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

mineralogy early marine diagenesis and the chemistry of shallow water carbonate sediments abstract shallow water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of earth s chemical and climatic history one of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post depositional diagenetic alteration in this study we use paired measurements of calcium 44ca 40ca or 44ca and magnesium 26mg 24mg or 26mg isotope ratios in sedimentary carbonates and associated pore fluids as a tool to understand the mineralogical and diagenetic history of neogene shallow water carbonate sediments from the bahamas and southwest australia we find that the ca and mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis the observed variability in bulk sediment ca isotopes is best explained by changes in the extent and style of early marine diagenesis from one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the fluid fluid buffered to one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the precursor sediment sediment buffered our results indicate that this process together with variations in carbonate mineralogy aragonite calcite and dolomite plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers 13c 18o major minor and trace elements in shallow water carbonate sediments in the geologic record our results also provide evidence that a large shallow water carbonate sink that is enriched in 44ca can explain the mismatch between the 44 40ca value of rivers and deep sea carbonate sediments and call into question the hypothesis that the 44 40ca value of seawater depends on the mineralogy of primary carbonate precipitations e g aragonite seas and calcite seas finally our results for sedimentary dolomites suggest that paired measurements of ca and mg isotopes may provide a unique geochemical fingerprint of mass transfer during dolomitization to better understand the paleo environmental information preserved in these enigmatic but widespread carbonate minerals

zn sr isotope records of the ediacaran doushantuo formation in south china diagenesis assessment and implications abstract recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle of the past oceans this study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks a thorough evaluation of diagenetic effects and systematic variations of zn and sr isotope ratios in lower part of the ediacaran stratigraphic unit deposited in the aftermath of the marinoan glaciation in south china the influence of post depositional diagenesis on zn isotope compositions of the studied samples is assessed by comparing 66zn to other geochemical indexes 87sr 86sr 13c 18o and mn sr ratios in the five studied cap carbonate sections member i of the doushantuo formation dolostones from four sections have 66zn values positively correlated with 18o values and negatively correlated with 87sr 86sr ratios 0 7081 0 7204 these correlations suggest that these cap dolostone samples have been modified by post depositional diagenesis the light 66zn value 0 02 suggests that initial zn isotope ratios of cap dolostones could have been reset by hydrothermal fluids with relatively high zn concentration and low 66zn values by contrast carbonates from member ii of the doushantuo formation above cap dolostones are relatively pristine based on their low 87sr 86sr ratios 0 7079 0 7086 being indistinct from the proposed early ediacaran seawater 87sr 86sr values chemical and isotopic variations in these samples are interpreted to reflect primary signals that record paleo environmental changes of the early ediacaran ocean a rapid increase of 66zn from 0 3 to 1 1 occurs in the middle part of member ii accompanying by relatively invariant 87sr 86sr ratios that imply insignificant changes in input from continental weathering considering the limited change in atmospheric oxygen during this period the rapid 66zn raise indicates an increase in buried organic matters which is consistent with the coupled positive shift of 13ccarb as well as the fossil records found in the same strata these results provide insights into zn cycling in the post marinoan oceans and facilitate the application of zn isotopes in carbonates as a proxy for the fate of marine organic matter

perspectives on proterozoic surface ocean redox from iodine contents in ancient and recent carbonate abstract the proterozoic eon hosted the emergence and initial recorded diversification of eukaryotes oxygen levels in the shallow marine settings critical to these events were lower than today s although how much lower is debated here we use concentrations of iodate the oxidized iodine species in shallow marine limestones and dolostones to generate the first comprehensive record of proterozoic near surface marine redox conditions the iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters to assess the validity of our approach neogene quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate iodine contents despite the potential for diagenetic loss maximum proterozoic carbonate iodine levels are elevated relative to those of the archean particularly during the lomagundi and shuram carbon isotope excursions of the paleo and neoproterozoic respectively for the shuram anomaly comparisons to neogene quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends the baseline low iodine levels in proterozoic carbonates relative to the phanerozoic are linked to a shallow oxic anoxic interface oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes however the diagnostically low iodine data from mid proterozoic shallow water carbonates relative to those of the bracketing time intervals are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans this redox instability may have challenged early eukaryotic diversification and expansion creating an evolutionary landscape unfavorable for the emergence of animals

tidal pumping of water between bahamian blue holes aquifers and the ocean summary exchange of water between conduits and aquifers occurs in many continental karst settings because allogenic recharge from confined catchments causes hydraulic heads in conduits to increase faster than in the aquifer most modern carbonate platforms lack allogenic catchments allowing rainfall to recharge the aquifer uniformly without sufficiently altering head gradients to drive exchange between conduits and aquifers some modern carbonate platforms experience tidal variations which could lead to head gradients that drive exchange to determine the impact of tides on exchange we measured water elevations at high temporal resolution in the ocean blue holes and wells on san salvador island and rum cay bahamas dampened tidal amplitudes inland indicate diffusivity values transmissivity storativity at wells were around 1 3 10 6 m 2 day and at blue holes were around 76 9 10 6 m 2 day assuming dampening results only from head loss these diffusivity values were used to estimate hydraulic conductivity values of around 4 0 294 10 4 m day although they may be lower if the aquifer thickness is greater than the estimated 10 m we assume wells provide values representing greater influence of matrix permeability than values from blue holes which represent a greater influence of conduit permeability differences in permeability drive exchange because hydraulic head in the aquifer lags the head in the conduits and blue hole through a tidal cycle if negligible head loss occurs with flow through conduits as reflected in lag times less than 10 min then differences in elevation at the blue holes and the ocean may represent exchange of water between the blue hole and matrix porosity with this assumption about 0 9 m 3 of water is exchanged per half tidal cycle or about 1 of the complete change in volume of water in the blue hole per half tidal cycle this volume represents an average penetration into the aquifer of 6 8 mm although it could be further in zones with elevated permeability exchange is reflected in systematic changes in specific conductivity and ph between high and low tide and the ph changes reflect reaction with the surrounding aquifer material since exchange occurs twice daily cumulative alteration of aquifer porosity could be large tidal exchange should decrease away from the coast on large carbonate platforms so that tidally driven alteration will be enhanced at the rims over interior of carbonate platforms

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

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

methane derived authigenic carbonate from the lower doushantuo formation of south china implications for seawater sulfate concentration and global carbon cycle in the early ediacaran ocean authigenic carbonate associated with anaerobic oxidation of methane aom usually via microbial sulfate reduction msr or ferric iron reduction is generally characterized by extremely low 13c values 30 vpdb this has been used as one of the major diagnostic features for the recognition of hydrocarbon seep carbonate in the geological past previous reports on precambrian authigenic carbonates are rare limiting our understanding of the effects of their deposition on the earth s carbon isotopic mass balance in this study mainly based on petrographic features and pronounced negative 13c values as low as 38 1 we discovered authigenic calcite cement immediately above the cap dolostone in the basal ediacaran doushantuo formation in the jiulongwan section yangtze gorges area south china our observations not only provide direct evidence for the involvement of aom during carbonate precipitation in the early ediacaran 635 ma but also suggest that the seawater sulfate concentrations in the early ediacaran may have been higher than previously thought

tracing crustal fluids applications of natural 129i and 36cl the two cosmogenic isotopes 129i and 36cl have half lives and geochemical characteristics that make their application of interest in the tracing of crustal fluids oil field brines and geothermal fluids the focus of this review is to compare 129i data from volcanic fluids with those from mud volcanoes and gas hydrate locations associated with the same subduction zone in order to demonstrate that fundamentally different hydrologic systems are present in active margins whereas 129i i ratios in volcanic fluids are site dependent and show a relation to the ages of subducting marine sediments ratios in fore arc fluids are similar in all sites investigated and are independent of the ages of the host sediments and the age of the subducting slab volcanic fluids contain iodine transported in sediments from the trench to the main volcanic zone whereas iodine in fore arc fluids is derived from organic material stored in the upper plates of subduction zones

tracking the onset of phanerozoic style redox sensitive trace metal enrichments new results from basal ediacaran post glacial strata in nw canada abstract a global rise in oxygen levels has been proposed to coincide with the ediacaran to cambrian radiation of animals yet the precise timing and nature of this change remains unresolved one hypothesis is that the ocean atmosphere system became temporarily well oxygenated in the earliest ediacaran directly following the marinoan snowball earth glaciation 635 ma the evidence for oxygenation is based on large enrichments of redox sensitive trace elements in black shale from south china presumably deposited under a euxinic water column these enrichments meet or exceed the bulk concentrations of redox sensitive trace elements found in phanerozoic shale deposited under euxinic water columns here we test the early ediacaran post snowball oxygenation hypothesis with new data from a high resolution multi proxy geochemical and sedimentological study of three stratigraphic sections in the earliest ediacaran sheepbed formation of the mackenzie and wernecke mountains nw canada iron speciation data from all sections suggest that the local water column was dominantly ferruginous with a notable exception of probable euxinic conditions recorded in part of one section redox sensitive elements show no appreciable enrichments in the basal sheepbed formation with maximum concentrations of molybdenum vanadium uranium and chromium only slightly above world average shale values the lack of substantial elemental enrichments within the sheepbed formation is consistent with results from coeval ferruginous strata in svalbard the dominance of local ferruginous conditions in nw canada cannot alone account for the muted redox sensitive element enrichments as v u and cr are thought to be enriched under these conditions and samples deposited under euxinic conditions also lack phanerozoic style enrichments these contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution

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

iodine content of the carbonates from the doushantuo formation and shallow ocean redox change on the ediacaran yangtze platform south china abstract the ediacaran period 635 541 ma is characterized by remarkable carbon isotope 13c excursions significant ocean redox changes and rapid diversification of eukaryotes including early animals these biological and geochemical events are commonly thought to be linked but their causal relationships remain controversial a particular point relates to the shallow ocean redox change across the negative 13c excursions most existing geochemical proxy data focused on tracking the redox conditions of deep or bottom water columns and provided limited information about the surface ocean ratios of iodine to calcium magnesium i ca mg from carbonates represent one of the few redox proxies that track the oxidation state of the shallow oceans here we report paired carbonate carbon isotopes 13ccarb and i ca mg ratios of the doushantuo formation from a drillcore section at daotuo paleogeographically located at the slope of the ediacaran yangtze platform south china in addition we compare the i ca mg ratios across the shuram upper doushantuo 13ccarb excursion in four sections of a shelf to slope transect in the slope section at daotuo the 13ccarb of the doushantuo formation shows three negative anomalies that are correlatable with those documented from the yangtze gorges area shelf and the siduping section slope the i ca mg ratios of the doushantuo formation from the drillcore are dominated by low but non zero

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

organic carbon isotope constraints on the dissolved organic carbon doc reservoir at the cryogenian ediacaran transition abstract prominent negative carbonate carbon isotope 13 c carb anomalies from some ediacaran successions are accompanied by invariant or decoupled organic carbon isotope 13 c org values and have been interpreted as resulting from the remineralization of a large dissolved organic carbon doc reservoir capable of buffering carbon isotopes of organic matter this inferred oceanic doc reservoir was thought to have initiated with the onset of cryogenian glaciations ca 720 ma and lasted for millions of years until the late ediacaran period 13 c org around 26 vpdb and relatively low 13 c 22 2 and 2 the overlying organic rich black shale and shaly dolostone have more negative 13 c org 28 to 35 and higher 13 c 28 30 both 13 c carb and 13 c org show a 6 shift within a 4 m thick interval overlying the doushantuo cap carbonate the 13 c org values of the cap carbonate are associated with low toc mostly 13 c carb and 13 c org following cap carbonate deposition is best interpreted as resulting from a rapid increase in organic carbon burial which may have resulted in the rise of oxygen and heralded the first appearance of animals a few meters above the doushantuo cap carbonate the data suggest that a large oceanic doc reservoir did not exist in the early ediacaran ocean excess oceanic doc required to explain the ediacaran shuram and upper doushantuo 13 c excursions if it existed had to be developed during the ediacaran period after cap carbonate deposition

iodine to calcium ratios in marine carbonate as a paleo redox proxy during oceanic anoxic events periods of globally distributed extreme oxygen depletion so called oceanic anoxic events oaes have been recognized in the mesozoic geological record and appear to be characteristic of episodes of extreme warmth here we explore the application of iodine calcium ratios i ca in marine carbonate as a new geochemical proxy to constrain seawater redox change and provide additional insights into the response of ocean chemistry to ancient climatic warming iodine has long been known as a redox sensitive and biophilic element mainly present as iodate and iodide in seawater iodate converting to iodide under anoxic conditions laboratory experiments growing calcite crystals from solutions spiked with iodate show that this is the ionic species incorporated into the carbonate structure likely substituting for the co 3 2 ion a fall in the i ca ratio measured in carbonates formed in shallow water by marine calcifiers during the early toarcian and cenomanian turonian oaes is interpreted both as a response to a decrease in the iodate iodide ratio in ocean waters and the drawdown of the global iodine inventory under conditions of accelerated organic matter burial the results suggest that i ca ratios in carbonates may be used to monitor seawater oxidation levels throughout earth history

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

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

trace metal records of regional paleoenvironmental variability in pennsylvanian upper carboniferous black shales abstract regional geochemical differences within a laterally continuous cyclic pennsylvanian upper carboniferous shale in midcontinent north america are interpreted in light of models of glacioeustatic forcing and new views on water column paleoredox stability and trace metal behavior in black shale environments specifically we characterize differences in transition metal fe mn mo v ni zn pb and u concentrations in black shales of the hushpuckney shale member of the swope limestone in iowa and equivalent black shale beds of the coffeyville formation in oklahoma although c s fe systematics and uniform 34 s depleted isotope ratios of pyrite indicate pervasive euxinic deposition anoxic and sulfidic bottom waters for these shales regional variations can be inferred for the efficiency of mo scavenging and for the rates of siliciclastic sedimentation as expressed in spatially varying fe al ratios black shales in iowa show mo enrichment roughly five times greater than that observed in coeval euxinic shales in oklahoma by contrast fe al ratios in oklahoma shales are as much as five times greater than the continental ratio of 0 5 observed in the over and underlying oxic facies and in the coeval black shales in iowa recent work in modern marine settings has shown that enrichments in fe commonly result from scavenging in a euxinic water column during syngenetic pyrite formation in contrast to fe the concentrations of other transition metals mo v ni pb zn u are typically more enriched in the black shales in iowa relative to oklahoma the transition metal trends in these paleozoic shales are reasonably interpreted in terms of early fixation in organic rich sediments due to euxinic water column conditions however regional variations in 1 rates of siliciclastic input 2 organic reservoirs including relative inputs of terrestrial versus marine organic matter and 3 additional inputs of metals to bottom waters from contemporaneous hydrothermal vents are additional key controls that lead to geographic variation in the extent of metal enrichments preserved in ancient organic rich sediments

experimental observations of marine iodide oxidation using a novel sparge interface mc icp ms technique abstract iodine speciation is used as a redox indicator and water mass tracer however the kinetics and mechanisms of oxidation of the reduced iodine species iodide i in seawater remain largely unresolved here we provide constraints on i oxidation rates from seawater incubation experiments using a sensitive radiotracer incubation approach with high precision mass spectrometry samples are inoculated with iodine 129 as i incubated under controlled conditions then oxidized and reduced iodine species are isolated using liquid chromatographic separation samples are subsequently aspirated using a novel sparging interface and the 129i 127i ratio of both iodine species is determined via multi collector inductively coupled plasma mass spectrometry this highly sensitive analytical approach is optimized for estimating the predicted slow rates of iodine redox transformations in small volume seawater samples at marine relevant total iodine concentrations application of this method to coastal samples from martha s vineyard sound usa evidences the heretofore elusive oxidation of i under marine analogue conditions our results demonstrate an increase in the 129i 127i of aqueous oxidized iodine species in unamended seawater most likely due to formation of hypoiodous acid and then dissolved organic iodine at rates of 118 189 nm yr these rates are comparable to those inferred from previous open ocean mass balance approaches but have the benefit of directly linking iodine speciation changes to in situ i oxidation though our treatment experiments do not definitively isolate the oxidation mechanism they provide preliminary insight into important roles for both abiotic and biotic factors as well as iodine transformations between dissolved particulate and gaseous forms ultimately the derived rates and novel experimental technique have potential to improve marine iodine cycling models and provide constraints on the importance of in situ processes in setting patterns of iodine speciation

oxygenation history of the neoproterozoic to early phanerozoic and the rise of land plants there has been extensive debate about the history of earth s oxygenation and the role that land plant evolution played in shaping earth s ocean atmosphere system here we use the rare earth element patterns in marine carbonates to monitor the structure of the marine redox landscape through the rise and diversification of animals and early land plants in particular we use the relative abundance of cerium ceanom the only redox sensitive rare earth element in well preserved marine cements and other marine precipitates to track seawater oxygen levels our results indicate that there was only a moderate increase in oceanic oxygenation during the ediacaran average cryogenian ceanom 1 1 average ediacaran ceanom 0 62 followed by a decrease in oxygen levels during the early cambrian average cryogenian ceanom 0 90 with significant ocean anoxia persisting through the early and mid paleozoic average early cambrian early devonian ceanom 0 84 it was not until the late devonian that oxygenation levels are comparable to the modern average of all post middle devonian ceanom 0 55 therefore this work confirms growing evidence that the oxygenation of the earth was neither unidirectional nor a simple two stage process further we provide evidence that it was not until the late devonian when large land plants and forests first evolved that oxygen levels reached those comparable to the modern world this is recorded with the first modern like negative ceanom values 0 6 occurring at around 380 ma frasnian this suggests that land plants rather than animals are the engineers responsible for the modern fully oxygenated earth system

seawater buffered diagenesis destruction of carbon isotope excursions and the composition of dic in neoproterozoic oceans carbonate sediments of nonglacial cryogenian 659 to 649 ma and early ediacaran 635 to 590 ma age exhibit large positive and negative 13ccarb excursions in a shallow water marine platform in northern namibia the same excursions are recorded in fringing deep sea fans and in carbonate platforms on other paleocontinents however coeval carbonates in the upper foreslope of the namibian platform and to a lesser extent in the outermost platform have relatively uniform 13ccarb compositions compatible with dissolved inorganic carbon dic in the modern ocean we attribute the uniform values to fluid buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection this attribution which is testable with paired ca and mg isotopes implies that large 13ccarb excursions observed in neoproterozoic platforms while sedimentary in origin do not reflect the composition of ancient open ocean dic

trace and rare earth elemental investigation of a sturtian cap carbonate pocatello idaho evidence for ocean redox conditions before and during carbonate deposition abstract this study examines the trace element and ree y geochemistry of a thin pink dolomite in the scout mountain member of the pocatello formation idaho the pink dolomite meets the criteria of a cap carbonate and provides an opportunity to examine ocean redox state and hydrothermal inputs on the margin of laurentia at the terminal stages of the sturtian glaciations here high resolution chemostratigraphy of the dolomite shows that the local marine conditions were rapidly changing during cap emplacement normalized trace metal concentrations of zn cd cu cr mn and ni suggest that transiently reducing conditions prevailed immediately following the initial deposition of the pink dolomite enrichments in eu suggest a hydrothermal vent source for marine waters present during deposition the pink dolomite lacks evidence for persistent sulfide mineralization and its associated enrichments in mo suggesting that anoxia was restricted to areas below the sediment water interface no notable enrichments are recorded in the glacial diamictite though the signal might be masked by high rates of sedimentation our data suggest that the deposition of the pink dolomite progressed through 1 the overturn of local ocean containing a strongly stratified hydrothermally influence water column 2 the delivery of micronutrients to the shallow ocean 3 a biotic bloom in response to micronutrient availability and 4 related organic matter scavenging of metals finally we argue that this period of anoxia was brief and conditions quickly returned to an environment of oxic deposition

chromium isotope stratigraphy of ediacaran cap dolostones doushantuo formation south china abstract the yangtze platform in south china is considered a key site for studying neoproterozoic ocean oxygenation comprehensive cr concentration and isotopic signatures are presented from cap dolostones of the doushantuo formation from a deep water and a shallow marine carbonate section that have previously been used to constrain the redox evolution of ediacaran seawater in space and time employing major and trace element abundances as well as sr o and c isotope compositions the majority of the studied samples yield elevated cr concentrations and negatively fractionated 53 cr signals due to i detrital contamination consistent with cr signatures of other post marinoan cap dolostones ii diagenetic carbonate alteration and post depositional remobilization of cr al concentrations were used as indicator of detrital contamination and to calculate authigenic cr concentration and isotope signatures when the detrital contribution of cr was 18 o carb signals that correlate with low cr concentrations and negatively fractionated 53 cr values post depositional remobilization of cr results in the loss of heavy cr isotopes and consequently light cr isotopes are retained during precipitation of secondary carbonates however some samples might still show pristine ediacaran seawater signals two samples pertaining to the deep water cap dolostone section at yanwutan and one sample pertaining to the shallow water section at jijiawan yield slightly positively fractionated 53 cr signals particularly when corrected to presumably authigenic signals 53 cr auth these values are consistent with those from other neoproterozoic carbonate sections deposited in an oxygenated environment this short sequence of positively fractionated 53 cr auth is concurrent with a cap dolostone 87 sr 86 sr signal close to ediacaran seawater values with relatively high sr and cr concentrations with decreased detrital contamination and with a pronounced negative ce anomaly they indicate either light pulses of enhanced oxidative weathering or inferred from a few least altered samples prevailing oxic conditions during cap dolostone deposition the cr isotopic composition of ediacaran cap dolostones pertaining to the doushantuo formation reveals fine scale fluctuations in post glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering cr isotopes have the potential to further enhance our understanding of neoproterozoic weathering regimes and past ocean oxygenation however before the cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates possible diagenetic alteration of the cr signal as well as cr contribution from detrital contamination need to be taken into consideration

positive cerium anomaly in the doushantuo cap carbonates from the yangtze platform south china implications for intermediate water column manganous conditions in the aftermath of the marinoan glaciation abstract oxygenation exerted an important control on the emergence and diversification of metazoans in the aftermath of the marinoan glaciation however the relationship between the oceanic dissolved o2 do levels and early metazoan evolution remains equivocal to provide a temporal and spatial reconstruction of the redox conditions for this critical interval this study investigates the early ediacaran cap carbonate deposits within intra shelf facies at three localities in the three gorges area the yangtze platform south china trace and rare earth elements are sequentially extracted from the carbonate fraction for pristine temporal seawater signals the member ii dolostones of the duoshantuo formation which overlies the cap carbonates show a negative ce anomaly no ce anomaly is observed in the lower units of the cap carbonates in contrast positive ce anomalies ce ce 1 3 in the uppermost part of the cap carbonates are observed in all studied sections these positive ce anomalies are accompanied by high mn fe ratios ranging from 5 04 to 14 96 and slightly positive mree anomalies suggesting that a fe mn oxyhydro oxide coparticipation occurred during the cap carbonate deposition the positive ce anomalies may result from the reductive dissolution of the ce enriched mn oxyhydro oxides across a mn iv mn ii redoxcline in a distinct manganous water wedge that was sandwiched between well oxygenated and anoxic ferruginous deep water layers this wedge may have exhibited low oxygen conditions with approximately 10 m do dissolved oxygen in comparison to the 90 m do expected in an oxic setting and 0 m do in an anoxic setting therefore the presence of a positive ce anomaly in the uppermost part of the cap carbonates indicates a new intermediate manganous condition and further elucidates the water column redox structure that existed following the end of the marinoan glaciation early ediacaran metazoans were possibly restricted to fully oxygenated conditions and were absent in the manganous or ferruginous rich environments

illuminating the shadows tomography attenuation and pore pressure processing in the south caspian sea the south caspian sea is a major oil and gas producing area characterized by thick sedimentary deposits and complex tectonic structure structural trends include anticlinal folds steep dips and mud volcanoes many of the large anticlinal folds contain world class producing fields however conventional processing of seismic data often fails to correctly image through the seismic shadow zones frequently observed in this region these shadow zones are characterized by low amplitude events or low signal to noise ratio arrival time delay attenuation of high frequency information and distortion of underlying reflectors this often makes interpretation difficult to impossible

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