controls of eustasy and diagenesis on the 238u 235u of carbonates and evolution of the seawater 234u 238u during the last 1 4 myr using a leaching protocol designed for the study of u isotopes in recent carbonates we measured the u isotope composition both 238 u 235 u and 234 u 238 u of modern and ancient corals n 6 a limestone and a dolostone as well as 43 shallow water carbonate sediments from the odp leg 166 site 1009 drill core on the slope of the bahamas platform although bulk corals record the seawater 238 u value within 0 02 differences of up to 0 30 in the 238 u of individual leachates suggest a control of the coral structure and a more positive 238 u 235 u ratio in the centers of calcification the drill core 238 u data shows that the 238 u 235 u ratio of shallow water carbonates is controlled mainly by 1 variations in sea level through the mixing of different amounts of platform derived sediments with 238 u 0 50 0 60 heavier than seawater and pelagic sediments with seawater like 238 u values 2 authigenic u enrichment via pore watercirculation and u reduction both on the platform and down to 5 m below the surface mbsf after deposition of the sediment and to a lesser extent by 3 early diagenetic processes i e carbonate dissolution and or recrystallization during sediment burial the global effect of these processes leaves the 238 u values of shallow water carbonates offset relative to that of seawater by carbonates sw 0 24 0 06 95 ci including all samples this shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow water platform shelf and slope environments i e most of the carbonate sedimentary record prior to the mesozoic to account for the average effect of carbonate diagenesis assuming that the 238 u 235 u ratio of carbonate platform sediments directly records the seawater 238 u 235 u ratio would underestimate the extent of ocean seafloor anoxia by at least a factor 10 the rapid fluctuations in 238 u values due to sea level changes i is a factor that should be considered before interpreting 238 u variations as reflecting changes in oceanic paleoredox conditions and ii reinforces the need for statistically meaningful data sets the 234 u data suggest that the 234 u 238 u ratio of the seawater has remained within 20 of the modern seawater value during the last 1 1 4 myr furthermore we find that small scale 1 15 variations in seawater 234 u mirror sea level changes during the penultimate glacial interglacial period 140 to 200 ka thus confirming the record of lower 234 u sw during periods of low sea level stand and expanding it to at least the last two glacial interglacial events i e 0 23 ma such fluctuations in 234 u initial values should be taken into account when screening carbonate sediments u th ages on the basis of the initial 234 u 238 u ratios of the samples

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

multiple s isotopic constraints on paleo redox and sulfate concentrations across the ediacaran cambrian transition in south china abstract changes in oceanic chemical conditions are frequently invoked as a primary driver of biodiversification and mass extinction events in earth history during the ediacaran cambrian transition two major biological events including the extinction of the ediacaran biota and the cambrian explosion occurred approximately coincident with perturbations of the carbon and sulfur cycle these dramatic biological changes and coeval geochemical events are commonly thought to be linked but their causal relationships remain controversial here we present multiple s isotopic compositions of pyrite 34spy and 33spy from the xiaotan section spanning the ediacaran cambrian transition interval in south china our isotopic results show that negative 33spy values together with positive 34spy values occurred across the ediacaran cambrian boundary the negative 33spy data suggest mixing of sulfide from deep waters during the upwelling of sulfidic waters into oxygenated shallow waters the coincidence between the negative 33spy and the approximate timing of the extinction of the ediacaran biota implies that paleo redox perturbations may have contributed to the disappearance of the ediacaran biota negative 33spy values coupled with positive 34spy values were also observed in the cambrian zhongyicun member when the phosphorites were deposited suggesting a strong linkage between the upwelling of nutrient rich sulfidic deep waters and the deposition of the phosphorites in oxygenated shelf areas in addition using box models and stoichiometry calculations we quantitatively estimated elevated burial rates of organic matter and pyrite based on the parallel positive 13ccarb 34spy and 33spy excursions observed at xiaotan within cambrian stage 2 our mass balance modelling results provide an independent constraint on sulfate concentrations which were 4 mm or lower in the early cambrian oceans the probable increase in oxygen levels resulting from the enhanced burial of pyrite and organic matter could have provided a more habitable environment for early animals and thus paved the way for the radiation of early cambrian animals

a geochemical framework for retrieving the linked depositional and diagenetic histories of marine carbonates in modern shallow seawater carbonate platforms successive changes in diagenetic zones are found as the result of a drop in sea level such a relationship is important in identifying diagenetic processes in interpreting global carbon isotope shifts and in finding the reasons for positive correlations between c and o isotopes in marine carbonates however it is not readily recognized in ancient marine carbonates because evidence for sea level changes can be cryptic in the rock record because of the fall of sea level fresh water may add to precipitating marine carbonates on continental substrates resulting in geochemical mixtures between shallow seawater and fresh water to resolve this issue we carry out a combined study of rare earth elements and yttrium ree y and co isotopes in carboniferous triassic marine carbonates from the lower yangtze platform in china the relationships between inter ree y ratios such as y ho vs nd yb paas for pure carbonates strongly indicate mixing between fresh water and shallow seawater the ree y patterns for pure carbonates show temporal variations reflecting changes in the proportion of fresh water addition into shallow seawater which were likely due to changes in relative sea level changes in diagenetic zones indicated by the relationships between carbonate c and o isotopes as well as other evidence follow the changes in relative sea level suggested by the ree y patterns therefore a combined study of the ree y and co isotopes in marine carbonates can provide robust constraints on the changes in the linked depositional environments and diagenetic processes on continental substrates

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

nitrogen cycle perturbations linked to metazoan diversification during the early cambrian abstract the rapid diversification of metazoans during the late ediacaran early cambrian is one of the most remarkable evolutionary events in earth history however the triggering mechanism remains highly contentious particularly concerning the relationship of this biological event to contemporaneous changes in ocean redox and nutrient cycling in this study we report new bulk nitrogen isotope 15n and organic carbon isotope 13corg data from the lower cambrian niutitang formation in two deep water sections in south china to provide a better understanding of the relationship of the nitrogen cycle ocean redox and biological evolution during this key period the 15n data from two study sections are characterized by consistently low values 1 to 2 throughout the niutitang formation despite large variations in 13corg we interpret these low 15n values to have resulted from assimilation of recycled ammonium nh4 and n2 fixation compilation of available 15n data reveals large temporal and spatial 15n variations from 1 to 7 in time equivalent strata in south china such 15n variations imply the co existence of aerobic and anaerobic n cycles and highly heterogeneous ocean redox conditions during the early cambrian in south china which is consistent with inferences from other geochemical proxies temporal fluctuations and spatial heterogeneity in ocean redox conditions may have controlled the distribution and evolution of metazoans during the early cambrian

a cenozoic seawater redox record derived from 238u 235u in ferromanganese crusts oceanic oxygen levels are projected to drop in certain areas due to warming climate but the net effect to the overall ocean redox state is difficult to predict here we measured the stable uranium isotope composition 238u 235u in globally representative hydrogenous ferromanganese crusts in order to reconstruct the redox evolution of the global ocean throughout the cenozoic samples averaging 3 myr intervals have analytically indistinguishable 238u 235u throughout the cenozoic combined with a u isotope mass balance model we suggest that the overall ocean redox state has remained remarkably stable on million year time scales throughout the cenozoic despite large surface temperature fluctuations during this time this suggests that stabilizing feedbacks for example nutrient limitation in low oxygen zones may have prevented dramatic large scale shifts in oxygen levels in the ocean however the fe mn crust record will be unlikely to reflect rapid perturbations in ocean redox state to investigate these events sediment archives with faster accumulation rates and redox proxies with faster response time must be explored

highly dynamic marine redox state through the cambrian explosion highlighted by authigenic δ238u records abstract the history of oceanic oxygenation from the late neoproterozoic to the early cambrian is currently debated making it difficult to gauge whether and to what extent environmental triggers played a role in shaping the trajectory of metazoan diversification uranium isotope 238 u records from carbonates have recently been used to argue for significant swings in the global marine redox state from the late neoproterozoic to the early cambrian however geochemical signatures in carbonates the u isotope archive most commonly employed to argue for redox shifts are susceptible to diagenetic alteration and may have variable offsets from seawater values therefore there is an impetus to reconstruct seawater u isotopic evolution using another sedimentary archive in order to verify that these excursions can indeed be linked to global shifts in marine redox landscape here we report new u isotope data from two fine grained siliciclastic upper ediacaran to lower cambrian ca 551 515 ma successions in south china we find large 238 u swings between 0 63 and 0 39 for calculated values of authigenic u in the siliciclastic rocks consistent with correlative records from the carbonates the replication of these patterns in both carbonate and siliciclastic units provides confirmatory evidence that the early cambrian seawater was characterized by highly variable u isotope compositions these new 238 u data also provide higher resolution records of global oceanic redox conditions during cambrian age 3 coeval with a critical interval of the cambrian explosion these 238 u data bolster the case that the ediacaran cambrian transition experienced massive swings in marine redox state providing a dynamic environmental backdrop for and potentially even a key driver of the emergence and radiation of metazoans

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

uranium isotopes distinguish two geochemically distinct stages during the later cambrian spice event a b s t r a c t anoxic marine zones were common in early paleozoic oceans 542 400 ma and present a potential link to atmospheric po2 via feedbacks linking global marine phosphorous recycling primary production and organic carbon burial uranium u isotopes in carbonate rocks track the extent of ocean anoxia whereas carbon c and sulfur s isotopes track the burial of organic carbon and pyrite sulfur primary long term sources of atmospheric oxygen in combination these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million year time scales here we report high precision uranium isotopic data in marine carbonates deposited during the late cambrian spice event at ca 499 ma documenting a well defined 0 18 negative 238 u excursion that occurs at the onset of the spice event s positive 13

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

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

uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late ediacaran period abstract to improve estimates of the extent of ocean oxygenation during the late ediacaran period we measured the u and mo isotope compositions of euxinic anoxic and sulfidic organic rich mudrocks orm of member iv upper doushantuo formation south china the average 238 u of most samples is 0 24 0 16 2sd relative to standard crm145 which is slightly higher than the average 238 u of 0 02 0 12 for restricted black sea deep water unit i euxinic sediments and is similar to a modeled 238 u value of 0 2 for open ocean euxinic sediments in the modern well oxygenated oceans because 238 u is preferentially removed to euxinic sediments compared to 235 u expanded ocean anoxia will deplete seawater of 238 u relative to 235 u ultimately leading to deposition of orm with low 238 u hence the high 238 u of member iv orm points to a common occurrence of extensive ocean oxygenation ca 560 to 551 myr ago the mo isotope composition of sediments deposited from strongly euxinic bottom waters h 2 s aq 11 m either directly records the global seawater mo isotope composition if mo removal from deep waters is quantitative or represents a minimum value for seawater if mo removal is not quantitative near the top of member iv 98 mo approaches the modern seawater value of 2 34 0 10 high 98 mo points to widespread ocean oxygenation because the preferential removal of isotopically light mo to sediments occurs to a greater extent in o 2 rich compared to o 2 deficient marine environments however the 98 mo value for most member iv orm is near 0 relative to standard nist srm 3134 0 25 suggesting extensive anoxia the low 98 mo is at odds with the high mo concentrations of member iv orm which suggest a large seawater mo inventory in well oxygenated oceans and the high 238 u hence we propose that the low 98 mo of most member iv orm was fractionated from contemporaneous seawater possible mechanisms driving this isotope fractionation include 1 inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater like 98 mo signature in sediments or 2 delivery of isotopically light mo to sediments via a particulate fe mn oxyhydroxide shuttle a compilation of mo isotope data from euxinic orm suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late neoproterozoic and early paleozoic time hence member iv does not capture irreversible deep ocean oxygenation instead complex ocean redox variations likely marked the transition from o 2 deficient proterozoic oceans to widely oxygenated later phanerozoic oceans

fluid migration directions inferred from gradient of time surfaces of the sub seabed the role of sub seabed topographically controlled fluid migration is assessed to improve our understanding of distributions of acoustic chimneys at the nyegga pockmark field on the mid norwegian continental margin 3d seismic data interpretations resulted in topographic gradients of seismic time surfaces and rms amplitude maps topographical gradient maps and flow tracing allowed identifying migration pathways and trapping locations for free gas within the shallow sub seabed the occurrence of acoustic chimneys pockmarks and mounds correlate with identified fluid migration pathways and gas trapping locations an important factor that controls the trapping locations and the lateral distribution of seeps on the seabed at nyegga is the variation through time of the depth of the base of the gas hydrate stability zone bghsz fluids can derive from gas hydrate systems that are suspected of being a biogenic source and or tertiary domes that are considered to show leakage of thermogenic fluids to the shallow geosphere

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

tracking the evolution of seawater mo isotopes through the ediacaran cambrian transition abstract the rapid increase in animal diversity and abundance over the ediacaran cambrian boundary has been attributed to among other parameters pronounced earth surface oxygenation at that time however whether atmospheric oxygen elevated to modern levels or whether the extent was smaller and oxygen remained relatively muted until the late paleozoic is currently debated here we present new mo isotope datasets of drill core material for two key black shale units the ediacaran doushantuo formation and the lower cambrian jiumenchong formation located on the yangtze block these data capture a 0 7 isotope fractionation between sediments experiencing quantitative and non quantitative mo removal by identifying this offset we provide integrated estimates for global seawater mo isotope compositions during the deposition of these two formations 1 2 1 4 and 1 8 2 respectively when paired with a mass balance model our results indicate that a large proportion of the early cambrian ocean was bathed under oxic waters we also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans

microbial and animal evolution in relation to redox fluctuations in a deep water setting of south china during the ediacaran cambrian transition ca 551 523 ma abstract the ediacaran cambrian e c transition witnessed significant ecosystem perturbations of the oceanic environment e g oxygen concentrations of seawater and biological evolution however in spite of abundant ediacara type megafossils trace fossils small shelly fossils ssfs macroscopic algae and metazoans recovered from shallow water e c transition strata throughout south china the manifestation of geological events in deep water settings remains elusive here we present new paleontological and geochemical evidence from the 38 m thick deep water liuchapo formation ca 551 523 ma of the yangtou section northeastern guizhou province china which consists mainly of chert through the e c transition the liuchapo formation can be divided into two members based on biostratigraphy and redox proxies the lower member 0 32 m is characterized by low mo al 0 15 1 82 ppm and u al 0 47 2 51 ppm values that suggest deposition under suboxic conditions these deposits are dominated by horodyskia minor paleopascichnus jiumenensis spherical cyanobacteria myxococcoides and random acritarchs the upper member 32 38 m records a transition to anoxic non sulfidic conditions as suggested by marked increases in mo al 5 47 ppm and u al 8 810 ppm the upper member contains ssfs sponge spicules filamentous cyanobacteria multicellular algae and sheet like fossils the survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that marine redox fluctuations may limit the growth of late ediacaran organisms and provide the living space and elevated nutrients for early cambrian biota

integrated geochemical petrographic insights from component selective δ238u of cryogenian marine carbonates emerging geochemical proxies have improved our understanding of the broad scale history of earth s oxygenation however paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting this is particularly an issue with marine carbonates which although being potentially ideal ocean redox archives are commonly altered during diagenesis here we provide new insight into the robustness of uranium isotopes 238u 235u ratios 238u values as paleoredox tracers by determining texture specific 238u values from a well described cryogenian balcanoona reef complex in south australia we found high variability in 238u values between different carbonate components even within a single sample petrographically the best preserved components from the balcanoona reef are marine cements which have a mean 238u value of 0 23 essentially unfractionated from riverine inputs these values are interpreted as reflecting a marine system with widespread anoxic and iron rich settings less well preserved phases have 238u values spanning almost the entire extent of the documented isotopic range this integrated petrographic geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case by case basis in future carbonate metal isotope geochemistry

a tale of three taphonomic modes the ediacaran fossil flabellophyton preserved in limestone black shale and sandstone abstract ediacaran macrofossils are typically preserved in three taphonomic modes casts molds in siliciclastic rocks casts molds in carbonate rocks and carbonaceous compressions in black shales only a few taxa are known to be preserved in more than one of these taphonomic modes flabellophyton is a genus that has been previously reported from lower ediacaran black shale of the lantian formation 635 551 ma in south china and upper ediacaran sandstone of the ediacara member 560 550 ma in south australia here we report flabellophyton from upper ediacaran limestone of the shibantan member of the dengying formation 551 539 ma in south china making flabellophyton the only genus that occurs in all three taphonomic modes we also provide a systematic description of flabellophyton based on material from the lantian and dengying formations in south china recognizing three morphospecies f lantianense f typicum sp nov and f obesum sp nov flabellophyton is reconstructed as an erect epibenthic marine organism attached to sandy carbonate and muddy substrates its phylogenetic affinity remains ambiguous though it was historically interpreted as an algal fossil the wide environmental and stratigraphic distribution of flabellophyton allow comparative taphonomic and paleoecological analysis taphonomic analysis of flabellophyton indicates that multiple taphonomic pathways can facilitate the preservation of ediacaran macrofossils as a window into ediacaran paleoecology flabellophyton and other ediacaran fossils played a crucial role in the construction of epibenthic communities in ediacaran oceans and helps to understand the ecological migration and evolutionary expansion from deeper to shallower oceans during the ediacaran period

uranium isotope evidence for an expansion of anoxia in terminal ediacaran oceans abstract anoxic and iron rich oceanic conditions prevailed throughout most of the archean and proterozoic 4000 to c 540 million years ago ma but the oceans are hypothesised to have become progressively oxygen rich during the ediacaran cambrian transition interval coincident with the rise of animal life we utilise the uranium isotope ratio of seawater 238u 235u reformulated as 238 u an effective tracer of oceanic redox conditions as a proxy for changes in the global proportion of anoxic seafloor we present a new 238 u dataset for carbonate rocks from the lower nama group namibia deposited in a shelf ramp succession during the terminal neoproterozoic 550 to 547 ma these data capture a transition from 238 u similar to the modern ocean towards persistently low 238 u average 0 81 0 06 such low 238 u are consistent with enhanced u drawdown from the water column under anoxic conditions and the preferential export of heavy 238u to sediments following u vi u iv reduction placing our results into a steady state ocean box model suggests at least a third of the global seafloor was covered by anoxic bottom waters compared with only 0 3 in today s oxygenated oceans comparison with 238 u from older sediments deposited in other basins further supports an expansion of anoxic bottom waters towards the end of the ediacaran our data are consistent with an emerging picture of a dominantly anoxic ediacaran ocean punctuated by brief ocean oxygenation events in the nama group the transition towards globally widespread anoxic conditions post dates the first appearance of both skeletal metazoans and soft bodied fauna of the nama assemblage this suggests that the global expansion of anoxia did not coincide with the decline of the ediacaran biota or drive the biotic turnover between the white sea and nama assemblages the impact of this global redox change on metazoan ecosystems is unclear since the expansion of anoxia if contained mainly within deeper waters may not have impinged significantly upon continental shelves that host the majority of biodiversity

spatiotemporal evolution and causes of marine euxinia in the early cambrian nanhua basin south china abstract marine redox conditions and their dynamic variations in the early cambrian are thought to have strongly influenced the evolution of early animals however the patterns and mechanisms of contemporaneous marine redox variation remain poorly understood the nanhua basin south china was a failed rift basin located between the yangtze block nw margin and cathaysia block se margin in the early cambrian in this study we conducted a comprehensive study of marine redox conditions sulfur cycling and chemical weathering intensity of source areas for the lower cambrian shales of the qingxi formation in the basinal shimensuidao section which was geographically close to the cathaysia block we integrated these results with previously published data from the nearby silikou section as well as the coeval inner shelf xiaotan upper slope daotuo lower slope longbizui and basinal zhalagou sections where were closer to the yangtze block at shimensuidao iron species and redox sensitive trace element data suggest dominantly euxinic conditions for the organic rich black shale interval lower member lm and variable redox conditions oxic ferruginous euxinic for the overlying organic lean shales upper member um of the qingxi formation integrated redox results show a similar lessening of euxinic conditions from lm to um across the entire basin and that euxinia developed along both the yangtze and cathaysia basin margins but it was mainly in upper slope areas e g daotuo on the yangtze margin and reached basinal areas e g silikou and shimensuidao on the cathaysia margin a decline of 34spy from lm to um was observed at shimensuidao and other sections which can be explained by decreasing microbial sulfate reduction rates due to decreasing organic availability and increasing riverine sulfate flux at study sections lower 34spy values in shallow marine relative to deep marine areas suggest increased terrestrial sulfate availability to the shallower sites chemical index of alteration cia values at shimensuidao are uniformly high for both the lm ciacorr1 82 5 0 4 ciacorr2 88 6 0 5 and um ciacorr1 80 2 2 3 ciacorr2 86 2 2 5 compared with the low to moderate values at daotuo ciacorr2 61 9 3 9 for lm 71 3 4 8 for um this pattern is consistent with intense chemical weathering of source areas on the cathaysia block and weaker chemical weathering of source areas on the yangtze block based on these observations we propose that strong chemical weathering of the cathaysia block led to regionally high terrigenous sulfate fluxes resulting in expansion of mid depth euxinia into deep basinal areas on the cathaysia margin of the nanhua basin in contrast weak chemical weathering of the yangtze block yielded lower terrigenous sulfate fluxes limiting euxinia to upper slope areas on the yangtze margin of the basin weaker euxinia during um deposition may have been due to its decreasing organic availability and or small seawater sulfate reservoir caused by persistent euxinic consumption during lm deposition our results highlight the complex controls and the key role of continental weathering and resulting terrigenous sulfate fluxes in marine euxinic variations in early cambrian marginal seas providing new insights into the co evolution of continents oceans and early animals