a basin redox transect at the dawn of animal life multiple eukaryotic clades make their first appearance in the fossil record between 810 and 715 ma molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation animals require oxygen to fuel their metabolism and low oxygen levels have been hypothesized to account for the temporal lag between metazoan origins and the cambrian radiation of large ecologically diverse animals here paleoredox conditions were investigated in the fifteenmile group ogilvie mountains yukon canada which hosts an 811 ma ash horizon and spans the temporal window that captures the inferred origin and early evolution of animals iron based redox proxies redox sensitive trace elements organic carbon percentages and pyrite sulfur isotopes were analyzed in seven stratigraphic sections along two parallel basin transects these data suggest that for this basin oxygenated shelf waters overlay generally anoxic deeper waters the anoxic water column was dominantly ferruginous but brief periods of euxinia likely occurred these oscillations coincide with changes in total organic carbon suggesting euxinia was primarily driven by increased organic carbon loading overall these data are consistent with proposed quantitative constraints on proterozoic atmospheric oxygen being greater than 1 of modern levels but less than present levels comparing these oxygen levels against the likely oxygen requirements of the earliest animals both theoretical considerations and the ecology of modern oxygen deficient settings suggest that the inferred oxygen levels in the mixed layer would not have been prohibitive to the presence of sponges eumetazoans or bilaterians thus the evolution of the earliest animals was probably not limited by the low absolute oxygen levels that may have characterized neoproterozoic oceans although these inferred levels would constrain animals to very small sizes and low metabolic rates

development of a modified sedex phosphorus speciation method for ancient rocks and modern iron rich sediments abstract we report the development of a modified method for evaluating different reservoirs of sedimentary phosphorus p in ancient marine sedimentary rocks and in modern fe rich sediments utilising the existing sedex scheme for p partitioning in modern sediments we initially demonstrate limitations in the application of the original scheme to sediments and rocks containing crystalline hematite and magnetite we tested additional extractions for these crystalline fe phases using both synthetic minerals and modern and ancient sediments the addition of 6 h oxalate and 6 h citrate dithionate acetate extractions considerably enhanced the total recovery of synthetic magnetite and hematite to 88 7 1 1 and 76 9 3 8 respectively in addition application of the 6 h oxalate extraction to synthetic p containing magnetite recovered 93 9 1 7 of the fe present and 88 2 12 8 of the co precipitated p based upon these results we developed a modified sedex extraction scheme the modified scheme was applied to modern fe rich sediments from golfo dulce costa rica which resulted in 16 higher fe bound p recovery application of the scheme to a variety of ancient marine rocks increased the recovery of fe bound p by up to 22 we also highlight the potential for authigenic carbonate fluorapatite to convert to more crystalline apatite in ancient rocks during deep burial and metamorphism we suggest that in such systems minimum and maximum estimates of the total reactive p pool may be calculated with and without the inclusion of crystalline p it is noted that the application of the revised method may have important implications for understanding the cycling of p in ancient marine environments

tectono sedimentary analysis of a complex extensional neogene basin formed on thrust faulted northern apennines hinterland radicofani basin italy abstract large nw se oriented neogene quaternary structural depressions up to about 200 km long and 25 km wide have developed on the western side hinterland of the northern apennines over thrust substrate the depressions are now for the most part laterally bounded by normal faults and are longitudinally separated into basins by transfer zones a debate exists in the literature as to whether these basins have developed as half graben under a predominantly extensional regime since late miocene or as thrust top basins under a predominantly compressional regime that has continued until the pleistocene the radicofani basin is one of the best preserved basins it developed mainly during the late miocene early pliocene in the southern half of the siena radicofani structural depression and is now bounded on the east by normal faults that transect a thrust anticline nose in the substrate to the north by a substrate high or transfer zone and to the south and west by quaternary igneous volcanic edifices the basin experienced variable differential tectonic and associated sedimentation along linking normal boundary faults along its eastern margin it shows the development of thick 600 m alluvial fans that developed in relay areas between boundary faults and transverse faults and transfer zones well exposed sections generally show upward transitions from conglomeratic alluvial fans to shoreface sandstone to offshore mudstones locally the transition is marked by deltas primarily characterised by thick gravelly sandy stacked cross sets the thicker sandy gravel to gravelly sand cross sets 5 8 m thick are interpreted as gilbert type deltas interstratified thinner 0 5 1 m thick generally openwork gravelly strata are part of delta topset assemblages and probably represent prograding fluvial bars tectonic movements provided the accommodation space for the total 2700 m thick basin fill sea level fluctuations that led to the repeated development of the cross sets may also have been influenced by climatic or eustatic changes possibly related to the effects of early antarctic glaciations some features of the radicofani basin can be found in both extensional and compressional basins however the position of the basin in the mountain chain and the development of alluvial fans fandeltas and associated deposits along the main boundary fault combined with structural evidence from seismic lines show that during the early pliocene this basin best conforms to existing models of half graben

assessing the utility of fe al and fe speciation to record water column redox conditions in carbonate rich sediments abstract geochemical proxies based on fe abundance fe al and fe speciation have been widely applied to marine sediments in order to unravel paleo depositional redox conditions though geological time to date however these fe proxies have only been calibrated in relation to modern and ancient fine grained siliciclastic marine sediments this clearly limits their use particularly in relation to carbonate rich sediments and rocks to address this we here explore the applicability of fe based redox proxies in carbonates through three approaches first we have compiled fe al data for modern marine sediments to investigate variability in fe enrichments as a function of carbonate content and depositional setting second we have expanded this approach with a compilation of new and existing fe speciation data for modern and ancient marine sediments deposited under oxic and euxinic anoxic sulfidic water column conditions finally we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the fe speciation redox proxy modern marine sediments deposited under oxic conditions show no relationship between fe al and carbonate content these sediments have an average fe al ratio of 0 55 0 11 with some higher values potentially being attributable to steady state early diagenetic remobilization of fe towards the sediment water interface in contrast significant fe al enrichments occur as a consequence of water column fe mineral formation and deposition either under anoxic conditions or due to input of anoxic hydrothermal fluids into oxic seawater iron speciation data also show no direct correlation with carbonate content and instead three groups can be distinguished based on total fe fe t and organic c contents sediments deposited under oxic water column conditions with fe t 0 5 wt generally plot below the lower fe hr fe t siliciclastic reference threshold 0 38 for distinguishing anoxic environments regardless of carbonate content also consistent with siliciclastic calibrations carbonate rich sediments deposited under anoxic water column conditions tend to have fe hr fe t ratios 0 38 in contrast oxic carbonate rich sediments with low fe t hr fe t ratios suggesting that the use of fe speciation for such samples is not appropriate for evaluating water column redox conditions taken together our data suggest that an fe t content of 0 5 wt rather than carbonate content represents the most suitable basis for identifying whether carbonate rich sediments are appropriate for fe based redox proxy analyses analysis of burial dolostones suggests that the fe speciation proxy may also be compromised by deep burial dolomitization where there has been a source of mobile fe to enrich rocks during recrystallization however pre screening can identify such samples this new assessment expands the utility of fe based redox proxies to also incorporate appropriate carbonate rich rocks provided that care is taken to assess the possible impact of deep burial dolomitization

oceanic molybdenum drawdown by epeiric sea expansion in the mesoproterozoic abstract molybdenum is a bioessential micronutrient whose abundance in the global oceans may have played a primary role in evolution of the earth s nitrogen cycle and ultimately in the timing of the ecologic expansion of early eukaryotes because molybdenum mo is delivered to the ocean under conditions of oxic weathering and is removed in suboxic and euxinic environments the concentration of mo in the oceans reflects a complex function of global redox and the hydrologic conditions that influence the areal extent of euxinic sedimentation recent compilations of mo within euxinic shales scott et al 2008 och and shields zhou 2012 sahoo et al 2012 indicate that the oceanic reservoir of mo did not rise substantially above crustal values until the great oxidation event 2 3 ga and a modern mo cycle did not develop until the contraction of ocean euxinia in the latest proterozoic at present a paucity of data from euxinic shales of the late mesoproterozoic 1 3 to 1 0 ga limits our ability to relate marine redox evolution to biological innovation during a critical interval of eukaryotic development here we present data from marine shales of the 1 1 ga atar and el mreiti groups mauritania which were deposited during sea level highstand when shallow epeiric seas covered much of the west african craton epicratonic strata of the el mreiti group were deposited under fluctuating redox conditions near a shallow chemocline as evidenced by iron speciation and record generally low mo concentrations

enhanced chemical weathering triggered an expansion of euxinic seawater in the aftermath of the sturtian glaciation abstract the cryogenian period comprised two episodes of global glaciation sturtian and marinoan glaciations separated by a non glacial interval which was characterized by early radiations of eukaryotic algae and putative metazoans geochemical data indicate that the non glacial interval might be marked by a transient marine oxygenation nevertheless oceanic redox conditions varied both in time and space further the links between non glacial climate and marine redox variations are not well constrained here we present high resolution lithium isotope 7 li fe speciation and trace element mo and u data for clastic sedimentary rocks from the cryogenian interglacial datangpo formation south china in order to track the evolution of continental chemical silicate weathering and driving factors behind marine redox variability during the cryogenian non glacial interval a significantly negative 7 li excursion of 5 is observed in the basal datangpo formation suggesting a dramatic increase in chemical silicate weathering intensity in the aftermath of the sturtian glaciation expansion and contraction of anoxic sulfidic conditions as demonstrated by fe speciation and trace element mo and u data mirror changes in silicate weathering intensity our study provides evidence that greater nutrient and sulfate availability due to high silicate weathering intensity associated with increased exposure of fresh rocks and a warm climate facilitated the spread of euxinic waters over the continental margins of the otherwise ferruginous cryogenian ocean

analyses of fluid inclusions in neoproterozoic marine halite provide oldest measurement of seawater chemistry we analyzed primary fluid inclusions in halite from marine evaporites in the ca 830 ma browne formation of the officer basin in western australia using the cryogenic scanning electron microscopy energy dispersive x ray spectrometry analysis technique the concentrations of the major ions except k and possibly so 4 2 fall within the range of phanerozoic seawater this is the first direct measurement of the composition of mid neoproterozoic seawater and extends present day knowledge of seawater chemistry by 300 m y our estimates suggest that mid neoproterozoic marine sulfate concentrations were lower 90 than modern values by the terminal neoproterozoic fluid inclusions in halite and evaporite mineralogy indicate seawater sulfate levels rose significantly to 50 80 of modern concentrations which parallels increases in atmospheric and oceanic oxygen

development of a sequential extraction procedure for iron implications for iron partitioning in continentally derived particulates abstract the development of a sequential extraction procedure for iron in modern and ancient sediments is presented the scheme recognizes seven operationally derived iron pools 1 carbonate associated fe fe carb including siderite and ankerite 2 easily reducible oxides fe ox1 including ferrihydrite and lepidocrocite 3 reducible oxides fe ox2 including goethite hematite and akaganeite 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u as such this is the first extraction scheme specifically developed to allow the separate identification of magnetite and the first to allow a complete evaluation of fe carbonate phases such as siderite and ankerite the scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times further tests on mixtures of pure minerals and on grain size separated sediments from two major us rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals and demonstrate a high degree of reproducibility for each analytical stage the data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine grained continental sediments

molybdenum evidence for expansive sulfidic water masses in 750 ma oceans abstract the ediacaran appearance of large animals including motile bilaterians is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances po 2 to date direct evidence for low oxygen in pre ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time because the abundance and isotopic composition of molybdenum mo in organic rich euxinic sediments can vary in response to changes in global redox conditions mo geochemistry provides independent constraints on the global evolution of well oxygenated environments here we establish a theoretical framework to access global marine mo cycle in the past from the abundance and isotope composition of ancient seawater further we investigate the 750 ma walcott member of the chuar group grand canyon which accumulated in a rift basin with open connection to the ocean iron speciation data from upper walcott shales indicate that local bottom waters were anoxic and sulfidic consistent with their high organic content up to 20 wt similar facies in phanerozoic successions contain high concentrations of redox sensitive metals but in the walcott member abundances of mo and u as well as mo toc 0 5 ppm wt are low 98 mo values also fall well below modern equivalents 0 99 0 13 versus 2 35 today these signatures are consistent with model predictions where sulfidic waters cover 1 4 of the global seafloor corresponding to a 20 80 fold increase compared to the modern ocean therefore our results suggest globally expansive sulfidic water masses in mid neoproterozoic oceans bridging a nearly 700 million year gap in previous mo data we propose that anoxic and sulfidic euxinic conditions governed mo cycling in the oceans even as ferruginous subsurface waters re appeared 800 750 ma and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic o 2 level consistent with the hypothesis that po 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the ediacaran period

redox dependent distribution of early macro organisms evidence from the terminal ediacaran khatyspyt formation in arctic siberia abstract the ediacaran period witnessed the first appearance of macroscopic animal life in earth s history however the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain in part due to the dearth of integrated paleobiological and chemostratigraphic datasets in this study a comprehensive geochemical analysis was conducted on the fossiliferous khatyspyt formation in arctic siberia in order to gain insights into the ediacaran paleoenvironments this study was designed to specifically address the relationship between paleoredox conditions and ediacaran fossil occurrences in the khatyspyt formation our data reveal a dramatic shift in pyrite sulfur isotope compositions 34 s pyrite from ca 20 to ca 55 and this shift is intriguingly associated with the first occurrence of ediacara type macrofossils at the studied section suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms based on multiple lines of sedimentological and geochemical evidence we propose that the development of oceanic euxinia which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal ediacaran period may have locally prohibited the colonization of ediacara type organisms and resulted in low 34 s pyrite values in the lower khatyspyt formation in the middle and upper khatyspyt formation progressive secular transition from euxinic to non euxinic and more habitable conditions may have allowed for the colonization of ediacara type and other macro organisms

middle proterozoic ocean chemistry evidence from the mcarthur basin northern australia we report on the iron sulfur systematics and s isotopic compositions of upper paleoproterozoic carbonaceous shales from the wollogorang and lower reward formations mcarthur basin northern australia iron speciation in these rocks is similar to that of modern euxinic black sea sediments and distinct from sediments depositing from oxygenated water providing persuasive evidence of euxinic depositional environments in a mid proterozoic marine basin high values of degree of pyritization dop also indicate anoxic bottom water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides the isotopic composition of the sulfides is however quite enriched in 3 4 s and for the lower reward formation similar to contemporaneous seawater sulfate these 3 4 s enriched sulfides are consistent with extensive or even complete sulfate depletion within the euxinic basins in which the sediments were deposited based on analogy with modern euxinic environments and further constrained by model results middle proterozoic seawater sulfate concentrations were probably within the range of 0 5 to 2 4 mm taken together our results indicate a relatively low sulfate late paleoproterozoic ocean and sulfidic deep waters in the mcarthur seaway a condition that could have encompassed much larger regions of the ocean

pervasive oxygenation along late archaean ocean margins the photosynthetic production of oxygen in the ocean is thought to have begun at least 2 7 billion years ago the geochemistry of marine sediments deposited 2 6 billion years ago suggests that ocean margins were oxygenated at least 100 million years before the first significant increase in atmospheric oxygen concentrations

widespread contamination of carbonate associated sulfate by present day secondary atmospheric sulfate evidence from triple oxygen isotopes the isotope composition of seawater sulfate is an important tracer of sulfur carbon and oxygen cycles in earth s deep past carbonate associated sulfate cas extracted by acid digestion is widely used as a proxy for sulfate in paleo seawater from which the carbonate minerals precipitated early and late diagenesis weathering and laboratory processing can in some cases compromise original seawater sulfate signals here we report that extracted cas can also be severely contaminated by recent atmospheric sulfate especially when the sampled carbonates are from outcrops in arid to semi arid climates or in heavily polluted regions our evidence comes from triple oxygen isotope compositions of sequentially extracted water leachable sulfate and acid leachable sulfate from carbonates of diverse ages from northwestern and north central china and southwestern north america independent of the age of the rocks almost all the water leachable sulfates and half of the acid leachable sulfates bear positive 17o anomalies clearly distinguishable from those of typical seawater sulfate because secondary atmospheric sulfate sas is the only source of sulfate known to bear positive 17o anomalies we conclude that sulfate extracted from carbonate outcrops in these regions has a significant component of sas because sas generally has a much lower 34s value than paleo seawater sulfate it could shift the 34s of the extracted cas to lower values and in some cases even lower than that of the co occurring pyrite i e the super heavy pyrite enigma reported in geological records our findings call for a re evaluation of many published outcrop based cas data and conclusions

neoproterozoic sulfur isotope variation in australia and global implications abstract seven sulfur isotope stages are apparent in australian neoproterozoic sections 1 from 840 ma to the sturtian glaciation 700 ma 34ssulfate varied little 19 to 17 5 and 34ssulfide ranged from 20 to 23 2 the sturtian glaciation was followed by 3 a rise in 34ssulfide to an average of 30 seen also in china namibia and canada and in 34ssulfate at least to 26 and possibly to 45 the sharp rise in 34s of all sulfur fractions sulfate sulfide and organic leads us to speculate that sulfide depleted in 34s was deposited on the abyssal plain and that residual sulfate enriched in 34s remained beneath a stagnant ice covered ocean during the sturtian glaciation and was brought to the shallow continental margin during the post glacial transgression 4 34ssulfide and 34ssulfate fall from 640 to 607 ma 34ssulfate down to 17 probably from oxygen excess through massive organic carbon burial which causes weathering of sulfide depleted in 34s continental sulfide and previously deposited sulfide on the abyssal plain 5 the oxic environment of the marinoan 605 595 ma glaciation minimises sulfate reduction unlike the sturtian so that 34ssulfate is unvarying 6 the post glacial environment has sulfide depleted in 34s average 34ssulfide

a global transition to ferruginous conditions in the early neoproterozoic oceans eukaryotic life expanded during the proterozoic eon1 2 5 to 0 542 billion years ago against a background of fluctuating ocean chemistry2 3 4 after about 1 8 billion years ago the global ocean is thought to have been characterized by oxygenated surface waters with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron containing ferruginous deeper waters5 6 7 the spatial extent of sulphidic waters probably varied through time5 6 but this surface to deep redox structure is suggested to have persisted until the first neoproterozoic glaciation about 717 million years ago8 9 10 11 here we report an analysis of ocean redox conditions throughout the proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops we find a global transition from sulphidic to ferruginous mid depth waters in the earliest neoproterozoic coincident with the amalgamation of the supercontinent rodinia at low latitudes we suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on rodinia we propose that this transition essentially detoxified ocean margin settings allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago

the low temperature geochemical cycle of iron from continental fluxes to marine sediment deposition suspended sediments from 34 major rivers geographically wide spread and 36 glacial meltwater streams have been examined for their variations in different operationally defined iron fractions fehr iron oxides soluble in dithionite fepr iron soluble in boiling hcl but not in dithionite and feu total iron less that soluble in boiling hcl river particulates show a close association between fehr and total iron fet reflecting the effects of chemical weathering which derive oxide iron from and retain it in close association with total iron consistent with this continental scale average fehr fet ratios vary with runoff ratios average river runoff per unit area average precipitation per unit area by contrast the diminished effects of chemical weathering produce no recognizable association of fehr with fet in glacial particulates and instead both fepr and feu are closely correlated with fet reflecting essentially pristine mineralogy a comparison of the globally averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in fehr fepr and fet but enriched in feu the river and glacial particulate data are combined with estimates of authigenic hydrothermal atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for fehr f e pr f e u and fet this budget suggests that the differences between riverine particulates and marine sediments can be explained by i preferentially removing fehr from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains salt marshes and estuaries and ii mixing the resulting riverine particulates with fehr depleted glacial particulates preliminary measurements of inner shore sediments are consistent with i above phanerozoic and modern normal marine sediments have similar iron speciation characteristics which implies the existence of a long term steady state for the iron cycle this steady state could be maintained by a glacioeustatic feedback where fehr enriched riverine particulates are either more effectively trapped when sealevel is high small ice masses diminished glacial erosion or are mixed with greater masses of fehr depleted glacial particulates when sealevel is low large ice masses enhanced glacial erosion further important controls on the steady state for fehr operate through the formation of euxinic sediments and ironstones which also provide sealevel dependent sinks for fehr enriched sediment

chemostratigraphy of the shaler supergroup victoria island nw canada a record of ocean composition prior to the cryogenian glaciations a b s t r a c t a new 13 ccarb curve combined with 13 corg values is presented for the upper shaler supergroup 900 to 720 ma amundsen basin northwestern canada the dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust there is a pronounced negative 13 c excursion in the wynniatt formation that can be correlated with a putative worldwide negative car bon isotope excursion namely the bitter springs stage however in the amundsen basin the 13 ccarb excursion drops to anomalously negative values 14 which we attribute to local overprints wherein isotopically light carbon in pore waters released by oxidation of methane and organic matter during sulphate and iron reduction was incorporated into authigenic carbonate cement we document basin euxinia and anoxia during the same time interval using a multi proxy approach specifically fe speciation and redox sensitive trace metal data patterns of pronounced enrichment in mo v and u concentrations in euxinic black shales suggest that the bitter springs stage was a transitional period in earth s redox evo lution from the more reduced global oceans during the mid proterozoic to the more oxygenated oceans during the phanerozoic 2015 elsevier b v all rights reserved

an assemblage of macroscopic and diversified carbonaceous compression fossils from the tonian shiwangzhuang formation in western shandong north china abstract macroalgae play a crucial ecological and biogeochemical role in modern marine ecosystems although eukaryotic algae have a deep history within the precambrian most pre cryogenian alga fossils are microscopic and macroalga fossils are poorly documented during this period here we report a well preserved assemblage of macroscopic and diversified carbonaceous compression fossils from the tonian shiwangzhuang formation in western shandong north china this assemblage consists of seventeen distinct morphotaxa including eleven macroalgae showing remarkable taxonomic diversity that is more diverse than many other tonian macrofossil assemblages all over the world three taxa from the shiwangzhuang assemblage show evidence of simple multicellularity with two of them consisting of giant cells and comparable to giant celled green algae and seven taxa with diagnostic transverse annulations are likely coenocytic algae the shiwangzhuang assemblage indicates that some eukaryotic clades had achieved macroscopic growth through simple multicellularity or coenocytism paving the way either ecologically or phylogenetically for the eventual appearance of complex multicellularity in addition there are some biostratigraphically significant genera including the chuaria tawuia and sinosabellidites protoarenicola pararenicola assemblages indicating a tonian age for the shiwangzhuang formation this study also suggests that the tonian diversity of multicellular or coenocytic macroalgae is probably underestimated pending for a systematic and thorough investigation of tonian macroscopic carbonaceous compressions macroalgae may have an initial ecological expansion at least locally if not globally in the tonian period

paleoclimate reconstruction during the messinian evaporative drawdown of the mediterranean basin insights from microthermometry on halite fluid inclusions 1 data constraining the paleoenvironmental conditions during the messinian evaporative drawdown of the mediterranean basin are still conflicting here we present a comprehensive paleoclimatic reconstruction during messinian halite deposition from several italian sites we performed fluid inclusion analyses to define better both the composition of the mediterranean water body and the paleotemperatures of the messinian brine during halite crystallization we measured homogenization temperatures on 218 primary all liquid fluid inclusions in messinian halite from the volterra crotone and caltanissetta evaporite basins these measurements provided mean homogenization temperatures of 17 18 c with a range between 10 11 c and 28 29 c which should be close to the sst of the mediterranean sea during halite deposition the occurrence of major elements such as cl na mg s ca and k within the halite fluid inclusions together with the presence of minerals such as pentahydrite polyhalite and ca k and mg sulfates indicates that these salt bodies originated from mainly marine water taking into account both the present day annual sst of the mediterranean sea around the sampling sites which ranges between 18 and 20 c and the lower latitude of the mediterranean basin during the messinian salinity crisis our homogenization temperatures point to a colder climate during the messinian halite deposition compared to the present interglacial climate stage this conclusion is consistent with halite deposition during a late miocene glacial interval tg12

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

superheavy pyrite δ34spyr δ34scas in the terminal proterozoic nama group southern namibia a consequence of low seawater sulfate at the dawn of animal life sulfur isotope analysis 34 s of well preserved carbonates spanning an 10 ma interval of the terminal proterozoic nama group reveals that disseminated pyrite is consistently enriched in 34 s relative to coeval seawater sulfate as preserved in carbonate associated sulfate cas this observation is not consistent with the current paradigm for interpreting the geologic record of sulfur isotopes which assumes that pyrite 34 s 34 s pyr will be equal to or less than co occurring cas 34 s 34 s cas due to the kinetic isotope effect of bacterial sulfate reduction bsr that favors the lighter isotope of sulfur 32 s during sulfur oxygen bond breakage although the precise mechanism of pyrite sulfur isotope enrichment is debatable our combined observations of extremely 34 s enriched pyrite low bulk rock concentrations of sulfur from cas and pyrite and high frequency fluctuations in 34 s cas and 34 s pyr throughout the nama group carbonates point to very low concentrations of sulfate in portions of the terminal proterozoic ocean the additional occurrence of 34 s enriched pyrite in contemporaneous terminal proterozoic sections from poland and canada reveal that low seawater sulfate may have been widespread in the oceans at this time however the absence of such extremely 34s enriched pyrite from well preserved coeval carbonate sections in oman suggests that such conditions were not globally uniform low geographically varied concentrations of marine sulfate in terminal proterozoic time are consistent with elevated geographically varied concentrations of reactive nonpyritized iron in marine shales recently reported for this interval suggesting that high dissolved fe ii and low o 2 persisted in a range of marine facies as late as 543 ma tens of millions of years after the origin of animals yet prior to their major diversification in early cambrian time

re os age constraints and new observations of proterozoic glacial deposits in the vazante group brazil abstract a new re os radiometric age date for an organic rich shale horizon from the vazante group in brazil coupled with geological observations provide evidence for late mesoproterozoic glacial episodes conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time field observations of a reverse fault juxtaposing older mesoproterozoic sedimentary rocks above younger neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of u pb detrital zircon ages through the succession combined the geochronological geochemical and stratigraphic evidence suggest that the vazante group sediments accumulated along a passive margin of the sao francisco craton and are correlative with the neighboring paranoa group biomarker sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post glacial transgression and deposition of one of the bituminous shale horizons the relationship of the glaciogenic vazante group to other late mesoproterozoic successions such as the non glacial atar group in west africa and bylot supergroup in arctic canada however remains enigmatic

proterozoic onset of crustal reworking and collisional tectonics reappraisal of the zircon oxygen isotope record a global u pb and 18o zircon database shows temporal changes in the magmatic record related to changes in the degree of crustal reworking the 18o composition of bulk sediment remains relatively constant through geologic time with a mean value of 14 9 in contrast the 18o values in magmatic zircons vary from relatively low values averaging 6 in the archean to increasingly higher and scattered values defining a series of peaks and troughs in post archean data the degree of crustal reworking increases at times of supercontinent assembly therefore we attribute the pattern of post archean 18o values recorded by magmatic zircons to a significant increase in the incorporation of high 18o sediment in response to enhanced crustal thickening and reworking associated with the onset of collisional tectonics especially during formation of supercontinents