in the marine photic zone upon burial ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological fe iii reduction coupled to the oxidation of buried microbial biomass however it has always been intriguing as to why the oldest bifs are characteristically magnetite rich while bifs formed after the neoarchean are dominated by hematite here we propose that some magnetite in early archean bif could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot fe ii rich hydrothermal fluids that existed in the deeper waters we conducted experiments that showed the reaction of fe ii with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite our model further posits that with the progressive cooling and oxidation of the earth s oceans the above reaction shuts off and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism

geobiology and palaeogenomics abstract geobiology is centered on interactions of the biosphere with the atmosphere hydrosphere and lithosphere in deep time geobiology has focused on critical intervals of change for earth and its biota such geobiological studies include data from palaeobiology geochemistry and sedimentary geology within an interdisciplinary framework palaeogenomics has a variety of research agendas and one of them is to understand the genes involved in key changes in the evolutionary history of life and when those genes and their interactions first evolved the combination of geobiology and palaeogenomic studies is a powerful approach which leads to a more fundamental understanding of how earth and life have changed through time for example linking palaeogenomic studies of biomineralization genes in modern organisms and when they first evolved in the cambrian with geobiological studies of the impact this had on the production of carbonate sedimentary facies such as encrinites and the development of the neritan ocean provides a unique perspective on how genes have shaped formation of the sedimentary record another component of the cambrian explosion the evolution of vertical bioturbation at the start of the cambrian has had significant effects on biogeochemical cycling and sediment production and provides an inviting target for future genomic studies the remaining phanerozoic includes a broad variety of evolutionary innovations which within a joint geobiological and palaeogenomic context can also be profitably studied in pelagic environments the genomic changes leading to evolution of the calcium carbonate biomineralized skeletons of coccolithophores contributed to the widespread deposition of carbonate sediment in deep settings and development in the mesozoic of the cretan ocean on land linking palaeogenomic studies of the genes that led to the palaeozoic evolution of lignin and their associated geobiological effects such as deposition of coal provide another example of the effects of genes upon development of the sedimentary record such work will ultimately provide a history of genomic change and the environmental factors that influenced it as well as how genomic change influenced environments

trace elements at the intersection of marine biological and geochemical evolution life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes these requirements differ between organisms and have evolved over geological time likely guided in some part by environmental conditions until recently most of what was understood regarding trace element concentrations in the precambrian oceans was inferred by extrapolation geochemical modeling and or genomic studies however in the past decade the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new and potentially more direct insights into secular changes in seawater composition and ultimately the evolution of the marine biosphere compiled records of many bioessential trace elements including ni mo p zn co cr se and i provide new insight into how trace element abundance in earth s ancient oceans may have been linked to biological evolution several of these trace elements display redox sensitive behavior while others are redox sensitive but not bioessential e g cr u their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere ocean redox conditions that are linked to biological evolution for example the activity of oxygen producing photosynthetic cyanobacteria in this review we summarize available precambrian trace element proxy data and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life we also examine several biologically relevant and or redox sensitive trace elements that have yet to be fully examined in the sedimentary rock record e g cu cd w and suggest several directions for future studies

rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2 5 billion years ago the history of the growth of continental crust is uncertain and several different models that involve a gradual decelerating or stepwise process have been proposed1 4 even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea subaerial landmasses with estimates ranging from about one billion to three billion years ago5 7 the area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans 8 and therefore connects earth s crustal evolution to surface environmental conditions9 11 here we use the triple oxygen isotope composition of shales from all continents spanning 3 7 billion years to provide constraints on the emergence of continents over time our measurements show a stepwise total decrease of 0 08 per mille in the average triple oxygen isotope value of shales across the archaean proterozoic boundary we suggest that our data are best explained by a shift in the nature of water rock interactions from near coastal in the archaean era to predominantly continental in the proterozoic accompanied by a decrease in average surface temperatures we propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near modern average elevation and aerial extent roughly 2 5 billion years ago the use of triple oxygen isotope data from continental shales spanning the past 3 7 billion years suggests that continental crust with near modern average elevation and extent emerged about 2 5 billion years ago

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

evaluating a primary carbonate pathway for manganese enrichments in reducing environments abstract most manganese mn enrichments in the sedimentary rock record are hosted in carbonate minerals which are assumed to have formed by diagenetic reduction of precursor mn oxides and are considered diagnostic of strongly oxidizing conditions here we explore an alternative model where mn carbonates form in redox stratified water columns linked to calcium carbonate dissolution in ferruginous brownie lake in minnesota usa we document mn carbonates as an hcl extractable phase present in sediment traps and in reducing portions of the water column mn carbonate becomes supersaturated in the brownie lake chemocline where dissolved oxygen concentrations fall below 5 m and mn oxide reduction increases the dissolved mn concentration supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline in the same zone sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon dic with negative 13 c these observations demonstrate that sedimentary mn enrichments may 1 develop from primary carbonate phases and 2 can occur in environments with dissolved oxygen concentrations 200 m and where mn and fe are partitioned by s cycling photoferrotrophy or microaerophilic fe oxidation a shallow lysocline enhances mn carbonate production by providing additional dic and nucleation sites for crystal growth this carbonate model for mn enrichments is expected to be viable in both euxinic and ferruginous environments and provides a more nuanced view of the relationships between mn and carbon cycling with applications throughout the rock record

a small s mif signal in martian regolith pyrite implications for the atmosphere abstract the past martian atmosphere is often compared to the archean earth s as both were dominated by co2 rich and o2 poor chemistries archean earth rocks preserve mass independently fractionated sulfur isotopes s mif non zero 33s and 36s originating from photochemistry in an anoxic atmosphere thus martian crustal rocks might also be expected to preserve a s mif signature providing insights into past atmospheric chemistry we have used secondary ion mass spectrometry sims to investigate in situ the sulfur isotope systematics of nwa 8171 paired to nwa 7034 a martian polymict breccia containing pyrite that formed through hydrothermal sulfur addition in a near surface regolith setting in this meteorite pyrite grains have a weighted mean of 33s of 0 14 0 08 and 36s 0 70 0 40 2 s e m so the s mif signature is subtle sulfur isotope data for four additional shergottites yield 33s values that are not resolvable from zero as in previous studies of shergottites at first glance the result for the polymict breccia might seem surprising but no martian meteorite yet has yielded a s mif signature akin to the large deviations seen on earth we suggest that s mif bearing aerosols h2so4 and s8 were produced when volcanic activity pushed a typically oxidising martian atmosphere into a reduced state after rain out of these aerosols s8 would tend to be oxidised by chlorate dampening the s mif signal which might be somewhat retained in the more abundant photolytic sulfate then in the regolith mixing of aqueous surface derived sulfate with igneous sulfide the latter with zero mif to form the abundant pyrite seen in nwa 8171 would further dampen the s mif signal nonetheless the small negative 33s anomalies seen in martian meteorites imply that volcanic activity was sufficient to produce a reducing atmosphere at times this volcanically driven atmospheric evolution would tend to produce high levels of carbonyl sulfide ocs given that ocs is a relatively long lived strong greenhouse gas the s mif signal implies that volcanism periodically generated warmer conditions perhaps offering an evidence based solution to the young wet mars paradox

in search of early life carbonate veins in archean metamorphic rocks as potential hosts of biomarkers abstract the detection of early life signatures using hydrocarbon biomarkers in precambrian rocks struggles with contamination issues unspecific biomarkers and the lack of suitable sedimentary rocks due to extensive thermal overprints importantly host rocks must not have been exposed to temperatures above 250 c as at these temperatures biomarkers are destroyed here we show that archean sedimentary rocks from the jeerinah formation 2 63 billion yrs and carawine dolomite 2 55 billion yrs of the pilbara craton western australia drilled by the agouron institute in 2012 which previously were suggested to be suitable for biomarker studies were metamorphosed to the greenschist facies this is higher than previously reported both the mineral assemblages carbonate quartz fe chlorite muscovite microcline rutile and pyrite with absence of illite and chlorite geothermometry suggest that the rocks were exposed to temperatures higher than 300 c and probably 400 c consistent with greenschist facies metamorphism this facies leads to the destruction of any biomarkers and explains why the extraction of hydrocarbon biomarkers from pristine drill cores has not been successful however we show that the rocks are cut by younger formation specific carbonate veins containing primary oil bearing fluid inclusions and solid bitumens type 1 veins in the carawine dolomite consist of dolomite quartz and solid bitumen whereas type 2 veins in the jeerinah formation consist of calcite within the veins fluid inclusion homogenisation temperatures and calcite twinning geothermometry indicate maximum temperatures of 200 c for type 1 veins and 180 c for type 2 veins type 1 veins have typical isotopic values for reprecipitated archean sea water carbonates with 13 c vpdb ranging from 3 to 0 and 18 o vpdb ranging from 13 to 7 while type 2 veins have isotopic values that are similar to hydrothermal carbonates with 13 c vpdb ranging from 18 to 4 and 18 o vpdb ranging from 18 to 12 evidently the migration and entrapment of hydrocarbons occurred after peak metamorphism under temperatures congruous with late catagenesis and from fluids of different compositions the relatively high temperatures of vein formation and the known geotectonic history of the rocks analysed suggest a probable minimum age of 1 8 billion yrs paleoproterozoic our results demonstrate that post peak metamorphic veins provide an exciting opportunity in the search for evidence of early life the integration of petrological and organic geochemical techniques is crucial for any future studies that use biomarkers to reconstruct the early biosphere

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

redox states of archean surficial environments the importance of h2 g instead of o2 g for weathering reactions abstract redox states of the archean eon have been constrained by various lines of evidence including atmospheric photochemical and ecological models mass independent fractionations of sulfur isotopes fe depletion of paleosols and preservation of diagnostic detrital minerals although these lines of evidence present seemingly consistent upper limits on po2 g they are conceptually contradictory about the redox state of archean surficial environments atmospheric photochemical and ecological modeling studies suggest weakly reducing environments under redox states represented by moderate h2 g levels however current interpretations of fe depletion in paleosols and the preservation of detrital minerals are based on low o2 g levels at which the reducing detrital minerals are thermodynamically unstable and survive because of slow kinetics of oxidative weathering in this study we show that under the redox state indicated by the archean ph2 g range fe2 and reducing fe ii minerals are actually thermodynamically stable and have no tendency to be oxidized we emphasize that ph2 g values are orders of magnitude higher than po2 g in the archean atmosphere and that h2 g and o2 g are not in equilibrium the redox states of archean surface environments behave as though they were controlled by the more abundant h2 instead of the very low o2 as in modern anoxic basins weathering in this case should have involved non redox acidic dissolution of fe ii species or reductive reaction of fe iii species fe ii depleted paleosols and the preservation of relatively reduced detrital minerals are natural consequences of their thermodynamic stabilities in the archean eon s reducing environments rather than slow kinetics of oxidizing reactions after the appearance of oxygenic photosynthesis probably in the middle late archean locally oxygenated environments could have existed while the atmosphere as a whole remained anoxic the profile of redox states on the archean surface seems to be a reverse analogue to the modern earth although oxidizing dissolution of transition metals could happen in o2 oases quick reduction of oxyanions by abundant reductants such as aqueous fe2 fe ii minerals and h2 might have restricted riverine transport of oxyanions and potentially complicate the interpretation of signals of o2 whiffs in marine sediments

geochemical stratigraphy sedimentology and mo isotope systematics of the ca 2 58 2 50 ga old transvaal supergroup carbonate platform south africa abstract the neoarchean paleoproterozoic transvaal supergroup in south africa contains the well preserved stromatolitic campbellrand malmani carbonate platform which was deposited in shallow seawater shortly before the 2 40 2 32 ga great oxidation event goe this platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for isotope geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the goe mo isotopes in sedimentary archives are widely used as a proxy for redox changes in modern and ancient environments and recent evidence suggests that oxy molybdate moo 4 2 is directly transferred from ocean water to inorganic carbonates with negligible fractionation thus reflecting oceanic mo isotope signatures in this study we analyzed major and trace element compositions as well as mo isotopic compositions of carbonate and mudstone samples from the kmf 5 drill core geochemical indicators such as fe and mn concentrations and fe to mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform heavy 30 si values of silicified carbonates between 0 53 and 2 35 point to si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint this assessment is supported by the frequent observation of rip up structures of silica chert layers within the entire sedimentary succession the 98 mo values of whole rock samples throughout the malmani campbellrand platform range between 0 82 and 1 40 similar to values reported for deeper slope carbonates from the griqualand west area but variations are independent from lithology or depositional water depth these large variations in 98 mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere ocean system at that time in agreement with earlier mo isotopic studies on campbellrand carbonates and shales a similar range in 98 mo values for carbonates between 0 40 and 0 87 however was also found on the hand specimen scale indicating the danger of a sample bias on the mo isotopic stratigraphy of this carbonate platform results of previously unpublished adsorption experiments of mo on caco 3 clearly indicate that the mo inventory of malmani campbellrand carbonates was not only influenced by primary adsorption from seawater but to a much larger degree by secondary processes during early diagenesis which also affected the mo isotopic composition of the samples on a local scale our results indicate that mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through earth s history nevertheless we interpret our heavy mo isotopic signatures from carbonates as a minimum value for neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic mo reservoir at that time

a critical evaluation of copper isotopes in precambrian iron formations as a paleoceanographic proxy abstract trace metals in iron formations if have been widely used as proxies for oceanic redox processes and oxygen evolution leading to the great oxidation event goe copper has hitherto received comparatively little attention with a single study reporting variation in 65cu between pre and post goe black shales this is attributed to postulated isotope fractionation effects in the pre goe ocean during widespread iron oxide deposition in if here we focus on the application of cu isotopes in two classic if containing sequences of the neoarchean paleoproterozoic namely the hamersley australia and transvaal south africa supergroups we specifically targeted the oxide rich joffre and kuruman if the carbonate rich griquatown if and the mn rich hotazel if which collectively record over 100 ma of sustained if deposition in the pre goe ocean the aim was to assess the utility of cu isotopes in if as a paleoceanographic proxy in view of existing oxygen evolution models iron formation cu concentrations are low compared to average crust and modern oceanic fe mn oxide deposits with average values between 1 and 5 ppm for all four if data sets copper concentrations show no systematic variability with mineralogy no statistical correlation with bulk fe and mn contents good correlations with ti rcu ti 0 73 for the joffre data set and with cr ni and v for the south african data sets and shale like cu ti ratios isotopic results show statistically invariant average 65cu values very close to 0 for all but the joffre if which has a marginally negative average 65cu value 0 24 0 22 combined with other trace transition metal systematics the cu isotope data point to a cu source that was controlled largely by inputs of fine volcanic derived particles ash thus placing limitations on its utility as a paleoceanographic redox proxy during if genesis

a revised classification scheme of pyrite in the witwatersrand basin and application to placer gold deposits abstract pyrite is the most abundant ore mineral in gold bearing quartz pebble conglomerates qpcs of the witwatersrand basin and similar archaean sedimentary units much of the pyrite in archaean conglomerates is detrital in origin implying that it survived weathering transport and reworking under anoxic conditions detrital pyrite is generally found together with authigenic pyrite formed in situ as a result of syn to post depositional processes as pyrite is frequently associated with gold mineralization many authors have developed different pyrite classification schemes in order to better understand the nature and origin of the mineralization although the different classifications cannot always be easily compared as they are largely based on inferred pyrite genesis we propose a revised entirely descriptive classification scheme for detrital and authigenic pyrite in archaean qpcs the scheme was created in order to provide a consistent framework for the petrographic description of pyrite in clastic sedimentary rocks detrital pyrite is subdivided into massive inclusion bearing and coarsely crystalline types authigenic pyrite includes euhedral overgrowth infill aggregate and pseudomorphic types we evaluate the methods used in pyrite classification propose formation pathways for detrital and authigenic pyrites and apply the classification scheme to some well known occurrences of detrital pyrite bearing conglomerates

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

on the origin and age of the steep rock buckshot ontario canada abstract the steep rock buckshot is a brecciated pisolithic ferruginous bauxite found in the errington iron mine steep rock lake ontario canada it occurs within the almost vertical ore zone included in the archean steep rock group reaching a depth in excess of 1100 ft 335 m successive authors considered its age as precambrian the biological observations now presented deny this supposed precambrian age treatment of the concretions by hci followed by staining put in evidence abundant microscopic figurate animal and vegetal remains namely shreds of insect integument and varied fragments of vascular plant material comparable results were obtained by casting and by scanning electron miscroscopy sem of the surface of the concretions and by thin sections all these remains are phanerozoic and one of them is probably not anterior to the late cretaceous the macro and micromorphology of the concretions obey models of termite constructions and the spectrum of organic remains is identical to the one found in tubulo alveolar laterites and in termite constructions the presence in the matrix of canaliculi attributable to thin roots gives force to the idea that the buckshot was originated near the soil surface

gold accumulation in the archaean witwatersrand basin south africa evidence from concentrically laminated pyrite abstract concentrically laminated pyrite is a relatively common although volumetrically minor component of auriferous conglomerates in the archaean ca 3 0 2 7 ga witwatersrand basin of south africa this type of pyrite contains high amounts several tens of ppm of au but the origin of the pyrite is debated and the timing of au deposition in these grains is not known in order to constrain the formation of pyrite we have studied concentrically laminated pyrite and other coexisting types of pyrite inclusion rich massive pyrite by analysing the contents and distribution of au and other trace elements by laser ablation icp ms the s and fe isotope composition by sims and the mineral inclusions by scanning electron microscope and laser raman spectroscopy trace element maps indicate that concentrically laminated pyrite is enriched in sb mn au ag tl cu mo mn and contains two types of gold finely dispersed au invisible gold with au ag 0 1 and likely of primary origin and au inclusions with au ag 10 of secondary origin the study of mineral inclusions revealed the presence of muscovite chlorite fine grained carbonaceous matter monazite ti oxides and quartz iron and multiple s isotopes suggest that concentrically laminated pyrite and inclusion rich pyrite were formed from two separate pools of s and fe with different isotope characteristics sulfur was derived from atmospheric s that had undergone mass independent isotope fractionation to form so 4 2 with negative 33 s that constituted concentrically laminated pyrite and elemental s with positive 33 s that formed inclusion rich pyrite iron pools were derived from partial oxidation of fe 2 so that concentrically laminated pyrite formed from a low 56 fe residual fe 2 average 0 2 and inclusion rich pyrite formed from a high 56 fe fe 3 pool average 2 7 biological activity may have been involved in the reduction of so 4 2 causing a wide spread of 34 s values 25 s reducing microorganisms as well as in the partial oxidation of fe 2 anaerobic photosynthetic fe reducers or photosynthetic o 2 producers and in the formation of pyrite from fe 3 dissimilatory fe reducers we propose that concurrent biogenically mediated pyrite formation and au trapping suggest that microbial activity was responsible for the accumulation of au and other trace elements e g sb mn ag tl cu mo mn which are commonly enriched in organic matter rich sediments