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

sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in early to middle ordovician greenhouse oceans article i nfo despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal neoproterozoic era late cambrian records point to the persistence of deep water anoxia and potential for development of euxinic conditions the late cambrian spice steptoean positive carbon isotope excursion event however is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere here we present high resolution carbon and sulfur isotope profiles from early to middle ordovician carbonate rocks from the argentine precordillera and western newfoundland to constrain oceanic redox conditions in the post spice world marine c isotope profiles record relatively stable behavior excursions b3 that is characteristic of greenhouse climates ma rine s isotope profiles record short term b10 6 yr rhythmic variation superimposed over a longer term

geochemistry of peruvian near surface sediments sixteen short sediment cores were recovered from the upper edge ueo within wo and below bo the oxygen minimum zone omz off peru during cruise 147 of r v sonne solids were analyzed for major trace elements total organic carbon total inorganic carbon total sulfur the stable sulfur isotope composition 34 s of pyrite and sulfate reduction rates srr pore waters were analyzed for dissolved sulfate sulfide and 34 s of sulfate in all cores highest srr were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and or diffusion of sulfate from bottom water 34 s of dissolved sulfate showed only minor downcore increases strong 32 s enrichments in sedimentary pyrite to 48 vs v cdt are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction manganese and co are significantly depleted in peruvian upwelling sediments most likely due to mobilization from particles settling through the omz whereas release of both elements from reducing sediments only seems to occur in near coastal sites cadmium mo and re are exceptionally enriched in wo sediments 600 m water depth high re and moderate cd and mo enrichments are seen in bo sediments 600 m water depth re mo ratios indicate anoxic and suboxic conditions for wo and bo sediments respectively cadmium and mo downcore profiles suggest considerable contribution to ueo wo sediments by a biodetrital phase whereas re presumably accumulates via diffusion across the sediment water interface to precipitation depth uranium is distinctly enriched in wo sediments due to sulfidic conditions and in some bo sediments due to phosphorites silver transfer to suboxic bo sediments is likely governed by diatomaceous matter input whereas in anoxic wo sediments ag is presumably trapped due to sulfide precipitation cadmium cu zn ni cr ag and t1 predominantly accumulate via biogenic pre concentration in plankton remains rhenium sb as v u and mo are enriched in accordance with seawater te availability lead and bi enrichment in ueo surface sediments is likely contributed by anthropogenic activity mining accumulation rates of toc cd mo u and v from peruvian and namibian sediments exceed those from the oman margin and gulf of california due to enhanced preservation off peru and namibia copyright 2004 elsevier ltd

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

contributors to chromium isotope variation of meteorites we report the results of a comprehensive high precision survey of the cr isotopic compositions of primitive chondrites along with some differentiated meteorites to ensure complete dissolution of our samples they were first fused with lithium borate tetraborate at 1050 1000 c relative to the nist cr standard srm 3112a carbonaceous chondrites exhibit excesses in 54cr 52cr from 0 4 to 1 6e 1e 1 part in 10 000 and ordinary chondrites display a common 54cr 52cr deficit of 0 4e analyses of acid digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54cr enriched carrier which is characterized by a large excess in 54cr 52cr up to 200e associated with a very small deficit in 53cr 52cr 2e we did not find 54cr anomalies in either bulk enstatite chondrites or in leachates of their acid digestion residues this either requires that the enstatite chondrite parent bodies did not incorporate the 54cr anomaly carrier phase during their accretion or the phase was destroyed by parent body metamorphism chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the nist srm 3112a cr standard the eucrite and martian meteorites studied exhibit small deficits in 54cr 52cr the 54cr 52cr variations among different meteorite classes suggest that there was a spatial and or temporal heterogeneity in the distribution of a 54cr rich component in the inner solar system we confirm the correlated excesses in 54cr 52cr and 53cr 52cr for bulk carbonaceous chondrites but the new data yield a steeper slope 6 6 than that reported in shukolyukov and lugmair 2006 the correlated excesses may affect the use of the mn cr chronometer in carbonaceous chondrites we could not confirm the bulk carbonaceous chondrite mn cr isochron reported by shukolyukov and lugmair 2006 and moynier et al 2007 mostly because we find much smaller total variations in e53cr 0 2 all bulk chondrites have small e53cr excesses up to 0 3 relative to the earth most likely reflecting the sub chondritic mn cr ratio of the earth the e53cr variations in chondrites do seem to grossly correlate with mn cr and yield an initial solar system 53mn 55mn value of 5 4 2 4 10 6 corresponding to an absolute age of 4566 4 2 2 ma nuclear interactions with cosmic rays result in coupled excesses in e54cr and e53cr with a 4 1 ratio in phases with high fe cr these are most dramatically demonstrated in the iron meteorite carbo showing excesses in e54cr of up to 140e these new results show that the mn cr chronometer should be used with caution in samples minerals with high fe cr and long cosmic ray exposure ages

synchrotron based speciation of chromium in an oxisol from new caledonia importance of secondary fe oxyhydroxides in new caledonia the weathering of ultamafic rocks under a tropical climate has led to the residual accumulation of trace elements in lateritic soils widely dominated by fe oxyhydroxides the speciation of trace elements such as cr ni and co in these oxisols remains a major subject of interest regarding mining and environmental issues we have assessed the speciation of chromium in the upper part of an oxisol by combining bulk and spatially resolved chemical analyses epma and sem eds with synchrotron based spectroscopic data exafs and xanes epma indicates that the main hosts for chromium in the bedrock sample are the silicates forsterite enstatite and lizardite hosting of chromium in these easily weatherable mineral species could lead to a significant loss of this element upon weathering however total analyses of major elements indicate only a slight depletion of cr together with an immobility of fe and al and drastic losses of si and mg after the weathering of the bedrock such a low mobility of chromium is likely related to its significant incorporation in goethite and hematite formed after the weathering of fe 2 bearing primary silicates this efficiency of secondary fe oxyhydroxides at immobilizing chromium is demonstrated by quantitative analysis of exafs data that indicates that these mineral species host between 67 and 75 wt of total cr compared to the 18 to 22 wt of total cr hosted by chromite in addition sem observation and sem eds analyses performed on the oxisol samples also show some evidence for chemical weathering of chromite chromite could then represent a past and or present source of chromium upon extended tropical weathering of the studied oxisol rather than a stable host these results emphasize the importance of secondary fe oxyhydroxides compared to cr spinels on chromium hosting in oxisols developed upon tropical weathering of ultramafic rocks although the trapping mechanism of chromium mainly corresponds to incorporation within the structural network of goethite and hematite sorption reactions at the surface of these mineral species could also be involved in such a process in addition considering their potential oxidative reactivity that can generate cr 6 or enhance the chemical weathering of chromite the occurrence of mn oxides could significantly modify the behavior of chromium upon weathering these considerations indicate that further studies are needed to assess the actual potential of chromium release from oxisols developed upon weathering of ultramafic rocks under a tropical climate

chromium isotope signature during continental crust subduction recorded in metamorphic rocks the chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern china were systematically measured to investigate the cr isotope behavior during continental crust subduction the cr isotope compositions of all samples studied were bulk silicate earth bse like with 53crnist979 of greenschists amphibolites and eclogites ranging from 0 06 to 0 17 0 05 to 0 27 and 0 01 to 0 24 respectively the lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic cr isotope fractionation was associated with the degree of metamorphism however the cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state rather than protolith heterogeneity i e magma differentiation in addition the differences in 53cr 53crcpx gt between coexisting clinopyroxene cpx and garnet gt from two garnet pyroxenites were 0 06 and 0 34 respectively indicating that significant inter mineral cr isotope disequilibria could occur during metamorphism to provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks we analyzed mantle derived chromites and the associated peridotites from luobusa and we obtained the following general order chromite free peridotites 0 21 to 0 11 chromite bearing peridotite 0 07 chromite 0 06 these findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite

fluctuations in late neoproterozoic atmospheric oxidation cr isotope chemostratigraphy and iron speciation of the late ediacaran lower arroyo del soldado group uruguay oxygenation of the earth s atmosphere occurred in two major steps near the beginning and near the end of the proterozoic eon 2500 to 542 ma ago but the details of this history are unclear chromium isotopes in iron rich chemical sediments offer a potential to highlight fine scale fluctuations in the oxygenation of the oceans and atmosphere and to add a further dimension in the use of redox sensitive tracers to solve the question regarding fluctuations of atmospheric oxygen levels and their consequences for earth s climate we observe strong positive fractionations in cr isotopes 53 cr up to 5 0 in iron rich cherts and banded iron formation horizons within the arroyo del soldado group ediacaran uruguay that can be explained by rapid effective oxidation of fe ii rich surface waters these fluctuations are correlated with variations in ratios of highly reactive iron fe hr to total iron fe tot which indicate a predominance of anoxic water columns fe hr fe tot 0 38 during the onset of oxidation pulses we favor the scenario by which isotopically heavy cr vi entered the basin after pulses of oxidative weathering on land and in which fe ii accumulated in the water column neodymium isotopes reveal that these oxygenation pulses were followed by increased influxes to the basin of continental crust derived detrital components of paleoproterozoic nd t dm model ages 2 1 2 2 ga provenance typical of the rio de la plata craton the association of positive 53 cr ferruginous fe hr fe tot 0 38 stratigraphic intervals with low diversity acritarch assemblages dominated by bavlinella faveolata strongly support models postulating a stratified eutrophic neoproterozoic ocean thus even within a few million years of the precambrian cambrian boundary paleoceanographic conditions resembled more those of paleoproterozoic oceans than phanerozoic and present oceans this highlights the sheer magnitude of ecological changes at the precambrian cambrian transition changes which ultimately led to the demise of the precambrian world and the birth of the metazoan dominated phanerozoic

the chromium isotopic composition of seawater and marine carbonates article history chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past however cr is a trace metal in seawater and its low concentrations make isotopic measurements challenging here we report the first determinations of 53 cr for seawater from open ocean argentine basin and coastal southampton water settings using a double spike technique the total chromium concentration in seawater from southampton water is 1 85 nm whereas the cr content of argentine basin samples is 5 8 6 6 nm the 53cr value of seawater from the argentine basin is 0 491 0 556 in intermediate and deep waters and varies between 0 412 and 0 664 in surface waters 150 m the 53

reconstruction of early cambrian ocean chemistry from mo isotopes the neoproterozoic cambrian transition was a key time interval in the history of the earth especially for variations in oceanic and atmospheric chemical composition however two conflicting views exist concerning the nature of ocean chemistry across the precambrian cambrian boundary abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late ediacaran while other studies provide seemingly conflicting evidence for anoxic deep waters with ferruginous conditions fe ii enriched persisting into the cambrian here two early cambrian sedimentary platform and shelf slope sections in south china were investigated to trace early cambrian ocean chemistry from mo isotopes the results reveal that early cambrian sediments deposited under oxic to anoxic euxinic conditions have 98 95mo values ranging from 0 28 to 2 29 which suggests that early cambrian seawater may have had 98 95mo values of at least 2 29 similar to modern oceans the heaviest and relatively homogeneous 98 95mo values were recorded in siltstone samples formed under completely oxic conditions which is considered that mn oxide free shuttling was responsible for such heavy 98 95mo value further combined with fe species data and the accumulation extent of mo and u the variation of 98 95mo values in the two studied sections demonstrate a redox stratified ocean with completely oxic shallow water and predominantly anoxic even euxinic deeper water having developed early on which eventually became completely oxygenated this suggests that oceanic circulation at the time became reorganized and such changes in oceanic chemistry may have been responsible for triggering the cambrian explosion of biological diversity

progressive oxidation of anoxic and ferruginous deep water during deposition of the terminal ediacaran laobao formation in south china abstract the laobao formation a black chert sequence was deposited in deep water in the terminal ediacaran nanhua basin the chert samples have finely laminated fabric and have no distinct recrystallization or metasomatic relict texture there is an extremely limited detrital component and no obvious hydrothermal influence therefore the chert appears to be an authigenic deep water precipitate the trace elements in the chert were mainly derived from seawater and thus could be a relatively unique proxy for late precambrian seawater conditions redox sensitive trace elements and relevant ratios were used to reconstruct the depositional environment of the chert sequence combined with iron species results of the laobao formation enrichments in redox sensitive trace elements u v mo depleted mn very low th u ratios 7 9 and v v ni 0 6 ratios suggest an overall anoxic and ferruginous conditions over the deposition of the chert the pyrite sulfur isotope compositions are between 15 4 and 11 0 and show a decreasing trend toward the top of the section possibly implying sulfate concentration gradually increased in the terminal ediacaran nanhua basin which is consistent with implication of their pyrite framboid diameter characteristics our data confirm that deep water in the nanhua basin was still anoxic and ferruginous and it was progressively oxidized during the terminal ediacaran period which could create necessary conditions for the emergence and rapid evolution of multicellular metazoans

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

hydrogen sulphide release to surface waters at the precambrian cambrian boundary changes in environmental conditions at the precambrian cambrian transition around 542 million years ago have been suggested as a possible explanation for the apparent rapid increase in abundance of multicellular organisms known as the cambrian explosion the nature of the environmental changes is still a matter of debate however wille et al now report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of about 542 million years with the help of a box model of the oceanic molybdenum cycle they find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal this suggests that the early cambrian animal radiation may have been triggered by a major change in ocean circulation following a long period during which the ocean was stratified with sulphidic deep water this paper reports molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 million years the findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water animal like multicellular fossils appeared towards the end of the precambrian followed by a rapid increase in the abundance and diversity of fossils during the early cambrian period an event also known as the cambrian explosion 1 2 3 changes in the environmental conditions at the precambrian cambrian transition about 542 myr ago have been suggested as a possible explanation for this event but are still a matter of debate1 2 3 here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 myr we find a transient molybdenum isotope signal immediately after the precambrian cambrian transition using a box model of the oceanic molybdenum cycle we find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal our findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water

the isotopic composition of authigenic chromium in anoxic marine sediments a case study from the cariaco basin chromium cr isotopes are an emerging proxy for tracking redox processes at the earth s surface however there has been limited exploration of the cr isotope record of modern and recent marine sediments the basic inorganic chemistry of cr suggests that anoxic marine basins should factor prominently in the global cr cycle and that sediments deposited within anoxic basins may offer a valuable cr isotope archive throughout earth s history here we present 53 cr data from sediments of the cariaco basin venezuela a type environment for large perennially anoxic basins with a relatively strong hydrological connection to the global oceans we document a marked positive shift in bulk 53 cr values following the termination of the last glacial maximum followed by relative stasis based on a suite of independent redox proxies this transition marks a switch from oxic to persistently anoxic and sulfidic euxinic depositional conditions within the basin we find good agreement between two independent approaches toward estimating the 53 cr composition of authigenic cr in euxinic cariaco basin sediments and that these estimates are very similar to the 53 cr composition of modern open atlantic ocean seawater these data together with considerations of reaction kinetics and mass balance within the cariaco basin are consistent with the hypothesis that anoxic marine settings can serve as a chemical archive of first order trends in seawater 53 cr composition additionally the cariaco basin data suggest that there has been secular stability in the average 53 cr value of atlantic seawater over the last 15 kyr

oxidative release of chromium from archean ultramafic rocks its transport and environmental impact a cr isotope perspective on the sukinda valley ore district orissa india abstract this study investigates cr isotope fractionation during soil formation from archean 3 1 3 3 ga ultramafic rocks in a chromite mining area in the southern singhbhum craton orissa india the cr isotope signatures of two studied weathering profiles range from non fractionated mantle values to negatively fractionated values as low as 53 cr 1 29 0 04 local surface waters are isotopically heavy relative to the soils this supports the hypothesis that during oxidative weathering isotopically heavy cr vi is leached from the soils to runoff the impact of mining pollution is observed downstream from the mine where surface water cr concentrations are significantly increased accompanied by a shift to less positive 53 cr values relative to upstream unpolluted surface water a microbial mat sample indicates that microbes have the potential to reduce and immobilize cr vi which could be a factor in controlling the hazardous impact of cr vi on health and environment the positive cr isotope signatures of the brahmani estuary and coastal seawater collected from the bay of bengal further indicate that the positively fractionated cr isotope signal from the catchment area is preserved during its transport to the sea isotopically lighter cr vi downstream from the mine is probably back reduced to cr iii during riverine transport leading to similar cr isotope values in the estuary as observed upstream from the mine

oceanic redox condition during the late ediacaran 551 541 ma south china abstract the late ediacaran 551 541 ma oceanic environment could represent a significant preparation for the enrichment of redox sensitive elements during the early cambrian and even for the evolution of life however the marine environment during the late ediacaran has not been thoroughly characterized to date to better understand late ediacaran oceanic redox conditions we investigated the fe and s isotopic compositions of cherts from the slope yinjiang and basin silikou sections of the nanhua basin in south china the slope yinjiang section is characterized by a wide range of 56fet values 0 12 to 1 06 and by continuously decreasing 34spy values from 40 4 to 4 1 the variable 56fet values in the yinjiang section primarily reflect the various oxidation degrees of dissolved fe2 in ferruginous seawater the homogeneous 56fet signatures 0 02 0 40 along the silikou section and the decreasing trend of 56fehr values from 0 86 to near zero primarily depend on the mixing of fe3 hydroxide oxide precipitates in a ferruginous water column possibly followed by partial reduction the authigenic framboidal pyrite trapped within an intermittent euxinic condition and detrital silicate fe minerals a 20 30 discrepancy in 34spy values between the yinjiang and silikou sections may be ascribed to the formation of diagenetic pyrites and framboidal pyrites respectively therefore the continuously decreasing 34spy values in the yinjiang and silikou sections may reflect a significant increase of sulfate diffusion into sediments at the slope location and the enlarged marine sulfate reservoir at the basin location these new results indicate that the deep water in the nanhua basin is primarily characterized by a ferruginous condition with intermittent euxinic conditions in the basin location during the late ediacaran

cr isotopic composition of the laobao cherts during the ediacaran cambrian transition in south china abstract in this study we present the cr isotopic compositions from the cherts in the laobao formation 551 522 ma that were deposited in deep water in south china the cr concentrations and isotopic compositions in the laobao cherts were dominated by the contemporaneous seawater and detrital phases the lower part of the laobao cherts show that the 53cr of contemporaneous deep seawater in the huanan basin was approximately 0 1 and the 53cr of detrital materials was approximately 0 2 these findings imply that surface 53cr enriched cr vi species did not affect the deep ocean until the late ediacaran because of the strong redox stratification in the ocean although the oxidative cr cycle in terrestrial settings had emerged before that time a slight rise in 53cr in the deep ocean to approximately 0 08 was estimated for the upper part of the laobao cherts if the 53cr rise and its amplitude can be further determined this will imply that shallow 53cr enriched cr vi species had an impact on the deep ocean and will suggest an oxidation of the ocean in the huanan basin around the ediacaran cambrian boundary 542 ma

marine redox stratification on the earliest cambrian ca 542 529 ma yangtze platform abstract in order to explore the marine redox structure on the earliest cambrian ca 542 529 ma yangtze platform south china new fe mo c s geochemical data from the slope xugongping section for the liuchapo and the base of niutitang formation are reported here integrated fe speciation and mo abundance data from the xugongping section in combination with previously published data from slope basinal longbizui section and others suggest oxic surface waters and ferruginous deep waters coexisting with occasional mid depth euxinic waters during the fortunian stage and with widespread euxinic waters during the early stage 2 marine transgression a compilation of new and published sulfur isotope data of pyrite for the liuchapo and the base of niutitang formations presents high isotopic values mostly 10 and distinctly different spatiotemporal gradient characteristics during the fortunian stage and the beginning of the stage 2 we propose that redox stratification developed in the earliest cambrian protected basin and eustatic change and the connectivity of the protected basin with the open ocean controlled the materials supply from riverine input and open ocean reservoir and thus spatiotemporal variability of ocean chemistry

from source to sink preserving the cosmogenic 10be derived denudation rate signal of the bolivian andes in sediment of the beni and mamore foreland basins abstract the denudation rate signal of the bolivian andes as measured by cosmogenic 10 be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years this conclusion is drawn from comparing published andean source area denudation rates with new cosmogenic 10 be data as measured in the floodplains of the large beni and mamore basins for the entire beni basin including the sediment producing andes and the vast flooded plains of the foreland the cosmogenic nuclide derived denudation rate is 0 45 0 06 mm yr while that of the mamore basin is 0 55 0 19 mm yr by comparison the respective andean source areas erode at averaged rates of 0 37 0 06 mm yr upper beni and at 0 56 0 09 mm yr upper mamore we notice a decrease in variability of denudation rate with increasing spatial scale as small scale processes are averaged out sediment mixing within the floodplain damps scatter of nuclide derived denudation rates picked up in the source area on the temporal scale a remarkable agreement between cosmogenic nuclide derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide derived denudation rates capture the long term erosional characteristics of the mountain belt the sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area this finding opens the unique possibility of constraining cosmogenic nuclide derived paleo sediment budgets for these large basins as the long term spatially averaged denudation rate signal of the sediment producing area is preserved in sedimentary archives

response of the cr isotope proxy to cretaceous ocean anoxic event 2 in a pelagic carbonate succession from the western interior seaway abstract chromium offers a redox sensitive isotopic proxy with potential for tracing past oxygen levels in the oceans we explore this potential in a pelagic succession of marine carbonate sediment deposited during cretaceous ocean anoxic event 2 oae 2 in the western interior seaway colorado wis using the usgs portland 1 core reduction of cr vi causes light isotopes of cr to preferentially partition into cr iii because cr vi is the thermodynamically favoured species in oxygenated seawater and cr iii is relatively insoluble under the same conditions increased removal of cr iii into anoxic marine sediment during ocean anoxic events should cause positive shifts in seawater 53cr values this assumes that isotopic fractionation associated with all cr removal fluxes from the oceans was constant during oae 2 and that there was no change in cr input fluxes to the oceans here we report findings that counter this prediction namely evidence for a negative shift in seawater 53cr during oae 2 of 1 1 in the wis the magnitude of the excursion depends on the speciation of cr removed in this setting over the duration of oae 2 as well as the fractionation factor accompanying the removal flux of chromate into carbonate sediment both of which are uncertain the 53cr excursion reaches values as low 0 09 during oae 2 which is indistinguishable from the high temperature igneous rock baseline value of 0 124 0 101 2 moreover the minima in the 53cr profile coincides with a peak in cr concentrations in the study core as well as anomalous enrichments of other trace metals of basaltic affinity building on previous studies linking oae 2 to a massive episode of submarine volcanic activity we attribute the decrease in carbonate 53cr values during oae 2 to the expansion of a hydrothermal superplume created during eruptions of the caribbean large igneous province lip this metal laden plume enriched in cr iii and probably anoxic moved from deep waters of the eruption site in the eastern pacific into deep waters of the proto north atlantic through an oceanic gateway in the central americas once inside anoxic metal rich waters upwelled against the surrounding continental margins where isotopically unfractionated cr iii whose solubility was improved by organic complexation and or oxidation to cr vi mixed with the isotopically heavy pool of oceanic chromate in surface waters above the redoxcline application of a simple mixing equation shows that hydrothermal cr contributed 90 of the authigenic cr in the trace metal enriched interval of the study core the increased flux of hydrothermal cr was large enough to mask the expected isotopic response of the ocean cr cycle to increasing anoxia we consider this interpretation to be robust in that it is not substantially affected by uncertainty in the speciation of hydrothermal cr nor uncertainty in the fractionation factor sw caco 3 associated with chromate removal into marine carbonate sediment which we measured in the modern caribbean sea to be 0 46 0 14 2

the cr isotope signature of surface seawater a global perspective abstract this study presents chromium isotope data expressed as 53 cr and chromium concentrations cr of surface seawater 0 2 m depth from a variety of locations worldwide in addition to samples from the open ocean arctic southern pacific and atlantic oceans samples were analysed from areas with more restricted water exchange rates the mediterranean sea oresund and baltic sea the data indicate a heterogeneous distribution of 53 cr in seawater with a total range from 0 13 to 1 24 the data are in agreement with a previous study which focused on depth profiles suggesting that seawater heterogeneity in 53 cr and cr concentration can be explained using a single fractionation factor for cr reduction e 0 79 0 06 2sd in basins with limited water exchange with the open ocean however local factors seem to control the cr isotope composition

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

weathering on land and transport of chromium to the ocean in a subtropical region misiones nw argentina a chromium stable isotope perspective abstract we have investigated the pathway of chromium from its mobilization on land and along its riverine transport in a subtropical region of south america misiones province argentina in an attempt to link cr stable isotope compositions recently measured in seawater with signals prevailing in rivers and ultimately with cr isotope effects observed during oxidative surface weathering in subtropical red soils cr concentrations and stable cr isotopic compositions expressed as 53cr values in two typical and representative surface profiles of weathered basalt show significant depletion of cr in the soils of up to 50 together with pronounced negatively fractionated 53cr values which are indicative of oxidative mobilization of heavy cr vi into the run off the behavior of cr in the studied weathering profiles is not correlated with that of other redox sensitive elements such as ce and u this is essentially due to the affinity of ree and u but not cr with secondary phosphates which form during weathering processes smaller tributaries in nw argentina to the parana river second largest river in south america carry dissolved cr in the order of 0 7 1 4 ppb 13 27 nm with 53cr values of 0 2 to 0 4 balancing the negatively fractionated weathering products the isotope composition and concentration of dissolved cr in the ca 1200 km long parana river from misiones to its estuary and discharge area into the south atlantic ocean remains relatively constant with an average cr concentration around 2 4 ppb 46 nm and an average 53cr value of 0 32 the cr concentration in the estuary itself drops by ca 50 but with only minor change in its cr isotope composition results from the parana estuary are identical with recently analyzed surface seawater from the argentine basin with cr contents of 0 3 ppb 6 nm and 53cr values 0 4 bonnand et al 2013 and indicate that there is only a minimal cr isotopic variability during riverine transport even during long transport distances as shown in our example of the parana river simple cr input flux calculations reveal that the parana river accounts for 5 of the total yearly cr flux to the world s oceans today and that its isotopic signature seems to be at least locally imparted to the surface seawater of the argentine basin whether or not this cr isotope signature is generally exhibited by the world s oceans needs further investigations particularly the characterization of seawater around the globe

new u pb zircon ages of the ediacaran cambrian boundary strata in south china the ediacaran cambrian transition is a critical interval marking drastic biological oceanic and geochemical co evolutions in geological history but it is poorly constrained geochronologically in south china we here present two new sets of sims u pb zircon ages from ediacaran cambrian boundary strata dengying liuchapo and niutitang formations deposited in the slope basin environments of carbonate platforms two weighted mean u pb ages of 542 1 5 0 ma and 542 6 3 7 ma in the basal and mid upper liuchapo formation respectively in slope and basinal settings provide the first direct age set for the ediacaran cambrian boundary in south china another two u pb ages of 524 2 5 1 ma and 522 3 3 7 ma from the base of the overlying niutitang formation indicate that this widespread unit in south china was deposited about 20 ma after the onset of the cambrian

new u pb age from the basal niutitang formation in south china implications for diachronous development and condensation of stratigraphic units across the yangtze platform at the ediacaran cambrian transition abstract the black shale dominated niutitang formation covers a large portion of the early cambrian yangtze platform in south china the base of this unit has been traditionally taken as the ediacaran cambrian boundary ca 542 ma recent radiometric ages from the basal niutitang formation and its correlative units however are significantly younger than 542 ma and raise questions about the diachrony of stratigraphic units across the yangtze platform at the ediacaran cambrian transition here we report a new u pb sensitive high resolution ion microprobe shrimp age of 522 7 4 9 ma from a tuffaceous bed of the basal niutitang formation in a deep water section in taoying guizhou province of south china this age in combination with existing ages and biostratigraphic data suggests that the base of the niutitang formation is of tommotian or upper meishucunian age and the ediacaran cambrian boundary is most likely within the liuchapo formation the depositional age of the unusual metal enriched ni mo pge sulfide deposits which are approximately 1 m above the ca 523 ma tuffaceous bed should be early tommotian upper meishucunian rather than early nemakit daldynian lower meishucunian regional correlation confirms the diachronous development and condensation of stratigraphic units across the early cambrian yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the ediacaran cambrian transition in south china

on the coevolution of ediacaran oceans and animals fe speciation and s isotope of pyrite data from the terminal proterozoic sheepbed formation in canada and doushantuo formation in china reveal that ocean deep waters were anoxic after the global glaciations snowball earth ending 635 million years ago but that marine sulfate concentrations and inferentially atmospheric oxygen levels were higher than before the glaciations this supports a long postulated link between oxygen levels and the emergence of eumetazoa subsequent ventilation of the deep ocean inferred from shifts in fe speciation in newfoundland previously published data and western canada this report paved the way for ediacaran macrobiota to colonize the deep seafloors

multiflood and multilayer method for scour rate prediction at bridge piers the sricos method was proposed in 1999 to predict the scour depth versus time curve at a cylindrical bridge pier for a constant velocity flow a uniform soil and a deep water condition in this article the method is extended to include a random velocity time history and a multilayer soil stratigraphy it is called the extended sricos or e sricos the algorithms to accumulate the effects of different velocities and to sequence through a series of soil layers are described the procedure followed by the computer program to step into time is outlined a simplified version of e sricos called s sricos is also presented calculations for the s sricos method can easily be done by hand eight bridges in texas are used as case histories to compare predictions by the two new methods e sricos and s sricos with measurements at the bridge sites

petrology and geochemistry of chert on the marginal zone of yangtze platform western hunan south china during the ediacaran cambrian transition the ediacaran cambrian transition was one of the most critical intervals in earth history during this interval widespread chert was precipitated commonly as a stratal wedge in carbonates along the southern margin of the yangtze platform south china the chert wedge passes into a full chert succession further basinward to the south east four lithotypes of chert are identified across the marginal zone in western hunan mounded vein brecciated and bedded chert the mounded chert is characterized by irregular to digitiform internal fabrics generally with abundant original vesicles and or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals the host chert or matrix of these mounds is dominated by amorphous cryptocrystalline silica commonly disseminated with pyrite the vein chert with minor quartz locally generally cross cuts the overlying dolostone and chert horizons and terminates under the mounded and or bedded chert bodies the brecciated chert commonly occurs as splayed intrusions or funnel shaped wedges and cross cuts the topmost dolostones the bedded chert the most common type generally is thin to medium bedded and laminated locally it is composed of amorphous silica with minor amounts of black lumps microthermometry of fluid inclusions from vein and void lining minerals mainly quartz and barite revealed homogenization temperatures from 120 to 180 c for the trapped primary fluids compositionally these chert deposits generally are pure with sio2 92 wt and only minor fe2o3 and al2o3 contents most of which show positive europium anomalies in rare earth element patterns especially for the mounded chert all these data suggest that the marginal zone chert deposits resulted from a low temperature silica rich hydrothermal system in which the mounded chert was precipitated around the releasing vents i e as silica chimneys the vein and splayed brecciated chert however was formed along the syndepositional fault fracture conduits that linked downward while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor these petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation rapid tectonodepositional and oceanic changes during the ediacaran cambrian transition in south china

chromium isotope variations δ53 52cr in mantle derived sources and their weathering products implications for environmental studies and the evolution of δ53 52cr in the earth s mantle over geologic time here we report chromium isotope compositions expressed as delta cr 53 52 in per mil relative to nist 979 measured in selected cr rich minerals and rocks formed by the primary magmatic as well as the secondary metamorphic and weathering processes the main objectives of this study were i to further constrain the isotope composition of the earth s mantle cr inventory and its possible variation during geological history based on the analysis of globally distributed and stratigraphically constrained mantle derived chromites and ii to investigate the magnitude and systematics of cr isotope fractionation during oxidative weathering and secondary alteration i e hydration serpentinization of the magmatic cr sources specifically we analyzed delta cr 53 52 in a set of globally distributed mantle derived chromites femgcr2o4 n 30 collected from various locations in europe asia africa and south america and our results confirm that a chromite hosted earth s mantle cr inventory is uniform at 0 079 0 129 2sd which we named here as a canonical mantle d 53 52 cr signature furthermore our dataset of stratigraphically constrained chromites whose crystallization ages cover most of the earth s geological history indicate that the bulk cr isotope composition of the chromite hosted mantle inventory has remained uniform within about 0 100 since at least the early archean times similar to 3500 million years ago ma to investigate the systematics of cr isotope fractionation associated with alteration processes we analyzed a number of secondary cr rich minerals and variably altered ultramafic rocks i e serpentinized harzburgites lherzolites that revealed large positive delta cr 53 52 anomalies that are systematically shifted to higher values with an increasing degree of alteration and serpentinization the degree of aqueous alteration and serpentinization was quantified by the abundances of fluid mobile rb k elements and by the loss on ignition loi parameter which determines the amount of structurally bound water oh h2o present in secondary hydrated minerals like serpentine overall we observed that altered ultramafic rocks that yielded the highest loi values and the lowest amounts of fluid mobile elements also yielded the heaviest delta cr 53 52 signatures therefore we conclude that secondary alteration i e hydration serpentinization of ultramafic rocks in near surface oxidative environments tend to shift the bulk cr isotope composition of the weathered products to isotopically heavier values pointing to a dynamic redox cycling of cr in the earth s crustal and near surface environments hence if validated by future

the chromium isotope composition of reducing and oxic marine sediments the chromium cr isotope composition of marine sediments has the potential to provide new insights into the evolution of earth surface redox conditions there are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport the major source of seawater cr and with partial cr reduction during burial in marine sediments the major sink for seawater cr a more comprehensive understanding of these processes is needed to establish global cr isotope mass balance and to gauge the utility of cr isotopes as a paleoredox proxy for these purposes we investigated the cr isotope composition of reducing sediments from the upwelling zone of the peru margin and the deep cariaco basin chromium is present in marine sediments in both detrital and authigenic phases and to estimate the isotopic composition of the authigenic fraction we measured 53cr on a weakly acid leached fraction in addition to the bulk sediment in an effort to examine potential variability in the cr isotope composition of the detrital fraction we also measured 53cr on a variety of oxic marine sediments that contain minimal authigenic cr the average 53cr value of the oxic sediments examined here is 0 05 0 10 2 n 25 which is within the range of 53cr values characteristic of the bulk silicate earth this implies that uncertainty in estimates of authigenic 53cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of cr in detrital versus authigenic phases rather than to the cr isotopic composition of the detrital pool leaches of cariaco basin sediments have an average 53cr value of 0 38 0 10 2 n 7 which shows no dependency on sample location within the basin and is close to that of atlantic deepwater cr 0 5 this suggests that authigenic cr in anoxic sediments may reliably reflect the first order cr isotope composition of deepwaters for peru margin samples the average 53cr values of bulk sediments 0 59 0 06 2 and leach fractions 0 61 0 06 2 are also comparable with those of cariaco basin samples and modern deepwater values finally we found that the 53cr of peru margin samples correlates with 15n on glacial interglacial timescales which we attribute to secular variation in basinal or global ocean redox conditions thus the 53cr stratigraphic record of peru margin sediments indicates that cr isotopes may be suited to tracking geologically short term changes in ocean oxygenation

constraints in using cerium anomaly of bulk sediments as an indicator of paleo bottom water redox environment a case study from the central indian ocean basin abstract the total rare earth elements ree abundance along with major and a range of trace element chemistry of twenty eight sub sections in a 5 m long sediment core aas 05 gc 02 from the central indian ocean basin shows a distinct ree fractionation sediments from the top 1 m of the core have comparatively low ree abundance close to average shale with a sample shale ree ratio very close to 1 strongly suggesting a dominantly terrigenous source the down core ree abundance shows a nearly two fold increase ree 167 341 ppm with depth and the ree strongly co varies with mn ti p and smectite inter element correlations and the shale normalized ree patterns suggest that light middle and heavy ree are selectively carried by terrigenous mn oxide and authigenic phosphate phases respectively suggesting a distinct ree fractionation these fractionation effects become more pronounced with increasing depth in the core the cerium ce anomaly in marine sediments is used as one of the promising tools to trace paleo bottom water redox conditions and this varies in the sediments from 0 10 to 0 08 the ce anomaly in general is positive in the top 4 m but the top 1 m have higher ce anomaly mn ti and ce ti suggest sediment deposition relatively under more oxic bottom water condition the negative ce anomaly between 4 and 5 m depth which could be interpreted to indicate a suboxic anoxic environment the redox environment inferred by the ce anomaly is compared to a number of other redox sensitive parameters to test its reliability these include mn content total organic carbon u th authigenic uranium cu zn and v cr ratio that all suggestive of deposition of sediments under oxygenated bottom water condition thus the negative ce anomaly observed between 4 and 5 m core depth does not represent a suboxic anoxic environment but is more likely due to the retention of a negative ce anomaly caused by authigenic phosphate and authigenic fe rich smectite which is formed by the reaction between iron oxyhydroxide and biogenic opal suggests an early diagenetic process and is in equilibrium with seawater ree early diagenetic process is responsible for the part of negative ce anomaly observed here therefore we suggest that the ce anomaly of bulk sediments as an indicator of paleo ocean bottom water redox conditions needs to be used with a caution

redox fluctuations during the ediacaran cambrian transition nanhua basin south china insights from cr isotope and ree y data abstract reconstructing fluctuations in marine and atmospheric redox conditions during the late precambrian early cambrian is essential to better our understanding of the link between oxygen and biological evolution this study presents ree y and cr isotopic data together with toc data and redox sensitive trace metal concentrations of marine black shales and cherts from the cambrian jiumenchong formation and ediacaran liuchapo formation from south china in order to improve our understanding of the redox conditions in the nanhua basin during this critical time period the liuchapo formation records negative ce anomalies and highly fractionated 53cr values of up to 1 06 0 09 suggesting deposition under oxic conditions and implying high oxidative weathering rates during the end of the ediacaran the overlying jiumenchong formation is characterised by a stratigraphic upward decrease in 53cr values to typical igneous values as well as a decrease in negative ce anomalies and an increase in positive eu anomalies suggesting an increase in detrital content due to more aeolian weathering and deposition under anoxic conditions at the beginning of the cambrian the latter is also confirmed by the trends in v cr and th u ratios which further indicate an eventual return to dys oxic seawater conditions during the early cambrian

chromium isotope stratigraphy of ediacaran cap dolostones doushantuo formation south china abstract the yangtze platform in south china is considered a key site for studying neoproterozoic ocean oxygenation comprehensive cr concentration and isotopic signatures are presented from cap dolostones of the doushantuo formation from a deep water and a shallow marine carbonate section that have previously been used to constrain the redox evolution of ediacaran seawater in space and time employing major and trace element abundances as well as sr o and c isotope compositions the majority of the studied samples yield elevated cr concentrations and negatively fractionated 53 cr signals due to i detrital contamination consistent with cr signatures of other post marinoan cap dolostones ii diagenetic carbonate alteration and post depositional remobilization of cr al concentrations were used as indicator of detrital contamination and to calculate authigenic cr concentration and isotope signatures when the detrital contribution of cr was 18 o carb signals that correlate with low cr concentrations and negatively fractionated 53 cr values post depositional remobilization of cr results in the loss of heavy cr isotopes and consequently light cr isotopes are retained during precipitation of secondary carbonates however some samples might still show pristine ediacaran seawater signals two samples pertaining to the deep water cap dolostone section at yanwutan and one sample pertaining to the shallow water section at jijiawan yield slightly positively fractionated 53 cr signals particularly when corrected to presumably authigenic signals 53 cr auth these values are consistent with those from other neoproterozoic carbonate sections deposited in an oxygenated environment this short sequence of positively fractionated 53 cr auth is concurrent with a cap dolostone 87 sr 86 sr signal close to ediacaran seawater values with relatively high sr and cr concentrations with decreased detrital contamination and with a pronounced negative ce anomaly they indicate either light pulses of enhanced oxidative weathering or inferred from a few least altered samples prevailing oxic conditions during cap dolostone deposition the cr isotopic composition of ediacaran cap dolostones pertaining to the doushantuo formation reveals fine scale fluctuations in post glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering cr isotopes have the potential to further enhance our understanding of neoproterozoic weathering regimes and past ocean oxygenation however before the cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates possible diagenetic alteration of the cr signal as well as cr contribution from detrital contamination need to be taken into consideration

the stable cr isotope inventory of solid earth reservoirs determined by double spike mc icp ms abstract we present the first comprehensive set of stable cr isotope data for the major igneous silicate earth reservoirs cr iii rich ores and minerals and hydrothermal chromates these were determined by mc icp ms using a double spike technique mantle xenoliths ultramafic rocks cumulates as well as oceanic and continental basalts share a common cr isotope composition with an average 53 52 cr value of 0 124 0 101 2 sd relative to the isotopically certified chromium standard nist srm 979 an isotopic difference between mantle xenoliths and basalts as was reported for iron was not observed for chromium thus the change in oxidation state that is observed when solid mantle rocks containing only trivalent chromium partially melt to form basaltic melts which predominantly contain bivalent chromium does not cause any measurable cr isotope fractionation chromite separates from major chromitite seams of the bushveld and great dyke layered igneous complexes are invariable in their cr isotope compositions and reproduce within uncertainties the average 53 52 cr srm 979 value of igneous silicate earth reservoirs this is important for environmental stable cr isotope studies because it labels the approximate isotope composition of industrial cr vi pollutants the cr isotope compositions of hydrothermal lead chromates crocoites pbcr vi o 4 from various localities yield 53 52 cr srm 979 values between 0 640 and 1 037 these cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached precipitation experiments revealed an isotope fractionation of 53 52 cr crocoite cr vi aq of only ca 0 10 0 05 at a temperature of 20 c thus the heavy cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes

global variability of chromium isotopes in seawater demonstrated by pacific atlantic and arctic ocean samples abstract seawater chromium cr isotope and concentration data are presented from multiple sites in the arctic ocean and three locations in the atlantic and pacific oceans a 2400 m profile illustrates the heterogeneity of 53cr in the arctic ocean with depth and water mass source pacific vs atlantic the highest 53cr values occur in pacific sourced waters which also have the lowest cr concentration chromium concentration and 53cr data from these locations in conjunction with published data for the south atlantic ocean yield a simple logarithmic function as predicted by rayleigh fractionation in a closed system the observed cr isotope signature is hypothesized to arise from fractionation during the reduction of cr vi in surface waters and oxygen minimum zones scavenging of isotopically light cr iii to deeper water and sediment and subsequent release of this seawater derived cr iii back into seawater either as organic complexes with cr iii or after oxidation to cr vi the isotopic fractionation factor e associated with cr cycling in seawater is estimated to be 0 80 0 03 2 samples from the sea ice affected surface mixed layer of the arctic ocean 10 m depth deviate from the general trend and samples proximal to rivers illustrate geographic variation in 53cr values for continental runoff but prompt loss of this signature away from the source

germanium silicon of the ediacaran cambrian laobao cherts implications for the bedded chert formation and paleoenvironment interpretations sedimentary strata of the terminal ediacaran 635 542 ma to early cambrian 542 488 ma laobao liuchapo bedded cherts in the south china block include the ediacaran oxidation event and the cambrian explosion understanding the origin and depositional environment of the bedded cherts may provide insight into how the earth s surface environment changed between the proterozoic and phanerozoic we measured major and trace element compositions and ge si ratios of the laobao cherts from northern guangxi province the laobao cherts were deposited in the deep basinal environment of the south china block we show that the composition of the laobao cherts is determined by a mixture of four components quartz clay carbonate and pyrite iron oxide the quartz component is the dominant component of the laobao cherts the maximum estimated ge si of the quartz component is between 0 4 and 0 5 mol mol which is close to the ge si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids these ge si systematics suggest that normal seawater rather than mid ocean ridge hydrothermal fluids is the primary si source for the laobao cherts the ge si of the clay component varies between 1 and 10 mol mol which is comparable to the ge si of typical marine clays but 10 100 times lower than that of chert nodules from early ediacaran beds the doushantuo formation predating the terminal ediacaran laobao cherts studied here our observations indicate that the clay component ge si ratio decreased from the early ediacaran to the late ediacaran we speculate that high ge si ratios in clays reflect the preferential chelation of ge by dissolved organic compounds adsorbed onto clays if so this suggests that the decrease in ge si ratio of the clay component in the ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the ediacaran cambrian transition consistent with oxidation of the oceans during the late ediacaran finally the seawater origin of the laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation instead direct precipitation from seawater or early diagenetic silicification of calcareous sediments perhaps due to the emergence of si accumulation bacteria may have been responsible for the bedded laobao liuchapo chert formation in south china block