sedimentology and c isotope geochemistry of the sturtian cap carbonate south australia abstract a combined sedimentary and isotopic analysis of the sturtian aged tindelpina cap carbonate in south australia has revealed a strong relationship between sedimentary facies and 13 c carb depositional water depths for the cap carbonate are difficult to constrain however a lack of diagnostic shallow water structures and the inferred magnitude of post glacial transgression several 100 m indicates a relatively deep water environment for each key facies the shallowest facies peloidal dolomite has 13 c values ranging between 3 6 and 0 3 in comparison to deeper water laminated limestones which have lighter values ranging between 5 5 and 3 5 equivalent calcareous shales deposited at basinal depths show even more 13 c depletion with values ranging between 6 7 and 3 7 the average difference in 13 c between the shallowest and deepest cap facies is up to 3 6 and may be an approximation of the seawater 13 c depth gradient at the time of deposition we interpret this high 13 c depth gradient as the result of prolonged physical ocean stratification both during and after deglaciation variations in the rates of upwelling mixing under this stratified regime may have significantly influenced the climate as well as controlling the precipitation and isotopic composition of the cap carbonate

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

the age of the nantuo formation and nantuo glaciation in south china a u pb sensitive high resolution ion microprobe shrimp age of 654 5 3 8 ma from an ash bed immediately below the nantuo formation in south china provides the lowest age constraint for the nantuo glaciation which has been correlated with the global marinoan glaciation a u pb shrimp age of 636 3 4 9 ma from a fallout tuff within the basal layer of the nantuo formation along with the existing age of 635 2 0 6 ma from its overlying doushantuo cap carbonate suggests that the traditionally defined nantuo formation may have been deposited in a short time period at the end of the nantuo glaciation in combination with available ages globally the data support a relatively short duration and rapid termination of marinoan glaciations

composition and pore characteristics of black shales from the ediacaran lantian formation in the yangtze block south china the ediacaran period was critical in the evolution of the biosphere and ocean in the history of the earth shales rich in organic matter om are well developed for this period in the yangtze block china and have recently been related to shale gas exploration in south china to date detailed characterisation of the ediacaran shales is not available the present work sets out a detailed investigation of the composition and pore characteristics of samples from a shallow drill core in the ediacaran lantian formation in the lower yangtze area and the effects of shale composition on the pore growth are discussed the 90 m thick lantian black shales are highly over mature equivalent vitrinite reflectance evro value approximately 4 0 with a total organic carbon toc content up to 12 they are dominated by quartz 33 72 generally with low clay or feldspar content bulk porosities of the shales range from 1 0 to 7 9 and are positively correlated with toc content high magnification scanning electron microscopy sem images support the crucial importance of om content on pore development generally pore volume shows excellent positive correlation with toc content for micropores good correlation for mesopores and relatively poor correlation for macropores two samples with mostly high toc content showed very low macropore volume and medium mesopore volume these results suggest a close relationship between om with relatively small pores and enhanced compaction effect on large pores in highly om enriched shales

a predominantly ferruginous condition in the ediacaran deep ocean geochemistry of black shales in the ediacaran doushantuo formation south china abstract the redox condition of the ocean played an important role in the evolution of the ediacaran biota but it remains largely unexplored particularly for the deep ocean environment in this study we present a comprehensive study of fe speciation and trace elements of the doushantuo black shales from the longe and jinjiadong sections in south china to investigate the redox condition of basinal environments in ediacaran in general both sections show high fe hr fe t ratios 0 38 low fe py fe hr ratios py fe hr ratios 0 7 in the jinjiadong section indicating occasional euxinia in the basin facies fe py fe hr ratios show an increase in the middle part both in the longe and jinjiadong sections indicating enhanced production of h 2 s via bacteria sulfate reduction bsr in the deep anoxic water which was likely responsible to the upper doushantuo negative carbon isotope excursion the enrichment factors efs of most redox sensitive trace elements in the sediments of both sections are low 1 while ef mo shows moderately enriched 3 60 which is similar to the modern cariaco basin because of paleogeographic restriction or persistent ocean stratification the trace elements renewal would be mostly obstructed in the isolated deep water and their inventory could become depleted which cause the low efs of most trace elements in sediments however for the non euxinic deep ocean a particulate shuttle would transport mo from shallow water to deep water through adsorption desorption on the mn fe oxyhydroxides which maintained moderate mo enrichment in seawater and related sediments

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

active faulting mountain growth and erosion at the margins of the tibetan plateau constrained by in situ produced cosmogenic nuclides abstract the india asia collision zone is a key area for understanding continental plateau formation and mountain building two fundamental questions in this context are how the northeastward motion of india is partitioned between strike slip and thrust faults and how mountain building is counteracted by erosion cosmogenic nuclides allow us to address these questions because they provide age constraints on tectonically offset landforms and constraints on erosion rates after considerable debate on whether or not major strike slip faults move at high rates of up to 20 30 mm yr and absorb most of the continental deformation it now appears that the three largest faults altyn tagh haiyuan kunlun have millennial slip rates of no more than 8 13 mm yr consistent with rates of elastic strain accumulation determined by geodetic methods furthermore a significant portion of the lateral slip on these faults is transferred to thrust faults within the collision zone both observations indicate that the eastward tectonic escape of material along these faults is less important than often assumed with respect to mountain building and erosion cosmogenic nuclide studies show that thrust faults at the northeastern and eastern margins of tibet qilian shan longmen shan have vertical slip rates of 0 3 to 2 mm yr while catchment wide erosion rates vary from 0 02 to 1 0 mm yr with high relief areas eroding significantly faster than the plateau interior and growing mountains in the foreland the deeply incised regions have apparently reached an erosional steady state in which rock uplift is balanced by erosion river terraces at active mountain fronts document repeated changes between sediment deposition and fluvial incision during the quaternary incision and terrace formation occurred predominantly at glacial interglacial transitions but also during interglacial periods hence flights of terraces at the fault bounded mountain fronts record the interplay between sustained rock uplift and a temporally variable climate

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

the origin of decoupled carbonate and organic carbon isotope signatures in the early cambrian ca 542 520 ma yangtze platform abstract the early cambrian ca 542 520 ma strata in south china record two prominent negative carbonate carbon isotope 13ccarb excursions of early nemakit daldynian n d and early tommotian ages across each of these excursions carbonate and organic carbon isotopes 13ccarb and 13corg are strongly decoupled regional correlation across a shelf to basin transect shows lateral heterogeneity of 13corg during the early middle n d but more homogenized 13corg values across the basin during the late n d and tommotian the temporal and lateral variations in 13corg suggest that decoupled 13ccarb and 13corg across the n d 13ccarb excursion were possibly caused by diagenetic alteration of organic matter and or amplification of detrital organic carbon isotope signature in low toc carbonates in contrast decoupled 13ccarb and 13corg of the upper n d and tommotian were likely resulted from chemoautotrophic methanotrophic biomass contribution to toc in organic rich black shale and carbonates the decoupled 13ccarb 13corg pattern from the lower n d strata ca 542 ma shows striking similarities with those from the basal ca 635 ma and upper ca 551 ma doushantuo formation in all three cases decoupled 13ccarb 13corg are seen in organic poor carbonates toc 0 1 and coupled 13ccarb 13corg occur in organic rich black shale and carbonates at the end of the negative 13ccarb excursion these similarities suggest that the shift from decoupled to coupled 13ccarb 13corg has no causal link with the terminal oxidation of a large oceanic doc reservoir given the pervasive anoxia euxinia in ediacaran early cambrian oceans local doc rich environments may have been common but a large oceanic doc reservoir capable of buffering the 13c of marine organic matter requires independent evidence

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

50 myr recovery from the largest negative δ13c excursion in the ediacaran ocean sedimentary rocks deposited during the ediacaran period 630 542 ma contain carbonates whose carbon isotopic ratios show a marked negative excursion consisting of a precipitous drop from 5 to 12 followed by a sub linear recovery to positive 13c values isotopic ages u pb and thermal subsidence modelling are combined to constrain the excursion in time and indicate an onset at 600 ma and duration of recovery of approximately 50 myr the excursion is widely recognized in oman and has potential correlatives in ediacaran strata elsewhere and may thus represent a characteristic feature of the ediacaran period the amplitude of this carbon isotope excursion far exceeds those of other neoproterozoic anomalies the isotopic trend of negative excursion and long term recovery spanned at least one short lived glacial episode at 580 ma but appears unrelated to glaciation which indicates that negative anomalies in the neoproterozoic marine carbon isotope record are not directly or uniquely linked to ice ages

subduction complex of pre jurassic age northern anatolia turkey in northern anatolia a zone of pre liassic orogenic deformation is located mainly between the internal anatolian massifs and the north anatolian fault this belt consists of metamorphic rocks in blueschist greenschist and amphibolite facies and an ophiolitic dynamometa morphic melange the metamorphic rocks represent deformed and metamorphosed oceanic crust and upper mantle whereas the melange rocks originated in diverse environments in abyssal plain trench arc trench gap and forearc environments the evolution of these assemblages is interpreted as a product of subduction probably associated with the closure of the tethys subduction related deformation and meta morphism terminated diachronously from pre late triassic in the west to pre late liassic in the east probably as a result of the diachronous closure of the tethys

carbon isotopic composition of neoproterozoic glacial carbonates as a test of paleoceanographic models for snowball earth phenomena consistently positive carbon isotopic values were obtained from in situ peloids ooids and stromatolitic carbonate within neoproterozoic glacial successions in northern namibia central australia and the north american cordillera because positive values continue upward into the immediately overlying postglacial cap carbonates the negative isotopic excursions widely observed in those carbonate rocks require an explanation that involves a short term perturbation of the global carbon cycle during deglaciation the data do not support the ecological consequences of complete coverage of the glacial ocean with sea ice as predicted in the 1998 snowball earth hypothesis of p f hoffman et al in the snowball earth hypothesis the postglacial cap carbonates and associated 5 negative carbon isotopic excursions represent the physical record of co2 transfer from the high p co2 snowball atmosphere 0 12 bar to the sedimentary reservoir via silicate weathering in the snowball aftermath stratigraphic timing constraints on cap carbonates imply weathering rates of 1000 times preglacial levels to be consistent with the hypothesis the absence of sr isotopic variation between glacial and postglacial deposits and calculations of maximum weathering rates do not support a post snowball weathering event as the origin for cap carbonates and associated isotopic excursions

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

stratification and mixing of a post glacial neoproterozoic ocean evidence from carbon and sulfur isotopes in a cap dolostone from northwest china abstract to improve our knowledge about the geochemical and environmental aftermath of neoproterozoic global glaciations we analyzed stable isotopes 13 c 18 o 34 s and elemental concentrations ca mg s sr fe and mn of the 10 m thick zhamoketi cap dolostone atop the tereeken diamictite in the quruqtagh area eastern chinese tianshan available chemostratigraphic data suggest that the tereeken diamictite is probably equivalent to the marinoan glaciation our new data indicate that organic and carbonate carbon isotopes of the zhamoketi cap dolostone show little stratigraphic variations averaging 28 2 and 4 6 respectively in contrast sulfur isotopes show significant stratigraphic variations carbonate associated sulfate cas abundance decreases rapidly in the basal cap dolostone and 34 s cas composition varies between 9 and 15 in the lower 2 5 m in the overlying interval cas abundance remains low while 34 s cas rises 5 and varies more widely between 10 and 21 the range of 34 s py of the cap dolostone overlaps with that of 34 s cas but direct comparison shows that 34 s py is typically greater than 34 s cas measured from the same samples hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation we propose that cas and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone in this model cas was derived from shallow oxic surface waters with moderate sulfate concentration and depleted in 34 s due to the post glacial influx of sulfur from continental weathering in contrast pyrite was derived from anoxic bottom waters or a sulfate minimum zone with low sulfate concentration and 34 s enrichment due to long term syn glacial sulfate reduction the rapid shift in cas abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation comparison with other post marinoan cap carbonates shows significant spatial heterogeneity in 34 s cas which together with strong temporal variation in 34 s cas points to generally low sulfate concentrations in post marinoan oceans

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

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

climatic cycles during a neoproterozoic snowball glacial epoch the profound glaciations of the neoproterozoic cryogenian period ca 850 544 ma represent an extreme climatic mode when it is claimed earth was fully or almost completely covered with ice for millions of years we show that the geochemistry and mineralogy of fine grained neoproterozoic sedimentary rocks in oman are best explained by climatic oscillations that drove variations in the intensity of chemical weathering on contemporary land surfaces the cold climate modes of the cryogenian were therefore cyclical punctuated with well defined warm humid interglacial periods the hydrological cycle and the routing of sediment were active throughout the glacial epoch which requires substantial open ocean water this reconstruction represents a significantly different target for numerical climate models at this critical time in the evolution of earth s biosphere

snowball earth termination by destabilization of equatorial permafrost methane clathrate the recovery from the marinoan snowball ice age of about 635 million years ago is one of the most severe climate change events known to have occurred on earth hypotheses accounting for the abruptness of deglaciation include ice albedo feedback deep ocean out gassing during post glacial oceanic overturn or methane hydrate destabilization martin kennedy and colleagues now report the broadest range of oxygen isotope values yet measured in marine sediments in methane seeps in marinoan deglacial sediments underlying a thin interval of carbonate these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise the range of values is likely the result of mixing between ice sheet derived meteoric waters and clathrate derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater their findings suggest that methane released from low latitude permafrost clathrates acted as a trigger and or strong positive feedback for deglaciation and warming this paper reports the broadest range of oxygen isotope values yet measured in marine sediments in methane seeps in marinoan deglacial sediments underlying a thin interval of carbonate these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise the range of values is likely the result of mixing between ice sheet derived meteoric waters and clathrate derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater the start of the ediacaran period is defined by one of the most severe climate change events recorded in earth history the recovery from the marinoan snowball ice age 635 myr ago ref 1 marinoan glacial marine deposits occur at equatorial palaeolatitudes2 and are sharply overlain by a thin interval of carbonate that preserves marine carbon and sulphur isotopic excursions of about 5 and 15 parts per thousand respectively3 4 5 these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise1 3 4 this abrupt transition records a climate system in profound disequilibrium3 6 and contrasts sharply with the cyclical stratigraphic signal imparted by the balanced feedbacks modulating phanerozoic deglaciation hypotheses accounting for the abruptness of deglaciation include ice albedo feedback3 deep ocean out gassing during post glacial oceanic overturn7 or methane hydrate destabilization8 9 10 here we report the broadest range of oxygen isotope values yet measured in marine sediments 25 to 12 in methane seeps in marinoan deglacial sediments underlying the cap carbonate this range of values is likely to be the result of mixing between ice sheet derived meteoric waters and clathrate derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater the equatorial palaeolatitude implies a highly volatile shelf permafrost pool that is an order of magnitude larger than that of the present day a pool of this size could have provided a massive biogeochemical feedback capable of triggering deglaciation and accounting for the global postglacial marine carbon and sulphur isotopic excursions abrupt unidirectional warming cap carbonate deposition and a marine oxygen crisis our findings suggest that methane released from low latitude permafrost clathrates therefore acted as a trigger and or strong positive feedback for deglaciation and warming methane hydrate destabilization is increasingly suspected as an important positive feedback to climate change11 12 13 that coincides with critical boundaries in the geological record14 15 and may represent one particularly important mechanism active during conditions of strong climate forcing

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

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

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

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

highly heterogeneous poikiloredox conditions in the early ediacaran yangtze sea abstract although there has been a sustained effort to reconstruct ocean redox conditions throughout the ediacaran a lack of high resolution data across time equivalent strata shelf to basin has hindered our ability to understand redox dynamics at the basinal scale we measured iron species trace and major element concentrations toc and corg p ratios of black shale from six high resolution sections from the early ediacaran yangtze sea i e the inner shelf jiulongwan outer shelf zhongling slope siduping and mingle and basinal yuanjia and long e sections which were integrated with published data from these sections and seven other correlative sections from the same region for a basinwide perspective on redox tracers these results document strong spatiotemporal redox variability during the early ediacaran including transitions between ferruginous euxinic suboxic and oxic conditions at high frequencies and short distances a pattern for which we coin the term poikiloredox conditions our results also suggest that multiple geochemical proxies including iron species redox sensitive trace element concentrations and enrichments and corg p ratios show different responses to poikiloredox conditions accounting for conflicting redox interpretations in early ediacaran studies this study thus demonstrates the importance of multiple proxies high stratigraphic resolution and a basin scale perspective in paleomarine redox studies underlining the potential for overgeneralization of redox patterns based on analysis of a single or a limited number of proxies or sections

a statistical analysis of the carbon isotope record from the archean to phanerozoic and implications for the rise of oxygen organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales permanent burial of organic carbon in the crust or mantle oxidizes the surface environment atmosphere ocean and biosphere by removing reduced carbon it has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout earth9s history thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen schidlowski 1988 catling and others 2001 holland 2002 holland 2009 kump and others 2009 rothman 2015 however the opposite conclusion has been drawn from the same carbon isotope record des marais and others 1992 bjerrum and canfield 2004 to test these opposing claims we compiled an updated carbon isotope database and applied both parametric and non parametric statistical models to the data to quantify trends and mean level changes in fractional organic carbon burial with associated uncertainties and confidence levels we first consider a conventional mass balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon for this model statistical analysis implies fractional organic burial has increased over earth history by a factor of 1 5 relative to organic burial at 3 6 ga with the 95 percent confidence interval ranging from factors of 1 2 to 2 0 an increase in organic burial by a factor of 1 2 cannot explain the rise of oxygen whereas an increase by a factor of 2 could conceivably explain the rise of oxygen there is however a highly significant and well constrained increase in organic burial from the proterozoic to the phanerozoic we also analyze changes in the difference between carbonate and organic carbon isotopic ratios over earth history there is a statistically significant increase in this difference from the early to late archean possibly caused by increased biological fractionation due to methanotrophic recycling this transition is consistent with the evolution of oxygenic photosynthesis at 2 8 ga or earlier finally we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes specifically ocean crust carbonatization and authigenic carbonates because the size of these fluxes has a large poorly constrained range our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all however it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time in which case conclusions from the conventional model would be valid

stable isotope geochemistry and palynology of the late precambrian to early cambrian sequence in newfoundland a sequence of clastic sediments in southeastern newfoundland straddling the precambrian cambrian boundary has been investigated for its stable isotope geochemistry of carbon and sulfur and acid resistant organic walled microfossils a detailed study of the chapel island formation which includes the boundary interval has revealed fluctuations in the isotopic composition of organic carbon these are largely interpreted as caused by differences in the depositional environments highly variable sulfur isotopic compositions indicate bacterial sulfate reduction as a pyrite forming process sometimes under sulfate limited conditions palynological results are quite limited with respect to diagnostic microfossils

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

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

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

facies dependent δ13c variation from a cryogenian platform margin south australia evidence for stratified neoproterozoic oceans abstract analysis of a cryogenian interglacial platform margin in the adelaide geosyncline reveals a strong carbonate 13 c facies relationship detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11 in time equivalent strata shallow water back reef facies have 13 c values up to 8 2 while equivalent basinal sediments have 13 c values between 3 and 0 allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy 13 c values 4 9 and are surrounded by basinal calcareous shales with light 13 c values ave 0 8 the regional and stratigraphic consistency of these 13 c trends suggests a primary marine origin we interpret this 13 c facies correlation to be the result of ocean stratification stagnation that significantly reduced the rate of deep ocean ventilation and produced a large deep water organically derived carbon reservoir we suggest that stratification may have been a persistent feature of the global ocean throughout much of neoproterozoic time ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era including large scale 13 c variation extreme climatic fluctuations and the presence of cap carbonates a highly variable climatic regime would be expected with the development of a large deep water carbon reservoir small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface ocean and atmospheric reservoirs leading to intense greenhouse or icehouse conditions respectively

fractionation between inorganic and organic carbon during the lomagundi 2 22 2 1 ga carbon isotope excursion article i nfo the lomagundi 2 22 2 1 ga positive carbon isotope excursion in shallow marine sedimentary carbonates has been associated with the rise in atmospheric oxygen but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen the amount of oxygen released to the exosphere during the lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon which is poorly characterized because dissolved inorganic and organic carbon reservoirs were arguably larger in the paleoproterozoic ocean at a time of lower solar luminosity and lower ocean redox state decoupling between these two variables might be expected we determined carbon isotope values of carbonate and organic matter in carbonates and shales of the silverton formation south africa and in the correlative sengoma argillite formation near the border in botswana these units were deposited between 2 22 and 2 06 ga along the margin of the kaapvaal craton in an open marine deltaic setting and experienced lower greenschist facies metamorphism the prodelta to offshore marine shales are overlain by a subtidal carbonate sequence carbonates exhibit elevated 13 c values ranging from 8 3 to 11 2 vs vpdb consistent with deposition during the lomagundi positive excursion the total organic carbon toc contents range from 0 01 to 0 6 and 13 c values range from 24 8 to 13 9 thus the isotopic fractionation between organic and carbonate carbon was on average 30 3 2 8 n 32 in the shallow marine environment the underlying sengoma shales have highly variable toc contents 0 14 to 21 94 and 13 c values 33 7 to 20 8 with an average of 27 0 3 0 n 50 considering that the shales were also deposited during the lomagundi excursion and taking 13 c values of the overlying carbonates as representative of the 13 c value of dissolved inorganic carbon during shale deposition a carbon isotope fractionation as large as 37 appears to characterize the production of bulk organic matter in the deeper part of the pretoria basin at that time this enhanced fractionation relative to that observed in shallow water environments likely reflects heterotrophic secondary and chemotrophic productivity at and below a pronounced redoxcline consistent with the euxinic conditions inferred from independent evidence for the deeper part of the pretoria basin greater variability in organic carbon vs carbonate carbon isotopic values on the shallow marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the lomagundi excursion our study suggests that in contrast to the late neoproterozoic and phanerozoic when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers in the paleoproterozoic redox stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline this strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the 13 c values of dissolved inorganic carbon in the paleoproterozoic seawater

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

carbonate platform growth and cyclicity at a terminal proterozoic passive margin infra krol formation and krol group lesser himalaya india the infra krol formation and overlying krol group constitute a thick 2 km carbonate rich succession of terminal proterozoic age that crops out in a series of doubly plunging synclines in the lesser himalaya of northern india the rocks include 18 carbonate and siliciclastic facies which are grouped into eight facies associations 1 deep subtidal 2 shallow subtidal 3 sand shoal 4 peritidal carbonate complex 5 lagoonal 6 peritidal siliciclastic carbonate 7 incised valley fill and 8 karstic fill the stromatolite rich peritidal complex appears to have occupied a location seaward of a broad lagoon an arrangement reminiscent of many phanerozoic and proterozoic platforms growth of this complex was accretionary to progradational in response to changes in siliciclastic influx from the south eastern side of the lagoon metre scale cycles tend to be laterally discontinuous and are interpreted as mainly autogenic variations in the number of both sets of cycles and component metre scale cycles across the platform may result from differential subsidence of the interpreted passive margin apparently non cyclic intervals with shallow water features may indicate facies migration that was limited compared with the dimensions of facies belts correlation of these facies associations in a sequence stratigraphic framework suggests that the infra krol formation and krol group represent a north to north westfacing platform with a morphology that evolved from a siliciclastic ramp to carbonate ramp to peritidal rimmed shelf and finally to open shelf this interpretation differs significantly from the published scheme of a basin centred on the lesser himalaya with virtually the entire infra krol krol succession representing sedimentation in a persistent tidal flat environment this study provides a detailed neoproterozoic depositional history of northern india from rift basin to passive margin and predicts that genetically related neoproterozoic deposits if they are present in the high himalaya are composed mainly of slope basinal facies characterized by fine grained siliciclastic and detrital carbonate rocks lithologically different from those of the lesser himalaya

an integrated chemostratigraphic δ13c δ18o 87sr 86sr δ15n study of the doushantuo formation in western hubei province south china abstract high resolution isotopic analyses were conducted on core samples from the ediacaran doushantuo formation at the wangjiapeng section western hubei province in south china whereby two laterally traceable negative 13ccarb excursions en1 and en3 were recognized the magnitude and duration of these excursions permit intra basinal and inter basinal correlation which indicates that they probably represent a global change in seawater composition the occurrence of decoupled 13ccar 13corg with almost invariable 13corg values at wangjiapeng zhongling yangjiaping sections is consistent with remineralization of a dissolved organic carbon doc pool by means of sulfate reduction as recorded in en3 the synchronous presence of en3 a shift to higher 87sr 86sr and decrease of mn and fe contents and 15n values together points to a glacial influence whereby oxygenation and remineralization of reduced carbon produced 13c depleted dic glaciations cause a drop in sea level which itself leads to increased continental shelf area to be exposed to surface weathering and ultimately to enhanced delivery of radiogenic 87sr the increase of 87sr 86sr ratios sulfate and phosphate are consequences of surface runoff into oceanic environments and such perturbations induce biogeochemical changes

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

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

iodine records from the ediacaran doushantuo cap carbonates of the yangtze block south china abstract late neoproterozoic cap carbonates formed following the cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the snowball earth events large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post marinoan cap carbonates across the globe but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations in this study we present i ca mg profiles of four sections from a shallow to deep water transect of the basal doushantuo cap carbonates one of the most extensively studied ediacaran units in south china most samples have i ca mg values lower than 1 mol mol with a few exceptions in the shelf sections up to 2 5 mol mol all calcite samples with extremely low 13c have iodine signal below detection limit suggesting carbonate precipitation from anoxic seawater or diagenetic fluids among the four sections the slope section has the lowest i ca mg values suggesting that it may have experienced more extensive fluid buffered diagenesis and more severe loss of io3 in combination with available major trace rare earth element and isotope data the temporal i ca mg variations support freshwater mixing and deep water upwelling in proximal sites and transient oxidation during deposition of the doushantuo cap carbonates