nitrogen cycle in the late archean ferruginous ocean abstract the hamersley group comprises a late archean sedimentary succession which is thought to record the prelude to the atmospheric oxygenation in the paleoproterozoic the so called great oxidation event goe at 2 4 ga we studied drill core samples of sedimentary rocks from the upper mount mcrae shale and brockman iron formation deposited before the goe at 2 5 ga in order to characterize the environments and ecosystems prevailing during their deposition the rocks from the mount mcrae shale and brockman iron formation represent respectively proximal euxinic conditions and distal ferruginous depositional environments thus providing an opportunity to examine lateral variability in the open marine basin we analyzed the concentration and isotopic composition of carbon in carbonate and organic matter bulk nitrogen content and its isotopic composition as well as major element concentrations the 13 c carb values and carbonate content range from 3 2 to 10 7 and 0 1 to 58 wt respectively organic carbon content also varies over a large range from 0 05 to 4 6 wt with a near constant 13 c org value of 28 7 0 8 negative 13 c org excursions down to 31 are generally correlated with high organic matter content bulk nitrogen shows highly variable concentration between 1 3 and 785 ppm and 15 n values between 0 4 and 13 4 the 13 c carb values reflect a diagenetic carbonate origin with negative values typical for fe rich carbonates formed by organic matter mineralization with ferric oxyhydroxides in contrast 13 c org and 15 n values record primary isotope signatures derived from ancient living organisms the relatively constant 13 c org values at around 28 7 are interpreted as reflecting photoautotrophs utilizing a large pool of dissolved inorganic carbon inverse stratigraphic co variation between 15 n and 13 c carb values was observed for the brockman iron formation we propose that n and c biogeochemical cycles were coupled by fe redox cycling in the water column and in sediments of the late archean ocean several models for biogeochemical n cycling linked to the redox structure of the water column are considered under fully anoxic conditions the dominant n species available for assimilation by micro organisms in the photic zone could be ammonium nh 4 highly positive 15 n values would reflect the assimilation of nh 4 enriched in 15 n by partial oxidation to nitrite followed by quantitative removal of the produced nitrite by denitrification or anamox processes ammonium oxidation could have been driven by i o 2 produced locally via oxygenic photosynthesis or ii microbial oxidation utilizing fe iii oxyhydroxides formed in the water column under redox stratified conditions n assimilated by primary producers could have been in the form of no 3 based on modern and phanerozoic analogs the positive 15 n values would have resulted in this case from partial denitrification of no 3 coupled to anaerobic microbial oxidation of fe ii to fe iii we conclude that similar positive 15 n signatures may record very different n biogeochemical cycles under anoxic stratified and fully oxic conditions in the ocean interpretation of the n isotopes in terms of n biogeochemical cycle thus requires independent constraints on the redox structure of the ocean

shallow stratification prevailed for 1700 to 1300 ma ocean evidence from organic carbon isotopes in the north china craton abstract the late paleoproterozoic to early mesoproterozoic from 1700 ma to 1300 ma was highlighted by the assembly of the nuna supercontinent expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes the isotopic composition of carbonate carbon 13ccarb was surprisingly constant during this interval but little is known about the secular variation in the organic carbon isotopic composition 13corg here we report 13corg data from the latest paleoproterozoic 1650 ma to early mesoproterozoic 1300 ma succession in north china the 13corg values range from 25 to 34 and are dependent on sedimentary facies in subtidal and deeper environments 13corg values are low and constant ca 32 but relatively enriched and more variable in shallower intertidal and supratidal environments we attribute the facies dependent variation in 13corg to the presence of a shallow chemocline a probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and or anaerobic photoautotrophic microbes in relatively deep environments from the latest paleoproterozoic to early mesoproterozoic continental shelf of north china

nitrogen isotope evidence for stepwise oxygenation of the ocean during the great oxidation event abstract the earth s oxygenation represents one of the most important environmental drivers of life s evolution with the first rise known as the great oxidation event goe corresponding to unpreceded accumulation of atmospheric o2 changes in the flux of marine nutrients and possibly global glaciations however the detailed evolution of the goe is still debated as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation here we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from two drill cores t2 and t3 in the early paleoproterozoic turee creek group western australia to track the redox evolution of marine conditions during the goe t2 core samples of the kungarra formation which consists of clastic sedimentary rocks overlaid by the glaciogenic meteorite bore member were deposited 2 31 ga ago t3 core intercepts from bottom to top quartzite of the koolbye formation and shales and stromatolitic carbonates of the kazput formation which were deposited around 2 25 ga samples from t2 show minor variations of 13corg avg 34 5 1 7 n 30 with no significant difference between siliciclastic and glaciogenic sedimentary rocks in contrast t3 samples display an increase in 13corg from 32 0 to 24 8 n 54 from shales to carbonates in both t2 and t3 cores 13corg values are inversely correlated with al2o3 suggesting a strong petrological control on 13corg values inferred here as resulting from variable contributions of detrital organic matter bulk n contents are low from 13 5 to 56 7 ppm and 15 7 to 53 4 ppm in t2 and t3 samples respectively the 15n values show a bimodal distribution with one mode at 2 6 in t2 and another at 8 8 in t3 independent from lithological variations this 15n values shift between t2 and t3 is interpreted as reflecting a change from dominating n2 fixers to no3 assimilating organisms this implies an increase of no3 availability and thus of o2 concentration during the time interval separating the deposition of t2 and t3 sediments dissolved no3 and o2 concentrations of the turee creek marine basin are estimated from two models using n isotope data the dissolved no3 concentration has an upper limit ranging from 1 91 to 3 04 m about one order of magnitude below the average value of modern oceans the lower limit for dissolved oxygen concentration ranges from 1 8 to 4 4 m which is two orders of magnitude lower than modern oceans together with previous studies the present data place quantitative constraints on the redox changes associated with the great oxidation event and illustrate a stepwise increase of no3 bioavailability between 2 31 to 2 25 ga in relation with increasing o2 level

clouds and snowball earth deglaciation neoproterozoic and possibly paleoproterozoic glaciations represent the most extreme climate events in post hadean earth and may link closely with the evolution of the atmosphere and life according to the snowball earth hypothesis the entire ocean was covered with ice during these events for a few million years during which time volcanic co 2 increased enough to cause deglaciation geochemical proxy data and model calculations suggest that the maximum co 2 was 0 01 0 1 by volume but early climate modeling suggested that deglaciation was not possible at co 2 0 2 we use results from six different general circulation models gcms to show that clouds could warm a snowball enough to reduce the co 2 required for deglaciation by a factor of 10 100 although more work is required to rigorously validate cloud schemes in snowball like conditions our results suggest that snowball deglaciation is consistent with observations

phanerozoic atmospheric co2 change evaluating geochemical and paleobiological approaches abstract the theory and use of geochemical modeling of the long term carbon cycle and four paleo pco 2 proxies are reviewed and discussed in order to discern the best applications for each method geochemical models provide pco 2 predictions for the entire phanerozoic but most existing models present 5 10 m y means and so often do not resolve short term excursions error estimates based on sensitivity analyses range from 75 200 ppmv for the tertiary to as much as 3000 ppmv during the early paleozoic the 13 c of pedogenic carbonates provide the best proxy based pco 2 estimates for the pre tertiary with error estimates ranging from 500 1000 ppmv pre devonian estimates should be treated cautiously error estimates for tertiary reconstructions via this proxy are higher than other proxies and models 400 500 ppmv and should not be solely relied upon we also show the importance of measuring the 13 c of coexisting organic matter instead of inferring its value from marine carbonates the 13 c of the organic remains of phytoplankton from sediment cores provide high temporal resolution up to 10 3 10 4 year high precision 25 100 ppmv for the tertiary to 150 200 ppmv for the cretaceous pco 2 estimates that can be near continuous for most of the tertiary its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short term excursions this method however must correct for changes in growth rate and oxygen level at elevated pco 2 750 1250 ppmv this proxy loses its sensitivity and is not useful the stomatal density and stomatal index of land plants also provide high temporal resolution 2 year high precision 10 40 ppmv for the tertiary and possibly cretaceous pco 2 estimates and so also is ideal for discerning short term excursions unfortunately this proxy also loses sensitivity at some level of pco 2 above 350 ppmv which currently is largely undetermined our analysis of the recently developed 11 b technique shows that it currently is not yet well constrained most importantly it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time in addition it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur a fifth co 2 proxy based on the redox chemistry of marine cerium has several fundamental flaws and is not discussed in detail here

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

exploring the usability of isotopically anomalous oxygen in bones and teeth as paleo co2 barometer fluctuations in atmospheric pco2pco2 may have played the key role in global climate throughout earth s history for the quantification of past variations in atmospheric pco2pco2 several geological proxy approaches and geochemical models have been developed here we evaluate a new co2 proxy approach that is based on the triple oxygen isotope composition 16o 17o 18o of skeletal apatite of terrestrial mammals our approach utilizes the relation between an anomaly in 17o of tropospheric air o2 and atmospheric pco2pco2 the anomaly is transferred from inhaled air o2 to skeletal apatite of mammals hence triple oxygen isotope data of mammalian bioapatite provide information regarding pco2pco2 during the animal s lifetime the approach was calibrated with a detailed mass balance model that was verified by analyses on a set of recent mammals we evaluate the potential of this new independent terrestrial paleo co2 proxy in a case study including eocene to miocene samples the present investigation provides promising results that are in good agreement with existing proxy and model data the uncertainty intrinsic to the proxy is mainly due to uncertainties in physiological parameters

microfacies diagenesis and hydrocarbon potential of the neoproterozoic cap carbonate of the southern amazon craton abstract the low latitude marinoan glaciation 635 ma record in the southern amazon craton has been one of the most important sites with strong evidence for the neoproterozoic snowball earth hypothesis in central brazil the 50 m thick carbonate succession of the post marinoan cap carbonate exposed in the tangara da serra region central brazil represents a complementary section of the classical occurrence of the puga cap carbonate in mirassol d oeste region that overlies glaciogenic deposits marking the well preserved cryogenian ediacaran boundary in south america the 8 m thick cap dolostone that represents the mirassol d oeste formation consists of pinkish microcrystalline peloidal dolomudstone exhibiting even parallel to low angle lamination locally tubestone and megaripple bedded dolopackstone the cap dolostone is succeeded upsection by the cap cementstone limestone basal guia formation comprising laminated lime mudstone rich in silt sized terrigenous grains quartz and subordinately bituminous shale dolomitic marls intraformational breccias and abundant calcite crystal fans and crusts crystal fans occur in isolated and decimetre scale arrays and are considered aragonite pseudomorphs primary precipitation of dolomite crystal fans and micrite and the formation of tubestone are related to depositional processes the cap dolostone was affected by the neomorphism of the dolomicrite development of vug and intercrystalline porosity euhedral dolomite precipitation synsedimentary fracturing calcite and quartz precipitation chemical compaction fe oxide substitution and precipitation the diagenetic processes of the cap cementstone limestone were pyrite precipitation calcite cementation neomorphism of micrite and crystal fan partial crystal fan dissolution dolomitization chemical compaction fracturing ferrous dolomite and calcite precipitation pyrite pseudomorph and micrite replacement with fe oxide and replacement of rhombohedral dolomite by mn oxides hydrocarbons are rare and its migration represents the last diagenetic event being found 1 in fractures and interpeloidal vug and intercrystalline pores 2 with low permeability and connectivity of the cap dolostone and 2 in intercrystalline pores fractures and rare pores in crystal fans of the cap cementstone limestone during the post marinoan sea level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm action the progressive transgression led the implantation of a deep anoxic and caco3 oversaturated aragonitic platform the similarities of cap carbonate microfacies among the post marinoan succession worldwide reflects an extraordinary and non recurring global precipitation event in geological history in contrast besides the majority of cap carbonate have the same textural and compositional framework the diagenetic history seems to be regional and completely dependent on the tectonic thermal and burial history of the depositional basins where they were deposited

small microbialites from the basal triassic mudstone tieshikou jiangxi south china geobiologic features biogenicity and paleoenvironmental implications abstract we report small microbialites from the calcareous mudstone slightly above the permian triassic boundary ptb in the tieshikou section southern jiangxi province south china the newly found microbialite is a bowl like structure which contrasts with the surrounding calcareous mudstone the small microbialite is composed of columnar forms and fan shaped structures mini columnar structures resemble mini stromatolites in plane view most branches are patchy or strip shaped with clotted structures resembling that of a thrombolite fan shaped cement precipitates are comprised of multiple crystal fans that have a radiating texture and that show distinct growth laminae the radiating fabrics are represented by rod shaped filaments under sem which are interpreted as calcified trichomes filaments prominent micropores occur within the rods and they may be the plane views of multiple sheaths of the false branching zone of trichomes thus both the columnar forms and radiating fans are probably microbial in origin moreover three types of nano sized structures intraparticle micropores fibrous biofilms and filamentous sheaths are also pronounced in both the columnar structures and cement fans of the tieshikou microbialite all suggestive of biogenicity these well preserved fibrous biofilms probably represent calcified extracellular polymeric substances eps and linear sheaths are interpreted as the traces of bacterial activities during the precipitation of dolomite crystals the tieshikou microbialite therefore shares similar biogenetic mechanisms with other ptb microbialites the combination of the absence of pyrite framboids slightly negative carbon isotope values and an association with abundant ammonoids bivalves and gastropods indicates oxic conditions during the growth of the tieshikou microbialite moreover the nodular preservational state resembles widely distributed nodular mudstones or muddy limestones of the lower triassic successions worldwide this implies that nodular mudstone or argillaceous limestones may also have been deposited in microbe rich environments in which terrigenous supply was abundant

carbon isotope excursions and the oxidant budget of the ediacaran atmosphere and ocean a possible global drop in marine carbon isotope values to as low as 12 peedee belemnite pdb recorded in the ediacaran shuram formation of oman has been attributed to the non steady state oxidation of oceanic dissolved organic carbon doc resulting from the rise in atmospheric oxygen to near modern values at the end of the precambrian geologic constraints indicate that the excursion lasted between 25 and 50 m y requiring a doc pool thousands of times to 10 000 times the modern inventory to conform with carbon isotope mass balance calculations for a 12 excursion at the consequent rates of doc oxidation oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of 800 k y oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the shuram excursion furthermore a doc driven excursion does not explain strong covariation between the carbon and oxygen isotope record these indicators show that negative isotope excursions recorded in the shuram and other ediacaran sections are unlikely to represent a global ocean signal

multiproxy isotope constraints on ocean compositional changes across the late neoproterozoic ghaub glaciation otavi group namibia abstract chromium isotopes are powerful in recording redox fluctuations in marine environments potentially tracing past atmospheric oxygenation and continental weathering we present 53 cr signals with 87 sr 86 sr 13 c and 18 o and element abundances in carbonates of two shallow marine successions of the otavi group namibia stretching over four stages from preglacial setting to retreat of the late neoproterozoic ghaub glaciation 635 ma the fluctuating but generally heavy 53 cr signals indicate oxygenated shallow seawater prior to the glaciation stage 1 sustained high 13 c carb signatures at that time suggest organic matter formed a greater proportion of buried carbon which implies a net increase in oxidant flux to the surface the 18 o signals fluctuate with a tendency towards lighter values approaching the ghaub fm correlating 53 cr and 13 c carb values indicate disruptions affecting the carbon cycle and oxygen availability stage 2 prior to the ghaub diamictite a sequence of 87 sr 86 sr in the range typical of late neoproterozoic seawater occurs with high sr concentrations low mn sr and low mg ca enhanced detrital input after the ghaub glaciation stage 3 indicates elevated weathering rates likely accompanied by enhanced nutrient influx the 53 cr 13 c carb patterns are resolved in a two step recovery towards the late glacial aftermath stage 4 which is characterized by values similar to those of the early preglacial interval there a sequence of postglacial limestones also records typical late neoproterozoic seawater 87 sr 86 sr ratios the carbonate 53 cr signatures at the base of the postglacial sequence are characterized by values even below the range of bulk silicate earth bse we hypothesize that this is due to i redox cycling of cr in seawater e g by partial reduction of cr vi during microbial mediated cap dolostone deposition and or at the redox boundary of redox stratified seawater and or due to ii increased contribution of negatively fractionated cr associated with an enhanced input of detrital derived cr from postglacial weathering and or subaqueous hydrothermal influx previously it was demonstrated that carbonate 53 cr signatures trace changes in continental weathering and provide valuable indication of fluctuating seawater redox conditions prior to and after one of the major syn marinoan glaciations furthermore changes in 53 cr values might scale with shifts in bioproductivity

redox dependent distribution of early macro organisms evidence from the terminal ediacaran khatyspyt formation in arctic siberia abstract the ediacaran period witnessed the first appearance of macroscopic animal life in earth s history however the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain in part due to the dearth of integrated paleobiological and chemostratigraphic datasets in this study a comprehensive geochemical analysis was conducted on the fossiliferous khatyspyt formation in arctic siberia in order to gain insights into the ediacaran paleoenvironments this study was designed to specifically address the relationship between paleoredox conditions and ediacaran fossil occurrences in the khatyspyt formation our data reveal a dramatic shift in pyrite sulfur isotope compositions 34 s pyrite from ca 20 to ca 55 and this shift is intriguingly associated with the first occurrence of ediacara type macrofossils at the studied section suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms based on multiple lines of sedimentological and geochemical evidence we propose that the development of oceanic euxinia which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal ediacaran period may have locally prohibited the colonization of ediacara type organisms and resulted in low 34 s pyrite values in the lower khatyspyt formation in the middle and upper khatyspyt formation progressive secular transition from euxinic to non euxinic and more habitable conditions may have allowed for the colonization of ediacara type and other macro organisms

detrital remanent magnetization in haematite bearing neoproterozoic puga cap dolostone amazon craton a rock magnetic and sem study summary neoproterozoic cap dolostones of the araras group mato grosso brazil deposited on top of glacial diamictites puga formation 630 ma yielded a dual polarity component providing al ow palaeolatitude for the amazon craton because palaeomagnetic negative directions are very close to the present day dipolar field a rock magnetic and sem study was conducted in order to check the primary nature of the remanent magnetization and to identify the nature and origin of the remanence carriers results show that detrital specular haematite grain size 10 m is the main magnetic carrier in the araras dolostones and was likely inherited from the underlying puga diamictites in some sites a mixture of haematite and magnetite carries the remanence of both polarities the primary nature of the magnetization is supported by the detrital character of the magnetic carriers in addition the remanence anisotropy is weak pointing to the lack of significant shallowing due to compaction these results reinforce the low palaeolatitude of the amazon craton at the time of the marinoan ice age

how tillite weathering during the snowball earth aftermath induced cap carbonate deposition the development and aftermath of a snowball earth during precambrian time still raise questions particularly the process involved in the deposition of the associated cap carbonates and the time required for the earth to return to more stable and less extreme climatic conditions we present numerical simulations modeling the chemical weathering of regolith exposed to a co 2 rich atmosphere after deglaciation we consider the effects of an early cation proton exchange reaction that promotes fast initial dissolution of the regolith a reaction often omitted in the modeling approach but one that becomes particularly important under acidic conditions and in environments where glacial erosion can substantially refresh the rock surfaces calculations show that this dissolution step may contribute significantly to the riverine ionic input and may promote the deposition of thick cap carbonates over a short period of time the long term weathering of the regolith becomes predominant representing an efficient way of removing atmospheric co 2 over periods ranging from tens to hundreds of thousands of years depending on the grain size of the regolith material

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

multiproxy constraints on alteration and primary compositions of ediacaran deep water carbonate rocks yangtze platform south china the occurrence of shallow and deep water sedimentary facies has established the yangtze platform in south china as a key site for the study of neoproterozoic ocean oxidation and ediacaran animal evolution following the marinoan glaciation the yanwutan section in hunan province is one of the few coherent sections on the yangtze platform where ediacaran deep water carbonate sediments predominantly dolostones are preserved together with organic carbon rich shales here we present new major and trace element abundance data as well as sr o and c isotope compositions of leachates from carbonates of the doushantuo formation we evaluate the role of diagenetic modification of the carbonate rocks and constrain the redox evolution of ediacaran seawater in space and time sr 87 sr 86 systematically varies with delta o 18 carb sr and ba abundances indicating variable but mostly strong modification of fluid mobile elements by continental basin fluids in contrast ree y patterns have preserved seawater like compositions cap dolostones unit i on top of the nantuo diamictites differ from cap dolostones at shallow water sections on the yangtze platform in that they show no ce anomalies and little alteration near the top sr 87 sr 86 0 7078 delta o 18 4 0 delta c 13 carb 1 1 suggesting that delta c 13 carb and delta o 18 of cap dolostones at many other sections were compromised by hydrothermal alteration the overlying organic carbon poor micritic dolostone unit ii shows negative ce anomalies that disappear towards the top of the unit no ce anomalies occur in subsequent organic carbon rich muddy dolostone units units iii to iv these observations enrichments in toc that correlate with variations in redox sensitive metals in the carbonates negative delta c 13 carb in units ii to iv and the decoupling of delta c 13 carb from delta c 13 org argue for the existence of mostly anoxic deep water at the yangtze passive continental margin during the ediacaran the negative ce anomalies at the base of unit ii with negative delta c 13 carb may reflect fluctuations towards suboxic or oxic conditions or an allochthonous origin of this unit however trace metal enrichments in carbonates of the same unit argue for reducing conditions in pore water whereas the carbonates may have preserved the ree y signatures inherited from suboxic to oxic seawater the trace element and negative delta c 13 carb values in units ii to iv are consistent with a stratified basin model with a large partially remineralised organic matter reservoir in anoxic bottom and pore waters c 2015 elsevier ltd all rights reserved

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

late glacial and late holocene moraines in the cerros cuchpanga central peru abstract small ice fields on the western cordillera northeast of lima were expanded to three times their present size in the recent past and the regional snow line was probably about 100 m lower than it is today outwash from the expanded glaciers formed deltas of silt in valley bottom lakes when the ice lobes retreated the reduced outwash was trapped behind recessional moraines and the clear meltwater infiltrated into the limestone bedrock and emerged at the heads of the deltas in spring pools the delta surfaces then became covered with peat and radiocarbon dates for the base of the peat 1100 70 and 430 70 yr b p for two different deltas indicate that the maximum ice advance was older than those dates and thus older than the little ice age of many north temperate regions much older moraines date from expansion of the same local summit glaciers to even lower levels in the main valleys which had previously been inundated by the cordilleran ice field the cordilleran deglaciation and this expansion of local glaciers probably occurred between 12 000 and 10 000 yr ago on the basis of slightly contradictory radiocarbon dates

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

paired δ13ccarb and δ13corg records of upper ordovician sandbian katian carbonates in north america and china implications for paleoceanographic change late sandbian early katian marine carbonates from two sections in north america and one in south china were analyzed for paired 13ccarb and 13corg and revealed similar 13ccarb but varying trends in 13corg stratigraphy while all sections recorded a well known positive 13ccarb shift these 13corg records are the first through the guttenberg 13ccarb excursion gice in china oklahoma and west virginia in south china a positive excursion in 13corg is associated with the latter part of the positive 13ccarb excursion and closely resembles trends from iowa and pennsylvania that have been interpreted to reflect a lowering of atmospheric pco2 levels the 13corg trends from oklahoma and west virginia differ significantly from what is observed in china this likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the dominant control on 12c fractionation in oklahoma and west virginia masking a possible global signal of lowered atmospheric pco2 levels observed in china

evolution of oceanic molybdenum and uranium reservoir size around the ediacaran cambrian transition evidence from western zhejiang south china abstract the cambrian explosion is one of the most fascinating episodes of diversification in the history of life however its relationship to the oxygenation of the oceans and atmosphere around the ediacaran cambrian transition is not fully understood marine inventories of redox sensitive trace elements reflect the relative balance of oxidative weathering on land and deposition in anoxic water masses and can be used to explore the evolution of oceanic and atmospheric redox conditions for this study we conducted a series of geochemical analyses on the upper lantian piyuancun and hetang formations in the chunye 1 well part of the lower yangtze block in western zhejiang iron speciation results indicate that the entire studied interval was deposited under anoxic conditions with three intervals of persistent euxinia occurring in the uppermost lantian fm the lower hetang formation fm and the upper hetang fm molybdenum mo and uranium u contents and mo toc and u toc ratios from the anoxic euxinic intervals of the chunye 1 well combined with published data from the sections in the middle and upper yangtze block suggest that the oceanic mo reservoir declined consistently from the ediacaran to cambrian stage 3 while the size of the oceanic u reservoir remained relatively constant both metals were depleted in the ocean in lower cambrian stage 4 before increasing markedly at the end of stage 4 the lack of an apparent increase in the size of the marine mo and u reservoir from the upper ediacaran to cambrian stage 3 suggests that oxic water masses did not expand until cambrian stage 4 the increase in marine mo and u availability in the upper hetang fm may have been due to the expansion of oxic water masses in the oceans associated with oxygenation of the atmosphere during cambrian stage 4 this expansion of oxic waters in the global ocean postdates the main phase of cambrian diversification suggesting that pervasive oxygenation of the ocean on a large scale was not the primary control on animal diversity following the ediacaran cambrian transition

a carbon isotope challenge to the snowball earth the snowball earth hypothesis contends that most of the planet was covered with thick ice for millions of years about three quarters of a billion years ago a dramatic warming event thought to have been fuelled by high atmospheric carbon dioxide levels eventually defrosted earth a new isotopic analysis of rocks that were laid down after the initial glaciation challenges the idea of a hard snowball earth the data are consistent with a rather low partial pressure of atmospheric co2 not the orders of magnitude above current levels that would provide the extreme warming necessary for escape from snowball earth this supports the idea that the initial glaciation was much less severe than has been proposed the snowball earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the neoproterozoic era in a self enhanced glaciation caused by the high albedo of the ice covered planet in a hard snowball picture the subsequent rapid unfreezing resulted from an ultra greenhouse event attributed to the buildup of volcanic carbon dioxide co2 during glaciation1 high partial pressures of atmospheric co2 from 20 000 to 90 000 p p m v in the aftermath of the marinoan glaciation 635 myr ago have been inferred from both boron and triple oxygen isotopes2 3 these values are 50 to 225 times higher than present day levels here we re evaluate these estimates using paired carbon isotopic data for carbonate layers that cap neoproterozoic glacial deposits and are considered to record post glacial sea level rise1 the new data reported here for brazilian cap carbonates together with previous ones for time equivalent units4 5 6 7 8 provide estimates lower than 3 200 p p m v and possibly as low as the current value of 400 p p m v our new constraint and our re interpretation of the boron and triple oxygen isotope data provide a completely different picture of the late neoproterozoic environment with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard snowball earth

chemostratigraphy of the shaler supergroup victoria island nw canada a record of ocean composition prior to the cryogenian glaciations a b s t r a c t a new 13 ccarb curve combined with 13 corg values is presented for the upper shaler supergroup 900 to 720 ma amundsen basin northwestern canada the dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust there is a pronounced negative 13 c excursion in the wynniatt formation that can be correlated with a putative worldwide negative car bon isotope excursion namely the bitter springs stage however in the amundsen basin the 13 ccarb excursion drops to anomalously negative values 14 which we attribute to local overprints wherein isotopically light carbon in pore waters released by oxidation of methane and organic matter during sulphate and iron reduction was incorporated into authigenic carbonate cement we document basin euxinia and anoxia during the same time interval using a multi proxy approach specifically fe speciation and redox sensitive trace metal data patterns of pronounced enrichment in mo v and u concentrations in euxinic black shales suggest that the bitter springs stage was a transitional period in earth s redox evo lution from the more reduced global oceans during the mid proterozoic to the more oxygenated oceans during the phanerozoic 2015 elsevier b v all rights reserved

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

carbon isotopic evidence for chemocline upward excursions during the end permian event abstract a negative shift in marine inorganic carbon isotope composition 13 c carb during the end permian mass extinction has been used as evidence for several different extinction mechanisms changes to the 13 c of organic matter and the difference between it and 13 c carb 13 c 13 c carb 13 c org have been examined at few locations with conflicting interpretations we examine the changes to both organic and inorganic carbon isotopes across the permian triassic boundary at two marine sections from south china meishan and shangsi and compare these to data from other previously published sections through these analyses we demonstrate that a decrease in 13 c occurred during the extinction event throughout the paleo tethys ocean the extent and intensity of the decrease varies by location averaging a negative shift of 5 several possibilities as to the cause of this shift exist including siberian trap volcanism a change in the terrestrial marine organic carbon input to the system or a change in the dominant marine biota brought about through environmental changes such as widespread ocean anoxia euxinia the decrease in 13 c observed at many of these sections across the event horizon is here interpreted to represent a shift from algae cyanobacteria to less fractionating phototrophic sulfur bacteria in marine shelf environments resulting from upward excursions of the chemocline these chemocline upward excursions would release euxinic water to the photic zone allowing phototrophic sulfur bacteria to thrive the limited available biomarker data are consistent with this interpretation

a red algal bloom in the aftermath of the marinoan snowball earth organic matter from the cap dolostones overlying the marinoan age glacigenic diamictites of the araras group amazon craton has been studied to reconstruct the post glacial ecosystem molecular fossils indicate that the post marinoan ecosystem was marked by an apparent decline in marine algal diversity the proliferation of red algae may be explained by environmental changes such as a massive nutrient input accompanying continental weathering after the ice thaw and a dimer light penetrating sea water due to the drowning of the platform in addition the presence of green sulphur bacteria indicates that sea water was stratified with an anoxic possibly euxinic i e sulphidic layer at the water sediment interface sulphur cycling probably occurred at the redox boundary as suggested by the recognition of active sulphate reduction this observation supports a microbially induced model for the formation of the cap dolostones

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

origin of cap carbonates an experimental approach abstract an experimental study was carried out to assess the abiotic natural reactions associated with carbonate precipitation leading to the formation of neoproterozoic cap carbonate sequences for this purpose synthetic seawater solutions were prepared reflecting the presumed composition of neoproterozoic seawater these solutions were heated to 40 c undergoing degassing and or evaporation while monitoring their chemical composition sulphate concentration mg ca molar ratio and identifying the precipitated minerals the experimental results are in good agreement with previous studies allowing us to constrain the type of carbonate precipitates formed in the ocean during the snowball earth aftermath among the phases precipitated during heating of the solutions mg bearing carbonate minerals i e mg calcite magnesite and dolomite remain of minor importance compared to ca carbonates especially aragonite consequently the basal dolostone units of the cap carbonates i e cap carbonates s s were originally in the form of caco3 and evolved spontaneously toward dolomite cap carbonates s l i e cap limestones containing aragonitic cement could result from the degassing of the whole ocean and the concomitant precipitation of aragonite the calculated thicknesses of carbonates formed in this way are of the order of several metres in line with the field observations although this process of abiotic carbonate precipitation does not rule out biologically mediated carbonate formation it may represent an alternate route to produce carbonate particularly in environments which are not yet colonized by bacteria after the global ice melting

a multilayered water column in the ediacaran yangtze platform insights from carbonate and organic matter paired δ13c abstract large carbon isotope fluctuations have been systematically reported for ediacaran carbonate rocks the meaning of which remains controversial in order to better understand the mechanisms behind such variations we present new paired 13c data on carbonates 13ccarb and their associated organic matter 13corg from a shelf margin section yangjiaping of the doushantuo formation in this section the doushantuo formation starts with cap dolostones presenting 13ccarb values around 5 and 13corg values between 30 3 and 27 6 up section phased variations in 13ccarb and 13corg describe positive and negative excursions while their difference 13cdol org 13ccarb 13corg remains around 29 2 these new data allow the first reconstruction of lateral variations of 13ccarb 13corg and 13ccarb org for a shelf to basin cross section of the yangtze platform south china after integration with results reported previously for two other sections across the yangtze platform the isotope signals reveal strong lateral heterogeneities with complex variations of 13ccarb and 13cdol org in the inner shelf section phased variations in the shelf margin section with positive 13ccarb and 13cdol org close to 29 and dominantly negative 13ccarb with 13corg as low as 35 in the basin these variations are incompatible with the idea that the 13ccarb can systematically be used as a proxy of ocean surface waters assuming that 13ccarb are acquired in bottom waters and or upper sediments we show that the heterogeneous 13ccarb and 13cdol org are compatible with a stratified water column composed of up to three layers i an oxic surface layer where dissolved inorganic carbon dic is probably in isotope equilibrium with the atmosphere ii an intermediate euxinic layer with a lower 13cdic due to organic matter oxidation by ongoing sulphate reduction iii a bottom euxinic layer that seems to be restricted to the inner shelf lagoonal facies lacking sulphate containing methane and with a higher 13cdic due to dic production by methanogenesis if our model holds true it suggests that not only negative but also positive ediacaran carbon isotope excursions may reflect ocean stratification the positive excursion possibly recording a sulphate free methanogenic layer at the bottom of restricted basins

boron and calcium isotope composition in neoproterozoic carbonate rocks from namibia evidence for extreme environmental change the level and evolution of atmospheric carbon dioxide throughout earth s history are key issues for palaeoclimate reconstructions especially during times of extreme climate change such as those that marked the neoproterozoic the carbon isotope ratios of marine carbonates are crucial in the correlation and identification of neoproterozoic glacial deposits and they are also used as a record for biogeochemical cycling and potential proxy for atmospheric pco2 likewise the boron and calcium isotope compositions of marine carbonates are potential proxies for palaeo seawater ph and the ratio of calcium fluxes into and out of seawater respectively and together they may be used to estimate atmospheric carbon dioxide here we use b and ca isotopes to estimate palaeoenvironmental conditions in the aftermath of a major neoproterozoic glaciation in namibia the validity of the b and ca isotope variation in the ancient marine carbonates is evaluated using the oxygen isotope composition of the carbonates and its correlation to the carbon isotope variation a negative 2 7 to 6 2 11b excursion occurs in the postglacial carbonates and is interpreted to reflect a temporary decrease in seawater ph associated variations in 44ca values ranging between 0 35 and 1 14 are linearly coupled with the carbon isotope ratios and imply enhanced postglacial weathering rates the reconstructed seawater ph and weathering profiles indicates that high atmospheric co2 concentrations were likely during the melt back of neoproterozoic glaciations and precipitation of cap carbonates however the b isotope trend suggests that these concentrations rapidly ameliorated and they do not co vary with 13c thus models attempting to link long lived negative 13c excursions to elevated pco2 need to be reconsidered

carbon isotopic evolution of the terminal neoproterozoic and early cambrian evidence from the yangtze platform south china profound geotectonic climatic and biological changes occurred during the terminal neoproterozoic and its transition into the early cambrian these changes are reflected in temporal variations of the carbon isotopic composition of seawater recorded in a sedimentary succession on china s yangtze platform however in addition to secular changes in carbon isotopes the studied succession reflects additional variations in accordance with the deposition in different sedimentary facies ranging from shallow water platformal to deeper water basinal settings likely a consequence of incorporation of variable amounts of 13 c depleted

resolved snowball earth clouds abstractrecent general circulation model gcm simulations have challenged the idea that a snowball earth would be nearly entirely cloudless this is important because clouds would provide a strong warming to a high albedo snowball earth gcm results suggest that clouds could lower the threshold co2 needed to deglaciate a snowball by a factor of 10 100 enough to allow consistency with geochemical data here a cloud resolving model is used to investigate cloud and convection behavior in a snowball earth climate the model produces convection that extends vertically to a similar temperature as modern tropical convection this convection produces clouds that resemble stratocumulus clouds under an inversion on modern earth which slowly dissipate by sedimentation of cloud ice there is enough cloud ice for the clouds to be optically thick in the longwave and the resulting cloud radiative forcing is similar to that produced in gcms run in snowball conditions this result is robust to large changes in the clo