global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

microfossils in cherts from the middle riphean mesoproterozoic avzyan formation southern ural mountains russian federation a diverse assemblage of well preserved microorganisms has been detected in black cherts from the approximately 1200 ma old avzyan formation suite of the southern ural mountains russian federation the lower kataskin member contains a diverse abundant microbiota dominated by mat forming filamentous cyanobacteria several types of colonial unicells and morphologically distinctive stalked cyanobacteria the upper revet member contains a less diverse biota dominated by unicellular cyanobacteria palaeoecological evidence indicates that the microbial community of the kataskin member inhabited a shallow water presumably marine carbonate environment revet microorganisms possibly lived in restricted peritidal environments the biostratigraphic significance of the avzyan microbiota is limited many of the taxa are long ranging they were already abundant in palaeoproterozoic successions and continue into the neoproterozoic nevertheless in many respects the kataskin assemblage is comparable to those reported from the middle late riphean deposits of northern america australia and eurasia the following taxa are here described chroococcaceae eogloeocapsa avzyanica sergeev gloeodiniopsis lamellosa schopf emend knoll et golubic entophysalidaceae eoentophysalis belcherensis hofmann dermocarpaceae polybessurus bipartitus fairchild ex green et al nostocaceae eosphaeronostoc kataskinicum sergeev nostocaceae or oscillatoriaceae siphonophycus robustum schopf emend knoll et golubic emend knoll et al siphonophycus sp incertae sedis eosynechococcus amadeus knoll et golubic

polybessurus like fossils as key contributors to permian triassic boundary microbialites in south china abstract massive occurrences of microbialites near the permian triassic boundary have been interpreted as being due to benthic microbial proliferation in unusual marine conditions after the end permian mass extinction here we present large populations of polybessurus like microfossils in the microbialites of the south china craton to reveal their morphological structures growth patterns and ecological implications these distinctive microfossils are composed of layered stalks and spherical endpoints the spheroids 20 40 m in diameter comprising external thin micritic walls and internal sparry parts are interpreted as coccoid cyanobacteria the stalks consist of multiple stacked cup micritic envelopes consisting of micron sized magnesian calcite dolomite grains and are catalogued as calcified remains of coccoid cyanobacterial extracellular polymeric substances eps early microbially mediated calcification of eps increased the preservation potential of microfossils the microfossils stand upright as coccoid unicells supported by the highly unidirectional secretion of eps revealing their benthic behaviours the successively secreted eps lift the cells away from the substrate expanding their ecological niches to gain more sunlight for photosynthesis in the barren marine floor the distinctive polybessurus like microfossils can be viewed both in laminae of stromatolites and in spotted digitate clots of thrombolites suggesting their critical roles in microbe dominated reefs in the aftermath of the end permian mass extinction the morphological features and growth patterns of the polybessurus like fossils resemble their proterozoic counterpart polybessurus bipartitus fairchild ex green et al 1987 the widespread polybessurus like fossils in the permian triassic boundary microbialites of the south china craton suggest pervasive retrogressive ecosystems on carbonate platforms after the largest extinction event during the earth s history

modeling the trajectories of satellite tracked drifters in the adriatic sea during a summertime bora event 1 during the summertime bora episode of 22 25 june 1995 characterized by hazardous weather conditions with strong winds of over 20 m s 1 two satellite tracked drifters were present in the adriatic shelf area one was on the open sea south of the istrian peninsula while the other which measured the surface current of 70 cm s 1 was tracked along the western coastal strip an oceanographic model forced by the outputs of mesoscale meteorological models and by river discharges was applied to simulate drifter trajectories during the event process oriented studies performed to determine governing forcings for the drifter movements in different areas of the adriatic revealed that strong current along the western adriatic coast resulted from a combined influence of the wind stress and the river discharges the wind was responsible for the strong drifter movements while the baroclinic current due to the coastal buoyancy fluxes prevented the drifter from moving close to the shore and from colliding with the coast at the same time drifter movements in the open sea resulted from the wind action solely differences in the alongshore and offshore vertical density profiles primarily due to the river inflows caused 3 times stronger currents in the coastal area as compared to the currents in the open sea the proper formulation of the drag coefficient was crucial for the successful simulations of the drifter trajectories as simulations with increased values of the drag coefficient reproduced well the effects of the sea surface roughness and atmospheric instability above the sea

composition of the core i solubility of oxygen in molten iron at high temperatures abstract experiments on the solubility of feo in molten iron have been carried out at temperatures between 2100 and 2550 c the results show that liquid feo is extensively soluble in molten iron at 2500 c and indicate that they probably become completely miscible above 2800 c liquid iron in equilibrium with crystalline magnesiowustite mg 0 8 fe 0 2 o which is believed to be an important mineral in the lower mantle would dissolve about 14 mol of feo at 2500 c and 40 mol of feo at 2800 c the geochemical implications of these results are discussed it is concluded that the outer core probably contains a large amount of dissolved feo and that oxygen is probably the principal light element in the outer core

microbenthic distribution of proterozoic tidal flats environmental and taphonomic considerations silicified carbonates of the late mesoproterozoic to early neoproterozoic society cliffs formation baffin island contain distinctive microfabrics and microbenthic assemblages whose paleo environmental distribution within the formation parallels the distribution of these elements through proterozoic time in the society cliffs formation restricted carbonates including microdigitate stromatolites laminated tufa and tufted microbial mats consist predominantly of synsedimentary cements these facies and the cyanobacterial fossils they contain are common in paleoproterozoic successions but rare in neoproterozoic and younger rocks less restricted tidal flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early yet demonstrably after compaction these strata contain cyanobacteria that are characteristic in neoproterozoic rocks within the formation the facies dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria a reasonable conclusion is that secular changes in microbenthic assemblages through proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments as well as concomitant changes in the taphonomic window of silicification through which early life is observed

organic geochemistry of the vindhyan sediments implications for hydrocarbons abstract the organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance source and thermal maturity which are essential prerequisites for a hydrocarbon source rock in the present study evaluation of organic matter in the outcrop shale samples from the semri and kaimur groups of vindhyan basin was carried out using rock eval pyrolysis also the adsorbed low molecular weight hydrocarbons methane ethane propane and butane were investigated in the near surface soils to infer the generation of hydrocarbons in the vindhyan basin the total organic carbon toc content in shales ranges between 0 04 and 1 43 the s1 thermally liberated free hydrocarbons values range between 0 01 0 09 mghc grock milligram hydrocarbon per gram of rock sample whereas the s2 hydrocarbons from cracking of kerogen show the values between 0 01 and 0 14 mghc grock based on the tmax temperature at highest yield of s2 and the hydrogen index hi correlations the organic matter is characterized by type iii kerogen the adsorbed soil gas ch4 c1 c2h6 c2 c3h8 c3 and nc4h10 nc4 concentrations measured in the soil samples from the eastern part of vindhyan basin son valley vary from 0 to 186 ppb 0 to 4 ppb 0 to 5 ppb and 0 to 1 ppb respectively the stable carbon isotope values for the desorbed methane 13c1 and ethane 13c2 range between 45 7 to 25 2 and 35 3 to 20 19 vpdb respectively suggesting a thermogenic source for these hydrocarbons high concentrations of thermogenic hydrocarbons are characteristic of areas around sagar narsinghpur katni and satna in the son valley the light hydrocarbon concentrations c1 c4 in near surface soils of the western vindhyan basin around chambal valley have been reported to vary between 1 2547 ppb 1 558 ppb 1 181 ppb 1 37 ppb and 1 32 ppb respectively with high concentrations around baran jhalawar bhanpur garot regions kumar et al 2006 the light gaseous hydrocarbon anomalies are coincident with the wrench faults kota dholpur ratlam shivpuri kannod damoh son banspur rewa wrench in the vindhyan basin which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils

coccoid microfossils from the doushantuo formation late sinian of south china abstract eight categories of organic walled coccoid microfossils in the c 700 ma old doushantuo formation are described and named of these one genus and species paratetraphycus giganteus is new most of the microfossils are interpreted as being the remains of the chroococcaceae and are morphologically comparable to those of the 650 ma old yudoma microbiota and the 850 ma old bitter springs microbiota these microfossils occur in non stromatolitic cherts adding preservational information to the study of precambrian life

kotuikan formation assemblage a diverse organic walled microbiota in the mesoproterozoic anabar succession northern siberia abstract abundant and diverse microfossils from shales of the lower member of the 1500 ma old kotuikan formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the kotuikan assemblage is dominated by prokaryotic cyanobacteria both filamentous oscillatorialeans and nostocaleans which are represented primarily by cellular trichomes and by empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens however unlike mesoproterozoic microbiota of shallow water settings the kotuikan compressed organic walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans as well as other morphologically complex microorganisms the kotuikan assemblage contains numerous taxa previously recorded from neoproterozoic successions e g elatera eosolena palaeastrum pterospermopsimorpha rugosoopsis and navifusa but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits the kotuikan assemblage documents the status of the marine biosphere at an early mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic microbiota as reported in this study 35 taxa were identified in the kotuikan assemblage of which five forms are described informally that are assigned to 18 genera of microscopic prokaryotes and eukaryotes two new genera and five new species are proposed hirudiforma lancetica gen and sp nov h simmetrica gen and sp nov lineaforma elongata gen and sp nov elatera minor sp nov eosolena minuta sp nov

terminal proterozoic cyanobacterial blooms and phosphogenesis documented by the doushantuo granular phosphorites ii microbial diversity and c isotopes an unprecedented period of phosphogenesis along with massive deposition of black shales major perturbations in the global carbon cycle and the rise of atmospheric oxygen occurred in the terminal proterozoic in the aftermath of the marinoan glaciation although causal links between these processes have been postulated evidence remains challenging correlated in situ micro analyses of granular phosphorites from the ediacaran doushantuo formation in yichang south china suggested that cyanobacteria and associated extracellular polymeric substances eps might have promoted aggregated granule growth and subsequent phosphatization she et al 2013 here we present new paleontological data for the doushantuo phosphorites from yichang which combined with raman microspectroscopy and carbon isotope data further document links between the biology of cyanobacteria and phosphogenesis mapping of microfossils in thin section shows that most phosphatic granules contain microfossils that are dominated by colonies of myxococcoides along with several filamentous genera generally considered to represent cyanobacterial sheaths in addition the phosphorites and associated rocks have 13corg values in the range of 26 0 to 29 7 vpdb consistent with photoautotrophic carbon fixation with the rubisco enzyme close association of phosphorites with the marinoan tillites in stratigraphic level supports a genetic link between deglaciation and phosphogenesis at least for the doushantuo occurrence our new data suggest that major cyanobacterial blooms probably took place in the terminal proterozoic which might have resulted in rapid scavenging of bioavailable phosphorus and massive accumulations of organic matter om within a redox stratified intra shelf basin the om bound phosphorus could have liberated by microbial sulfate reduction and other anaerobic metabolisms and subsequently concentrated by fe redox pumping below the chemocline upwelling of the bottom waters or upward fluctuation of the chemocline might have brought p enriched waters to the photic zone where it was again scavenged by cyanobacteria through their eps to be subsequently precipitated as francolite the feedbacks between enhanced continental weathering cyanobacterial blooms carbon burial and accelerated phosphorus cycle thus controlled the marine biogeochemical changes which led to further oxygenation of the atmosphere and oceans ultimately paving the way for the rise of metazoans

paleobiology of the mesoproterozoic neoproterozoic transition the sukhaya tunguska formation turukhansk uplift siberia silicified carbonates of the latest mesoproterozoic sukhaya tunguska formation northwestern siberia contain abundant and diverse permineralized microfossils peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of lyngbya and phormidium in subtidal to lower intertidal settings mat dwelling microbenthos and possible coastal microplankton are abundant in contrast densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface most microbenthic populations are cyanobacterial although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton protists are more securely represented by large up to 320 micrometers in diameter but poorly preserved acritarchs in basinal facies the sukhaya tunguska assemblage contains 27 species in 18 genera by virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions including substrates proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and thereafter resist competitive replacement the resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of phanerozoic plants and animals

proterozoic ministromatolites with radial fibrous fabric small digitate stromatolites with diameters in the range of 0 2 20 mm ministromatolites are common in early and middle proterozoic carbonate sequences and extend stratigraphically from the archaean to the holocene an occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial fibrous fabric and microscopically crinkled microcrenate lamination is described from the 1 9 gyr old belcher supergroup mcleary formation in southeastern hudson bay canada the structures which can be considered to be a variety of tufa are unusually well preserved because of early diagenetic silicification columnar types are referable to pseudogymnosolen asperia and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern canada where the fabric is normally preserved by a secondary mosaic of dolomite the textural evidence of angulate cross sections and rectilinear divergent patterns indicates that the radial fibrous fabric represents primary or very early diagenetic precipitation and that pseudogymnosolenids with mosaic dolomite originally also had radial fibrous structure the precipitation may have been within or on microbial mats

late palaeoproterozoic 1800 1600ma stromatolites cuddapah basin southern india cyanobacterial or other bacterial microfabrics abstract late palaeoproterozoic stromatolites from the vempalle formation of the cuddapah supergroup south eastern india contain well preserved microfabrics these exhibit a range of complex clotted to bush like micritic fabrics set in microsparite and sparite cements no calcified microfossils confidently referable to calcified cyanobacteria have been recognized although micritic masses crudely resembling the phanerozoic calcified cyanobacterium angusticellularia are present it is suggested that these vempalle stromatolitic microfabrics compare more closely with modern calcified non cyanobacterial biofilms this interpretation questions the widespread assumption that cyanobacteria were important components of stromatolite building microbiotas during the proterozoic and instead emphasizes the possibility that examples similar to those studied were dominated by non cyanobacterial bacteria it is generally thought likely that cyanobacteria were important components of stromatolite building microbiotas during the proterozoic and that early lithification by carbonate precipitation was widespread and intense if these assumptions are correct then the apparent absence of diverse calcified cyanobacteria suggests that these organisms which are conspicuous in the early palaeozoic underwent evolutionary radiation close to the neoproterozoic cambrian boundary

c o sr and pb isotope systematics of carbonate sequences of the vindhyan supergroup india age diagenesis correlations and implications for global events abstract the vindhyan supergroup of central india records a substantial portion of proterozoic time and contains some of the most disputed precambrian cambrian fossil discoveries despite the significance of these fossil finds many issues including reliable geochronology remained unresolved similarly the geochemical aspects of the sedimentary sequences and their implications to the evolution of proterozoic oceans have been studied only at a reconnaissance level here we report the results of the pb pb dating for the rohtasgarh limestone from the lower vindhyan supergroup giving an age of 1601 130 ma an estimate that is in accord with the recent u pb zircon dating of the lower vindhyan supergroup by rasmussen et al geology 30 2002 103 and ray et al geology 30 2002 131 sr isotope stratigraphy of carbonates from the upper vindhyan supergroup suggests a mid neoproterozoic 750 650 ma age for these formations this in turn argues for a long hiatus between the deposition of the lower and upper vindhyan supergroups our results conflict with the reports of cambrian age small shelly fossils and ediacaran fossils from the upper vindhyan rocks at the same time they suggest that the alleged trace fossils in the lower vindhyan rocks must be of paleoproterozoic age the most common 18 o values for calcitic components from limestone formations of the supergroup range from 10 to 7 similar to paleo neoproterozoic carbonates worldwide the modes of 13 c in the lower and upper vindhyan supergroups are 0 2 and 3 2 respectively again consistent with the worldwide dataset the secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the vindhyan basin c and sr isotope data provide new calibration points for the seawater evolutionary curves particularly for the paleoproterozoic for strontium isotopes the data from the lower vindhyan supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed

microfossil populations in the context of synsedimentary micrite deposition and acicular carbonate precipitation mesoproterozoic gaoyuzhuang formation china abstract silicified stratiform stromatolites of the mesoproterozoic gaoyuzhuang formation 1 4 1 5 ga china contain well preserved microfossils the cherts also harbor varied synsedimentary precipitates and void filling cements replaced by early diagenetic silica minerals these precipitates disclose microenvironments characterized by supersaturated solutions in protected shallow depressions within an intertidal setting the precipitates provided surfaces for selective microbial settlement and rigid sediment matrix for microbial growth together with early silicification the frequent precipitation events contributed to preservation of microfossils by reducing sediment compaction and shearing fossiliferous cherts display fine flat and wavy lamination characterized by an alternation of highly silicified thin organic rich layers with thick sediment rich layers organic rich layers are dominated either by coccoid or by filamentous microfossils whereas sediment rich layers contain abundant synsedimentary precipitates within which the microfossils are preserved in their growth position four dominant microfossils coccostratus dispergens n gen et sp eoentophysalis belcherensis eoschizothrix composita and siphonophycus inornatum occur contiguously through several tens of laminae and are identified as main frame building biological components of gaoyuzhuang stromatolites community composition microbial density distribution orientation and developmental patterns of the frame building microfossils are closely correlated with the changing depositional events of gaoyuzhuang cherts contrasting conditions of sedimentary kinetics with those of sedimentary stasis each assemblage of frame building microfossils responded to sedimentation with different mechanisms to escape burial high sedimentation rates correlate with scattered colonies of coccoids and with loose webs of predominantly upright filaments low sedimentation rates correlate with dense laterally connected colonies of coccoids and with a change in filament orientation from vertical to horizontal in multi trichomous microfossil eoschizothrix composita low sedimentation rates are also accompanied with an increase in number of trichomes per filament the observed morphological variability of the frame building microfossils is explained by microbial development reproduction and behavior by interactions between sedimentological and biological controls

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

microbial fossils from the kheinjua formation middle proterozoic semri group lower vindhyan son valley area central india abstract abundant coccoid and filamentous microfossils are found in petrographic thin sections of stratiform stromatolitic cherts from the 1200 ma old fawn limestone of the kheinjua formation lower vindhyan supergroup central india the assemblages is dominated by coccoid forms of probable chroococcacean and entophysalidacean affinities most filaments closely resemble the oscillatoriacean gunflintia minuta unlike other middle to late proterozoic stromatolitic microbiotas the kheinjua is dominated by microfossils taxonomically the kheinjua microbiota most closely resembles proterozoic microbiotas described from the belcher islands kasegalik formation mcarthur group bitter springs formation and the yudoma suite the following taxa are formally described chroococcaceae myxococcoides minor schopf eosynechococcus isolatus n sp tetraphycus congregatus n sp kheinjuasphaera vulgaris n gen n sp glenobotrydion aenigmatis schopf melasmatosphaera media hofmann entophysalidaceae eoentophysalis belcherensis hofmann e magna n sp oscillatoriaceae cf gunflinta minuta barghoorn eomycetopsis siberiensis lo

the biostratigraphic conundrum of siberia do true tonian cryogenian microfossils occur in mesoproterozoic rocks abstract abundant and diverse microfossils from shales of the 1500 ma old ust il ya formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the ust il ya assemblage is dominated by cyanobacteria both filamentous oscillatorialeans and nostocaleans represented primarily by cellular trichomes and empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens microbiotas from shallow water settings that contain morphologically simple and small microfossils were originally taken to be representative of mesoproterozoic life however the ust il ya and other compressed organic walled microfossil assemblages in open shelf facies include diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and other morphologically complex microorganisms the ust il ya assemblage contains numerous taxa previously reported from tonian and cryogenian successions e g elatera pterospermopsimorpha rugosoopsis and chuaria but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a few taxa reported from the calymmian microfossil assemblages e g lineaforma aff l elongata satka favosa and elatera aff e minor the ust il ya mixed assemblage of prokaryotic and eukaryotic microorganisms records an evolutionary stage transitional between the prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic world as reported in this study 38 taxa were identified in the ust il ya assemblage of which six forms are described informly that are assigned to 17 genera of microscopic eukaryotes presumably 17 20 entities and prokaryotes a new species is proposed heliconema ustilense

the distribution of microfossil assemblages in proterozoic rocks abstract a biostratigraphic model of the temporal distribution of distinctive proterozoic microfossil assemblages is suggested based on studies of upper precambrian chert embedded and compression preserved organic walled microfossils from the reference sections of eurasia north america and australia microfossils from 2 0 to 0 542 ga can be divided into seven successive informal global units which can be compared to standard units of the international and russian time scales each unit is characterized by a particular association of taxa typified by the fossil assemblage that gives it its name these form broad biostratigraphic units comparable to assemblage zones of phanerozoic successions in general but with minor differences they correspond to chronostratigraphic units accepted by the internal commission on stratigraphy the units are 1 labradorian the upper part of the paleoproterozoic orosirian and statherian 2 0 1 65 ga 2 anabarian lower mesoproterozoic calymmian ectasian lower riphean lower middle riphean 1 65 1 2 ga 3 turukhanian upper mesoproterozoic stenian upper middle riphean 1 2 1 03 ga 4 uchuromayan lower neoproterozoic late stenian tonian lower upper riphean 1 03 0 85 ga 5 yuzhnouralian upper neoproterozoic cryogenian upper upper riphean 0 85 0 63 ga 6 amadeusian lower ediacaran lower vendian 0 63 0 55 ga 7 belomorian upper ediacaran upper vendian 0 55 0 542 ga

evidence for primary aragonite precipitation lower proterozoic 1 9 ga rocknest dolomite wopmay orogen northwest canada abundant precipitation of aragonite is suggested by well preserved fabrics in dolostones of the lower proterozoic 1 9 ga rocknest formation fabrics include dolomite and silica after botryoidal aragonite in tepee structures and seafloor cement fans and cement crusts in cryptalgal tufas aragonite was common on the 1 9 ga rocknest shelf as it is on modern tropical shelves a search for similar marine cement fabrics in other precambrian carbonates should help resolve whether aragonite was an important precipitate from precambrian tropical seas implying an ocean carbonate chemistry similar to today or if aragonite precipitation was restricted to rare times in the past calcite and even dolomite being the main carbonates precipitated as recently suggested by some authors

evolution of early proterozoic passive margin carbonate platform rocknest formation wopmay orogen northwest territories canada abstract the superbly exposed rocknest formation wopmay orogen northwest territories canada is an early proterozoic 1 89 ga carbonate platform that formed along the eastern passive margin of slave craton archean the palinspastically restored shelf sequence is an eastward thinning prism 0 1 100 m thick extending for over 220 km parallel to strike and over 200 km perpendicular to strike it contains from west to east slope outer shelf shoal complex and inner shelf facies slope facies are carbonate rhythmites megabreccias and platy rhythmite breccias outer shelf facies include stromatolitic reefal boundstone and backreef ooid intraclast grainstore shoal complex facies are shallowing upward cycles of tufa cryptalgalaminite laminated dolosiltite lutite overlain by tepees b eccias and pisolite these formed a paleotopographic high restricting an eastern lagoon from western ocean inner shelf facies are dominantly asymmetric shallowing upward cycles of lagoonal mixed carbonates and siliciclastics that grade into intertidal stromatolitic dolomites cyclic facies pass eastward into noncyclic siliciclastics of central and eastern lagoon the rocknest lagoon spanned the width of slave craton and was bounded by a western carbonate shoreline shoal complex and by a tectonically active eastern siliciclastic shoreline the rocknest carbonate shelf developed on an underlying siliciclastic shelf an initial ramp rapidly evolved into a progradational accretionary rim and shoal complex followed by vertical aggradation of the rim concurrently the shoal complex underwent numerous west to east progradational events to produce many shallowing upward cycles in the shelf interior cyclic sedimentation was interrupted during three events of incipient shelf drowning associated with local backstepping of the rim and noncyclic sedimentation over most of the shelf probably caused by fourth order eustatic sea level oscillations the shelf was terminally drowned and overlain by deep water synorogenic siliciclastics during attempted subduction of the margin carbonates capping the shelf are stromatolitic patch re fs which overlie a possible unconformity suggesting emergence of the shelf prior to drowning

middle proterozoic 1200 1400 ma microfossils from the western hills near beijing china eleven species of microfossils have been recognized in cherts from shallow marine and intertidal dolostones of the wumishan formation 1200 1400 ma in the western hills near beijing of these one is a new taxon oscillatoriopsis subtilis n sp the microbiota has some affinities with that of the approximately coeval dismal lakes assemblage in arctic canada

genesis of carbonate precipitate patterns and associated microfossils in mesoproterozoic formations of india and russia a comparative study abstract the jaradag fawn limestone formation 1 6 ga semri group vindhyan supergroup india contains abundant microfossils and precipitates in early diagenetic bedded and stromatolitic cherts the silicified carbonate precipitates and associated micritic event laminae were formed at or near the sediment water interface and presumably lithified very rapidly the precipitates deposited inorganically without active participation of cyanobacterial mats can be grouped into three morphological categories radial fibrous fans microlaminated stratiform laminae and poorly differentiated laminated stratiform laminae a diverse assemblage of cyanobacteria is preserved both within precipitates where preservation is excellent and in other synsedimentary textures precipitates and microfossil assemblages from four mesoproterozoic formations of siberia are described and compared with those from jaradag fawn limestone comparison of the indian and the siberian precipitates and associated microfossil assemblages reveals an almost identical pattern for contemporaneous mesoproterozoic carbonate precipitates and microbiotas in cherts the microbial assemblages almost exclusively composed of the remnants of cyanobacteria are dominated by entophysalidacean members and short trichomes and can be termed as typical mesoproterozoic microbiotas co occurrence of these microbiotas and precipitates is probably related to the depositional environment of the mesoproterozoic tidal flats with high carbonate saturation neoproterozoic depositional conditions changed drastically as did the composition of microbial communities

neoproterozoic fossils in mesoproterozoic rocks chemostratigraphic resolution of a biostratigraphic conundrum from the north china platform abstract siliciclastic rocks of the ruyang group southern shanxi and the broadly equivalent gaoshanhe group shaanxi contain exceptionally well preserved large o 150 m acanthomorphic acritarchs recently interpreted as late neoproterozoic sinian c 800 544 ma in age this biostratigraphic interpretation is based on the presence of large acanthomorphs in sinian successions of south china and elsewhere and the perceived absence of comparable forms in older rocks it casts doubt on the long accepted interpretation of ruyang and correlative rocks as mesoproterozoic in age 1600 1000 ma in contrast thick marine dolomites in overlying units contain abundant radial fibrous fabrics and a narrow range of 13c values c 0 1 vs pdb features which characterize unambiguously mesoproterozoic carbonates elsewhere on the north china platform and on other continents age estimates based on petrofabrics and chemostratigraphy are corroborated by a u pb zircon age of 999 ma no recorded error for granites which intrude overlying carbonates thus in combination the available data constrain the ruyang siliciclastics and overlying carbonates to be older than about one billion years making shuiyousphaeridium yan and other large process bearing acritarchs from these units among the oldest known distinctly ornamented eukaryotic microfossils