sulfur record of rising and falling marine oxygen and sulfate levels during the lomagundi event carbonates from approximately 2 3 2 1 billion years ago show markedly positive 13c values commonly reaching and sometimes exceeding 10 traditional interpretation of these positive 13c values favors greatly enhanced organic carbon burial on a global scale although other researchers have invoked widespread methanogenesis within the sediments to resolve between these competing models and more generally among the mechanisms behind earth s most dramatic carbon isotope event we obtained coupled stable isotope data for carbonate carbon and carbonate associated sulfate cas cas from the lomagundi interval shows a narrow range of 34s values and concentrations much like those of phanerozoic and modern marine carbonate rocks the 34s values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition these observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone toward the end of the carbon isotope excursion there is an increase in the 34s values of cas we propose that these trends in c and s isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion

sample scale carbon isotopic variability and diverse biomass in the paleoproterozoic zaonega formation russia abstract the stratigraphic record of organic matter in the c 1 97 ga zaonega formation zf onega basin northwestern russia exhibits a distinct negative 13c excursion 13corg from 25 to 40 vpdb which was previously interpreted either to reflect a disturbance in the global carbon cycle after the great oxidation event or to have been caused by an increase in basinal methanotrophic activity in order to assess the nature of primary biomass and the effects of post depositional alteration we here report the sample scale carbon isotopic characteristics of organic matter in two drill cores from the zf covering 500 m of stratigraphy by using secondary ion mass spectrometry sims the results confirm that the organic matter has to a large extent preserved the primary isotopic signatures whereas secondary effects are limited

chemistry of sands from the modern indus river and the archean witwatersrand basin implications for the composition of the archean atmosphere both the indus river and the witwatersrand basin contain sand with grains of detrital uraninite because this mineral is easily oxidized its presence in archean strata as a detrital particle has been used as evidence for a low oxygen atmosphere before 2 5 ga however its presence in modern sand from the indus river system has been used to argue that detrital uraninite does not provide information about the oxygen concentration of earth s early atmosphere petrographic and chemical study of sand from these two sources reveals differences that suggest the modern indus sand cannot be used as an analog for the archean witwatersrand occurrences the witwatersrand quartzites are depleted in ca mg and na indicating that the original sand from which they formed had been subjected to intense weathering the chemical index of alteration cia a commonly used indicator of degree of weathering yields an average value of about 0 80 for witwatersrand quartzites comparable to modern tropical streams such as the orinoco that drain deeply weathered terrains under tropical conditions cia 0 75 in contrast the cia for indus sand is 0 45 indicating virtually no chemical weathering the significance of archean quartz pebble conglomerates is not just that they contain unstable detrital phases like uraninitemore and pyrite but that these particles are associated with rocks whose compositions suggest intense weathering these conglomerates must have been subjected to intense weathering under tropical conditions either in their source area or at the site of deposition and the preservation of minerals like uraninite such conditions is indeed strong evidence for a low oxygen atmosphere less

garnet orthopyroxene bearing granulites from enderby land antarctica metamorphic pressure temperature time evolution of the archaean napier complex

diagenetic history of the proterozoic carbonates and its role in the oil field development in the baikit anteclise southwestern siberia abstract the meso to neoproterozoic kamo group and ediacaran carbonate rocks of the baikit anteclise southwestern siberian platform russia were studied to reconstruct the history of development of the reservoir rocks and oil field the kamo group is composed of 3 5 to 4 km thick dolostone and rarely limestone succession that was deformed before the ediacaran time post depositional processes played the key role in forming the reservoir properties of the kamo group during the stage of deposition and subsidence 1500 720 ma silicification cementation and recrystallization caused development of a dense and low permeability matrix pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments including increase in mn up to 0 093 and fe up to 3 11 contents and 87sr 86sr ratios up to 0 7239 decrease in sr concentrations down to 11 ppm and the development of flat ree normalized patterns folding and uplift in the baikit basin began about 720 ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot 70 130 c and highly saline up to 28 7 wt fluids geochemical e g paas normalized patterns of ree and isotopic 87sr 86sr 18o and 13c signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks uplift caused subaerial erosion intense dissolution and karstification which penetrated down to 200 300 m below the surface folding uplift and peneplenization were important for porosity and permeability development in the dolostones of the kamo group the cap rocks were subsequently formed during the ediacaran and paleozoic sedimentation cycle the oil field developed in the paleozoic oil produced from the source rocks of the madra vedreshev and iremeken formations of the kamo group was dissipated during the uplift and erosion before the ediacaran sedimentation after deposition of the ediacaran and cambrian strata source rocks of the iremeken formation might have released oil when they passed the oil window but most of oil in the kuyumba field was derived from younger ediacaran source rocks and migrated through fault network from the flanks the kureika and cis sayan yenisey syneclises into the reservoir of the baikit anteclise that was a topographic high at that time

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

the continental plates are getting thicker petrologists and seismologists now agree that the old cores of the continents have deep roots extending well below the thickest ocean plates

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

sulfur isotopic trends and pathways of iron sulfide formation in upper holocene sediments of the anoxic black sea down core trends for extents of iron sulfidation in upper holocene microlaminated deposits of the black sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and or very close to the sediment water interface sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core with a mean 34scdt value of 37 2 0 9 1 n 18 these isotopic trends support the hypothesis of early pyrite formation isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and to 34s values for water column dissolved sulfide within a narrow zone immediately below the o2 h2s interface these three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column 175 m with 34s values for the deep dissolved sulfide reservoir being consistently lower by 2 to 4 these isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow shallow region at the top of the sulfidic water mass isotopic data from a fine grained turbidite collected in the deep basin and from rapidly accumulating iron monosulfide rich muds on the anoxic upper slope are enriched in34s relative to the microlaminated deposits the enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial a broad suite of geochemical data including sulfur isotopic results are consistent with an upper slope sediment source for the ubiquitous muddy turbidites of the deep basin the collective effects of reworking transport and redeposition result in a complete conversion of avs to pyrite with little additional iron sulfidation this conclusion is supported by a detailed isotopic mass balance the present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin sediments of the upper slope and the genetically linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep basin floor suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin importantly isotopic patterns from the modern black sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record

oxygenated conditions in the aftermath of the lomagundi jatuli event the carbon isotope and rare earth element signatures of the paleoproterozoic zaonega formation russia abstract the c 2 0 ga zaonega formation of the onega basin nw russia has been central in efforts to understand what led to the initial rise great oxidation event goe and postulated fall in free atmospheric oxygen and associated high amplitude carbon cycle excursions the lomagundi jatuli event lje and subsequent shunga event during paleoproterozoic time the formation accumulated shortly after the lje and encompasses both the recovery in the carbon cycle and hypothesised contraction of the oceanic oxidant pool however interpreting the correct environmental context recorded by geochemical signatures in the zaonega rocks is difficult due to a complex depositional and diagenetic history in order to robustly constrain that history we undertook a multiproxy study mineralogy petrography carbon isotope and rare earth element composition of carbonate beds in the upper part of the zaonega formation recovered in the 102 m composite section of the onzap drill cores our findings differentiate primary environmental signatures from secondary overprinting and show that i the best preserved carbonate beds define an upwards increasing 13ccarb trend from c 5 4 to near 0 and that ii large intra bed 13ccarb variations reflect varying contributions of methanotrophic dissolved inorganic carbon dic to the basinal dic pool rare earth element and yttrium reysn patterns confirm a marine origin of the carbonate beds whereas a consistent positive eusn anomaly suggests a strong high temperature hydrothermal input during accumulation of the zaonega formation importantly the presence of a negative cesn anomaly in the reysn pattern indicates an oxygenated atmosphere ocean system shortly after the lje and indicates that models invoking a fall in oxygen at that time require reassessment

paleoproterozoic high δ13c dolomites from the lastoursville and franceville basins se gabon stratigraphic and synsedimentary subsidence implications abstract new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern gabon franceville and lastoursville sub basins indicate that the seawater value of 13 c in these sediments underwent a large positive excursion around 2 1 ga these carbonate rocks belong to the fc formation of the francevillian group deposited along the margin of the archean chaillu craton in open marine shelf and evaporite settings and experienced locally lower greenschist facies metamorphism four microfacies mf1 mf4 are recognized from subtidal cyanobacterial dolobindstones to inter and supratidal collapse breccias mf4 the dolomites exhibit elevated 13 c values most values are above 0 50 the highest value is 9 74 and the average value is near 4 vs vpdb consistent with deposition during the large worldwide positive excursion between 2 22 and 2 06 ga these positive 13 c values are independent of the microfacies type such high 13 c values are also recorded in reworked carbonates dolomites embedded in coarse grained sandstones and conglomerates interstratified in the black shales of the underlying fb formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits interaction between early structural development and sediment fill lead to the formation of several sub basins with distinct differences in terms of amount and timing of subsidence the 13 c enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the fb sediments in this extensional geotectonic context as a consequence the cherty and dolomitic fc formation cannot longer be considered as a marker interval the end of the positive excursion is located near the top of the francevillian series i e fe and could be used in future studies as a potential chemostratigraphic tool in the paleoproterozoic

oxygen dynamics in the aftermath of the great oxidation of earth s atmosphere the oxygen content of earth s atmosphere has varied greatly through time progressing from exceptionally low levels before about 2 3 billion years ago to much higher levels afterward in the absence of better information we usually view the progress in earth s oxygenation as a series of steps followed by periods of relative stasis in contrast to this view and as reported here a dynamic evolution of earth s oxygenation is recorded in ancient sediments from the republic of gabon from between about 2 150 and 2 080 million years ago the oldest sediments in this sequence were deposited in well oxygenated deep waters whereas the youngest were deposited in euxinic waters which were globally extensive these fluctuations in oxygenation were likely driven by the comings and goings of the lomagundi carbon isotope excursion the longest lived positive 13c excursion in earth history generating a huge oxygen source to the atmosphere as the lomagundi event waned the oxygen source became a net oxygen sink as lomagundi organic matter became oxidized driving oxygen to low levels this state may have persisted for 200 million years

molybdenum isotope fractionation in pelagic euxinia evidence from the modern black and baltic seas here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the black sea and two deeps of the baltic sea dissolved molybdenum in all water samples is depleted in salinity normalized concentration and enriched in the heavy isotope 98mo values up to 2 9 compared to previously published isotope data of sedimentary molybdenum from the same range of water depths furthermore 98mo values of all water samples from the black sea and anoxic deeps of the baltic sea are heavier than open ocean water the observed isotope fractionation between sediments and the anoxic water column of the black sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions an extrapolation to a theoretical pure mos42 solution indicates a fractionation constant between mos42 and authigenic solid mo of 0 5 0 3 measured waters with all thiomolybdates coexisting in various proportions show larger but non linear fractionation the best explanation for our field observations is mo scavenging by the thiomolybdates dominantly but not exclusively present in the form of mos42 the mo isotopic compositions of samples from the sediments and anoxic water column of the baltic sea are in overall agreement with those of the black sea at intermediate depth and corresponding sulphide concentrations the more dynamic changes of redox conditions in the baltic deeps complicate the black sea derived relationship between thiomolybdates and mo isotopic composition in particular the occasional flushing mixing of the deep waters affects the corresponding water column and sedimentary data 98mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations the results can be explained by non conservative behaviour of mo under suboxic to anoxic conditions in the shallow bottom parts of the basin most pronounced on the nw shelf of the black sea

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

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

evolution of the oceanic sulfur cycle at the end of the paleoproterozoic here we present new measurements of 32 s 33 s 34 s and 36 s in sedimentary sulfides and couple these measurements with modeling treatments to study the sulfur cycle of a late paleoproterozoic marine basin we target the transition in ocean chemistry from the deposition of paleoproterozoic iron formations gunflint formation biwabik formation trommald formation and mahnomen iron formations to the inferred sulfidic ocean conditions recorded by overlying shale rove formation the data suggest that certain features of the global sulfur cycle such as a control by sulfate reducing prokaryotes and low mm concentrations of oceanic sulfate were maintained across this transition this suggests that the transition was associated with changes in the structure of the basin scale sulfur cycle during deposition of these sediments sulfide data from the iron formations are interpreted to reflect sedimentary sulfides formed from microbial reduction of pore water sulfate that was supplied through steady state exchange with an overlying oceanic sulfate reservoir the sulfide data for the euxinic rove formation shales reflect the operation of a sulfur cycle that included the loss of sulfide by a rayleigh like process we suggest that the prevalence of large and variable heavy isotope enrichments observed in rove formation sulfide minerals reflect a sustained and significant net loss of sulfide from the euxinic water column either as a result of a shallow chemocline and degassing to the atmosphere or as a result of a water column pyrite sink the inclusion of 36 s measurements in addition to 32 s 33 s and 34 s illustrates the mass dependent character of these sedimentary environments ruling out contributions from the weathering of archean sulfides and pointing to at least modest levels of sustained atmospheric oxygen 10 5 present atmospheric levels of o2

al zoning in orthopyroxene in a sapphirine quartzite evidence for 1120 c uht metamorphism in the napier complex antarctica and implications for the entropy of sapphirine a unique sapphirine orthopyroxene quartz granulite from mt riiser larsen in the tula mountains of enderby land east antarctica preserves two generations of coarse and texturally equilibrated orthopyroxene and sapphirine coexisting with quartz initial subhedral orthopyroxene porphyroblasts retain core compositions enriched in al2o3 12 2 0 5 wt compared with their rims and finer orthopyroxene 9 6 0 5 wt al2o3 that forms granoblastic textures with sapphirine sapphirine and quartz also form symplectites on and along cleavage planes within orthopyroxene these compositional and textural features are consistent with the reaction 2mgal2sio6 mg2al4 sio10 sio2 leading to the formation of sapphirine quartz at the expense of aluminous orthopyroxene calculations in the mas and fmas systems and theoretical considerations involving the phases enstatite sapphirine sillimanite quartz and cordierite indicate that the reaction above progresses from left to right with decreasing temperature in the orthopyroxene sapphirine quartz field at pressures of ca 8 10 kbar the temperature difference required to account for the ca 2 5 3 wt decrease in al2o3 in orthopyroxene is at least 60 80 c and implies peak temperatures for the initial assemblage of at least 1120 c if the second granoblastic assemblage equilibrated at 1040 c the p t conditions required by the sapphirine quartz association and other p t sensitive assemblage indicators in the napier complex it is not possible to distinguish whether the two assemblages are simply related by cooling and re equilibration or reflect a polyphase evolution involving the superposition of a second uht event on an earlier even higher temperature uht metamorphism preliminary thermodynamic modelling of the reaction above incorporating the observed range in orthopyroxene al2o3 zoning indicates that present estimates for the entropy of high temperature sapphirine are potentially too high by 15 18 compared with sapphirine entropy estimates that are consistent with mas system experiments the mt riiser larsen sapphirine quartz rocks preserve the first definitive record of regional metamorphic temperatures in excess of 1120 c in the napier complex or indeed any uht granulite terrain worldwide similarly high peak temperatures may be retrieved from detailed studies of sapphirine quartz granulites from other regions further expanding the thermal realm of crustal metamorphism but progress will critically depend on the experimental acquisition of new entropy data for sapphirine

trace element perspective into the ca 2 1 billion year old shallow marine microbial mats from the francevillian group gabon abstract the sedimentary fabrics of precambrian mat related structures mrs represent some of the oldest convincing evidence for early life on earth the ca 2 1 billion year ga old mrs in the fb2 member of the francevillian basin in gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated shallow marine settings but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source despite these insights what remains lacking is a full characterization of the geochemical composition of the mrs to test whether the bulk composition of fossilized mrs is distinct from the host sediments sandstones and shales here we show that the trace element te content of microbial textures belonging to pyritized mrs poorly pyritized mrs and elephant skin textures est is highly variable and differs from that of the host sediments the poorly pyritized mrs contain a unique matrix with embedded ti and zr rich minerals and syngenetically enriched in te the est some of which are developed along the same stratigraphic horizon as the poorly pyritized mrs display a distinct distribution of te bearing heavy minerals suggesting a local difference in physical conditions during sedimentation similarly high chalcophile element ce content in pyritized mrs relative to the host sediments of the fb2 member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis the geochemical relationship between the mrs and the francevillian sediments e g fb fc and fd formations indicates that specific biological pathways for ce enrichments i e microbially controlled accumulation are not apparent our findings highlight bulk rock te distinction between the 2 1 billion year old mrs and their host sediments but also indicate that environmental conditions such as hydrodynamic regime and water column redox chemistry may simply overwhelm any potential biological signal our data suggest that the microbial impact may have only passively influenced te enrichment in the studied sediments implying that te concentrations in mrs are a poor biosignature importantly this work indicates that bulk te geochemistry does not unveil specific microbiological processes in the rock record which is consistent with the observed patterns in modern analogues

multiple sulphur and iron isotope composition of detrital pyrite in archaean sedimentary rocks a new tool for provenance analysis abstract multiple s 34 s and 33 s and fe 56 fe isotope analyses of rounded pyrite grains from 3 1 to 2 6 ga conglomerates of southern africa indicate their detrital origin which supports anoxic surface conditions in the archaean rounded pyrites from meso to neoarchaean gold and uranium bearing strata of south africa are derived from both crustal and sedimentary sources the latter being characterised by non mass dependent fractionation of s isotopes 33 s as negative as 1 35 and large range of fe isotope values 56 fe between 1 1 and 1 2 most sediment sourced pyrite grains are likely derived from sulphide nodules in marine organic matter rich shales sedimentary exhalites and volcanogenic massive sulphide deposits some sedimentary pyrite grains may have been derived from in situ sulphidised fe oxides prior to their incorporation into the conglomerates as indicated by unusually high positive 56 fe values sedimentary sulphides without significant non mass dependent fractionation of s isotopes were also present in the source of some conglomerates the abundance in these rocks of detrital pyrite unstable in the oxygenated atmosphere may suggest factors other than high p o 2 as the cause for the absence of significant non mass dependent fractionation processes in the 3 2 2 7 ga atmosphere rounded pyrites from the c 2 6 ga conglomerates of the belingwe greenstone belt in zimbabwe have strongly fractionated 34 s 33 s and 56 fe values the source of which can be traced back to black shale hosted massive sulphides in the underlying strata the study demonstrates the utility of combined multiple s and fe isotope analyses for provenance reconstruction of archaean sedimentary successions

constraining provenance for the uraniferous paleoproterozoic francevillian group sediments gabon with detrital zircon geochronology and geochemistry abstract the paleoproterozoic sandstones of the ca 2 1 ga francevillian group southeast gabon host the oldest known redox controlled high grade uranium u deposits uranium bearing detrital minerals e g monazite in the francevillian group sediments were derived by erosion of archean basement rocks from the east gabonian block of the northwestern congo craton surrounding the francevillian basin they are viewed as primary sources for u but the nature of these sources remains poorly constrained here u pb ages and hf isotope data for detrital zircon grains are combined with literature geochemical data for igneous rocks from the archean basement to characterize the nature age and origin of the provenance for the francevillian basin and the ultimate source of u for the high grade uranium deposits detrital zircon ages range from ca 3 1 to 2 6 ga with a dominant population between ca 2 90 and 2 80 ga suggesting a major contribution from ca 2 88 to 2 81 ga high k granitoids with a limited contribution from ca 3 1 to 2 84 ga ttgs and 2 75 to 2 7 ga granitoids of the east gabonian block most detrital zircon grains share similar depleted mantle hf model ages between ca 3 08 and 3 30 ga indicating significant recycling of a paleoarchean to mesoarchean crust we suggest that ca 2 9 to 2 8 ga high k u and th granitoids which reflect hybrid melts of both mantle and crustal parentage were the major detrital sources of u to the francevillian basin and ultimately for the u deposits we propose that ca 2 9 to 2 8 ga detrital zircons with hf tdm ages between ca 3 1 and 3 3 ga may thus allow for vectoring u deposits in sedimentary basins across the congo craton

dynamic carbon and sulfur cycling in the aftermath of the lomagundi jatuli event evidence from the paleoproterozoic hutuo supergroup north china craton abstract the unprecedented positive 13c excursion in carbonates deposited between 2 2 and 2 0 ga known as the lomagundi jatuli event lje has been documented globally and linked to the rise of atmospheric oxygen increasing oxidation inevitably changed the atmosphere hydrosphere system but few chemostratigraphic or quantitative constraints for the aftermath of this event exists here we describe a 200 m thick carbonate succession in the huaiyincun formation hutuo supergroup 2 0 1 9 ga from the north china craton there is a lithological transition from pink purple dolostones to grey dolostones at ca 91 6 m above the base of the huaiyincun formation the former are more enriched in hematite and detrital minerals whereas the latter contain more organic matter but almost no detrital phases meanwhile the frequent occurrence of tempesite structures along with the abrupt decline of stromatolites in the upper huaiyincun formation suggest a storm dominated environment these distinct features within the huaiyincun formation reveal increased water depth during a transgression event two types of raman spectra of organic matter were found in the lower and upper huaiyincun formation respectively which is proposed to be the result of variable oxidation at 65 6 m 26 m below the lithological transition 91 6 m remarkable decreases in both 13ccarb and 34scas are observed this discordance between c s isotopic excursions and sedimentological and mineralogical variations argues against a seawater depth gradient effect of the 13ccarb and 34scas curves instead the decline of 13ccarb rather correlates with the negative 13ccarb excursions in ca 2 0 ga carbonates from gabon and russia known as the shunga francevillian event sfe the result of the quantitatively constrained paleo seawater so42 sw suggests a crash of the seawater sulfate reservoir compared with that during the preceding lje however the decreased 34scas and increased cas concentration towards the top of the study unit represent the recovery of seawater sulfate reservoir the coordinated decline in 34scas and 13ccarb values is likely related to enhanced oxidation of continental pyrite and organic matter in the aftermath of the lje the huaiyincun formation therefore represents a critical interval that recorded dynamic carbon and sulfur cycles after the lje

micro scale quadruple sulfur isotope analysis of pyrite from the 3480 ma dresser formation new insights into sulfur cycling on the early earth abstract we report in situ quadruple sulfur isotope analysis 32s 33s 34s and 36s of a pyritized microbial mat from the 3480 ma dresser formation pilbara craton western australia these data yield positive 34s and 33s indicative of sulfur sourced from a pool with similar character as the putative atmospheric elemental sulfur channel of pavlov and kasting 2002 contrary to previous data from the dresser formation however this pyrite is heavily depleted in 36s with a 36s 33s slope of c 3 6 much steeper than slopes typically seen in other early archean rock successions 36s 33s 1 which suggests either a different atmospheric signature for deposited s or a different pool altogether significant micro scale isotopic heterogeneity is observed within the microbial mat 34s 1 6 to 6 7 33s 0 4 to 2 6 36s 3 1 to 8 1 implying a role for microbial s metabolism while metabolic s cycling has been shown to shift 36s to lower values microbial metabolization of s does not appear sufficient to account for the full range of 36s we conclude that the isotopic composition of the pyrite was controlled by the relative proportions of mass independently fractionated mif s0 and sulfate derived sulfur incorporated into polysulfide pyrite precursors during reactions in the microbial mat the dominance of the mif s0 isotopic signature 34s 33s 36s indicates that contributions from the sulfate derived sulfur pool were relatively small consistent with low concentrations of sulfate in archean seawater and that contributions from a non sulfate pool were significant micro scale isotopic heterogeneity in the pyrite points to mixing between the two sulfur pools in selected micro environments within the microbial mat the particularly negative 36s observed here reveals a 3480 ma sulfur reservoir with novel 36s 33s chemistry whose significance now requires further investigation

identifying global vs basinal controls on paleoproterozoic organic carbon and sulfur isotope records abstract paleoproterozoic sedimentary successions are important archives of the redox evolution of earth s atmosphere and oceans efforts to unravel the dynamics of our planet s early oxygenation from this archive rely on various geochemical proxies including stable carbon and sulfur isotopes however ancient metasedimentary rocks often experienced early and late stage bio geochemical processes making it difficult to discern primary environmental signals from bulk rock 13corg and 34s values such complexity in carbon and sulfur isotope systematics contributes to uncertainty about the redox structure of paleoproterozoic oceans a currently popular idea is that following the great oxidation event global changes led to low oxygen environments and temporally fluctuating ocean redox conditions that lasted until the neoproterozoic the volcano sedimentary rocks of the onega basin have figured prominently in this concept particularly the exceptionally organic rich rocks of the 1 98 ga zaonega formation however a growing body of evidence shows that local depositional processes acted to form the 13corg and pyrite 34s records of the zaonega formation thus calling for careful assessment of the global significance of these isotope records placing new and existing organic carbon and sulfur isotope data from the zaonega formation into the context of basin history and by comparing those results with key paleoproterozoic successions of the francevillian basin gabon the pechenga greenstone belt nw russia and the animikie basin canada we show that the stratigraphic 13corg and pyrite 34s trends can be explained by local perturbations in biogeochemical carbon and sulfur cycling without requiring global drivers despite their temporal disparity we also demonstrate that individual successions share certain geological traits e g magmatic and or tectonic activity hydrocarbon generation basin restriction suggesting that their pyrite 34s and 13corg trends were governed by common underlying mechanisms e g similar basinal evolution and biogeochemical feedbacks and are not necessarily unique to certain time intervals we further show that pyrites in these successions that are most likely to capture ambient seawater sulfate isotopic composition have consistent 34s values of 15 18 which hints at remarkable stability in the marine sulfur cycle over most of the paleoproterozoic era

oxidative forcing of global climate change a biogeochemical record across the oldest paleoproterozoic ice age in north america carbon isotope compositions of organic matter in fine grained siliciclastic units deposited above and below glacial diamictite at the base of the ca 2 45 2 22 ga huronian supergroup in ontario canada were studied to constrain relationships between profound fluctuations in the exogenic carbon cycle and dramatic climate changes at the beginning of the proterozoic eon in both drill core and outcrop sections the organic matter preserved in proximal lithofacies dominated by coarse grained sand and silt are enriched in 13 c relative to distal lithofacies dominated by argillites in the drill core sand dominated lithofacies of the mckim formation beneath the glacial diamictite of the ramsay lake formation have a narrow range of 13 c values 28 4 to 26 0 vpdb but organic matter in argillite dominated lithofacies of the outcrop section 40 km to the southeast is somewhat more 13 cdepleted with values ranging from 34 5 to 26 4 similarly sand dominated lithofacies of the pecors formation above the glacial diamictite in the drill core section with 13 c values of ca 28 are notably 13 c enriched relative to argillite dominated lithofacies which record values as low as 40 5 the sand dominated lithofacies of the pecors formation in the outcrop sections have 13 c compositions ranging from 34 4 to 27 9 the isotopic differences appear to be unrelated to organic carbon abundances so we suggest that these are controlled by environmental differences in proximal and distal settings the strong 13 cdepletion in the organic lean mckim and pecors argillites especially in the drill core section of the pecors formation is consistent with significant biological methane production and oxidative recycling by methanotrophs both before and after the ice age in shallow water environments stratified with respect to oxygen the rise of atmospheric oxygen and subsequent enhanced biogeochemical methane cycling in shallow water settings likely contributed to unstable climate conditions during the paleoproterozoic glacial epoch 2007 elsevier b v all rights reserved

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

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

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

selenium isotopes record extensive marine suboxia during the great oxidation event abstract it has been proposed that an oxygen overshoot occurred during the early paleoproterozoic great oxidation event goe in association with the extreme positive carbon isotopic excursion known as the lomagundi event moreover it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative shunga francevillian carbon isotopic anomaly these redox fluctuations could have profoundly influenced the course of eukaryotic evolution as eukaryotes have several metabolic processes that are obligately aerobic here we investigate the magnitude of these proposed oxygen perturbations using selenium se geochemistry which is sensitive to redox transitions across suboxic conditions we find that 82 78se values in offshore shales show a positive excursion from 2 32 ga until 2 1 ga mean 1 03 0 67 selenium abundances and se toc total organic carbon ratios similarly show a peak during this interval together these data suggest that during the goe there was pervasive suboxia in near shore environments allowing nonquantitative se reduction to drive the residual se oxyanions isotopically heavy this implies o2 levels of 0 4 m in these settings unlike in the late neoproterozoic and phanerozoic when negative 82 78se values are observed in offshore environments only a single formation evidently the shallowest shows evidence of negative 82 78se this suggests that there was no upwelling of se oxyanions from an oxic deep ocean reservoir which is consistent with previous estimates that the deep ocean remained anoxic throughout the goe the abrupt decline in 82 78se and se toc values during the subsequent shunga francevillian anomaly indicates a widespread decrease in surface oxygenation

the neoproterozoic huttenberg δ13c anomaly genesis and global implications abstract the neoproterozoic huttenberg formation in northeastern namibia records a remarkable 13 c carb positive excursion with a sustained plateau of values up to 12 i e the huttenberg anomaly high resolution chemostratigraphic analyses of drill core samples spanning the upper elandshoek and huttenberg formations reveal multiple new observations 1 overall high but oscillatory 13 c carb values 2 18 o carb values ranging from 8 to 2 3 significant enrichment of 13 c in organic carbon and a broad co variation between 13 c carb and 13 c org 4 a profound negative excursion in 34 s pyrite from 30 to 10 5 an overall inverse 13 c 34 s relationship and 6 87 sr 86 sr values down to 0 7074 in limestone samples the new data suggest that the huttenberg anomaly records dynamic fluctuations in marine redox conditions which may include an oxygenation event during the height of the 13 c carb positive excursion and a deoxygenation event at its termination the 34 s pyrite negative excursion suggests the buildup of the marine sulfate reservoir likely due to enhanced pyrite oxidation during the oxygenation event the 34 s pyrite increase at the end of the huttenberg anomaly may result from a seawater sulfate concentration drawdown towards pre anomaly conditions on one hand the huttenberg anomaly may reflect restricted basin signals that are deviated from the ediacaran open ocean on the other hand the ediacaran huttenberg anomaly together with the cryogenian 13 c carb positive excursions suggests a stepwise pattern of the neoproterozoic oxygenation event both local and global environmental factors may have contributed to the huttenberg anomaly the huttenberg anomaly therefore represents a local enhancement of global oxygenation signals our data support the emerging view that the neoproterozoic oxygenation event may have facilitated the evolution of early life at that time

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

multiple sulphur isotope records tracking basinal and global processes in the 1 98 ga zaonega formation nw russia abstract the exceptionally organic rich rocks of the 1 98 ga zaonega formation deposited in the onega basin nw russia have refined our understanding of earth system evolution during the paleoproterozoic rise in atmospheric oxygen these rocks were formed in vent or seep influenced settings contemporaneous with voluminous mafic volcanism and contain strongly 13c depleted organic matter here we report new isotopic 34s 33s 36s 13corg and mineralogical major element total sulphur and organic carbon data for the upper part of the zaonega formation which was deposited shortly after the termination of the lomagundi jatuli positive carbon isotope excursion the data were collected on a recently obtained 102 m drill core section and show a 13corg shift from 38 to 25 sedimentary sulphides have 34s values typically between 15 and 25 reflecting closed system sulphur isotope behaviour driven by high rates of microbial sulphate reduction high sulphate demand hydrothermal activity and hydrocarbon seepage four intervals record 34s values that exceed 30 we interpret these unusually 34s enriched sulphides to be a result of limited sulphate diffusion into pore waters due to changes in sedimentation and or periods of basinal restriction additionally there are four negative 34s excursions that are interpreted to reflect changes in the open closed system behaviour of sulphate reduction or availability of reactive iron our findings highlight the influence of basinal processes in regulating sulphur isotope records and the need for care before interpreting such signals as reflecting global conditions

coupled molybdenum iron and uranium stable isotopes as oceanic paleoredox proxies during the paleoproterozoic shunga event the paleoproterozoic era was a time of remarkable importance in the redox evolution of earth s atmosphere and oceans here we present a multi proxy study of mo u and fe isotopes together with fe speciation of black shales and siltstones from the upper zaonega formation of the onega basin in karelia we attempt to better understand oceanic redox conditions during the 2 05 ga shunga event as the next step following the great oxidation event goe and the lomagundi carbon isotope excursion event a cautious examination of the fe speciation data shows that the studied section was deposited under dominantly euxinic conditions anoxic and sulfidic and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite during this episode the system was closed with respect to fe but not sulfur the mo and u isotopic compositions corrected for detrital input were not affected by the metamorphism and loss of s and are fairly uniform throughout the entire section the fe isotope compositions are exceptionally heavy in the lower part of the section up to 56 fe irmm 14 0 83 and become lighter towards the upper intervals which also show significant mo and u enrichments we suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column the upper part was deposited within a locally euxinic portion of the basin where h 2s availability was highest removal of mo and u was more efficient and precipitated pyrite captured relatively non fractionated dissolved fe in other words quantitative uptake of fe was favored in contrast the lower interval was deposited on the lower margin of a euxinic wedge where h 2s availability was lower and removal of mo and u was less efficient pyrite precipitation in this part of the water column reflected a more fractionated dissolved fe reservoir due to more protracted non quantitative fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of fe the u isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co precipitation into carbonates was the main process of u removal at this time we estimate the mo isotope composition of the contemporaneous ocean to be 98 mo sw 0 85 0 21 this is the lowest value yet reported for the proterozoic ocean suggesting that the oceanic mo cycle was dominated by euxinic and anoxic sinks with negligible mo removal into oxic environments recent studies have proposed a sharp increase in ocean atmosphere oxygen levels during the lomagundi event followed by a dramatic crash our results from black shales of the 2 05 ga shunga event are consistent with a post lomagundi decrease in biospheric oxygen levels

geochemistry of precambrian carbonates vi aphebian albanel formations quebec canada abstract the 1 85 0 15 ga old albanel formations of northern quebec canada an example of paleoproterozoic carbonate buildups are lithologically composed of cherty dolostones subordinate limestones are encountered only near the base of the sequence coincident with maximal transgression the chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases such as chert quartz plagioclase illite k spar pyrite and fe mn oxides all abundant particularly in the basal part of the section the chemistry of the carbonate portion of limestones is strongly reset by post depositional phenomena but the relict high sr contents suggest that aragonite may have been an original precursor phase geochemically the carbonate portion of dolostones can be subdivided into two populations reflecting the stratigraphie bottom and top of the sequence trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures but in a more open system the latter reflects the greater lithological variability and permeability of the younger strata the 13c varies up section from about 3 to 1 perhaps a reflection of progressively more open marine circulation the associated four generations of post depositional pyrites have overlapping 34s from 7 1 to 54 with a mean of 24 a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and or replenishment the best measured 87 sr 86 sr of 0 70600 although possibly not a direct reflection of the coeval seawater may still be a near original value as indicated by its coherency with the roughly co stratigraphic paleo proterozoic sequences evaluation of the existing data indicates that strontium isotopic composition of the precambrian seawater was characterized by oscillations similar to those of the phanerozoic the interval of 0 8 1 5 ga ago being dominated by relatively nonradiogenic values followed by a radiogenic period at 1 6 2 2 ga ago near mantle values during the late archean and possibly another radiogenic peak at 3 5 ga ago

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

pyrite multiple sulfur isotope evidence for rapid expansion and contraction of the early paleoproterozoic seawater sulfate reservoir abstract earth s oxygenation is often described in terms of two unidirectional steps at the beginning and end of the proterozoic eon separated by a long lived intermediate redox state recent work defines a more complicated path to oxygenation exemplified by an apparent drop in oxidation state following the early paleoproterozoic lomagundi carbon isotope excursion the timing of this proposed drop in oxidation state is not well constrained and it is not clear how it relates to redox conditions during the remainder of the proterozoic here we present a study of pyrite multiple sulfur isotopes supported by fe speciation and organic carbon isotopes from early paleoproterozoic black shales we find evidence for the rapid expansion of the seawater sulfate reservoir during the great oxidation event at ca 2 3 ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca 2 05 ga this scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early paleoproterozoic comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early paleoproterozoic did not return until the late neoproterozoic

the metamorphic evolution of granulites at fyfe hills implications for archaean crustal thickness in enderby land antarctica granulites at fyfe hills in enderby land antarctica crystallized at temperatures in excess of 850 c and possibly as high as 1000 c and at pressures of 8 10kbar during the mid to late archaean a number of features including repeated retrograde metamorphism at 5 5 8kbar retrograde reaction textures and rimward zoning in pressure sensitive systems suggest that following peak metamorphism the granulites stabilized at a depth of 18 26 km after stabilization the granulites cooled near isobarically to temperatures of 600 700 c assuming a total crustal thickness of 35 40 km during this late archaean interval of isobaric cooling the peak metamorphic crustal thickness is estimated at 35 56 km this estimate is significantly less than the 60 70 km obtained by summing the depths of the present levels of exposure 26 34 km and the thickness of the crust presently beneath fyfe hills approxi mately 35km and is therefore consistent with independent evidence for extensive post archaean thickening of the enderby land crust

fractionation between inorganic and organic carbon during the lomagundi 2 22 2 1 ga carbon isotope excursion article i nfo the lomagundi 2 22 2 1 ga positive carbon isotope excursion in shallow marine sedimentary carbonates has been associated with the rise in atmospheric oxygen but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen the amount of oxygen released to the exosphere during the lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon which is poorly characterized because dissolved inorganic and organic carbon reservoirs were arguably larger in the paleoproterozoic ocean at a time of lower solar luminosity and lower ocean redox state decoupling between these two variables might be expected we determined carbon isotope values of carbonate and organic matter in carbonates and shales of the silverton formation south africa and in the correlative sengoma argillite formation near the border in botswana these units were deposited between 2 22 and 2 06 ga along the margin of the kaapvaal craton in an open marine deltaic setting and experienced lower greenschist facies metamorphism the prodelta to offshore marine shales are overlain by a subtidal carbonate sequence carbonates exhibit elevated 13 c values ranging from 8 3 to 11 2 vs vpdb consistent with deposition during the lomagundi positive excursion the total organic carbon toc contents range from 0 01 to 0 6 and 13 c values range from 24 8 to 13 9 thus the isotopic fractionation between organic and carbonate carbon was on average 30 3 2 8 n 32 in the shallow marine environment the underlying sengoma shales have highly variable toc contents 0 14 to 21 94 and 13 c values 33 7 to 20 8 with an average of 27 0 3 0 n 50 considering that the shales were also deposited during the lomagundi excursion and taking 13 c values of the overlying carbonates as representative of the 13 c value of dissolved inorganic carbon during shale deposition a carbon isotope fractionation as large as 37 appears to characterize the production of bulk organic matter in the deeper part of the pretoria basin at that time this enhanced fractionation relative to that observed in shallow water environments likely reflects heterotrophic secondary and chemotrophic productivity at and below a pronounced redoxcline consistent with the euxinic conditions inferred from independent evidence for the deeper part of the pretoria basin greater variability in organic carbon vs carbonate carbon isotopic values on the shallow marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the lomagundi excursion our study suggests that in contrast to the late neoproterozoic and phanerozoic when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers in the paleoproterozoic redox stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline this strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the 13 c values of dissolved inorganic carbon in the paleoproterozoic seawater

exceptional preservation of expandable clay minerals in the ca 2 1 ga black shales of the francevillian basin gabon and its implication for atmospheric oxygen accumulation abstract clay minerals are exceptionally well preserved in marine black shale of the ca 2 1 ga francevillian group in southeastern gabon the fb formation of the francevillian group is characterized by smectite rich clay minerals including randomly ordered r0 type and ordered r1 type mixed layer illite smectite i s the preservation of r0 type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis which is unique considering the paleoproterozoic age of the sedimentary rocks r0 and r1 type smectite rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures suggesting formation of clay organic matter complexes our data suggests that clay minerals may have enhanced organic matter preservation providing the oldest example where a link between clay minerals and organic matter sequestration can be established our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time

molybdenum record from black shales indicates oscillating atmospheric oxygen levels in the early paleoproterozoic the early paleoproterozoic witnessed earth9s first major oxygenation referred to as the great oxidation event or goe the goe began around 2 45 billion years ago ga and progressed over hundreds of millions of years as evidenced by multiple redox indicators before coming to an abrupt end by ca 2 06 ga the details of the goe and the extent of oxygenation are still not resolved however and it is not clear how redox conditions across the goe compare with those during the middle proterozoic in order to investigate the evolution of deep ocean redox conditions during the goe we present mo concentration and isotope data together with fe speciation values for three key organic matter rich shale units of the early paleoproterozoic age 2 32 2 06 ga in addition we present a new graphical representation of modeling suggesting that the oceanic mo isotope system is highly sensitive to the balance between anoxic suboxic and euxinic conditions until deep ocean oxygenation similar in scale to modern ocean oxygenation is reached our approach indicates rising yet oscillating atmospheric oxygen at 2 32 ga leading to an abrupt increase in mo supply to the oceans and large mo isotope variations under non steady state conditions the low seawater 98mo value based on the ca 2 32 ga black shales 0 32 0 58 suggests that the oceans were still largely anoxic with locally developed euxinic conditions between 2 2 and 2 1 ga during the peak of the lomagundi carbon isotope excursion we observe higher 98mosw values 1 23 0 36 together with lower mo concentrations in euxinic shales mo 6 3 9 0 ppm we suggest that a decrease in the continental mo input flux in the later part of the goe was the main cause of this trend lower sulfide availability on the continents after protracted sulfide weathering associated with the early stages of the goe and efficient mo removal in poorly oxygenated oceans under weakly euxinic conditions would both have contributed to the contraction of the mo oceanic reservoir by ca 2 06 ga the mo isotope composition of seawater as inferred from euxinic black shale intervals became significantly lighter 0 70 0 21 reflecting an increased rate of quantitative mo removal due to the more widespread development of strongly euxinic conditions counterintuitively seawater mo concentrations recovered likely due to an increase in the mo input which in turn might reflect enhanced weathering of organic carbon rich shales deposited during the lomagundi event

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

high isotope fractionations during sulfate reduction in a low sulfate euxinic ocean analog a detailed record of the early earth sulfur s cycle is chronicled by the s isotope values of sulfide and sulfate preserved in the rock record interpretation of this record rests on our understanding of sulfur cycling in modern systems experiments and the resulting isotopic signatures very large fractionations in 34s of 70 are commonly measured between sulfide and sulfate in modern systems and in ancient sediments theoretical calculations suggest that sulfate reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide although they have only been demonstrated to generate fractionations up to 48 here we report the first direct determination of 60 70 fractionations by natural populations of sulfate reducers these high fractionations occur under the relatively low sulfate conditions 1 1 2 mm of meromictic lago di cadagno in switzerland the major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction with little evidence for modification by further oxidative sulfur cycling these observations help us to constrain the evolution of seawater sulfate concentrations

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

extending the record of the lomagundi jatuli carbon isotope excursion in the labrador trough canada the labrador trough in northern quebec and labrador is a 900 km long rhyacian orosirian orogenic belt containing mixed sedimentary volcanic successions despite having been studied intensively sinc