a sulfate control on marine mid depth euxinia on the early cambrian ca 529 521 ma yangtze platform south china abstract given the high oxygen demands of complex metazoans a sound understanding of marine redox conditions is essential to reconstruct the factors that lead to the dramatic increases in ecosystem complexity and biological diversity in the early cambrian in this study we report new fe mo c s geochemical data from black shale rich strata of the early cambrian continental margin yangjiaping section in the yangtze platform south china integrated fe speciation and mo abundance data from the yangjiaping section in combination with previously reported data from shatan inner shelf songtao slope and longbizui basinal sections suggest mid depth euxinic waters coexisting with ferruginous deep waters and oxic surface waters this redox structure is similar to conditions previously suggested for the ediacaran yangtze platform and other proterozoic marine basins indicating that proterozoic layered ocean redox structure in some basins persisted into the early cambrian furthermore the fe mo data demonstrate that the mid depth euxinia was widespread during a marine transgression but shrank gradually along with widespread shelf oxygenation when the basin evolved into subsequent regression excess highly reactive iron fe hr in deep water with a hydrothermal origin may have had a control on the mid depth euxinia given geochemical and geological evidence for basinal hydrothermal activities on the other hand a sulfate limitation on bacterial sulfate reduction bsr and sulfide generation was suggested by a weak correlation r 2 0 22 between pyrite iron versus fe hr and total organic carbon toc form anoxic deep water sections and high toc averagely 6 in the deepest ferruginous water longbizui section furthermore a spatial gradient was observed in sulfur isotopic composition of pyrite averagely from 0 2 to 21 0 consistent with a lateral gradient of sulfate in concentration or and in isotope from shore to distal waters on the transgressive yangtze platform we accordingly proposed that two distinct sources riverine input and ocean reservoir provided sulfate for bsr dominantly in near shore and distal areas respectively and the euxinic retreat observed for the regression can be explained as a response to the progressive sulfate consumption of the ocean reservoir by intensive bsr and pyrite formation during preceding transgression therefore our results support a sulfate control on the spatiotemporal distribution of mid depth marine euxinia on the yangtze platform in early cambrian when complex metazoans rapidly evolved

sizing up the sub tommotian unconformity in siberia sedimentary rocks in the western anabar region northwestern siberia preserve an exceptional record of evolution and biogeochemical events near the proterozoic cambrian boundary carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower anabar succession staraya reckha and lower manykai formations with sub tommotian carbonates of the ust yudoma formation in southeastern siberia in contrast the upper manykai and most of the overlying medvezhya formation appear to preserve a sedimentary and paleontological record of an evolutionarily important time interval represented in southeastern siberia only by the sub tommotian unconformity correlation of the anabar section with other northern siberian successions that contain well dated volcanic rocks permits the estimate that the sub tommotian unconformity in southeastern siberia spans approximately 3 to 6 m y diverse small shelly fossils but not archaeocyathans previously thought to mark the base of the tommotian stage evolved sequentially throughout this earlier interval

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

textural elemental and isotopic variations among constituents in middle devonian limestones north america abstract biogenic components brachiopods crinoids and corals in middle devonian limestones were manually separated from their enclosing matrix and analyzed by textural elemental and isotopic methods in order to assess the preservational state of these components in general brachiopods in these rocks are better preserved texturally than other constituents as determined by sem and cathodoluminescence observations elemental variations among the major components are compatible with a differential response to diagenesis governed by the relative stability of the original mineralogy of the component however among the most stable low mg calcite brachiopods the relationship between textural preservation and trace element content is complex this apparent complexity may be due at least par ially to our method of textural characterization of whole brachipod samples the nonluminescent portions of a set of well preserved brachipods have textural characteristics and elemental contents which are virtually indistinguishable from modern representatives consequently their 18o values 3 7 0 2 are assumed to represent primary biogenic calcite these 18o values are in the heavy end of the range of values from texturally well preserved whole brachiopods 6 6 to 2 5 texturally altered whole brachiopods 6 7 to 3 7 and texturally altered crinoids and cora s 6 7 to 2 7 this comparison suggests that marine derived pore waters were a predominant component during the principal phase of diagenesis the relatively low l8o values of well preserved brachiopods compared with modern representatives implies that middle devonian seawater was either warmer by about 10 c or depleted in 18o by about 2 3 relative to modern seawater

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

spread f sporadic e coupling at chung li especially for postsunset periods of sunspot maximum years using ionogram recorded data the coupling between spread f occurrence and enhanced e blanketing frequencies fe indicative of fe changes has been investigated for an equatorial ionospheric anomaly crest station chung li for the postsunset periods of sunspot maximum years apart from the evidence for coupling from inspections of the f plots alone for the years analyzed 1981 1982 1988 and 1989 statistical results of more significance were found from superposed epoch analyses isolated spread f events of duration of 2 hours or less which started at or before 2200 lt were used as controls on time series these series were made up of comparisons between 15 min f plot fe values and interpolated monthly median fe values evidence for coupling was found for the analyses which involved 97 controls the evidence in the f plots for three events involving e patches which investigators have examined in some detail using the vhf radar at chung li suggests that these events are also coupled to isolated spread f occurrence copyright 2003 by the american geophysical union

effects of large floods on sediment transport and reach morphology in the cobble bed sainte marguerite river abstract sediment transport rates were estimated for two flood events on the cobble bed sainte marguerite river in the saguenay region canada morphologic methods were used to derive one set of estimates and a combination of the meyer peter and muller equation with a dimensionless sediment transport ratio after dietrich et al nature 340 1989 215 was used to derive another set of estimates both sets of estimates give consistent results for the first event which had a decade scale return period and for the second event which was the largest flood on record and had a century scale return period the transport occurring during the second event was an order of magnitude greater than that occurring during the first event despite this disparity in the transport intensity of the two events the channel morphology remained qualitatively similar the observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s

stable isotopic calibration of the earliest skeletal fossil assemblages in eastern siberia precambrian cambrian boundary sections from the eastern margin of the siberian platform provide important reference sections for stable isotopic correlation of the newly defined precambrian cambrian boundary level since they contain some of the earliest pre tommotian shelly fossil assemblages known and are amenable to 13 c stratigraphy stable isotopes are examined from the dvortsy and ulakhan sulugur sections river aldan where sparse pre tommotian assemblages occur and from the uchur river region where the putative anabarites trisulcatus and purella antiqua zones of the nemakit daldynian stage are well developed diagenetic resetting of 18 o and 13 c must be taken into account when comparing such sections simple corrections allow for stable isotopic correlation of basal nemakit daldynian and tommotian strata across eastern siberia

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

c o and sr isotope stratigraphy across the lower middle cambrian transition of the cantabrian zone spain and the montagne noire france west gondwana c o and sr isotopes have been analysed for three sections of the cantabrian zone northern spain and the southern montagne noire france that cross the lower middle cambrian transition or bilbilian leonian transition in its iberian sense the base of the leonian middle cambrian is regionally defined by the first appearance of the trilobite acadoparadoxides mureroensis which is well constrained in the cantabrian zone but absent in the montagne noire in the bilbilian parts of the three sections nearshore to peritidal strata are capped by erosive unconformities the succeeding leonian relative sea level rise is recorded in similar fashion in both environments leading to the deposition of aggradational bioclastic limestones and or transgressive limestone shale couplets the so called griotte facies overlain by offshore shales which relate to platform drowning and coastal smothering during the leonian caesaraugustan transition bilbilian leonian carbonates show a long term trend to higher 13c values that was punctuated by a succession of positive excursions in cremenes valdore cantabrian zone and ferrals les montagnes montagne noire two bilbilian 13c shifts an older uncertain peak at 0 1 0 01 and a younger certain shift at 0 6 1 7 and two leonian shifts peaking at 1 1 1 2 and 1 3 respectively are apparent at genestosa cantabrian zone a fifth early middle caesaraugustan shift is identified peaking at 2 1 these positive excursions and increasing trend are similar to published trends from the lower middle cambrian transition in siberia and australia although in those areas only two 13c excursions have been reported so far the marked negative 13c excursion reported from the lower middle cambrian boundary of laurentia and china is not recorded in our dataset oxygen isotope and trace element ca mn and sr data indicate that our 13c trends are unlikely to be diagenetic features while the lack of correlation with relative sea level fluctuations facies changes or benthic community dynamics means that they could be of potential for global stratigraphic correlation sr isotope compositions for the cantabrian zone succession exhibit consistent values upsection which suggests that seawater 87sr 86sr remained around 0 7089 0 7090 our more limited dataset for the montagne noire region confirms this value and both are consistent with published constraints from laurentia for seawater 87sr 86sr across the lower middle cambrian boundary the only other 87sr 86sr study to cover the boundary interval reported significantly lower 87sr 86sr values not vert similar0 7087 0 7088 indicating perhaps that the lower middle cambrian boundary in siberia lies stratigraphically below its equivalent levels in europe and laurentia however we consider it more likely that these siberian samples have been diagenetically altered to less radiogenic values

sediment transport along lower fraser river 1 measurements and hydraulic computations a comprehensive program of sediment transport measurements was conducted along lower fraser river british columbia between 1966 and 1986 the data yield a detailed sediment budget annual total suspended loads at three stations are virtually identical averaging 17 106 tonnes year t yr 1 the suspended sand load is about 5 5 106 t yr 1 about one third of the total in the gravel bed reach of the river the sand behaves as wash load significant transport of gravel begins at agassiz at a discharge of about 5000 m3 s 1 the annual gravel transport was estimated to be about 0 23 106 t yr 1 only 1 of the total load all of this material is deposited in the reach upstream of mission at mission sands finer than 0 177 mm make up more than 50 of the suspended sand load but are virtually absent from the sand bed therefore a portion of the sand load at mission is wash load the total bed material load here was estimated to be 3 0 106 t yr 1 about 18 of the total sediment load virtually all of the bed material load was transported in intermittent suspension near the bed less than 5 occurring as bed load in the long term the suspended sand load upstream is approximately equal to the total sand load at mission however within individual years some of the sand is stored within the reach temporarily and then reentrained later

a global carbon isotope excursion spice during the late cambrian relation to trilobite extinctions organic matter burial and sea level abstract the steptoean positive carbon isotope excursion spice marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue water trilobites the spice excursion is documented from sections in laurentia kazakhstan china and australia where it begins with the mass extinction at the base of the pterocephaliid biomere steptoean stage in laurentia and at coeval extinction horizons in gondwana and perigondwana terranes the peak of spice 5 coincided with a time of maximum regression in laurentia spice is similar in this regard to excursions that coincide with glacio eustatic falls such as in the late ordovician a plausible scenario involves the transformation of ocean circulation between two states which led to enhanced coastal upwelling and benthic extinctions the lack of evidence for glaciation indicates that the coeval sea level fall sauk ii sauk iii event resulted from tectonic or hydrologic changes that remain poorly understood at this time

a negative carbon isotope excursion defines the boundary from cambrian series 2 to cambrian series 3 on the yangtze platform south china a globally recorded negative carbon isotope excursion characterizes the transition from cambrian series 2 to cambrian series 3 this transition is also well exposed in sedimentary successions on the yangtze platform and the wuliu zengjiayan section guizhou province south china has been proposed as a potential global stratotype section and point gssp for this boundary here we report delta 13 c carb values for the jianshan and the wuliu zengjiayan sections both sections display a progressive decrease in delta 13 c from values around 3 parts per thousand upwards in stratigraphy to a pronounced delta 13 c minimum with values as low as 6 9 parts per thousand at the proposed boundary level and a return to delta 13 c values between 0 and 1 parts per thousand in the upper part of the sections the delta 13 c minimum is thought to be caused by a transgressive event flooding the shelf area with 13 c depleted basinal anoxic bottom water our delta 13 c data are in good agreement with carbon isotope profiles recorded elsewhere these define the so called roece event redlichiid oleneliid extinction carbon isotope excursion cf zhu et al 2006 2007 and may reflect the perturbation of the global carbon cycle during the cambrian series 2 to cambrian series 3 transition c 2009 elsevier b v all rights reserved

sediment transport in a large impounded river the lower ebro ne iberian peninsula abstract the sediment transport of the highly regulated lower ebro river is estimated on the basis of a measuring programme carried out between 2002 and 2004 total sediment transport including both suspended load and bedload was measured upstream and downstream from the mequinenza and riba roja reservoirs with special attention to the transport during floods annual total load upstream from the dams is estimated at around 1 64 106 t of which at least 99 is transported in suspension annual total load downstream from the dams is estimated at around 0 45 106 t of which 60 is transported in suspension and the remaining 40 as bedload mean d50 bl in the range of 32 mm total load represents 3 of what was transported at the beginning of the 20th century in the delta plain sediment yield is three to four times lower below the dams a fact that is caused by the trapping of sediment within the reservoirs around 90 in the case of suspended sediment and 100 in the case of bedload as a consequence sediment that is transported downstream from the dams is all entrained from the riverbed and eroded from the banks the sediment deficit causes a mean riverbed incision of 30 mm per year as estimated from bedload measurements and by means of scour chains and painted pebbles located between the lowermost dam and the measuring section 28 km downstream since floods have been reduced on average by 25 but sediment supply from upstream has been reduced to almost nothing the river channel of the lower ebro river will continue exporting sediment both during floods 2000 m3 s mostly bedload and frequent high flows 1000 m3 s to 2000 m3 s making the incision progressive unless restoration steps are taken

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

introduction from snowball earth to the cambrian explosion evidence from china the neoproterozoic palaeozoic transition npt around 600 ma ago was a critical time interval when the earth experienced fundamental change manifested as climatic extremes snowball earth followed by the emergence and rapid diversification of animals cambrian explosion how animals and environments co evolved and what caused these fundamental changes to the earth system during the npt is a great scientific puzzle which has been a rapidly developing frontier of interdisciplinary research between bio and geosciences south china preserves a complete stratigraphic succession of the npt developed in various facies ranging from shallow to deep marine realms with extraordinarily well preserved successive fossil biotas in various taphonomic settings zhu 2010 fig 1 making it a key area and global focus of studies in the field over recent decades indeed the current narrative of early animal evolution has largely been based on the fossil biotas from south china these include 1 the world9s oldest microscopic animal fossils with cellular details from the early ediacaran weng9an biota doushantuo formation 2 putative macroscopic animal fossils preserved as carbonaceous imprints from the early ediacaran lantian wenghui and miaohe biotas also doushantuo formation 3 typical late ediacaran faunas preserved in dark limestone shibantan biota and as large and poorly mineralized tubular animal fossils gaojiashan biota both from the dengying formation 4 phosphatized small shelly and soft bodied animal fossils from the early cambrian meishucun and kuanchuanpu faunas and 5 cambrian fossil lagerstatten chengjiang guanshan and kaili faunas with typical burgess shale type soft bodied preservation

extreme enrichment of polymetallic ni mo pge au in lower cambrian black shales of south china an os isotope and pge geochemical investigation extreme metal enrichments occur in the lower cambrian niutitang formation a thick black shale sequence deposited on the yangtze platform of south china and form a regionally distributed conformable polymetallic ni mo pge au sulfide horizon the origin for the extreme enrichment of metals in these black shales is discussed based on a comprehensive geochemical investigation of re os isotopes and platinum group elements the black shales of the lower cambrian niutitang formation in guizhou province yielded an re os isochron age of 535 11 ma with an initial 187 os 188 os ratio of 0 80 0 04 which is similar to the initial 187 os 188 os ratio 0 78 0 19 of the ni mo sulfide ores but slightly lower than the initial 187 os 188 os ratio 1 18 0 02 of lower cambrian black shales from the lesser himalaya india a comparison of os isotope data among the chinese ni mo sulfide ores the seafloor massive sulfide deposits and contemporaneous seawater suggests a possible similar hydrothermal forming mechanism for these deposits the pge concentrations pt anomaly pt pt pt pd ir pd au pd ratios of the chinese ni mo sulfide ores are similar to those metal rich black shales of submarine volcanogenic hydrothermal origin from czech republic namibia and canada but are different from those data of the ocean floor fe mn crusts that have scavenged their pge and other metals directly from seawater it is therefore suggested that the ni mo sulfide ores from the niutitang black shale sequence in south china had a submarine hydrothermal origin the anoxic environments and abundant organic matter in the ocean basin also played a key role for the extreme metal enrichments the discharge of hydrothermal fluids into the cambrian ocean may have had a great effect on life during the cambrian explosion 2007 elsevier b v all rights reserved

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

seawater carbon and strontium isotope variations through the late ediacaran to late cambrian in the tarim basin abstract the radiation of early animals during the cambrian explosion was accompanied by widespread perturbations in oceanic and atmospheric conditions however the cause effect relationships between evolutionary innovations carbon c cycling and continental weathering are still a matter of debate further paleoenvironmental reconstructions of poorly studied paleo continents covering the precambrian cambrian pc c transition may improve the correlation of the events and their impact on the earth system on a global scale the tarim basin was presumably located proximal to the well studied south china craton during the ediacaran cambrian boundary and is expected to contribute equally to our paleoenvironmental reconstructions here we apply carbon and strontium sr isotope records 13c and 87sr 86sr obtained from well preserved marine carbonates spanning the late ediacaran to the late cambrian from the keping region in the northwestern tarim basin nw china by comparison to other carbonate successions from various paleo continents our obtained seawater 87sr 86sr curve in this study shows a stepwise first order increase to more radiogenic values superimposed by second order presumably regional variations our obtained 13c variations exhibit four prominent negative and three positive excursions identical to the global 13c record based on existing paleontological evidence and our new tarim 13c and 87sr 86sr curves we propose a stratigraphic correlation from the late ediacaran to the late cambrian between the tarim basin and other paleo continents besides our paleoenvironmental reconstruction reveals both regional and global sea level changes that may have controlled the influx of radiogenic sr and presumably the influx of nutrients controlling primary production and the rate of organic carbon burial in the tarim basin at the time of deposition these feedbacks may have ultimately influenced the rapid diversification of metazoans and the oxygenation of the cambrian ocean and atmosphere approaching present po2 levels

influence of a dam on fine sediment storage in a canyon river glen canyon dam has caused a fundamental change in the distribution of fine sediment storage in the 99 km reach of the colorado river in marble canyon grand canyon national park arizona the two major storage sites for fine sediment i e sand and finer material in this canyon river are lateral recirculation eddies and the main channel bed we use a combination of methods including direct measurement of sediment storage change measurements of sediment flux and comparison of the grain size of sediment found in different storage sites relative to the supply and that in transport in order to evaluate the change in both the volume and location of sediment storage the analysis shows that the bed of the main channel was an important storage environment for fine sediment in the predam era in years of large seasonal accumulation approximately 50 of the fine sediment supplied to the reach from upstream sources was stored on the main channel bed in contrast sediment budgets constructed for two short duration high experimental releases from glen canyon dam indicate that approximately 90 of the sediment discharge from the reach during each release was derived from eddy storage rather than from sandy deposits on the main channel bed these results indicate that the majority of the fine sediment in marble canyon is now stored in eddies even though they occupy a small percentage similar to 17 of the total river area because of a 95 reduction in the supply of fine sediment to marble canyon future high releases without significant input of tributary sediment will potentially erode sediment from long term eddy storage resulting in continued degradation in marble canyon

accounting for uncertainty in cumulative sediment transport using bayesian statistics abstract that sediment transport estimates have large uncertainty is widely acknowledged when these estimates are used as the basis for a subsequent analysis such as cumulative sediment loads or budgets treatment of uncertainty requires careful consideration the propagation of uncertainty is a problem that has been studied in many other scientific disciplines in recent years bayesian statistical methods have been successfully used to this end in hydrology ecology climate science and other disciplines where uncertainty plays a major role their applications in sediment transport however have been few previous work demonstrated how deterministic sediment transport equations can be brought into a probabilistic framework using bayesian methods in this paper we extend this basic model and apply it to sediment transport observations collected on the snake river in wyoming usa these data were used previously to develop a 50 year sediment budget below jackson lake dam we revisit this example to demonstrate how viewing sediment transport probabilistically can help better characterize the propagation of uncertainty in the calculation of cumulative sediment transport we present the development of probabilistic sediment rating curves that rely on deterministic sediment transport equations and then show how these can be used to compute the distribution of sediment input and output for each year from 1958 to 2007 the bayesian approach described provides a robust way to quantify uncertainty and then propagate it through to subsequent analyses results show that transport uncertainty is quantified naturally in the bayesian approach making it unnecessary for modelers to assume some specified error rate e g 5 when developing estimates of cumulative transport further we demonstrate that a bayesian approach better constrains uncertainty and allows sediment deficit and surplus to be examined in terms of quantified risk

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

downstream effects of impounding a natural lake the snake river downstream from jackson lake dam wyoming usa a sediment mass balance constructed for a 16 km reach of the snake river downstream from jackson lake dam jld indicates that river regulation has reduced the magnitude of sediment mass balance deficit that would naturally exist in the absence of the dam the sediment budget was constructed from calibrated bed load transport relations which were used to model sediment flux into and through the study reach calibration of the transport relations was based on bed load transport data collected over a wide range of flows on the snake river and its two major tributaries within the study area in 2006 and 2007 comparison of actual flows with unregulated flows for the period since 1957 shows that operations of jld have reduced annual peak flows and increased late summer flows painted tracer stones placed at five locations during the 2005 spring flood demonstrate that despite the reduction in flood magnitudes common floods are capable of mobilizing the bed material the sediment mass balance demonstrates that more sediment exits the study reach than is being supplied by tributaries however the volume of sediment exported using estimated unregulated hydrology indicates that the magnitude of the deficit would be greater in the absence of jld calculations suggest that the snake river was not in equilibrium before construction of jld but was naturally in sediment deficit the conclusion that impoundment lessened a natural sediment deficit condition rather than causing sediment surplus could not have been predicted in the absence of sediment transport data and highlights the value of transport data and calculation of sediment mass balance in informing dam operations copyright 2011 john wiley sons ltd

marine redox variations and nitrogen cycle of the early cambrian southern margin of the yangtze platform south china evidence from nitrogen and organic carbon isotopes the early cambrian is a crucial interval during which the marine environment encountered significant perturbations alongside key biological innovations the nitrogen isotopic composition of ancient sediments has the potential to record co variation between the marine ecosystem and environmental conditions here we report bulk nitrogen and organic carbon isotopic compositions accompanied by mo concentration data a proxy for redox conditions for a shelf marginal drill core section sancha and a deep basinal outcrop section yuanjia in hunan province south china for the cambrian fortunian stage variable delta n 15 bulk values 4 to 2 parts per thousand and low delta c 13 org values 35 parts per thousand to 33 from the yuanjia section indicate that anoxic waters shoaled into the photic zone with ammonia assimilation competing with anammox in the basinal area in the late cambrian stage 2 low delta n 15 bulk and delta c 13 org values together with high mo bulk concentrations delta n 15 bulk 5 to 0 parts per thousand delta c 13 org 33 5 to 31 0 parts per thousand 100 ppm mo bulk in sancha section and delta n 15 bulk 0 parts per thousand delta c 13 org similar to 33 parts per thousand 100 ppm mo bulk in yuanjia section reflect intense n 2 fixation accompanied by intermittent ammonia assimilation under expanded euxinic condition with shoaling chemocline possibly into the photic zone in the early cambrian stage 3 relatively high delta n 15 bulk values alongside positive delta c 13 org excursions at both sections indicate an increasing contribution from n 15 enriched nitrate assimilation and hence more oxic conditions shelf margin sediments from the middle and late parts of cambrian stage 3 are characterized bylaw delta n 15 bulk values similar to 0 parts per thousand being lowest in the basinal area 3 to 0 parts per thousand in combination with high mo bulk concentration 100 ppm and high mo tocbulk ratios averaging similar to 18 in deep basinal sediments these data suggest that the photic zone became anoxic again but that sulfidic conditions occurred in the basinal area only compared with stage 2 euxinia was less extensive during this interval with photosynthetic biomass dominating over chemoautotrophic biomass at both localities based on a combination of delta n 15 data from this study and published data from studies on contemporaneous sections in yunnan province and the three gorges area it is possible to reconstruct the spatial variability and temporal evolution of redox conditions in the various marine environments along the southern margin of the yangtze platform the basinal area appears to have been dominated by anoxia and intermittent euxinia in the photic zone during most of the early cambrian fortunian stage to stage 3 with the exception of more oxic conditions during early stage 3 on the platform oxygenated shallow water prevailed with the exception of the notable cambrian stage 2 anoxic event the outer shelf area experienced anoxia from stage 2 to stage 3 of the early cambrian although of possibly limited areal extent the high mo bulk concentrations and mo tocbulk ratios suggest that the south china basin was well connected with the considerably oxygenated open ocean and that the mean low delta n 15 bulk values similar to 1 6 parts per thousand from the basin may reflect generally lower delta n 15 composition for seawater nitrate in the early cambrian compared with modern value c 2015 elsevier b v all rights reserved

reconstructing marine redox conditions for the transition between cambrian series 2 and cambrian series 3 kaili area yangtze platform evidence from biogenic sulfur and degree of pyritization the transition between series 2 and series 3 of the cambrian period was a time of environmental perturbations and biological innovations a generally positive sulfur isotopic composition of sedimentary pyrite with delta s 34 pyrite values between 1 7 and 373 is consistent with a biological origin via reduction of strongly s 34 enriched seawater sulfate no substantial changes in the redox conditions of the water column are discernible from the dop record and the sulfide sulfur isotopic composition is independent of dop in particular an excursion towards less is enriched values immediately preceding the proposed level of the cambrian series 2 to cambrian series 3 transition this level is also characterized by a negative carbonate carbon isotope excursion believed to reflect the transgressive flooding of the shelf with basinal anoxic waters low dop values would suggest that these might have been ferruginous rather than sulfidic c 2013 elsevier b v all rights reserved

marine carbon and sulfur cycling across the ediacaran cambrian boundary in tarim block and its implications for paleoenvironmental changes abstract the ediacaran cambrian transition was a critical period of earth history because of the coincidence among climate change bioevolution and tectonic activity the nature of carbon and sulfur cycling processes spanning the ediacaran cambrian boundary remains debatable this study considers high resolution stable isotope chemostratigraphy of the sugetbrak and yutixi sections of the keping area of the tarim basin northwestern china the sections display a pronounced negative 13ccarb shift the basal cambrian carbon isotope excursion that spans the ediacaran cambrian boundary deposits overlying the negative excursion are characterized by heavier 13ccarb values ranging from 7 8 to 4 1 the biostratigraphically calibrated 13ccarb profiles likely reflect widespread perturbation of the global oceanic carbon cycle and associated mass extinction and subsequent biological evolution the 34spy profiles of the studied sections display a wide distribution of values ranging from 16 4 to 23 8 the variable 34spy signatures of these sections are partially controlled by microbial sulfate reduction rates which are sensitive to local sulfate concentration availability of organic matter and the marine redox state the strong coupling between enhanced organic carbon and pyrite burial the appearance of cambrian type animals and a shift to a more oxidizing marine environment might suggest a temporal causality of higher oxygen levels of the atmosphere ocean realm with bio events during the ediacaran cambrian transition the development of ecosystems across the ediacaran cambrian transition may have significantly impacted the oceanic carbon and sulfur biogeochemical processes in earth s history and in turn affecting themselves

carbon isotope stratigraphy of upper cambrian steptoean stage sequences of the eastern great basin record of a global oceanographic event a large global positive excursion in 13 c from 0 5 to 4 5 during the late cambrian pterocephaliid biomere steptoean stage aphelaspis elvinia zones is documented at high stratigraphic resolution in three sections in the eastern great basin the excursion which we refer to as the steptoean positive carbon isotope excursion or spice excursion began coincident with a world wide extinction event the 13 c data from the great basin reflect global seawater values in a wide range of lithologies including oolitic grainstones wackestones thrombolitic boundstones and flat pebble conglomerates we use a section at shingle pass in the southern egan range to divide the pterocephaliid biomere into eight isotope steps that represent equal divisions of the 13 c rise and fall 4 this provides a basis for recognition of a revised chronostratigraphic framework for the pterocephaliid biomere steptoean stage strata deposited during the beginning of the spice excursion record a major change in the pattern of sedimentation in the eastern great basin this is reflected in a siliciclastic carbonate transition at shingle pass nevada and a carbonate siliciclastic transition at the house range and lawson cove sections in utah a regional siliciclastic influx recognized throughout the great basin occurs near the peak of the spice excursion carbon isotope analyses from cratonal sections in wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the spice excursion the correlated changes in 13 c relative sea level and the marine biota during the spice excursion provide remarkably detailed records of a major paleoceanographic event we speculate that changes in sea level climate or tectonics may have triggered the spice excursion and coeval extinction event subsequent burial of organic carbon caused the increase in 13 c and may have led to an interval of global cooling the results of this study lend confidence to carbon isotopic studies of pre mesozoic rocks

dissolution methods for strontium isotope stratigraphy guidelines for the use of bulk carbonate and phosphorite rocks diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy sis this is especially important for precambrian and cambrian studies for which no suitable skeletal material is available one proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution we applied a similar technique to dissolve a range of lithologies limestone dolostone and phosphorite and compared the strontium isotopic composition of various leaches to contemporaneous seawater our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near primary seawater 87sr 86sr composition once contaminant strontium from secondary calcite and other phases has been removed commonly applied trace element e g mn sr mg ca and isotopic c o screening cut offs were also examined in the light of these results and proved unreliable particularly in the case of dolostones and partially dolomitized limestones we recommend that rigid sample selection and sequential leaching procedures be applied to all sis studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future

the late cambrian spice δ13c event and the sauk ii sauk iii regression new evidence from laurentian basins in utah iowa and newfoundland carbon isotope data from upper cambrian sections in three laurentian basins in northern utah central iowa and western newfoundland record a large positive d 13 c excursion spice event of up to 1 5 peak d 13 c ratios are well dated by trilobite collections to the middle of the steptoean stage dunderbergia zone and occur during maximum regression associated with formation of the sauk ii sauk iii subsequence boundary on the north american craton max imum regression was marked by an influx of quartz sand into carbon ate platform settings in all three widely separated basins in northern utah this quartz sand formed a thick sequence known as the worm creek quartzite which marks a conspicuous interruption of carbonate deposition during the middle to late cambrian in the region in west ern newfoundland the thickness of the quartz sand unit is much re duced but still marks a brief shutdown of the carbonate factory that is unique to the cambrian shelf succession of the area in the central iowa area of the cratonic interior an upward shallowing carbonate succession culminates in cross stratified trilobite grainstones at the peak of the spice in dunderbergia zone time and the lowest point on the relative sea level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface although it is difficult to determine precisely the departure from baseline d 13 c that marks the beginning of the spice excursion in the stratigraphic successions analyzed our results are consistent with a rise and subsequent fall in d 13 c tracking a major regressive transgressive event recorded across northern laurentia the correlation of a major d 13 c excursion with regression is similar to that described for the late ordovician for which the pattern has been attributed to either in creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic matter containing silicates at high latitudes or high productivity and organic carbon burial driven by oceanic overturn the lack of known steptoean age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic carbon burial was the likely cause of the spice event we suggest that increased weathering and erosion rates during relative sea level fall sauk ii iii increased the burial fraction of organic carbon in an expanded region of fine grained siliciclastic deposits in shelf and upper slope environments during the steptoean

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

the neoproterozoic lower cambrian in northern govi altay western mongolia regional setting lithostratigraphy and biostratigraphy this paper provides a framework for the stratigraphy and correlation of key sections through the neoproterozoic lower cambrian of the govi altay area western mongolia studied by members of igcp project 303 dzabkhan formation volcanic rocks at the base of the succession are suggested to have formed during rifting of the baydrik tarbagatay microcontinent in the baykalian interval c 850 650 ma pre vendian accretion transformed this rift and adjacent microcontinental fragments into a marginal marine back arc basin in which accumulated relatively uninterrupted neoproterozoic and earliest palaeozoic sediments lithostratigraphy for the latter is defined with detailed maps at five sections bayan gol northern and southern blocks tsagaan gol salaany gol and tayshir the tsagaan oloom formation lower member contains two diamictites of possible sturtian age middle and upper carbonates are compared with the yudomian vendian of siberia the former ending with a unit of boxonia grumulosa stromatolites in the upper member phosphorite shale is followed mainly by limestones with the earliest skeletal fauna the bayan gol formation comprises siliciclastic limestone intercalations with increasingly diverse invertebrate biota correlation of lithological marker beds across the study area produces a composite stratigraphic section to which the earliest skeletal fossil occurrences are tied the anabarites trisulcatus zone of the nemakit daldynian stage begins near the base of the tsagaan oloom formation upper member unit 11 the purella antiqua zone begins near the base of the bayan gol formation unit 18 assemblages typical of the nochoroicyathus sunnaginicus zone base of the tommotian stage range through units 19 to 23 elements typical of the dokidocyathus regularis zone middle tommotian appear in unit 24 near the top of the bayan gol formation

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

colorado river sediment transport 1 natural sediment supply limitation and the influence of glen canyon dam analyses of flow sediment transport bed topographic and sedimentologic data suggest that before the closure of glen canyon dam in 1963 the colorado river in marble and grand canyons was annually supply limited with respect to fine sediment i e sand and finer material furthermore these analyses suggest that the predam river in glen canyon was not supply limited to the same degree and that the degree of annual supply limitation increased near the head of marble canyon the predam colorado river in grand canyon displays evidence of four effects of supply limitation 1 seasonal hysteresis in sediment concentration 2 seasonal hysteresis in sediment grain size coupled to the seasonal hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation analyses of sediment budgets provide additional support for the interpretation that the predam river was annually supply limited with respect to fine sediment but it was not supply limited with respect to fine sediment during all seasons in the average predam year sand would accumulate and be stored in marble canyon and upper grand canyon for 9 months of the year from july through march when flows were dominantly below 200 300 m3 s this stored sand was then eroded during april through june when flows were typically higher after closure of glen canyon dam because of the large magnitudes of the uncertainties in the sediment budget no season of substantial sand accumulation is evident because most flows in the postdam river exceed 200 300 m3 s substantial sand accumulation in the postdam river is unlikely

carbonate carbon isotope evolution of seawater across the ediacaran cambrian transition evidence from the keping area tarim basin nw china sedimentary rocks from the ediacaran cambrian boundary record important biological climatic and geotectonic changes during this time to date only few geochemical investigations on the upper ediacaran upper cambrian rocks in the tarim basin have been carried out here we report high resolution 13 c carb records from the penglaiba the wushi phosphorite and the dongergou sections from ediacaran cambrian series 3 in the keping area of the tarim basin the sections display several obvious 13 c carb shifts 13 c carb values increased from 3 to 6 7 across the qigebulage formation moreover a negative 13 c carb shift across the ediacaran cambrian boundary is apparent 13 c carb values decreased to a minimum of 9 8 in the wushi phosphorite section 7 7 in dongergou section and 5 4 in penglaiba section followed by a positive carbonate carbon isotopic excursion across the yuertusi formation into the middle of the overlying xiaoerbulak formation furthermore more or less invariable positive 13 c carb values characterize the middle and upper xiaoerbulak formation the most negative 13 c carb value 14 3 occurred near the base of the shayilik formation which is the absolute minimum value among the studied sections of the cambrian series 2 to cambrian series 3 transition in the world the 13 c data from keping tarim basin are in good agreement with carbon isotope profiles recorded in south china and these events may reflect the perturbation of the carbon cycle in the tarim basin during the ediacaran cambrian and the cambrian series 2 cambrian series 3 transitions

stable isotope geochemistry of the lower cambrian sekwi formation northwest territories canada implications for ocean chemistry and secular curve generation abstract three curves delineating 13 c variations across the early cambrian sekwi formation carbonate ramp mackenzie mountains northwest territories canada are incorporated into a biostratigraphic and sequence stratigraphic framework that provides a higher resolution temporal and spatial framework for the sekwi formation the three 13 c curves were correlated across a range of depositional environments from tidal flat to deep subtidal on the sekwi carbonate ramp eight 13 c cycles in the curves a h were correlated across the basin complimenting the sequence stratigraphic framework the most complete 13 c curve from the sekwi formation correlates well with a composite early cambrian 13 c curve from the siberian platform however the differing magnitude and absolute timing of some of the correlative isotopic excursions records the influence of regional tectonism sea level changes and diagenesis on the sekwi ramp additionally the sekwi formation 13 c curve has more high frequency variations than the siberian curve due in part to the higher sample density used to construct the sekwi curve the early middle cambrian boundary was not sampled in this study but a large negative isotopic excursion in the very latest early cambrian coincides with a regional drowning that may be correlative throughout western laurentia but recognizing this event globally is problematic

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

spatial and temporal variations in carbon and sulfur isotopic compositions of sinian sedimentary rocks in the yangtze platform south china abstract eight stratigraphic sections from the yangtze platform south china were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of sinian sedimentary rocks carbon isotope compositions of carbonate from lower sinian strata deposited between pulses of glacial diamictite range from 3 9 to 9 9 the more negative values recorded from mn carbonates suggest that in part the carbon was derived from the oxidation of organic matter during early diagenesis in the upper sinian strata 13c varies with the facies they are positive for the platform carbonates whereas they are negative from about 3 to 12 for the carbonates formed in or near basin environments it is evident that post depositional alteration accounts for the strongly negative 13c values however some information on primary isotopic compositions can be obtained by evaluating sample quality the primary 13c values of carbonates from the doushantuo formation are ca 3 to 4 and from the dengying formation as low as 6 34s values for pyrite from the lower sinian rocks are highly positive and range from values near the coeval seawater sulfate up to ca 60 in contrast 34s values for pyrite from the upper sinina rocks are negative down to 27 to 30 the feature of sulfur isotopic composition for the lower sinian rocks is consistent with the late proterozoic sulfur isotopic record it might be explained by the supercontinent model many basins on the yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher 34s values than the coeval seawater for a geologically long period an alternative explanation is based on the sulfate minimum zone smz modal because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower sinian shallow water environment particularly in the smz the residual sulfate might have been significantly enriched in 34s so that pyrite produced in closed diagenetic system would yield very positive 34s values

carbon isotope stratigraphy of the precambrian cambrian sukharikha river section northwestern siberian platform a high resolution carbon isotope profile through the uppermost neoproterozoic lower cambrian part of the sukharikha section at the northwestern margin of the siberian platform shows prominent secul