late precambrian pennatulids sea pens from the eastern yangtze gorge china paracharnia gen nov abstract pennatulids preserved as membranous carbonised compressions from the shibantan member dengying formation sinian system in the eastern yangtze gorge hubei province represent the first genuine discovery of the late precambrian ediacara type metazoan remains in china the specimens were initially referred to charnia ford 1958 and described as c dengyingensis by ding and chen in 1981 a re examination of the type material and comparing it with other late precambrian frond like forms particularly those from the ediacara assemblage of south australia and from the charnwood assemblage of england indicate that this chinese species represents a distinct genus paracharnia gen nov its characteristic polyp leaves individually free standing on distinct leaf stalks along the lateral sides of the median stem resemble those of many living sea pens represented by pennatula and contribute a significant feature to the precambrian charniidae glaessner 1979 its close association with abundant macroscopic algal remains of vendotaenia sp and its proximity to the overlying basal cambrian small shelly fossil assemblages in the same succession emphasise its palaeontological and biostratigraphic significance

assessing the utility of fe al and fe speciation to record water column redox conditions in carbonate rich sediments abstract geochemical proxies based on fe abundance fe al and fe speciation have been widely applied to marine sediments in order to unravel paleo depositional redox conditions though geological time to date however these fe proxies have only been calibrated in relation to modern and ancient fine grained siliciclastic marine sediments this clearly limits their use particularly in relation to carbonate rich sediments and rocks to address this we here explore the applicability of fe based redox proxies in carbonates through three approaches first we have compiled fe al data for modern marine sediments to investigate variability in fe enrichments as a function of carbonate content and depositional setting second we have expanded this approach with a compilation of new and existing fe speciation data for modern and ancient marine sediments deposited under oxic and euxinic anoxic sulfidic water column conditions finally we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the fe speciation redox proxy modern marine sediments deposited under oxic conditions show no relationship between fe al and carbonate content these sediments have an average fe al ratio of 0 55 0 11 with some higher values potentially being attributable to steady state early diagenetic remobilization of fe towards the sediment water interface in contrast significant fe al enrichments occur as a consequence of water column fe mineral formation and deposition either under anoxic conditions or due to input of anoxic hydrothermal fluids into oxic seawater iron speciation data also show no direct correlation with carbonate content and instead three groups can be distinguished based on total fe fe t and organic c contents sediments deposited under oxic water column conditions with fe t 0 5 wt generally plot below the lower fe hr fe t siliciclastic reference threshold 0 38 for distinguishing anoxic environments regardless of carbonate content also consistent with siliciclastic calibrations carbonate rich sediments deposited under anoxic water column conditions tend to have fe hr fe t ratios 0 38 in contrast oxic carbonate rich sediments with low fe t hr fe t ratios suggesting that the use of fe speciation for such samples is not appropriate for evaluating water column redox conditions taken together our data suggest that an fe t content of 0 5 wt rather than carbonate content represents the most suitable basis for identifying whether carbonate rich sediments are appropriate for fe based redox proxy analyses analysis of burial dolostones suggests that the fe speciation proxy may also be compromised by deep burial dolomitization where there has been a source of mobile fe to enrich rocks during recrystallization however pre screening can identify such samples this new assessment expands the utility of fe based redox proxies to also incorporate appropriate carbonate rich rocks provided that care is taken to assess the possible impact of deep burial dolomitization

oxygenation variations in the atmosphere and shallow seawaters of the yangtze platform during the ediacaran period clues from cr isotope and ce anomaly in carbonates abstract the late neoproterozoic era witnessed a significant oxygenation in earth s surface in line with the naissance and diversity of early animals however detailed variations of the oxygenation during this period remain controversial chromium isotope in ancient carbonates has shown its ability to trace the oxygenation history of the atmosphere hydrosphere system in this study we analyzed cr isotope and ce anomaly data of carbonates from the ediacaran doushantuo and dengying formations in the yangtze gorges area south china in order to reconstruct oxygenation variations in the atmosphere and shallow seawaters of the yangtze platform during the ediacaran period after evaluating and excluding detrital and diagenetic effects well preserved 53crauth signatures were obtained post marinoan carbonates record slightly positive 53crauth values 0 18 0 43 and slightly negative ce anomaly suggesting generally oxidizing conditions in the atmosphere with po2 over 1 pal and in shallow seawaters of the yangtze platform during the post marinoan phase which may have been related to the diversity of early ediacaran organisms shuram wonoka correlative carbonates are characterized by the fluctuations of 53crauth values 0 25 to 0 97 and a significant negative 13c excursion which may have resulted from partial oxidation of the large deep water doc reservoir under enhanced oxidative weathering conditions this process released 13c depleted carbon into dic pool and isotopically unfractionated cr iii into water columns the latter of which immediately precipitated by adsorption onto the detrital materials meanwhile an increase in local anoxia and even euxinia in the intrashelf basin of the yangtze platform driven by enhanced microbial sulfate reduction and the resultant h2s possibly limited regional biological evolution during this phase late ediacaran carbonates record elevated 53crauth values 0 40 0 96 together with the decreasing trend in ce ce values indicating a more oxygenated environment in the atmosphere and shallow seawaters of the yangtze platform during the late ediacaran phase which may have been beneficial to the diversification of late ediacaran animals

importance of vertical geochemical processes in controlling the oceanic profiles of dissolved rare earth elements in the northeastern indian ocean vertical profiles of dissolved rare earth elements rees were obtained in the bay of bengal and the andaman sea the ree concentrations at various depths in the bay of bengal are the highest in the indian ocean this is attributable ultimately to the large outflow of the ganges brahmaputra and irrawaddy rivers but the dissolved ree flux to surface waters alone cannot explain the large and near constant ree enrichment throughout the entire water column the underlying fan sediments serve as not a source but a sink for dissolved ree iii s absence of excess 228ra in the deep waters suggests that lateral input of dissolved rees from slope sediments is also small in these regions partial 0 3 dissolution of detrital particles which are carried by the rivers and lateral surface currents and subsequently settle through the water column appears to be a predominant source for the dissolved rees vertical profiles showing an almost linear increase with depth are common features for the light and middle rees everywhere but their concentration levels are variable from basin to basin and from element to element this suggests that their oceanic distributions respond quickly to the variation of particle flux and its ree composition through reversible exchange equilibrium with suspended and sinking particles much like the case for th the relative importance of the vertical geochemical processes of reversible scavenging over the horizontal basin scale ocean circulation with passive regeneration like nutrients decreases systematically from the light to the heavy rees using a model the mean oceanic residence times of rees in the bay of bengal are estimated to range from 37 years for ce to 140 1510 years for the strictly trivalent rees in the deep water of the andaman sea isolated from the bay of bengal by the andaman nicobar ridge maximum sill depth of 1800 m the ree concentrations are almost uniform presumably due to rapid vertical mixing the ree iii concentrations are similar to that of 1250 m depth water in the bay of bengal consistent with other oceanographic properties however the ree composition of the deep water appears to be altered slightly by preferential scavenging of the light ree iii at the bottom boundary of the basin

dissolved rare earth elements in the southern ocean cerium oxidation and the influence of hydrography abstract analysis of dissolved ree distributions in the southeastern atlantic ocean has revealed that progressive oxidative removal of dissolved ce from deepwater 1 km occurs over the mixing time of the oceans ce anomaly ce ce values measured in deep waters from the southeastern atlantic are similar to but slightly higher than those from deep ocean stations throughout the indian and pacific oceans typically falling in the range 0 08ce ce 0 12 only deepwater samples from the north atlantic exhibit values greater than ce ce 0 12 with the exception of southeastern atlantic bottomwaters which are influenced by antarctic bottomwater covariation of dissolved ree concentrations with salinity and or dissolved si concentrations at this site has allowed dissolved ree compositions for various southern ocean water masses to be predicted end member concentrations for antarctic bottom water aabw derived in this way indicate high dissolved ree concentrations similar to average pacific deep water but with significantly enhanced dissolved ce concentrations and less pronounced hree enrichment this result is consistent with the proximity of our study site to active aabw deepwater formation and evolution of that water mass during transport to the pacific ocean

iron rich carbonate tidal deposits angepena formation south australia a redox stratified cryogenian basin abstract the cryogenian angepena formation ca 650 ma of south australia records deposition in a peritidal environment equivalent to and landward of the balcanoona reef complex and offers valuable insights into shallow marine chemistry during the nonglacial interlude between two global ice ages the sedimentary facies comprise an iron rich marine red bed succession and include red mudcracked dolomitic shales tepee dolostone crossbedded intraclastic ooid grainstone and tepee related sheet cavities with marine cements these facies are interpreted to have been deposited in a cryogenian tidal flat to subtidal setting that is laterally equivalent to the reefal and back reef environments of the balcanoona formation marine cements from this environment are characterized by well preserved non and bright cathodoluminescent zoning and strong negative to slightly positive cerium ce anomalies ce ce we suggest that the angepena formation is indicative of intermittent synsedimentary iron oxide precipitation in an oxidized peritidal environment in an otherwise strongly ferruginous and anoxic oceanic setting large negative ce anomalies develop over a narrower depth range when compared to modern oxic marine settings highlighting the importance of dissolved mn and fe concentrations in controlling the magnitude of the negative ce anomalies overall coupled sedimentological and geochemical evidence suggest a neoproterozoic shallow marine system at the interface of an oxidizing atmosphere and an anoxic ferruginous marine system

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

cambrian small skeletal fossil and carbon isotope records of the southern huangling anticline hubei china and implications for chemostratigraphy of the yangtze platform abstract rich small skeletal fossil assemblages are described from the yanjiahe formation and tianzhushan member of southern huangling anticline the anabarites trisulcatus protohertzina anabarica assemblage zone small skeletal fossil zone 1 is recognized in the middle to upper yanjiahe formation while the thin topmost conglomeratic limestone unit yielded a mixed assemblage resulting from reworking and redeposition of source material of small skeletal fossil zones 1 to 3 including anabarites trisulcatus protohertzina anabarica p unguliformis watsonella crosbyi aldanella attleborensis anabarella plana our revised biostratigraphic correlation indicates the existence of longer hiatuses in the yanjiahe formation and between the yanjiahe and shuijingtuo formations the obtained 13ccarb values in our profile deviate in some details from those of some previous reports based on the new biostratigraphic correlation the 13ccarb trend records values oscillating between 3 to 4 4 in the upper anabarites trisulcatus protohertzina anabarica assemblage zone which is unusual for the yangtze platform this interval of positive 13ccarb values in the upper yanjiahe formation and the dahai member of eastern yunnan are here interpreted to reflect the impact of a local aquafacies moreover it is here argued that the selection of a carbon isotope peak as a primary index tool in the laolin section for defining the cambrian stage 2 must be treated with caution as this isotope excursion is influenced by a regional water mass and does not reflect an open marine seawater signature underlining other technical and theoretic concerns on the general application of chemostratigraphy in cambrian shallow water carbonate platforms overall we suggest that chemostratigraphic correlations need to be supported by biostratigraphic tools

secular change in northwestern tethyan water mass oxygenation during the late hauterivian early aptian abstract the mesozoic is punctuated by several extreme episodes of oxygen drawdown in the ocean known as oceanic anoxic events oae beside these discrete events little is however known about longer term oxygen level fluctuations and their controls a high resolution dataset of rare earth elements ree content in carbonate deposited in the vocontian basin se france has allowed to survey the evolution of the cerium anomaly ce ce during part of the early cretaceous latest hauterivian early aptian this ratio is used as a proxy for the degree of oceanic oxygenation the cerium anomaly is compared to the coeval evolution of relative sea level organic rich occurrences and nutrient level proxies in order to infer a cause and consequence relationship between these parameters and the evolution in sea water oxygenation the long term evolution of the cerium anomaly shows that northwestern tethyan water masses have evolved from less oxygenated during the latest hauterivian early barremian interval to more oxygenated during the late barremian earliest aptian time before being strongly oxygen depleted during the early aptian oae 1a time interval this trend is correlated with both the long term trend in nutrient levels as well as with the frequency of organic rich layers within the rock record on a medium term scale sub stage level there is a good correlation between the cerium anomaly and tectono eustatism as deduced by the mn content of the basinal carbonate and sequence stratigraphic observation on surrounding shallow water areas this suggests that sea level changes by altering ocean stratification and currents as well as modulating the amount of nutrient delivered from the continent exert control on oceanic oxygenation via changes in primary productivity

nitrogen and organic carbon isotope stratigraphy of the yangtze platform during the ediacaran cambrian transition in south china abstract n and corg isotope results are presented from six sections along a west east transect in the south china basin scb covering both shallow and deeper domains in order to investigate biogeochemical cycling stratigraphic correlation and isotope systematics over the crucial ediacaran cambrian transition 15n bulk values range between 3 and 7 while 13corg values range between 21 and 39 similar isotopic trends have been identified for both these proxies across the basin although hiatuses differences in depositional setting and syn depositional bacterial fermentation may have caused some inconsistencies a trend towards negative n isotope values can be recognized above the pc c boundary in both shallow and deeper basin realms across the yangtze platform this negative 15n excursion is probably a response to photic zone anoxia and intense nitrogen fixation assimilation by diazotrophic cyanobacteria and green purple sulfur bacteria gsb and psb the xiaotan section and a composite section from the yangtze gorges area show meaningful similarities in their nitrogen isotope trends interpreted as chemocline fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform using carbon isotope stratigraphy we correlate boundary strata across the platform to test the wider significance of nitrogen isotopic variations increased bioturbation and food chain complexity across the ediacaran cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early cambrian reflecting the beginning of a new more biologically controlled era

the ree composition and petrography of apatite in 2ga zaonega formation russia the environmental setting for phosphogenesis the first significant p rich deposits appear in the global rock record during the paleoproterozoic around 2 ga however the specific triggers that led to apatite precipitation are still under debate the ca 2 ga zaonega formation karelia russia contains p rich intervals in its upper part with abundantly occurring apatite these apatites have been studied for their rare earth element ree composition using laser ablation inductively coupled plasma mass spectrometry la icp ms with an emphasis on the environmental condition during phosphogenesis petrographic observations by scanning electron microscopy sem integrated with la icp ms results allow recognition of variously recrystallized apatite and distinction of best preserved diagenetic apatite which presumably records syn depositional ree characteristics diagenetic apatite exhibits moderately negative ce anomalies that indicate an at least partially oxygenated water column a variable but typically positive eu anomaly is consistent with geologic evidence suggesting intensive magmatic activity and hydrothermal influence during apatite precipitation we conclude that phosphatic sediments in the paleoproterozoic zaonega formation record phosphogenesis in a vent seep influenced setting that experienced fluctuating redox conditions at the sediment water interface

on the coevolution of ediacaran oceans and animals fe speciation and s isotope of pyrite data from the terminal proterozoic sheepbed formation in canada and doushantuo formation in china reveal that ocean deep waters were anoxic after the global glaciations snowball earth ending 635 million years ago but that marine sulfate concentrations and inferentially atmospheric oxygen levels were higher than before the glaciations this supports a long postulated link between oxygen levels and the emergence of eumetazoa subsequent ventilation of the deep ocean inferred from shifts in fe speciation in newfoundland previously published data and western canada this report paved the way for ediacaran macrobiota to colonize the deep seafloors

rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments abstract analyses have been made of ree contents of a well characterized suite of deep sea 4000 m principally todorokite bearing ferromanganese nodules and associated sediments from the pacific ocean ree in nodules and their sediments are closely related nodules with the largest positive ce anomalies are found on sediments with the smallest negative ce anomalies in contrast nodules with the highest contents of other rare earths 3 ree are found on sediments with the lowest 3 ree contents and vice versa 143 nd 144 nd ratios in the nodules 0 51244 point to an original seawater source but an identical ratio for sediments in combination with the ree patterns suggests that diagenetic reactions may transfer elements into the nodules analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to ree contents of nodules and sediments is negligible inter element relationships and leaching tests suggest that ree contents are controlled by a p rich phase with a ree pattern similar to that for biogenous apatite and an fe rich phase with a pattern the mirror image of that for sea water it is proposed that 3 ree concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate processes which favour the enrichment of transition metals in equatorial pacific nodules favour the depletion of 3 ree in nodules and enrichment of 3 ree in associated sediments in contrast ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions

formation of shallow water glaucony in weakly oxygenated precambrian ocean an example from the mesoproterozoic tieling formation in north china abstract authigenic glaucony precipitation in phanerozoic oceans takes place mostly in middle shelf to upper slope environments with low depositional rate or sediment starvation precambrian glaucony however is more common in stratigraphic successions deposited from shallower water environments with high and variable depositional rates this phenomenon has long been noticed in literature but the controlling factors of shallow water glaucony precipitation in precambrian oceans have not been adequately investigated to better understand the glauconitization processes in precambrian oceans we have conducted an integrated study of the glaucony in stromatolitic carbonates of the mesoproterozoic tieling formation ca 1437 ma in north china using sedimentological mineralogical and geochemical data obtained from field observations petrography xrd sem quantitative eds and icp ms analyses macro and microscopic observations show that the tieling glaucony fills voids of varying sizes and shapes and records different maturation stages of glauconitization geochemical analyses show that the tieling glaucony has high k 2 o avg 8 but low and variable total fe 2 o 3 tfe 2 o 3 contents 1 92 13 65 wt the tfe 2 o 3 contents increase with maturation of glaucony titration results show that the tieling glaucony contains both fe 3 and fe 2 ions but has fe 2 fe 3 ratios much higher than that of the phanerozoic glaucony ree results of glaucony hosting carbonates show weak negative and positive ce anomalies with average ce ce ratio close to 1 0 suggesting carbonate precipitation near the redoxcline of fe mn oxides all these features suggest that the tieling glaucony was precipitated in seawater around the fe redoxcline where both fe 2 and fe 3 were available throughout the glaucony maturation stages high fe 2 fe 3 ratios in the depositional environments led to fe 2 occupation at octahedral sites of glaucony and negative charges on octahedrons which resulted in high k content to balance the negative charges on octahedrons and low tfe 2 o 3 limited by dioctahedral structure the formation of the tieling glaucony and other similar precambrian glauconies is likely controlled by low oxygen concentration in seawater and a shallow redoxcline that controls the availability of fe and k cations during initial precipitation and maturation of glaucony the shift of authigenic glaucony precipitation from shallow water in the precambrian to deep water in the phanerozoic may record the deepening of ocean chemocline in response to increased ocean oxygenation

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

redox conditions of the late cretaceous western interior seaway recorded by rare earth elements of bearpaw molluscan fossils understanding the causes of the low biodiversity faunas in the western interior seaway wis overwhelmingly dominated by ammonite and bivalve molluscs will help us to understand paleoenvironmental conditions in the wis in this study we examined rare earth elements rees in well preserved molluscs from the bearpaw formation to reconstruct wis redox conditions both nektonic and benthic molluscs share similar ree patterns with enrichment in light rees there is only a slight ce depletion in both types of molluscs indicating no significant fractionation of ce from the other rees a lack of significant ce anomalies in molluscs points to oxygen deficient probably dysoxic conditions in the middle to bottom part of the water column where the molluscs lived given the general lack of significant ce anomalies also in molluscs from older formations oxygen deficiency was likely prevalent in the late cretaceous in the wis a warmer climate in the cretaceous is probably a driver of such conditions as predict

mo isotopic variations of a cambrian sedimentary profile in the huangling area south china evidence for redox environment corresponding to the cambrian explosion abstract the radiation of macroscopic animals in the early cambrian commonly be called as cambrian explosion had a close relationship with earth oxygenation these macrofossils are widely preserved within the early cambrian in the northern and western yangtze craton in order to show the casual relationship between animal evolution and earth redox environment mo content and isotopic composition analyses have been conducted on dolomites of the tianzhushan formation and the black shales of the shuijingtuo formation from the lowermost cambrian profile in the eastern huangling dome northern yangtze craton the calcitic dolostones from the tianzhushan formation display lower concentrations of mo contents 0 055 0 666 ppm than that of the black silty shales from the shuijingtuo formation 6 0 172 ppm this magnitude difference is most likely related to the different lithologies due to mo enrichment in shales the mo isotope values 98 95mo of the calcitic dolostones for the tianzhushan formation vary from 0 73 to 0 41 in comparison the black sandy shales from the shuijingtuo formation exhibit 98 95mo of 1 04 1 84 including 5 samples from the earliest shuijingtuo formation displaying systematically higher 98 95mo values 0 09 1 84 than those of the other 5 samples from upper layers 98 95mo 1 04 to 0 19 these geochemical data suggest that the redox environment of the early cambrian is fluctuating and the peak oxygenation of the ocean had already reached a level similar to modern environments according to the mo concentrations and 98 95mo data the sedimentary succession can be divided into five intervals which are closely correlated to the episodic evolution of early life in the cambrian together with the available published data from other profiles in the yangtze craton our new data reveal that the higher oxidation environment had a positive effect on the evolution of early life

siliceous sponge expansion and phosphogenesis in a shallow water environment in the malyi karatau range kazakhstan during the precambrian cambrian transition abstract ocean oxygenation during the precambrian cambrian transition is thought to be one of the key factors for the rapid diversification of animals known as the cambrian explosion siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth resulting in an increase in oxygen levels of shallow water to assess the global importance of the impact of ecosystem engineering by sponges we evaluated the palaeo environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the precambrian cambrian transition we investigated three lithosections in the malyi karatau range in southern kazakhstan the lowermost chert samples of these sections are typically characterized by comparably low 30si values and exhibit molds of siliceous sponge spicules upsection the occurrence of cross bedding structures and granular phosphorites document the shallowing of the depositional environment where overall increasing 30si values and increasing al si indicate less settlement of sponges and higher suspended particle loads corresponding carbon isotope records of the carbonate fraction decrease from near zero to negative carbon isotope values averaging 5 9 the 13ccarb values overall coincide with y ho the redox indicator ce ce and mass fractions of carbonate fluorapatite suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment the studied sections document the ecological requirements for sponge settlement and proliferation such as the presence of a hard substrate and sufficiently low suspended particle concentrations and or low wave energy this work suggests that the widespread appearance of karsted surfaces around the precambrian cambrian transition that were subsequently submerged during the cambrian transgression would have created permissive habitats for sponges and large phosphate deposits the capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the precambrian cambrian transition

rare earth element characteristics of paleoproterozoic cap carbonates pertaining to the sausar group central india implications for ocean paleoredox conditions abstract the paleoproterozoic era is marked by significant changes in the level of oxygen in the atmosphere and chemistry of the ocean and the presence of world wide glaciation geochemistry of chemogenic sediments provide clues to these changes the sausar group 2478 2250 ma of central india is one of the few places in the world where paleoproterozoic glacial unit overlained by a cap carbonate horizon has been identified major trace and rare earth element ree contents of paleoproterozoic cap carbonates of the sausar group have been analysed for their implications for ocean redox conditions during interglacial periods of the huronian glaciation the la la gd gd lusn lasn and y ho ratios of the cap carbonate reflect their pristine marine nature conspicuous ce anomaly ce ce up to 1 75 accompanied by the enrichment of fe mn zn and u provides insight into the redox state of the ocean at the time of deposition of the cap carbonate succession based on these observations we infer anoxic depositional conditions in the basin during the interglacial period of the paleoproterozoic glaciation before the great oxidation event goe and the level of oxygen in the atmosphere fluctuating between 10 5 and 10 2 of the present atmospheric level pal

oxygenation of ediacaran ocean recorded by iron isotopes abstract the increase in atmospheric oxygen during the late neoproterozoic era ca 800 542 ma may have stimulated the oxygenation of the deep oceans and the evolution of macroscopic multicellular organisms however the mechanism and magnitude of neoproterozoic oxygenation remain uncertain we present fe isotopes fe species and other geochemical data for two sections of the doushantuo formation ca 635 551 ma deposited after the nantuo glacial episode in the yangtze gorge area south china it is highlighted that highly positive 56 fe values reflect a lower oxidation rate of fe ii aq under ferruginous conditions and in turn near zero 56 fe values indicate oxidizing conditions our study suggests that during the deposition of the bottom of member ii of the doushantuo formation the shallow seawater was oxic but the deep water was characterized by ferruginous conditions which is consistent with a redox chemical stratification model subsequent anoxic conditions under shallow seawater represented by positive 56 fe and negative 13 c carb excursions should be ascribed to the upwelling of fe ii aq and dissolved organic carbon doc rich anoxic deep seawater the oxidation of fe ii aq and doc rich anoxic deep seawater upon mixing with oxic shallow water provides an innovative explanation for the well known negative 13 c carb excursions enc2 and positive 56 fe excursions in the middle of doushantuo formation meanwhile the upwelling fe ii aq and doc rich anoxic deep seawater could increase photosynthetic activity the balance between oxygen consumption and production was most important criteria for the oxygenation of early ediacaran ocean and diversity of eukaryotic organisms

the paleoenvironmental evolution of the cambrian longwangmiao formation stage 4 toyonian on the yangtze platform south china petrographic and geochemical constrains abstract during the cambrian period stage 4 biodiversity and biomass declined rapidly and globally in an event known as the botomian toyonian crisis to gain a better understanding of the paleoenvironment in south china during this stage we present and interpret complex multi parameter carbonate analyses petrography carbon and oxygen isotopic composition and major trace and rare earth elements from the toyonian longwangmiao formation in the bandenggou section the longwangmiao formation comprises primarily carbonate deposits with massive detrital mineral occurrences in its lower middle sequence principal component analysis of the minerals and oxides leads to the definition of two intervals i and ii interval i is composed mainly of dolostones interbedded with sandstones and contains exceedingly high amounts of terrestrial original minerals and oxides as well as high ree y th and sc concentrations interval ii which is composed primarily of limestones has trace amounts of terrestrial original minerals and oxides and low ree y th and sc concentrations the 13c and 18o values show a significant positive shift from interval i to interval ii after eliminating the effects of diagenesis contributions through the analyses of diagenesis sensitive indexes 18o values mgo cao mn sr and fe sr ratios and the correlations of 13c 18o 18o eu eu eu eu ce ce and eu eu feo the sr and na concentrations of calcites in interval i are extremely high exceeding even the corresponding ranges of marine aragonites while calcites from interval ii have low sr and na concentrations that are within the range of marine low mg calcite however both show strong positive correlations between ca sr ca na and sr na the different calcite elemental behaviors in intervals i and ii suggest different carbonate precursors associated with seawater temperature and climate furthermore the sr ca ratios are negatively correlated with 13c and 18o values and the 13c values also negatively correlated with sio2 content these features indicate a significant change in the depositional environment during the cambrian toyonian stage from an early restricted marine environment under a warm and arid climate with massive terrestrial influxes to a later relatively open marine environment under a cool and humid climate with negligible terrestrial influxes this shift likely resulted in the observed changes in biomass and biodiversity

the link between metazoan diversity and paleo oxygenation in the early cambrian an integrated palaeontological and geochemical record from the eastern three gorges region of south china abstract the link between environmental perturbations especially oceanic redox conditions and evolution during the cambrian biodiversification event is still highly controversial although this issue has attracted considerable attention for decades we here report on the fossil record of the early cambrian biota discovered in the uppermost yanjiahe and shuijingtuo formations that crop out in the luojiacun and qiaojiaping sections eastern three gorges area south china the uppermost member of the yanjiahe formation contains abundant benthic algae based on the fossil record four distinct intervals were recognized in the shuijingtuo formation i few sessile suspension feeders such as sponges were recovered from interval i ii interval ii is dominated by bivalved arthropods living in the water column in this interval are also found a few sponges and eodiscoid trilobites iii no fossils other than few sponge spicules were recovered from interval iii iv diverse benthic communities composed of sponges cnidarians brachiopods as well as bradoriids dominate interval iv comprehensive research on framboidal pyrite morphology whole rock geochemistry as well as in situ la icp ms analyses were carried out on samples from the luojiacun section in order to assess paleo redox conditions the results show that the overall evolution of the paleo redox conditions was characterized by a double episode of sharp anoxic events ae observed at the base of intervals i and iii followed by a gradual oxygenation event oe evidence of oxygenation fluctuations were detected especially within interval iv a case study is presented herein which brings together early cambrian changes in metazoan diversity and changes in environmental conditions it is suggested that periodical oxygenation of the water column increased the biological diversity in the restricted basins of the eastern three gorges thus four intervals of habitat succession can be tentatively reconstructed in the studied area for the cambrian age 2 age 3 interval indicating a transition from uninhabited sediments to well populated habitats finally both the temporal and spatial characteristics of the cambrian aes oes and the fossil record in south china are compared and links between them are discussed in the context of their specific paleo geographical setting

rare earth elements in foraminifera tests abstract the concentrations of nine rare earth elements ree la ce nd sm eu gd dy er and yb have been determined in mixed species assemblages of foraminifera tests taken from atlantic ocean sediment core tops reductive cleaning techniques have revealed that ree are present in three phases in foraminifera tests collected from sea floor sediments ree included in the foraminiferal calcite matrix termed lattice ree ree associated with an authigenic femn rich phase adsorbed onto the surface of the test following the death of the organism termed coating ree and ree associated with alumino silicate detritus termed detrital ree which commonly infills the test chambers after burial in the sediment the concentrations of ree in the nondetrital lattice plus coating and lattice phases have been measured in this study approximately 90 of the ree measured in the non detrital phase reside in the coating phase the remainder being present in the lattice phase these data have been used to investigate the relationship between the distribution of dissolved ree in the ocean and in the coating and lattice phases in addition to the ree the concentrations of mn fe cu al and po4 have been measured as an aid to characterisation of the various phases

extreme ocean anoxia during the late cryogenian recorded in reefal carbonates of southern australia abstract the neoproterozoic was a time of great change in the earth s surface and marine environments including extensive climate variability the widespread oxygenation of the oceans and the accompanying rise of animal life however the timing of ocean oxygenation remains uncertain particularly in regard to cryogenian seas which were disrupted by large periods of global glaciation interglacial cryogenian reef complexes in the northern adelaide fold belt of australia contain abundant primary marine dolomite cements these cements have well preserved textural and growth zonation indicating they preserve their original marine chemistry and can be used as geochemical proxies for late cryogenian paleooceanography analysis of marine cements from peritidal nearshore facies shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo depth while nearshore cements have low fe contents and commonly contain iron oxide inclusions the shallow and deep cements have very high fe concentrations chalcophile elements cu cd pb zn etc are most abundant in the nearshore cements whereas rare earth elements are found in highest concentrations in the deeper facies cements rare earth element profiles are unusual with shallow and deep facies having convex profiles with negligible ce ce anomalies and positive eu eu anomalies being constrained by sedimentology this carbonate geochemistry provides a window into interglacial cryogenian ocean chemistry and structure the marine cements reveal pronounced chemical stratification in this late cryogenian ocean a thin veneer of oxic surface waters existed at the ocean surface in peritidal facies with increasingly anoxic and fe rich seawater at depth the distribution of strongly chalcophile elements like cd and cu across the chemocline suggests that although ferruginous deeper anoxic waters probably contained some dissolved sulphide these conditions describe a ferro sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late precambrian a return to archean like ocean conditions at this time suggests large scale disruption of the ocean system during the neoproterozoic

dissolved rare earth elements and hydrography in the sulu sea because most southeast asian basins are flushed rapidly by waters from the western pacific the effects of respiration and silica dissolution within those basins are hardly discernible based on general hydrographic and geochemical observations broecker et al 1987 however a different situation is expected for the sulu sea because its deep water is isolated from other deep sea basins by shallow sills of 400 m depth hence any heterogeneity of elemental distributions within the basin must be ascribed to vertical physical and biogeochemical processes we have obtained detailed vertical profiles of dissolved rare earth elements rees together with conductivity temperature depth ctd and hydrographic measurements in the sulu sea during the 1996 97 hakuho maru cruise vertical profiles clearly indicate that the ree iii s are enriched in the deep water due to regeneration and are involved in vertical biogeochemical cycling the light ree iii s la pr and nd showed a subsurface maximum at 500 m and a minimum 1500 m in depth none of the hydrographic properties or nutrients shows such features therefore the ree iii s and their elemental ratios may be useful as tracers of water masses the relationship of ree iii s versus dissolved si suggests that the ree iii s are regenerated in a delayed fashion relative to si and it is likely that their dissolution largely occurs at the bottom interface the mean residence time of the deep water is estimated to be 300 6 150 yr consequently the calculated benthic flux of ree iii s becomes roughly comparable with the river and atmospheric fluxes because of oxidation to ce iv the ce profile shows a decrease from 6 pmol kg at the surface to a constant value at 3 9 pmol kg below 1500 m unlike the other rees the rees in the sulu sea are enriched in the middle rees and ce relative to those of the north pacific deep water and must derive from local sources around the sulu sea copyright 1999 elsevier science ltd

rare earth elements in the terminal ediacaran bambui group carbonate rocks brazil evidence for high seawater alkalinity during rise of early animals abstract rare earth elements plus yttrium rey mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater here we investigate rey patterns in two carbonate sections from the ediacaran bambui group sao francisco basin brazil which comprise its two lowermost transgressive regressive second order sedimentary sequences shale normalised distributions vary with the sequence stratigraphy framework in the basal 2nd order sequence carbonate samples from the basal sequence transgressive systems tract display light rey lrey distributions slightly depleted to enriched that reflect input of freshwater possibly in a post glacial episode upwards carbonate rocks from the early highstand systems tract ehst yielded lrey enriched distributions which progressively turns into lrey shale normalized depleted distributions on samples from the late highstand systems tract lhst this portion of the sequence also displays y positive anomaly in some cases carbonate samples from the upper second order sequence do not display coherent patterns ce ce values 1 in most samples throughout the two sections suggest permanent anoxia of seawater the rey change from the ehst to lhst in the basal sequence marks an important paleoenvironmental overturn in the basin with increasing alkalinity in seawater driving rey fractionation and lrey depletion confinement of the basin in the inner areas of west gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent thus delivering a higher ionic influx to a restricted setting increasing alkalinity during lhst cloudina sp fragments were reported in this stage of the bambui group and in carbonate rocks with high sr mass fractions in other west gondwana basins supporting the hypothesis that the high alkalinity of seawater during late ediacaran may have driven the appearance of the first biomineralizing organisms

oxidative forcing of global climate change a biogeochemical record across the oldest paleoproterozoic ice age in north america carbon isotope compositions of organic matter in fine grained siliciclastic units deposited above and below glacial diamictite at the base of the ca 2 45 2 22 ga huronian supergroup in ontario canada were studied to constrain relationships between profound fluctuations in the exogenic carbon cycle and dramatic climate changes at the beginning of the proterozoic eon in both drill core and outcrop sections the organic matter preserved in proximal lithofacies dominated by coarse grained sand and silt are enriched in 13 c relative to distal lithofacies dominated by argillites in the drill core sand dominated lithofacies of the mckim formation beneath the glacial diamictite of the ramsay lake formation have a narrow range of 13 c values 28 4 to 26 0 vpdb but organic matter in argillite dominated lithofacies of the outcrop section 40 km to the southeast is somewhat more 13 cdepleted with values ranging from 34 5 to 26 4 similarly sand dominated lithofacies of the pecors formation above the glacial diamictite in the drill core section with 13 c values of ca 28 are notably 13 c enriched relative to argillite dominated lithofacies which record values as low as 40 5 the sand dominated lithofacies of the pecors formation in the outcrop sections have 13 c compositions ranging from 34 4 to 27 9 the isotopic differences appear to be unrelated to organic carbon abundances so we suggest that these are controlled by environmental differences in proximal and distal settings the strong 13 cdepletion in the organic lean mckim and pecors argillites especially in the drill core section of the pecors formation is consistent with significant biological methane production and oxidative recycling by methanotrophs both before and after the ice age in shallow water environments stratified with respect to oxygen the rise of atmospheric oxygen and subsequent enhanced biogeochemical methane cycling in shallow water settings likely contributed to unstable climate conditions during the paleoproterozoic glacial epoch 2007 elsevier b v all rights reserved

the incorporation of rare earth elements in modern coral we report measurements of rare earth elements rees which show that these trace elements are being incorporated in modern coral in proportion to their seawater concentrations four bermuda north rock coral species diploria strigosa diploria labyrinthiformis montastrea annularis and porites astreoides and two tarawa atoll samples of the species hydnophora microconos were analyzed by ti idms following cleaning techniques to isolate the lattice bound rees based on the replicate analyses of the same piece of diploria strigosa excellent reproducibility was achieved the ree ca ratios 0 1 3 nmol mol of the bermuda and tarawa corals are similar to those of cd ca the trace metal with the lowest seawater concentration used in coral studies with the exception of ce the distribution coefficients e g dnd nd coral nd seawater ca seawater ca coral between bermuda coral lattice and sargasso seawater have fairly flat patterns across the ree series the values of d range from 1 to 3 like those reported for other trace elements in corals this suggests but does not prove that rees are incorporated in the aragonite lattice of these corals the shale normalized ree patterns of the tarawa corals also have seawater like distributions however no local seawater data are available to calculate values of d two bermuda species d labyrinthiformis and p astreoides have values of d for ce that are high with respect to the d values of la and nd implying that there is preferential uptake of ce into the lattice this may be related to the fact that ce is the only ree with an active redox chemistry in seawater there is considerable interest in using the chemical and isotopic composition of coral as indicators of climatic variations the combination of ree ca ratios and neodymium isotopic composition of coral has the potential to help understand important natural processes a primary application could be directed toward a tracer for river water discharge in tropical regions and by inference a proxy for rainfall climate variation and weathering history this application would exploit the fractionated composition of rees in river water as a terrigenous signature in coastal corals

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

cerium anomaly variations in ediacaran earliest cambrian carbonates from the yangtze gorges area south china implications for oxygenation of coeval shallow seawater abstract the late neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean however apart from the emergence of animal fossils little supporting evidence has been found for the rise of oxygen in shallow seawater here we present ce anomaly data a redox proxy with a higher reduction potential than many other redox proxies for carbonates from well preserved marine successions the doushantuo formation dengying formation and lower yanjiahe formation of the ediacaran period and the upper yanjiahe formation of early cambrian on the neoproterozoic yangtze platform in the yangtze gorges area in order to constrain the redox evolution of the shallow marine environment calculated ce anomalies were screened using the following criteria al 36 in acetic acid leached carbonate to target samples that preserve primary seawater ree features and ce anomalies the samples satisfying this screening show seawater like ree distribution patterns in leached carbonates and may have recorded ce anomalies of the seawater from which the carbonates precipitated these data show that the doushantuo formation has ce ce values between 0 92 and 0 71 the dengying formation between 0 90 and 0 40 and the yanjiahe formation between 0 52 and 0 72 the ce ce values of the doushantuo formation suggest that shallow waters during the doushantuo stage 635 551 ma were anoxic to suboxic from the bottom to the top of the dengying formation ce ce values decrease systematically which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the dengying stages from 551 ma to ediacaran cambrian boundary these results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the ediacaran period

geobiology of a palaeoecosystem with ediacara type fossils the shibantan member dengying formation south china abstract despite the importance of palaeoecosystems with ediacara type fossils for the early evolution of metazoans only little is known about the interplay of geological and biological processes in these environments the reason is that sedimentary structures biogenic structures and bio geochemical signatures e g hydrocarbon biomarkers are commonly not well preserved due to the predominance of volcanic and siliciclastic lithologies the shibantan member dengying formation south china is one of only few carbonate settings with ediacara type organisms worldwide and its lithology promises an excellent preservation of sedimentary facies and bio geochemical signatures here we provide the first comprehensive geobiological characterisation of the shibantan member in order to reconstruct the interplay between sedimentary and bio geochemical processes and to assess the microbial activities in the palaeoecosystem with ediacara type fossils facies analyses revealed that carbonate and organic matter were autochthonously formed by bio geochemical processes linked to microbial mats however the material was frequently reworked and re deposited within the same setting i e para autochthonous as evidenced by small scale hummocky cross stratification erosional contacts lenticular bedding and load casts negative ce anomalies ce ce and low v cr ratios demonstrate that molecular o 2 was present in the water column whereas characteristic ni co v v ni and v sc ratios suggest the contemporaneous presence of sub to anoxic water taken together these observations imply a temporarily stratified water body frequently mixed and ventilated by storms 13 c enrichments in the shibantan carbonates 13 c 3 29 to 3 98 vpdb together with 13 c depletions of syngenetic n alkanes cleaved from the extraction residue using catalytic hydropyrolysis hypy 13 c 31 7 to 36 3 vpdb could indicate a significant withdrawal of 12 c by primary producers that thrived within the mats at the same time sulphurised biomarkers in the bitumen and hypy pyrolysate hint at organic matter decomposition and concomitant sulphide production by sulphate reducing bacteria when oxygen was available at the sediment water interface due to mixing by storms sulphide oxidising bacteria were possibly temporarily favoured the results demonstrated that palaeoenvironmental conditions dynamically changed through a complex interplay of biogenic and abiogenic processes

expanded oxygen minimum zones during the late paleocene early eocene hints from multiproxy comparison and ocean modeling anthropogenic warming could well drive depletion of oceanic oxygen in the future important insight into the relationship between deoxygenation and warming can be gleaned from the geological record but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions we use i ca in benthic foraminifera to reconstruct late paleocene through early eocene bottom and pore water redox conditions in the south atlantic and southern indian oceans and compare our results with those derived from mn speciation and the ce anomaly in fish teeth we conclude that waters with lower oxygen concentrations were widespread at intermediate depths 1 5 2 km whereas bottom waters were more oxygenated at the deepest site in the southeast atlantic ocean 3 km epifaunal benthic foraminiferal i ca values were higher in the late paleocene especially at low oxygen sites than at well oxygenated modern sites indicating higher seawater total iodine concentrations in the late paleocene than today the proxy based bottom water oxygenation pattern agrees with the site to site o2 gradient as simulated in a comprehensive climate model community climate system model version 3 but the simulated absolute dissolved o2 values are low 35 mol kg while higher o2 values 60 100 mol kg were obtained in an earth system model grid enabled integrated earth system model multiproxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved

sr and nd isotopes of cold seep carbonates from the northern south china sea as proxies for fluid sources abstract in this study sr isotopes combined with nd isotopes of modern cold seep carbonates were studied to provide reliable proxies for sources and pathways of seep fluids we presented sr and nd isotopes together with stable carbon and oxygen isotope data of cold seep carbonates recovered from the northern continental slope of the south china sea scs based on the evidence for negligible detrital contamination and diagenetic alteration carbonate samples are considered dependable archives which record sr and nd isotope signals of the original pore waters from which carbonates have precipitated excellent correlation between 87sr 86sr ratios and 1 sr observed in carbonate samples indicates two end members with distinct sr isotopic signatures including modern seawater as a major source and a lesser contribution from methane rich seep fluid driven from a deep source besides these two end members release of nd from organic matter and fe mn oxyhydroxide is an additional contribution to pore water nd furthermore sr and nd isotopic compositions suggest water rock interactions between upward seep fluid and detrital component in sediments which resulted in the isotopic shift towards pronounced 143nd depleted and slightly 87sr enriched in the fluid phase this study provides insight into the knowledge for tracing seepage fluid circulation by sr nd isotope system at modern seep systems

secular changes of water chemistry in shallow water ediacaran ocean evidence from carbonates at xiaofenghe three gorges area yangtze platform south china ediacaran carbonates from the shallow water section near xiaofenghe three gorges area hubei province on the yangtze platform south china have been studied to understand post marinoan changes in seawater chemistry major and trace element abundances and sr nd isotopic compositions were obtained on acetic acid leachates of carbonate sediments from both the doushantuo formation members d1 d4 and the overlying dengying formation c and o isotopic compositions of these samples were analysed using the phosphoric acid method with the exception of the d1 cap carbonates 87sr 86sr 0 7088 0 713 most samples show low mn sr and 87sr 86sr 0 7078 in d2 d4 and 0 709 in the dengying formation similar to seawater values inferred from ediacaran carbonates elsewhere these data together with the absence of a correlation between 87sr 86sr and 18ocarb indicate minor diagenetic fluid overprinting the initial nd values of the carbonate leachates show a limited variation range from 4 3 to 7 5 with the highest values in d1 these data overlap with published data for doushantuo sediments in south china and possibly indicate an isotopically homogenous nd source in the source area during most of the ediacaran d1 samples of cap carbonates display negative 13ccarb values of 2 0 to 3 6 vpdb and 18ocarb values ranging from 6 5 to 7 3 vpdb whereas the overlying d2 strata show positive 13ccarb values around 6 carbonate rocks from d3 and d4 show a large variation of 13ccarb from 1 4 to 9 4 with some samples having unusually high 13ccarb values coupled to low toc contents most carbonate leachates at the base of the doushantuo display superchondritic y ho and variable hree enrichment pr yb 1 similar to modern seawater this suggests a similar seawater chemistry as under modern open ocean conditions at the beginning of the ediacaran whereas lower y ho in d3 correlate with flat ree y patterns indicating a change to variable freshwater seawater mixing and suppression of hree lree fractionation as in modern estuaries carbonate rock samples from d4 are rich in clay minerals up to 15 of possible authigenic origin such as nontronite and saponite formation of these clay minerals may only be possibly under extreme alkaline and saline conditions the d4 carbonate leachates show the highest y ho and lowest ce ce values and have unusually high 13ccarb values which may also support the conditions of extreme shallowing and perhaps temporarily restricted basins that led to a rise in salinity due to high evaporation rates ce anomalies in carbonate leachates show strong variation throughout the profile a secular increase of seawater oxygenation through the ediacaran is not obvious however the most pronounced ce anomalies can be found in the upper doushantuo ce ce as low as 0 64

diagenetic history of the proterozoic carbonates and its role in the oil field development in the baikit anteclise southwestern siberia abstract the meso to neoproterozoic kamo group and ediacaran carbonate rocks of the baikit anteclise southwestern siberian platform russia were studied to reconstruct the history of development of the reservoir rocks and oil field the kamo group is composed of 3 5 to 4 km thick dolostone and rarely limestone succession that was deformed before the ediacaran time post depositional processes played the key role in forming the reservoir properties of the kamo group during the stage of deposition and subsidence 1500 720 ma silicification cementation and recrystallization caused development of a dense and low permeability matrix pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments including increase in mn up to 0 093 and fe up to 3 11 contents and 87sr 86sr ratios up to 0 7239 decrease in sr concentrations down to 11 ppm and the development of flat ree normalized patterns folding and uplift in the baikit basin began about 720 ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot 70 130 c and highly saline up to 28 7 wt fluids geochemical e g paas normalized patterns of ree and isotopic 87sr 86sr 18o and 13c signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks uplift caused subaerial erosion intense dissolution and karstification which penetrated down to 200 300 m below the surface folding uplift and peneplenization were important for porosity and permeability development in the dolostones of the kamo group the cap rocks were subsequently formed during the ediacaran and paleozoic sedimentation cycle the oil field developed in the paleozoic oil produced from the source rocks of the madra vedreshev and iremeken formations of the kamo group was dissipated during the uplift and erosion before the ediacaran sedimentation after deposition of the ediacaran and cambrian strata source rocks of the iremeken formation might have released oil when they passed the oil window but most of oil in the kuyumba field was derived from younger ediacaran source rocks and migrated through fault network from the flanks the kureika and cis sayan yenisey syneclises into the reservoir of the baikit anteclise that was a topographic high at that time

variations of redox conditions in the atmosphere and yangtze platform during the ediacaran cambrian transition constraints from cr isotopes and ce anomalies abstract the ediacaran cambrian transition was characterized by the evolution from ediacaran soft body to cambrian skeletal organisms and fluctuations of the marine carbon cycle marine redox changes should have played an essential role in both ecological and environmental evolutions however temporary changes and spatial distribution of redox conditions in the ediacaran cambrian oceans remain elusive and highly debated in this study we analyzed chromium cr isotope and cerium ce anomaly profiles in carbonates from the upper ediacaran lower cambrian dengying and zhujiaqing formations at the xiaotan section south china in an attempt to provide new insights helping unravel earth s surface redox changes during the ediacaran cambrian transition our results show that these carbonates record generally positive 53crauth values up to 1 15 with fluctuations and prominent negative ce anomalies ce ce values down to 0 35 implying a broad increase in oxygenation in the atmosphere and the inner shelf of the yangtze platform during the ediacaran cambrian transition the first appearance and subsequent diversification of the cambrian animals may have benefited from this oxygenation event the pronounced fluctuations of 53crauth and 13ccarb values can be explained by episodic oxidation of a deep water doc pool in a stratified water column across the chemocline through upwelling releasing light carbon isotopes and organic matter bound isotopically unfractionated cr iii into shallow waters

rare earth element and sr nd isotope geochemistry of phosphatic rocks in neoproterozoic ediacaran doushantuo formation in zhangcunping section from western hubei province south china abstract phosphatic rocks are widely distributed in neoproterozoic ediacaran doushantuo formation in yangtze platform south china in this study we examined samples from the zhangcunping section in western hubei province with major element rare earth element ree and sr nd isotopic analyses on the phosphorites and the host dolostone rocks we employed an acid leaching method to separate the carbonate acetic acid leachable and phosphate hydrochloric acid leachable fractions in all the samples the geochemical differences between the different fractions can be recognized based on the p2o5 ree contents and sr nd isotopic characteristics the carbonate fractions in the rocks display low total ree concentrations with seawater like ree patterns super chondritic y ho ratios moderate negative ce anomalies and positive eu anomalies indicating these carbonates have retained the signatures of paleo seawater on the contrary the phosphate fractions display high total ree concentrations with hat shaped ree patterns generally lower y ho ratios weak negative ce anomalies and positive eu anomalies which may have recorded the features of diagenetic fluids negative ce anomalies and positive eu anomalies displayed in the carbonate fractions of the studied samples may also indicate the anoxic environments of ocean basins in south china during the deposition of the neoproterozoic ediacaran doushantuo formation a geochemical comparison between lower phosphorite layer and upper phosphorite layer indicates a greater degree of diagenesis occurred in the lower phosphorite layer than the upper phosphorite layer the negative end t values of the samples suggest continental crustal sources for the rees and the lower values in the phosphorite samples 6 9 to 6 0 than the dolostone samples 5 5 to 3 8 may suggest different processes for them and it is likely that the rees in the phosphorites have been modified by diagenetic fluids with ree contributions of pore fluids derived from the clastic silicates in the strata which generally have lower end t values whereas the dolostone took the rees from the ambient seawater mainly

selenium arsenic and molybdenum variation and bio radiation in the ediacaran cambrian interval abstract the ediacaran early cambrian period is critical for the evolution of multicellular life however the distribution and enrichment patterns of biological trace elements and their correlations with the bio changes are not yet fully understood here we present the variation patterns of trace elements in the yangtze gorges area south china compared to the upper continental crust selenium se cadmium cd arsenic as and molybdenum mo are the most enriched and varied trace elements in the whole ediacaran and early cambrian formations the concentrations of which vary from 10 5 to 30 1 0 02 to 163 1 5 10 5 to 196 5 and 0 02 to 289 0 mg kg respectively with average values of 1 3 1 0 14 0 and 8 0 mg kg average enrichment factors ef of 27 0 10 7 9 7 and 5 4 and ef variations of 601 2 1663 6 131 0 and 192 6 respectively the significantly enriched ef 10 strata sequences of se and as mainly include the lower and upper ediacaran doushantuo member ii dst ii dst iv bottom and upper yanjiahe formation precambrian cambrian and shuijingtuo formation early cambrian mo displays similar significantly enriched strata with se and as from dst iv and above compared to the cryogenian nantuo formation se as and mo are enriched and concentrated in the whole ediacaran and early cambrian strata especially at the early ediacaran doushantuo formation the diachronous yanjiahe formation precambrian cambrian and the early cambrian shuijingtuo formation which are the horizons of the great bio changes moreover the significantly enriched strata of se as and mo are also consistent with intervals of great bio changes

fifty years of ferromagnetic single domain theory kittel 1946 gave the original presentation of the theory of the single domain sd structure of fine ferromagnetic particles and their response to an applied magnetic field the theory predicted that the sd coercive fields would be determined by the crystalline and shape anisotropies of the particle and could be much higher than those for the multidomain specimens used in magnetic recording at that time the current magnetic recording industry has developed the use of magnetic fine particles in geophysics the observational evidence for plate tectonics continental drift and reversals rely in lava flows on the thermoremanent magnetization acquired on cooling through the curie temperature in sediments where physical deposition of magnetic particles occur the measurements are of the magnetic orientations at times of deposition or soon thereafter of single domain grains of inorganic or biogenic origin to obtain the maximum total magnetic moment in a magnetobacterium it is counterproductive for magnetosome growth to exceed the single domain size limit if in future extraterrestrial single domain magnetic particles are found that are characteristic of magnetotactic bacteria as in meteorite arrivals from mars or other planets they could be cited as evidence for extraterrestrial life

effective use of cerium anomalies as a redox proxy in carbonate dominated marine settings abstract rare earth elements and yttrium rey have a distinct distribution pattern in seawater and this pattern may be faithfully preserved in carbonate sediments and rocks anomalous concentrations of redox sensitive cerium ce compared with neighbouring rey originate in oxic water column conditions and as such ce anomalies can provide a potentially useful redox proxy in carbonate dominated marine settings the methods used to extract rey from carbonates vary widely and may suffer from widespread leaching of rey from accessory non carbonate minerals and organic matter limiting the application of ce anomalies for palaeo redox reconstruction we have systematically compared different methods on 195 carbonate samples with varying purity carbonate from both modern and ancient environments we used sequential leaching experiments in nitric acid to identify the most seawater like portion of the carbonate sample where contributions from non carbonate minerals and organic matter are minimised we also compared the results of sample dissolution in different types and strengths of acid our results confirm that rey concentrations can be inadvertently contaminated by partial leaching of clays and fe oxyhydr oxides during a single step digestion and we suggest a pre leach of 20 of the sample followed by a partial leach of 40 of the sample to selectively dissolve carbonate we suggest that rey studies are optimised in carbonates with 85 caco 3 and show that dolomites behave differently during the leaching process and must be treated separately we present rey patterns for modern carbonate rich sediments from a range of environments and show that seawater rey are faithfully preserved in some non skeletal carbonate but modified leaching procedures are necessary for impure unlithified or organic rich carbonate sediments we combine rey with fe speciation data to identify how fe oxides and clays contribute to the rey signal and explore how the two proxies can be used together to provide a complex and high resolution redox reconstruction in carbonate dominated marine environments

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

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

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

geochemical evidence from marine carbonate for enhanced terrigenous input into seawater during the ediacaran cambrian transition in south china abstract high resolution carbon and oxygen analyses along with residue contents major and trace element concentrations were carried out for marine dolostone from a profile through the upper ediacaran to the lower cambrian strata in south china the results provide geochemical evidence for enhanced input of terrigenous material into seawater in this period four positive and four negative carbon isotope excursions are identified in this profile the first remarkably negative 13c excursion exhibits the lowest value down to 2 at the ediacaran to cambrian boundary ecb in agreement with the global 13c and stratigraphy correlation the second remarkably negative 13ccarb excursion is as low as 1 81 subsequent to a positive 13ccarb excursion as high as 1 74 in the lower cambrian the ecb position in the stratigraphic profile is constrained by carbon isotope correlations the decreases of dolostone 18o and 13c values are synchronous across the ecb suggesting a change of the seawater composition at that time the increase of residue contents is concordant with decreases of y ho ratios and 18o values for the dolostone across the ecb suggesting the increase of terrigenous input in this period the relatively smooth ree y patterns of dolostone suggest its deposition in the environment of continental platform an integrated interpretation of these geochemical data including the major and trace element concentrations mn sr and mg ca ratios and the well preserved dolostone 13c and 18o values indicates that the terrigenous material from the continental weathering is rich in nutrients and 13c depleted organic matter which would have contributed to the negative 13ccarb excursion of dolostone at the stratigraphic boundaries the strong positive 13ccarb excursion in the lower cambrian may be linked to the biological radiation with lots of organic matter buried from the seawater in this period the fall and rise of sea level during different periods in the same region may result in differential input of terrigenous materials including freshwater into marine carbonates recording the coupling between regional tectonism and paleoclimatic change on the continental platform

reconstructing paleoceanographic conditions during the middle ediacaran evidence from giant ooids in south china abstract the ediacaran was a period of intense environmental change in earth history transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies including rare earth elements and yttrium rey but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder in this study we explore the possibility that a newly discovered deposit of giant ooids in the nanshanping outcrop in south china contains a relatively intact record of the ree composition of ediacaran seawater providing an important window into the redox state of the surface ocean at this time these ooids are relatively large in size 1 6 mm and exhibit densely banded laminae a regional chemostratigraphic correlation shows that the giant ooid bearing succession in south china is in the upper part of the second ediacaran positive carbon isotopic shift ep2 and thus its occurrence was not associated with the shuram excursion additionally the occurrences of giant ooid bearing oolites in different blocks were not synchronous during the middle ediacaran in situ laser ablation analysis of giant ooid cortices reveals that the shale normalized rey patterns are similar to those of modern seawater including depleted light rees superchondritic y ho ratios and positive la anomalies yet no ce anomalies 1 0 0 15 in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in south china at that time the petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial they formed in open ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and rey signatures from further diagenetic overprinting the characteristics of ce anomalies and rey compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in south china during the late stage of ep2 implying the very low atmospheric o2 level at that time our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early earth history

redox condition in the nanhua basin during the waning of the sturtian glaciation a chromium isotope perspective abstract the neoproterozoic oxygenation event noe is often considered to be synchronously and possibly causally associated with the cryogenian glaciations reconstructing early cryogenian redox conditions is therefore essential for understanding the onset of the noe we analyzed cr isotope compositions 53cr as well as major and rare earth element ree concentrations of the banded iron formations bifs in the basal fulu formation of the xiajiajiang section nanhua basin which are thought to have been deposited during the waning stage of the sturtian glaciation strongly positively fractionated authigenic 53cr values were obtained after detrital correction ranging from 0 64 to 1 22 and averaging at 0 91 0 40 2 n 10 or after detrital evaluation 0 71 this suggests extensive oxidative cr cycling occurred at the earth s surface and implies that the po2 was probably higher than 1 pal during this period the lack of ce anomalies 0 90

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

recurrent anoxia recorded in shallow marine facies at zhangcunping western hubei china throughout the ediacaran to earliest cambrian abstract in order to reconstruct the redox history of surface seawater for ediacaran shallow marine facies on the yangtze platform as well as to understand its implications for geological events oxidation and phosphatization and early evolution of life we have conducted a systematic high resolution investigation on microfacies analyses pyrite morphology as well as sulfur and carbon contents in a continuous carbonate ediacaran cambrian succession drilled 407 core in the zhangcunping area of western hubei south china the framboidal pyrites and snav toc nav non acid volatile sulfur based redox reconstruction show a dynamic history of recurrent shallow marine anoxia the early aftermath of the marinoan glaciation cap dolomite to phosphorites was characterized by either absence of pyrite or dominated by large framboids low toc contents and phosphorite deposition as well as well preserved microfossils indicating a minor oxygenated dysoxic to oxic photic shallow marine ecosystem meanwhile the uppermost doushantuo and dengying formations were potentially fully oxygenated due to the lack of pyrites as well as low snav and toc contents correlating with macrofossil assemblages miaohe biota and ediacara biota found in western hubei a major oxygenated marine ecosystem can be outlined for the late ediacaran in addition a complicated diagenetic microfabric overprints the fine microstructure of microfossils preserved in phosphorites leaving remarkable taphonomic puzzles for future study

a petrographic and geochemical study of carbonate and silica phases from the ediacaran doushantuo formation in the three gorges area of south china implications for diagenetic conditions abstract the carbonate of the ediacaran doushantuo formation contains various silica phases in the three gorges area of south china to better understand the diagenetic conditions under which these silica phases formed we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the xiaofenghe section xfh in the three gorges area of south china five types of silica phases can be distinguished based on petrographic characteristics s1 and s2 are composed of microcrystalline quartz in chert nodules and disseminated in the host dolostone respectively s3 is composed of the megaquartz while s4 and s5 are microcrystalline quartz and fibrous silica respectively both replacing calcite cement six distinct types of carbonate can be recognized including the host matrix carbonate c1 sparry calcite nodules c2 calcite around chert nodules c3 dispersed calcite cement within chert nodules c4 fine micropeloidal calcite cements c5 and calcite veins c6 sedimentary structures suggest that formation of the s1 through s5 silica phases and c2 through c5 carbonate phases occurred during early burial diagenesis and therefore their formation recorded early diagenetic conditions assuming that ediacaran surface seawater temperature ranged from 28 to 39 c surface seawater 18o values can be calculated based on the oxygen isotopic composition of the host dolostone matrix c1 which range from 6 to 0 4 to 0 and 2 to 0 for the d2 d3 and d4 subunits of the doushantuo formation respectively based on the 18o values of chert nodules and beds mainly phase s1 diagenetic temperatures can be constrained to between 27 and 60 c under such low diagenetic temperatures the primary geochemical signatures of the host dolostone such as ree y patterns 13ccarb and 34spy values are likely to be preserved those silica phases without evidence of contamination of continental silicate detritus show characteristic ree y patterns with minor light ree depletions positive la eu and gd anomalies negative ce anomalies and slightly super chondritic y ho ratios implying that the source of silica may be derived from seawater negative ce anomalies in the silica phases in d2 through d4 subunits indicate that the bottom seawater or early diagenetic porewater was oxic at the time of silica precipitation

ree y uptake and diagenesis in recent sedimentary apatites abstract rare earth elements and yttrium ree y distribution in authigenic phases are frequently used as proxies for reconstructing past seawater conditions sedimentary apatite precipitates near the sediment water interface and is therefore capable of recording the ree y composition of the overlying water column while the overprinting of primary ree y signal during late stage diagenesis is a widely known phenomenon less is known about early diagenetic effects in this study we investigate the ree y distribution in recent sedimentary apatites found on the namibian shelf using in situ mapping by laser ablation inductively coupled plasma mass spectrometry la icp ms the namibian phosphorite deposits consist of reworked and redeposited pelletal apatitic grains ranging from miocene to pleistocene in age and pristine concretionary apatitic grains that formed during pleistocene to recent the results of this study show that the ree y signatures of pelletal and concretionary apatitic grains are different with both types showing intragranular variability and differences between grain centers and rims the ree y are concentrated in the external part of the apatitic grains forming a 10 20 m wide enrichment zone while the central parts of apatitic grains are low in ree y ree y

rare earth element and nd isotopic variations in regionally extensive dolomites from the burlington keokuk formation mississippian implications for ree mobility during carbonate diagenesis abstract marine carbonates of the burlington keokuk fm miss of the midcontinent region have been subject to two major episodes of regionally extensive 100 000 km2 dolomitization previous studies of the cathodoluminescent petrography sr c and o isotopic compositions and sr fe mn and ca concentrations of burlington keokuk dolostones outline a diagenetic history in which lime mud was pervasively dolomitized by a seawater dominated fluid at many localities this early dolomite generation dolomite i was recrystallized by nonmarine diagenetic fluids at shallow burial depths to produce dolomite ii dolomites i and ii have similar rare earth element ree patterns and abundances and a narrow range of initial 143nd 144nd isotope ratios of 0 511906 0 000025 1 this consistency in the nd isotope ratios and ree patterns along with the presence of anomalously low abundances of cerium in the dolomites cherts glauconite pellets fish apatite and crinoids suggests that 1 seawater derived ree dominated the system and 2 the extensive water rock interaction that produced profound textural and compositional changes during the formation of dolomite ii did not significantly alter the ree patterns or nd isotopic signatures of the dolomites the results of quantitative modeling of isotopic and trace lement exchange during water rock interaction in carbonate systems indicate that extremely large water rock ratios are required to alter the ree signatures of diagenetic carbonates such large water rock ratios can reflect either 1 large extents of fluid flow during carbonate recrystallization or 2 minimal interaction between diagenetic fluids and host rocks during carbonate cement crystallization late vug filling calcite and dolomite have low sm nd and high ce nd n ratios relative to the dolostones such distinct ree signatures probably reflect limited interaction between late diagenetic fluids and burlington keokuk host rocks penecontemporaneous clastic sediments have similar initial 143nd 144nd isotope ratios 0 511857 0 511877 and lower sm nd ratios compared to the dolomites suggesting 1 a common crustal source of ree for the clastics and carbonates and 2 ree fractionation during the formation of carbonate apatite glauconite and other authigenic phases in the marine environment

extremely low oxygen concentration in mid proterozoic shallow seawaters abstract the mid proterozoic 1 8 0 8 ga witnessed the first appearance but unusually low diversification of eukaryotes the stagnant biotic evolution during this billion year long period commonly referred to as the boring billion was arguably ascribed to low oxygen levels in atmosphere and ocean however evidence supporting low oxygen in shallow marine environments where early eukaryotes first evolved is generally lacking or insufficient here we report rare earth element and yttrium ree y data particularly cerium ce anomalies from a suite of mid proterozoic sedimentary rocks of the north china platform the new data from north china in combination with available ce anomaly data from other proterozoic successions demonstrate that during mid proterozoic negative ce anomalies did not occur until 1 54 ga and after 1 54 ga only episodic negative ce anomalies were present in shallow water carbonates trace element enrichments u ef v ef and mo ef remained at the average continental crust level before 1 54 ga but showed a sudden increase at 1 54 ga the data suggest that oxygen concentration in shallow marine environments of the mid proterozoic ocean was extremely low probably

paleomagnetic evidence of a low temperature origin of carbonate in the martian meteorite alh84001 indirect evidence for life on mars has been reported from the study of meteorite alh84001 the formation temperature of the carbonates is controversial some estimates suggest 20 to 80 c whereas others exceed 650 c paleomagnetism can be used to distinguish between these possibilities because heating can remagnetize ferrimagnetic minerals study of two adjacent pyroxene grains from the crushed zone of alh84001 shows that each possesses a stable natural remanent magnetization nrm implying that mars had a substantial magnetic field when the grains cooled however nrm directions from these particles differ implying that the meteorite has not been heated significantly since the formation of the internal crushed zone about 4 billion years ago the carbonate globules postdate this brecciation and thus formed at low temperatures

precambrian cambrian trace fossils from the yangtze platform south china and the early evolution of bilaterian lifestyles abstract siliciclastic carbonaceous and phosphoritic shallow marine sediments from several neoproterozoic ediacaran and precambrian cambrian pc c boundary sections on the yangtze platform south china contain remarkable trace fossil assemblages the present paper provides a general overview on the distribution and the possible palaeobiological and palaeoenvironmental significance of these trace fossil assemblages from selected pc c boundary sections on the passive yangtze platform margins comparative investigations of the typology of neoproterozoic ediacaran ichnofaunas as well as of the related sedimentary environments accompanying the ichnoassemblages demonstrate in general correspondence with other studies worldwide that the neoproterozoic ichnofaunas are mainly composed of slightly curved to straight and unbranched horizontal traces which are usually related to the occurrence of shallow marine softgrounds sealed by microbial mats rare traces show ichnomorphological patterns that indicate developments toward a more effective horizontal exploitation of the feeding ground during the late neoproterozoic and earliest cambrian the majority of these traces appear to represent undermat miners with simple feeding strategies during the earliest phanerozoic early meishucunian stage equivalent to the nemakit daldynian stage of siberia the oxic shallow water environments along the ancient continental passive margins of the yangtze platform were populated by large as yet unknown arthropods which produced rusophycus type trace fossils the late precambrian to early phanerozoic sediments of the yangtze platform generally provide excellently preserved trace fossil records which together with the presence of index fossils ssf from the nemakit daldynian stage provide a more detailed insight into the development of pc c palaeoenvironments and the evolution of lifestyles among early bilaterian metazoans a preliminary comparison of the neoproterozic biotas occurring on the yangtze platform and in other time equivalent ediacaran habitats demonstrates general similarities but also some striking differences probably due to palaeoenvironmental variations the data presented here from south china are critically discussed in the light of current palichnological approaches and theories

ree and trace element patterns from organic rich rocks of the ediacaran cambrian transitional interval abstract it has been established that important changes in the marine environment and the biosphere occurred during the cambrian however the relationships between the so called cambrian explosion and the concomitant environmental changes are not yet fully understood this study presents new geochemical data from the black shale successions from different facies belts of the yangtze platform in south china variations in the concentrations of ree and trace elements varying ce ce th u v sc and v cr ratios in kerogen as well as in bulk rocks from different depositional environments along a transect from platform to basin indicate two oxidation events which led to the oxygenation of the water column in shallow marine environments and euxinic conditions weak correlation between toc v u and mo in the deeper sea during the first oxidation event in the late terreneuvian anoxic conditions in bottom waters rapidly changed to euxinic conditions subsequently the second oxidation event during the early epoch 2 of the cambrian led to oxic suboxic conditions in deeper seawater

the tonian beck spring dolomite marine dolomitization in a shallow anoxic sea abstract the reason for the abundance of dolomite lithologies in earth s early geological record compared to modern environments remains contentious this study provides new insight into this precambrian dolomite problem by revisiting one of the most controversial dolomite localities the beck spring dolomite of death valley usa consistent with some previous studies petrographic evidence indicates that although the beck spring dolomite now consists almost entirely of dolomite it was originally precipitated largely as aragonite and high mg calcite depositional constituents microbialites and ooids were likely originally aragonitic and early marine length fast cements now dolomite are suggested to have precipitated as high mg calcite then replaced syntaxially by dolomite based on petrographic and geochemical evidence we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit and for the most part involved syntaxial and mimetic replacement further a length slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement this is indicated by the length slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation this new evidence for tonian marine dolomite precipitation reinforces the idea of precambrian marine environmental conditions including the chemical composition of seawater promoting dolomite formation at this time the trace metal geochemical composition of well preserved marine components especially dolomite marine cements reveals information about redox conditions in this tonian shallow seawater in terms of rare earth element geochemistry the beck spring dolomite has no significant ce anomaly and a ubiquitous positive eu anomaly consistent with widespread oceanic anoxia during deposition furthermore the relatively low levels of iron and chalcophile elements co cu pb and zn in marine components compared to other neoproterozoic carbonates suggest euxinic conditions both anoxic and sulphidic prevailed during deposition of the beck spring dolomite

geochemical and stable isotope signatures of proterozoic stromatolitic carbonates from the vempalle and tadpatri formations cuddapah supergroup india implications on paleoenvironment and depositional conditions abstract spectacular stromatolitic dolomites with varied morphological features are preserved in the vempalle and tadpatri formations of the proterozoic cuddapah basin eastern dharwar craton they exhibit stratifera columnar conical domal and spheroidal lobate morphologies in which columns are branched at some places in tadpatri formation based on their trace elements such as rey and stable isotope compositions carbon and oxygen the vempalle stromatolites have been classified as three types vempalle i stromatolites exhibit pronounced positive eu anomalies with small scale positive ce anomalies whereas vempalle ii counterparts have slight negative to positive eu feeble positive ce and positive gd anomalies vemaplle iii stromatolites display positive ce eu and gd anomalies the tadpatri formation stromatolites have pronounced gd anomalies with low order positive ce and eu anomalies significant depletion in 18o is observed in vempalle ranging from 10 70 to 7 07 and 20 73 to 11 87 in tadpatri formations the tadpatri stromatolites display negative 13c varying from 4 56 to 1 26 contrarily the vempalle stromatolites recorded both positive and negative 13c ranging from 0 24 to 2 00 thereby reflecting their deviation from the popular belief of the boring billion concept of displaying midproterozoic 13c stasis the flat ree patterns along with ce eu and gd anomalies chondritic to superchondritic y ho ratios collectively reflect on marine water conditions for their deposition with variable inputs of siliciclastic terrigenous materials and hydrothermal signatures these cherty dolomitic stromatolites were deposited in subtidal intertidal supratidal zones of a shallow shelf of proterozoic open ocean under warm alkaline and anoxic conditions

frequency distribution of river bed materials the size distribution of river bed material plays an important role in the study of fluvial hydraulic processes it is not always easy to sample the river bed material for the determination of its size distribution but knowledge of the frequency distribution and of bed material sizes can be used to predict its size distribution therefore data on bed material size distributions of 63 streams from 10 different countries have been analysed to determine the frequency distribution which can be best fitted to them the log normal distribution cannot be fitted to the bed material mixtures when the entire range of sizes is considered the method of power transformation is found to normalize these mixtures relationships have been proposed between the parameters of the normalization and the stream slope these relations can be used to predict the size distribution of the river bed material

on the upper precambrian sinian suberathem in china abstract this paper gives a general outline of the upper precambrian sinian suberathem in china including its geographic distribution development stratigraphic sequence and classification the sinian suberathem with a time range of 1950 615 ma constitutes the greater part of the proterozoic it is characterized by the development of stromatolites appearance of eukaryotes and the presence of metazoa in its upper part its sedimentological characteristics are quite similar to those of the overlying paleozoic on the basis of the biological evolution isotopic dating and stages of tectonic development the sinian suberathem can be divided into four systems in ascending order the changcheng ca 1950 1400 ma jixian ca 1400 1000 ma qingbaikou ca 100 800 ma and sinian ca 800 615 ma systems the stratotype for the first three systems is located in jixian county and the type section for the sinian system in the yangzi gorges area the contact between the sinian and cambrian systems is in some places a continuous transition with abundant small shelly fauna above the precambrian cambrian boundary e g in the yangzi gorges yunnan and sichuan with regard to sedimentological characteristics the rocks of the sinian suberathem may be subdivided into three main facies such as 1 carbonate facies of stable region 2 volcanic sedimentary facies of mobile zones and 3 argillo arenaceous carbonate facies of transitional zones progress in the study on the succession of the sinian suberathem has been made in recent years in regard to palaeontology stratigraphy isotopic geochronology magnetostratigraphy and palaeoglaciology they provide important evidence for the stratigraphic correlation of the upper precambrian of china and other continents

petrographic rare earth elements and isotope constraints on the dolomite origin of ordovician majiagou formation jizhong depression north china abstract dolomites from the ordovician majiagou formation are considered as major hydrocarbon reservoir rocks in the yangshuiwu area jizhong depression however the origin of the dolomites remains unclear this study utilizes a composite analysis of seawater normalized rare earth elements rees petrography and isotopes c o and sr to trace dolomitizing fluids and processes three types of dolomite are identified planar s to nonplanar microcrystalline type d1 dolomite

rare earth elements of carbonate rocks from the bambui group southern sao francisco basin brazil and their significance as paleoenvironmental proxies abstract rare earth elements plus y rey data obtained for carbonate rocks of the bambui group in the southern portion of the sao francisco basin match the c and sr isotope variations that subdivide the unit in three chemostratigraphic intervals ci 1 to ci 3 the lower ci 1 samples exhibit flat rey shale normalised distributions that record a high freshwater input in the basin after a glacial period the subsequent marine transgression diminished the continental influence and initiated the development of la gd and y positive anomalies in the ci 2 samples interestingly a seawater like rey pattern is present in the ci 3 samples which correspond to the closure of the sao francisco basin rather than open marine conditions the ci 3 records a decrease in the silicate chemical weathering of sources located on the marginal belts of the sao francisco craton due to higher denudation rates and a change in the dissolved influx towards carbonate rocks the resultant high concentration of carbonate ions in the restricted sea induced a rey fractionation process similarly to that of the modern ocean constant positive ce ce values throughout the shallow and deep sections suggest anoxic early diagenetic conditions inefficient circulation and stratified water column which could have worked as a barrier for life diversification in the basin at the ediacaran cambrian limit finally our data show that the rey concentrations of ancient biochemical environments are not an unequivocal tool either to track open and restricted marine settings or to distinguish proximal and distal environments

early cambrian ocean anoxia in south china the cause of the most marked changes in the evolution of life which define the first order stratigraphic boundary between the precambrian and the phanerozoic eon remains enigmatic and a highly topical subject of debate a global ocean anoxic event triggered by large scale hydrogen sulphide h 2 s release to surface waters has been suggested by wille et al on the basis of two data sets from south china and oman to explain the fundamental biological changes across the precambrian cambrian pc c boundary here we report a new precise shrimp u pb zircon age of 532 3 0 7 million years myr ago fig 1 for a volcanic ash bed in the critical unit that reflects the ocean anoxic event the lowermost black shale sequence of the niutitang formation in the guizhou province south china this age is significantly younger than the precise pc c boundary age of 542 0 0 3 myr ago approximately 10 myr younger than the extinction of the ediacaran fauna and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the pc c boundary

global ocean redox variations across the smithian spathian boundary linked to concurrent climatic and biotic changes abstract the smithian spathian boundary ssb was an interval characterized by a major global carbon cycle perturbation climatic cooling from a middle late smithian boundary hyperthermal condition and a major setback in the recovery of marine necto pelagic faunas from the end permian mass extinction although the ssb has been linked to changes in oceanic redox conditions key aspects of this redox variation e g duration extent and triggering mechanisms and its relationship to coeval climatic and biotic changes remain unresolved here we report a high resolution middle smithian to middle spathian u isotope 238u record based on marine carbonates of the zuodeng south china and jesmond british columbia sections to investigate the timing and global extent of ocean redox variation across the ssb our 238u record reveals values similar to or slightly heavier than modern seawater 0 39 during the middle smithian a rapid negative shift to highly negative values during the early late smithian a positive shift at the ssb and a rapid shift back to more negative values in the early middle spathian a simple u isotope mass balance modeling suggests that the global area of anoxic seafloor expanded strongly during the late smithian and the early middle spathian covering 11 of total seafloor area but that it contracted sharply during the ssb 2 the redox pattern documented by our 238u record shows a good first order correspondence to tropical sea surface temperature sst data for the smithian spathian in particular peak anoxia coincided with the middle late smithian boundary hyperthermal event and diminished anoxia with a pronounced decline in ssts at the ssb the temporal correlation between anoxia and low biodiversity levels of many marine clades e g conodonts and ammonoids during the late smithian indicates that oceanic anoxia may have played a role in the ssb biocrisis

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

transient and stepwise ocean oxygenation during the late ediacaran shuram excursion insights from carbonate δ238u of northwestern mexico abstract the ediacaran shuram excursion se was a key episode in the history of atmospheric and oceanic oxygenation that has also been linked to contemporaneous bioevolutionary events however key aspects of the redox state of ediacaran oceans during the se remain in doubt here the uranium isotope compositions 238u of marine carbonates from sonora northwestern mexico were measured to investigate ocean oxygenation during the se we found that the se in sonora is characterized by a shift toward higher 238u values from 0 7 to 0 3 consistent with a major oxygenation event that has now been documented in widely separated coeval upper ediacaran sections including the doushantuo formation in south china the bol shoy patom formation in siberia and the johnnie formation in california in the sonora succession this 238u shift exhibits a 0 8 myr lag relative to the 13ccarb excursion that defines the se to account for these observations we propose a two step oceanic oxygenation model for the se 1 initial oxidation of an aqueous reductant e g dissolved organic carbon that resulted only in a shift of 13ccarb from 5 to 3 and 2 subsequent oxidation of methane or particulate organic matter that caused a continuation of the negative shift in 13ccarb from 3 to 12 as well as a positive excursion of 238u due to progressive oceanic oxygenation this transient period of enhanced oceanic oxygenation of the late ediacaran ocean which may have promoted the evolution of early metazoans terminated after 5 0 to 6 5 myr

petrography and the ree composition of apatite in the paleoproterozoic pilgujarvi sedimentary formation pechenga greenstone belt russia the first globally significant phosphorous rich deposits appear in the paleoproterozoic at around 2 ga however the specific triggers leading to apatite precipitation are debated we examine phosphorous rich rocks up to 8 wt p2o5 in 1 98 1 92 ga old pilgujarvi sedimentary formation pechenga greenstone belt russia phosphates in these rocks occur as locally derived and resedimented sand to gravel pebble sized grains consisting of apatite cemented muddy sediments phosphatic grains can be subdivided into four petrographic types a d each has a distinct ree signature reflecting different early to late diagenetic conditions and or metamorphic overprint pyrite containing petrographic type d which typically has a flat ree pattern negative ce anomaly and positive eu anomaly is the best preserved of the four types and best records conditions present during apatite precipitation type d phosphatic grains precipitated under sub oxic basinal conditions with a significant hydrothermal influence these characteristics are similar to zaonega formation phosphates of nw russia s onega basin and consistent with phosphogenesis triggered by the development of anoxic sulfidic sub oxic redoxclines at shallow sediment depth during the paleoproterozoic

constraints on the late ediacaran sulfur cycle from carbonate associated sulfate we report new sulfur isotope compositions 34s in carbonate associated sulfate cas and pyrite from the lower nama group namibia 550 to 35 is highly unusual in earth history and in the terminal ediacaran is best explained by a high pyrite burial flux pyrite in the nama group is close in isotopic composition to coeval sulfate but the sulfur isotope fractionation between sulfate and pyrite varies widely among different studied basins suggesting highly heterogeneous redox and depositional conditions

new carbon isotope stratigraphy of the ediacaran cambrian boundary interval from sw china implications for global correlation the yangtze platform preserves relatively thick carbonate successions and excellent fossil records across the ediacaran cambrian boundary interval the intensely studied meishucun section in east yunnan was one of the global stratotype section candidates for the precambrian cambrian boundary however depositional breaks were suspected in the section and the first appearance of small shelly fossils could not be verified the laolin section located in ne yunnan is more continuous and shows great potential for global correlation of carbon isotope features across the precambrian cambrian boundary however the stratigraphic framework and correlations were controversial we studied and systematically sampled the laolin section and present here new carbon isotope data for this section the laolin section consists of in ascending order the baiyanshao dolostone of the dengying formation the daibu siliceous dolostone zhongyicun dolomitic phosphorite lower dahai dolostone and upper dahai limestone of the zhujiaqing formation and the black siltstone of the shiyantou formation our data reveal a large negative 13 c excursion 7 2 l1 in the daibu member which matches the previously published data for the laolin section and a large positive excursion 3 5 l4 in the dahai member which was not shown in the published data the excursion l1 correlates well with the similarly large negative excursion near the first appearance of small shelly fossils in siberia and mongolia similar magnitude excursions are also known from morocco and oman for which there are no robust fossil constraints but from where volcanic ash beds have been dated precisely at 542 ma thus confirming a global biogeochemical event near the ediacaran cambrian boundary our data also indicate that deposition was more continuous at the laolin section compared with the meishucun section where there are no records of a comparable negative excursion near the ediacaran cambrian boundary nor any comparable positive excursion in the dahai member therefore the laolin section has proven potential to be a supplementary global stratotype section for the ediacaran cambrian boundary on the yangtze platform

copper zinc chromium and osmium isotopic compositions of the tepla barrandian unit black shales and implications for the composition and oxygenation of the neoproterozoic cambrian ocean abstract two neoproterozoic cambrian shale suites that consist of normal nbs and metal rich hmbs black shales from the tepla barrandian unit bohemian massif czech republic are studied in terms of their cu zn cr and re os isotopic compositions to improve our knowledge of elemental and isotopic mass balance of these elements in earth s oceans in the past and the extent of surface oxygenation at the neoproterozoic cambrian boundary the rock compositions indicate that the inputs of cu and zn have been controlled by variable proportions of metal rich authigenic and low metal terrigenous sources the latter having exceptionally low 65cu and partially 66zn in this respect our data confirm previous studies conclusions that organic rich sediments with high a terrigenous flux play an important role in the sinking of light zn in the oceans and provide new evidence that these types of rocks also represent the currently unidentified burial of very light cu in the ocean the chromium isotopic composition reveals a complex history of cr uptake and fractionation through the dominant cr input from different terrigenous sources with variable 53cr followed by cr fractionation that was controlled by redox states nbs on the other hand mixing and or continuous equilibrium isotopic exchange between seawater and hydrothermal cr was the dominant process for the hmbs the re os contents of the nbs were controlled by hydrogenous components which yield a poorly defined re os age for the least disturbed samples of 555 60 ma indicating an open system behavior while the hmbs re os composition was largely influenced by hydrothermal fluids during deposition the calculated authigenic 65cu which is similar to that in the present day ocean and highly positive 53cr values of the hmbs suggest high levels of surface oxygenation during the neoproterozoic cambrian transition furthermore the authigenic 66zn which is indistinguishable from that in the present day ocean may imply a constant zn isotopic composition of the oceans from the neoproterozoic because of the well balanced cycle of phosphates which were strongly adsorbed by fe hydroxides the radiogenic nature of the initial 187os 188os 0 6 although with a high associated error seems to not confirm previous indications of an abrupt increase of seawater 187os 188os during the late neoproterozoic due to the large scale radiogenic os flux into the ocean

transient shallow ocean oxidation associated with the late ediacaran nama skeletal fauna evidence from iodine contents of the lower nama group southern namibia abstract the terminal ediacaran interval from 550 ma to 541 ma witnessed the first appearance of skeletal metazoans and complex ecosystems in earth history this biotic innovation event is thought to be related to increase of oxygen in earth s surface environments but many studies suggested that pervasive oceanic anoxia continued through the late ediacaran and early cambrian to further evaluate the redox conditions of the terminal ediacaran interval and their potential relationship with biotic changes we analyzed the iodine contents in the lower nama group 550 547 ma from the driedoornvlagte and zebra river sections southern namibia the i ca mg values fluctuate from 0 08 to 6 20 mol mol with an average of 0 94 mol mol in the driedoornvlagte section and 0 70 mol mol in the zebra river section high i ca mg values 2 6 mol mol occur immediately above the boundary between the lower and upper omkyk members with a peak up to 6 20 mol mol indicative of well oxygenated surface waters comparable with those of the modern marine environments abundant skeletal fossils including cloudina namacalathus namapoikia and trace fossils are observed only in the intervals after the high i ca mg peak the results are consistent with the interpretation that the nama skeletal communities grew in oxic o2 20 70 m and dysoxic o2 10 m waters above the chemocline the i ca mg ratios show significant tempo spatial variations which is also consistent with previous studies that suggested redox stratified terminal ediacaran sedimentary basins and highly heterogeneous oceanic redox conditions

magnetotactic bacteria and single domain magnetite in hemipelagic sediments remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time scale study geomagnetic reversals and secular variation and measure the rates of seafloor spreading although these sediments may contain different magnetic minerals magnetite is the most commonly observed and magnetically stable phase and its size shape and post depositional fate affect the magnetic remanence of the sediments biogenic magnetites are single domain with a high natural magnetic remanence nrm and have been suggested as a significant source of magnetic remanence in marine sediments we have studied surface sediments from the santa barbara basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra fine grained single domain magnetite using a novel extraction technique transmission electron microscopy and squid magnetometry we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments

uranium isotope evidence for temporary ocean oxygenation in the aftermath of the sturtian snowball earth abstract the appearance and radiation of animals are commonly attributed to neoproterozoic oceanic oxygenation yet independent geochemical evidence for such an event remains equivocal strata deposited between the sturtian and marinoan snowball earth glaciations 660 to 640 ma contain the earliest animal biomarkers and possible body fossils to quantify the extent of seafloor oxygenation during this critical interval we present uranium isotope ratios 238u 235u denoted as 238 u from limestone of the taishir formation in mongolia through two stratigraphic sections that are separated by 75 km within the same depositional basin above the sturtian glacial deposits through 150 m of stratigraphy 238 u compositions have a mean value of 0 47 this interval is followed by a 0 3 decrease in 238 u coincident with the taishir negative carbon isotope excursion thereafter 238 u values remain relatively low until the erosional unconformity at the base of the marinoan glacial deposits using a box model we show that the best explanation for the higher 238 u values of the post sturtian limestones is extensive but temporary oxygenation of the seafloor and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post snowball weathering the decline in 238 u in overlying strata coincident with the taishir negative 13 c excursion indicates a subsequent decrease in seafloor oxygenation the u isotopic data combined with modeling results challenge the notion of a simple unidirectional oxygenation of neoproterozoic oceans

redox architecture of an ediacaran ocean margin integrated chemostratigraphic δ13c δ34s 87sr 86sr ce ce correlation of the doushantuo formation south china abstract early diagenetic silicification and phosphatization of the ediacaran doushantuo formation ca 635 to 551 ma in south china offer extraordinary taphonomic windows into the early evolution of multicellular eukaryotes including various algal groups and potentially animals in order to understand how the ecological and taphonomic distribution of these ediacaran eukaryotes was controlled by oceanic redox conditions it is critical to reconstruct the redox architecture of the sedimentary basin recently two alternative redox models have been proposed to account for the geochemical and sedimentary features of the doushantuo formation one argues that the unit was deposited on a continental margin where a metastable sulfidic wedge was dynamically maintained by a sulfate concentration gradient between shelf and basinal environments the other contends that the sulfidic water mass was largely restricted to the intra shelf basin behind a rimmed margin these two models make different predictions about the stratigraphic completeness and correlation of the doushantuo formation to test these predictions we generated high resolution time series trends of multiple isotopic and elemental tracers including 34s 87sr 86sr and ce ce to facilitate an integrated chemostratigraphic correlation between inner shelf xiaofenghe intra shelf jiulongwan and outer shelf yangjiaping and zhongling sections our correlations suggest that both the inner and outer shelf sections are stratigraphically incomplete relative to the intra shelf section the euxinic wedge model should be reconsidered insofar as it is based on a miscorrelation between sections viewed from our revised chemostratigraphic framework euxinic conditions on the platform appear to have been largely restricted to the intra shelf basin carbonates in the upper doushantuo formation at jiulongwan and their stratigraphic equivalents are characterized by a profound negative carbon isotope anomaly i e the shuram excursion coincident with a drop in pyrite sulfur isotope values and a significant rise in 87sr 86sr from 0 7080 to 0 7090 the integrated stratigraphic data from south china suggest that the onset of the shuram excursion is associated with enhanced oxidative continental weathering that delivered radiogenic strontium as well as sulfate to the ediacaran basin

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

oceanic environment changes caused the late ordovician extinction evidence from geochemical and nd isotopic composition in the yangtze area south china the ordovician silurian o s transition was a critical interval in geological history multiple geochemical methods are used to explore the changes in oceanic environment the nd isotopic compositions in the yangtze sea are controlled by two sources the continental erosion and the panthalassa ocean high end t values during the katian late hirnantian and rhuddanian intervals are associated with the high sea level which resulted in less terrestrial input based on the low ti al and zr al ratios in contrast low end t values during the early hirnantian interval are related to the sea level fall in this case the exposure of submarine highs and the growth of yangtze oldlands could lead to more continental materials being transported into the yangtze sea based on high ti al and zr al ratios in addition the negative end t excursion can also be attributed to the weak circulation between the yangtze sea and panthalassa ocean when sea level was low furthermore the sea level eustacy plays a significant role in the changes in redox water conditions the redox indices mainly uef ce ce and corg pt across the o s transition show a predominance of anoxic ocean over the yangtze sea during the katian late hirnantian and rhuddanian intervals and an oxygenated episode was briefly introduced during the early hirnantian period because of the fall in sea level the late ordovician biotic crisis was marked by two phase extinction events and the change in sea level and redox chemistry may be the important kill mechanisms

the geochemical balance of the rare earth elements and neodymium isotopes in the oceans profiles of ree concentrations and neodymium isotope measurements in the western indian ocean are consistent with published data from the other major oceans ree concentrations show a strong hydrographic control the influence of intermediate water masses is particularly significant surface water concentrations vary with salinity paniculate rees are less than 5 of dissolved concentration except for ce where up to 20 of total rees are present as 0 4 m particles in contrast to dissolved profiles concentration profiles of paniculate rees including ce all increase with depth ree concentrations correlate with silica over all but the deepest waters although in detail correlations are complex the neodymium isotopic composition of indian ocean seawater avg nd 0 8 3 is intermediate between that for the atlantic and pacific oceans comparison of ree data from the major oceans shows that the hrees behave more like si than do the lrees sm nd ratios are near constant for all oceans whereas ernd ratios vary between the atlantic indian and pacific oceans the fractionation of the rees in surface seawaters can be explained by fractionation factors relative to silica from 0 12 for la to 0 05 for er use of a seven box model shows that the interoceanic variations in neodymium isotopic compositions and nd concentrations cannot be reconciled unless particle water exchange is invoked the degree of exchange required is consistent with particle residence times of 1 10 yr

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