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

ocean chemistry during glacial time abstract measurements of co2 to air ratios in the gas trapped in bubbles in ice of glacial age suggest that the co2 content of the atmosphere was considerably lower during peak glacial time than during holocene time the purpose of this paper is to show that such a change must in all likelihood be the result of alterations in the nutrient element chemistry of sea water two possible scenarios are presented one involves alternate storage and erosion of phosphorous leaving residues from shelf sediments the other involves changes in the c p ratio in the organic debris falling to the deep sea means of verifying the nutrient cycle hypothesis are also given it is shown that the 13c record as we know it in planktonic and benthic foraminifera the oxygen record as inferred from benthic foraminifera species distributions and the early post glacial caco3 preservation event as recorded by aragonitic pteropods are consistent with both of the hypotheses presented only if an early post glacial spike in the 13c record for planktonic shells could be found would it be possible to eliminate one of these hypotheses i e that involving shelf storage the implications of these nutrient hypotheses to climate theory are as follows if shelf storage is responsible for the glacial to interglacial co2 increase then the co2 change must be considered an amplifier of some primary cause the reason is that sea level changes are needed to drive deposition on and erosion from the shelves on the other hand if changes in the c p ratio for falling debris are responsible then the co2 change could either be an amplifier or a primary cause for the major glacial to interglacial climatic cycle the latter is possible as self sustained oscillations in ocean chemistry might be driven by interactions between ocean ecology and ocean nutrient chemistry

a chemostratigraphic overview of the late cryogenian interglacial sequence in the adelaide fold thrust belt south australia within the neoproterozoic sequence of the adelaide fold thrust belt south australia two of the most severe ice ages in earth history sturtian and marinoan are recorded in the glacigenic rocks which mark the base and top of the 4 5 km thick umberatana group the focus of the present chemostratigraphic study rock units from three drillholes and ten measured outcrop sections located in the southern and central flinders ranges and on the stuart shelf were sampled for isotopic analysis of their co existing calcite 13ccal dolomite 13cdol and kerogen 13corg strontium and sulphur isotopic analyses were also run on selected samples the sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen the resulting data when integrated with the lithostratigraphy of the umberatana group yield one of the most complete records of secular variation in the c isotope signature of the late cryogenian ocean between 750 and 680 ma its composite 13ccarb curve begins with a sharp positive excursion 3 to 1 5 in the dolomitic tindelpina shale which caps sturtian glacial diamictites wilyerpa formation then follows a steady climb through the shoaling upward tapley hill formation reaching 4 in the brighton limestone and culminating in a plateau of 13c enrichment within the platformal carbonates of the etina formation 8 to 10 the upper part of the temporal trend shows a decline to 6 to 7 in the stromatolite reef facies of the enorama shale and a dramatic drop to 8 to 9 5 followed by a recovery to 3 in the youngest carbonate unit trezona formation beneath the marinoan glacigenic elatina formation the latter is capped by a thin dolostone the nuccaleena formation which records a consistent up section decrease in 13c 1 to 3 superimposed on this overall pattern of c isotopic variation are second and third order fluctuations attributable to eustatic sea level change pyritic sulphur in the lower tapley hill formation 34s 9 to 40 is distinctly heavier than that in the underlying wilyerpa formation 4 to 20 minimum 87sr 86sr values obtained from the brighton limestone 0 7071 and etina formation 0 7076 are consistent with their late cryogenian age this sturtian to marinoan interglacial succession is more complete than equivalent sequences elsewhere and thus provides a useful reference for global chemostratigraphic correlation notable features of its c isotope curve are the strong post wilyerpa positive excursion also seen in key sections from svalbard and western mongolia the etina plateau of 13c enrichment which is remarkably similar to maxima in the terminal proterozoic c isotope records of western mongolia north western canada and brazil and the spectacular depletion of 13c in the lower trezona relative to similar carbonate facies in the underlying etina formation and enorama shale a preglacial offset in 13ccarb values of such proportions 14 is almost unprecedented except perhaps for that recently documented in the upper ombaatjie formation beneath tillites of the ghaub formation otavi group namibia it represents a key piece of evidence in the case presented by the proponents of a neoproterozoic snowball earth

carbonate deposition climate stability and neoproterozoic ice ages the evolutionary success of planktic calcifiers during the phanerozoic stabilized the climate system by introducing a new mechanism that acts to buffer ocean carbonate ion concentration the saturation dependent preservation of carbonate in sea floor sediments before this buffering was primarily accomplished by adjustment of shallow water carbonate deposition to balance oceanic inputs from weathering on land neoproterozoic ice ages of near global extent and multimillion year duration and the formation of distinctive sedimentary cap carbonates can thus be understood in terms of the greater sensitivity of the precambrian carbon cycle to the loss of shallow water environments and co 2 climate feedback on ice sheet growth

evaluation of δ13c chemostratigraphy for intrabasinal correlation vendian strata of northeast siberia integrated sedimentologic stratigraphic and geochemical 13 c 18 o fe mn and sr data from vendian cambrian carbonate ramp deposits of the northeast siberian platform olenek uplift and kharaulakh mountains region are used to constrain primary time dependent oscillations in the carbon isotope record and to evaluate 13 c chemostratigraphy for high resolution intrabasinal correlation the vendian 13 c record of northeast siberia reflects global variations seen elsewhere by displaying in ascending order a strong positive isotopic shift to values near 6 herein named the p interval an intermediate interval of relatively little isotopic change i interval which shows in some sections a monotonic decrease in 13c from 2 at the base to near 0 at the top and a negative excursion to 4 n interval just beneath the vendian cambrian boundary in addition to secular isotopic shifts these strata exhibit local small scale signals related to intrabasinal variations in subsidence erosion and diagenetic alteration e g degradation of organic matter dolomitization and hydrothermal and burial effects these local intrabasinal processes in some cases have modified both the magnitude and form of the primary isotopic excursions which are used as the basis for correlation in all studies carbon isotope profiles provide an important method to evaluate intrabasinal variations in subsidence erosion and stratigraphic completeness these profiles reflect increasing subsidence along the platform to basin transition whereas progressive truncation of isotopic profiles along this trend illustrate pronounced uplift and erosion of ramp strata along unconformity surfaces

stratigraphy sedimentology and isotopic geochemistry of australian neoproterozoic postglacial cap dolostones deglaciation delta 13 c excursions and carbonate precipitation abstract thin d13c excursions in the geologic record they are interpreted to record a brief postglacial anomaly in ocean chemistry in australia the marinoan varanger equivalent 600 ma cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe including fine grained dolomitic mineralogy lateral persistenc at basinal scales thin laminae graded beds intervals with abundant marine cements crystal fans and a distinctive negative d13c isotopic signature of up to 5 pdb on the basis of these features the australian examples are interpreted here to be deeper water deposits below storm wave base resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression detailed isotopic analysis of australian cap dolostones indicates d13c values ranging from 1 to 5 and gen rally becoming more depleted in 13c upsection trace element data indicate some diagenetic stabilization but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event the range in d13c values between sections as well as more negative 13c values appear to correlate with greater paleobathymetry within basins this implies either 1 only partial preservation of the complete oceanic variation of d13c or 2 precipitation of this peculiar facies owes its genesis to basinal specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone in the later case d13c values would not represent whole ocean values stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression this implies that the cause s of d13c variation likely operated on time scales significantly less than the residence time of carbon in the oceans 105 yr the continent wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period such large scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused or have been the product of profound changes in the carbon cycle and global climate at tha time similar evidence for transgression and increased carbonate deposition in the holocene are attributed to the changing basin shape and ph of the oceans the coral reef hypothesis of berger 1982 cap dolostones may record a proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

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

the effectiveness of the paleoproterozoic biological pump a δ13c gradient from platform carbonates of the pethei group great slave lake supergroup nwt samples of carbonate cements collected along a depth transect of the pethei platform a 1 9 ga stromatolitic reef reveal a small 0 5 carbon isotope gradient between shallow and basinal facies the magnitude of this gradient would conventionally be interpreted as indicating low export of organic matter from ocean surface waters but steady state simulations using a two box model of the paleoproterozoic ocean suggest that the small carbon isotope gradient could instead be due to high partial pressures of carbon dioxide in the paleoproterozoic atmosphere which would increase the ocean9s dissolved inorganic carbon content and damp the effects of biological pumping if the paleoproterozoic atmosphere were indeed enriched in atmospheric co 2 in compensation for a less luminous sun these results indicate that the planktonic biota was a significant component of the precambrian global carbon and nutrient cycles affecting if not regulating as today the composition of both surface and deep ocean waters

the problems and potential of using animal fossils and trace fossils in terminal proterozoic biostratigraphy abstract despite the discovery of soft bodied megascopic fossils of late neoproterozoic age at numerous localities world wide there has been slow acceptance of their potential for intercontinental correlation the stratigraphic thickness of sediments separating individual occurrences of such fossils from the base of the cambrian is very varied basic questions have also been posed as to the classification of the organisms represented and hence their likely time significance recent geochronological studies support notions of a narrow time distribution well studied neoproterozoic sequences in eurasia eastern and western north america and southern australia show strikingly similar patterns of increase in the numbers and diversity of trace fossils and soft bodied megascopic remains up section though strata immediately preceding the cambrian are relatively barren other than for a sparse palynoflora and compressions of primitive plants trace fossils from australian sections relating to an early part of the ediacaran apparently overlap a time of abundance of large process bearing acritarchs and big leiosphaerids this overlap of early animal traces and distinctive acritarchs may prove to be of significant biostratigraphic importance rare discoveries of forms resembling rangea gurich in the flinders ranges of south australia precede the classical ediacara assemblage and may provide a possible correlation with occurrences of this genus in namibia the distinctive remains and trace fossils at this level constituting the rangeid hiemalora assemblage zone resemble a biota from the podolia region of the ukraine and show a similarity to finds in parts of the windermere supergroup of the northwest territories canada similarly the offshore frondose assemblage of newfoundland is comparable with the biota of charnwood forest england the type ediacara assemblage of the flinders ranges characterizing the dickinsonia costata assemblage zone is a climax biota with diverse taxa and large individual species as well as some 20 ichnogenera it closely correlates with a late redkinian or early kotlinian assemblage in the upper part of the section on the zimny gory coast of the white sea northern russia the trace fossils of these levels show that significant diversification began before the cambrian

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

comments and reply on precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites reply

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

paleomagnetism of the paleoproterozoic gowganda and lorrain formations ontario low paleolatitude for huronian glaciation abstract to further our understanding of the paleolatitudes of precambrian glaciations a paleomagnetic study has been conducted on the glaciomarine coleman member of the gowganda formation and the conformably overlying deltaic firstbrook member and lorrain formation huronian supergroup 2 4 2 3 ga in ontario canada many of the rocks notably the gray and grayish red argillic facies display unstable magnetization or overprints carried by magnetite and or hematite that give negative fold tests some red sandstones however have stable high temperature 690 c components marked by very shallow northerly or southwesterly directions five sites in pale red sandstone from the coleman member near elliot lake give a dip corrected a direction of d 216 3 i 5 5 95 6 7 n 30 and 17 sites in lorrain red sandstone near desbarats give a dip corrected a direction of d 5 4 i 5 5 95 5 9 n 75 unfavorable structural attitudes rendered fold tests inconclusive these very shallow directions may be ascribed to early chemical remanent magnetization crm because a some specimens display polarity reversals b such directions are unknown either as original or as overprint components in post huronian rocks of the region and c a prior site in coleman red beds gave a high temperature shallow nnw direction that is not significantly different from lorrain a and which a positive conglomerate test suggested is not an overprint the very shallow directions are near primary directions for the slightly older 2 45 ga matachewan dyke swarm in the region lorrain argillic rocks at five sites give a normal component b t 670 c d 18 6 i 59 6 95 6 1 n 16 dip corrected and a reverse component e t 500 c d 190 9 i 60 9 95 7 3 n 14 in situ which are close to a prefolding direction carried by magnetite previously identified in coleman gray argillites morris a however a primary origin for morris a is dubious because of the widespread occurrence of overprints carried by magnetite in coleman gray argillites and because the fold test employed folds formed during the penokean orogeny at 1900 1800 ma our results suggest that morris a lorrain b and lorrain e may data from immediately before and just after penokean folding respectively hence coleman sandstone a and lorrain a may be better estimates of original directions those components and matachewan paleomagnetic data constrain huronian paleolatitudes and suggest that huronian glaciation occurred within 11 and possibly within 4 of the paleoequator

redox stratification and anoxia of the early precambrian oceans implications for carbon isotope excursions and oxidation events abstract an updated compendium of 13c data offer compelling evidence that 13c positive excursions of unsurpassed magnitude in the recorded earth history 8 and up to 18 pdb occurred in the early proterozoic the lomagundi event questions whether or not these unprecedented positive 13c shifts were contemporaneous in various basins and represent local or global events remain unresolved the framework of major geologic events that occurred in the paleoproterozoic argues against a snowball earth scenario as a backdrop to these exceptional 13c enrichments substantial increases of carbon flux ratio fo fc eight times the phanerozoic average and organic carbon burial rate fo three times the phanerozoic average are required to account for the observed 13c enrichments under steady state and dynamic equilibrium modes respectively these enhanced ratios and rates are conditional on the availability of a flux of nutrients to the contemporaneous biota producers in excess of the riverine flux and a decoupling of the p and c cycles it is argued that these two conditions were met between 2 25 and 2 11 ga in a redox stratified ocean with weak upwelling and sluggish meridional surface circulation the alternative that the major disturbance in the terrestrial carbon cycle occurred in a rapidly ventilated ocean is assessed as being unlikely a large pulse of o2 equivalent to six to seven times the present terrestrial oxygen budget was rapidly scavenged and spent in the oxidation of reduced fe and s transported to the shelves by rivers this contention is consistent with data inferring low oxygen levels 10 5 to 10 2 pal in the paleoproterozoic atmosphere and the occurrence of ocean anoxia until the late precambrian

stratified oceans and oxygenation of the late precambrian environment a post glacial geochemical record from the neoproterozoic of w mongolia the late neoproterozoic 750 545 ma was a time of revolutionary environmental change recurrent glaciation and tectonic upheaval which culminated in the evolution of metazoans and animal biomineralization geochemical data are presented for a post glacial 700 ma limestone succession from w mongolia 87 sr 86 sr is used to constrain the timing of early diagenetic exchange stratigraphic trends to high d 13 c carb and d 13 c org mirror sea level fluctuations which we argue is consistent with the hypothesis that late precambrian ocean chemistry was controlled by vertical stratification repeated establishment of stratification 700 545 ma would have led to greater oxygen availability in the surface environment a causal link between this oxygenation and the further evolution of animals is proposed

anoxic events in the mediterranean sea in relation to the evolution of late neogene climates stable isotopic and micropaleontological studies were made of selected sapropels organic rich sediments deposited in the mediterranean sea during the last 5 0 m y to determine the processes responsible for their formation distinct isotopic and faunal changes occur across sapropels of late pleistocene early pleistocene and latest pliocene age while smaller isotopic changes and more stable faunal assemblages are associated with the early and mid late pliocene sapropels the large 18o depletions and euryhaline fauna associated with latest pliocene pleistocene sapropels supports a density stratification model with a low salinity surface layer in contrast early pliocene and mid late pliocene sapropels appear to have been formed as the result of sluggish circulation and low oxygen contents in bottom waters of the eastern mediterranean due to the stable warm climatic conditions of that time period

neoproterozoic chemostratigraphy and correlation of the port askaig glaciation dalradian supergroup of scotland the 13 c of preglacial carbonates beneath the glaciogenic port askaig formation falls from 5 to negative values while least altered 87 sr 86 sr values are close to 0 7067 postglacial cap carbonates of the bonahaven formation begin with negative 13 c carb values jumping sharply to 11 7 near the top which we correlate with excursions above the elbobreen wilsonbreen glaciations of svalbard and putative sturtian not varanger glaciations elsewhere dated at c 720 ma much of the succeeding terminal neoproterozoic is missing in ne svalbard and east greenland raising doubts about their use for global sr and c isotope curves in the neoproterozoic

new evidence for changes in plio pleistocene deep water circulation from southern ocean odp leg 177 site 1090 abstract changes in atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal 13c record at odp site 1090 in the south atlantic with similar records from the north atlantic sites 982 607 925 929 and deep pacific site 849 oceans important deep water circulation changes occurred in the early pleistocene at 1 55 myr and during the mid pleistocene transition at 0 9 myr at 1 55 myr glacial 13c values in the southern ocean became significantly lower than those in the deep pacific establishing a pattern that persisted throughout the late pleistocene we propose that the lowering of 13c values of southern component water scw at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52 accompanying this change in southern ocean deep water circulation was enhanced interhemispheric coupling between the north and south atlantic after 1 55 myr at 0 9 myr the magnitude of glacial to interglacial variability in 13c increased and shifted to a longer frequency 100 kyr along with oceanic 18o ice volume calculation of percent northern component water ncw using site 1090 as the scw end member yielded 20 30 less reduction of ncw during glacial periods of the late pleistocene also a trend toward reduced glacial suppression of ncw during the past 400 kyr is not evident the apparent decoupling of ice volume and deep water circulation reported previously may be an artifact of using a pacific rather than a southern ocean carbon isotopic record to calculate past mixing ratios of ncw and scw

landscape evolution in martian mid latitude regions insights from analogous periglacial landforms in svalbard periglacial landforms on spitsbergen svalbard norway are morphologically similar to landforms on mars that are probably related to the past and or present existence of ice at or near the surface many of these landforms such as gullies debris flow fans polygonal terrain fractured mounds and rock glacier like features are observed in close spatial proximity in mid latitude craters on mars on svalbard analogous landforms occur in strikingly similar proximity which makes them useful study cases to infer the spatial and chronological evolution of martian coldclimate surface processes the analysis of the morphological inventory of analogous landforms on svalbard and mars allows the processes operating on mars to be constrained different qualitative scenarios of landscape evolution on mars help to better understand the action of periglacial processes on mars in the recent past

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

the geology and geochemistry of the palaeoproterozoic makganyene diamictite abstract the palaeoproterozoic earth experienced a global glacial event at 2400 ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans the transvaal supergroup in the griqualand west basin south africa hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes the focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic makganyene formation in order to constrain its palaeoenvironmental settings the makganyene formation forms the base of the postmasburg group and has been regarded as resting on an erosive regional unconformity throughout the northern cape province systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal the makganyene formation is in fact conformable with underlying formations of the koegas subgroup in the deep southern prieska basin and rests on an unconformity only on the shallow ghaap platform to the north east the makganyene formation displays lateral facies changes that reflect the palaeogeography of the study area and the advance and retreat of ice sheets shelves geochemical investigations of glacial strata of the makganyene formation demonstrate that underlying banded iron formations of the transvaal supergroup acted as the main clastic source for the diamictite detritus geographic variations in bulk composition of the diamictites correlate well with field observations and show that sorting processes were controlled largely by the morphology of the palaeobasin carbon isotope results emphasize the transitional nature of the makganyene formation in terms of the environmental conditions that resulted in widespread global glaciation in the palaeoproterozoic on the basis of the above geological evidence it is proposed that the transvaal supergroup in the northern cape province represents a continuous depositional event that lasted approximately 250 ma and hence provides a unique opportunity for assessing the transitional changes experienced by the palaeoproterozoic earth

enigmatic nature of thick sedimentary carbonates depleted in 13c beyond the canonical mantle value the challenges to our understanding of the terrestrial carbon cycle carbon and oxygen isotope measurements of 66 samples from the 60 m thick variegated marble in the upper allochthon of the norwegian caledonides have a mean 13ccarb of 8 4 0 9 v pdb and a mean 18o of 20 2 2 2 v smow the variegated marble is overlain by 150 m thick pale grey marble characterised by mean 13ccarb of 6 5 0 8 n 25 and underlain by dark grey marbles with a mean 13ccarb of 4 8 1 1 n 61 this tripartite unit of an poorly constrained age but between neoproterozoic and early silurian discontinuously developed over a distance of 500 km is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported the marbles depleted in 13c beyond the canonical mantle value of 6 show no obvious evidence of post sedimentary repartitioning of carbon isotopes several other carbonate formations deposited within approximately 680 540 ma time interval chenchinskaya nikolskaya and torginskay in siberia ingta in nw canada shuram in central oman trezona and wonoka in south australia are several hundred meters thick developed over a distance of hundreds of kilometres and all show a similar depletion in 13c beyond the mantle value for reasons that are not well understood the existence of these carbonates represents a challenging problem for our current understanding of global carbon geodynamics changes in the ratio of reduced oxidised carbon sequestered in sediments a methane hydrate release or zero biological productivity if applied separately cannot explain carbon isotope characteristics and formation of these carbonates we tentatively propose that several factors associated with unusual geodynamic and palaeoclimatic scenarios developed between 600 and 540 ma might have been involved in the extreme lowering of the isotopic composition of carbon entering the global earth surface environment this period was marked by the retreat of neoproterozoic glaciers and break up of the supercontinent rodinia the late postglacial warming might have induced a massive release of methane hydrates extremely enriched in 12c the death of rodinia was marked by unusually rapid approximately 20 cm year motion of newly formed continental plates suggesting vigorous mantle convection and an enhanced restructuring of the lowest compartments of the earth this could provide a flux of 12c rich material from the isotopically light asthenosphere mantle source 13c 25 to 15 to the surface considerable reconfiguration of the continental and sea areas of the earth might have triggered enhanced weathering of previously deposited 12c rich organic matter in presenting this working hypothesis we also question the completeness of our understanding of the terrestrial carbon cycle and the popular assumption that the isotopic composition of carbon entering the global earth surface environment was always around 6

the neoproterozoic cambrian record in australia a stable isotope study abstract shallow intracratonic basins developed across a large area of the central part of the australian craton during the neoproterozoic and continued to accumulate marine sedimentary successions until well into the early palaeozoic a half billion year sedimentary record carbonate successions ranging in age from neoproterozoic cryogenian to ostensibly early ordovician have been sampled at eleven sites across four of the northernmost of these basins amadeus georgina wiso and daly basins a total of 996 samples were then analyzed for stable carbon 13 c carb and oxygen 18 o carb isotopes to develop a regional secular stable carbon isotopic curve the curve complements earlier work and provides an important framework for global correlation at a critical time in earth history during which the biosphere and atmosphere were evolving rapidly at one site in the amadeus basin diagenesis proved to have significantly modified the isotopic composition of the carbonates and the data were eliminated analysis of the remaining samples suggests that they reflect the global secular stable carbon isotope curve and tie well with established palaeontological timescales in particular key isotopic excursions can be identified in the earliest part of the neoproterozoic succession notably the negative excursion following the sturtian glaciation in the early palaeozoic record a major excursion during ordian early templetonian 509 506 ma time can be identified across the region as can the so called spice excursion which has been associated globally with the steptoean stage of laurentia 497 5 495 ma a smaller late templetonian floran 506 503 ma positive excursion has also been recognized the stable carbon isotope record provides a strong regional correlation in both fossiliferous and unfossiliferous neoproterozoic and early palaeozoic successions and allows a global correlation to be established

mantle plume uplift in the sedimentary record origin of kilometre deep canyons within late neoproterozoic successions south australia in south australia canyons 200 250 km long are incised to depths of c 1 5 km in late neoproterozoic after c 590 ma marine deposits of the adelaide fold belt and canyons 90 km long and 700 m deep occur in correlative marine strata of the officer basin 1000 km to the nw in the adelaide fold belt the isotopic signature of limestone veneering canyon walls canyon sinuosity and sedimentary structures in canyon fill suggest that the canyons were cut subaerially and filled by shallow water marine deposits during coastal onlap eustacy cannot account for the kilometre scale change in sea level required for subaerial canyon erosion and chemostratigraphy and the lack of evaporites at that stratigraphic level in southern australia argue against a messinian style drawdown in each region uplift and crustal extension preceding or coeval with canyon incision are indicated by a regional unconformity turbidite or mass flow deposition and extensional faults these features and canyon incision may be explained by regional uplift above a rising mantle plume over a distance of 1000 km in southern australia flood basalt volcanism in the officer basin tentatively dated at 563 40 ma may have followed this uplift mantle plume uplift is expressed in the sedimentary record by emergent trends in facies regional thinning regional unconformities turbidites gravity slides normal faults and incised canyons and palaeovalleys

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

the elatina glaciation late cryogenian marinoan epoch south australia sedimentary facies and palaeoenvironments abstract the late cryogenian elatina glaciation in south australia of marinoan age is named after the elatina formation of the glaciogenic yerelina subgroup which covers 200 000 km2 in the adelaide geosyncline and on the cratonic stuart shelf and is up to 1500 m thick the elatina glaciation is marked by numerous facies that are like those of phanerozoic glaciations basal diamictite displaying glacitectonites with penetrative deformation of preglacial beds indicating grounded ice or scouring by icebergs glaciomarine diamictites containing numerous faceted and striated clasts up to several metres across of intrabasinal and extrabasinal origin laminated siltstone and mudstone containing scattered ice rafted dropstones in outer marine shelf environments sandstones deposited in fluvial deltaic and inner marine shelf settings tidalites deposited in estuaries and on tidal deltas during interstadial rise of sea level with cyclic tidal rhythmites recording the annual oscillation of sea level and displaying wave generated ripple marks which together indicate long lived and extensive open seas siltstone containing acicular crystal pseudomorphs implying the formation of evaporite minerals in littoral deposits permafrost regolith of frost shattered quartzite breccia 20 m deep displaying large scale periglacial structures including primary sand wedges 3 m deep that indicate a frigid strongly seasonal climate near sea level periglacial aeolian sandstone covering 25 000 km2 and containing primary sand wedges near its base these deposits record a spectrum of settings ranging from permafrost regolith and periglacial sand sheet on the stuart shelf in the west through fluvial deltaic and inner marine shelf in the western and central parts of the adelaide geosyncline to outer marine shelf in the north and southeast the widespread and persistent rainout of fine grained sediment and ice rafted debris indicates that the sea was not frozen over during the elatina glaciation no direct age determination is available for the elatina glaciation and only maximum and minimum age limits of 640 and 580 ma respectively can at present be applied high quality palaeomagnetic data for red beds from the elatina formation supported by positive fold tests on soft sediment slump folds that demonstrate the early acquisition of magnetic remanence indicate deposition within 10 of the palaeoequator the yerelina subgroup in the adelaide geosyncline and on the stuart shelf is disconformably to unconformably overlain by the nuccaleena formation cap carbonate that marks the early ediacaran post glacial marine transgression the presence in near equatorial palaeolatitudes of glaciomarine deposition grounded ice and permafrost near sea level a strongly seasonal periglacial climate and widespread open seas implies a paradoxical palaeoclimate and palaeogeographic setting for the elatina glaciation the strong evidence for a non actualistic late cryogenian glacial climate in south australia has been a stimulus to worldwide multidisciplinary research on cryogenian glaciogenic successions

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

chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in earth history the neoproterozoic shuram formation nafun group oman abstract oman records in its neoproterozoic nafun group huqf supergroup an essentially complete carbonate rich ediacaran succession the nafun group overlies the presumed marinoan rift related fiq member ca 635 ma and ends just below the precambrian cambrian boundary 542 ma tectonic subsidence caused by thermal contraction following fiq aged rifting allowed the preservation of about 1 km of postrift stratigraphy with no major stratigraphic breaks the nafun group above the marinoan cap carbonate hadash formation represents two siliciclastic to carbonate grand cycles both initiated by significant transgressions these cycles comprise the masirah bay khufai formations and the shuram buah formations the khufai shuram boundary is associated with the start of a major carbon isotope perturbation the uppermost ramp carbonates of the khufai formation record a smooth decrease in 13c from about 4 to values around 0 followed by two descending steps across which values plunge to a nadir of 12 in the overlying red siltstones and shales interbedded with thin limestones of the shuram formation this fall in isotopic values is temporally rapid and coincident in both shallow and deep water sections in the time span of a single parasequence the 13c nadir is then followed by 50 million years of monotonic recovery the shuram shift represents the largest 13c inorganic carbon negative excursion in earth history although the snowball earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation the non glaciated context of the shuram formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non unique although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic the long term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate

integrated stratigraphy of the upper neoproterozoic succession in yunnan province of south china re evaluation of global correlation and carbon cycle abstract this study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub cambrian marine sedimentary succession in yunnan province of south china our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late neoproterozoic the succession has a basal glacial diamictite nantuo formation and comprises two large scale depositional cycles towards the precambrian cambrian pc c boundary each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal in the absence of biostratigraphic control attribution of the tillite has remained equivocal between the 710 ma sturtian and 635 ma marinoan glaciations a compilation of known paleomagnetic and other geochronologic constraints suggests that the nantuo glaciation in south china was at least diachronous in which the nantuo tillite in yunnan belongs to the older glaciation stratigraphic patterns of the diagenetically screened 13c variation and depositional sequences during the carbonate dominated interval show a striking similarity with those of the upper cryogenian and entire terminal proterozoic carbonates at widely separated other localities rather than the terminal proterozoic alone supporting the view that deposition of the nantuo tillite in yunnan reflects the sturtian rather than marinoan glaciation further considerations of the 13c results provide three important implications for the late neoproterozoic marine carbon cycle first comparison of different 13c profiles suggests considerable variability in the absolute values of carbonate on both intra and inter basinal scales which differs profoundly from their relative homogeneity in marine phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late neoproterozoic second an empirical correlation between the 13c and relative sea level changes was applied to evaluate hypothesized explanations for 13c temporal variation in surface sea yielding short term inverse and long term positive relationships the short term trend marks periods of the negative 13c excursions and is compatible to most of the existing arguments for the 13c events on the other hand mechanisms causing the long term trend remain enigmatic in the absence of comparable data from other localities our observation however suggests that at least two different mechanisms operated marine carbon cycle during the late neoproterozoic third the negative 13c anomaly characterizing the period following the marinoan glaciation worldwide was not coincident between kerogen and carbonate but occurs as a lagged drop in yunnan where a 7 depletion in kerogen preceded an 3 depletion in carbonate their lagged trends likely reflect different sources as kerogen and carbonate signals are considered to monitor surface and deeper waters respectively the negative 13c event would therefore have occurred in surface water followed by a smaller 13c depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site this mechanism supports the proposed hypothesis that injection of the 13c depleted atmospheric co2 into the surface sea deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post glacial warming contributed to the negative 13c anomaly