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

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

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

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

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

new c isotope stratigraphy from southwest china implications for the placement of the precambrian cambrian boundary on the yangtze platform and global correlations a high resolution c isotope profile from a fossiliferous section spanning the precambrian cambrian pc c boundary on the yangtze platform southwest china is used to constrain c isotope chemostratigraphy and its global correlation the discovery of a strong negative c isotopic anomaly just below the china marker a indicates that this negative anomaly can be correlated globally and demonstrates that the pc c boundary of the chinese successions should be close to marker a on the basis of c isotope chemostratigraphic constraints and biostratigraphic data we find that c isotopic peak s2 also first reported here from the chinese successions can be correlated with c isotopic peak i in northern siberia and c isotope feature d in southwest mongolia by integrating the data from archaeocyaeothan fauna the c isotopic peak s3 in the middle dahai carbonates may be correlative with feature g in the mongolian section

the bi azimuthal scattering distribution of an abyssal hill high resolution bistatic images of a typical abyssal hill on the western flank of the mid atlantic ridge are made with a low frequency towed array system operating remotely at 1 2 convergence zone 33 3 km stand off comparison with modeled images generated from high resolution supporting bathymetry sampled at 5 m intervals roughly the wavelength scale reveals that steep scarps return the strongest echoes because they project the largest area along the acoustic path from the source to receiver prominent returns deterministically image scarp morphology when the cross range axis of the system s resolution footprint runs along the scarp axis statistical fluctuations inherent in the scattered field prevent the system from distinguishing smaller scale anomalies on the scarps such as canyons and gullies 100 200 m scale that would otherwise be resolvable in range in certain bistatic geometries the mean bi azimuthal scattering distributions of the two major scarps on the abyssal hill are identical a

using sims to decode noisy stratigraphic δ13c variations in ediacaran carbonates abstract carbonate carbon isotope 13ccarb chemostratigraphy is a valuable tool in precambrian stratigraphic correlation the effectiveness of this tool rests on the assumption that 13ccarb data record global seawater signals however in some cases 13ccarb data exhibit rapid and noisy stratigraphic variations that may have been influenced by authigenic or diagenetic carbonate minerals to assess the contribution of non primary minerals towards bulk carbonate carbon isotope values we acquired sims secondary ion mass spectrometry 13ccarb data electron microscopic data and cl cathodoluminescence microscopic data from two ediacaran successions the lower member ii of the doushantuo formation in south china and the mooifontein member of the zaris formation in southern namibia the doushantuo samples came from a stratigraphic interval with noisy meter scale 13ccarb variations of up to 10 whereas the mooifontein member is characterized by consistent 13ccarb values with limited meter scale variations of less than 4 our data show that the meter scale stratigraphic variations in 13ccarb are also mirrored in the sims data at m mm scales in both doushantuo and mooifontein samples in the doushantuo samples sims 13ccarb values of authigenic calcite vary by up to 10 over m mm scales and can be either higher or lower than those of the co existing dolomite matrix which also appears to be affected by authigenic carbonate on the basis of petrographic observation bulk sample 13ccarb values measured on powders microdrilled from the same sims specimens are within the range of sims 13ccarb values thus we infer that bulk sample 13ccarb values of doushantuo samples represent mixtures of different carbonate components the doushantuo and mooifontein sims 13ccarb data validate the traditional method of using chemostratigraphic consistency to evaluate authigenic diagenetic alteration but given that secondary phases may be 13c enriched relative to matrix carbonate caution is warranted when taking maximum values to approximate primary chemostratigraphic trends

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

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

microbial sequestration of phosphorus in anoxic upwelling sediments organic phosphorus is removed from the ocean by its conversion to phosphorite laboratory incubations suggest that bacteria catalyse phosphorite formation and that the rate of conversion is greatest under anoxic conditions phosphorus is an essential nutrient for life in the ocean phosphorus burial regulates marine primary production1 2 phosphorus is removed from the ocean by sedimentation of organic matter and the subsequent conversion of organic phosphorus to phosphate minerals such as apatite and ultimately phosphorite deposits3 4 bacteria are thought to mediate these processes5 but the mechanism of sequestration has remained unclear here we present results from laboratory incubations in which we labelled organic rich sediments from the benguela upwelling system namibia with a 33p radiotracer and tracked the fate of the phosphorus we show that under both anoxic and oxic conditions large sulphide oxidizing bacteria accumulate 33p in their cells and catalyse the nearly instantaneous conversion of phosphate to apatite apatite formation was greatest under anoxic conditions nutrient analyses of namibian upwelling waters and sediments suggest that the rate of phosphate to apatite conversion beneath anoxic bottom waters exceeds the rate of phosphorus release during organic matter mineralization in the upper sediment layers we suggest that bacterial apatite formation is a significant phosphorus sink under anoxic bottom water conditions expanding oxygen minimum zones are projected in simulations of future climate change6 potentially increasing sequestration of marine phosphate and restricting marine productivity

middle proterozoic ocean chemistry evidence from the mcarthur basin northern australia we report on the iron sulfur systematics and s isotopic compositions of upper paleoproterozoic carbonaceous shales from the wollogorang and lower reward formations mcarthur basin northern australia iron speciation in these rocks is similar to that of modern euxinic black sea sediments and distinct from sediments depositing from oxygenated water providing persuasive evidence of euxinic depositional environments in a mid proterozoic marine basin high values of degree of pyritization dop also indicate anoxic bottom water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides the isotopic composition of the sulfides is however quite enriched in 3 4 s and for the lower reward formation similar to contemporaneous seawater sulfate these 3 4 s enriched sulfides are consistent with extensive or even complete sulfate depletion within the euxinic basins in which the sediments were deposited based on analogy with modern euxinic environments and further constrained by model results middle proterozoic seawater sulfate concentrations were probably within the range of 0 5 to 2 4 mm taken together our results indicate a relatively low sulfate late paleoproterozoic ocean and sulfidic deep waters in the mcarthur seaway a condition that could have encompassed much larger regions of the ocean

sedimentary corg p ratios paleocean ventilation and phanerozoic atmospheric po2 the corg p ratios of organic rich facies tocn1 exhibit considerable variation through the phanerozoic from b10 1 in the permian phosphoria formation to n1000 1 in some devonian black shales relative to the composition of phytoplankton c p 106 1 which provide the bulk of organic c and p to organic rich marine sediments the range of phanerozoic sedimentary corg p ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of c and p remobilization enhanced preservationoforganiccatlowerredoxpotentials combinedwithenhancedsedimentaryretentionofremineralizedorganicpathigher redox potentials results in a strong relationship between benthic redox conditions and sedimentary corg p ratios the strong secular coherence of the phanerozoic corg p record i e limited variation within narrow time slices suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale such as atmospheric po2 inversion and scaling of the phanerozoic corg p record yields a new atmospheric po2 model that has strong similarities to existing models based on elemental and isotopic mass balances yet differs in some potentially significant respects in particular the new model suggests that atmospheric po2 wasmarkedlylowerduringtheearlytomiddlepaleozoicthanpreviouslyinferred duringmostofthedevonian atmospherico2levels mayhavebeensufficientlylow b13 astopreventthesustainedcombustionofplantmaterial thisfeatureofthemodelmayaccount for ananomalous gapinthefossilcharcoalrecordduringthedevonian theredox dependentburialofpintheoceanresults inpositive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales temporal variations in the burial of p relative to c suggest that the strength of these feedback mechanisms varied over the phanerozoic 2007 elsevier b v all rights reserved

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

cryogenian glaciation and the onset of carbon isotope decoupling global carbon cycle perturbations throughout earth history are frequently linked to changing paleogeography glaciation ocean oxygenation and biological innovation a pronounced carbonate carbon isotope excursion during the ediacaran period 635 to 542 million years ago accompanied by invariant or decoupled organic carbon isotope values has been explained with a model that relies on a large oceanic reservoir of organic carbon we present carbonate and organic matter carbon isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the sturtian and marinoan glacial events of the cryogenian period approximately 720 to 635 million years ago growth of the organic carbon pool may be related to iron rich and sulfate poor deep ocean conditions facilitated by an increase in the fe s ratio of the riverine flux after sturtian glacial removal of a long lived continental regolith

high resolution organic carbon isotope stratigraphy from a slope to basinal setting on the yangtze platform south china implications for the ediacaran cambrian transition organic carbon isotope data from two slope to basinal sections across the ediacaran cambrian transition on the yangtze platform south china record secular variations in delta c 13 data supplement highly comparable organic carbon isotope profiles from time equivalent shallow shelf and slope settings no environmentally determined carbon isotope gradient across the yangtze platform is discernible suggesting that a transgressive anoxic event provided an isotopically homogenous carbon source for primary productivity that originated from the deeper part of an anoxic and or sulfidic basin c 2011 elsevier b v all rights reserved

paleoceanographic constraints on precambrian phosphorite accumulation baraga group michigan usa abstract the palaeoproterozoic baraga group ca 1850 1 ma of northern michigan is a 1200 m thick sedimentary succession of marine clastic iron formation chert and phosphatic sedimentary rocks that accumulated at the peak of the world s first major phosphogenic episode lithofacies stacking patterns are interpreted to record the flooding of the nuna continental margin during two sea level cycles the base of the first sequence is marked by a transgressive lag on archean basement that is transitional into a highstand accumulation of sandstones deposited in peritidal environments the bottom of the second sequence is characterized by a chert and carbonate unit with numerous subaerial exposure surfaces deposited in intertidal to supratidal environments this chert grades upward into subtidal deposits composed of interbedded organic rich mudstone and sandstone the highstand and falling stage systems tracts are composed of progradational deltaic deposits sequences were framed using the newly discovered sudbury impact ejecta horizon as a datum its emplacement approximately coincides with unrelated environmental changes that increased the delivery of river borne sediment to the nuna margin iron bearing minerals ankerite siderite and pyrite show that ferrous iron was present in peritidal to deep marine environments ankerite dominates nearshore settings where pyrite is absent pyrite occurs in deeper progradational deltaic deposits where sulfide was produced by bacterial sulfate reduction phosphorite is restricted to shallow water sediments of the first sequence and the lowstand of the second sequence precipitation is interpreted to be the result of fe redox pumping just below the sediment water interface where photosynthetically produced nearshore oxygen oases impinged on the seafloor such shallow water phosphorite accumulation is in stark contrast to many phanerozoic depositional systems in which phosphogenesis occurs across the shelf this difference likely reflects the dissimilarity in the oxygenation state of the seafloor in the precambrian fe redox pumping and thus phosphogenesis was restricted to shallow water settings with a suboxic seafloor in the phanerozoic phosphorite forms in the full spectrum of shelf environments because the entire seafloor is generally well oxygenated the concentration of bioavailable p in neritic environments during the proterozoic may have played a major role in the development of benthic microbial ecosystems and evolving eukaryotes

comments on the life cycle of the northern hemisphere sudden stratospheric warmings limpasuvan et al 2004 hereafter lth consider thelife cycle of sudden stratospheric warmings ssws from onset to maturity and eventually decay by com positing multiple events over the course of 80 days their main emphasis is on the mature phases of the lifecycle when there is a clear chain of downward strato sphere to troposphere propagation of anomalies inthis comment we aim to focus on the onset resultsreported in lth by placing the results in the context ofmore regional phenomena reported in earlier studiesand providing a more comprehensive framework forunderstanding wintertime northern hemisphere cli mate variability and improved predictability among the results presented lth show that sswsare associated with antecedent eliassen palm ep fluxconvergence from the troposphere these upward propagating ep flux anomalies precede the large scaleanomalies in the stratosphere and subsequent down ward propagating anomalies yield an arctic oscillation ao signal in the lower troposphere a number ofprevious studies provide evidence that the forcing forthe real or apparent downward propagation of zonallysymmetric large scale height and wind anomalies fromthe stratosphere to the troposphere has its origins re gionally in the lower troposphere kuroda and kodera1999 cohen et al 2002 furthermore these lower tropospheric precursors have also been linked toanomalies in eurasian snow cover saito et al 2001 gong et al 2003 which antecede even the atmosphericprecursors the ep flux anomalies shown in lth aresimilar to the wave fluxes presented in kuroda andkodera 1999 saito et al 2001 and gong et al 2003 which were shown to perturb the stratosphereand originate over eurasia but were not considered bylth who limit their analysis to zonal means kurodaand kodera 1999 also show wave fluxes over the at lantic sector but these are observed only during themature phase of an ao event and their influence isconfined to the troposphere also lth show statistically significant heightanomalies in the both the stratosphere and the tropo

a comparison of quasi continuous wave and broadband travel time techniques in the prediction of long range reverberation previous work has shown that the general structure of deep ocean reverberation can be reasonably modeled using single frequency propagation techniques recent results indicate that secondary multipath influences can affect reverberation levels significantly primarily these influences are observed in shadow regions of the primary direct path ensonification temporal separation of these paths is required in order to distinguish these effects this suggests that regions of the reverberation signal may not be well predicted by single frequency quasi cw continuous wave methods in this paper a quasi cw method of predicting long range reverberation is compared to an exact two way travel time technique each model assumes a simple constant plane wave scattering strength thereby emphasizing the influence of propagation results from both models are compared to measured reverberation data from an onr srp bottom reverberation experiment conducted near the mid atlantic ridge it is shown that the single frequency method underestimates reverberation levels on the backside of large ridge structures which act as shadow zones for all propagation paths returns from multipaths however begin to fill in these regions in time and may explain the lack of such dramatic signal drop outs from measured data

synsedimentary diagenesis in a cryogenian reef complex ubiquitous marine dolomite precipitation abstract analysis of the synsedimentary diagenetic phases from a pervasively dolomitised cryogenian 650 ma reef complex south australia reveals a fundamentally different style of marine diagenesis to that of phanerozoic carbonates textural evidence from dolomitised and undolomitised lithologies of the oodnaminta reef complex indicates that depositional components were precipitated initially as calcium carbonate aragonite and calcite dissolution textures and preserved fabrics in ooids indicate an originally aragonitic mineralogy early fibrous marine cements that have well preserved fibrous crystallographic fabrics with a length fast optical character are likely to have precipitated as high magnesium calcite however following this very early phase of diagenesis the dominant marine process occurring in these reefs was dolomite precipitation this involved the dolomitisation of earlier depositional and diagenetic phases as well as the precipitation of fibrous dolomite cements these fibrous marine cements include the newly described radial slow fascicular slow radiaxial slow and rhombic dolomite cements the cements have optical properties chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates dolomite precipitation during marine diagenesis in this reef complex suggests that the oceans of the cryogenian were chemically different to those of the phanerozoic abundant dolomite formation appears to be linked to anoxic magnesium rich seawater during this time marine dolomite precipitation under these conditions may explain the abundance of dolomite in the precambrian sedimentary record

the influence of large scale seafloor slope and average bottom sound speed on low grazing angle monostatic acoustic scattering variations in large scale seafloor slope and average seabed sound speed account for a significant portion of the variations in scattering intensity observed in low grazing angle monostatic reverberation numerical modeling using a finite difference solution to the elastic wave equation is used to quantify the effect of these large scale parameters in interpretations of reverberation data for hard rough seafloor e g basalt the results of the modeling suggest that the monostatic backscattering strength increases with increasing large scale seafloor slope up to a slope of about 15 dipping toward the incident direction once the grazing angle of the incident wavefield exceeds the critical grazing angle for the flat reference seafloor the backscattering intensity increases only slowly with increasing grazing angle similarly average subseafloor sound speed has a significant effect seafloor with low sound speeds characteristic of soft bottoms e g sediment generate significantly weaker backscatter signals than seafloor with sound speeds characteristic of hard bottoms e g basalt the difference is that the shear waves can always be passed into soft bottoms because even for a flat seafloor there is no shear wave critical grazing angle

ediacaran growth of the marine sulfate reservoir abstract the sulfur isotope record as preserved in sedimentary sulfate and sulfide minerals reflects the progressive oxidation of earth s surface environment the latest proterozoic to early paleozoic marine sulfur isotope record is distinguished by unusually high 34 s values in marine sulfates and sulfides geological considerations and sulfur isotopic data indicate that the 34 s enrichment of the marine sulfate reservoir was the result of a rapid increase in the net fractionation between sedimentary sulfate and pyrite 34 s without a compensatory decline in the fractional burial rate of pyrite f pyr a simple one box model in which 34 s is linked non linearly to marine so 4 is used to explore the response in sedimentary sulfur isotope records 34 s sulf and 34 s pyr to an increasing marine sulfate reservoir the model results when compared with a compilation of existing 34 s sulf and 34 s pyr data suggest that the marine sulfate reservoir grew rapidly to roughly phanerozoic levels in the middle ediacaran period before the gaskiers glaciation and prior to the onset of the shuram wonoka negative 13 c anomaly it is further hypothesized that p o 2 levels rose prior to the onset of this extreme perturbation to the global carbon cycle permitting the oxidation of a previously euxinic deep ocean and appearance of the first ediacaran fauna by ca 575 ma despite the enormous demand on oxidants implied by this large magnitude and long duration carbon isotope anomaly

reverberation at the mid atlantic ridge during the 1993 arsrp experiment seen by r v alliance from 200 1400 hz and some modeling inferences in july 1993 saclantcen participated in an experiment for the acoustic reverberation special research program arsrp the primary objective was to take high resolution measurements to shed light on the detailed physical processes dominating the low frequency scattering from rough topographic features and from deep sediment pond areas a very detailed set of monostatic and bistatic scattering experiments were conducted just west of the mid atlantic ridge near 26 degrees n 47 degrees w results from saclantcen s r v alliance are presented which show monostatic and bistatic scattering from selected bathymetric features using pulsed sources from 200 1400 hz and sus charges area water depths ranged from 3300 to greater than 5200 m the receivers were a horizontal array of 128 elements spaced at 0 5 1 and 2 m source depths varied from 120 to 1200 m and receiver depths ranged from 400 to 500 m received reverberation levels are presented as functions of the estimated scatterer position on area bathymetry maps results show the same high scattering areas from one seamount were observed in three distinct frequency bands centered at 227 350 and 700 hz even at 1350 hz the correlation of scattering highlights with nearby bathymetric features is quite good comparisons of measured reverberation and modeled reverberation are presented which suggest that the dominant longer range scattering from the large features is driven by the steep slopes present above the critical depth but that small scale roughness is also essential for strong backscatter

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

isotope stratigraphy of the ediacarian neoproterozoic iii of the adelaide rift complex australia and the overprint of water column stratification abstract this study establishes a high resolution 13c chemostratigraphic framework for the type ediacarian terminal proterozoic section in the adelaide rift complex a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the earth s surface environment only 15 m of the 3 5 km thick type section consists of shallow water carbonates of the kind that have formed the basis of most previous studies of neoproterozoic chemostratigraphy however the organic 13corg record reveals a surprising richness of information and shows excellent utility in correlation between sections in the adelaide rift complex lateral variations in 13corg profiles correspond to expected diachronous patterns of basin fill with the addition of 87sr 86sr data correlation can also be made to the officer and amadeus basins consistent with known lithostratigraphic and sequence stratigraphic constraints sublittoral micritic carbonates of the upper nuccaleena formation cap dolostone and lower and middle wonoka formation show strong basinwide 13c depletion 13ccarb 3 to 10 despite compelling evidence for preservation of depositional 13ccarb such as good textural preservation evidence for sea floor lithification and in the wonoka formation pristine 87sr 86sr and low mn sr associated with the depleted carbonates are rare subaqueous evaporite relatively heavy 13corg and anomalously 13c depleted organic carbon of benthic microbial mat origin and a common cause salinity stratification of the water column is postulated the indicated palaeogeographic setting an evaporative mediterranean like basin tenuously connected to the world ocean is entirely consistent with previous suggestions of one or more messinian type desiccation events to explain very deep 1 km incised valleys or canyons in the adelaide rift complex and officer basin at horizons within the wonoka formation and correlates positive excursions in 13corg during basin stratification may in part be due to 12c depletion of surface waters by photosynthetic carbon fixation despite the proposed semi isolation of australian ediacarian basins from the world ocean major 13c excursions which can to some extent be differentiated by the monotonic rise in ediacarian marine 87sr 86sr appear to be correlatable to the upper windermere supergroup northwest canada

a composite reference section for terminal proterozoic strata of southern namibia integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal proterozoic to cambrian witvlei and nama groups of namibia coupled with precise u pb zircon age constraints these correlations make it possible to construct a composite reference section for use in calibrating terminal proterozoic chronostratigraphy the namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the proterozoic cambrian boundary the two glacial horizons may represent each of two distinct varanger age glaciations better known from the north atlantic region from the higher of the two glacial horizons up the composite stratigraphy preserves one of the thickest and most complete available records of carbon isotope variability in post varanger terminal proterozoic seawater four carbon isotope chemostratigraphic intervals are recognized 1 a postglacial negative 13 c excursion npg interval 2 a rising interval pr interval of increasing positive delta 13 c values 3 a falling interval pf interval characterized by decreasing positive 13 c and culminating in near zero or negative values and 4 an interval of moderately positive relatively invariant 13 c values i interval that extends up to the unconformity that contains the proterozoic cambrian boundary each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world by comparing sediment accumulation rate in the radiometrically calibrated namibian stratigraphy with sediment accumulation rates in correlative sections in arctic canada and oman a maximum age of 564 ma is estimated for the end of the younger varanger glaciation 25 m y younger than previous estimates

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

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

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

propagation and analysis issues in the prediction of long range reverberation data collected from the office of naval research special research program onr srp bottom reverberation research cruises consist of various environmental measurements sound speed profiles bottom properties etc extensive bathymetric mapping of the 300 km 150 km natural laboratory and high quality acoustic reverberation data recorded in both monostatic and bistatic geometries the analysis of the acoustic data and the effects of propagation are investigated accurate gps tracking of the participating research vessels provided the precision necessary to attempt to correlate spiky reverberation events with bathymetric features understanding the limitations of our ability to resolve such features due to imperfect signal processing environmental variability and complex multipath structures is the main objective of this work using a simplified time to range conversion the measured reverberation data can be displayed over the local bathymetry this process shows good correlation between large scal

ediacaran negative c isotopic excursions associated with phosphogenic events evidence from south china abstract the yangtze platform of south china preserves relatively continuous strata of the ediacaran period and has been one of the key areas to explore neoproterozoic earth history in this study high resolution carbon isotope 13c and oxygen isotope 18o data of 701 samples were reported for sedimentary rocks spanning the entire ediacaran period using a continuous drill core collected from wangji south china the c isotopic data of the doushantuo formation are consistent with previous work showing three negative 13c excursions that can be correlated intra basinally a large negative 13c excursion from 8 16 to 7 20 identified at the top of the doushantuo formation which is equivalent to the shuram negative excursion allows us to correlate the 13c profile globally in the overlying dengying formation we identified two distinct 13c intervals with an average value of 3 32 and of 0 40 respectively as well we observed a negative 13c excursion in the middle dengying formation which has not been reported previously three major negative 13c excursions two in the doushantuo formation and one in the dengying formation are tightly associated with phosphorite deposits the temporal correlation between the negative 13c excursions and the phosphorite deposits suggests a potential link between carbon and phosphorus cycling in the ediacaran oceans we argue that multiple periods of strong upwelling may have played an important role in c and p cycling in the ediacaran oceans oxidative processes of the 13c depleted dissolved organic carbon doc transported into oxic near surface waters by upwelling resulted in the negative 13c excursions and contributed to synchronous burial of abundant phosphorites in the inner shelf areas the different magnitudes of the negative 13c excursions observed in the doushantuo and dengying formations may reflect spatial changes of doc reservoir and variations of upwelling intensity and mixing processes

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

phosphorus recycling and burial in baltic sea sediments with contrasting redox conditions in this study redox dependent phosphorus p recycling and burial at 6 sites in the baltic sea is investigated using a combination of porewater and sediment analyses and sediment age dating pb 21

the vendian cambrian δ13c record north iran evidence for overturning of the ocean before the cambrian explosion continuous fossilliferous successions across the precambrian cambrian pc c boundary in the elburz mountains of northern iran show a remarkable negative 13c excursion just below the pc c boundary high concentrations of manganese phosphorus barium and high abundances of fossil phytoplankton and black shale coincide with the excursion worldwide stratigraphic correlation shows that the isotopic anomaly is a global event the initial metazoan diversification coupled with13c enrichment occurs stratigraphically just above the excursion we propose the following scenario for oceanic environmental changes before the cambrian faunal explosion based on new data from iran a global warm climate following the last precambrian glaciation resulted in a generally stagnant oceanic condition so that surface water was oxic deep water was dysoxic depleted in13c and enriched in nutrients massive upwelling of deep water vertical advection of nutrients and13c depleted co2 caused enhanced phytoplankton productivity and a sharp drop in 13c in shallow water carbonate and organic carbon we conclude that latest cryptozoic overturning of ocean stratification preceded the cambrian explosion

was the ediacaran shuram excursion a globally synchronized early diagenetic event insights from methane derived authigenic carbonates in the uppermost doushantuo formation south china abstract the ediacaran period is characterized by the most profound negative carbon isotope 13c excursion in earth history the shuram excursion various hypotheses including the massive oxidation of dissolved organic carbon doc in the oceans the weathering of terrestrial organic carbon or the release and oxidation of methane hydrates and or expelled petroleum from the subsurface have been proposed as sources of the 13c depleted carbon more recently it has been suggested that global scale precipitation of early authigenic carbonates driven by anaerobic microbial metabolism in unconsolidated sediments may have caused the shuram excursion but empirical evidence is lacking here we present a comprehensive analysis of a shuram associated interval from the uppermost doushantuo formation in south china our study reveals petrographic evidence of methane derived authigenic calcite formed as early diagenetic cements and nodules that are remarkably depleted in 13c suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane aom and systematically depleted in 18o relative to co occurring dolomite early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels in light of the finding of methane derived authigenic carbonates at zhongling and based on our basin scale stratigraphic correlation we hypothesize that the marked 13c and 18o depletion including their co variation noted worldwide in the shuram excursion may reflect an episode of authigenesis occurring within a sulfate methane transition zone smtz if true the shuram excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the ediacaran ocean driven by the neoproterozoic oxidation of surface environments this paleo oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals our study highlights the significance of an integrated approach that combines petrography mineralogy and texture specific micro drilling geochemistry in chemostratigraphic studies

constraints on 87sr 86sr of late ediacaran seawater insight from siberian high sr limestones abstract in se siberia carbonate formations with 13 c carb values ranging between 12 and 7 v pdb and sr concentrations of up to 2 5 occupy an area of 40 000 km 2 several successions exceed 1000 m in thickness and represent the world9s largest known exposures of sedimentary carbonates exhibiting extreme depletion in 13 c the carbonates were deposited on a carbonate platform evolving from a mixed carbonate siliciclastic ramp to a carbonate ramp and then from a peritidal rimmed shelf to a deep water open shelf all sequences reveal a facies independent upward rise in marine 13 c carb from 12 to 7 the trend and magnitude of the values mimic those that are characteristic of the 600 550 ma shuram wonoka isotope event a coincident stratigraphic rise in 87 sr 86 sr from 0 70802 to 0 70862 in several sections of limestones containing 4400 g g 1 sr on average is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the late ediacaran if the greatest temporal rate of change in seawater 87 sr 86 sr observed in the cenozoic is applied to the siberian sections the calculated minimum duration for the suram wonoka event is 10 ma supplementary material xrf and icp aes analysis c o and sr isotopic data are available at http www geolsoc org uk sup18324

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

a comparison of bistatic scattering from two geologically distinct abyssal hills the bistatic scattering characteristics of two geologically distinct abyssal hills located on the western flank of the mid atlantic ridge known as b and c are experimentally compared using data acquired with low frequency towed array systems at 12 convergence zone 33 km stand off the comparison is significant because the abyssal hills span the two classes of elevated seafloor crust that cover the mid atlantic ridge the highly lineated b feature is representative of abyssal hills composed of outside corner crust the most commonly occurring category whereas the domed c promontory is representative of the rougher low aspect ratio abyssal hills composed of inside corner crust the latter are less common and usually restricted to segment valley margins the mean biazimuthal scattering distributions of the two abyssal hills each exhibit lambertian behavior with comparable albedos suggesting that the distinction between abyssal hills composed of differing crust is not significant in modeling long

migration of backscatter data from the mid atlantic ridge in studies of low frequency reverberation within the marine environment a central concern is the relationship between reverberation events and morphological features of the seafloor a time domain migration algorithm for the reverberation intensity field is developed that produces scattering coefficient maps coregistered with a bathymetry database the algorithm is tailored to broadband transient sources with good range resolution and was developed to analyze an extensive set of reverberation records from a 200 255 hz source collected on the flanks of the mid atlantic ridge the precise sample by sample tracking of wavefronts across elements of the bathymetry database that forms the foundation of the algorithms implementation results in reverberation maps that show a clear and detailed correlation between scattering and morphology with narrow scarp slopes consistently highlighted environmentally induced asymmetries in transmission loss and incidence angle are exploited to break the inherent left righ

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

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

wide but not ubiquitous distribution of glendonite in the doushantuo formation south china implications for ediacaran climate abstract glendonites are pseudomorphs of syndepositional or early authigenic ikaite caco3 6h2o that often forms at near freezing temperatures silicified glendonite has been reported from inner shelf deposits of the lower ediacaran doushantuo formation at a single stratigraphic section in south china where they are stratigraphically associated with the positive 13c excursion ep1 but predate the negative 13c excursion en3 shuram excursion indicating a period of cool climate somewhere between 609 ma and 551 ma this interpretation predicts a wider geographic distribution of ediacaran glendonites in equivalent strata in the yangtze block of south china to test this prediction we conducted a regional survey of doushantuo formation and found that although glendonites are not universally present they occur in expected stratigraphic intervals and in association with ep1 at two new sections representing inner shelf and intrashelf basin facies the wide but not ubiquitous distribution of doushantuo glendonites indicates that ikaite precipitation and glendonite formation was controlled by both regional climatic and local geochemical factors glendonites at the new localities are stellate clusters pseudomorphed by calcite spar and sometimes are rimmed with silica the calcite spar is characterized by highly variable and mostly negative 13c values as low as 37 indicating that diagenetic transformation of precursor ikaite to calcitic glendonite may be related to anaerobic oxidation of organic matter or methane in sediment the new data suggest an early ediacaran cold period prior to en3 or the shuram excursion and facilitation of glendonite ikaite formation by both climatic and local geochemical conditions such as redox conditions and phosphate concentrations

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

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

influence of water column anoxia on the burial and preservation of carbon and phosphorus in marine shales abstract organic p and organic c concentrations were measured in several well characterized phanerozoic marine shale sequences with the primary focus being the camp run member of the devonian mississippian new albany shale sequences were selected with close spatial association of bioturbated and laminated sediments which reflect deposition from oxic and anoxic waters respectively average organic c p mole ratios calculated from the data are 150 for bioturbated shales and 3900 for the laminated shales of the new albany differences in the extent and mechanisms of early diagenesis related to the oxygenation of waters at the sediment water interface can account for the c p ratios of buried organic matter low c p ratios of bioturbated sediments are attributed to 1 1 the increased ability of bacteria to store p in well oxygenated environments which leads to the production of low c p ratio bacterial biomass and 2 2 the extensive oxidation of sedimentary organic matter resulting in the formation of residual organic phases with low c p ratios high organic c p ratios of laminated sediments are explained by a combination of 1 1 the limited ability of bacteria to store p under anoxic conditions 2 2 extensive p regeneration from sedimentary organic matter 3 3 enhanced c preservation relative to the bioturbated shales it is also shown that relative to organic c laminated shales are not as effective a p sink as are bioturbated shales this provides a mechanism in anoxic environments for burying large quantities of organic matter without simultaneously sequestering the p needed to sustain further productivity

carbon isotope chemostratigraphy and sedimentary facies evolution of the ediacaran doushantuo formation in western hubei south china abstract the doushantuo formation of south china consisting of intercalated siliciclastic and carbonate rocks which exhibit considerable variation in thickness and sedimentary facies is one of the key sedimentary units in the world for understanding ediacaran geobiological processes however stratigraphic correlation and sedimentary facies evolution of the doushantuo formation remain controversial sedimentary facies analysis of the well known sections of the yangtze gorges area and new sections in northern yichang western hubei indicate that the doushantuo succession in the yangtze gorges area was deposited in an offshore intra shelf basin within the yangtze platform in contrast the doushantuo formation in the northern yichang area which is characterized by one or a few phosphate rich horizons represents deposition on the subtidal or intertidal shelf and so is similar to the succession in the weng an area central guizhou province the carbon isotope profiles of the different sections exhibit significant variation which is interpreted to result from facies differences sequence and high resolution carbon isotope stratigraphy have clarified problematic correlations and the temporal relationship between fossil assemblages of the doushantuo formation on the yangtze platform the results confirm that a prominent negative 13ccarb excursion baince nadir down to 5 in the middle portion of the second sequence of the doushantuo formation is a widely traceable event baince together with three other 13ccarb excursions namely cance at the base wance near the first sequence boundary near the middle and dounce at the top of the doushantuo formation constitute a complete carbon isotope profile for the doushantuo formation that reflects contemporaneous seawater and so serves as a reference chemostratigraphic profile for global stratigraphic correlation and for evaluating the chemical evolution of seawater during the ediacaran period

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

phosphogenesis associated with the shuram excursion petrographic and geochemical observations from the ediacaran doushantuo formation of south china abstract the ediacaran period witnessed one of the largest phosphogenic events in earth s history coincidently some phosphorite deposits in south china are associated with the largest known carbon isotope negative excursion i e shuram excursion suggesting an intimate coupling of the biogeochemical carbon and phosphorous cycles however the geomicrobiological linkage between these anomalies remain poorly understood in this study we investigated the phosphorite samples from the uppermost doushantuo formation in south china carbon isotope compositions of authigenic calcite cements and nodules in the phosphorites are as low as 34 vpdb petrographic and geochemical investigations indicate that the 13 c depleted carbonates likely formed as the result of microbial sulfate and iron reduction that released phosphorous from iron oxyhydroxide concentrating phosphorous in pore waters and thereby promoting phosphate mineralization the timing of this event appears to coincide with enhanced sulfate delivery to seawater through continental weathering the basin scale distribution of doushantuo phosphorites suggests a redox control on the availability of iron oxyhydroxide and the recycling of pore water phosphorous both inner and outer shelf regions were likely characterized by an oxic water column and were the main loci for phosphogenesis on the contrary intra shelf and slope regions which are lean in phosphorite were subjected to euxinic or ferruginous water column conditions the intimate coupling between ediacaran phosphogenesis and the shuram excursion suggests strong links among seawater redox conditions c s p fe cycling and fossil phosphatization increased microbial sulfate reduction driven by enhanced sulfate reservoir in the ediacaran ocean may have played an essential role on these biogeochemical events

secular variations in precambrian seawater chemistry and the timing of precambrian aragonite seas and calcite seas an extension of the seawater secular variation model that successfully predicted the observed timing of phanerozoic mgso 4 vs kcl marine evaporites and aragonite seas vs calcite seas has been applied to the prediction of the secular variations in the major ion chemistry of seawater and aragonite seas vs calcite seas during the precambrian testing of the predictions was based on those precambrian seafloor carbonate precipitates that have been interpreted by others to have formed originally as aragonite of 16 examples of precambrian seafloor aragonite 14 fall within the 6 periods of aragonite seas predicted for the late archean proterozoic by the model 1 falls on the transition between an aragonite sea and calcite sea and 1 falls in a period of calcite seas this strong correlation supports the following predictions of the model 1 precambrian seawater was a saline nacl water with ca hco 3 since at least the late archean 2 the major ion compositions of precambrian seawater chemistry and their secular variations are in the same ranges as those of phanerozoic seawater 3 the mg ca mole ratio in seawater has controlled the types of caco 3 polymorphs that have precipitated from earth9s oceans throughout the phanerozoic and most if not all of the precambrian

the effect of rising atmospheric oxygen on carbon and sulfur isotope anomalies in the neoproterozoic johnnie formation death valley usa abstract carbonates within the rainstorm member in the terminal neoproterozoic johnnie formation of death valley california record a remarkable negative carbon isotope anomaly to a nadir of near 11 that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate 34 s values the carbonates including the laterally extensive johnnie oolite were deposited during marine flooding atop a sequence boundary best observed in cratonward sections the 12 c rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric o 2 which resulted in 1 the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides and 2 the ventilation of the oceans similar carbon isotope anomalies are recorded in broadly equivalent successions that post date known marinoan glacial deposits and pre date the precambrian cambrian boundary in oman india china australia and namibia supporting the view that the oxygenation event was global thus the biogeochemical anomaly post dates the last widely recognized neoproterozoic ice age but is nonetheless associated with sea level drawdown that is likely to be eustatic in origin the radiation of a high diversity microbiota and the first appearance of large metazoan fossils ediacaran animals follow this carbon isotope anomaly suggesting that a critical threshold with respect to atmospheric o 2 had been crossed by this time a negative excursion of similar magnitude occurs in overlying strata at the precambrian cambrian boundary which likely reflects similar processes

the stratigraphic complexity of the middle ediacaran carbon isotopic record in the yangtze gorges area south china and its implications for the age and chemostratigraphic significance of the shuram excursion abstract the middle ediacaran shuram excursion represents the most pronounced negative carbon isotopic shift in earth history and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for ediacaran stratigraphic subdivision and global correlation previous study has revealed a pronounced negative 13c shift en3 in the upper doushantuo formation of south china which has been interpreted as an equivalent of the shuram excursion detailed 13c investigation of multiple sections of the ediacaran doushantuo formation around the huangling anticline western hubei province south china indicates that the 13c variation in the upper doushantuo formation is more complex than previously reported in the western region of the huangling anticline a moderately positive 13c excursion here termed the diaoyapo positive excursion bisects two strongly negative 13c excursions here termed the jiuqunao negative excursion and the younger miaohe negative excursion with a 551 ma ash bed capping the miaohe negative excursion in the central region there is only one negative 13c excursion whereas in the eastern region the negative 13c excursion is absent the variable stratigraphic expression of these 13c excursions is most parsimoniously interpreted as reflecting local facies variation diagenesis and or a cryptic unconformity the more complete but more complex 13c chemostratigraphic record in the western region of huangling anticline implies that depending on how this record is correlated with the shuram excursion the latter may be significantly older than 551 ma if it does represent a global chemostratigraphic marker

evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the californian continental margin peruvian continental slope north carolina continental slope and from the santa monica basin california are reported comparison of these sites indicates that o2 depleted bottomwaters enhance p regeneration from sediments diminishing overall phosphorus burial efficiency based on these observations a positive feedback linking ocean anoxia enhanced benthic phosphorus regeneration and marine productivity is proposed on shorter timescales these results also suggest that o2 depletion in coastal regions caused by eutrophication may enhance p regeneration from sediments thereby providing additional p necessary for increased biological productivity