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

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

upb geochronology of layered mafic intrusions in the eastern baltic shield implications for the timing and duration of paleoproterozoic continental rifting abstract paleoproterozoic layered mafic ultramafic intrusions are widespread in the eastern baltic fennoscandian shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts u pb zircon baddeleyite ages have been determined for seven layered mafic intrusions and a subvolcanic felsic unit associated with two paleoproterozoic volcanic sedimentary belts the pechenga imandra varzuga belt in the kola peninsula and the sumi sariola belt in karelia the layered intrusions in the sumi sariola belt show an age progression from 2449 0 1 1 ma for the burakovka complex located at the southeastern edge of the belt to 2442 1 1 4 ma and 2441 3 1 2 ma for the lukkulaisvaara and tsipringa intrusions of the olanga complex in northern karelia northeastern part of the belt these age results indicate a total duration of mafic magmatism along the ca 800 km belt in karelia to be less than 10 m y the pechenga imandra varzuga belt kola peninsula contains two distinct age groups of layered mafic intrusions and related rocks the younger of these overlaps with the ages obtained for the olanga intrusions and includes the 2441 0 1 6 ma imandra lopolith and a spatially associated felsic subvolcanic unit dated at 2442 2 1 7 ma the older group of mafic layered intrusions in the kola peninsula includes the 2501 5 1 7 ma pana tundra intrusion the 2504 4 1 5 ma monche pluton and the 2505 1 1 6 ma general skaya mountain intrusion the temporal and spatial distribution of the 2 50 2 44 ga rift related igneous activity in the baltic shield is consistent both with the model of starting mantle plume and with the model of passive rifting controlled by pre existing lithospheric weaknesses

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

travertine precipitation in the paleoproterozoic kuetsjarvi sedimentary formation pechenga greenstone belt ne fennoscandian shield abstract precambrian travertines tufas and speleothems either formed rarely or they have not been identified in previous studies in the absence of high pco2 soils in paleoproterozoic karst solution and speleothem formation occurred by processes distinct from those commonly found in present day low temperature karst environments however the high pco2 atmosphere could itself have encouraged karst formation the paleoproterozoic kuetsjarvi sedimentary formation of the pechenga greenstone belt nw russia includes abundant terrestrial carbonate precipitates these precipitates were sampled from a drillcore representing a complete section of the ca 120 m thick formation and were investigated for c and o isotopes acid soluble elemental contents and petrography the newly obtained results were used to constrain the origins of the precipitates and to illuminate different terrestrial carbonate types the investigated drillcore includes abundant small scale cavities and veins which are commonly filled with dolomite and quartz dolomite crusts are found both in the cavities and on bedding erosional surfaces dolomite cements coat uneven surfaces and surficial rock fragments the surficial dolomite crusts form distinct and discrete layers whereas the cements do not the cavity and vein fills are likely post depositional in origin whereas the surficial dolomite crusts and dolomite cements are likely syn depositional precipitates the investigated precipitates often show 13c values lower than those reported from their host rocks suggesting the influence of an external carbon source petrographic features and geochemical data suggest dissolution and precipitation of carbonate material originating from deep sourced co2 bearing fluids likely at high earth surface temperatures

the lomagundi jatuli carbon isotopic event recorded in the marble of the tandilia system basement rio de la plata craton argentina abstract the lomagundi jatuli event corresponds to the most important 13c positive anomaly 5 globally reported in palaeoproterozoic marine carbonates between 2 30 and 2 06 ga in the tandilia system argentina rio de la plata craton this event was recorded in the basement marble of the san miguel area the calcite diopside marble hosted by biotite gneiss and intruded by 2 12 ga garnet leucogranite was metamorphosed in amphibolite facies during the transamazonian cycle paas normalised rare earth elements ree and y for the carbonate rocks are hree enriched and display positive eu and y anomalies typical of primary precipitates from a mixed hydrothermal marine environment carbonate additionally a truly negative ce anomaly for all the samples indicates that the depositional environment was oxidising positive 13c values ranging from 5 90 to 4 30 v pdb and 18o from 17 45 to 13 84 v smow were determined in this marble both gradually decreasing towards the contact with the leucogranites these values indicate that devolatilization reactions took place during the crystallisation of a wollastonite vesuvianite grossular diopside skarn generated by the leucogranite intrusions into the marble 18o values obtained from diopside and calcite crystals in the marble sectors furthest from the contacts with leucogranite allowed a 663 623 c formation temperature to be calculated considering oxygen in a calcite diopside geothermometric pair these temperatures are consistent with the metamorphic degree amphibolite facies reached in this portion of the basement although the san miguel marble shows petrographic and mineralogical evidence of regional and contact metamorphism important geochemical and isotopic characteristics together with its estimated palaeoproterozoic age indicate that the marble protolith was a marine carbonate deposited during the lomagundi jatuli event

a short term post lomagundi positive c isotope excursion at c 2 03 ga recorded by the wooly dolomite western australia it is generally accepted that carbon isotope variations in seawater were muted between c 2 06 ga after the end of the lomagundi carbon isotope excursion lcie and c 1 3 ga evidence is presented here that c 30 myr after the end of the lcie the biogeochemical cycle of carbon experienced a short term c 2 myr high amplitude up to 8 4 v pdb perturbation recorded in the horseshoe rift basin western australia the basin was initiated at c 2 03 ga with deposition of fluvial and shallow marine sandstones followed by the eruption of flood basalt and culminated with the deposition of platform carbonates and accompanying volcaniclastic and siliciclastic sediments wooly dolomite the horseshoe rift basin during deposition of the wooly dolomite was fault compartmentalized but connected to an ocean six depositional sequences make up the wooly dolomite sequence 1 records establishment of a carbonate platform conformably on basalt and coevally with volcaniclastic sedimentation all other sequences have dominant carbonate platform deposits and are unconformity bounded sequence 3 contains a c 57 m thick section with 13 c enriched carbonates bracketed between carbonates with close to 0 carbon isotope values further high resolution chemostratigraphic studies may reveal a more complex pattern of carbon isotope variations during the boring billion years but without precise geochronology similar short term carbon isotope excursions in carbonate successions could be incorrectly correlated to the lcie supplementary material table 2 including chemical and isotopic data sample locations and their position in figure 8 is available at https doi org 10 6084 m9 figshare c 2868055

new age constraints for the proterozoic aravalli delhi successions of india and their implications abstract proterozoic sedimentary successions of india are important archives of both the tectonic history of the indian subcontinent and the geochemical evolution of earth surface processes however the lack of firm age constraints on many of these stratigraphic units limits their current utility here we present new detrital zircon age data from strata of the southern aravalli delhi orogenic belt adob and the rajasthan vindhyan successions the alwar group of the southern delhi supergroup yielded a large population of 1 2 ga detrital zircon grains which refutes the 1 9 1 7 ga age assertion for this unit detrital zircon age distributions from the southern alwar group differ strongly from the alwar group of the north delhi belt demonstrating miscorrelation between these two regions the jhamarkotra formation of the lower aravalli group contains a large population of 1 9 1 7 ga detrital zircon grains therefore the unit cannot be 2 1 ga as traditionally assumed age distributions of the aravalli and delhi supergroups are similar to those of the lower and upper vindhyan successions and we postulate contiguous sediment sources for both regions with strata of the tectonically deformed adob representing the distal margin equivalents of the vindhyan successions additionally a late paleoproterozoic age for the jhamarkotra formation nullifies the hypothesis that the markedly positive carbonate 13c values in this unit are linked to the 2 3 2 0 ga lomagundi jatuli positive isotope excursion the potential of a large late paleoproterozoic ca 1 7 ga positive 13c excursion contrasts with the long held view of a prolonged period of carbon isotope stasis during the so called boring billion

potential influence of sulphur bacteria on palaeoproterozoic phosphogenesis all known forms of life require phosphorus and biological processes strongly influence the global phosphorus cycle1 although the record of life on earth extends back to 3 8 billion years ago2 and the advent of biological phosphate processing can be tracked to at least 3 5 billion years ago3 the earliest known p rich deposits appeared only 2 billion years ago4 5 the onset of p deposition has been attributed to the rise of atmospheric oxygen 2 4 2 3 billion years ago and the related profound biogeochemical shifts6 7 8 9 which increased the riverine input of phosphate to the ocean and boosted biological productivity and phosphogenesis5 10 however the p rich deposits post date the rise of oxygen by about 300 million years here we use microfabric trace element and carbon isotope analyses to assess the environmental setting and redox conditions of the 2 billion year old p rich deposits of the vent or seep influenced zaonega formation northwest russia we identify phosphatized microorganism fossils that resemble modern methanotrophic archaea and sulphur oxidizing bacteria analogous to organisms found in modern seep settings and upwelling zones with a sharp redoxcline11 12 we therefore propose that the p rich deposits in the zaonega formation were formed by phosphogenesis mediated by sulphur bacteria similar to modern sites13 and by the precipitation of calcium phosphate minerals on microbial templates during early diagenesis

global nature of the paleoproterozoic lomagundi carbon isotope excursion a review of occurrences in brazil india and uruguay abstract positive carbon isotope excursion is reported from paleoproterozoic carbonates of the aravalli supergroup northwestern india the minas supergroup brazil and new sections of the paso severino formation uruguay the 2 42 ga gandarela formation minas gerais brazil contains red carbonate facies bif grading into dolostones and limestones and yielding 13 c values ranging from 1 6 to 0 4 v pdb the positive c isotope excursion up to 11 v pdb in marine shallow water carbonates in india and brazil jhamarkotra formation in northwestern india and cercadinho and fecho do funil formations in minas gerais state brazil is comparable to that observed in 2 22 2 1 ga carbonate successions worldwide that were deposited during the lomagundi excursion in uruguay 13 c values up to 11 6 v pdb in the deep water paso severino formation of the piedra alta terrane are compatible with deposition at ca 2 15 ga as indicated by the 2146 7 ma u pb age of dacites occurring at the top of the unit negative 13 c values are also present in carbonates of the paso severino formation but an origin related to organic matter remineralization cannot be ruled out thin carbonate beds in the rio itapicuru greenstone belt bahia state brazil are associated as in the paso severino formation with deep water black shales and have carbon isotope values up to 9 v pdb high metamorphic grade carbonates of the jacurici terrane in the medrado ipueira area bahia brazil have carbon isotope values up to 6 9 v pdb consistent with their minimum age of 2085 5 ma inferred from the intrusive contact with and the age of the medrado norite no evidence was found in india brazil or uruguay for paleoproterozoic glacial events recognized in the 2 45 2 22 ga sedimentary successions worldwide unconformities between the gandarela and cercadinho formations in brazil and the banded gneissic complex and the lower aravalli supergroup in india might explain the absence of glacial record compositional and isotopic data presented here for studied paleoproterozoic carbonate successions allow their integration into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age the study highlights the global nature of the lomagundi excursion furthermore it indicates that the lomagundi excursion is recorded in both shallow water aravalli and minas supergroups and deep water carbonates paso severino formation and rio itapicuru greenstone belt negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow water open marine and isolated basins

palaeoproterozoic evaporites in fennoscandia implications for seawater sulphate the rise of atmospheric oxygen and local amplification of the δ13c excursion this paper addresses global oxygenation and establishment of a marine sulphate reservoir in the palaeoproterozoic we report syn depositional marine anhydrite containing pseudomorphs after ca sulphates as widespread throughout the tulomozero formation in the se fennoscandian shield implying that surface waters were oxidized and a large so42 marine reservoir was developed as early as 2100 ma the ca sulphates and associated magnesite and halite precipitated syn depositionally from oxidized evolved and modified seawater in coastal playa sabkha and intertidal flat settings sr 87 sr 86 and delta 13 c of associated c 13 rich stromatolitic dolostones were environmentally controlled with the highest ratios occurring in playa and sabkha carbonates the results imply that the palaeoproterozoic delta 13 c carb excursion was amplified by 8 by local environmental factors and calls into question many observations of putative delta 13 c global signals reported previously from similar palaeoproterozoic evaporitic dolostones the local environmental amplification can explain a large regional and intercontinental delta 13 c discrepancy observed in synchronous carbonates

a role for scavenging in the marine biogeochemical cycling of zinc and zinc isotopes zinc zn and cadmium cd are important biologically active trace metals in the ocean to date the marine distributions of these elements have been understood primarily in terms of biological assimilation by growing phytoplankton and regeneration of sinking biological material initial studies of zn and cd concentrations and stable isotope ratios 66zn and 114cd have therefore focused on their use as simple tracers of assimilation and regeneration in the oceans however these two processes are insufficient to explain new data on the marine distribution of zn and 66zn here using the first high resolution paired marine depth profiles of zn cd 66zn and 114cd we suggest that scavenging of zn onto organic matter plays a major yet largely unconsidered role in the marine cycling of zn this hypothesis is supported by culture experiments which show that zn released from degrading phytoplankton is rapidly scavenged back onto organic matter and that adsorbed zn is isotopically heavier than the dissolved phase by 0 58 in contrast very little cd or phosphate was scavenged and cd isotopes were not significantly fractionated during degradation our hypothesis is further supported by one dimensional modeling which reproduces observed marine 66zn profiles with 1 of zn adsorbed to particles understanding how zn cycling in the oceans is a balance between assimilation scavenging and regeneration is necessary in order to investigate 66zn as a tracer of marine productivity we anticipate that paired analyses of 66zn and 114cd will prove to be valuable new tools in constraining patterns of global primary productivity providing key information for the marine carbon cycle during periods of past and present global climate change

multiple palaeoproterozoic carbon burial episodes and excursions organic rich rocks averaging 2 5 total organic carbon and positive carbonate carbon isotope excursions view the mathml source and locally much higher i e the lomagundi jatuli event are hallmark features of palaeoproterozoic successions and are assumed to archive a global event of unique environmental conditions following the c 2 3 ga great oxidation event here we combine new and published geochronology that shows that the main palaeoproterozoic carbon burial episodes cbes preserved in russia gabon and australia were temporally discrete depositional events between c 2 10 and 1 85 ga in northwest russia we can also show that timing of the termination of the lomagundi jatuli event may have differed by up to 50 ma between localities and that ni mineralisation occurred at c 1920 ma further cbes have traits in common with mesozoic oceanic anoxic events oaes both are exceptionally organic rich relative to encasing strata associated with contemporaneous igneous activity and marked by organic carbon isotope profiles that exhibit a stepped decrease followed by a stabilisation period and recovery although cbe strata are thicker and of greater duration than oaes 100 s of metres versus metres 106 years versus 105 years their shared characteristics hint at a commonality of cause s and feedbacks this suggests that cbes represent processes that can be either basin specific or global in nature and a combination of circumstances that are not unique to the palaeoproterozoic our findings urge circumspection and re consideration of models that assume cbes are a deep time singularity

depositional setting of the 2 1 ga francevillian macrobiota gabon rapid mud settling in a shallow basin swept by high density sand flows the depositional setting of the 2 1 ga fill to the franceville basin of gabon is important for understanding the habitat energy and availability of light and oxygen and taphonomy of recently discovered early macro organisms buried in black shales in unit fb the available data bearing on the stratigraphy and sedimentology of unit fb provide new insight into processes acting on the palaeo seafloor the shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands the latter poubara sandstones were emplaced during a forced regression during the terminal infill of fault bounded sub basins following a stage characterised by a ferruginous to anoxic water column the structureless sandstones were deposited from high density gravity currents along with a locally strong bottom oscillation of the water column tuft structures preserved in cyanobacterial mats together with the position of the macro organisms at the top of the sandstone beds within associated black shales point to a water depth of less than 80 m the relative sea level fall that drove deposition of the poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows this article is protected by copyright all rights reserved

temporal constraints on the paleoproterozoic lomagundi jatuli carbon isotopic event the paleoproterozoic lomagundi jatuli positive 13c excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in earth history although precise absolute chronology of this event remains poorly constrained in northeastern fennoscandia an 1300 m thick sedimentary volcanic succession of the pechenga greenstone belt records decline of this isotopic excursion zircons from sedimentary rocks that occur within the decline have yielded 207pb 206pb dates at 2058 2 ma 6 ma including u decay constant uncertainties and provide the first maximum age constraint on the termination of the lomagundi jatuli event combined with existing constraints these data indicate an 140 m y interval characterized by 13c rich carbonate accumulation

primitive os and 2316 ma age for marine shale implications for paleoproterozoic glacial events and the rise of atmospheric oxygen abstract re os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the rooihoogte and timeball hill formations transvaal supergroup south africa provides a precise isochron age of 2316 7 ma 4 statistical uncertainty if error on decay constant is excluded and a chondritic initial 187os 188os ratio of 0 1121 0 0012 these units were deposited between what are interpreted as the second and third of three paleoproterozoic global glacial events and thus provide minimum and maximum ages respectively for these events the rooihoogte formation is correlative with the duitschland formation which records previously undated carbon isotope excursions because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative 34s values the rise of atmospheric oxygen most likely began prior to 2 32 ga the chondritic initial 187os 188os ratio requires that primitive hydrothermal sources of os dominated the os budget of seawater at 2 32 ga os is mobile only under oxidizing conditions therefore the chondritic initial 187os 188os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic os to offset the primitive hydrothermal magmatic flux even if atmospheric oxygen levels had increased significantly by 2 32 ga anoxic conditions throughout the archean most likely limited os enrichment in archean marine shales so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic os to paleoproterozoic seawater

a widespread positive 13ccarbanomaly at around 2 33 2 06 ga on the fennoscandian shield a paradox carbon isotope measurements carried out on 201 carbonate samples from the early proterozoic of the kola peninsula n karelia and norway yield 13c pdb spanning 20 5 to 11 a general 13c secular trend shows that prior to 2 33 ga values are typically normal marine averaging around 3 0 between 2 33 and 2 06 ga in jatulian time there follows a rapid excursion to positive 13c of around 6 post jatulian time is characterized by 13c of sedimentary carbonates fluctuating between 5 and 3 also it is remarkable for the first pronounced development of diagenetic carbonates which have 13c between 14 and 6 the c 6 positive 13c shift with a duration of about 270 myr coincides with a maximum in the diversity and abundance of stromatolites and with widespread development of red beds but does not coincide with the maximum of buried corg mass the fennoscandian shield represents the largest isotoically anomalous carbonate province yet reported and the positive 13c excursion together with a series of major global palaeoenviromental changes seems to be more intense than the precambrian cambrian transition events however it is still not clear what kind of mechanism this phenomenon could be attributed to an increase of the ronov ratio and or broecker ratio and other possible models are discussed as the target for future investigations

sedimentary and volcanic facies recording the neoarchaean continent breakup and decline of the positive δc excursion abstract the palaeoproterozoic kalix greenstone belt at the northern end of the bothnian bay in sweden comprises three groups the lower group a ca 3000 m thick unit of subaerially erupted tholeiitic intraplate basalts interbedded with fluviatile conglomerates represents the volcano sedimentary succession deposited in a rift setting these units are truncated by a break up unconformity above which is the middle group an 800 m thick succession of dolograinstones stromatolitic dolostones arenites volcanoclastic and mafic volcanic rocks stromatolite morphologies and the occurrence of distinctive sedimentary structures including planar and herringbone cross bedding hummocky cross stratification symmetrical and asymmetrical ripples desiccation cracks and tepees indicate markedly variable depositional environments associated with transition from a marine influenced rifted margin to a rimmed carbonate shelf sedimentation was accompanied by subaerial mafic volcanism and syndepositional tectonics the upper group a more than 2000 m thick unit of deep water shales was deposited on the drowned carbonate platform of the middle group in response to tectonically enhanced subsidence this study provides a detailed palaeoproterozoic depositional history of the fennoscandian shield from rifting to passive margin development associated with both dispersal of the neoarchaean supercontinent and decline of the proterozoic positive isotopic excursion of 13 c 12 c in sedimentary carbonates in contrast to many previously reported 13 c rich palaeoproterozoic carbonates which were deposited in non marine to restricted or partly restricted settings the carbonate facies reported herein were well connected to the oceanic realm and thereby represent a good opportunity to test restricted marine versus open marine models for how the carbon cycle functioned in deep time

reos isotopes and major and trace element geochemistry of carbonaceous shales aravalli supergroup india impact of post depositional processes abstract this study presents re os isotopes and major and trace elemental geochemistry of carbonaceous black shales and a weathering profile developed on them from the paleoproterozoic deposits of the jhamarkotra formation lower aravalli group india these measurements provide information on the impact of post depositional processes particularly hydrothermal fluid interactions on the 187re 187os geochronology and element mobility in shales in contrast to previously reported compositions for black shales the re concentrations of the jhamarkotra shales are in general lower than their corresponding os concentrations the 187os 188os ratios of the black shales vary widely from 3 5 to 10 5 without any discernible systematic trend with 187re 188os range 0 4 to 17 7 excluding an outlier these 187re 188os ratios are lower by a few orders of magnitude than those generally reported for fresh black shales these observations indicate that the jhamarkotra shales though they seem visually well preserved have undergone post depositional modification resulting in open system behavior of re and os with re more mobile than os higher sio2 tio2 and k2o na2o ratios of the jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal possibly si and k rich fluids the sio2 tio2 ratios an index for hydrothermal alteration show negative correlation with re corg and positive correlation with corg n ratios 42 154 indicating loss of re and nitrogen during hydrothermal fluid interaction earlier studies have shown the presence of re porphyrins n containing organic compounds in black shales thus the observed co variations of sio2 tio2 with re corg and corg n ratios suggest mobilization of re from the porphyrins in the shales further positive correlation between sio2 tio2 and 187os 188os ratios indicates that significant amounts of radiogenic os component were re distributed throughout the jhamarkotra shales during the post depositional processes in the weathering profile developed on black shales the al2o3 normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering on the contrary the al2o3 normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile suggesting limited mobility over the depth of the profile the depth profiles of re al2o3 and 188os al2o3 and 187os 188os ratios in the weathering profile by and large reflect compositions of their presumed parent shales which have gone through significant post depositional changes

high δ13c paleoproterozoic carbonates from the transvaal supergroup south africa marbles from the transvaal supergroup kaapvaal craton south africa record two positive carbon isotope excursions to average 13 c carbonate values of 6 to 8 relative to v pdb separated by carbonates with normal marine 13 c values 0 2 these elevated values have been preserved through metamorphic devolatilization together with an already documented high 13 c excursion in overlying rocks three separate positive carbon isotope excursions occurred on the kaapvaal craton in the interval 2 43 to 1 93 ga it is not yet clear how many of these excursions reflect global rather than local processes

insights from isotope stratigraphy a symposium on precambrian isotope stratigraphy was heldwithintheframeofthe33rdigcheldatoslo norway august2008 this symposium spawned the idea for a special issue on this themeand ultimately to the ten papers assembled here this special issueprovides the results of some of the latest cutting edge research ontraditional c sr s isotopechemostratigraphyandintroducesomenewer isotope systems ree ca and hg chemostratigraphy as apotential tracer for glacial events and estimate of the buildup ofvolcanicgases includingco

archean proterozoic collision boundary in central fennoscandia revisited abstract archean proterozoic ap collision boundary in central fennoscandia is a complex collage of archean basement rocks its paleoproterozoic supracrustal cover rocks and svecofennian 1 93 1 78 ga supracrustal and plutonic rocks the apparent steep nature and complexity of the ap boundary has led to contradictory tectonic models varying from arc continent collision via back arc closure to accretion of arc terranes along a strike slip shear zone we present new geochronology data from detrital zircons combined with whole rock geochemical and sm nd isotopic data on paleoproterozoic sedimentary and volcanic rocks across the boundary to revise its nature the 2 06 ga siilinjarvi volcanic complex and associated turbidites with an inferred deposition age at 2 1 2 06 ga are part of the cover sequence the siilinjarvi volcanic rocks derive from both plume oib and sub continental lithospheric mantle sclm magma sources and are often contaminated by the archean crust the sahara suite sedimentary cover rocks are derived from local 1 95 1 92 ga bimodal volcano plutonic source which source also explains the common age population found in younger sedimentary rocks the nalantojarvi suite psammites with maximum depositional ages of 1 97 1 93 ga are continental slope to deep water sediments deposited at the continental margin contemporaneous mafic magmatism seen as lavas and dykes show a change from sclm dominated present east to asthenospheric depleted mantle dm dominated present west melts the interpretation of the nalantojarvi suite as cover rocks of the lower plate indicates that the archean crust continues at least 50 km more to the west than thought earlier parautochthonous sedimentary rocks from salahmi eastern kansanneva and loutemaki with maximum depositional ages at 1 92 1 91 ga are inferred as collision related foredeep deposits from local sources both the lampaanjarvi suite and pyhajarvi suite sedimentary rocks have deposited at 1 89 1 88 ga or later whereas the lampaanjarvi suite rocks are dominated by a local source and the pyhajarvi suite rocks derive from a mixture of older sediments and 1 89 1 88 ga granitoids at least two tectono metamorphic stages separated with a major unconformity can be differentiated in the study area the early deformation and metamorphic stage in the nalantojarvi suite has occurred before the intrusion of the late to post collisional pyroxene granitoids at 1 88 ga the pyhajarvi suite sedimentary rocks have been deposited after the intrusion of these granitoids between 1880 and 1875 ma and then subsequently deformed and metamorphosed at 1875 ma we propose a tectonic model where the continental breakup occurred at 2 1 2 06 ga followed by a new spreading within the pre existing passive margin at 1 98 1 93 ga during initial continent arc collision the subducted slab steepened strongly at high angle subduction ceased rapidly and finally detached and shallow slab breakoff occurred at 1 92 1 91 ga followed subsequently by ophiolite obduction and a formation of a foreland fold and thrust belt mantle crust decoupling of the archean lithosphere occurred and a crocodile jaw formed where the arc rocks are partly buried beneath the bounding archean continent similar features have been recorded in other ap collision zones which could indicate that the buoyant nature of the archean lithosphere combined with denser paleoproterozoic lithosphere may be a general cause for decoupling of archean crust and mantle during arc continent collision in paleoproterozoic

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

paleomagnetism and u pb geochronology of the lac de gras diabase dyke swarm slave province canada implications for relative drift of slave and superior provinces in the paleoproterozoicgeological survey of canada contribution 20080350 lac de gras diabase dykes trend north to nne across the central slave province of the canadian shield u pb baddeleyite ages of 2023 2 and 2027 4 ma are interpreted as dyke emplacement ages th

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

did global tectonics drive early biosphere evolution carbon isotope record from 2 6 to 1 9 ga carbonates of western australian basins the 13 ccarb record of well preserved carbonates in outcrop and core is here examined from the 2 6 to 1 9 ga old basins of western australia these data which are constrained by a well defined stratigraphic and tectonic framework and by u pb zircon ages provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere in the latest archaean ca 2 6 ga the secular 13 ccarb curve is flat much like that seen in the later palaeoproterozoic basins of northern australia 1 8 ga this implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable in the early palaeoproterozoic beginning after 2 5 ga and continuing until at least 1 9 ga the 13 ccarb the curve is much more dynamic with significant positive and negative excursions including a major positive excursion 9 pdb close to 2 2 ga these excursions can be correlated with the lomagundi event identified in africa europe and north america previously published studies of the overlying meso to palaeoproterozoic bangemall basin and of 1 8 1 5 ga old basins in northern australia suggest that the 13 ccarb curve became relatively monotonic again after ca 1 8 ga and remained so for most of the following mesoproterozoic comparisons with data from other ancient cratons especially africa suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the neoproterozoic when the western australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins the data are consistent with an earth in which the carbon mass balance was in equilibrium the 13 ccarb curve began to oscillate following the onset of glaciation as the pilbara and yilgarn cratons began to converge during the capricorn orogeny suggesting periods of rapid carbon burial during continental dispersal however the major positive excursion is preserved in carbonates from back arc basins formed as the ocean closed and subduction began since similar tectonic processes can be recognised not only in northern australia but also on other early cratons it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development we explain this coincidence of carbon isotopic excursions

new isotopic and geochemical data from the palaeoproterozoic pechenga greenstone belt nw russia implication for basin development and duration of the volcanism abstract the north pechenga group of the pechenga greenstone belt comprises a thick supracrustal succession developed within a time interval of ca 500 ma in the early part of the palaeoproterozoic era it records several important global events leading to the onset of modern style earth system but the temporal evolution of the belt is still insufficiently established new nd isotope data clearly indicate an abrupt change from a strong crustal signature in the lower two volcanic units to a depleted mantle like signature in the upper two volcanic units which conforms with the coeval shift of the volcanism from subaerial to submarine indicating a significant thinning of the lithosphere due to continental rifting the timing of the change in the geotectonic regime can be bracketed between 2058 6 ma u pb data and 2018 54 ma sm nd data in situ ion microprobe analyses of zircon grains from a thin felsic tuff member in the upper part of the succession pilgujarvi volcanic formation produced an age of 1988 3 ma which is within error the same as has been reported previously for the ferropicritic magmatism that produced the pechenga ni cu ores it is concluded that the upper 3 km thick volcanic pile dominated by tholeiitic basalts and minor ferropicrites was generated within a short time span unresolvable with current geochronological tools geochemical and isotopic data suggest that the felsic tuffs were generated from a ferropicritic parental magma via extensive fractional crystallisation and the presence of gneiss xenoliths in the tuffs reveals that this happened in a magma chamber located in the underlying archaean basement thus although the mafic subaqueous volcanism of the upper part of the belt does not bear nd isotopic or trace element evidence for contamination with ancient crustal material the rifting of the basement did not yet proceed to a total break up of the archaean craton

rock magnetic and paleomagnetic study of the keurusselka impact structure central finland abstract there are 31 proven impact structures in fennoscandia one of the most densely crater populated areas of the earth the recently discovered keurusselka impact structure 62 08 n 24 37 e is located within the central finland granitoid complex which formed 1890 1860 ma ago during the svecofennian orogeny it is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism e g planar deformation features in quartz new petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented the suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility the presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization four different remanent magnetization directions were isolated 1 a characteristic svecofennian target rock component a with a mean direction of d 334 8 i 45 6 95 14 9 yielding a pole plat 51 1 plon 241 9 a95 15 1 2 component b d 42 4 i 64 1 95 8 4 yielding a pole plat 61 0 plon 129 1 a95 10 6 3 component c d 159 5 i 65 4 95 10 7 yielding a pole plat 21 0 plon 39 3 a95 15 6 and 4 component e d 275 5 i 62 0 95 14 4 yielding a pole plat 39 7 plon 314 3 a95 19 7 components c and e are considered much younger possibly neoproterozoic overprints compared with the components a and b the pole of component b corresponds with the 1120 ma pole of salla diabase dyke and is in agreement with the 40ar 39ar age of 1140 ma from a pseudotachylitic breccia vein in a central part of the structure therefore component b could be related to the impact and thus represent the impact age

chemostratigraphy of carbonates from the minas supergroup quadrilatero ferryifero iron quadrangle brazil a stratigraphic record of early proterozoic atmospheric biogeochemical and climactic change the ca 2 42 ga old gandarela formation overlies the finely laminated caue banded iron formation bif of the itabira group minas supergroup brazil and consists of red carbonate facies bif grading upsection into buff dolomites and limestones which are locally stromatolitic carbonates of the gandarela formation have 1 3 c values ranging narrowly from 1 6 to 0 4 permil versus pdb n 93 the cercadinho formation at the base of the overlying piracicaba group is a thick arenaceous succession with rare thin carbonates that have 1 3 c values ranging from 3 3 to 5 4 permil n 12 stromatolitic dolomites of the overlying fecho do funil formation have pb pb carbonate age 2 11 0 11 ga babinski and others 1995 that provide the minimum age for this unit and 1 3 c values ranging from 5 6 to 7 4 permil n 69 at present we find no evidence in brazil for the any of the three early paleoproterozoic glacial events recognized in 2 45 to 2 22 ga sedimentary successions in north america given the radiometric constraints however it is possible that geological and geochemical traces of these ice ages are missing along the unconformity between the gandarela and cercadinho formations if correct carbonates of the gandarela formation may provide a record of the carbon isotope composition of the seawater prior to any of the paleoproterozoic ice ages combined with other available 1 3 c data results from the gandarela formation suggest that seawater preceding the ice ages was not significantly enriched in 1 3 c on the other hand the carbon isotopic signature of the cercadinho carbonates and chemical composition of the cercadinho arenites are consistent with deposition after the end of the glacial epoch and likely during the early stage of ca 2 22 to 2 1 ga carbon isotope excursion the 1 3 c values of marine carbonates of the fecho do funil formation are consistent with carbon isotope record of contemporaneous carbonate successions worldwide providing additional evidence for a global biogeochemical anomaly at ca 2 22 to 2 1 ga these compositional and isotopic changes allow integration of the minas supergroup into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age

on the lomagundi jatuli carbon isotopic event the evidence from the kalix greenstone belt sweden we report a significant new database on the carbon and oxygen isotopic analyses 233 whole rock and 43 microcored samples obtained from sedimentary dolostones in the kalix greenstone belt from the fennoscandian shield of sweden this characterises the second half of the prominent palaeoproterozoic lomagundi jatuli positive carbon isotope excursion within a 600 m thick succession of alternating volcanic volcaniclastic siliciclastic and dolomitic rocks from greenschist epidote amphibolite metamorphic facies the least altered dolostone samples show a gentle oscillation between 2 and 4 throughout stratigraphy with a second order positive excursion from 4 through 8 and gradually back again to 4 in the c 150 m thick unit in the middle and upper parts of the succession this second order excursion is superimposed on the general relatively rapid 13c decline at the second half of the lomagundi jatuli isotopic event and coincides with the transition from a marine influenced rift to a passive margin setting if the excursion is global it would reveal internal fine structure of the lomagundi jatuli isotopic excursion if not basinal local factors are the main players lithostratigraphic and c isotope correlation between kalix and neighbouring perapohja schist belt successions 40 km apart suggest an incomplete geological record in both successions despite the absence of obvious non depositional unconformities

redox stratification and anoxia of the early precambrian oceans implications for carbon isotope excursions and oxidation events abstract an updated compendium of 13c data offer compelling evidence that 13c positive excursions of unsurpassed magnitude in the recorded earth history 8 and up to 18 pdb occurred in the early proterozoic the lomagundi event questions whether or not these unprecedented positive 13c shifts were contemporaneous in various basins and represent local or global events remain unresolved the framework of major geologic events that occurred in the paleoproterozoic argues against a snowball earth scenario as a backdrop to these exceptional 13c enrichments substantial increases of carbon flux ratio fo fc eight times the phanerozoic average and organic carbon burial rate fo three times the phanerozoic average are required to account for the observed 13c enrichments under steady state and dynamic equilibrium modes respectively these enhanced ratios and rates are conditional on the availability of a flux of nutrients to the contemporaneous biota producers in excess of the riverine flux and a decoupling of the p and c cycles it is argued that these two conditions were met between 2 25 and 2 11 ga in a redox stratified ocean with weak upwelling and sluggish meridional surface circulation the alternative that the major disturbance in the terrestrial carbon cycle occurred in a rapidly ventilated ocean is assessed as being unlikely a large pulse of o2 equivalent to six to seven times the present terrestrial oxygen budget was rapidly scavenged and spent in the oxidation of reduced fe and s transported to the shelves by rivers this contention is consistent with data inferring low oxygen levels 10 5 to 10 2 pal in the paleoproterozoic atmosphere and the occurrence of ocean anoxia until the late precambrian

kolosjoki sedimentary formation a record in the aftermath of the paleoproterozoic global positive δ13c excursion in sedimentary carbonates abstract the kolosjoki sedimentary formation of the pechenga greenstone belt north eastern fennoscandia records the aftermath of the paleoproterozoic global positive 13 c excursion in sedimentary carbonates the lomagundi jatuli isotope excursion the goals of this research were to provide detailed new data on the 13 c trend in the kolosjoki sedimentary formation and generate a secular 13 c curve for the time period immediately succeeding the lomagundi jatuli isotope excursion for these purposes sedimentary carbonate samples from two new drillcores representing the kolosjoki sedimentary formation were collected in total 88 whole rock samples and 67 micro drilled subsamples ca 1 3 mg were analyzed for the isotope composition of c and o the whole rock elemental abundances and for the composition of the acid soluble fraction the kolosjoki carbonate rocks are dolostone or calcitic dolostone mg ca 0 50 0 67 with minor limestone mg ca 0 04 0 06 only weak correlation was observed between the mn sr ratios and the 13 c and 18 o values and the carbonate samples have likely retained their near primary 13 c values the sedimentary carbonate rocks show first low 13 c values at 2 to 1 and then a subtle upward increasing 13 c trend from ca 1 to 3 the increasing 13 c trend is interpreted to record a recovery from a 13 c minimum following the lomagundi jatuli isotope excursion

paleomagnetism and rock magnetic cyclostratigraphy of the ediacaran doushantuo formation south china constraints on the remagnetization mechanism and the encoding process of milankovitch cycles abstract south china provides exceptional and unique sedimentary archives of earth s fundamental changes in ediacaran time however the lack of robust geochronological constraints renders a poorly defined chronostratigraphic framework for ediacaran aged strata in south china thus making it difficult to understand earth s critical changes during this time interval magnetostratigraphy and rock magnetic cyclostratigraphy turn out to be a few reliable methods in chronostratigraphic studies especially in the precambrian here we present a detailed paleomagnetic and rock magnetic study of the ediacaran doushantuo formation in the huanglianba section guizhou province south china the section consists of 75 m thick carbonate succession and has a record of one of the largest carbon isotope excursions in the earth history known as the shuram excursion stepwise thermal demagnetization of doushantuo carbonates reveals single component magnetizations that yield a paleomagnetic pole consistent with the late triassic segment of the apparent polar wander path of south china the secondary origin of doushantuo carbonates magnetization is supported by their magnetic mineralogy based on various rock magnetic experiments and scanning electron microscopic analyses we interpret that the secondary magnetization resides mainly in authigenic pyrrhotite that likely precipitated from the reducing fluids during the late stage of indosinian orogeny in the triassic interestingly the companion high resolution cyclostratigraphic study of a 4 m thick succession at the upper layers of the doushantuo formation in the huanglianba section indicates that the milankovitch cycles were encoded in the variation of magnetic susceptibility immune from the influence of remagnetization variation of magnetic susceptibility is attributed to changes in the abundance of paramagnetic minerals which by implication means that the amount of terrestrial materials transported from the continent was regulated by milankovitch cycles during the deposition of doushantuo carbonates therefore our results suggest that even though carbonates are prone to remagnetization astronomical modulation of deposition could be encoded in rock magnetics of the remagnetized carbonates as a result rock magnetic cyclostratigraphy has potential in remagnetized carbonates for providing a floating astronomical timescale that can be applied to constrain sediment accumulation rates estimate the duration of geological events and construct high resolution chronostratigraphies

hydrothermal dedolomitisation of carbonate rocks of the paleoproterozoic zaonega formation nw russia implications for the preservation of primary c isotope signals abstract the paleoproterozoic zaonega formation in karelia nw russia has played a key role in understanding the environmental conditions postdating the great oxidation and lomagundi jatuli events its carbonate and organic rich rocks shungite define the postulated shunga event representing an accumulation of very organic rich sediments at c 2 ga and are central in ideas about changing ocean atmosphere composition in the wake of those worldwide biogeochemical phenomena our work focussed on a key interval of carbonate rocks in the upper part of the formation to i obtain new high resolution carbon oxygen and strontium isotope data complemented by detailed petrography and mineralogical characterisation and ii expand upon previous studies by using our data to constrain geochemical modelling and show in greater detail how magmatic hydrothermal fluids induced dedolomitisation and altered geochemical signals our findings show that the 13ccarb of calcite rich intervals are the most altered with values between 16 9 to 0 6 whereas the dolomite dominated parts retain the best preserved i e most original values those define a trend of steadily increasing 13ccarb from 6 to 0 5 which we interpret as a return to normal marine conditions and carbonate carbon values following the lomagundi jatuli event

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

chemostratigraphy of the paleoproterozoic duitschland formation south africa implications for coupled climate change and carbon cycling the paleoproterozoic duitschland formation lies stratigraphically beneath the timeball hill formation which contains the only unequivocal glacial unit of this era in the transvaal basin south africa lithologic evidence in paleoproterozoic successions of north america however indicates the existence of three discrete and potentially global ice ages within this 300 my interval carbonates of the duitschland formation are significantly enriched in 13c up to 10 1 permil in the upper part of the succession above a notable sequence boundary in contrast the lower part of this unit contains carbonates with consistently negative 13c values trace and major element compositions of these carbonates as well as carbon isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly the stratigraphic constraints indicate that 13c enriched carbonates were deposited prior to paleoproterozoic glaciation in southern africa similar to carbonates stratigraphically beneath neoproterozoic glacial diamictites worldwide also mirroring the neoproterozoic record are strongly negative 13c values in cap carbonates atop glacial diamictites in paleoproterozoic strata of wyoming and ontario the litho and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well preserved carbonates of the lower duitschland formation may reflect a second paleoproterozoic ice age in the transvaal succession this interpretation is further supported by recently discovered bullet shaped clasts with striations in diamictite from the basal part of the succession thus the emerging temporal pattern of carbon isotope variations and glaciation in the paleoproterozoic has a close analogue to neoproterozoic events suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

the ree composition and petrography of apatite in 2ga zaonega formation russia the environmental setting for phosphogenesis the first significant p rich deposits appear in the global rock record during the paleoproterozoic around 2 ga however the specific triggers that led to apatite precipitation are still under debate the ca 2 ga zaonega formation karelia russia contains p rich intervals in its upper part with abundantly occurring apatite these apatites have been studied for their rare earth element ree composition using laser ablation inductively coupled plasma mass spectrometry la icp ms with an emphasis on the environmental condition during phosphogenesis petrographic observations by scanning electron microscopy sem integrated with la icp ms results allow recognition of variously recrystallized apatite and distinction of best preserved diagenetic apatite which presumably records syn depositional ree characteristics diagenetic apatite exhibits moderately negative ce anomalies that indicate an at least partially oxygenated water column a variable but typically positive eu anomaly is consistent with geologic evidence suggesting intensive magmatic activity and hydrothermal influence during apatite precipitation we conclude that phosphatic sediments in the paleoproterozoic zaonega formation record phosphogenesis in a vent seep influenced setting that experienced fluctuating redox conditions at the sediment water interface

sr isotopic composition of paleoproterozoic 13c rich carbonate rocks the tulomozero formation se fennoscandian shield abstract rb sr systematics has been studied in 13c rich carbonate rocks of the paleoproterozoic 2 09 0 07 ga tulomozero formation in the northern onega lake area the se fennoscandian shield the formation is divided into eight members a f consisting of greenschist facies grade variegated sandstones siltstones mudstones stromatolitic dolostones and subordinate crystalline limestones samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the tulomozero formation prior to isotope analysis the rocks powders were treated with 1n ammonium acetate for partial removal of the late epigenetic carbonate phases major resetting of the rb sr systems in the tulomozero carbonate rocks appears to take place during the svecofennian regional metamorphic event and it was screened by using mn sr fe sr mg ca and 18o 16o ratios high sr content up to 2080 g g in limestones and 530 g g in dolostones coupled with low fe mn 0 40 in dolostones of the upper part of member b and of members g and h is indicative of seawater influxes sea transgression into the tulomozero basin the 87sr 86sr values in the least altered mn sr

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

a review of temporal constraints for the palaeoproterozoic large positive carbonate carbon isotope excursion the lomagundi jatuli event abstract the palaeoproterozoic lomagundi jatuli event is one of the largest magnitude and earliest known positive carbonate carbon isotope excursions preserving 13 c values between 5 and 16 and even higher it is recorded in sedimentary rocks on all continents bar antarctica and spans stratigraphic thicknesses ranging from several to many tens of metres this unique positive 13 c interval signals fundamental changes in the global carbon cycle and is a key event in earth system evolution following oxygenation of earth s atmosphere here we present a comprehensive review of the age constraints on the lomagundi jatuli event the first such effort in two decades this new chronology compilation focuses on the u pb and re os chronometers and demonstrates that global synchronicity of the lomagundi jatuli event is permissible an interpretation supported by the apparent wide dispersion of lomagundi jatuli event bearing successions in the most recent palaeoproterozoic plate reconstructions assuming the lomagundi jatuli event is synchronous worldwide then the bounds on its duration range from a maximum of 249 9 myr 2306 9 ma to 2057 1 ma to a minimum of 128 9 4 myr 2221 5 ma to 2106 8 ma

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

synsedimentary diagenetic and metamorphic pyrite pyrrhotite and marcasite at the homestake bif hosted gold deposit south dakota usa insights on au as ore genesis from textural and la icp ms trace element studies the paleoproterozoic homestake deposit northern black hills south dakota is the largest banded iron formation bif hosted gold deposit in the world and one of the largest single gold deposits globally 1 300 t au mined at an average grade of 8 4 g t the origin of the deposit has remained in dispute from its discovery with views broadly falling into two groups 1 the gold and associated elements were externally sourced and deposited in a suitable structural and or chemical trap e g iron formation and 2 gold was indigenous to the host iron formation from which it was remobilized into dilatant structural zones during deformation and metamorphism in recent times most workers have favored the former model appealing to the broad synchroneity of paleoproterozoic metamorphism felsic magmatic activity and gold event timing la icp ms analysis of synsedimentary to early diagenetic sulfides at homestake and surrounding area indicates that the au in the orebodies may have originally had a significant syngenetic component in particular la icp ms mapping of multistage pyrites from carbonaceous shale in the upper poorman formation shows that au and a suite of trace elements co ni cu zn se mo ag sb te au hg tl pb and bi similar to those in diagenetic pyrite from around the globe are contained in anhedral sponge textured cores and nodules surrounded by relatively trace element barren au poor euhedral pyrite rims furthermore interelement ratios e g co ni ni ag of the trace element rich spongy and nodular pyrites are also similar to those of typical diagenetic pyrite la icp ms imaging of pyrrhotite in the same rocks reveals multistage growth in this mineral as well with the earlier generation having higher concentrations of ag sb pb tl and bi than its presumed metamorphic counterpart imaging of marcasite in the shales of the upper poorman formation demonstrates this mineral s high abundance of w and tl relative to all generations of pyrite and pyrrhotite mass balance calculations indicate that the volume of upper poorman formation in the mine area approx 12 km3 could potentially yield 4 500 t of au a value greater than the total mined resource by more than a factor of three these geochemical data and calculations suggest that a significant portion of the gold at homestake may have been sourced from the relatively thick carbonaceous and sulfidic black shale facies in the stratigraphic footwall to the host iron formation in a manner similar to other sediment hosted gold districts

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

macrofaunal control of microbial community structure in continental margin sediments through a process called bioturbation burrowing macrofauna have altered the seafloor habitat and modified global carbon cycling since the cambrian however the impact of macrofauna on the community structure of microorganisms is poorly understood here we show that microbial communities across bioturbated but geochemically and sedimentologically divergent continental margin sites are highly similar but differ clearly from those in nonbioturbated surface and underlying subsurface sediments solid and solute phase geochemical analyses combined with modeled bioturbation activities reveal that dissolved o2 introduction by burrow ventilation is the major driver of archaeal community structure by contrast solid phase reworking which regulates the distribution of fresh algal organic matter is the main control of bacterial community structure in nonbioturbated surface sediments and in subsurface sediments bacterial and archaeal communities are more divergent between locations and appear mainly driven by site specific differences in organic carbon sources