the sulfur isotope signatures of marinoan deglaciation captured in neoproterozoic shallow to deep cap carbonate from south china a b s t r a c t we present high resolution sulfur and carbon isotope records from the cap carbonate of the doushantuo formation south china a unique suite of depositional facies spanning across a pronounced paleo oceanic depth gradient the i34s of carbonate associated sulfate cas decreases across a shelf to slope depth transect we propose that the deeper water slope profile represents syn glacial sulfur isotope compositions of the seawater sulfates with impact of 34 s depleted hydrothermal sulfur inputs while the i 34 s of the shallow inner shelf were dominantly controlled by the 34 s enriched terrigenous input we suggest that the terrigenous sulfur inputs were mainly from weathering and erosion of the pre marinoan datangpo formation glacially ground and then freshly exposed during rapid deglaciation the widespread datangpo formation deposited between the sturtian and marinoan glacial intervals is noted for the unusually high i34s values of its pyrite the low sulfate conditions in the ocean allowed for strong local controls and consequent heterogeneities in seawater chemistry the post marinoan ocean was stratified in the immediate wake of glaciation with anoxic deep waters overlain by an oxic melt water layer during the transgression the shallow water was disturbed by upwelling the oxidation of dissolved organic carbon doc from deep water would lead to transiently enhanced 13c depletions but 34s enrichments in the shallow waters

the record of the varanger glaciation at the rio de la plata craton vendian cambrian of uruguay abstract a sedimentary succession included in the lower section of the playa hermosa formation from the playa verde basin uruguay is described two facies associations one mainly coarse to medium grained and other one fine grained have been defined fa i ii in the first one breccias conglomerates sandstones and minor mudstones were deposited in a subaqueous depositional setting proximal suggesting slope instability and resedimentation processes the second one contains diamictites rhythmites sandstones and mudstones and presents abundant evidence of soft deformation also interpreted to be deposited in a subaqueous environment distal dropstones clast layers diamictites rhythmites and varve like deposits are interpreted as ice rafting processes generated during a glacial episode this glacial related succession constitutes the first record from the varanger glaciation at the rio de la plata craton of the late neoproterozoic age and also represents one of the oldest sedimentary records after the collision of the rio de la plata and kalahari cratons a combined interaction of extensional faulting and glaciation in a tectonically active basin with locally high subsidence rates resulted in high rates of sedimentation and resedimentation processes as a whole the sedimentary succession sets a relevant datum to be used in future paleogeographic reconstructions of the vendian glacial record in southern south america

the timing and environment of tepee formation in a marinoan cap carbonate thin carbonate units capping marinoan age glacial diamictites contain atypical tepee like structures that have been interpreted as evidence of seafloor dolomite cementation at parachilna gorge in the adelaide fold thrust belt a marinoan age cap dolomite contains paired tepee and growth fault structures that demonstrate the tepees grew 3 5 7 1 m below the sediment water interface in the early diagenetic zone although generally much larger such tepees and their associated sheet veins have an external morphology and internal structure identical to that of inter tidal tepee structures formed through expansive crystallisation on the other hand cap carbonate hosted tepees lack the size chaotic internal bedding carbonate crust and concretionary morphologies and expected carbon isotope geochemistry to be cold seep structures expansive crystallisation during early diagenesis seems the most likely means of forming tepees in cap carbonates the only cement common to all cap carbonate tepees is microcrystalline dolomite the crystallisation of this mineral phase drove both tepee growth and the fluid overpressure fracturing that formed the sheet veins the same microcrystalline dolomite is the component of cap carbonates routinely selected for isotopic analysis with the resulting 13c values assumed to be representative of coeval marine bicarbonate however the parachilna gorge tepees suggest that the microcrystalline dolomite contains both primary and early diagenetic components their separation is not achieved by current analytical methods

c isotopic stratification in a neoproterozoic postglacial ocean we report c isotopic data from the cap carbonate overlying the terminal proterozoic nantuo diamictites in south china the c isotopic measurements reveal a large c isotopic gradient of 3 along paleoenvironmental transect from shelf to deep basinal sedimentary facies the c isotopic stratification may attribute to the effect of biological pumping in a postglacial ocean our results confirm that the sea level rise was rapid and that atmospheric co2 was significantly high immediately following deglaciation

aragonite crystal fans in neoproterozoic cap carbonates a case study from brazil and implications for the post snowball earth coastal environment abstract neoproterozoic cap carbonates are characterized by anomalous sedimentary features including sea floor precipitates represented by aragonite pseudomorph cement crusts and crystal fans these features are found in restricted periods of geological history and their generation in the context of the neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation new sedimentary and geochemical results on well preserved neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the sete lagoas formation bambui group central brazil are reported the micrite settling facies shows the highest abundance of crystal fans arranged in centimeter scale layers interfingered with the micrite matrix this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters a numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pco2 in these conditions the mg ca ratio of the neoproterozoic seawater is calculated at ca 1 2 alkalinity oversaturation is monitored by a variable evaporation degree aragonite formation is kinetically favored and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pco2 hence alkalinity is higher these modeling conditions do not preclude the contribution of incomplete organic matter degradation to alkalinity as suggested by negative carbon isotope signatures in addition the large number of aragonite micrite pairs suggests a seasonal or paleoclimatic forcing other cases of abiotic aragonite precipitation through time are also briefly examined by comparison

cap carbonate platform facies model noonday formation se california the neoproterozoic outcrop belt of the death valley region california preserves an oblique cross section of the noonday formation a mixed carbonate siliciclastic platform that hosts distinctive basal ediacaran cap carbonate affiliated sedimentary structures stromatolite textures and 13 c carb values the noonday platform encompasses two depositional sequences that reveal two cycles of relative sea level change within strata conventionally considered to record a single rapid postglacial sea level rise in updip localities facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite bearing tubestone textured reef rimmed platform downdip localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge base level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef overlying strata record the backfilling of a submarine incised valley and reestablishment of a back stepping carbonate dominated ramp prior to a second subaerial exposure event that defines the contact between the noonday and johnnie formations we address the relative contributions of syndepositional tectonism and recovery from low latitude deglaciation in dictating noonday platform architecture and the intra noonday formation sequence boundary noonday formation deposition coincided with extension of the laurentian margin during disaggregation of the rodinian supercontinent within this framework previous work has suggested that the intra noonday formation sequence boundary records growth faulting that reinforced differential topography uplifting reef rimmed horsts exposing the reef crest to karstic dissolution and downdropping grabens however we trace the intra noonday formation sequence boundary seaward of the reef crest and demonstrate that for a time wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks thus if the noonday margin were undergoing extension then the creation of the intra noonday formation sequence boundary required a concomitant decrease in accommodation due perhaps to postglacial isostatic uplift attendant with low latitude deglaciation we speculate that noonday formation sequence architecture records 1 immediate deglacial flooding 2 shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice sheet collapse against a background of global deglaciation and 3 resumed flooding following complete deglaciation as rift related tectonism could amplify or counter glacial isostasy inferences of the amplitude of local postglacial sea level change will require robust estimates of syndepositional extension across the noonday margin

enigmatic origin of the largest known carbon isotope excursion in earth s history carbonate rocks from the middle ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in earth history this finding suggests a dramatic reorganization of earth s carbon cycle named the shuram excursion for its original discovery in the shuram formation oman the anomaly closely precedes impressive events in evolution including the rise of large metazoans and the origin of biomineralization in animals instead of a true record of the carbon cycle at the time of sedimentation the carbon isotope signature recorded in the shuram excursion could be caused by alteration following deposition of the carbonate sediments a scenario supported by several geochemical indicators however such secondary processes are intrinsically local which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe both possibilities are intriguing if the shuram excursion preserves a genuine record of ancient seawater chemistry it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern earth if however it represents secondary alteration during burial of sediments then marine sediments must have been globally preconditioned in a unique way to allow ordinary and local processes to produce an extraordinary and widespread response

how late are k feldspar megacrysts in granites abstract various petrologists have suggested that k feldspar megacrysts grow in granites that are extensively crystallized even at subsolidus conditions however experimental evidence indicates that though k feldspar nucleates relatively late in the crystallization history abundant liquid is available for development of large crystals a great deal of evidence involving many different factors favours a magmatic phenocrystic origin for k feldspar megacrysts in granites namely simple twinning oscillatory zoning euhedral plagioclase inclusions and concentric crystallographically controlled arrangements of inclusions in addition abundant evidence has been presented of 1 mechanical accumulation of k feldspar megacrysts in granites 2 alignment of megacrysts and megacryst concentrations in magmatic flow foliations 3 involvement of megacrysts in zones of magma mixing in granite plutons and 4 occurrence of megacrysts in some volcanic rocks implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains

ediacaran δ13c chemostratigraphy of south china abstract we report new 13 c chemostratigraphic data of the ediacaran system in south china using litho and biostratigraphic markers as independent calibrators we combine new and previously published 13 c data to construct a composite 13 c profile for the ediacaran system in south china the composite curve shows in chronostratigraphic order 1 a negative 13 c excursion en1 in the doushantuo cap carbonate that overlies the ghaub age nantuo diamictite 2 a pronounced positive 13 c excursion ep1 in the lower doushantuo formation where doushantuo type acanthomorphic acritarchs red algae and animal embryos first appear 3 a mild negative 13 c excursion en2 in the middle doushantuo formation 4 a positive 13 c excursion ep2 in the upper doushantuo formation where doushantuo type acanthomorphic acritarchs micrometazoans and stem group florideophyte red algae become abundant and diverse 5 a pronounced negative 13 c excursion en3 near the uppermost doushantuo formation 6 a strongly positive 13 c excursion up to 6 3 pdb ep3 in the lower dengying formation 7 a stable 13 c plateau ca 2 5 pdb ei in the middle and upper dengying formation where abundant vendotaenid algae macroscopic ediacaran fossils biomineralized animal tubes and mostly horizontal animal traces occur and 8 a 10 negative 13 c excursion en4 immediately below the ediacaran cambrian boundary the general pattern of this composite 13 c curve is robust despite a considerable amount of variation among the 13 c profiles of different sections this composite 13 c curve provides a first order chemostratigraphic framework for the subdivision and correlation of the ediacaran system in south china

zn isotopic evolution in early ediacaran ocean a global signature abstract the zn isotope composition of sedimentary archives reveals key biosphere lithosphere interactions throughout geologic history a recent zn isotope study of ediacaran cap carbonates from south australia revealed that the local isotopic profile reflects global changes in oceanic chemistry to examine this critical assumption this study investigates a high resolution zinc isotope profile of marinoan post glacial cap carbonates from the yangtze platform south china stratigraphic variations in zn isotope data from this study are similar to previously reported sections in south australia and south china the remarkable isotopic similarity between the two sections that were deposited on distant geographically separated continental blocks demonstrates that the zn isotopic variations reflect changing global signatures of the early ediacaran ocean this study highlights the value of zn isotopes in sediments as a global proxy for paleoceanographic studies we interpret the systematic stratigraphic changes in the zn isotope composition of the studied post glacial marinoan carbonates as a global oceanic trend controlled mainly by varying inputs and outputs of zn to the ocean decreasing 66zn values in the lowermost part of the section are attributed to the global precipitation of carbonate enriched in the heavier zn isotope a positive shift in the middle part of the section is explained by the non quantitative removal of isotopically lighter zn by sulfide minerals lower 66zn values in the uppermost part likely reflect the changing contributions of continental weathering and sulfide precipitation our data suggest that non quantitative removal of zn by sulfides is a previously unrecognized sink for zinc in the oceans

carbonate deposition climate stability and neoproterozoic ice ages the evolutionary success of planktic calcifiers during the phanerozoic stabilized the climate system by introducing a new mechanism that acts to buffer ocean carbonate ion concentration the saturation dependent preservation of carbonate in sea floor sediments before this buffering was primarily accomplished by adjustment of shallow water carbonate deposition to balance oceanic inputs from weathering on land neoproterozoic ice ages of near global extent and multimillion year duration and the formation of distinctive sedimentary cap carbonates can thus be understood in terms of the greater sensitivity of the precambrian carbon cycle to the loss of shallow water environments and co 2 climate feedback on ice sheet growth

c o sr isotopic stratigraphy of cap carbonates overlying marinoan age glacial diamictites in the paraguay belt brazil abstract the neoproterozoic paraguay belt located along the southeastern border of the amazon craton and eastern border of the rio apa block comprises a thick passive margin succession of glaciomarine turbidites carbonates and siliciclastic sedimentary rocks that were deformed during the brasiliano pan african orogeny the carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks araras formation which directly overlie marinoan glacial diamictites puga formation the late neoproterozoic age of this carbonate sequence is indicated by the presence of cloudina lucianoi and corumbela werneri found in the upper part of the corumba group southern paraguay belt carbon and sr isotope data were obtained on marine carbonate samples across three different sections of the araras formation 13 c pdb values of the carbonate from the border of the basin range between 10 5 and 2 7 including a short stratigraphic interval of cap dolomite 22 m directly overlying diamictites the isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay limestone cap carbonate that overlie glacial diamictites also show negative 13 c pdb values 5 3 and 0 6 in contrast with the lower portion of the carbonate sequence a 800 m thick unit of shallow water dolostones forming the upper part of the carbonate sequence present uniform positive 13 c pdb values ranging from 1 9 to 2 4 dolostone and arenaceous dolostone of the upper parts of the araras formation have high 13 c pdb values 4 1 to 9 6 which are followed by an abrupt decrease in 13 c values down to 1 0 the carbon isotope profile of the paraguay belt is quite different from the bambui group an other important neoproterozoic sequences in central brazil which also overlie glacial sediments negative 13 c pdb values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion values ranging from 6 9 to 16 0 on the other hand the 13 c pdb profile observed across the araras formation exhibits an approximately 200 m thick section of carbonates with low 13 c pdb values carbonates of the araras formation have 87 sr 86 sr ratios ranging from 0 70753 to 0 70803 comparable to cap carbonates that overly marinoan glacial deposits from other neoproterozoic sequences

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

ediacaran palaeomagnetism and apparent polar wander path for australia no large true polar wander summary we report new palaeomagnetic data for red beds from the ediacaran brachina and wonoka formations in the adelaide geosyncline south australia and discuss their place with previously determined poles in the ediacaran apparent polar wander path for australia both formations behave similarly on thermal demagnetization displaying high temperature components that decay to the origin at 680 c consistent with haematite being the only magnetic mineral present restoring the strata to the palaeohorizontal yielded positive fold tests for both units at 99 per cent confidence indicating that acquisition of magnetization occurred before the early palaeozoic delamerian orogeny for the brachina formation n 91 specimens the mean direction after unfolding is declination d 178 2 inclination i 22 6 95 4 4 indicating a palaeolatitude 11 8 2 5 and a pole position at latitude p 46 0 s longitude p 315 4 e with confidence semi axes dp 2 4 and dm 4 6 the mean direction for the wonoka formation after unfolding n 70 is d 255 9 i 23 7 95 6 4 indicating 12 3 3 8 3 4 and a pole position at latitude p 5 2 s longitude p 30 5 e dp 3 6 and dm 6 8 the mean directions for these units and other ediacaran units in the adelaide geosyncline are significantly different from each other which excludes blanket remagnetization of the units before delamerian folding and therefore gives strong preference to their magnetization dating from close to the time of deposition the late cryogenian ediacaran cambrian apparent polar wander path for south australia spans 150 myr from 635 to 490 ma and places australia in low palaeolatitudes throughout the interval studied the poles differ significantly from each other suggesting australia underwent continual drift during that time whereas the directional difference between the late cryogenian elatina formation and early ediacaran nuccaleena formation is mainly in inclination for most other contiguous stratigraphic units the differences are mainly in declination with minor inclination differences indicating australia was rotating about a nearby euler pole in low palaeolatitudes the large and perhaps rapid polar shifts at 615 590 and 575 565 ma in the laurentian apparent polar wander path are not evident in the ediacaran apparent polar wander path for australia because true polar wander should be recorded globally in the absence of evidence for any major stratigraphic break in the south australian succession we conclude that large true polar wander did not occur during the ediacaran

extreme winds and waves in the aftermath of a neoproterozoic glaciation the most severe excursions in the earth s climatic history are thought to be associated with proterozoic glaciations according to the snowball earth hypothesis the marinoan glaciation which ended about 635 million years ago involved global or nearly global ice cover at the termination of this glacial period rapid melting of continental ice sheets must have caused a large rise in sea level here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples these structures occur at homologous stratigraphic levels in australia brazil canada namibia and svalbard our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds significantly longer than those typical for today s oceans the reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch unlimited ocean basins we propose that these extraordinary wind and wave conditions were characteristic of the climatic transit and provide observational targets for atmospheric circulation models

stratigraphy sedimentology and isotopic geochemistry of australian neoproterozoic postglacial cap dolostones deglaciation delta 13 c excursions and carbonate precipitation abstract thin d13c excursions in the geologic record they are interpreted to record a brief postglacial anomaly in ocean chemistry in australia the marinoan varanger equivalent 600 ma cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe including fine grained dolomitic mineralogy lateral persistenc at basinal scales thin laminae graded beds intervals with abundant marine cements crystal fans and a distinctive negative d13c isotopic signature of up to 5 pdb on the basis of these features the australian examples are interpreted here to be deeper water deposits below storm wave base resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression detailed isotopic analysis of australian cap dolostones indicates d13c values ranging from 1 to 5 and gen rally becoming more depleted in 13c upsection trace element data indicate some diagenetic stabilization but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event the range in d13c values between sections as well as more negative 13c values appear to correlate with greater paleobathymetry within basins this implies either 1 only partial preservation of the complete oceanic variation of d13c or 2 precipitation of this peculiar facies owes its genesis to basinal specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone in the later case d13c values would not represent whole ocean values stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression this implies that the cause s of d13c variation likely operated on time scales significantly less than the residence time of carbon in the oceans 105 yr the continent wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period such large scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused or have been the product of profound changes in the carbon cycle and global climate at tha time similar evidence for transgression and increased carbonate deposition in the holocene are attributed to the changing basin shape and ph of the oceans the coral reef hypothesis of berger 1982 cap dolostones may record a proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms

a predominantly ferruginous condition in the ediacaran deep ocean geochemistry of black shales in the ediacaran doushantuo formation south china abstract the redox condition of the ocean played an important role in the evolution of the ediacaran biota but it remains largely unexplored particularly for the deep ocean environment in this study we present a comprehensive study of fe speciation and trace elements of the doushantuo black shales from the longe and jinjiadong sections in south china to investigate the redox condition of basinal environments in ediacaran in general both sections show high fe hr fe t ratios 0 38 low fe py fe hr ratios py fe hr ratios 0 7 in the jinjiadong section indicating occasional euxinia in the basin facies fe py fe hr ratios show an increase in the middle part both in the longe and jinjiadong sections indicating enhanced production of h 2 s via bacteria sulfate reduction bsr in the deep anoxic water which was likely responsible to the upper doushantuo negative carbon isotope excursion the enrichment factors efs of most redox sensitive trace elements in the sediments of both sections are low 1 while ef mo shows moderately enriched 3 60 which is similar to the modern cariaco basin because of paleogeographic restriction or persistent ocean stratification the trace elements renewal would be mostly obstructed in the isolated deep water and their inventory could become depleted which cause the low efs of most trace elements in sediments however for the non euxinic deep ocean a particulate shuttle would transport mo from shallow water to deep water through adsorption desorption on the mn fe oxyhydroxides which maintained moderate mo enrichment in seawater and related sediments

the ediacaran sedimentary architecture and carbonate productivity in the atar cliffs adrar mauritania palaeoenvironments chemostratigraphy and diagenesis abstract the neoproterozoic strata of the taoudeni basin west african craton display a single glaciogenic deposit late cryogenian in age and included within a stratigraphic marker triad of tillite carbonate and bedded chert this work is focused on the analysis of the sedimentary architecture and episodic carbonate productivity related to its transgressive deglaciation recorded in the atar cliffs of adrar mauritania it offers a joint chemo event and sequence stratigraphic analysis that allows us to control problems of stratigraphic completeness and time resolution necessary for regional and worldwide stratigraphic correlation in the atar cliffs the cap carbonate sequence is underlain by a composite unconformity that comprises the pan african i discordance and an inherited palaeorelief associated with the glaciation the transgressive sequence began with accumulation of a sheet like apron onlapped by a toe of slope to basin transition with cap carbonates bedded cherts and distal turbidites tichilit el beida shales the carbonate strata of the atar cap carbonate sequence have recorded both dolomitization and dedolomitization processes destroying the original cement textures carbon isotope values from the sheet like apron range from 0 to 0 6 were directly controlled by the reworking of the underlying atar strata and do not reflect primary variations in seawater composition the general pattern of 13 c fluctuations within the overlying incomplete cap carbonate sequence is consistent with other post late cryogenian strata except in the initial shift from slightly to moderately negative values recognized elsewhere that is not preserved in the study area carbon isotope values in the preserved upper part of the cap carbonate record a pronounced rise in 13 c from 6 2 to highly enriched values 3 7 followed by the final demise of carbonate productivity the cap carbonate sequence is topped by an erosive unconformity succeeded by the recovery of carbonate accumulation represented by an onlapping dolostone deposited in coastal to foreshore environments there the carbon isotope values reflect the return to moderately negative values ranging from 0 9 to 2 0 the palaeogeographic development of a palaeorelief associated with the end of glacial conditions and deposition of the onlapping atar cap carbonate is related to the presence of interbedded felsic bentonites and phenocrysts and reflects the intermittent influence of a neighbouring explosive volcanism

the unusual sedimentary rock record of the early triassic a case study from the southwestern united states abstract sedimentary systems as well as the earth s biota changed dramatically following the end permian mass extinction this global sedimentological transformation is reflected in the widespread occurrence of flat pebble conglomerates subtidal wrinkle structures microbialites and carbonate seafloor fans in various lower triassic sections as a case study a facies analysis of the lower triassic virgin limestone member of the moenkopi formation and the middle and upper members of the union wash formation southwestern united states was conducted and all unusual facies were documented within a palaeoenvironmental framework the occurrence of the flat pebble conglomerates and wrinkle structures is significant because these reflect a long term reduction in vertical bioturbation in the case of the subtidal microbialites and carbonate seafloor fans their formation is linked to an increase in alkalinity the presence of these facies in the uppermost lower triassic strata of the southwestern united states indicates that reduced levels of infaunal bioturbation as well as and perhaps a consequence of unusual ocean chemistry extended for 5 6 million years after the end permian mass extinction suggesting that depositional environments as well as the biota were significantly affected by the extinction event

chemostratigraphy of the neoproterozoic mirassol d oeste cap dolostones mato grosso brazil an alternative model for marinoan cap dolostone formation abstract we have conducted a detailed study of the neoproterozoic mirassol d oeste cap dolostones that overlay the glacial diamictites of the puga formation 635 ma amazon craton brazil in order to understand the formation of these post glacial dolostones petrographic features indicate that the dolostones are primary to early diagenetic in origin and precipitated in a moderately shallow water platform corresponding to a carbonate ramp during transgressive conditions major and trace element contents as well as c and o isotopic signatures are consistent with an anoxic sediment influenced by sulphate reducing bacteria such an environment is known to provide favourable conditions for the precipitation of dolomite as observed nowadays in modern hypersaline lagoons isotopic compositions of tube like structures suggest local upward fluid seepage from the underlying cap dolostone our data concur with geochemical data from other neoproterozoic cap dolostones to support a microbially mediated model in specific environmental conditions for the formation of these unusual deposits worldwide

time specific aspects of facies state of the art examples and possible causes abstract the term time specific facies tsf was introduced to the scientific community by the late otto h walliser to refer to unique facies typical of particular narrow intervals some of which were related to bioevents in some senses however the concept was recognized much earlier and is even engrained in the very names of some geologic periods the concept of time specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time the recognition of time specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale these range from very short term but widespread facies that overlap with event deposits to general facies types that may persist for intervals up to 10s of millions of years in duration this paper briefly summarizes the history of development of the tsf concept provides examples to illustrate a few of the key aspects of time specific facies and offers a few tentative explanations for this phenomenon among the factors that control tsfs abrupt changes in redox conditions and early diagenetic mineralization sedimentary condensation often associated with abrupt sea level change altered climate and paleoceanography and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of tsfs inevitably however many tsfs reflect multiple effects and some remain largely unexplained

a burial diagenesis origin for the ediacaran shuram wonoka carbon isotope anomaly marked negative 13c excursions in ediacaran age carbonate sediments have been identified in several sections globally but are not recognized in all sections of similar age the presence of 13ccarb values as low as 12 has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today the 13ccarb anomalies are strongly correlated with 18ocarb values but are not represented in 13corg records while no primary depositional processes have been identified that can produce the correlated 18o 13c arrays simulations show that fluid rock interaction with high pco2 fluids is capable of producing such arrays at geologically reasonable pco2 and water rock ratios variations in the mg ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs calcite and anhydrite in the resulting mineral assemblage incorporation of an initially aragonitic mineralogy demonstrates that high sr low mn sr and modest alteration of 87sr 86sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the 13c anomalies and do not necessarily imply preservation of primary values the profound ediacaran negative 13c anomalies can be adequately explained by well understood diagenetic processes conflated with the difficulty of correlating precambrian sections independently of chemostratigraphy they are not a record of primary seawater variations and need not have independent stratigraphic significance

geobiology of the late paleoproterozoic duck creek formation western australia the ca 1 8 ga duck creek formation western australia preserves 1000 m of carbonates and minor iron formation that accumulated along a late paleoproterozoic ocean margin two upward deepening stratigraphic packages are preserved each characterized by peritidal precipitates at the base and iron formation and carbonate turbidites in its upper part consistent with recent studies of neoarchean basins carbon isotope ratios of duck creek carbonates show no evidence for a strong isotopic depth gradient but carbonate minerals in iron formations can be markedly depleted in 13 c in contrast oxygen isotopes covary strongly with depth 18 o values as positive as 2 vpdb in peritidal facies systematically decline to values of 6 to 16 in basinal rocks reflecting we posit the timing of diagenetic closure the duck creek formation contains microfossils similar to those of the gunflint formation canada they are restricted to early diagenetic cherts developed in basinal facies strengthening the hypothesis that such fossils capture communities driven by iron metabolism indeed x ray diffraction data indicate that the duck creek basin was ferruginous throughout its history the persistence of ferruginous waters and iron formation deposition in western australia for at least several tens of millions of years after the transition to sulfidic conditions in laurentia suggests that the late paleoproterozoic expansion of sulfidic subsurface waters was globally asynchronous

a neogene seawater sulfur isotope age curve from calcareous pelagic microfossils abstract until now our knowledge of the sulfur isotopic composition of seawater through geologic time has depended on stable isotopic analysis of sulfate from evaporites owing to the sporadic occurrence of evaporites through time the secular sulfur isotope age curve contains many gaps with little or no data in order to fill in some of these gaps particularly the neogene we have analyzed the sulfur isotopic composition of carbonate associated sulfate in carbonate tests of planktonic foraminifera other investigators have shown that sulfate may occur in biogenic calcites either lattice bound as micro fluid inclusions in adsorbed phases or as protein polysaccharides whatever the origin the sulfur isotopic composition of this sulfate appears to be representative of that of the water in which the organism lived as shown by results on recent calcareous foraminifera and macrofossils using this approach for study of miocene to recent pelagic marine sediments supplemented by new data for miocene marine evaporites from the gulf of suez we have found that the 34 s of seawater has decreased about 2 5 over the past 25 m y and that most of the decrease has occurred over the past 5 m y parallelling a decrease in the 13 c of dissolved oceanic bicarbonate from the same interval sedimentary redox models based on isotope records suggest that organic carbon and sulfide burial have both decreased over the past 5 m y alternatively an increase in weathering rates over the past 5 m y would not require a decrease in organic carbon or sulfide burial as long as the isotopic effect of the increased river input exceeds the isotopic effect of the burial of the reduced species in either case the net result would be a decrease in atmospheric p o 2

seafloor precipitates and c isotope stratigraphy from the neoproterozoic scout mountain member of the pocatello formation southeast idaho implications for neoproterozoic earth system behavior abstract macroscopic seafloor precipitated crystal fans are rare in post paleoproterozoic strata but they are more common in the unusual carbonates that cap certain neoproterozoic glacial strata the uppermost carbonate and marble unit of the scout mountain member of the neoproterozoic pocatello formation younger than 667 5 ma contains seafloor fans records negative 13 c values and lies within a transgressive sequence all features consistent with such post glacial carbonate units it does not however rest directly upon a known glaciogenic deposit or sequence boundary instead it occurs within a cyclic upward fining succession that overlies an erosional contact on the pink dolostone post glacial carbonate atop the late sturtian scout mountain member diamictites it is possible that the carbonate unit in question is a cap carbonate and thus requires recognition of an additional glaciation recorded in the pocatello formation alternatively it may imply that means independent of glaciation such as overturn of an anoxic ocean foster the deposition of cap like carbonates independent of post glacial processes thus neoproterozoic carbonate units with unusual lithofacies and negative carbon isotope values need not indicate the aftermath of low latitude glaciation additionally the radiometrically calibrated 13 c record of the pocatello formation demonstrates that the interval from 700 to 580 ma has more variation than previous compilations would suggest

an early diagenetic deglacial origin for basal ediacaran cap dolostones abstract the beginning of the ediacaran period 635 ma is marked by conspicuous dolostone units that cap marinoan glacial deposits worldwide the extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over saturation coincided with deglacial sea level rise and ocean warming however the geochemical variability within cap dolostones both between continents across single continental margins and within individual stratigraphic sections has been difficult to reconcile with depositional models using a compilation of new calcium and magnesium isotope measurements in marinoan cap dolostone successions worldwide we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid and sediment buffered conditions dolostones from the outer platform formed under fluid buffered conditions whereas dolostones on the inner platform and foreslope environment formed under sediment buffered conditions this spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea level rise and development of a meltwater surface ocean using a numerical diagenetic model to evaluate the geochemical differences between sediment and fluid buffered cap dolostone units we constrain the chemical and isotopic composition of both the dolomitizing fluid glacial seawater 13 c 0 2 the meltwater lens 13 c 11 and the primary aragonite sediment 13 c 6 to 3 these model end members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal ediacaran cap dolostones our results provide a novel framework for understanding the geochemical variability of cap dolostone units including large excursions in carbon isotopes and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last snowball earth

ocean oxidation during the deposition of basal ediacaran doushantuo cap carbonates in the yangtze platform south china abstract precipitation of cap carbonate lithologies is a key feature of cryogenian global glaciations negative carbonate carbon isotopic compositions 13ccarb of these cap carbonates have been variably interpreted as massive drawdown of atmospheric co2 via extensive continental chemical weathering methane oxidation or postglacial upwelling each of these interpretations argues a non steady state of carbon cycle in the aftermath of marinoan global glaciation to further explore the postglacial marine carbon cycle we measured 13ccarb of cap carbonates from six localities in the yangtze platform south china the studied cap carbonates were deposited in a variety of sedimentary environments ranging from the open shelf slope to basin facies cap carbonates deposited in different environments show distinct stratigraphic trends of 13ccarb in the open shelf 13ccarb profile of the songlin section remains almost constant 3 to 4 while the 13ccarb of the jiulongwan section records a negative excursion decreasing from 3 5 to 7 13ccarb of cap carbonates deposited in the slope environment does not show stable stratigraphic trend in the basin environment 13ccarb demonstrates a sharp decline in the middle part of cap carbonates to the nadir value of 11 the negative 13ccarb excursion is best interpreted in terms of oxidation of dissolved organic carbon doc thus recording a pulse of ocean oxidation during cap carbonate precipitation clearly absence of negative 13ccarb excursion in all slope and most open shelf sections may imply that such oxidation event was not ubiquitous in the yangtze platform we speculate that the renewed thermohaline circulation during deglaciation brought oxic surface water into ocean interior which oxidized the basin environment of the yangtze platform however the deglacial thermohaline circulation was not strong enough to cause complete oxidation of the ocean the sporadic oxidation in the open shelf on the other hand might result from the terrestrial influx of oxidant from postglacial continental weathering our study suggests that ocean oxidation though sporadic might have occurred during cap carbonate precipitation and predated the first appearance of putative animal embryos

the sea level fingerprint of a snowball earth deglaciation cap dolostones are thought to represent deposition from seas transgressing over formerly glaciated continental margins during marinoan snowball deglaciation nevertheless facies associations within some cap dolostones indicate that an episode of regional regression punctuated these transgressive sequence tracts to date inferences of sea level change during and after the marinoan snowball deglaciation have been interpreted using simple qualitative arguments in the present study we explore the full spatio temporal variability of sea level change during snowball deglaciation and its aftermath using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model the theory is applied to model marinoan snowball deglaciation on a generalized ediacaran paleogeography with a synthetic continental ice sheet distribution we find that sea level change following a synchronous rapid 2 kyr collapse of snowball ice cover will exhibit significant geographic variability including site specific histories that are characterized by syn deglacial sea level fall or stillstand moreover some sites that experience syn deglacial transgression will continue to experience transgression in the post deglacial phase taken together these results suggest that sea level change recorded by strata capping snowball glaciogenic units may reflect a more complicated trajectory than previously thought including deposition that was not limited to the deglaciation phase these simulations as well as others that explore the response to asynchronous melting and deglaciation phases of longer duration demonstrate that an episode of regional regression interrupting a cap dolostone transgressive sequence tract may reflect one of several processes or their combination 1 near field adjustment associated with rapid local melting during an otherwise global hiatus in deglaciation 2 post glacial uplift of sites during a period of slowing deglaciation and 3 a transition at some sites from a sea level fall dominated by post glacial uplift to a phase of sea level rise due to eustasy and peripheral bulge subsidence throughout an extended order 50 kyr or greater snowball deglaciation

the effectiveness of the paleoproterozoic biological pump a δ13c gradient from platform carbonates of the pethei group great slave lake supergroup nwt samples of carbonate cements collected along a depth transect of the pethei platform a 1 9 ga stromatolitic reef reveal a small 0 5 carbon isotope gradient between shallow and basinal facies the magnitude of this gradient would conventionally be interpreted as indicating low export of organic matter from ocean surface waters but steady state simulations using a two box model of the paleoproterozoic ocean suggest that the small carbon isotope gradient could instead be due to high partial pressures of carbon dioxide in the paleoproterozoic atmosphere which would increase the ocean9s dissolved inorganic carbon content and damp the effects of biological pumping if the paleoproterozoic atmosphere were indeed enriched in atmospheric co 2 in compensation for a less luminous sun these results indicate that the planktonic biota was a significant component of the precambrian global carbon and nutrient cycles affecting if not regulating as today the composition of both surface and deep ocean waters

carbon isotopic composition of neoproterozoic glacial carbonates as a test of paleoceanographic models for snowball earth phenomena consistently positive carbon isotopic values were obtained from in situ peloids ooids and stromatolitic carbonate within neoproterozoic glacial successions in northern namibia central australia and the north american cordillera because positive values continue upward into the immediately overlying postglacial cap carbonates the negative isotopic excursions widely observed in those carbonate rocks require an explanation that involves a short term perturbation of the global carbon cycle during deglaciation the data do not support the ecological consequences of complete coverage of the glacial ocean with sea ice as predicted in the 1998 snowball earth hypothesis of p f hoffman et al in the snowball earth hypothesis the postglacial cap carbonates and associated 5 negative carbon isotopic excursions represent the physical record of co2 transfer from the high p co2 snowball atmosphere 0 12 bar to the sedimentary reservoir via silicate weathering in the snowball aftermath stratigraphic timing constraints on cap carbonates imply weathering rates of 1000 times preglacial levels to be consistent with the hypothesis the absence of sr isotopic variation between glacial and postglacial deposits and calculations of maximum weathering rates do not support a post snowball weathering event as the origin for cap carbonates and associated isotopic excursions

episodic volcanism during break up of the north atlantic evidence from the east greenland plateau basalts abstract the lower tertiary upper paleocene lower eocene plateau basalts in the scoresby sund region of east greenland formed during three distinct magmatic episodes the activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence lower sequence the activity of the second episode was centred to the southeast offshore the present atlantic coast and this also produced a voluminous regional basalt sequence upper sequence the third episode was centred on the atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting the lavas are all tholeiitic basalts after an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height from tio2 rich feti basalts to tio2 poor basalts and ending with a reversal to tio2 rich basalts each of these cyclic patterns apparently resulted from a temporary rifting episode and much of the compositional variation found can be explained by magma development in open magma chambers the dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts and their tio2 p2o5 ratios indicate formation at a spreading ridge centre the pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region and the volcanism itself is closely related to this process

the origin of k feldspar megacrysts hosted in alkaline potassic rocks from central italy a track for low pressure processes in mafic magmas abstract in situ sr isotope and microchemical studies were used to determine the provenance of k feldspar megacrysts hosted in mafic alkaline potassic ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central italy at monte cimino volcanic complex mafic leucite free ultrapotassic megacryst bearing rocks of olivine latitic composition are associated with evolved latite and trachyte here latites and trachytes straddle the sub alkaline field age corrected 87sr 86sr values sri of the analysed cimino olivine latites vary from 0 71330 and 0 71578 and strongly increase at constant mg value latite and trachyte have lower sri than olivine latites ranging between 0 71331 and 0 71361 sri of k feldspar megacrysts from olivine latites are between 0 71352 and 0 71397 but core and rim 87sr 86sr ratios within individual megacryst are indistinguishable in all the mafic rocks the megacrysts are not in isotopic equilibrium with the hosts k feldspar megacrysts from both the latite and trachyte have similar sr isotope compositions sri 0 71357 0 71401 to those in the olivine latites however sri of megacryst in the trachyte vary significantly from core to rim sri from 0 71401 to 0 71383 as with the olivine latites the k feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte at vico volcano megacryst bearing rocks are mafic leucite free potassic rocks mafic leucite bearing ultrapotassic rocks and old trachytic rocks the mafic leucite bearing and leucite free rocks are a tephri phonolite and an olivine latite respectively a megacryst in vico trachyte is isotopically homogeneous sri core 0 71129 rim 0 71128 and in equilibrium with the host rock sri bulk rock 0 71125 sri of megacryst from tephri phonolite is clearly not in isotopic equilibrium with its host sri bulk rock 0 71158 and it increases from core sri 0 71063 to rim sri 0 71077 a megacryst in vico olivine latite is isotopically homogeneous sri core 0 71066 rim 0 71065 but not in equilibrium with the host rock sri bulk rock 0 71013 the sr isotope microdrilling technique reveals that cimino megacrysts were crystallised in a cimino trachytic magma and were subsequently incorporated by mixing mingling processes in the latitic and olivine latitic melts a model invoking the presence of a mafic sub alkaline magma which was mixed with the olivine latite is proposed to justify the lack of simple geochemical mixing relation between cimino trachytes and olivine latites this magmatological model is able to explain the geochemical characteristics of cimino olivine latites otherwise ascribed to mantle heterogeneity the similarity of core sri of megacrysts hosted in vico tephri phonolite and olivine latite suggests that the k feldspar megacrysts are co genetic isotopic equilibrium between megacryst and vico host trachyte indicates that the trachyte is the parent of this megacryst on the contrary the megacrysts hosted in tephri phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim sr isotope increase of the xenocryst hosted in the tephri phonolite the megacrysts hosted in the vico mafic rocks might derive from a trachytic melt similar in composition to the old vico trachytes

carbon isotope variability across the ediacaran yangtze platform in south china implications for a large surface to deep ocean δ13c gradient new isotope data obtained from relatively conformable carbonate rich strata of the ediacaran yangtze platform in south china reveal substantial 13 c variability in platform sections four negative 13 c anomalies with a nadir down to 8 pdb are present in the interval between the cap carbonate level 635 ma and the precambrian cambrian boundary 542 ma while in slope and basinal sections 13 c values are negative through the entire doushantuo formation 635 551 ma if these 13 c values are close to their primary seawater signature they imply a strong 10 surface to deep ocean 13 c gradient that is consistent with long term deep ocean anoxia and the presence of a large dissolved organic carbon doc reservoir the two prominent negative 13 c excursions within the doushantuo formation above the cap carbonate level are associated with shoaling and local exposure of the platform the anomalies may thus record remineralization of a large oceanic doc pool via sulfate reduction that transferred 13 c depleted carbon from the oceanic doc reservoir to the surface ocean during regression inconsistencies in ediacaran 13 c profiles globally and variations in south china in particular highlight the need for further evaluation of local departures in 13 c from an inferred average seawater signature

phosphogenesis aragonite fan formation and seafloor environments following the marinoan glaciation abstract carbonates in the sete lagoas formation sao francisco craton brazil preserve a record of chemical biological and oceanographic changes that occurred during the ediacaran period the base of this formation constitutes a post glacial cap carbonate which contains seafloor precipitates carbonate and barite crystal fans as well as various authigenic and diagenetic minerals apatite pyrite and barite here we present petrographic and geochemical data on this unit and discuss the significance of its mineral association for marine environments following the marinoan snowball earth glaciation for the first time we report well developed apatitic cements in a neoproterozoic cap carbonate isopachous and intergranular void filling cements encrust and surround seafloor precipitated fan crystals that precipitated as aragonite we propose a model for the origin of this mineral association which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions according to this model the boundary between oxic and anoxic conditions was located at or just below the sediment water interface burial of iron oxyhydr oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction iron reduction released fe2 which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor concurrently iron pumping shuttled phosphate from the water column to the sediment and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways created conditions conducive to phosphate mineralization this model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation in addition to high alkalinity and that fe2 serves as this inhibitor overall our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the ediacaran illuminates the paleoenvironments of cap carbonates with seafloor precipitates and contributes to understanding of phosphogenesis following low latitude glaciations

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

neoproterozoic sulfur isotope variation in australia and global implications abstract seven sulfur isotope stages are apparent in australian neoproterozoic sections 1 from 840 ma to the sturtian glaciation 700 ma 34ssulfate varied little 19 to 17 5 and 34ssulfide ranged from 20 to 23 2 the sturtian glaciation was followed by 3 a rise in 34ssulfide to an average of 30 seen also in china namibia and canada and in 34ssulfate at least to 26 and possibly to 45 the sharp rise in 34s of all sulfur fractions sulfate sulfide and organic leads us to speculate that sulfide depleted in 34s was deposited on the abyssal plain and that residual sulfate enriched in 34s remained beneath a stagnant ice covered ocean during the sturtian glaciation and was brought to the shallow continental margin during the post glacial transgression 4 34ssulfide and 34ssulfate fall from 640 to 607 ma 34ssulfate down to 17 probably from oxygen excess through massive organic carbon burial which causes weathering of sulfide depleted in 34s continental sulfide and previously deposited sulfide on the abyssal plain 5 the oxic environment of the marinoan 605 595 ma glaciation minimises sulfate reduction unlike the sturtian so that 34ssulfate is unvarying 6 the post glacial environment has sulfide depleted in 34s average 34ssulfide

aftermath of a snowball earth 1 using a simple 3 box model of the ocean atmosphere system we simulate the cycling of carbon and strontium in the aftermath of a global glaciation model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pco2 conditions as well as ocean mixing air sea gas exchange and biological productivity the 13c of the first carbonate precipitated after the glaciation depends on the pco2 temperature the saturation state of the surface ocean and kinetic effects associated with mineral precipitation with no biological productivity the model produces 13c values between 1 and 3 consistent with observations this is in direct contradiction with arguments by kennedy et al 2001a who suggest that the 13c value of dissolved carbon in a snowball ocean and directly afterward must be 5 kennedy et al assume the carbon isotope cycle is in steady state which does not apply to a global glaciation and also neglect any effect of high pco2 on the carbonate chemistry of seawater a major difference between our findings and the qualitative predictions of hoffman et al 1998 is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves as a result the 2 drop in the 13c observed in the cap dolostone is unlikely to be the product of rayleigh distillation of atmospheric co2 via silicate weathering instead we interpret the 2 drop in the 13c values as indicative of an increase in sea surface temperature which lowers the fractionation between co2 and carbonate kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important rayleigh distillation of atmospheric co2 via silicate weathering is a viable explanation for the continued drop in the 13c values in the limestone sequence above the cap dolostone with biological productivity and carbonate weathering driving a slow increase in 13c values once pco2 levels decline our study also simulates the cycling of strontium in seawater in contrast to the finding of jacobsen and kaufman 1999 and kennedy et al 2001a model simulations show a drop in 87sr 86sr of less than 0 001 during 5 million years of global glaciation and an increase of less than 0 001 over the entire episode of silicate weathering our calculations emphasize the importance of considering the changes in seawater chemistry due to high pco2 in evaluating the snowball earth hypothesis

chemostratigraphic correlation of neoproterozoic successions in south america abstract the neoproterozoic successions of south america largely accumulated during extensional events associated with the breakup of rodinia these units are preserved as thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments on passive cratonal margins which have been intensely deformed in mobile belts surrounding the craton volcaniclastic and siliciclastic materials are associated with some of the marginal fold belts three mega sequences are represented in the cratonal area glaciogenic carbonate and molasse at least two transgressive regressive sea level cycles occurred during the evolution of the carbonate mega sequence which lies above iron cemented glacio marine diamictite of probable sturtian age we report high resolution carbon oxygen strontium and sulfur isotope trends from analyses of well preserved sample sets and use principally strontium and carbon isotopes from the best preserved samples in concert with lithostratigraphic and biostratigraphic observations to provide detailed correlations of the south american neoproterozoic successions lead zinc deposits some of which with important metal reserves seem to be restricted to the sturtian carbonate succession and associated with a sequence boundary at the end of the first transgressive regressive cycle phosphate deposits are found in both sturtian and varangerian marinoan carbonate successions above glaciogenic sequences

condensation origin for neoproterozoic cap carbonates during deglaciation neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates this stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions in contrast to this sequence stratigraphic field studies of an ediacaran ca 635 ma glacial to postglacial transition in the amadeus basin of central australia reveal a complex deglacial stratigraphy in which more than 175 m of conglomerate sandstone marl and carbonate at the basin margin and portions of four unconformity bounded sequences pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies the unconformities which are characterized by as much as several tens of meters of erosional relief oblique sections of incised valleys separate intervals of contrasting sediment provenance and are confidently mapped on the basis of both criteria comparable unconformities are absent in the overlying neoproterozoic succession which is 2 km thick and encompasses many tens of millions of years the amadeus basin cap carbonate was thus deposited during a protracted interval of multiphase cyclical transgression more similar to phanerozoic cyclical sea level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate the superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions facies lithologically similar to cap carbonates may be less obvious in phanerozoic successions because of a secular change in carbonate composition to reefal and deep sea pelagic deposits

deposition and palaeogeography of a glacigenic neoproterozoic succession in the east kimberley australia facies provenance and palaeocurrent analysis of the predominantly siliciclastic neoproterozoic succession of the east kimberley district of northwestern australia indicates southwesterly directed regional palaeoice flow directions associated with continental glaciation facies relationships at three key study sites in the east kimberley record glacial advance retreat and marine transgression and the likely effects of ice dynamics on local basin depositional history ice scour linked with provenance data indicate ice advance from the northeast additional ice flow data from the west kimberley and the daly river area of northern territory are consistent and record a regional palaeoice flow direction towards the southwest palaeocurrent data and facies relationships in the east kimberley however suggest that sediment transport directions within the east kimberley basin were influenced by a combination of ice induced lithospheric flexure pre existing topography and modification of basin topography and sediment dispersal paths by ice induced redistribution of debris a basin evolution model incorporating these concepts within the framework of ice advance retreat and marine inundation is proposed identification of regional ice flow trends of this scale should ultimately prove a useful tool in testing possible palaeogeographic reconstructions of continent configurations in a post rodinian world

zinc and cadmium stable isotopes in the geological record a case study from the post snowball earth nuccaleena cap dolostone abstract zinc and cadmium are important biologically cycled nutrients in the modern ocean recent analytical advances and sampling efforts such as geotraces have led to measurements of the global distribution of zn and cd and their stable isotope ratios 66zn and 114cd providing an understanding of how biological processes cycle these elements and their isotopes in the modern ocean this opens the door to better application of 66zn and 114cd as tracers of biogeochemical processes archived in the geological record here we present new measurements of cd and 114cd on samples from the nuccaleena cap dolostone in south australia which was deposited during deglaciation of the ca 635 ma old marinoan snowball earth our new data are combined with previously measured 66zn data from the same samples the combined record of 114cd and 66zn is used to explore various hypotheses about the sources and sinks of cd and zn in the post marinoan ocean key features of this record are that zn and cd remain relatively similar throughout that 66zn and 114cd change at the same stratigraphic level and in the same direction and that 66zn changes are about four times larger than 114cd changes even though both quantities reflect a similar fractional mass difference we suggest that both zn and cd concentration and isotopic data are consistent with a change in the sink by which zn and cd are removed from the oceans we propose that no isotope fractionation is expressed during removal of zn and cd as sulfides while zn and cd removed with biological material are about 0 6 and 0 2 lighter than deep ocean 66zn and 114cd respectively many previous studies attribute the burial of isotopically light biological zn to preferential assimilation of isotopically lighter zn by phytoplankton in contrast we propose that lighter biological 66zn occurs mostly because isotopically heavy zn was scavenged by organic matter which causes dissolved 66zn in the surface ocean to be lighter than deep ocean 66zn the organic matter originated from primary producers that thrived in the surface ocean during deglaciation for cd whose isotope cycling in the modern ocean is dominated by preferential assimilation of isotopically lighter cd by phytoplankton much less biological fractionation is expressed the combined analyses of 66zn and 114cd provide insight into the biogeochemical processes which occurred after the marinoan snowball earth glaciation and may be a useful new tool to explore the history of life on earth in other geological records as well

pb isotopic zoning of k feldspar megacrysts determined by laser ablation multi collector icp ms insights into granite petrogenesis this study investigates pb isotopic zoning in magmatic k feldspar megacrysts from the monte capanne pluton elba italy using laser ablation multi collector icpms the studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open system processes and better characterize the components involved in the genesis of complex magma systems earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal derived magmas the investigated k feldspar megacrysts exhibit strong zoning in 207pb 206pb and 208pb 206pb correlated with lead elemental variations we interpret these variations as reflecting growth zoning as opposed to secondary diffusive exchange despite a great variety of zoning patterns we were able to correlate different events of megacryst growth reflecting crystallization in a dynamic magma system our two step model includes 1 growth of a granitic magma chamber by addition of low 208pb 206pb magma to a high 208pb 206pb magma contaminated with crustal material i e the megacryst cores and 2 recharge by mantle derived magma i e the megacryst rims we interpret the thorogenic nature of the megacryst rims to reflect the mantle derived component involved in the mixing process taking account of other data from the tuscan magmatic province the mantle source is inferred to have been metasomatized by continental material during subduction tims sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled

stratification and mixing of a post glacial neoproterozoic ocean evidence from carbon and sulfur isotopes in a cap dolostone from northwest china abstract to improve our knowledge about the geochemical and environmental aftermath of neoproterozoic global glaciations we analyzed stable isotopes 13 c 18 o 34 s and elemental concentrations ca mg s sr fe and mn of the 10 m thick zhamoketi cap dolostone atop the tereeken diamictite in the quruqtagh area eastern chinese tianshan available chemostratigraphic data suggest that the tereeken diamictite is probably equivalent to the marinoan glaciation our new data indicate that organic and carbonate carbon isotopes of the zhamoketi cap dolostone show little stratigraphic variations averaging 28 2 and 4 6 respectively in contrast sulfur isotopes show significant stratigraphic variations carbonate associated sulfate cas abundance decreases rapidly in the basal cap dolostone and 34 s cas composition varies between 9 and 15 in the lower 2 5 m in the overlying interval cas abundance remains low while 34 s cas rises 5 and varies more widely between 10 and 21 the range of 34 s py of the cap dolostone overlaps with that of 34 s cas but direct comparison shows that 34 s py is typically greater than 34 s cas measured from the same samples hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation we propose that cas and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone in this model cas was derived from shallow oxic surface waters with moderate sulfate concentration and depleted in 34 s due to the post glacial influx of sulfur from continental weathering in contrast pyrite was derived from anoxic bottom waters or a sulfate minimum zone with low sulfate concentration and 34 s enrichment due to long term syn glacial sulfate reduction the rapid shift in cas abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation comparison with other post marinoan cap carbonates shows significant spatial heterogeneity in 34 s cas which together with strong temporal variation in 34 s cas points to generally low sulfate concentrations in post marinoan oceans

continental weathering intensity during the termination of the marinoan snowball earth mg isotope evidence from the basal doushantuo cap carbonate in south china abstract the neoproterozoic global glaciations 720 635 ma represent the most severe icehouse climatic conditions in earth s history collectively known as the snowball earth events the snowball earth hypothesis hoffman et al 1998 proposed that the termination of the global glaciation was driven by the extremely high atmospheric pco2 level 300 times of present atmospheric level pal that was accumulated during the global glaciation by volcanic degassing for tens of million years because of extremely high temperature the deglaciation might associate with intense continental weathering that brought abundant bicarbonate and alkaline earth metals mg ca to the ocean resulting in the rapid precipitation of cap carbonate all over the world thus cap carbonate precipitation marks the termination of global glaciation the deglacial intense continental weathering has been recently supported by mg isotopes however the mg isotope data also indicate that the deglaciation might have significantly predated the cap carbonate precipitation and limited data suggest that the weathering intensity was already low during the cap carbonate precipitation in order to further test whether the cap carbonate precipitation was associated with low weathering intensity here we analyzed mg isotopic composition of siliciclastic component 26mgsil of the doushantuo cap carbonate in the yangtze block south china four cap carbonate sections the tongle tl drill core the fanyang fy section the wuluo wl drill core and the jiulongwan jlw section spanning from shelf to basin environments were analyzed 26mgsil ranges from 0 43 to 0 39 n 10 from 0 31 to 0 48 n 9 from 0 06 to 0 10 n 7 and from 1 42 to 0 03 n 8 for the jlw fy wl and tl samples respectively there are negative correlations between mg al and 26mgsil in jlw wl and tl the geochemical data indicate that the variation of 26mgsil might be attributed to authigenic clay formation during the cap carbonate precipitation the binary mixing model indicates that the isotopic compositions of terrestrial clays range from 0 to 0 3 comparable to the values of modern soil clays but significantly lower than the values of the upper nantuo formation up to 0 90 suggesting the moderate to low weathering intensity during the cap carbonate deposition our study suggests low atmospheric pco2 level during cap carbonate precipitation probably due to the consumption by intense continental weathering

carbon sulfur and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the ediacaran hankalchough glaciation in order to understand spatial variations of stable isotope geochemistry in the quruqtagh basin northwestern china in the aftermath of an ediacaran glaciation we analyzed carbonate carbon isotopes 13ccarb carbonate oxygen isotopes 18ocarb carbonate associated sulfate sulfur 34scas and oxygen isotopes 18ocas and pyrite sulfur isotopes 34spy of a cap dolostone atop the ediacaran hankalchough glacial diamictite at four sections the four studied sections ykg mk h and zbs represent an onshore offshore transect in the quruqtagh basin our data show a strong paleobathymetry dependent isotopic gradient from the onshore to offshore sections 13ccarb values decrease from 2 to 16 vpdb whereas 18ocarb values increase from 4 to 1 vpdb both 34scas and 34spy show stratigraphic variations in the two onshore sections mk and ykg but are more stable in the two offshore sections h and zbs 18ocas values of onshore samples are consistent with terrestrial oxidative weathering of pyrite we propose that following the hankalchough glaciation seawater in the quruqtagh basin was characterized by a strong isotopic gradient the isotopic data may be interpreted using a three component mixing model that involves three reservoirs deep basin water surface water and terrestrial weathering input in this model the negative 13ccarb values in the offshore sections are related to the upwelling of deep basin water where anaerobic oxidation of dissolved organic carbon resulted in 13c depleted dic whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite the new data provide evidence for the oceanic oxidation following the hankalchough glaciation

sulfur cycling in the aftermath of a 635 ma snowball glaciation evidence for a syn glacial sulfidic deep ocean we have analyzed 34 s sulfate and pyrite 18 ocarbonate and 13 ccarbonate and major and trace elemental concentrations including extractable fe and mn phases in four sections of the maieberg formation the cap carbonate sequence to the marinoan glaciation in northern namibia 34 ssulfate profiles and other geochemical characteristics in the basal transgressive cap dolostone keilberg member are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations in the overlying interval that consists of rhythmites deposited during the transgressive high stand large geochemical disparities occur between sections that we interpret to have been deposited in open ocean versus restricted settings in the former a large negative shift in 34 ssulfate of 20 above the cap dolostone accompanies a change in mineralogy from dolomite to limestone a sharp decrease in 18 o and 13 c and a positive spike in fe and mn concentrations in the latter dolomite persists above the cap dolostone 34 ssulfate increases abruptly 18 o and 13 c are invariant and a spike in mn and fe concentrations is present but subtle these contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold euxinic deep water and a cap of warm oxic brackish water flooded the continental shelf during the post glacial transgression the geochemical di e rences between open ocean and restricted sections diminishes upsection presumably recording the gradual mixing and homogenization of the waters along the otavi platform

geochemistry of ediacaran cap dolostones across the yangtze platform south china implications for diagenetic modification and seawater chemistry in the aftermath of the marinoan glaciation cap dolostones of the ediacaran doushantuo formation yangtze platform south china from various palaeo water depths were studied to evaluate the extent of their diagenetic alteration and to assess temporal and spatial variations of seawater chemistry in the aftermath of the marinoan glaciation diagenetic fluid overprint is common in cap dolostone lithologies however the mobilization of trace elements and the modification of sr and o isotopic compositions are variable and were controlled by multiple stages of fluid overprint the highest 87 sr 86 sr ratios up to 0 7246 occur in cap dolostones from the palaeo slope environment at the huanglianba section whereas cap dolostones deposited in platform settings and in one of the distal basinal settings reveal 87 sr 86 sr ratios close to the proposed late neoproterozoic seawater composition 0 7077 shale normalized ree y patterns of carbonate leachates display enrichments of heavy over light ree and superchondritic y ho ratios typical of seawater however y ho ratios in the cap dolostones are always lower than modern seawater values which is interpreted to reflect dilution of the seawater signal by continent derived meltwater influx during deglaciation negative ce anomalies in carbonate leachates from platform and slope sections suggest that oxidized conditions existed in shallow marine environments shortly after the marinoan glaciation whereas positive or no resolvable ce anomalies in basin settings indicate that the latter remained anoxic redox stratification of the yangtze margin at the beginning of the ediacaran is further supported by the relative enrichment of redox sensitive trace metals in basinal sections these data may indicate moderately anoxic less than 10 m dissolved o 2 and presumably manganous conditions during the deposition of cap dolostones in the deeper realms of the yangtze basin supplementary material supplementary data for this paper are available at https doi org 10 6084 m9 figshare c 3770957

neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites abstract neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large cryogenian glaciations several thin dolomite units that have features resembling cap dolomites occur within the cryogenian ediacaran succession of the adelaide geosyncline while each of these dolomite units has distinctive features they also share many similarities with cap dolomites including stratigraphic thickness ranging from several centimetres to several metres impure dolomicrite composition graded beds with a sharp top that become more dolomite rich upwards and evidence of volume expansion in the form of sheet cracks tepees or other soft sediment deformation structures the carbon isotopic compositions for these thin dolomite horizons have values ranging from 5 to 2 per mil vpdb which overlap those commonly found in both sturtian and marinoan cap dolomites the generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds volume expansion structures are a product of this near surface dolomicrite precipitation brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited marine carbonate source for the precipitating dolomite we suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of cryogenian early ediacaran oceans and caused by alkalinity producing anoxic bacterial reactions when sedimentation rates slow sufficiently e g during transgressions thin dolomicrite horizons are produced in this hypothesis cap dolomites may largely be the result of sediment starvation combined with a possible increase in carbonate saturation during and following the post glacial transgressions associated with the sturtian and marinoan glacial events this scenario explains most of the sedimentological features of these unusual carbonates the widespread development of such authigenic carbonate would have profound implications for the interpretation of precambrian marine carbon isotopic compositions

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

stratigraphic expression of earth s deepest δ13c excursion in the wonoka formation of south australia the most negative carbon isotope excursion in earth history is found in carbonate rocks of the ediacaran period 635 541 ma known colloquially as the shuram excursion workers have long noted its broad concordance with the rise of abundant macro scale fossils in the rock record collectively known as the ediacaran biota thus the shuram excursion has been interpreted by many in the context of a dramatically changing redox state of the ediacaran oceans for example a result of methane cycling in a low o2 atmosphere the final destruction of a large pool of recalcitrant dissolved organic carbon doc and the step wise oxidation of the ediacaran oceans more recently diagenetic interpretations of the shuram excursion have challenged the various redox models with the very negative 13c values of ediacaran carbonates explained via sedimentary in growth of very 13c depleted authigenic carbonates meteoric alteration or late stage burial diagenesis a stratigraphic and sedimentological context is required to discriminate between these explanatory models and to determine whether the shuram excursion can be used to evaluate oxygenation in terminal neoproterozoic oceans here we present chemo stratigraphic data 13c 18o and trace element abundances from 15 measured sections of the ediacaran aged wonoka formation fm of south australia in some locations the wonoka fm is 700 meters m of mixed shelf limestones and siliciclastics that record a 17 permil 13c excursion 12 to 5 further north in the basin the wonoka fm is host to deep 1 km paleocanyons which are partly filled by tabular clast carbonate breccias canyon filling ceased during ongoing sedimentation on the shelf interior the canyon shoulder as evidenced by upper canyon shoulder units that overlie and cap certain canyon fill sequences the unprecedented size of the chemostratigraphic dataset presented here 2671 13c 18o measurements from the canyon shoulder 1393 13c 18o measurements from canyon clasts and 11 different trace elements measured on 247 wonoka fm carbonate samples when coupled with the unique canyon shoulder to canyon fill depositional system of the wonoka fm allows for new insights into ediacaran carbon isotope systematics the excursion is preserved in a remarkably consistent fashion across 12 000 km2 of basin area fabric altering diagenesis where present occurs at the sub meter vertical scale results in

shelf to basin transition in the cambrian lower ordovician of sardinia italy reply in southern and central sardinia two different cambrian lower ordovician sedimentary belts occur the former is exposed in southwestern sardinia iglesiente and sulcis and is characterized by shallow marine siliciclastic and carbonate sediments settled on a continental shelf with lowangle morphology the latter exposed in the nappe zone central and southeastern sardinia consists of a thick monotonous sequence of siliciclastic deposits settled in a deeper water basin

ore forming processes in the wang ershan gold deposit jiaodong china insight from microtexture mineral chemistry and sulfur isotope compositions abstract the microtexture trace element characteristics and sulfur isotopic features of ore related minerals provide important clues to trace physico chemical conditions and source characteristics of ore formation in this study we investigate the wang ershan gold deposit in the jiaodong gold province of eastern north china craton where three hydrothermal stages were distinguished stage i is characterized by k feldspar sericite quartz with minor pyrite precipitation py i sericite quartz qtz a and pyrite py ii with minor gold precipitated during stage ii and stage iii involved quartz qtz b gold and polymetallic sulfides with minor pyrite py iii in situ 34s results show a decreasing trend from 21 7 to 14 6 py i to 9 2 8 3 py ii and 8 9 8 6 py iii the py ii and py iii are poor in mineral inclusions and porous microtextures and have no oscillatory composition zoning the co content is 1 2 order of magnitudes higher in py i 371 2 823 2 ppm av 597 2 ppm than late stage pyrites 0 4 524 2 ppm av 95 4 ppm qtz a show cataclastic breccia cemented by qtz b with little dissolution during the injection of stage iii ore forming fluids the al contents decrease from qtz a 49 0 426 6 ppm to qtz b 18 2 46 8 ppm based on the sulfur isotope trace element and microtextural characteristics in py ii and py iii combined with previous fluid inclusion studies we infer that the ore forming fluids of stage ii and iii experienced gentle immiscibility and were suggested to be of different evolutionary stages of a single ore forming fluid sharing common sources with similar and near equilibrium physicochemical conditions the trace element characteristics with lack of oscillatory compositional zoning in the two stages of quartz suggest gentle cooling with lack of any abrupt change of physicochemical conditions during fluid immiscibility the decoupling of au and as contents in py ii and py iii was likely controlled by chemisorption of gold in pyrite structure complex variation in geochemical features of ore forming fluids from early alteration to main mineralization period is indicated by the large range of 34s and co contents in py i and late stage pyrites and the differences in alteration processes based on a comparison of the co ni ni and as among pyrites from the wang ershan gold deposit typical porphyry deposits and orogenic gold deposits together with the large positive deviation of 34s relative to mantle values we propose that ore forming materials of the wang ershan gold deposit were derived from mixed sources involving magmatic hydrothermal related and sedimentary strata related components

stratigraphic investigations of carbon isotope anomalies and neoproterozoic ice ages in death valley california an unusual richness of biogeochemical events is recorded in neoproterozoic cambrian strata of the death valley region california united states eight negative carbon isotope d13c excursions are found in carbonate units between 1 08 ga and the precambrian cambrian boundary four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial cap carbonates including one or more of the following laminite with rollup structures apparent tube rocks seafloor precipitates and sheet crack cements however only two of these units the sourdough limestone member of the kingston peak formation and the noonday dolomite rest directly upon glacial strata the basal beck spring dolomite and the rainstorm member of the johnnie formation each contain negative excursions and cap carbonate like lithofacies but do not rest on known glacial deposits if the negative d13c excursions are assumed to record depositional processes two equally interesting hypotheses are possible 1 the death valley succession records four glacial pulses in neoproterozoic time but glacial units are not preserved at two stratigraphic levels 2 alternatively other global oceanographic processes can cause negative excursions and cap carbonate like facies in addition to or independent of glaciation

neoproterozoic loess and limits to snowball earth abstract neoproterozoic tillites overlain by limestones and dolostones cap carbonates have been interpreted as evidence of abrupt climate change from glaciers to tropical seas on the assumption that cap carbonates were marine this assumption is here challenged by evidence for loess deposition of the nuccaleena formation of south australia from granulometry angular randomly oriented silt modal diameter 7 and sedimentary structures climbing translatent cross stratification linear dunes in addition a variety of palaeosols in the nuccaleena formation have red clayey horizons replacive micritization expansion cracks and thufur mounds as well as low carbon and oxygen isotope values high strontium isotope ratios and geochemical evidence of leaching and clay formation this cap carbonate has no definitively marine features and is here interpreted as periglacial loess overlying the fluvioglacial intertidal elatina formation so it is not a record of abrupt global warming from snowball earth other cap carbonates around the world differ in various ways and may have formed in different depositional environments but could profitably be reconsidered from the new perspective of the loess depositional model proposed here supplementary material tabular comparison of marine and non marine interpretations of cap carbonates petrographic textures and composition and new isotopic and chemical analyses of nuccaleena formation are available at http www geolsoc org uk sup18443

industrial mineral occurrences associated with cenozoic volcanic rocks of sardinia italy geological mineralogical geochemical features and genetic implications the most important industrial mineral im deposits associated with the cenozoic volcanics of sardinia include kaolin bentonite and zeolites though of less significance perlite and potassium feldspar are also important in local and national markets bentonite and to a lesser extent kaolin and perlite form economically viable deposits and zeolites appear as very important industrial mineral commodities for future economic development the market situation for potassium feldspar is still under evaluation for use in ceramics all these deposit types occur as alteration products of cenozoic volcanic rocks which were emplaced during three main volcanic cycles in the eocene ec 55 to 51 ma oligocene miocene omc 32 to 13 ma and pliocene pleistocene ppc 5 to 0 1 ma kaolin bentonite zeolites and potassium feldspar occur as alteration products of the omc volcanites perlite occurs as an alteration product of the ppc volcanites all deposits have generally derived from the alteration of the most acidic members of the cenozoic volcanic rocks rhyolites to rhyodacites but differences among the associated ore forming phenomena are recognized hydrothermal sensu stricto deuteric alteration and weathering are the most important processes involved in ore formation weathering although of minor importance can also contribute to enhance the alteration of the deposits formed by hydrothermal and deuteric processes the extent and type of occurrences are mainly controlled by the local geological and structural features and the environmental geochemical conditions kaolin occurs in three main districts at romana and tresnuraghes it is hosted within ignimbritic sequences while at serrenti furtei it is found in pyroclastic flows and ash flow tuffs kaolin formed by hydrothermal processes occurs in high sulfidation romana and serrenti furtei and low sulfidation tresnuraghes type epithermal systems the alteration is proximal to the feeder conduits and structural setting controls kaolin formation bentonite is widespread throughout sardinia deposits occur as stratiform mineral bodies in situ altered within poor welded pumice rich and ash rich layers and also as distal fall out deposits as lenticular bodies interlayered within the miocene sedimentary sequences the former originated by deuteric alteration and formation is mainly controlled by lithology whereas the second type originated by weathering which altered the volcanic ash lenses deposited in lacustrine and or fluvial deltaic environments

multiple early paleozoic volcanic events at the northern gondwana margin u pb age evidence from the southern variscan branch sardinia italy abstract in sardinia one of the southernmost remains of the european variscan belt a crustal section through different gondwanan paleodomains is largely preserved laser ablation icp ms u pb ages on undoubtedly igneous sites of the zircons were determined on the lower palaeozoic volcanic rocks constrained by defined field relationships thus evidencing three subsequent volcanic events i intermediate and felsic 491 7 3 5 ma 479 9 2 1 concordia ages transitional volcanic rocks embedded within a cambro ordovician terrigenous succession that occurs with continuity in external and inner nappes bounded to the top by the sardic unconformity ii this cambrian lower ordovician succession is cut by calc alkalic rhyodacites which yielded a concordia age of 465 4 1 4 ma confirming their pertinence to the huge bimodal mid ordovician arc volcanism commonly interpreted as the widespread marker of the rheic ocean subduction iii alkalic metaepiclastites in the external nappe within the post caradocian transgressive sequence dated at 440 1 7 ma likely related to rifting and collapse of the mid ordovician volcanic arc in the reshaped lower palaeozoic stratigraphy of sardinia the timing of the early steps of the variscan wilson cycle can be inferred

does the global stratigraphic reproducibility of δ13c in neoproterozoic carbonates require a marine origin a pliocene pleistocene comparison the large and systematic negative shifts in the 13 c values 12 of carbonate dominated rocks that preceded neoproterozoic glacial successions have been interpreted to record a dramatic series of global environmental and evolutionary events these values are widely considered to be marine rather than diagenetic in origin because stratigraphic patterns of change are systematic and reproducible from basin to basin distinct in magnitude and associated with recognizable stratigraphic markers such as glacial deposits in contrast diagenetic systems are commonly considered to have a more local and stochastic influence on 13 c values cores taken in quaternary carbonate platform sediments however reveal a curious similarity in magnitude thickness and core to core reproducibility where diagenetic alteration has occurred in response to sea level fall sea level changes produced similar 13 c and 18 o stratigraphic records at globally disparate locations which are unrelated to the global marine 13 c signal and bear no relation to the global carbon cycle by analogy with the pliocene pleistocene we propose that spatial reproducibility of 13 c in some neoproterozoic successions might be attributed to causes other than secular variation of the global carbon cycle including diagenesis this observation does not negate the stratigraphic utility of the carbon isotopic values only the origin of the signal

life geology and snowball earth according to the snowball earth hypothesis a series of global glaciations occurred 750 580 million years ago each lasting for millions of years and ending in a scorching heat caused by an extreme enrichment of atmospheric greenhouse gases hyde et al 1 have used climate models to simulate this global glaciation finding in one case an alternative climate scenario in which a partially frozen earth has ice free oceans equatorward of 25 latitude we do not believe that this slushball earth is consistent with the most striking geological and palaeomagnetic observations explained by the snowball earth hypothesis

chemostratigraphy of neoproterozoic cap carbonates from the volta basin west africa abstract the neoproterozoic era includes some of the most extreme ice ages in earth history the exact number of glaciations is unknown although there were at least two events of global reach and possibly an equal number of lesser advances neoproterozoic glacial deposits in west africa have proven particularly difficult to correlate with better constrained successions elsewhere in most west african successions only a single glaciogenic deposit is present generally within a stratigraphic triad of tillite carbonate and bedded chert the age of these deposits and their synchroneity across the craton remain controversial we report isotopic data from carbonates that cap tillites in the volta basin sud banboli group in the three sections measured 13 c pdb begins at 2 to 3 and drops to 5 to 8 correlated with a drop in both 18 o and mg ca this pattern both the drop in 13 c and its correlation with 18 o and lithology has been observed in neoproterozoic cap carbonates on other cratons suggesting that it reflects primary depositional variations in addition the strong positive correlation among lithology 13 c and 18 o suggests that lithological variation may have exerted a principal control on isotopic variation implying that the dolomite formed in isotopic equilibrium with seawater as a primary precipitate or very early diagenetic replacement this would require low levels of sulfate estimated to be 1 mm or less consistent with available s isotopic data the presence of barite crusts in association with some cap carbonates and the conditions of seawater chemistry hypothesized in the aftermath of a snowball earth the stratigraphic pattern of carbon isotope variation in the volta basin cap carbonates suggests that underlying tillites are likely marinoan 650 600 ma in age this interpretation is supported by distinctive lithological characters observed in volta basin cap carbonates and accepted marinoan correlatives tillites elsewhere on the craton have been assigned latest proterozoic or early cambrian ages either the basis for these proposals requires reexamination or west african tillites record more than one glacial event

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

continental weathering following a cryogenian glaciation evidence from calcium and magnesium isotopes a marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger cryogenian glaciation have been inferred from boron isotope measurements calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time data from neoproterozoic sections in namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate dominated one combined with the effects of primary dolomite formation in the cap dolostones this caused the ocean to depart from a state of acidification and return to higher ph after climatic amelioration differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern congo craton shelf point to local influences modifying and amplifying the global signal which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux

superheavy s isotopes from glacier associated sediments of the neoproterozoic of south china oceanic anoxia or sulfate limitation black shales and mn carbonates interbedded with glacial deposits from the neo proterozoic of southern china exhibit extremely heavy values of pyrite s isotopes that may reflect the peculiar environment of earth at this time 34 s averages 30 at tanganshan and 44 at xiangtan compared with typical values of 0 to 5 found in younger deposits furthermore there is no distinction between the shales and the mn carbonate ores in the neoproterozoic unlike the younger deposits which show much lighter 34 s in the shales than in the mn ores the spread is 25 most other chemical parameters are very similar to both the younger mn deposits and those from the paleoproterozoic the exception is the rare earth elements ree all neoproterozoic fe ores and most neoproterozoic mn ores lack the positive eu anomaly that characterizes archean and paleoproterozoic fe mn accumulations on the other hand neoproterozoic mn deposits have positive ce anomalies on north american shale composite nasc normalized plots in contrast to other mnco 3 ores the ree is also higher than in other mn deposits but lower than in modern deep sea crusts sulfide s values in all neoproterozoic shales tend to be exceptionally variable and to often show much heavier values than can be found in marine strata from the phanerozoic therefore the anomalous 34 s values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects times of enrichment of seawater sulfate in 34 s do not correspond to periods of glaciation so the likely cause of the s isotope patterns is not worldwide glaciation but a generally low level of dissolved sulfate s in the neoproterozoic oceans that allowed modest increases in the amounts of s removed as pyrite to drive down the oceanic s reservoir enough to produce strong rayleigh reservoir effects the abundance worldwide of sturtian age mn and fe deposits indicates an increase in fe flux to the oceans that would have been sufficient to depress so 4 2 levels severely and to result in residu al dissolved s extremely enriched in 34 s ree evidence indicates that most of this enhanced fe and mn flux came from diagenetic remobilization of detrital oxides

precambrian tidal and glacial clastic deposits implications for precambrian earth moon dynamics and palaeoclimate abstract over the past decade the study of precambrian clastic tidal rhythmites stacked laminae of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided accurate palaeotidal and palaeorotational data palaeotidal records obtained from tidal rhythmites may be systematically abbreviated however and derived periods and frequencies can be misleading the validity of such values including past length of day can be assessed by testing for internal self consistency through application of the laws of celestial mechanics such a test supports the estimated length of day of 21 9 0 4 h derived from the late neoproterozoic 620 ma elatina reynella rhythmites in south australia and the indicated mean rate of lunar retreat of 2 16 0 31 cm year since 620 ma the validity of estimated lengths of day obtained from other precambrian tidal rhythmites remain unverified because the data sets contain only one primary value directly determined from the rhythmites the elatina reynella data militate against significant earth expansion at least since 620 ma and suggest that the free nutation or tipping of the earth s fluid core has undergone a resonance with the earth s annual forced nutation since the neoproterozoic glaciogenic deposits are readily distinguishable from ejecta resulting from impacts with earth crossing bodies palaeomagnetic data based on the geocentric axial dipole model for the geomagnetic field indicate that neoproterozoic and palaeoproterozoic glaciation and cold climate near sea level occurred in low palaeolatitudes this enigmatic finding may imply global glaciation or an increased obliquity of the ecliptic and is relevant to modelling the effect of ice sheet formation on the earth s obliquity history by obliquity oblateness feedback mechanisms through multidisciplinary studies clastic sedimentology and geophysics together can make substantial contributions to understanding precambrian earth moon dynamics and global palaeoenvironments

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

facies dependent δ13c variation from a cryogenian platform margin south australia evidence for stratified neoproterozoic oceans abstract analysis of a cryogenian interglacial platform margin in the adelaide geosyncline reveals a strong carbonate 13 c facies relationship detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11 in time equivalent strata shallow water back reef facies have 13 c values up to 8 2 while equivalent basinal sediments have 13 c values between 3 and 0 allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy 13 c values 4 9 and are surrounded by basinal calcareous shales with light 13 c values ave 0 8 the regional and stratigraphic consistency of these 13 c trends suggests a primary marine origin we interpret this 13 c facies correlation to be the result of ocean stratification stagnation that significantly reduced the rate of deep ocean ventilation and produced a large deep water organically derived carbon reservoir we suggest that stratification may have been a persistent feature of the global ocean throughout much of neoproterozoic time ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era including large scale 13 c variation extreme climatic fluctuations and the presence of cap carbonates a highly variable climatic regime would be expected with the development of a large deep water carbon reservoir small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface ocean and atmospheric reservoirs leading to intense greenhouse or icehouse conditions respectively

trace sulfate in mid proterozoic carbonates and the sulfur isotope record of biospheric evolution concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the earth s surface this balance is expressed in the sulfur isotope composition of seawater sulfate given the near absence of gypsum in pre phanerozoic sediments trace amounts of carbonate associated sulfate cas within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in precambrian seawater although absolute cas concen trations which range from those below detection to 120 ppm sulfate in this study may be compromised by diagenesis the sulfur isotope compositions can be buffered sufficiently to retain primary values stratigraphically controlled 34 s measurements for cas from three mid proterozoic carbonate successions 1 2 ga mescal limestone apache group arizona usa 1 45 1 47 ga helena and newland formations belt supergroup montana usa and 1 65 ga paradise creek formation mcnamara group nw queens land australia show large isotopic variability 9 1 to 18 9 1 1 to 27 3 and 14 1 to 37 3 respectively over stratigraphic intervals of 50 to 450 m this rapid variability ranging from scattered to highly systematic and overall low cas abundances can be linked to sulfate concentrations in the mid proterozoic ocean that were substantially lower than those of the phanerozoic but higher than values inferred for the archean results from the belt supergroup specifically corroborate previous arguments for seawater contributions to the basin limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep ocean sulfate reduction the scarcity of bedded gypsum and the stratigraphic 34 s trends and 34 s enrichments commonly observed for iron sulfides of mid proterozoic age copyright 2005 elsevier ltd

geochemical characterization of the marinoan cap carbonate of the niari nyanga basin central africa abstract despite recent works on the schisto calcaire group of the west congolian supergroup in central africa neoproterozoic cap carbonate have not been well documented in the niari nyanga basin that extends from gabon to angola the couplet cap carbonate scia and upper diamictite that is a marker of the snowball earth event was studied in 5 sections from the niari nyanga basin and in one section from the external zone of the mayombe folded belt mfb our paper presents a high resolution data set based on results from petrography geochemistry mineralogy and stable isotope studies performed on cap carbonate samples the estimation of the illite crystallinity indexes indicates different degrees of post sedimentary transformations evolving from deep diagenesis in the syncline basin to an epimetamorphism in the external zone of the mfb thin sections of rocks sampled in this latter section display metamorphic features materialized by recrystallization of minerals in a s1 cleavage plane despite such an observation all our samples clearly show preservation of primary petrographic structures such as biolaminations fenestrae and peloids geochemical data leads us to highlight that the cap carbonate is composed by a variety of carbonate types these results support the fact that the cap carbonate was derived from a combination of biological and chemical processes organomineralization and chemical precipitation from supersaturated solutions with a negligible impact of post depositional transformations for all the studied sections stable isotope profiles display a drop in 13c values from about 2 6 to 4 associated with 18o values ranging from 6 to 10 moreover the similarity of the isotopic profiles of the studied cap carbonate from the niari nyanga basin and the mfb and the close fit to the expected neoproterozoic 13c and 18o seawater values constitute a strong indication of a weak impact of a diagenetic to epimetamorphic evolution on the carbon isotopic signature of the scia cap carbonate in the light of this data set our isotopic records must be considered as a reliable tracer of the marinoan glaciation within the west congolian supergroup

no evidence of extraterrestrial noble metal and helium anomalies at marinoan glacial termination abstract high concentrations of extraterrestrial iridium have been reported in terminal sturtian and marinoan glacial marine sediments and are used to argue for long likely 3 12 myr durations of these cryogenian glaciations reanalysis of the marinoan sedimentary rocks used in the original study supplemented by sedimentary rocks from additional terminal marinoan sections however does not confirm the initial report new platinum group element concentrations and 187os 188os and 3he 4he signatures are consistent with crustal origin and minimal extraterrestrial contributions the discrepancy is likely caused by different sample masses used in the two studies with this study being based on much larger samples that better capture the stochastic distribution of extraterrestrial particles in marine sediments strong enrichment of redox sensitive elements particularly rhenium up section in the basal postglacial cap carbonates may indicate a return to more fully oxygenated seawater in the aftermath of the marinoan snowball earth sections dominated by hydrogenous osmium indicate increasing submarine hydrothermal sources and or continental inputs that are increasingly dominated by young mantle derived rocks after deglaciation sedimentation rate estimates for the basal cap carbonates yield surprisingly slow rates of a few centimeters per thousand years this study highlights the importance of using sedimentary rock samples that represent sufficiently large area time products to properly sample extraterrestrial particles representatively and demonstrates the value of using multiple tracers of extraterrestrial matter

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

an unusual occurrence of an exploitable k feldspar deposit hosted in the ordovician porphyroids southern sardinia geology mineralogy geochemical features and economic potential abstract an atypical potassium feldspar deposit of real economic potential has been found at arcu de su bentu southern sardinia italy the massive mineralization occurs in the middle ordovician porphyroids metavolcanic complex which consists of dominant metarhyolite and minor metarhyodacite lavas and or ignimbritic flows erupted in a sub aerial environment during variscan orogenesis the mineralized area was affected by low grade anchizone to greenschist facies metamorphism the deposit generally shows uniform macroscopic textural features over the entire outcrop the significant presence of k feldspar sanidine and quartz megacrystals the latter being generally smaller than the former mafic minerals do not occur in significant proportions although minor amounts of biotite and muscovite occur locally besides the uncommon lithological type the typical feature of this deposit is the unusual coarseness 10 to 20 cm in diameter of the potassium feldspar crystals the favourable geomorphological conditions of the area high k 2 o contents 6 to 8 wt high proportion of k feldspar in the mineralized rock 35 to 51 vol and large reserves estimated at 4 368 000 t in just one restricted part of the deposit are all favourable conditions for future economic exploitation in spite of this mineralogical and chemical impurities may adversely affect the possible use of k feldspar from arcu de su bentu in some conventional applications e g manufacture of high quality products such as ceramics to assess the real future economic development of the deposit market specific beneficiation is required although it is important to note that the bulk raw materials even if they have a lower market value could be still be targeted to specific industrial sectors e g red gres

spatial and temporal variations in carbon and sulfur isotopic compositions of sinian sedimentary rocks in the yangtze platform south china abstract eight stratigraphic sections from the yangtze platform south china were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of sinian sedimentary rocks carbon isotope compositions of carbonate from lower sinian strata deposited between pulses of glacial diamictite range from 3 9 to 9 9 the more negative values recorded from mn carbonates suggest that in part the carbon was derived from the oxidation of organic matter during early diagenesis in the upper sinian strata 13c varies with the facies they are positive for the platform carbonates whereas they are negative from about 3 to 12 for the carbonates formed in or near basin environments it is evident that post depositional alteration accounts for the strongly negative 13c values however some information on primary isotopic compositions can be obtained by evaluating sample quality the primary 13c values of carbonates from the doushantuo formation are ca 3 to 4 and from the dengying formation as low as 6 34s values for pyrite from the lower sinian rocks are highly positive and range from values near the coeval seawater sulfate up to ca 60 in contrast 34s values for pyrite from the upper sinina rocks are negative down to 27 to 30 the feature of sulfur isotopic composition for the lower sinian rocks is consistent with the late proterozoic sulfur isotopic record it might be explained by the supercontinent model many basins on the yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher 34s values than the coeval seawater for a geologically long period an alternative explanation is based on the sulfate minimum zone smz modal because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower sinian shallow water environment particularly in the smz the residual sulfate might have been significantly enriched in 34s so that pyrite produced in closed diagenetic system would yield very positive 34s values

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

stratigraphy of the port nolloth group of namibia and south africa and implications for the age of neoproterozoic iron formations uncertainties in the number and age of glacial deposits within the port nolloth group have hindered both structural and stratigraphic studies in the neoproterozoic gariep belt of namibia and south africa these uncertainties are compounded by major lateral facies changes that complicate correlations locally herein we report the results of integrated geological mapping chemo and litho stratigraphic and sedimentological studies that shed light on the age and stratigraphic architecture of the port nolloth group particularly we have distinguished an additional glacial deposit herein referred to as the namaskluft diamictite which is succeeded by a ca 635 ma basal ediacaran cap carbonate this interpretation indicates that the stratigraphically lower iron bearing numees diamictite is not marinoan or gaskiers in age as previously suggested but is instead a ca 716 5 ma sturtian glacial deposit a sturtian age for the numees formation is further supported by the discovery of microbial roll up structures in the dark limestone of the bloeddrif member that caps the diamictite a re evaluation of the age constraints indicates that all neoproterozoic iron formations may be of sturtian age and thus indicative of secular evolution of the redox state of the ocean

iodine records from the ediacaran doushantuo cap carbonates of the yangtze block south china abstract late neoproterozoic cap carbonates formed following the cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the snowball earth events large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post marinoan cap carbonates across the globe but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations in this study we present i ca mg profiles of four sections from a shallow to deep water transect of the basal doushantuo cap carbonates one of the most extensively studied ediacaran units in south china most samples have i ca mg values lower than 1 mol mol with a few exceptions in the shelf sections up to 2 5 mol mol all calcite samples with extremely low 13c have iodine signal below detection limit suggesting carbonate precipitation from anoxic seawater or diagenetic fluids among the four sections the slope section has the lowest i ca mg values suggesting that it may have experienced more extensive fluid buffered diagenesis and more severe loss of io3 in combination with available major trace rare earth element and isotope data the temporal i ca mg variations support freshwater mixing and deep water upwelling in proximal sites and transient oxidation during deposition of the doushantuo cap carbonates