87sr86sr in late proterozoic carbonates evidence for a mantle event at 900 ma ago abstract strontium isotopic measurements were made on late proterozoic carbonates from west african craton comparison of samples with acceptable trace element patterns with coeval data from southern africa and with the published australian results suggests that the 87 sr 86 sr ratio of the late proterozoic sea water evolved in the following manner about 0 7075 at 1000 50 ma 0 7056 to 0 7074 at 900 50 ma 0 7068 to 0 7091 0 7106 at 800 50 ma 0 7074 to 0 7077 at 700 50 ma and 0 7076 to 0 7089 0 7096 at 600 50 ma ago the variations are comparable in magnitude and frequency to those described previously for the phanerozoic strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of sr into late proterozoic oceans was of comparable isotopic composition to its present day counterpart 0 711 consequently the non radiogenic 87 sr 86 sr value at 900 50 ma ago signifies a large flux of mantle strontium into the ocean at this time because the present time resolution is only about 75 25ma additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed late proterozoic sea water curve

chromium isotopes in carbonates a tracer for climate change and for reconstructing the redox state of ancient seawater abstract strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout earth s history mainly for the purpose of reconstructing ancient climatic changes we here present results of a new isotopic tracer system stable chromium isotopes applied to a late ediacaran marine carbonate sequence exposed in the calera de recalde syncline arroyo del soldado group uruguay the aim was to compare cr isotope signatures directly to 13 c 87 sr 86 sr and 143 nd 144 nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold warm periods accompanied by sea level changes in response to glaciation deglaciation at higher latitudes the studied section is characterized by a pronounced negative down to 3 3 13 c excursion in carbonates paralleled by a decrease of 87 sr 86 sr values chromium isotope signatures over this section also show a correlated decrease in 53 cr 53 cr 53 cr 52 cr sample 53 cr 52 cr srm979 1 1000 values from 0 29 to 0 17 which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high temperature magmatic sources linear correlations between 53 cr and end 570 ma 87 sr 86 sr and cr concentrations can be explained by mixing between two major input sources of cr nd and sr into the shallow seawater 1 a source characterized by negative 53 cr values of 0 2 low 87 sr 86 sr values of 0 707 and elevated 147 sm 144 nd values of 0 13 recognized as a subaqueous hydrothermal dominated input source and 2 a source characterized by positively fractionated 53 cr values of 0 2 higher 87 sr 86 sr values of 0 708 and lower 147 sm 144 nd values of 0 11 a source which is strongly affected by continentally derived input chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that at least locally neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and or that oxygen levels of the atmosphere were sufficient to transform cr iii into the more mobile hexavalent cr vi formed during weathering processes on land the fact that 87 sr 86 sr values despite 13 c fluctuations remain low indicative of a strong hydrothermal input into the basin at his time implies that co 2 limitation was the cause of negative 13 c and 53 cr excursions in otherwise nutrient rich late neoproterozoic basins and that glaciation is only one more consequence of a tectonically driven biologically mediated system in such a scenario glaciation acts as an amplifier of 53 cr signals these signals in marine carbonates are a sensitive tracer for redox processes in the ocean and or on land and have the potential to contribute significantly in combination with the other commonly used isotopic tracers to the reconstruction of climatic changes particularly those that are associated with major glaciation periods in earth s history

carbon and oxygen isotope profiles across meso neoproterozoic limestones from central brazil bambui and paranoa groups we present carbon and oxygen isotopic data on carbonates along three profiles of the neoproterozoic bambui group in central brazil this unit covers an area of more than 300 000 km2 and comprises carbonate silicliclastic sediments at the base that grade into siliciclastic sediments towards the top the bambui group overlies by unconformity the paranoa group which consists mostly of siltstone quartzite and minor limestone the data presented here improve the stratigraphic correlation within the bambui basin and show that it evolved in an environment significantly different from that of the paranoa basin our data show large fluctuations of 13cpdb in limestones from the bambui group from 0 8 to 13 5 in all the three studied areas some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale this observation is relevant considering the lack of fossil record and other stratigraphic markers in neoproterozoic sequences we also present the first isotopic profiles along the paranoa bambui transition which shows that the 13cpdb values grade from 1 0 in the paranoa group to 2 6 in the lower portion of the bambui group increasing up to 12 in the upper part of this unit based on our carbon isotope data as well as other geological mineralogical and nd isotope studies we argue that the sediments of the paranoa group were deposited on an open platform that was fully connected to the ocean on the other hand the sediments of the bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790 600 ma this unit displays an increased amount of clastic sediments upwards we argue that the high carbon isotope values observed in limestones and marlstones from the bambui group are correlated to worldwide high carbon isotope values reported for the neoproterozoic however we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the bambui carbonates

late neoproterozoic cambrian granitic magmatism in the aracuai orogen brazil the eastern brazilian pegmatite province and related mineral resources

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

carbonate versus silicate weathering in the raikhot watershed within the high himalayan crystalline series the major element and sr isotope geochemistry of surface waters bedrock and river sands was investigated in the raikhot watershed within the high himalayan crystalline series hhcs in northern pakistan mass balance calculations of mineral weathering contributions to the dissolved flux of ions from the watershed indicate that 82 of the hco 3 flux is derived from the weathering of carbonate minerals and only 18 is derived from silicate weathering even though the bedrock is predominantly quartzofeldspathic gneiss and granite with only 1 carbonate in the watershed this study demonstrates the importance of trace amounts of bedrock carbonate in controlling the water chemistry of glacial watersheds we suggest that the flux of sr with a high 87 sr 86 sr ratio in the major himalayan rivers may be derived in large part from weathering of trace amounts of calcite within the largely silicate hhcs models that use the flux of radiogenic sr from the himalayas as a proxy for silicate weathering rates may therefore overestimate the amount of co 2 consumption due to silicate weathering in the himalaya

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

neoproterozoic glacial deposits from the aracuai orogen brazil age provenance and correlations with the sao francisco craton and west congo belt article i nfo glacigenic diamictite successions of the macaubas group are widespread in the western domain of the aracuai orogen east of the sao francisco craton brazil diamictites also occur on this craton and in the african counterpart of the aracuai orogen the west congo belt detrital zircon grains from the matrix of diamictites and sandstones from the macaubas group were dated by the u pb shrimp technique the geochronological study sets the maximum depositional age of the glacial diamictites at 900 ma and indicates multiple sources for the macaubas basin with ages ranging from 900 to 2800 ma sm nd tdm model ages determined on whole rock samples range from 1 8 ga to 2 5 ga and get older up section comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the jequitai and carrancas diamictites in the sao francisco craton and to the lower mixtite formation of the west congolian group exposed in africa the 900 1000 ma source is most probably represented by the zadinian mayumbian volcanic rocks and related granites from the west congo belt however one of the most voluminous sources with ages in the 1 1 1 3 ga interval has not been detected in the sao francisco congo craton possible sources for these grains could occur elsewhere in africa or possibly from within the brasilia belt in western central brazil

u pb dating of granites in the neoproterozoic sergipano belt ne brazil implications for the timing and duration of continental collision and extrusion tectonics in the borborema province abstract the sergipano belt is the outcome of collision between the pernambuco alagoas massif and the sao francisco craton during neoproterozoic assembly of west gondwana field relationships and u pb geochronology of granites intruded in garnet micaschists of the macurure domain are used to constrain the main collisional event d 2 in the belt the granites are divided into two groups the pre collisional granites pre to early d 2 and the syn collisional granites syn to tardi d 2 the latter were emplaced as sheets along the s 2 axial plane foliation or they were collected at the hinge zones of f 2 folds a u pb shrimp zircon age of 628 12 ma was obtained for the pre collisional camara tonalite two u pb tims titanite ages were obtained for the syn collisional granites 584 10 ma for the angico granite and 571 9 ma for the pedra furada granite and these ages are close to the garnet whole rock sm nd isochron of 570 ma found for the peak of metamorphism in the sergipano belt the ages of the camara tonalite 628 ma and the pedra furada granite 571 ma mark respectively the maximum age for beginning of the d 2 event and minimum age for the end in the macurure domain using these ages the main neoproterozoic d 2 collisional event has been in operation in the sergipano belt for at least 57 million years correlation with coeval granitoids farther north in the borborema province indicate that while in the sergipano belt the syn d 2 granites ca 590 570 ma were emplaced under compression in the borborema province they emplaced under extensional conditions related to regional strike slip shear zones these contrasting emplacement settings for contemporaneous neoproterozoic granitoids are explained by a combination of continent continent collision and extrusion tectonics

a burial diagenesis origin for the ediacaran shuram wonoka carbon isotope anomaly marked negative 13c excursions in ediacaran age carbonate sediments have been identified in several sections globally but are not recognized in all sections of similar age the presence of 13ccarb values as low as 12 has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today the 13ccarb anomalies are strongly correlated with 18ocarb values but are not represented in 13corg records while no primary depositional processes have been identified that can produce the correlated 18o 13c arrays simulations show that fluid rock interaction with high pco2 fluids is capable of producing such arrays at geologically reasonable pco2 and water rock ratios variations in the mg ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs calcite and anhydrite in the resulting mineral assemblage incorporation of an initially aragonitic mineralogy demonstrates that high sr low mn sr and modest alteration of 87sr 86sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the 13c anomalies and do not necessarily imply preservation of primary values the profound ediacaran negative 13c anomalies can be adequately explained by well understood diagenetic processes conflated with the difficulty of correlating precambrian sections independently of chemostratigraphy they are not a record of primary seawater variations and need not have independent stratigraphic significance

tectono sedimentary evolution of sedimentary basins from late paleoproterozoic to late neoproterozoic in the sao francisco craton and aracuai fold belt eastern brazil abstract two first order regional unconformities separate proterozoic supracrustal rocks of the sao francisco craton and the aracuai fold belt eastern brazil into three genetic units these constitute three superimposed first order basin fill cycles the espinhaco macaubas salinas and bambui megasequences the paleo mesoproterozoic espinhaco megasequence ca 1730 1500 ma was deposited in an intracratonic rift sag basin in four stages pre rift rift transitional and flexural each of these stages is bounded by regional second order unconformities basin evolution was controlled primarily by tectonics although sea level variations were also important in the transitional and flexural stages after a hiatus of several hundred million years subsidence renewed during extension related to the early neoproterozoic breakup of the supercontinent rodinia with rift to passive margin sedimentation in the macaubas salinas basin ca 950 700 ma initial glaciogenic sedimentation possibly induced by uplift related to a mantle plume occurred in a rift setting subsequent a shelf slope deep sea passive margin deposits represent opening of a northern branch of the adamastor ocean tectonic magmatic climatic and eustatic processes all combined to control basin evolution the bambui megasequence ca 800 650 ma was deposited in a foreland basin during convergent and collisional tectonics related to closure of the brazilide ocean and generation of the brasilia fold belt west of the sao francisco paleocontinental region the evolution of the bambui basin whose fill is characterized by three transgressive progradational shallowing upward sequences was mainly controlled by tectonic processes

global perturbation of the marine ca isotopic composition in the aftermath of the marinoan global glaciation abstract ca isotopic compositions of marinoan post glacial carbonate successions in brazil and nw canada were measured both basal dolostones display 44 40ca values between 1 and 0 7 overlying limestones show a negative ca isotope excursion to values around 0 1 and 44 40ca values rapidly increase up section to near 2 0 in the brazilian successions those high 44 40ca values rapidly decrease and stabilize to values between 0 6 and 0 9 these ca isotope secular variation trends are unlike those of sturtian post glacial carbonate successions but similar to those of marinoan post glacial carbonate successions in namibia suggesting that the perturbation of the marine ca cycle was global this recommends ca isotope stratigraphy as a tool to correlate neoproterozoic post glacial carbonate successions worldwide while the lowermost and uppermost strata have 44 40ca values typical of phanerozoic carbonates the extremes 0 1 and 2 0 have not been thus far reported for other marine carbonates these extreme values suggest a short lived non actualistic perturbation in the marine ca cycle simple box modelling of the marinoan post glacial marine ca cycle can reproduce the extreme values only by postulating a two step process with ca input initially exceeding ca removal trough carbonate precipitation followed by precipitation overtaking a decreased ca input

multiproxy constraints on alteration and primary compositions of ediacaran deep water carbonate rocks yangtze platform south china the occurrence of shallow and deep water sedimentary facies has established the yangtze platform in south china as a key site for the study of neoproterozoic ocean oxidation and ediacaran animal evolution following the marinoan glaciation the yanwutan section in hunan province is one of the few coherent sections on the yangtze platform where ediacaran deep water carbonate sediments predominantly dolostones are preserved together with organic carbon rich shales here we present new major and trace element abundance data as well as sr o and c isotope compositions of leachates from carbonates of the doushantuo formation we evaluate the role of diagenetic modification of the carbonate rocks and constrain the redox evolution of ediacaran seawater in space and time sr 87 sr 86 systematically varies with delta o 18 carb sr and ba abundances indicating variable but mostly strong modification of fluid mobile elements by continental basin fluids in contrast ree y patterns have preserved seawater like compositions cap dolostones unit i on top of the nantuo diamictites differ from cap dolostones at shallow water sections on the yangtze platform in that they show no ce anomalies and little alteration near the top sr 87 sr 86 0 7078 delta o 18 4 0 delta c 13 carb 1 1 suggesting that delta c 13 carb and delta o 18 of cap dolostones at many other sections were compromised by hydrothermal alteration the overlying organic carbon poor micritic dolostone unit ii shows negative ce anomalies that disappear towards the top of the unit no ce anomalies occur in subsequent organic carbon rich muddy dolostone units units iii to iv these observations enrichments in toc that correlate with variations in redox sensitive metals in the carbonates negative delta c 13 carb in units ii to iv and the decoupling of delta c 13 carb from delta c 13 org argue for the existence of mostly anoxic deep water at the yangtze passive continental margin during the ediacaran the negative ce anomalies at the base of unit ii with negative delta c 13 carb may reflect fluctuations towards suboxic or oxic conditions or an allochthonous origin of this unit however trace metal enrichments in carbonates of the same unit argue for reducing conditions in pore water whereas the carbonates may have preserved the ree y signatures inherited from suboxic to oxic seawater the trace element and negative delta c 13 carb values in units ii to iv are consistent with a stratified basin model with a large partially remineralised organic matter reservoir in anoxic bottom and pore waters c 2015 elsevier ltd all rights reserved

neoproterozoic oceanic crust remnants in northeast brazil the borborema province of northeast brazil occupies a strategic position in the central portion of west gondwana linking three of its major cratonic constituents the sao francisco congo amazon and west africa cratons the southern portion of the province the riacho do pontal fold belt comprises an association of exhalative rocks with metabasalts having transitional mid oceanic ridge basalt geochemistry monte orebe complex that suggests the preservation of remnants of neoproterozoic oceanic crust this view is supported by geophysical data the riacho do pontal fold belt corresponds to the inflexion of a paired positive negative bouguer anomaly similar to other precambrian suture zones sm nd isotope data for the metabasalts yield a whole rock isochron age of 819 120 ma with an initial e nd t 4 4 indicating derivation from a depleted mantle source the onset of subduction within the borborema province ca 630 ma caused the inversion of basins and obduction of slices of oceanic crust in this scenario the riacho do pontal fold belt represents a complete late neoproterozoic plate tectonics cycle involving the collision of the sao francisco craton lower plate with the pernambuco alagoas block upper plate this interpretation challenges current views that the borborema province has acted as a coherent block since paleoproterozoic time part of the atlantica supercontinent suggesting instead a dynamic setting where multiple plates interacted during the proterozoic the monte orebe ophiolite provides a link with other cryogenian oceanic crust occurrences in central brazil and west africa indicating the preservation of a transcontinental neoproterozoic suture zone in the heart of west gondwana

controls over the strontium isotope composition of river water abstract strontium concentrations and isotope ratios have been measured in river and ground waters from the ganges orinoco and amazon river basins when compared with major element concentrations the data set has allowed a detailed examination of the controls over the strontium isotope systematics of riverine input to the oceans in the following environments 1 1 typical drainage basins containing limestones evaporites shales and alumino silicate metamorphic and igneous rocks 2 2 shield terrains containing no chemical or biogenic sediments and 3 3 the floodplains that constitute the largest areas of many large rivers the strontium concentration and isotope composition of river waters are largely defined by mixing of strontium derived from limestones and evaporites with strontium derived from silicate rocks the strontium isotope composition of the limestone endmember generally lies within the phanerozoic seawater range which buffers the 87 sr 86 sr ratios of major rivers a major exception is provided by the rivers draining the himalayas where widescale regional metamorphism appears to have led to an enrichment in limestones of radiogenic strontium derived from coexisting silicate rocks the strontium isotope systematics of rivers draining shield areas are controlled by the intense transportlimited nature of the weathering reactions and thereby limits variations in the strontium flux from these terrains floodplains are only a minor source of dissolved strontium to river waters and precipitation of soil salts in some floodplains can reduce the riverine flux of dissolved strontium to the oceans the most effective mechanisms for altering the isotope ratio and flux of riverine strontium to the oceans are increased glaciation and large scale regional metamorphism of the type produced during continental collision both mechanisms provide a means for increasing the 87 sr 86 sr ratio of the global riverine flux

marinoan glaciation in east central brazil abstract remnants of a neoproterozoic glaciation in east central brazil are represented by thin diamictite layers jequitai formation and correlative units locally overlying striated pavements on the sao francisco craton the diamictites are covered by the sete lagoas formation of the basal bambui group which is generally accepted to be a typical cap carbonate sequence although most authors have preferred a mid cryogenian post sturtian age for it based mainly on pb pb whole rock data the sete lagoas formation bears lithostratigraphic and isotopic characteristics that are identical to early ediacaran cap carbonates worldwide including a basal thin 0 10 m pale and flinty cap dolostone preserving a drop in 13c values from around 3 2 to 4 5 with associated 18o around 5 and crystal fan facies interpreted as aragonite pseudomorphs ediacaran zircons have been recovered from the middle of the sete lagoas formation constraining the deposition of its upper half to be younger than 610 ma rodrigues 2008 although there is an unconformity below the point where the zircons were collected it is short lived as suggested by the identical typically ediacaran 87sr 86sr values above and below 0 7074 0 7076 carbonate clasts from the jequitai formation and correlative diamictite bearing units in the fold belts that surround the sao francisco craton canabravinha and serra do catuni formations display similar ranges in 13c 6 7 to 2 6 suggesting the erosion of a pre glacial carbonate platform with negative 13c values i e the islay and or trezona anomalies the cratonic carrancas formation on the other hand yielded pale dolostone clasts with 13c in a small range between 4 2 and 3 4 and 18o values around 6 5 these clasts could be derived from the cap dolostone unit itself in which case the carrancas formation would represent resedimented basal sete lagoas formation and imply that sections of the sete lagoas formation sitting atop the carrancas formation are incomplete the base truncated sections have confused previous attempts to correlate the sete lagoas formation with other cap carbonate successions in light of the available lithostratigraphic isotopic and u pb zircon data we propose that the sete lagoas formation represents a basal ediacaran cap carbonate sequence 635 610 ma deposited after the marinoan glaciation in east central brazil

chemostratigraphic correlation of neoproterozoic successions in south america abstract the neoproterozoic successions of south america largely accumulated during extensional events associated with the breakup of rodinia these units are preserved as thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments on passive cratonal margins which have been intensely deformed in mobile belts surrounding the craton volcaniclastic and siliciclastic materials are associated with some of the marginal fold belts three mega sequences are represented in the cratonal area glaciogenic carbonate and molasse at least two transgressive regressive sea level cycles occurred during the evolution of the carbonate mega sequence which lies above iron cemented glacio marine diamictite of probable sturtian age we report high resolution carbon oxygen strontium and sulfur isotope trends from analyses of well preserved sample sets and use principally strontium and carbon isotopes from the best preserved samples in concert with lithostratigraphic and biostratigraphic observations to provide detailed correlations of the south american neoproterozoic successions lead zinc deposits some of which with important metal reserves seem to be restricted to the sturtian carbonate succession and associated with a sequence boundary at the end of the first transgressive regressive cycle phosphate deposits are found in both sturtian and varangerian marinoan carbonate successions above glaciogenic sequences

controls on sr isotopic evolution in lacustrine systems eocene green river formation wyoming abstract strontium isotopes from lacustrine carbonates record detailed weathering histories of exposed bedrocks and thus are potentially useful for understanding interactions between tectonics and climate that may be driven by local or regional factors in basin scale hydrologic systems we combine extensive 87 sr 86 sr and 18 o datasets from the green river formation in order to identify environmental factors driving the evolution of sr concentration and isotopic composition in lacustrine systems through the use of sr mass balance modeling two models are developed the first tests the effect of drainage capture that drove a constant volume lake from balance filled to overfilled conditions and is applied to the laney member of the green river formation the second model focuses on lacustrine evolution in response to changing the concentration composition and mass of water influx in a variable size sr reservoir lake this model is applied to the wilkins peak member of the green river formation 87 sr 86 sr ratios are correlated with 18 o values only during the laney member when both are readily explained by a common forcing mechanism drainage capture modeling of sr isotope compositions and concentrations indicates a short residence time for sr 10 3 10 4 years or less in both balance overfilled and underfilled phases of the green river formation lacustrine system this in turn suggests that paleo lacustrine sediments in most lakes can preserve sr isotope records with high resolution 10 2 years timescales rates of sr sequestration in carbonate are shown to have a strong influence on lacustrine sr concentration and high sr concentrations of lacustrine carbonate consistent with high salinity are observed in the underfilled wilkins peak member and the balance filled tipton and lower laney members in the laney member 87 sr 86 sr mass balance modeling results provide additional support for previous interpretations that the introduction of a large drainage system produced an isotopic shift across the lower laclede upper laclede boundary major drainage reorganization is not required to drive high variability in 87 sr 86 sr ratios however modeling shows that variability in 87 sr 86 sr ratios of 0 004 observed in the wilkins peak member can be explained by change in the characteristics of intrabasinal water sources during highstand vs lowstand conditions

the role of intracontinental deformation in supercontinent assembly insights from the ribeira belt southeastern brazil neoproterozoic west gondwana in this study we challenge the multiple collision model for the tectonic evolution of the neoproterozoic ribeira belt in southeastern brazil new u pb shrimp data reveal palaeoproterozoic 2153 15 ma and cryogenian 783 6 and 768 8 ma granitic rocks in the embu domain and detrital zircon data of metasedimentary units from the embu and costeiro domains suggest a coherent tectonic evolution for the whole ribeira belt rather than by multiple collisions these data are best explained by a simpler tectonic model involving continent craton volcanic arc collisions in the dom feliciano and brasilia belts that led to intracontinental crustal thickening of the adjacent thinned hinterland ribeira belt at 640 610 ma followed by widespread post collisional magmatism and rift related sedimentation at 600 540 ma we suggest that intracontinental orogeny is a relevant process during supercontinent assembly as illustrated here by the evolution of significant parts of the brasiliano orogen

tracking connection and restriction of west gondwana sao francisco basin through isotope chemostratigraphy abstract c and sr isotope compositions of carbonate rocks from the intracontinental sao francisco basin can track ocean connections and restriction the lower three formations of the bambui group can be grouped into three chemostratigraphic intervals ci recording different evolution stages of the basin lowermost ci 1 comprises the basal cap carbonates of the sete lagoas formation displaying an initial c negative excursion followed by a coeval c and sr positive excursions 13 c values from 5 to 0 and 87 sr 86 sr ratios from 0 7074 to 0 7082 in 10 m of stratigraphic record it marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression ci 2 comprises carbonates of the middle portion of the sete lagoas formation with 13 c values around 0 and 87 sr 86 sr ratios around 0 7082 that matches those observed worldwide for the late ediacaran it records the onset of a gondwana sea pathway connecting several epicontinental basins allowing migration of index fossil cloudina sp uppermost ci 3 starts after a major positive excursion in the 13 c values reaching 16 and a steepened decrease of 87 sr 86 sr ratios to 0 7075 which are lower than those expected for the ediacaran cambrian boundary this interval comprises the upper sete lagoas serra de santa helena and lagoa do jacare formations and records the end of the connection of the sao francisco basin to the gondwana sea pathway setting a restricted epeiric sea restriction was probably caused by late ediacaran uplifting of orogenic belts surrounding the basin other west gondwana cloudina bearing units also display the same mismatch in the sr isotope ratios suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations

highly 13c enriched carbonate and organic matter in the neoproterozoic sediments of the bambuigroup brazil abstract paleontological and radiometric dating methods rb sr k ar and pb pb suggest neoproterozoic ages for sedimentary carbonate rocks of the bambuigroup in the southern part of the sa o francisco basin of central brazil the 13c values for carbonates fall within two ranges 2 to 2 and 10 to 16 for both groups coexisting organic carbon is depleted in 13c by 28 2 relative to the carbonates similar to numerous neoproterozoic examples worldwide secular variations in 13c in the bambuigroup show a significant positive excursion beginning with values near 0 in carbonates immediately above a diamictite and rising to values as high as 16 upsection the sr isotope data for carbonates from the bambuigroup agree with the age inferred from the secular variation in oceanic 13c values the striking feature in this study is that both the carbonate and total organic carbon are considerably more enriched in 13c than previously reported for neoproterozoic carbonates elsewhere it is not clear if these unusual 13c values are caused by primary or secondary processes

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

effect of fines content and void ratio on the saturated hydraulic conductivity and undrained shear strength of sand silt mixtures the hydraulic conductivity represents an important indicator parameter in the generation and redistribution of excess pore pressure of sand silt mixture soil deposits during earthquakes this paper aims to determine the relationship between the undrained shear strength liquefaction resistance and the saturated hydraulic conductivity of the sand silt mixtures and how much they are affected by the percentage of low plastic fines finer than 0 074 mm and void ratio of the soil the results of flexible wall permeameter and undrained monotonic triaxial tests carried out on samples reconstituted from chlef river sand with 0 10 20 30 40 and 50 non plastic silt at an effective confining pressure of 100 kpa and two initial relative densities d r 20 91 are presented and discussed it was found that the undrained shear strength liquefaction resistance can be correlated to the fines content intergranular void ratio and saturated hydraulic conductivity the results obtained from this study reveal that the saturated hydraulic conductivity k sat of the sand mixed with 50 low plastic fines can be in average four orders of magnitude smaller than that of the clean sand the results show also that the global void ratio could not be used as a pertinent parameter to explain the undrained shear strength and saturated hydraulic conductivity response of the sand silt mixtures

chemostratigraphy and diagenetic constraints on neoproterozoic carbonate successions from the sierras bayas group tandilia system argentina carbon and oxygen isotopic data combined with a detailed diagenetic study obtained from undeformed and unmetamorphosed dolomite and limestone from the neoproterozoic sierras bayas group argentina provide a new record of isotopic stratigraphic variations useful for regional and global correlations the sierras bayas group is subdivided into the villa monica sandstones and dolomites cerro largo siliciclastic rocks and loma negra limestones formations unconformably overlain by the cerro negro formation siliciclastic rocks with basal marls and grouped into four depositional sequences bounded by unconformities the

late proterozoic glaciation in central eastern brazil synthesis and model abstract in the surroundings of the middle proterozoic sao francisco craton of central eastern brazil numerous traces of an ancient glaciation can be found almost all criterua necessary to prove an ancient glaciation occur including scratched and facetted pebbles a striated surface quarrying outwash plains eskers varvites dropstones decollement structures etc poorly sorted tillites with varying pebble composition are widely distributed continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton at the coast the wet base glaciers passed over to floating shelf ice in the northeast glaciers also came from the serrinha craton in northeastern bahia the glaciation covered an area of at least 1300 700 km the sequences containing traces of glaciation discordantly overlie archean to middle proterozoic rocks they in turn are overlain by late proterozoic calcareous and psammopelitic sediments of the bambui group present data indicate that glaciation took place around 1000 ma ago

the influence of oblique waves on the azimuthal response of a ku band scatterometer a laboratory study the authors conducted experiments in the large wind wave tank at irphe to examine the influence of longer waves propagating at oblique angles to the wind direction on microwave backscattering measurements were made of wind wave directional spectra and cross sections from a 13 5 ghz scatterometer at 30 spl deg from nadir incidence angle the authors characterize the scatterometric azimuthal scans with respect to the magnitude and the direction of the maximum backscattered power scans of solely wind waves are used to assess the effects of oblique waves for which the scans yield trends that are related to the oblique wave direction relative to the wind and the oblique wave steepness in particular cross section maxima are enhanced by oblique waves and the direction of maxima rotates from the wind axis toward the oblique wave direction these scatterometer data in addition to other oblique wave studies led the authors to conclude that wind vectors produced from standard retrieval algorithms for scatterometers should be used with prudence in regions with significant veering winds such as near meteorological fronts

constraining the marine strontium budget with natural strontium isotope fractionations 87sr 86sr δ88 86sr of carbonates hydrothermal solutions and river waters we present strontium sr isotope ratios that unlike traditional 87sr 86sr data are not normalized to a fixed 88sr 86sr ratio of 8 375209 defined as 88 86sr 0 relative to nist srm 987 instead we correct for isotope fractionation during mass spectrometry with a 87sr 84sr double spike this technique yields two independent ratios for 87sr 86sr and 88sr 86sr that are reported as 87sr 86sr and 88 86sr respectively the difference between the traditional radiogenic 87sr 86sr normalized to 88sr 86sr 8 375209 and the new 87sr 86sr values reflect natural mass dependent isotope fractionation in order to constrain glacial interglacial changes in the marine sr budget we compare the isotope composition of modern seawater 87sr 86sr 88 86sr seawater and modern marine biogenic carbonates 87sr 86sr 88 86sr carbonates with the corresponding values of river waters 87sr 86sr 88 86sr river and hydrothermal solutions 87sr 86sr 88 86sr hydend in a triple isotope plot the measured 87sr 86sr 88 86sr river values of selected rivers that together account for not vert similar18 of the global sr discharge yield a sr flux weighted mean of 0 7114 8 0 315 8 the average 87sr 86sr 88 86sr hydend values for hydrothermal solutions from the atlantic ocean are 0 7045 5 0 27 3 in contrast the 87sr 86sr 88 86sr carbonates values representing the marine sr output are 0 70926 2 0 21 2 we estimate the modern sr isotope composition of the sources at 0 7106 8 0 310 8 the difference between the estimated 87sr 86sr 88 86sr input and 87sr 86sr 88 86sr output values reflects isotope disequilibrium with respect to sr inputs and outputs in contrast to the modern ocean isotope equilibrium between inputs and outputs during the last glacial maximum 10 30 ka before present can be explained by invoking three times higher sr inputs from a uniquely glacial source weathering of shelf carbonates exposed at low sea levels our data are also consistent with the weathering peak hypothesis that invokes enhanced sr inputs resulting from weathering of post glacial exposure of abundant fine grained material

chemostratigraphy of the late mesoproterozoic atar group taoudeni basin mauritania muted isotopic variability facies correlation and global isotopic trends a b s t r a c t sedimentary strata of the atar el mreiti groups taoudeni basin mauritania are characterized by car bon isotope values that fall largely between 2 5 and 4 0 and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from 1 2 ga to 1 1 ga chemostratigraphic results are consistent with recent re os dates of 1 1 ga for atar el mreiti group strata rooney et al 2010 and support an emerging dataset that identifies the late mesoproterozoic as an isotopically distinctive interval in earth history the relatively low amplitude carbon isotope variabil ity limits the use of chemostratigraphy in detailed correlation during this time interval in this study we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of deposi tional facies to construct a rigorous and testable model for the intrabasinal correlation of atar and el mreiti group strata data support a depositional model that results in broadly uniform stratigraphic thicknesses across the west african craton with predominantly stromatolitic facies of the atar group mauritania and the hank and dar cheikh groups algeria representing deposition on shallow water craton margins and clayey carbonate and shale facies of the el mreiti group mauritania representing deposition on a broad shallow water epicratonic platform within this stratigraphic framework regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely asso ciated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean similarly distinct differences in trace element composition recorded in multilayered marine cements within deeper water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox gradients in the water column we use this multifaceted approach to test a recent hypothesis that suggests correlation of atar el mreiti group strata with glaciogenic strata of the vazante group sao francisco craton critically preservation of shallow water carbonate and evaporite strata across more than 1200 km of the predominantly epicra tonic taoudeni basin with minimal evidence for high amplitude sea level change is consistent with inferences of late mesoproterozoic highstand conditions in a global greenhouse climate such charac teristics are inconsistent with suggestions of a time equivalency with glaciogenic strata of the vazante group sao francisco craton despite broad similarities in re os geochronology and carbon isotope com position together these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution 2012 elsevier b v all rights reserved

continental weathering following a cryogenian glaciation evidence from calcium and magnesium isotopes a marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger cryogenian glaciation have been inferred from boron isotope measurements calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time data from neoproterozoic sections in namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate dominated one combined with the effects of primary dolomite formation in the cap dolostones this caused the ocean to depart from a state of acidification and return to higher ph after climatic amelioration differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern congo craton shelf point to local influences modifying and amplifying the global signal which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux

closing the clymene ocean and bending a brasiliano belt evidence for the cambrian formation of gondwana southeast amazon craton we report new paleomagnetic and geochronological data from ediacaran rift drift carbonates in the paraguai belt at the southern end of the suture zone between the amazon craton and the sao francisco and rio de plata cratons south america early thrusting resulted in remagnetization ca 528 36 ma or later the mean age is established by 40 ar 39 ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold thrust belt this remagnetization overlaps with a 525 ma gondwana reference pole metamorphic illite from the slate belt yields 40 ar 39 ar ages of 496 484 ma the timing of peak regional metamorphism oroclinal bending of the paraguai belt was caused by a 90 clockwise rotation of the east west limb after ca 528 ma probably refl ecting the irregular margin of the southeast amazon craton the age of the paraguai belt overlaps with that of the pampean orogeny farther south along the western margin of the rio de plata craton suggesting a coeval closure for the clymene ocean separating the amazon craton from the sao francisco and rio de plata cratons

age constraints on the neoproterozoic baikal group from combined sr isotopes and pb pb dating of carbonates from the baikal type section southeastern siberia abstract depositional ages for the middle part of the neoproterozoic sedimentary succession exposed along lake baikal in the southern margin of the siberian platform have been constrained by the application of sr isotope chemostratigraphy and pb pb dating of carbonate rocks the thick siliciclastic carbonate baikal group is the type section of the regional siberian succession although many of the baikal group carbonate rocks have undergone pervasive dolomitization by mixed meteoric marine fluids oolitic limestones in the 800 m thick carbonate member of the uluntui formation preserve primary textures and high sr contents up to 3200 ppm suggesting that aragonite might have been the original precursor the 87sr 86sr in the least altered high sr limestones mg ca 8 pdb range within 0 70842 0 70876 values we interpret as representing the best proxy for coeval seawater stepwise dissolution using 0 5 n hbr was applied to enrich samples in primary carbonate from the uluntui formation and nine data points from the five least altered high sr limestones yielded a pb pb isochron age of 560 30 ma with mswd 0 6 and probability 0 8 this age matches that inferred from sr isotope stratigraphy 580 550 ma and further supports the inference that the baikal paleobasin was connected to the contemporary oceans our sr chemostratigraphy and pb pb isochron age of the uluntui limestones cast doubt on previously proposed correlations of the baikal group as late mesoproterozoic to cryogenian instead the data indicate that the entire siliciclastic carbonate succession of the baikal group was most likely deposited during the ediacaran or vendian of the russian stratigraphic scale

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

meso neoproterozoic isotope stratigraphy on carbonates platforms in the brasilia belt of brazil abstract carbonate platforms were present worldwide during the proterozoic eon and variations in their c and sr isotope ratios are commonly used as a correlation tool particularly through the neoproterozoic when rapid secular change in marine c and sr isotope values permit distinction between cryogenian glacial events in central brazil the late mesoproterozoic and neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited and later deformed along the eastern margin of the sao francisco craton these strata are divided into the three major groups 1 the paranoa group which consists of a succession of sandstone siltstone rhythmite and selected intervals of carbonate 2 the macaubas group which consists of a glacial diamictite jequitai formation and 3 the bambui group with an important carbonate bearing succession that includes characteristic cap carbonate facies in its lower strata carbonate facies of paranoa and bambui groups typically occur in unconformable contact and when the diamictite of jequitai formation is absent it can be difficult to determine the stratigraphic position of these lithological similar groups furthermore uncertainties in the age of the bambui group has lead to several distinct interpretations regarding the age of the jequitai glacial diamictites we investigated the c o and sr isotopes and chemical composition of carbonate rocks in five measured sections including both pre and post glaciation carbonate successions the 13 c pdb values in the upper paranoa group occur in a narrow interval between 0 6 and 3 6 whereas the post glacial bambui group begins with substantially negative values as low as 5 7 in cap dolomite facies and rises to values up to 11 permil in limestone of the upper sete lagoas formation similarly carbonate rocks of the paranoa and bambui groups are distinct in terms of their 87 sr 86 sr ratio generally non radiogenic ratios between 0 7056 and 0 7068 are recorded in the upper paranoa group and ratios between 0 7074 and 0 7080 occurring within the bambui group the stratigraphic pattern of the c and sr ratios indicates distinctive isotopic characteristics for these two carbonate successions the isotopic data for the cabonates in the paranoa group are consistent with a sedimentation age in upper mesoproterozoic or lowermost neoproterozoic preceding the first cryogenian glaciation carbonate facies and the isotopic data for the lower bambui group suggest a relationship with the second cryogenian glaciation

direct dating of the sete lagoas cap carbonate bambui group brazil and implications for the neoproterozoic glacial events the end of the neoproterozoic is punctuated by glacial deposition but the chronology of these deposits is hindered presently by the paucity of geochronological data here we present new radiometric dating for the basal sete lagoas cap carbonate deposits that overlie glacial units in the sao francisco craton six samples from aragonite pseudomorph crystal rich facies showing pristine textures and constant 87sr 86sr ratios around 0 7075 yielded a pb pb isochron age of 740 22 ma which is interpreted as the depositional age for these remarkably preserved rocks this age can be used to infer a low to moderate palaeolatitude of 20 30 for carbonate and glacial deposition in addition as it overlaps the ages obtained for the oldest neoproterozoic glacial successions our result reinforces the idea of a long standing sturtian interval suggesting that this event represents either different discrete glaciations or a protracted event encompassing almost 80 ma

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana