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

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

hydrogeological and geotechnical assessments of the kadifekale landslide area izmir turkey the study area is located in the south of izmir city centre in an area built without planning permission or any overall city plan in this area a number of mass movements occurred in the past and the region is still an active landslide field at present the real sliding factor is geological structure which is made up of hard volcanic rocks overlying soft neogene clayey soils and forms a typical structure prone to sliding unplanned human activities change the hydrogeological and geotechnical stabilities of the geological formations in a negative way rain and water leaking either directly from the mains or septic holes infiltrate into the ground and act as one of the factors in causing landslides it is clearly shown that the irregular urbanization in and around kadifekale is one of the factors contributing to the landslides in the area to prevent the occurrence of landslides in the study area an effective surface and under ground drainage should be established rain and wastewater should be removed from the area by separate systems slopes should be reduced water loving trees should be planted and construction of high rise buildings should be avoided

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

a vendian cambrian boundary succession from the northwestern margin of the siberian platform stratigraphy palaeontology chemostratigraphy and correlation siberia contains several key reference sections for studies of biological and environmental evolution across the proterozoic phanerozoic transition the platonovskaya formation exposed in the turukhansk region of western siberia is an uppermost proterozoic to cambrian succession whose trace and body fossils place broad limits on the age of deposition but do not permit detailed correlation with boundary successions elsewhere in contrast a striking negative carbon isotopic excursion in the lower part of the platonovskaya formation permits precise chemostratigraphic correlation with upper most yudomian successions in siberia and possibly worldwide in addition to providing a tool for correlation the isotopic excursion preserved in the platonovskaya and contemporaneous successions documents a major biogeochemical event likely involving the world ocean the excursion coincides with the palaeontological breakpoint between ediacaran and cambrian style assemblages suggesting a role for biogeochemical change in evolutionary events near the proterozoic cambrian boundary

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

microfossils from silicified stromatolitic carbonates of the upper proterozoic limestone dolomite series central east greenland silicified flake conglomerates and in situ stratiform stromatolites of the upper proterozoic c 700 800 ma limestone dolomite series central east greenland contain well preserved microfossils five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages providing a broad palaeontological representation of late proterozoic peritidal mat communities comparison of assemblages demonstrates that the taxonomy and diversity of mat builder dweller and allochthonous populations all vary considerably within and among horizons the primary mat builder in most assemblages is siphonophycus inornatum a sheath forming prokaryote of probable but not unequivocally established cyanobacterial affinities an unusual low diversity unit in bed 17 is dominated by a different builder tenuofilum septatum while a thin cryptalgal horizon in bed 18 is built almost exclusively by siphonophycus kestron although variable taphonomic histories contribute to observed assemblage variation most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments twenty two taxa are recognized of which two are described as new myxococcoides stragulescens n sp and scissilisphaera gradata n sp

microfossils from the neoproterozoic gangolihat formation kumaun lesser himalaya their stratigraphic and evolutionary significance abstract a well preserved microbial assemblage from the gangolihat dolomite kumaun lesser himalaya is described in the present paper the present assemblage contains both filamentous septate and non septate and coccoidal microfossils besides acritarchs and sponge spicules the assemblage includes 23 species belonging to 12 genera siphonophycus typicum is the most common species in the entire assemblage the assemblage shows an abundance of filaments over coccoids and acritarchs the assemblage includes siphonophycus septatum siphonophyscus typicum siphonophycus robustum siphonophycus capitaneum siphonophycus kestron and siphonophycus punctatum gunflintia minuta gunflintia grandis gunflintia barghoornii oscillatoriopsis media oscillatoriopsis obtusa oscillatoriopsis cuboides oscillatoriopsis sp nostocomorpha chlorogloeaopsis contexta obruchevella parvissima sphaerophycus parvum myxococcoides minor leiosphaeridia crassa trachyhystrichosphaera vidalii trachyhystrichosphaera sp micrhystridium pallidum cymatiosphaera minuta and sponge spicules of monaxons and hexactinellid affinity the assemblage suggests a neoproterozoic age for gangolihat dolomite

a diverse and exquisitely preserved organic walled microfossil assemblage from the meso neoproterozoic mbuji mayi supergroup democratic republic of congo and implications for proterozoic biostratigraphy abstract a well preserved and diversified microfossil assemblage is reported from the meso neoproterozoic mbuji mayi supergroup in the kasai oriental province central part of democratic republic of congo a total of 49 taxa belonging to 27 genera were identified including 11 species of unambiguous eukaryotes 10 species of possible eukaryotes or prokaryotes and 28 species of probable bacteria this assemblage is more diverse than previously reported but includes taxa reported in coeval worldwide assemblages it is characterized by abundant sphaeromorphs filamentous colonial aggregates and filamentous forms as well as a relatively low diversity of acanthomorphs including the late mesoproterozoic and early neoproterozoic index fossil trachyhystrichosphaera aimika reported for the first time in central africa this species co occurs with other taxa also reported for the first time in africa trachyhystrichosphaera botula jacutianema solubila cf tappania sp valeria elongata and numerous other taxa correlation with other geochronologically constrained successions that contain trachyhystrichosphaera confirms t aimika as promising index fossil to define the late mesoproterozoic early neoproterozoic interval the available biostratigraphic data enable to suggest a minimum tonian age for the mbuji mayi supergroup this age is consistent with the published and new geochronological data comparison with worldwide proterozoic assemblages permits to define microfossil assemblages useful for biostratigraphy this study significantly improves our understanding of the diversity of the late mesoproterozoic early neoproterozoic biosphere and in particular the diversification of early eukaryotes preserved in the democratic republic of congo rock record and more broadly in africa where micropaleontological investigations are sparse

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

stable isotopic compositions of carbonates from the mesoproterozoic bangemall group northwestern australia marine carbonate rocks from the mesoproterozoic bangemall group of northwestern australia show little deviation 1 3 in whole rock delta 13c carb values about a mean of 0 5 this narrow range persists despite close sampling every 10 20 m through long sections up to 2500 m that are geographically widespread up to 250 km apart over many depositional environments supralittoral to outer shelf sediment sources stromatolitic bioherms to detrital calcilutites and rock types pure limestones to dolomitic shales the only major excursions from the norm seem related to unusual environmental or post depositional processes as they are correlated with large enrichments to 3 or depletions to 16 in 18o relatively heavy delta 13c values up to 2 5 occur in a single bed of brecciated ferruginous dolostone at a single locality these abnormal values may result from local evaporitic conditions limey and shaley nodular dolostones have delta 13c values as low as 4 3 probably caused by remineralization of organic matter during late and patchy dolomitization most notably sharp negative excursions in delta 13c up to 8 4 occur in bleached kerogen free rocks with mineral assemblages of dolomite quartz calcite tremolite talc reflecting isotopic re equilibration in thick metamorphic aureoles around dolerite intrusions general environmental variations are minor with delta 13c values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative there are no strong secular trends but subtle fluctuations within the range 2 to l can be correlated along the northwestern margin of the basin this resembles the pattern seen in other mesoproterozoic successions but is markedly unlike the heavy background 5 and extreme variations up to l0 in delta 13c evident in neoproterozoic successions of similar thickness and environmental setting hence in contrast to the neoproterozoic the global rate of organic carbon burial was probably fairly constant during deposition of the bangemall group and perhaps generally during the mesoproterozoic as was the redox state of the atmosphere and hydrosphere

proterozoic stromatolitic micro organisms from hebei north china cell preservation and cell division abstract well preserved microfossils of coccoid cyanophytes have been found in partially silicified stratiform stromatolites of the 1 7 ga old dahongyu formation changcheng group pangjapu iron mine hebei province north china some of the microfossils show preferential preservation of cells as compared with their sheaths which is contrary to the common preservation mode of most cyanophyte microfossils the colony forms and the cell arrangements reveal the cell division patterns of these proterozoic micro organisms the endosporangia like structures associated with the vegetative cells indicate the possibility of an endosporulation reproductive method among these micro organisms two new taxa are described here gloeotheceopsis aggregata gen et sp nov and glenobotridion granulosum sp nov

the carbon isotopic composition of proterozoic carbonates riphean successions from northwestern siberia anabar massif turukhansk uplift thick carbonate dominated successions in northwestern siberia document secular variations in the c isotopic composition of seawater through mesoproterozoic and early neoproterozoic early to early late riphean time mesoproterozoic dolomites of the billyakh group anabar massif have delta 13c values that fall between 0 and 1 9 permil versus pdb with values in the upper part of the succession yusmastakh formation consistently higher than those of the lower ust il ya and kotuikan formations consistent with available biostratigraphic and radiometric data delta 13c values for billyakh carbonates compare closely with those characterizing early mesoproterozoic carbonates about 1600 1200 ma worldwide in contrast late mesoproterozoic to early neoproterozoic limestones and dolomites in the turukhansk uplift exhibit moderate levels of secular variation only the lowermost carbonates in the turukhansk succession linok formation have delta 13c values that approximate billyakh values higher in the turukhansk succession delta 13c values vary from 2 7 to 4 6 permil with outliers as low as 5 0 permil interpreted as diagentically altered again consistent with paleontological and radiometric data these values compare well with isotopic values from 1200 to 850 ma successions elsewhere five sections measured in different parts of the turukhansk basin show nearly identical patterns of variation confirming that carbonate delta 13c correlates primarily with time and not facies the siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra and interbasinal correlation of mesoproterozoic and early neoproterozoic rocks

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

thermal and mechanical evolution of an orogenic wedge during variscan collision an example in the maures tanneron massif se france synthesis of structural petrological and geochronological data for the maures tanneron massif and its integration in the framework of adjacent massifs i e sardinia and corsica has allowed us to propose a new model of evolution for the southern variscan belt after siluro devonian subduction associated with high pressure low temperature hp lt metamorphism m 0 c 10 15 8c km 21 and subsequent carboniferous nappes stacking the belt underwent strong reworking related to back thrusting nappes stacking and back thrusting were associated with typical barrovian metamorphism m 1 c 20 30 8c km 21 starting at 360 ma that progressively evolved to higher temperature metamorphisms m 2 c 40 60 8c km 21 and m 3 c 60 80 8c km 21 during 330 300 ma in the internal part of the belt progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold dome structures the mass transfer is accommodated by orogen parallel transpressive shearing synchronous with m 3 during late carboniferous time the orogenic wedge is characterized by two main tectono metamorphic units decoupled by a major shear belt an internal zone with migmatites and syntectonic granitoids where hp relicts have been exhumed and an external zone that escaped the late ht event and preserved precious structures

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

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

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

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago