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

toward constraining the long term reversing behavior of the geodynamo a new maya superchron 1 billion years ago from the magnetostratigraphy of the kartochka formation southwestern siberia abstract we present new magnetostratigraphic data obtained from the late mesoproterozoic 1 ga kartochka formation in the east angara terrane of the yenisey ridge region southwestern siberian platform a 200 m thick section encompassing the carbonate kartochka formation was densely sampled and another more limited outcrop several kilometers away was sampled in order to conduct a paleomagnetic fold test between the two sites paleomagnetic analyses revealed the existence of two magnetization components a low unblocking temperature component which likely has a viscous origin was first isolated below 200 c a characteristic component carried either by magnetite or by a mixture of magnetite and hematite was then isolated in the medium to high temperature range up to 565 585 c or 680 c the hematite bearing component has a shallower inclination than the magnetite component showing that magnetite was less sensitive to flattening a positive paleomagnetic fold test was obtained between the two studied sections indicating that the characteristic magnetization was likely acquired during or very soon after sediment deposition this primary origin was further verified by comparison between the paleomagnetic poles derived for the kartochka formation and other late mesoproterozoic siberian sections all data from the kartochka formation contain a single magnetic polarity assumed to be normal this long normal polarity interval is consistent with the polarity zonation previously determined from magnetostratigraphic and paleomagnetic data from the siberian uchur maya region the new data fortify evidence of a normal polarity superchron spanning the 1000 ma old mesoproterozoic neoproterozoic boundary which we propose to call the maya superchron these data confirm the occurrence of sharp transitions between a frequently reversing regime and a non reversing regime of the geodynamo this behavior also observed in other regions may represent a consistent property of the proterozoic geomagnetic field together with changes in the amount of time spent in the superchron regime they may testify to a different field behavior during the late precambrian than during the phanerozoic this difference could reflect a stronger influence of the heterogeneous heat flux patterns at the core mantle boundary during the precambrian possibly as a consequence of the inner core not yet being nucleated at this time

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

isotopic evidence for the paleoenvironmental evolution of the mesoproterozoic helena formation belt supergroup montana usa high resolution isotopic and geochemical data constrain long debated marine vs lacustrine depositional models for the helena formation a carbonate unit within the mesoproterozoic belt supergroup montana paired carbon and oxygen isotope and minor element analyses of microsamples from a diverse array of petrographic components effectively distinguish between primary and secondary isotopic signatures 13c values reflecting primary depositional conditions increase progressively from an average of 0 near the base of the helena formation to an average of 1 4 near the top over a stratigraphic thickness of 100 200 m because the base of this 1 6 shift coincides with a basin wide facies change marked by the appearance of stromatolitic units and an increased occurrence of features indicative of evaporative conditions and subaerial exposure the shift toward more positive 13c values is interpreted to reflect an increase in biologic productivity coupled with increasingly evaporative conditions in the eastern part of the belt basin 13c values from the lower part of the helena formation are similar to those from other marine carbonates of similar age and suggest that the belt basin was marine during deposition of the helena formation taken as a whole these relations suggest that helena sediments were deposited in a shallow marine basin that was increasingly isolated from the open mesoproterozoic ocean original 13o values have been retained only in syndepositional and early diagenetic intergranular cements in oolitic grainstones that occur in the upper part of the helena formation the most positive 13o values derived from these cements 6 1 pdb are up to 3 higher than previous estimates for the 13o value of mesoproterozoic marine calcite however because these phases are associated with facies deposited under restricted marine conditions interpretation of 13o values from these cements with regard to the composition of the open mesoproterozoic ocean awaits study of additional coeval marine carbonate successions

isotopic evolution of the biogeochemical carbon cycle during the proterozoic eon abstract time related changes in the proterozoic sedimentary isotopic record of organic carbon and carbonates are consistent with the following 1 an increase in the oxidation state of the environment 2 a transition towards microbial populations dominated by producers and utilizers of o 2 and so 4 2 and 3 a relatively minor increase in organic 13 c during postdepositional thermal alteration 13 c increased 3 as h c declined from 0 8 to 0 1 the progressive oxidation of the environment was episodic for example between 2 2 and 1 9 ga crustal and atmospheric reservoirs of o 2 fe 3 and so 4 2 increased relatively rapidly and this increase was too large to be attributed solely to the declining rate of oxidant consumption by volcanic and hydrothermal fluxes of reduced species additional fundamental changes seem to be required in the tectonic style of organic sedimentation and preservation episodic changes in both the biogeochemical c cycle and atmospheric o 2 levels during the proterozoic eon likely influenced the evolution of eukarya

carbon isotope records in a mesoproterozoic epicratonic sea carbon cycling in a low oxygen world abstract carbon isotope profiles of sedimentary strata are widely used as both a tool for stratigraphic correlation and as a mechanism for inferring important changes in earth s biogeochemical cycles chemostratigraphic interpretations however often rely on the assumption that the isotopic composition of dissolved inorganic carbon dic in the surface oceans is broadly homogeneous at times of globally high sea level when epicratonic epeiric environments dominate our sedimentary record carbon isotope records reveal substantial lateral variability here we investigate lateral variability in the marine carbon isotope record from pericratonic to epicratonic environments of the mesoproterozoic 1 1 ga atar el mreiti groups of the taoudeni basin west africa restricted marine epicratonic environments are consistently 2 4 lighter than open marine pericratonic environments suggesting input of an isotopically light carbon source that preferentially affected epicratonic environments in contrast to epicratonic environments in the paleozoic where input of isotopically light carbon is generally attributed to input of terrestrial organic matter we suggest that in situ remineralization of organic carbon via anaerobic microbial cycling drove observed isotopic variability within epicratonic el mreiti group strata the extent of organic matter remineralization and thus degree of 13 c depletion is correlated with water depth and associated with distinct differences in both total organic carbon toc and pyrite concentration suggesting a potential linkage to both the persistence of anoxia and the availability of sulfate within epicratonic environments in such settings the isotopic effects of organic carbon remineralization are potentially enhanced by either methane oxidation following methanogenic decomposition or the intermittent oxidation of an enhanced dissolved organic carbon doc pool

structure and magnitude of the carbon isotope excursion during the paleocene eocene thermal maximum abstract the paleocene eocene thermal maximum petm was a geologically rapid and transient warming event that occurred ca 56 million years ago ma it was associated with a pronounced negative carbon isotope excursion cie and with profound changes in the atmosphere hydrosphere and biosphere this event reflects injections of large amounts of carbon into the atmosphere ocean system but whether single or multiple episodes of carbon release caused this event is uncertain here we present carbon isotope data of bulk carbonate from two parallel limestone sections at tingri south tibet where the limestone was deposited on a tropical carbonate ramp during the petm carbon isotope data of bulk carbonate from both sections are almost identical displaying an expanded stepped cie profile with large magnitude the stepped cie profile consists of three discrete intervals of decreasing carbon isotope values which can be correlated on an inter regional scale between sections from different depositional environments lake carbonate ramp continental slope and open ocean the consistency of these correlations implies occurrence of at least three carbon releases within the cie and indicates complex processes of carbon injection during the petm moreover the cie magnitude of 7 recorded at tingri is tentatively interpreted to reflect carbon isotope changes in the atmosphere and the surface ocean which is larger than the generally accepted value of 4 but is still within the range of some previous records

carbonate chemostratigraphy of the vazante group brazil a probable tonian age abstract the thick carbonate siliciclastic succession of the vazante group was deposited on the western border of the sao francisco craton central brazil which was deformed during the brasiliano orogeny that ended at 540 ma in this research we studied five drill cores and outcrops to obtain integrated isotope data 13ccarb 18ocarb and 87sr 86sr as well as u pb ages of detrital zircons to understand the chronostratigraphic relationships between the tonian and cryogenian periods the vazante group forms a succession of more than 3 km thick of limestones dolostones shales and rare sandstones these rocks were deposited on a marine shelf that shows distinct 13c variations throughout the stratigraphic succession the lower limestone of this group lagamar formation has low positive 13c values 0 3 1 0 and 87sr 86sr ratios of 0 70646 0 70692 with 423 1 719 ppm sr and possible vase shaped microfossils vsms that may be equivalent to those found in huaibei group in northern china lower tonian or has stratigraphic correlations to the vsms bearing units of the upper tonian the serra do poco verde formation has a plateau of moderate 13c positive values 0 0 4 followed by dolostones of the morro do calcario formation which is a 50 m thick interval with negative values as low as 4 the upper unit of the vazante group serra da lapa formation begins with a sharp negative 13c excursion locally overlying a few layers of diamictite which may be the equivalent of the sturtian glacial age restrictions on establishing the depositional age of the vazante group continue however the combination of biostratigraphic and chemostratigraphic data and the detrital zircons spectra suggest a tonian age for the vazante group

reos and s isotopic constraints on the origins of two mineralization events at the tangdan sedimentary rock hosted stratiform cu deposit sw china abstract the tangdan stratiform cu deposit in the kangdian region of south china is hosted in sedimentary rocks of the late paleoproterozoic to early mesoproterozoic dongchuan group orebodies mainly occur in dolostone and black shale above a thick sequence of hematitic sandstone and siltstone mineralization consists mainly of bedding parallel stratiform ores with minor amounts of discordant vein type ores both stratiform and vein type ores are composed of bornite and chalcopyrite with minor chalcocite and digenite associated with gangue minerals including ankerite dolomite calcite and quartz cu sulfides from vein type ores contain 122 293 ppb re much higher than those of stratiform ores 0 6 14 ppb re they have highly radiogenic os with re os model ages ranging from 1413 to 1457 ma and a weighted mean re os model age of 1432 19 ma 2 mswd 0 30 which is within uncertainty in agreement with isochron ages of 1401 30 ma 2 mswd 0 67 and 1397 71 ma 2 mswd 0 22 using 187 re 188 os versus 187 os 188 os and 187 re versus 187 os regression respectively in contrast sulfides from stratiform ores did not yield meaningful model ages due to the extremely low concentrations of re and radiogenic os however early studies have shown that the stratiform ores were likely formed at 1700 ma during the diagenesis of the host strata sulfides from the vein type ores have 34 s values ranging from 15 to 8 significantly lower than those of stratiform ores 3 to 10 suggesting that organic matter may have been involved in the formation of the vein type ores contrasting re os and s isotopes suggest that the two types of ores at the tangdan deposit were likely formed from different ore forming fluids we propose that stratiform ores were precipitated from basinal fluids during the early evolution of the rift basin whereas the vein type ores were formed due to a much younger thermal event during which hydrothermal fluids interacted with overlying black shale to account for the high re and negative 34 s values this study demonstrates that re os isotopes of sulfides can be used to study the origin of hydrothermal system

differential unroofing within the central metasedimentary belt of the grenville orogen constraints from 40ar 39ar thermochronology an 40ar 39ar thermochronological investigation of upper greenschist to granulite facies gneiss amphibolite and marble was conducted in the central metasedimentary belt cmb ontario to constrain its cooling history incremental 40ar 39ar release spectra indicate that substantial differential unroofing occurred in the cmb between 1000 and 600 ma a consistent pattern of significantly older hornblende and phlogopite 40ar 3ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation variations in hornblende 40ar 39ar age plateaus exceeding 200 ma occur over distances less than 50 km with major age discontinuities occurring across the robertson lake shear zone and the sharbot lake mylonite zone which separate the sharbot lake terrane from the elzevir and frontenac terranes extensional displacements of up to 14 km are inferred between the frontenac and elzevir terranes of the cmb no evidence for significant post argon closure vertical displacement is indicated in the vicinity of the perth road mylonite within the frontenac terrane variations of nearly 100 ma in phlogopite 40ar 39ar plateau ages occur in undeformed marble on either side of the bancroft shear zone phlogopites from sheared and mylonitized marble within the shear zone yield 40ar 39ar diffusional loss profiles but have older geologically meaningless ages thought to reflect incorporation of excess argon by 900 ma southeast directed extension was occurring throughout the cmb possibly initiated along previous zones of compressional shearing an easterly migration of active zones of extension is inferred possibly related to an earlier overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone the duration of extensional shearing is not well constrained but must have ceased before 600 ma as required by the deposition of overlying undeformed cambrian and or ordovician sedimentary rocks

geochronology of early to middle proterozoic fold belts in northern australia a review abstract this geochronological review integrates geologically relevant isotopic data from the several early to middle proterozoic fold belts of northern australia a number of firm chronological benchmarks largely established by means of u pb zircon studies now provide a time framework within which to better quantify the tectonostratigraphic history of many of these terranes the apparent worldwide dearth of crustal events in the earliest proterozoic between 2500 and 2000 ma was broken in northern australia with the advent of a major crust forming episode between 1880 and 1850 ma ago these rocks may have evolved from an initial mantle or lower crustal melting and fractionation process about 400 300 ma earlier that is evidenced from sm nd model ages the extent involvement and or degree of assimilation of any pre existing archaean crust is small but whether or not it is negligible or zero is not satisfactorily known the 1880 1850 ma event the barramundi orogeny is well represented in northern australia both lithologically and analytically and the rocks that pre and post date it have a similar tectonic setting over very wide areas of the continent two consistent and widespread elements of this tectonism are regional metamorphism and deformation of the first cycle of supracrustals barramundi orogeny and crystallization of i type k rich felsic magmas closely following and in some cases overlapping with deformational processes in the halls creek inlier the ages of this rapid deep crustal to supracrustal tectonic transition cannot be distinguished within the range 1860 1850 ma this major yet short lived tectonism is mirrored in other fold belts such as pine creek tennant creek and mount isa in which u pb zircon ages give slightly older constraints of 1885 1870 ma 1920 1870 ma and 1885 10 ma respectively the ensuing felsic magmatism in these latter terranes continued for a few tens of millions of years with the crystallization and emplacement of comagmatic volcanic plutonic pairs throughout in the interval 1870 1860 ma in the georgetown and arunta inliers direct evidence of this otherwise ubiquitous event if present is masked by younger high grade metamorphism in the latter however metamorphic crustal components having approximate protolith ages of this order can be inferred from their evolved isotopic signatures the geochronological and geochemical coherency of the extensive early proterozoic 1870 ma magmatism contrasts with that of post 1800 ma magmatism which is less extensive generally anorogenic commonly bimodal in character and varies in age from one terrane to another several major sedimentary basins were being filled at this time major igneous events at 1790 1740 ma 1670 ma and 1500 ma are recognized elongate sublinear volcanic and plutonic belts 1790 1740 ma old are present at mount isa in high grade granitic gneisses of the arunta inlier a few u pb zircon ages and extrapolated rb sr protolith ages are coincident at about 1760 1750 ma indicating that this event may be important in the development of the division 2 sequences of that fold belt the platformal hart dolerite and oenpelli dolerite two of the largest mafic intrusions in the world were emplaced at 1762 25 ma and 1688 13 ma younger felsic plutonism and volcanism gave rise to rapakivi type granites in the mount isa tennant creek and the granites tanami inliers at 1670 ma a time close to that of the aileron deformational metamorphic event in central australia further at 1650 ma there is growing evidence of an unexpectedly coherent hydrothermal episode that reset or disturbed many rb sr ages gave rise to pegmatites and was possibly associated with mineralization in the davenport and eastern arunta inliers discrete middle proterozoic deformational events between 1610 and 1470 ma punctuate the tectonostratigraphic history in the arunta mount isa and georgetown fold belts where they are best documented

c isotope composition of carbonates from indravati basin india implications for regional stratigraphic correlation abstract the indravati basin represents an unmetamorphosed and unfossiliferous sequence and shows a broad similarity in lithological association with other purana basins of central india the carbonates belonging to indravati basin have on average moderately positive 13 c values up to 4 5 numerous successions reported from other parts of world show similar 13 c values and contain evidence to suggest that the latest mesoproterozoic to early neoproterozoic period may represent an isotopically recognizable interval globally the elevated 13 c values of indravati basin are also correlatable with other purana basins in india including upper vindhyan supergroup

detrital zircon record of meso and neoproterozoic sedimentary basins in northern part of the siberian craton characterizing buried crust of the basement abstract we present new la icp ms detrital zircon data from meso and neoproterozoic sedimentary basins located in the northern and western parts of the siberian craton along the western cratonic margin turukhansk uplift northern yenisei ridge the basins accumulated predominantly 2 6 2 5 ga and 1 9 1 85 ga erosional products while the main sources for the fill of intracratonic basin to the northeast near the anabar shield east anabar basin were 2 9 2 7 ga and 2 1 1 95 ga old igneous rocks the studied meso to early neoproterozoic sandstones were deposited in rift related or passive margin settings and underwent major craton wide recycling to produce late ediacaran silicilastic successions meso to early neoproterozoic sandstones are immature to submature pointing to erosion of proximal archean and paleoproterozoic crustal units the unique detrital age spectra for the northeast and western basins indicate provinciality and provide a basis for unravelling the age of buried domains of the siberian craton basement

biostratigraphic and chemostratigraphic constraints on the age of early neoproterozoic carbonate successions in north china abstract thick carbonate successions of proterozoic age were deposited in the huaibei and huainan regions along the southern margin of the north china block these successions including the huaibei group in the huaibei region and the huainan and feishui groups in the huainan region have been correlated with the qingbaikou group which outcrops in the jixian area of north china and is the eponymous succession of the qingbaikouan system traditionally regarded as 1000 800 ma recently it has been shown that lower qingbaikouan strata in the jixian area may be as old as 1300 1400 ma by correlation the huaibei huainan and feishui groups may include mesoproterozoic strata but these units have also been interpreted as tonian or cryogenian ediacaran cambrian in age these contradictory age interpretations hamper our ability to explore the rich paleontological and sedimentary records preserved in these rocks and to assess paleogeographic reconstructions in this report we describe new microfossils and stable isotope data 13ccarb 18ocarb 34scas from the huaibei huainan and feishui groups possible vase shaped microfossils from the jiayuan formation of the lower huaibei group and characteristic early neoproterozoic acritarchs particularly trachyhystrichosphaera aimika from the gouhou and liulaobei formations are consistent with a tonian age for the huaibei and huainan groups similarly carbon isotope trends in the huaibei huainan and feishui groups are comparable with tonian 13ccarb patterns in south australia and laurentia with a plateau of moderately positive 1 5 values followed by pronounced negative 13ccarb excursion in the gouhou formation which may be equivalent to the bitter springs anomaly in south australia 34scas values of the huaibei group are mostly 25 40 similar to values from the stratigraphically equivalent jingeryu formation in the jixian area overall the combination of biostratigraphic chemostratigraphic and available radiometric data suggests that the huaibei huainan and feishui groups are early neoproterozoic tonian in age

evolution and environmental conditions of conophyton jacutophyton associations in the atar dolomite upper proterozoic mauritania within the upper proterozoic sedimentary cover of the west african craton well exposed in the atar area mauritania the carbonate rich atar formation exhibits a peculiar development of stromatolites this formation is subdivided into 6 units based on lithology and stromatolite content units 1 3 and 5 are mainly terrigenous shales and silts and contain carbonate layers of different facies laminated and structureless micrite microbially laminated and detrital carbonates stromatolitic layers the environmental conditions were characterized by low energy sporadically increasing and shallow water with rare desiccation features units 2 and 4 contain conophyton jacutophyton and tilemsina unit 6 has a succession of conophyton jacqueti to baicalia these carbonate units were deposited under water in low energy conditions without any allochthonous sediment supply the rhythmic alternation of terrigenous and carbonate units corresponds to rhythmic variations in environmental conditions related to sea level or climatic changes the morphological diversity of stromatolites is not related systematically to changes in the environment some of the morphological variations are controlled by the evolution of the biological community e g the evolution of conophyton shape with time and its eventual disappearance new forms of stromatolite are described baicalia safia conophyton jacqueti jaculophyton sahariensis and tilemsina agdeireia

carbonate rocks and related facies with vestiges of biomarkers clues to redox conditions in the mesoproterozoic ocean abstract the raipur group of the chattisgarh basin preserves two major late mesoproterozoic carbonate platforms the lower platform is about 490 m thick separated from the upper platform 670 m thick by a 500 m thick calcareous shale carbonate strata cover almost 40 of the chattisgarh basin outcrop and represent two major platform types a a non stromatolitic ramp the charmuria sarangarh limestone and b a platform developed chiefly in the intertidal to shallow subtidal environment with prolific growth of stromatolites the chandi saradih limestone the first platform consists primarily of the black timarlaga limestone that is locally replaced by early diagenetic dolomite this carbonate platform experienced strong storm waves and was subsequently drowned by a major transgression during which extensive black limestone marl rhythmite was deposited followed by deposition of the gunderdehi shale the carbonate factory was later re established with development of an extensive stromatolite dominated charmuria sarangarh platform that ranged from restricted embayment to open marine conditions sea level change played a major role in controlling the broad facies pattern and platform evolution the 13 c signatures of the chattisgarh limestones falling within a relatively narrow range 0 to 4 are typical for upper mesoproterozoic carbonate rocks 18 o values however have a greater range 5 7 to 13 3 indicating significant diagenetic alteration of some samples likely dysoxic or anoxic conditions prevailed during deposition of the black timarlaga limestone and well oxygenated conditions during deposition of the gunderdehi shale and saradih chandi stromatolite the lack of 17 21 moretanes and high t max values suggest mature organic matter in the non stromatolitic ramp a paucity of diagnostic eukaryotic steroids indicates that algae were rare in the chattisgarh basin a high content of hopanes supports a generally bacterially dominated proterozoic ocean in which various stromatolites flourished

a billion years of environmental stability and the emergence of eukaryotes new data from northern australia carbon isotopes through 6km of fully cored drill holes in 1 7 to 1 5 ga carbonates of the mount isa and mcarthur basins australia which host the earliest known eukaryote biomarkers provide the most comprehensive and best dated delta 13c stratigraphy yet obtained from such ancient rocks both basins reveal remarkably stable temporal delta 13c trends mean of 0 6 2 pdb peedee belemnite and confirm the impression of delta 13c stasis between 2 0 and 1 0 ga which together with other evidence suggest a prolonged period of stability in crustal dynamics redox state of surface environments and planetary climate this delta 13c stasis is consistent with great stability in the carbon cycle controlled we suggest by p limitation of primary productivity recent evidence shows that p depletion is a major factor in obligate associations between photosymbionts and host cells we argue that a billion years of stability in the carbon and nutrient cycles may have been the driving force that propelled prokaryotes toward photosymbiosis and the emergence of the autotrophic eukaryote cell

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

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

the baltica laurentia connection sveconorwegian grenvillian metamorphism cooling and unroofing in the bamble sector norway the sveconorwegian bamble sector of south norway is a segment of the high grade metamorphic basement of the baltic shield generally considered an integral piece of the global grenvillian orogenic belt in particular the bamble sector is generally directly linked to the grenville province of north america in middle proterozoic tectonic reconstructions however few high precision data exist on the metamorphic chronology of the bamble sector to test such models in this paper high precision u pb data for monazite allanite and titanite indicate that rocks from the bamble sector near arendal underwent granulite facies metamorphism at 1140 ma a second phase of metamorphism may be indicated by u pb titanite ages of 1105 ma or alternatively these may simply record delayed cooling or growth of titanite below temperatures for significant lead diffusion hornblende 40ar 39ar ages indicate cooling of the same rocks below 550 c between 1090 and 1080 ma metamorphic cooling rates of 3 8 c ma between 725 an

a possible sequence of events for the generalized glacial interglacial cycle 1 there is not yet widespread agreement as to the underlying cause of the 80 100 ppmv roughly 100 kyr duration glacial interglacial cycles in atmospheric pco2 most of the mechanisms which have been proposed to account for the observed pco2 variations appear to in some way violate interpretations of paleo proxy data the inability of a single mechanism to explain the observed cycles in atmospheric co2 which show amazing similarity over the past 430 000 years is perplexing and leads us to consider whether a combination of mechanisms might be consistent with available evidence consistent with previous work we find that physical changes ocean circulation temperature mixing can only explain part of the observed atmospheric pco2 variability changes in ocean chemistry are invoked to explain the remainder in order to account for the initial pco2 drawdown from interglacial to intermediate levels we invoke physical changes in the ocean mixing temperature the transition from intermediate atmospheric pco2 levels to full glacial conditions involves a small increase in mean ocean nutrient levels and mean ocean alkalinity accomplished by falling sea level and subsequent erosion of organic rich shelf sediments the first part of the transition out of full glacial conditions is achieved through increased temperature and increased mixing in the southern ocean the final part of the atmospheric pco2 rise up to full interglacial conditions is accomplished through rising sea level and the subsequent change in mean ocean alkalinity and phosphate and a rise in the northern hemisphere temperature and ocean mixing the proposed sequence of events is consistent with most existing proxy evidence for paleo nutrient levels and changes in export production over the last glacial interglacial cycle furthermore it is consistent with evidence for a whole ocean shift in 13c toward significantly more negative values in the late glacial the proposed scenario is also consistent with ice core based timing constraints as summarized by broecker and henderson 1998 we show that we are able to explain the full magnitude of the glacial interglacial cycle in atmospheric pco2 without the need to invoke iron fertilization in the southern ocean

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

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

oxygen carbon and strontium isotope records of ediacaran carbonates in central iberia spain carbon oxygen and strontium isotope ratios were determined in the uppermost neoproterozoic carbonate rocks at pastores sw salamanca central iberia the low 18o v smow values 15 19 indicate an alteration of the whole stratigraphic series however the alteration process does not seem to have affected most of the 13c and 87sr 86sr data these data reveal that only one particular level of the carbonate section shows simultaneous relative increases in mn sr and 87sr 86sr ratios decreases in the 18o and 13c values and covariations between 87sr 86sr and the other parameters despite this most of the carbonate section lacks these covariations and its 13c 4 6 to 0 5 and 87sr 86sr 0 70845 0 70853 ratios probably reflect the seawater isotopic features at the time of sedimentation the data for c and sr isotopes at the pastores carbonate section and the presence of cloudina in platform carbonates together with an overlying major sub cambrian unconformity following a relative sea level fall are features comparable to those frequently shown by proterozoic cambrian boundary series across the world thus our results consistently support the notion that the carbonates at pastores are equivalent to other carbonates deposited close to the ediacaran cambrian boundary

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

geochemical preservation potential of high grade calcite marble versus dolomite marble implication for isotope chemostratigraphy the neoproterozoic hekkelstrand marble from the north central norwegian catedonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit which underwent polyphase amphibolite facies scandian metamorphism major and trace element geochemistry delta 18 o delta 13 c and sr 87 sr 86 ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post depositional isotopic resetting a high strontium concentration in the calcite marble up to 1200 ppm suggests an aragonitic lime mudstone precursor and its least altered delta 13 c 4 parts per thousand v pdb delta 18 o 27 parts per thousand v smow and sr 87 sr 86 0 70615 to 0 70624 ratios are consistent with 700 600 ma seawater the dolomite marbles contain minor relics of the calcite aragonite precursor despite pervasive dolomitisation the average mn 44 ppm and fe 1041 ppm contents of the dolomite marbles are comparable with those measured in the host calcite marbles and much lower than those reported from non metamorphosed dolostones of a similar neoproterozoic age the dolomite marbles have an average sr content of 115 ppm an average mg ca ratio of 0 56 is lower than that for stoichiometric dolomite 0 62 and is controlled mainly by remnant inclusions of precursor calcite sr 87 sr 86 ratios range between 0 70725 and 0 71034 the least altered delta 13 c of 6 parts per thousand and delta 18 o of 30 parts per thousand suggest a high degree of isotopic preservation of the dolomite the dolomite marbles are isotopically heavier both delta 18 o and delta 13 c by 2 parts per thousand on average compared with the calcite marble thus suggesting a pre metamorphic lower temperature isotopic equilibrium the dolomite was apparently originally precipitated from 600 to 700 ma seawater dominated fluids the sr 87 sr 86 ratio was modified by post depositional processes the high delta 13 c and delta 18 o values and the low mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration the highest sr 87 sr 86 ratio of 0 70868 suggests that the earlier dolomite was apparently modified by ca 560 ma seawater or by meteoric water considerably buffered by ca 560 ma limestones the most significant geochemical and isotopic modification of the dolomite took place during metamorphism this has been observed across the dolomite marble silicate rock contacts over a limited distance of lt 0 5 in where c and 0 isotope systems were reset by up to 4 5 parts per thousand and 9 7 parts per thousand respectively the sr 87 sr 86 ratio increased by up to 0 0012 whereas the fe mn and sr concentrations have increased by factors of similar to 80 similar to 15 and similar to 1 5 respectively the dolomite marbles despite amphibolitc facies polyphase metamorphic alterations have preserved high delta 18 o and delta 13 c values which can be utilised for chronostratigraphic purposes whereas sr 87 sr 86 ratios provide insights into the composition of the post depositional fluids

neoproterozoic early cambrian isotopic variation and chemostratigraphy of the lesser himalaya india eastern gondwana the rodinia supercontinent had fragmented by 750 ma and east gondwana india australia and antarctica separated from west laurentia baltica africa and south america occupied other side of the rodinia neoproterozoic rifting breakup of rodinia low latitude glaciation and global warming events have been recorded from the lesser himalaya of india chemostratigraphy of the blaini krol tal succession indicates precambrian cambrian transition lies in the lower tal formation 13 c 4 pdb the krol belt in the lesser himalaya is characterized by positive 13 c values 1 to 6 pdb the appearance of multicelluler ediacaran life in the upper krol is consistent with hypothesis that increase in atmospheric oxygen played a major role in metazoan evolution and cambrian diversification of life on earth the base of the terminal proterozoic system in the lesser himalaya is established in the blaini formation the pink cap carbonate of the blaini formation shows negative 13 c value 3 pdb and this blainian glaciation is correlated with marinoan glacial event other regions of the eastern gondwana and south china parts of siberia and north africa neoproterozic early cambrian chemostratigraphy in the lesser himalaya is consistent in the isotopic variation where there is carbon isotopic similarity between neoproterozoic bambui group in central brazil south america and the krol formation of the lesser himalaya india in the northeastern lesser himalaya india the neoproterozoic sedimentary succession shows well developed carbonate the buxa dolomite shows significantly positive c isotope ratios 13 c 3 7 to 5 4 pdb the o isotopic data also shows remarkable consistency with the 18 o values fluctuating within a narrow range between 8 9 and 7 2 vpdb these isotopic results from the eastern lesser himalaya correspond to the terminal proterozoic c isotopic evolution followed by oscillations during the precambrian cambrian transition in the lesser himalaya in eastern gondwana

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

isotopic composition of organic and inorganic carbon from the mesoproterozoic jixian group north china implications for biological and oceanic evolution a b s t r a c t analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of earth surface environments particularly in the latter portion of the proterozoic eon earlier mesoproterozoic successions however have received comparatively little attention due to the relatively subdued nature of carbon isotope variation in this study we present high resolution isotopic profiles from three sections in the yanshan basin north china craton that combined comprise the entirety of the early mesoproterozoic 1600 1400 ma calymmian period jixian group high resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con strain both the pattern and origin of isotopic variation in the mesoproterozoic marine carbonate rocks of the jixian group show generally muted isotopic variation with average values near 0 consistent with previous observations from the early mesoproterozoic data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long term decrease in pco2 and conse quently in the isotopic buffering capacity of marine dissolved inorganic carbon dic by contrast the isotopic composition of marine organic matter suggests facies dependent differences in carbon cycling organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom position in shallow water environments and increased remineralization by anaerobic heterotrophs in deeper water environments correlation between organic carbon composition and depositional environ ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters

quantifying the dynamic evolution of graded gravel beds using particle tracking velocimetry abstractthe motion of graded gravels under steady and spatially uniform turbulent flow is investigated in laboratory conditions using particle tracking velocimetry ptv the gravel bed is subjected to flows approaching critical armor velocity and water worked until static armor is reached a digital particle tracking dpt algorithm based on image subtraction between subsequent frames is developed and applied on the particle scale as the graded gravel bed is water worked the dynamic evolution of graded gravels is quantified continuously for up to 8 h characteristics of particle motion are identified and movement patterns under complex flow conditions are analyzed particle tracking is applied to low frequency 0 67 fps and high frequency 30 fps data this study provides a new perspective on gravel particle transport distances trajectory sinuosity sediment transport rate fractional sediment transport rates particle movement patterns and instantaneous particle velocities

cooling and inferred uplift erosion history of the grenville orogen ontario constraints from 40ar 39ar thermochronology stepwise 40ar 39ar degassing experiments of 57 mineral separates of hornblende muscovite biotite and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the grenville orogen in ontario the 40ar 39ar data support an interpretation of slow nearly uniform cooling 1 4 c m y from temperatures of 500 c to below 150 c in the central gneiss belt cgb hornblendes cooled through ar closure between 930 and 1025 ma whereas in the central metasedimentary belt cmb hornblendes record the following range in 40ar 39ar cooling ages 1104 ma in the frontenac terrane 1007 1067 ma in the sharbot lake terrane 919 1026 ma in the elzevir terrane and 972 ma in the central metasedimentary belt boundary zone regional uplift erosion rates of 0 03 0 14 km m y have been estimated for the grenville orogen in ontario based on the 40ar 39ar data a model retrograde p t path for rocks of the cgb and an upper time constraint provided by flat overlying cambrian and ordovician sediments these erosion rates are consistent with rates estimated for other proterozoic or archean granulite terranes but are an order of magnitude slower than active orogens such as the alps and himalayas a regular variation in hornblende 40ar 39ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the cmb the pattern of 40ar 39ar ages is interpreted to reflect late tectonic extension consistent with field observations in the central metasedimentary belt boundary zone and elsewhere in the cmb up to 13 km of vertical displacement is inferred for some rocks in the cmb between the time they cooled below closure to argon diffusion in hornblende 500 c and their exposure at the surface 25 c

stratigraphic correlations in mid to late proterozoic carbonates of the democratic republic of congo using magnetic susceptibility abstract in this paper we have tested the application of magnetic susceptibility measurements in cu ag zn pb fe mineralized carbonates of the bie subgroup democratic republic of congo as an efficient tool for regional and global high resolution stratigraphic correlations in the neoproterozoic marine carbonates to achieve this goal we integrate the low field magnetic susceptibility xlf data with facies analyses geochemistry and isotope stratigraphy the microfacies analyses of two cores tshinyama s70 and kafuku 15 drilled in the early neoproterozoic carbonates of the mbuji mayi supergroup reveal a deep carbonate ramp setting associated with a microbial stromatolitic mid ramp environment high resolution stratigraphic correlations using magnetic susceptibility and c isotope curves established for both cores 190 km apart suggest a sedimentary hiatus at the base of the tshinyama s70 succession c o and sr isotopes and sr ca and fe abundances show that a diagenetic meteoric overprint affected the series of the tshinyama s70 core and a thermal effect related to mineralizing fluids affected the kafuku 15 core carbonates

radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late mesoproterozoic borden basin arctic canada

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

geochemistry of precambrian carbonates vii belt supergroup montana and idaho usa carbonates from the 1100 1450 ma old proterozoic belt supergroup were collected from stratigraphic sections throughout montana and idaho usa the sampled sequences in ascending stratigraphic order include the newland altyn spokane greyson transition empire wallace helena siyeh snowslip shepard and libby formations an increase in the degree of postdepositional alteration of belt limestones is reflected in a diminution of sr and mg contents an increase in mn and depletion in 13c and 18o two diagenetic trends can be resolved for the limestones one affecting the presumed originally aragonite rich sediments includes carbonates from the lower belt newland formation in contrast the middle belt carbonate wallace helena siyeh formations may have been originally of high mg calcitic mineralogy projection of the alteration trends for the lower and middle belt limestones suggest 21 smow and 2 5 to 1 0 pdb as the best preserved values for the 18o and 13c of seawater respectively both comparable to results from other mesoproterozoic carbonate sequences the oxygen isotope data for limestones show a regional westward depletion of 8 in 18o possibly reflecting a higher temperature of postdepositional alteration in the western belt basin this depletion in 18o is accompanied by a comparable decrease in 13c values most likely because a higher proportion of carbon was incorporated from co2 generated by thermal cracking of hydrocarbons at depth dolostones in the belt basin are dominantly micritic with good preservation of depositional textures chemically and isotopically their alteration trends mimic those of limestones leading to comparable projected best values for 13c 87sr 86sr values of belt carbonates range between 0 70484 and 0 74991 progressive diagenesis as indexed by decreasing concentrations of sr and depletions in 18o and 13c results in an increase in 87sr 86sr values the least radiogenic measurement from a lower belt newland limestone sample appears to fit reasonably well into the general trend for proterozoic seawater as discussed in mirota and veizer 1994 this and the consistency of 18o and 13c in limestones with other coeval sequences suggests that the lower and middle belt carbonate sections are marine deposited in an environment that was not continuously separated from the open ocean

sulfur and carbon isotope records from 1700 to 800 ma carbonates of the jixian section northern china implications for secular isotope variations in proterozoic seawater and relationships to global supercontinental events abstract this study is a comprehensive stable isotope survey of the marine carbonate dominated upper paleo to lower neoproterozoic stratigraphy of jixian county china carbonate associated sulfate cas was extracted and measured for 34 s cas using the same samples analyzed for 13 c carbonate this integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of proterozoic seawater we specifically sampled marine carbonate intervals from the lower section of the chuanlinggou formation changcheng group ca 1700 ma to the top of the jingeryu formation qingbaikou group ca 800 ma 13 c carbonate values are mostly negative in the upper paleoproterozoic changcheng group with an ascending trend from 3 to 0 we observed variation of approximately 0 1 in the mesoproterozoic jixian group and positive values of 2 2 characterize the lower neoproterozoic qingbaikou group stratigraphic variations in 34 s cas are more remarkable in their ranges and magnitudes including conspicuously high values exceeding 30 in the three intervals at ca 1700 ma 1300 1100 ma and 1000 900 ma in the changcheng group 34 s cas values are typically higher than 25 with only a few values of less than 15 in contrast most of the data spanning from the mesoproterozoic tieling formation of the jixian group to the lower neoproterozoic jingeryu formation of the qingbaikou group are highly variable between 10 and 25 with some values exceeding 25 in the late paleoproterozoic 1700 1600 ma a 10 decrease in 34 s cas and 3 increase in 13 c carbonate are coincident with and likely related to the breakup of columbia a supercontinent that predated rodinia carbon and sulfur isotope data from the mesoproterozoic when global tectonic activity was comparatively weaker fall mostly in the ranges of 15 10 and 0 1 respectively but fluctuations of 20 for 34 s cas and 3 for the 13 c carbonate at ca 1450 1400 ma may reflect subduction and large scale magmatic activity in island arcs marking the end of columbia breakup from the late mesoproterozoic ca 1300 1100 ma to the early neoproterozoic ca 800 ma the 13 c and 34 s of seawater increased gradually with increasing variability most impressive are 34 s cas values that exceed 30 in two intervals at ca 1300 1100 ma and ca 1000 900 ma which may reflect the assembly and early breakup of rodinia although gaps in the record remain and studies of even higher resolution are warranted our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of c and s furthermore periods of the proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity

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

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

petrogenesis of 1 9 ga limestones and dolostones and their record of paleoproterozoic environments abstract fibrous calcite cements from limestones of the paleoproterozoic pethei group in northwest canada have relatively constant 13 c values near 1 that indicate the c exogenic cycle of ca 1 9 ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the phanerozoic 87 sr 86 sr ratios of these cements are near 0 7051 and support a progressively increasing influx of radiogenic sr from continental weathering into the paleoproterozoic oceans 18 o values of the cements vary from 14 to 8 5 although the original values of the cements were probably higher because the cements have been subseqently altered as evidenced by their variable trace element contents proposed higher temperatures or substantially lower 18 o values of ancient seawater based on such values may be in error fabric retentive dolomites generally regarded as syndepositional also have variable 18 o values the highest of which are ca 30 greater than the highest values of the fibrous calcite cements the dolomites have more radiogenic sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones these fluids may have partially altered the 18 o values and chemical compositions of the fibrous calcite cements late stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar 18 o values and radiogenic sr isotopic ratios as the fabric retentive dolomites if these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions

molar tooth structures in calcareous nodules early neoproterozoic burovaya formation turukhansk region siberia abstract molar tooth structures are abundant in large 1 2 m diameter carbonate nodules within fine grained subtidal carbonates of the early neoproterozoic lower upper riphean burovaya formation along the sukhaya tunguska river turukhansk uplift northwestern siberia although molar tooth structures are regionally abundant in this unit here they occur only within the nodules stable isotopic compositions of molar tooth filling dolomicrospar cements and of thinly bedded dolomicrite within and surrounding the nodules are indistinguishable from one another the carbon isotopic compositions mean 13c 2 8 pdb 0 4 reflect mean average oceanic surface water composition during their formation the light oxygen isotopic compositions mean 18o 6 4 pdb 2 2 are generally similar to those of other little altered meso to neoproterozoic limestones and dolostones these molar tooth structures have no features that would support a tectonic origin they more likely formed through bacterial processes carbonate cement filling of these voids occurred soon after their formation but the mechanism responsible for this carbonate precipitation is currently uncertain local restriction of molar tooth structures to early diagenetic nodules suggests that penecontemporaneous lithification was required for the formation or at least preservation of these widespread mesoproterozoic to neoproterozoic features

stable isotope geochemistry of the lower cambrian sekwi formation northwest territories canada implications for ocean chemistry and secular curve generation abstract three curves delineating 13 c variations across the early cambrian sekwi formation carbonate ramp mackenzie mountains northwest territories canada are incorporated into a biostratigraphic and sequence stratigraphic framework that provides a higher resolution temporal and spatial framework for the sekwi formation the three 13 c curves were correlated across a range of depositional environments from tidal flat to deep subtidal on the sekwi carbonate ramp eight 13 c cycles in the curves a h were correlated across the basin complimenting the sequence stratigraphic framework the most complete 13 c curve from the sekwi formation correlates well with a composite early cambrian 13 c curve from the siberian platform however the differing magnitude and absolute timing of some of the correlative isotopic excursions records the influence of regional tectonism sea level changes and diagenesis on the sekwi ramp additionally the sekwi formation 13 c curve has more high frequency variations than the siberian curve due in part to the higher sample density used to construct the sekwi curve the early middle cambrian boundary was not sampled in this study but a large negative isotopic excursion in the very latest early cambrian coincides with a regional drowning that may be correlative throughout western laurentia but recognizing this event globally is problematic

sequence organization and growth patterns of late mesoproterozoic stromatolite reefs an example from the burovaya formation turukhansk uplift siberia abstract the latest mesoproterozoic burovaya formation is a 600 1000 m thick stromatolite mud dominated carbonate unit demarcated by an erosional unconformity into two subformations containing stromatolite reefs of variable size and geometry the lower subformation consists of two distinct stratigraphic sequences the lower sequence records shelf to ramp evolution and includes small scale stromatolite reefs developed within an upper subtidal shelf settings at the transgressive systems tract and overlying mud dominated highstand deposits accumulated in lower subtidal to peritidal environments the second sequence of muddy and fine grained sediments makes up a lowstand systems tract and documents a progressive structural and environmental differentiation of the burovaya platform the upper subformation records a third stratigraphic sequence containing transgressive and highstand systems tracts represented by reef bearing and reef cap facies complexes the reef bearing complex characteristic of the transgressive and the bulk of the highstand systems tracts contains a giant 20 25 km wide 10 km long up to 550 m thick stromatolite reef and surrounding deep water carbonate mudstones containing sporadic slump block deposits the reef was located on the distally steepened part of the ramp and contains distinct internal facies organization defined by particular stromatolite microstructure and morphology and prevailing environments late highstand development of the burovaya platform was marked by the termination of reef accretion infilling of basinal relief by shallow water carbonate and shale sediments of the cap complex and subsequent subaerial exposure the history of the burovaya reef is indicative of the interplay between reef growth potential and platform geometry defined by regional tectonic and sea level dynamics

simple models for the geochemical response of the ocean to climatic and tectonic forcing abstract simple models for the geochemical response of the ocean to periodic forcing are used as a framework to discuss the shortest periods of change resolvable by present analytical precision of elemental or isotopic ratio measurements of the chemical evolution of seawater for chemical properties whose budget is dominated by fluxes into and out of the ocean we show that the geochemical response to changing fluxes is strongly attenuated at periods shorter than the residence time and that the response at these periods is also phase shifted so as to lag the forcing by one quarter cycle the amplitude response as a function of period is used to estimate the periods of forcing resolvable by present analytical precision in measuring the 87sr 86sr and 234u 238u evolution of seawater and we find that the shortest periods resolvable are of the order of 1 myr if variations at periods shorter than this are found we would argue that these are very likely of local as opposed to global origin we illustrate the importance of phase shifts as a function of forcing period by discussing some recently published data that show a high degree of correlation over the last 300 kyr between changes in 87sr 86sr and 18o measured in the same sediment core at first sight this correlation might be taken as good evidence that both 87sr 86sr and 18o are responding to some common climatic forcing but given the long residence time of sr in the ocean sr and o should in fact have different phase shifts relative to any proposed common forcing thus the apparent correlation between 87sr 86sr and 18o over the past several 100 ma is not necessarily an indication that they are responding to a common cause amplitude response curves are also used to place limits on permissible global seawater 234u 238u variations and these are then compared with published data to identify local diagenetic effects application to 187os 186os and u records in seawater detectors is proposed

age constraints on the neoproterozoic baikal group from combined sr isotopes and pb pb dating of carbonates from the baikal type section southeastern siberia abstract depositional ages for the middle part of the neoproterozoic sedimentary succession exposed along lake baikal in the southern margin of the siberian platform have been constrained by the application of sr isotope chemostratigraphy and pb pb dating of carbonate rocks the thick siliciclastic carbonate baikal group is the type section of the regional siberian succession although many of the baikal group carbonate rocks have undergone pervasive dolomitization by mixed meteoric marine fluids oolitic limestones in the 800 m thick carbonate member of the uluntui formation preserve primary textures and high sr contents up to 3200 ppm suggesting that aragonite might have been the original precursor the 87sr 86sr in the least altered high sr limestones mg ca 8 pdb range within 0 70842 0 70876 values we interpret as representing the best proxy for coeval seawater stepwise dissolution using 0 5 n hbr was applied to enrich samples in primary carbonate from the uluntui formation and nine data points from the five least altered high sr limestones yielded a pb pb isochron age of 560 30 ma with mswd 0 6 and probability 0 8 this age matches that inferred from sr isotope stratigraphy 580 550 ma and further supports the inference that the baikal paleobasin was connected to the contemporary oceans our sr chemostratigraphy and pb pb isochron age of the uluntui limestones cast doubt on previously proposed correlations of the baikal group as late mesoproterozoic to cryogenian instead the data indicate that the entire siliciclastic carbonate succession of the baikal group was most likely deposited during the ediacaran or vendian of the russian stratigraphic scale

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

meso neoproterozoic isotope stratigraphy on carbonates platforms in the brasilia belt of brazil abstract carbonate platforms were present worldwide during the proterozoic eon and variations in their c and sr isotope ratios are commonly used as a correlation tool particularly through the neoproterozoic when rapid secular change in marine c and sr isotope values permit distinction between cryogenian glacial events in central brazil the late mesoproterozoic and neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited and later deformed along the eastern margin of the sao francisco craton these strata are divided into the three major groups 1 the paranoa group which consists of a succession of sandstone siltstone rhythmite and selected intervals of carbonate 2 the macaubas group which consists of a glacial diamictite jequitai formation and 3 the bambui group with an important carbonate bearing succession that includes characteristic cap carbonate facies in its lower strata carbonate facies of paranoa and bambui groups typically occur in unconformable contact and when the diamictite of jequitai formation is absent it can be difficult to determine the stratigraphic position of these lithological similar groups furthermore uncertainties in the age of the bambui group has lead to several distinct interpretations regarding the age of the jequitai glacial diamictites we investigated the c o and sr isotopes and chemical composition of carbonate rocks in five measured sections including both pre and post glaciation carbonate successions the 13 c pdb values in the upper paranoa group occur in a narrow interval between 0 6 and 3 6 whereas the post glacial bambui group begins with substantially negative values as low as 5 7 in cap dolomite facies and rises to values up to 11 permil in limestone of the upper sete lagoas formation similarly carbonate rocks of the paranoa and bambui groups are distinct in terms of their 87 sr 86 sr ratio generally non radiogenic ratios between 0 7056 and 0 7068 are recorded in the upper paranoa group and ratios between 0 7074 and 0 7080 occurring within the bambui group the stratigraphic pattern of the c and sr ratios indicates distinctive isotopic characteristics for these two carbonate successions the isotopic data for the cabonates in the paranoa group are consistent with a sedimentation age in upper mesoproterozoic or lowermost neoproterozoic preceding the first cryogenian glaciation carbonate facies and the isotopic data for the lower bambui group suggest a relationship with the second cryogenian glaciation

the spar lake strata bound cu ag deposit formed across a mixing zone between trapped natural gas and metals bearing brine ore formation at the spar lake red bed associated strata bound cu deposit took place across a mixing and reaction zone between a hot oxidized metals transporting brine and a reservoir of sour h2s bearing natural gas trapped in the host sandstones fluid inclusion volatile analyses have very high ch4 concentrations 1 mol in most samples and a sample from the fringe of the deposit has between 18 and 36 mol ch4 the ratio of ch4 co2 in fluid inclusions appears to vary regularly across the deposit with the lowest ch4 co2 ratios from high grade chalcocite bearing ore and the highest from the chalcopyrite bearing fringe the helium r ra isotope ratios 0 23 0 98 and concentrations define a mixture between crustal and atmospheric helium the volatiles in fluid inclusions ch4 co2 h2s so2 h2 h2o and other organic gases and values of f o2 and temperature calculated from the volatiles data all show gradations across the deposit that are completely consistent with such a mixing and reaction zone other volatiles from the fluid inclusions hcl hf 3he 4he n2 ar characterize the brine and give evidence for only shallow crustal fluids with no magmatic influences the brine entered the gas reservoir from below and along the axis of the deposit and migrated out along bedding to the southwest northeast and northwest metals transporting brines may have been fed into the host sandstones from the east fault but that remains uncertain abundant ore stage fe and mn calcite cements from the reduced fringe have 13c values as low as 18 4 and many values less than 10 which indicate that significant carbonate was generated by oxidation of organic carbon from the natural gas the zone of calcite cements with very low 13c values approximately envelopes chalcocite bearing ore sulfur isotope data of cu pb and fe sulfides and barite indicate derivation of roughly half of the orebody sulfide directly from sour gas h2s that sour gas h2s had developed in steps known from other sedimentary basins starting with 1 bacterial sulfate reduction bsr of seawater sulfate having 34s of about 20 and sequestering of the sulfide in organic matter in source rocks stratigraphically below the deposit host rocks followed by 2 maturation of the sulfide bearing organic matter into liquid petroleum with relatively homogeneous sulfide having 34s of 5 5 then by 3 thermal cracking of the oil to ch4 and h2s with relatively homogeneous sulfide having 34s closely distributed about 6 the ch4 and h2s migrated and were trapped in sandstones of the upper member of the revett formation where they were later met by the 200 c metals transporting brine there was additional contribution of sulfide to ore from later thermochemical sulfate reduction tsr operating on sulfate 34s of 20 to 29 in both formation waters and metals transporting solutions a large range of 34s in sulfides resulted as the 6 sour gas sulfide was supplemented with varying proportions of 20 to 29 sulfide from tsr

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

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane